Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Bill S on 14/02/2015 20:29:23
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Extract from the Haifa Lectures:
"Consider the following ‘thought experiment’: A cat is crossing an intersection in the road. At the center of the intersection there is a manhole cover and at the curb there is a workman with his hand on a lever that would open the manhole cover. As soon as the cat steps toward the manhole cover, the workman simultaneously pulls the lever that opens it. The cat then falls below the street, never to reach the other side of the road. Suppose now that a helicopter is flying over the road, at a speed close to the speed of light! In looking down at the road, the pilot sees the cat crossing the road and the workman pulling the lever to open the manhole cover non-simultaneously, i.e. at a different time than when the cat reaches the manhole cover in the road. The pilot then expects the cat to reach the other side of the road. But instead he sees that the cat disappears midway across the road! He then asks himself: “why didn’t the cat get to the other side of the road?” He answers that it was because what he saw was influenced by the fact that he was in a moving frame of reference, relative to the cat and the road. To learn what really happened he applies the Lorentz transformation to put himself into the frame of reference of the cat and the manhole cover, independent of any outside observer! This is called the ‘proper’ frame of reference — it involves only the interacting things — the cat and the Earth that pulls it downwards. In this (proper) reference frame, he learns that the cat did not reach the other side of the road because the workman pulled the lever at the precise time when the cat stepped down toward it, and so it fell below the street before reaching the other side. Thus we see that the relativity of simultaneity in this theory is not physical; it is only descriptive regarding a viewing from the frame of reference of the observer. To say that relative simultaneity is a physical fact is to predict a paradox — that, in this example, the cat would reach the other side of the road and it would not reach the other side of the road!"
What Sachs seems to be saying here is that there is a “proper” frame of reference in which it is possible to identify what “really happened”, as distinct to what appeared, in another F of R, to have happened.
I was under the impression that one had to consider every F of R as having the same validity in terms of reality; isn’t that right?
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Modify the experiment by having a video screen next to the workman which shows what he sees. From the helicopter, it is now clear that he pulls the lever at the point when he sees the cat stepping onto the manhole cover. It's better to ignore the helicopter altogether and think instead about the cat and workman playing out the action in different frames of reference. In their own frame, the workman is dealing with a delay as the light has to get to him from the cat (plus a much longer delay in his head as all the cogs grind round), but he can predict the right moment to pull the lever to get the cat down the hole. If we think it through again and imagine the whole street to be moving at close to the speed of light, the action of the cat and workman is slowed down substantially, so this cancels out the extra delays caused by the lengthened communication distances - the light takes longer to reach him from the cat but the lever opens the manhole more quickly (or the light may reach him sooner if the street's moving in the opposite direction, in which case the lever will take longer to open the manhole). We therefore have different theories about the temporal separation of the two events, but none of them result in any idea that the cat shouldn't fall down the hole, so the thought experiment is flawed.
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If we think it through again and imagine the whole street to be moving at close to the speed of light, the action of the cat and workman is slowed down substantially, so this cancels out the extra delays caused by the lengthened communication distances - the light takes longer to reach him from the cat but the lever opens the manhole more quickly (or the light may reach him sooner if the street's moving in the opposite direction, in which case the lever will take longer to open the manhole).
That isn't correct at all. The speed of light will remain constant to the local observers so the situation won't change. The speed of reaction on the lever will be as if in a frame moving at non-relativistic speeds. Time dilation comes into effect.
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What Sachs seems to be saying here is that there is a “proper” frame of reference in which it is possible to identify what “really happened”
I didn't read it that way. Instead I thought of a collision between two spaceships. It doesn't matter how some observer is moving, the spaceships collide. There is no way in which you can move so that you see the spaceships miss one another.
as distinct to what appeared, in another F of R, to have happened. I was under the impression that one had to consider every F of R as having the same validity in terms of reality; isn’t that right?
Yes, but what happened is the reality. A frame of reference is merely an abstract thing. It's little more than a state of motion. You can't point up to the clear night sky and say hey look, there's a frame of reference.
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If we think it through again and imagine the whole street to be moving at close to the speed of light, the action of the cat and workman is slowed down substantially, so this cancels out the extra delays caused by the lengthened communication distances - the light takes longer to reach him from the cat but the lever opens the manhole more quickly (or the light may reach him sooner if the street's moving in the opposite direction, in which case the lever will take longer to open the manhole).
That isn't correct at all. The speed of light will remain constant to the local observers so the situation won't change. The speed of reaction on the lever will be as if in a frame moving at non-relativistic speeds. Time dilation comes into effect.
It was (and remains) completely correct. What I described with the street moving at very high speed was exactly how it would be perceived from the helicopter if the people in it regarded themselves as stationary. It should be no surprise to any observer that the cat falls down the hole.
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You might find this article interesting: Length Contraction Paradox by Wolfgang Rindler, Am. J. Phys. 29(6), Jun. (1961)
Abstract - A certain man walks very fast-so fast that the relativistic length contraction makes him very thin. In the street he has to pass over a grid. A man standing at the grid fully expects the fast thin man to fall into the grid. Yet to the fast man the grid is much narrower even to the stationary man, and he certainly does not expect to fall in. Which is correct? The answer hinges on the relativity of rigidity.
It can be downloaded at http://www.newenglandphysics.org/Science_Literature/Journal_Articles/Rindler_AJP_29_6.pdf
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You might find this article interesting: Length Contraction Paradox by Wolfgang Rindler, Am. J. Phys. 29(6), Jun. (1961)
Abstract - A certain man walks very fast-so fast that the relativistic length contraction makes him very thin. In the street he has to pass over a grid. A man standing at the grid fully expects the fast thin man to fall into the grid. Yet to the fast man the grid is much narrower even to the stationary man, and he certainly does not expect to fall in. Which is correct? The answer hinges on the relativity of rigidity.
It can be downloaded at http://www.newenglandphysics.org/Science_Literature/Journal_Articles/Rindler_AJP_29_6.pdf
Now that's just out there Pete. I will have to read it.
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The high speed would make it hard for him to fall down through the hole, but if we scale everything up so that he can be travelling at 87%c at normal walking speed, he would then be compressed to half his normal width in the direction of travel, and the gaps in the grid could be that size too. We can also make him extremely short (un-tall) so that he can get through the gap without any issues with delays for parts of him further up. A tiny acceleration downwards could now put him through a grid of extreme thinness if the gap is even slightly wider than his contracted length, but this downward acceleration has changed his direction of travel and that change will not occur simultaneously along his entire length (width), so the leading part of him will go down first and be followed in turn by each following part. The components of him are all held together by forces which travel at the speed of light, so there is no means by which the parts at the back can hold up the parts at the front fast enough to prevent him bending. So, even when looked at from his own frame of reference, if he gets his sums right, he should see that he can fit through the gap.
Edit: having modified the experiment to this degree though, it occurs to me that he would now fit through the gap even if it's still narrower than his contracted length in all frames of reference.
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Length contraction is a can of worms, David. Motion is relative, the cat could claim that it's the grid that's moving, and is so length contracted that he couldn't fit down a gap. What PmbPhy referred to by Rindler was an attempt to explain the paradox, but I find it utterly unconvincing myself. Take this to the limit with two passing cats riding 1m rods, each with a 0.5m butterfly net: they can't scoop each other. Hence I think it's best to stick with spaceship collisions, then you don't get sucked into length-contraction issues. BOOM! The collision really happens, and everybody sees that it happens, regardless of how they're moving. Nobody sees one spaceship miss the other.
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Extract from the Haifa Lectures:
"Consider the following ‘thought experiment’: A cat is crossing an intersection in the road. At the center of the intersection there is a manhole cover and at the curb there is a workman with his hand on a lever that would open the manhole cover. As soon as the cat steps toward the manhole cover, the workman simultaneously pulls the lever that opens it. The cat then falls below the street, never to reach the other side of the road. Suppose now that a helicopter is flying over the road, at a speed close to the speed of light! In looking down at the road, the pilot sees the cat crossing the road and the workman pulling the lever to open the manhole cover non-simultaneously, i.e. at a different time than when the cat reaches the manhole cover in the road. The pilot then expects the cat to reach the other side of the road. But instead he sees that the cat disappears midway across the road! He then asks himself: “why didn’t the cat get to the other side of the road?” He answers that it was because what he saw was influenced by the fact that he was in a moving frame of reference, relative to the cat and the road. To learn what really happened he applies the Lorentz transformation to put himself into the frame of reference of the cat and the manhole cover, independent of any outside observer! This is called the ‘proper’ frame of reference — it involves only the interacting things — the cat and the Earth that pulls it downwards. In this (proper) reference frame, he learns that the cat did not reach the other side of the road because the workman pulled the lever at the precise time when the cat stepped down toward it, and so it fell below the street before reaching the other side. Thus we see that the relativity of simultaneity in this theory is not physical; it is only descriptive regarding a viewing from the frame of reference of the observer. To say that relative simultaneity is a physical fact is to predict a paradox — that, in this example, the cat would reach the other side of the road and it would not reach the other side of the road!"
What Sachs seems to be saying here is that there is a “proper” frame of reference in which it is possible to identify what “really happened”, as distinct to what appeared, in another F of R, to have happened.
I was under the impression that one had to consider every F of R as having the same validity in terms of reality; isn’t that right?
Everything that Mr Sachs says there is silly IMO.
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Length contraction is a can of worms, David. Motion is relative, the cat could claim that it's the grid that's moving, and is so length contracted that he couldn't fit down a gap. What PmbPhy referred to by Rindler was an attempt to explain the paradox, but I find it utterly unconvincing myself. Take this to the limit with two passing cats riding 1m rods, each with a 0.5m butterfly net: they can't scoop each other. Hence I think it's best to stick with spaceship collisions, then you don't get sucked into length-contraction issues. BOOM! The collision really happens, and everybody sees that it happens, regardless of how they're moving. Nobody sees one spaceship miss the other.
It's true that length contraction was not the issue in the story of cat and manhole. The issue was that cat seemed to step on a cover covering a hole, but still the cat seemed to disappear, like it fell into some hole.
The cat actually did fall into that hole that the cover in the non-proper frame seemed to be covering at the critical moment. In the proper frame the cover was not covering the hole at the critical moment. The proper frame is the frame of the cat and the manhole cover.
Does that make sense?
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Kind of. But like I said earlier, I prefer a collision example because it's simpler. You could say that the cat "collided" with the hole as it were. The pilot would see that. Suggesting he doesn't just muddies the waters IMHO.
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Take this to the limit with two passing cats riding 1m rods, each with a 0.5m butterfly net: they can't scoop each other.
Are you sure? If they're doing 87% the speed of light relative to each other, they can scoop each other. As soon as they've done so though, the nets will be wrenched out of their paws and will take up the speed of the cat they're wrapped around, whereupon they (the nets) will contract and burst apart. Alternatively, if the nets are stronger and can't be ripped out of their paws, both cats will stop and be crushed by the nets. There will of course be different stories to tell about what happened depending on which frames you analyse the events from, because in some frames the net will not be applied around a cat evenly - for example, the leading edge may cross ahead of it first and the trailing edge will then close behind it after most of the cat has been crushed by the front part of the net, but in another frame the cat is simultaneously surrounded by the net all round and then expands to crush itself.
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Kind of. But like I said earlier, I prefer a collision example because it's simpler. You could say that the cat "collided" with the hole as it were. The pilot would see that. Suggesting he doesn't just muddies the waters IMHO.
I happen to have exceptionally good reading comprehension. Therefore I can say that Mr Sachcs does not know that what the pilot is seeing is the reality of what is happening.
Also such thing as the frame of cat and cover does not exist, when the cat is moving relative to the cover.
Contraction of cat and hole is more interesting though.
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Are you sure? If they're doing 87% the speed of light relative to each other, they can scoop each other.
They can't! For that to happen, each net has to be smaller than the other.
As soon as they've done so though, the nets will be wrenched out of their paws...
Yes, bad things will happen, but before that, the crucial point is that each cat's net has to fit over the other cat's rod, which is bigger than the other cat's net. Problemo! And wherever you look, you will not find a proper resolution to what is in essence the pole and the barn paradox (http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/polebarn.html). If you think you have found one, do let me know.
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You can think of it in terms of causality. You have two choices, one where frames disagree, one where they do not. The one where they do not is the universe we exist in.
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That holds true for QM too, the universe we exist in has one outcome (event), as described by us inside it (observers), not several, although all have a real probability of existing before one of those probabilities (outcome) becoming finally observed by us. Although that tells us nothing of where those other probabilities 'went', if they now 'went somewhere'. To happen they need a arrow though. And most probably a whole universe defined around them, to be as 'real' as the events we observe.
To see what the first statement really says you should consider the famous muon example, in where we have two complementary explanations. From the thought up muon the distance contracts as it 'falls' a geodesic path ending at the ground, from the earthbound observer the muon's 'clock' slows down instead. Together those explanations becomes a 'universal' logic, keeping causality intact, explaining why a muon can reach the ground, although it 'shouldn't' (too short-lived when ignoring relativistic effects).
So the logic is there, and so is causality, but it isn't the one we're used too from before Einstein.
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/muon.html
Defining it this way all frames are 'equivalent' in terms of being 'proper frames'. As in the muon example all 'frames of reference' thought up must use their local clock and ruler to measure, and there is no way around it. We all do it.
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What if you put the cat in a box? Then you have uncertainty totally wrapped up in one thought experiment. As Schrodinger used the cat to show how he felt about the HUP I use the box to show the absurdity of this thought experiment.
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It's a tricky proposition to digest :)
What the thought experiment aim for though is that probabilities do have a life of their own, described through the statistics we prove. And you don't know the outcome before it happened. It's another try for a description equivalent to defining a smeared out electron cloud, with a 'orbital' instead of 'orbit', http://www.chemguide.co.uk/atoms/properties/orbitsorbitals.html instead of some singular electron orbiting a nucleus. But yea, it's weird and you need some alternatives when trying to make it into a mind picture. The one I use is thinking of it as a 'field' of sorts, instead of stuff 'moving'.
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such an description can only be meaningful if we can translate between the normal definition of things moving and a 'field' expressing itself at different positions (time & space) though. Maybe one could think of it as another expression of causality's demands. It has to be 'water tight' to work, hmm, I better put a cork in it now :)
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...It's another try for a description equivalent to defining a smeared out electron cloud, with a 'orbital' instead of 'orbit', http://www.chemguide.co.uk/atoms/properties/orbitsorbitals.html instead of some singular electron orbiting a nucleus.
What's wrong with that is the little red dots:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.chemguide.co.uk%2Fatoms%2Fproperties%2Fnastruct2.gif&hash=232ee89c6fe2ac10ca00054e56940d4a)
They're misleading, and they interfere with the message, which is that in atomic orbitals (http://en.wikipedia.org/wiki/Atomic_orbital#Electron_properties) electrons "do not orbit the nucleus in the sense of a planet orbiting the sun, but instead exist as standing waves." A better analogy would be Saturn's rings. Only made out of electromagnetic standing waves. Which aren't quite standing waves.
But yea, it's weird and you need some alternatives when trying to make it into a mind picture. The one I use is thinking of it as a 'field' of sorts, instead of stuff 'moving'. Such a description can only be meaningful if we can translate between the normal definition of things moving and a 'field' expressing itself at different positions (time & space) though. Maybe one could think of it as another expression of causality's demands.
IMHO hula-hoops is quite a nice analogy, only the electron isn't a hoop. On its own it has a spherical symmetry. You can diffract electrons, so they have a definite wave nature. But if the wave is a standing wave, it looks like a standing field. Only it isn't actually static. For an analogy, I could spin a hula-hoop around your waist, and if it was spinning very fast, you might think it wasn't spinning. Then if you jiggle just right, it could start looking like a figure of 8. Next time you've got some sparklers, try whizzing them around in circles and figure-8s. Then try to imagine this kind of thing where you starting with a sphere rather than a ring or a point of light. Not easy I know, but I think you can get some of the way there:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F2%2F2d%2FNeon_orbitals.JPG&hash=bb42e39985bf2612db55b8aa00102b0b)
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Are you sure? If they're doing 87% the speed of light relative to each other, they can scoop each other.
They can't! For that to happen, each net has to be smaller than the other.
They can - each cat sees its net successfully fit around the other cat (just for a moment). What each cat also sees though is the other cat's net fitting around it (the one being scooped by this) by means of the leading part passing just ahead of it first and the trailing part passing just behind it later on, so the net is warped out of its normal shape.
As soon as they've done so though, the nets will be wrenched out of their paws...
Yes, bad things will happen, but before that, the crucial point is that each cat's net has to fit over the other cat's rod, which is bigger than the other cat's net. Problemo!
No hay problemo! Each cat's net does fit over the other cat's rod, but each cat sees the other cat's rod as doing so by cheating - it does not do a simultaneous surroundation.
And wherever you look, you will not find a proper resolution to what is in essence the pole and the barn paradox (http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/polebarn.html). If you think you have found one, do let me know.
There is no paradox there - all there is is a set of different accounts of events which contradict each other on the simultaneity point but crucially with no difference whatsoever to the actual results or indeed to the predictions made as to the outcome. It makes no detectable causal difference whether an expanding cat is crushed progressively from one side to the other inside a strong net or is crushed simultaneously throughout, or indeed if it's crushed progressively from the opposite side. [There will be actual causal differences in an LET universe, but not in Einstein's universe where all causality is necessarily fake - this is an interesting example of that because in a case where something is crushed by this kind of expansion it is crushed from one side in one account but from the opposite side in another account as the expansion is blocked by the container, so part B is crushed by part A in one account (and not by part C), while in another account part B is crushed by part C (and not by part A).]
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I happen to have exceptionally good reading comprehension. Therefore I can say that Mr Sachcs does not know that what the pilot is seeing is the reality of what is happening.
And Dr. Mendel Sachs is an exceptionally good physicist so it's highly unlikely that your suggestion is true.
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It makes no detectable causal difference whether an expanding cat is crushed progressively from one side to the other inside a strong net or is crushed simultaneously throughout, or indeed if it's crushed progressively from the opposite side. [There will be actual causal differences in an LET universe, but not in Einstein's universe where all causality is necessarily fake - this is an interesting example of that because in a case where something is crushed by this kind of expansion it is crushed from one side in one account but from the opposite side in another account as the expansion is blocked by the container, so part B is crushed by part A in one account (and not by part C), while in another account part B is crushed by part C (and not by part A).]
I got that wrong - went over it last night repeatedly until I'd covered all possibilities. There is a problem with causality in relativity, but this turns out not to be an example of it. One of the mistakes I made was in thinking that if something is crushed simultaneously across its length in one frame and is crushed progressively from one end to another in another frame, there will be a frame which allows it to be crushed progressively in the opposite direction. That is not the case. If you view the scene from the frame of the cat being crushed you will see it as simultaneous crushing along the length of the cat. If you view it from the frame from which the net was swung over the cat (the net then appears to get caught on the cat and is whipped away from us) we see the cat being progressively crushed by the net. But, if we observe from any frame moving at any speed in the opposite direction to the original net flinger we see simultaneous crushing of the cat rather than progressive crushing in the opposite direction from before. I'll leave it for those who are keen to do the maths themselves to work out why this should be so.
The other important case I considered (which influenced what I said in my previous post) involves a few tricks to avoid impact compression, so the net is accelerated to match the speed of the cat it's been flung over without relying on picking up any acceleration from the impact. In this version of the thought experiment we then have a net that suddenly finds itself to be stretched out of shape and it will either break up (an option which we'll ignore - we can make it elastic) or it will contract in a hurry and crush the cat inside it. In this case though, we don't have even compression of the cat because we have over-compression of some parts which then spread at the speed of sound rather than the speed of light, and this results in a wave of adjustments to the degree of compression which travel through the cat from both ends towards each other. In the case where the cat is stationary in the frame from which we are observing, the net is applied unevenly onto it with the front edge being in place first and the trailing edge being in place after a delay - this results in the point where the waves of comprression adjustment meet being nearer to the back end of the cat than the front. If we observe from the other frame from which the net was swung (meaning that we're now observing a cat which we consider to be moving while it's being compressed) we see the waves of compression adjustment start at the same time and move towards each other, but they don't meet in the middle because the speed of light across our moving cat is different in different directions, so the waves meet at the same point in the cat as before.
So, it's an awkward case to think your way around, but it ends up not revealing anything interesting.
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Extract from the Haifa Lectures:
"Consider the following ‘thought experiment’: A cat is crossing an intersection in the road. At the center of the intersection there is a manhole cover and at the curb there is a workman with his hand on a lever that would open the manhole cover. As soon as the cat steps toward the manhole cover, the workman simultaneously pulls the lever that opens it. The cat then falls below the street, never to reach the other side of the road. Suppose now that a helicopter is flying over the road, at a speed close to the speed of light! In looking down at the road, the pilot sees the cat crossing the road and the workman pulling the lever to open the manhole cover non-simultaneously, i.e. at a different time than when the cat reaches the manhole cover in the road. The pilot then expects the cat to reach the other side of the road. But instead he sees that the cat disappears midway across the road! He then asks himself: “why didn’t the cat get to the other side of the road?” He answers that it was because what he saw was influenced by the fact that he was in a moving frame of reference, relative to the cat and the road. To learn what really happened he applies the Lorentz transformation to put himself into the frame of reference of the cat and the manhole cover, independent of any outside observer! This is called the ‘proper’ frame of reference — it involves only the interacting things — the cat and the Earth that pulls it downwards. In this (proper) reference frame, he learns that the cat did not reach the other side of the road because the workman pulled the lever at the precise time when the cat stepped down toward it, and so it fell below the street before reaching the other side. Thus we see that the relativity of simultaneity in this theory is not physical; it is only descriptive regarding a viewing from the frame of reference of the observer. To say that relative simultaneity is a physical fact is to predict a paradox — that, in this example, the cat would reach the other side of the road and it would not reach the other side of the road!"
What Sachs seems to be saying here is that there is a “proper” frame of reference in which it is possible to identify what “really happened”, as distinct to what appeared, in another F of R, to have happened.
I was under the impression that one had to consider every F of R as having the same validity in terms of reality; isn’t that right?
All inertial frames give consistent accounts of the same events. The local frame, where the events occur, ls the simplest, avoiding long spatial and temporal values.
The relative simultaneity of a frame IS real, since the perception IS real. It's misleading in the sense of assigning times to remote events, via the synch convention. Even illusions are "real" since they involve real images. It's the perceptional interpretation that is misleading.
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All inertial frames give consistent accounts of the same events. The local frame, where the events occur, ...
There's no such frame as "The frame where the event occurred." An event exists independent of a frame of reference. For example: consider the event: "Fire cracker exploded." There is no unique frame which can be said that this event occurred in that frame. The event occurred in "all" frames of reference, inertial or otherwise.
ls the simplest, avoiding long spatial and temporal values.
You're not speaking of a single event here but two events since its only between two events that a spatial distance or a temporal separation exists between them.
It's misleading in the sense of assigning times to remote events, via the synch convention.
Not at all. No physicist would ever say that the synchronization procedure outlined by Einstein was misleading in any way. And it's quite reasonable to assign a time to any event regardless of where its located. You may have made such an assertion but you never said anything to support it.
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I don't know if I will agree with Sachs there. He takes it a whole step further than the Barn paradox http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html He's stating that the two reference frames will show two different chains of events, doesn't he, in where the 'not so proper chain' breaks down? In one instant the cat just 'disappear'? While in the other 'proper version' logic and causality prevails, event by event.
What the guy sitting in the helicopter sees would be 'magic' to anyone not knowing relativity, and actually still is, well, to me:) That the timing sequences will differ depending on your frame of reference makes sense, but that I would see the cat 'go up in smoke' without cause and effect from that frame makes me wonder. It's not that it's impossible, but to me it breaks causality for one observer. Also time symmetry I think? because one 'FOR' will only be playable to, and from, the moment the cat disappear in thin air. Although in its 'proper frame' it then exist all the time, living or dead, falling down that manhole.
I definitely think that interpretation is arguable.
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On the other tentacle, it fits locality, the one I propose, as locality doesn't need all points of view to make a logic out of it. But causality, and action and reaction definitely need it, if we want a 'container universe'? Because, if Sachs is correct it seems to break down there. . Ouch, no it doesn't fit my version either, as each frame is equally true, and unique. Which then leaves me one frame of observation in where the guy will suspect magic to exist :). I still need event by event to be explainable experimentally in the frame of that observes, and here it won't be possible to explain. So we'll have to assume it fit a 'container model', but one in where magic can exist, at least from the view of that observer in the helicopter.
What he does there is actually to suggest that one frame of reference is more 'real', than the other. What would that then make a Lorentz transformation? It's no longer in a equilibrium, is it? For lack of better expressions. I don't think it fits a 'container universe', neither does it fit locality as I presume that there always will be experiments explaining cause and effects, locally measured. If it doesn't? I don't know, it would be as splinters, mostly seamlessly fitting giving us causality and Lorentz transformations, but in some situations losing causality locally measured, although presumably still able to fit together by a Lorentz transformation.
It's one of the most irritating statements I've seen :) thinking about it. What does it make of a repeatable experiment for example, if it in one place disappear in smoke, relativistically but still uniformly moving :) And where does it place, ahem, the 'real reality'? In a mathematical space, disconnected from my local 'reality'? Thought provoking isn't it? We should invite him.
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You could possibly argue that this is one of the things differing a 'mathematical container universe' from an idea of strict locality? I don't think that would be correct though, as we in both cases expect a explainable logic to the events we observe locally, 'cause and effect' as it is called. And it's also so that locality if placed in a 'common space', as in sharing information, which it is and does, also becomes a 'universe' defined by causality. Still? It's a really interesting argument.
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Heh, thinking it through again. He doesn't suggest that one frame is more correct than the other. What he states is that locality is what defines it. So, even though all frames of reference are equivalently true, if you want to know what really happened to the cat you need to apply that Lorentz transformation to see it with the eyes of the workman or cat, 'at rest' with each other more or less. But it still leaves us the question whether it would happen as he describes it? That the cat just would disappear. That's the one I'm still stuck on.
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I could possibly argue that locality to be defined need be 'at rest' with what it observes?
This one is tricky.
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So okay, even though he defines a frame here as being the 'proper one', he doesn't mean it differently from Einsteins definitions, as I read him now. It's exchangeable frames, the helicopters frame becomes the 'proper one' if the cat wants to see what the helicopter saw, and he will need to make a Lorentz transformation too. Then we have causality left, and the ordering of events. Would it be as magic, not knowing relativity? Does it break casuality for the guy in the helicopter, or not. From where do you define it, if so.
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Ultimately a frame of reference can be thought of as a point in spacetime. Any point adjacent to it could be considered infinitesimally different to it. How absurd do we want to be? One hair on the cat might be caught in a breeze whilst another can be considered stationary with reference to its body. The moving hair will be in a different frame of reference to the 'stationary' hair. Whilst an amusing exercise this really moves nothing forward.
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I know Jeffrey, you can take this being 'at rest' into a micro management. But lets not go there :)
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I've actually discussed that one extensively elsewhere if you're interested, but it will take us from the questions created by the idea of the cat disappearing in one frame of reference. To me it's about there being a causality, and how you then should define it? As presenting us locally understandable events, action and reaction, cause and effect, or not?
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Ultimately a frame of reference can be thought of as a point in spacetime.
What?? Since when? I think that you've been talking to jccc for far too long. Lol!
Any point adjacent to it...
What is the "it" that you're talking to?
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It's simple if we stay in a mathematical space. There's something here that I'm still not happy with, that reality now becomes moved from me observing, into this 'space'. It doesn't break the idea of repeatable experiments though, as they always are performed locally, we just 'compare notes' on them, validating.
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Is it about what logic defining a universe? If someone can think up a experiment validating Sachs idea here I would be much obliged. Let's put it this way. Assume everything is communication and information. And that the stuff doing it is 'c'. That is your reality, the 'force carriers' are 'c'. This is valid when watching the cat 'disappear' too.
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It's somewhat like the question of different uniform motions, and what they do to a universe, isn't it? Why is it just angular momentum that come close to 'c'? What would the universe be if we had found (distant) planets and suns moving relativistically relative us?
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I think it has to do with how to define locality to me. If I accept Sachs definition then I see it as a proof of frames of reference being disconnected from the laws and rules defining how it 'is'. 'What you see is what you get' in my definitions, but it places causality somewhere else. It's not the simple 'universe' we exist in any longer. The 'container' becomes a mathematical space, even though WYSWYG rules locally defined.
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Yep, I think he's right.
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It's a beautiful example he created, isn't it? Which is why I would like to see it experimentally validated. It fits my views on this universe being created out of rules and laws, properties and 'emergences'. It doesn't tell us what 'reality' is, unless you believe yourself to consist of mathematics too :) but it sure as h** tells us that it isn't the mechanical clock work we once expected it to be. It's much more than that.
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And finally, where it doesn't fit my ideas. That's the helicopter, not knowing relativity, watching the cat magically disappear. And the reason why we don't see it can be connected to my question on different relative motions in this universe, and why they're not relativistic. Unless they are we won't see anything like this. And being 'at rest' with Earth and our solar system, and galaxy, the only way to see it should be in a particle accelerator, well, possibly?
So that one has to do with preconceptions.
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Had to look him up after this, and it seems his ideas are somewhat incompatible to mine :) And with what I got out of this example you cited Bill. Because to me he do place causality inside a 'mathematical space' making the cat 'disappear' in one frame, not inside a 'physical space' as defined by matter alone, and so do any application of a Lorentz transformation thinking of it. A awesome thinker though.
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Length contraction is a can of worms, David. Motion is relative, the cat could claim that it's the grid that's moving, and is so length contracted that he couldn't fit down a gap. What PmbPhy referred to by Rindler was an attempt to explain the paradox, but I find it utterly unconvincing myself. Take this to the limit with two passing cats riding 1m rods, each with a 0.5m butterfly net: they can't scoop each other. Hence I think it's best to stick with spaceship collisions, then you don't get sucked into length-contraction issues. BOOM! The collision really happens, and everybody sees that it happens, regardless of how they're moving. Nobody sees one spaceship miss the other.
While it is tempting to simply stick one's head in the ground and ignore length contraction and rigidity, this is not a wise long-term strategy for understanding special relativity.
Imagine that there were two pairs of spacecraft, the "forward" of each pair moving towards each other as in the colliding scenario and each "trailing" spacecraft at rest relative to the other of the pair. In a reference frame of one pair, the trailing spacecraft of the other pair is closer to the forward spacecraft. One can do the calculations simply by calculating the trajectories of the crafts. This contraction occurs whether or not the points we are considering are within one physical object or within two separate physical objects.
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To make the description fit a 'container universe' of matter I would expect both frames, timing notwithstanding, to be able to see cause and effect without needing relativity. that means that each local frame will find a logic and causality to what is happening, without having to turn to a mathematical space(time:) translation.That's the way I first thought of it too, but? Maybe this idea of Sachs better describe the universe in where we live? I really wish this could be made into a experiment.
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to see the way I think we can use that 'muon example' in where we find complementary explanations. You can actually ignore those two explanations, instead concentrating on whether both frames are in constant communication with (observing) the other 'frame'. They are at all times 'communicating' as far as I can see, keeping causality intact in both frames. But when it comes to the cats 'instant disappearance' one frames information 'stops' there (helicopter), breaking cause and effect as observed from that frame, although continues following the type of causality we're used to, from the frame of the workman.
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Let’s try looking at causality.
1. The man pulling the lever causes the cover to open.
2, The cover opening causes the cat to fall into the hole.
We then have to ask two questions.
1. Is there any frame of reference in which the cover is seen to open before the man pulls the lever?
2. Is there any frame of reference in which the cat is seen to fall into the hole before the cover is removed?
I suspect that we need to ask ourselves if we have really adjusted our thinking about past/present/future from Newtonian to Einsteinian physics.
I’m going to think about that for a bit, then try to work out how to insert diagrams, as I guess they might be valuable.
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Let’s try looking at causality.
1. The man pulling the lever causes the cover to open.
2, The cover opening causes the cat to fall into the hole.
We then have to ask two questions.
1. Is there any frame of reference in which the cover is seen to open before the man pulls the lever?
According to relativity theory, no, since the impulse from the lever to the hole travels at less than the speed of light, and signals like this always preserve time order in every well-formed system of coordinates (i.e., frame of reference). (This assumes that the lever opens the hole.)
2. Is there any frame of reference in which the cat is seen to fall into the hole before the cover is removed?
Again, this should not be the case for the same reason.
I suspect that the author of the original piece spoke of the cat "disappearing" to simply mean that the cat fell away from view, not that the cat vanished in some instantaneous way.
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Ultimately a frame of reference can be thought of as a point in spacetime.
What?? Since when? I think that you've been talking to jccc for far too long. Lol!
Any point adjacent to it...
What is the "it" that you're talking to?
LOL probably
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I suspect that the author of the original piece spoke of the cat "disappearing" to simply mean that the cat fell away from view, not that the cat vanished in some instantaneous way.
Either that is just semantics, or you are saying that the cat fell into the hole in one F of R, but not in the other. Are we talking reality here?
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Either that is just semantics, or you are saying that the cat fell into the hole in one F of R, but not in the other. Are we talking reality here?
According to relativity theory, if an event happens in one frame, it happens in every frame. A frame is a way of describing events; it is a necessary condition for describing events in a way that they can be measured.
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According to relativity theory, if an event happens in one frame, it happens in every frame.
Yet at any given time, the cover could be open in one frame, but not in another.
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A frame isn't something that exists, Bill. It's little more than an observer's state of motion.
To understand this gedankenexperiment, think collisions. Start with a fixed manhole cover. Imagine that the cat jumps onto the manhole cover. The cat and the manhole cover are at the same place at the same time, and all observers observe this, regardless of their state of motion, regardless of their reference frame. Now remove the manhole cover and rerun the experiment. The cat and the hole are at the same place at the same time, and all observers observe this, regardless of their state of motion, regardless of their reference frame.
When you have some guy pulling some lever, then either the cat lands on the manhole cover, or it falls into the hole. Whichever happens, all observers observe it, regardless of their state of motion, regardless of their reference frame.
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According to relativity theory, if an event happens in one frame, it happens in every frame.
Yet at any given time, the cover could be open in one frame, but not in another.
No, because there is no "given time" between frames.
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According to relativity theory, if an event happens in one frame, it happens in every frame.
Yet at any given time, the cover could be open in one frame, but not in another.
The cover moving out of the way, and the cat falling through the hole, are coincident events, at the same place and at the same time. The difference in spatial and temporal coordinates is zero for both. No motion of any other viewer can change that! We also have to remember, this is an idealized thought experiment. I've seen videos of cats do amazing movements. Perhaps in a real life scenario, it might extend its reach to an edge and recover.
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you're right PhysBang, it must happen in every frame to keep causality intact. That was my view too, but then I read this piece and started to wonder, again. There are other descriptions too that already had made my head ache, as Being and Becoming in Modern Physics. (http://plato.stanford.edu/entries/spacetime-bebecome/) from that lovely place Stanford Encyclopedia of Philosophy. Have a look at the Andromeda paradox and welcome to my headache. I know that it practically wouldn't matter as it always must be information that decides a appropriate action, and that one is still 'c', but?
If he now was serious in his description of that cat, disappearing, we still need some type of causality to prove a container model. And then it seems to move to this mathematical space? Which I find quite fascinating, but which makes me of two minds :)
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A frame isn't something that exists, Bill. It's little more than an observer's state of motion.
This is a misleading way to think of frames of reference, even though "observer" language was used in the beginning of relativity theory. One can identify frames of reference where no observe is at rest. Frames of reference provide a standard for any measurement in principle, both spatial measurements and temporal measurements. These measurements do not actually have to be done or observed.
The location of every event is given in every frame of reference. (Unless that frame of reference is, for some reason, not well-formed; it is possible to construct frames of reference that do not include all events and this makes them not helpful in discussing those events.) In special relativity, there are ways to calculate the location given to an event in one frame if we are given the location in another frame. (There are a couple of other restrictions on what kind of frame it can be but we can gloss over those right now).
We can say that an object is at rest in a frame if its spacial coordinates do not change over time and we can say that an object is in motion in a frame if its spacial coordinates do change over time. This is essentially a definition of "at rest" and "in motion"--it is one that is entirely dependent on the frame one chooses.
To understand this gedankenexperiment, think collisions. Start with a fixed manhole cover. Imagine that the cat jumps onto the manhole cover. The cat and the manhole cover are at the same place at the same time, and all observers observe this, regardless of their state of motion, regardless of their reference frame. Now remove the manhole cover and rerun the experiment. The cat and the hole are at the same place at the same time, and all observers observe this, regardless of their state of motion, regardless of their reference frame.
This doesn't quite make sense, since if one removes the manhole cover, then the cat and the manhole cover would seemingly be in different places. It is better to say that, if the cat and the manhole cover touch as some point, then they touch at some point in every frame.
When you have some guy pulling some lever, then either the cat lands on the manhole cover, or it falls into the hole. Whichever happens, all observers observe it, regardless of their state of motion, regardless of their reference frame.
Again, I caution against using "observers", since what one observes is dependent on a number of factors, but the coordinates assigned by a frame do not depend on what someone observes, they are in principle restrictions on measurements, not empirical ones.
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you're right PhysBang, it must happen in every frame to keep causality intact. That was my view too, but then I read this piece and started to wonder, again. There are other descriptions too that already had made my head ache, as Being and Becoming in Modern Physics. (http://plato.stanford.edu/entries/spacetime-bebecome/) from that lovely place Stanford Encyclopedia of Philosophy. Have a look at the Andromeda paradox and welcome to my headache. I know that it practically wouldn't matter as it always must be information that decides a appropriate action, and that one is still 'c', but?
I do not think that we should demand that our physics be relativistic but our metaphysics not relativistic. That seems to do away with any worries about the Andromeda situation.
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:)
heh, don't you want that experiment too, I know I do. If you look at it my first reaction was an almost instinctive 'no' :) the text didn't fit my views of relativity, but then as I started to get how he thought he made more and more sense. And then there was only that da**ed pimpernel, the cat left :)
It's like they say, you want loyalty, get a dog.
Not a cat :)
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(1) A Newtonian past/future diagram would have a central, horizontal line representing the universal present, above which would be the future, and below which would be the past.
(2) A relativistic diagram conventionally takes the form of an X, the centre of which is the subjective here-and-now. The arms of the cross represent the speed of light, and the upper half is the future light cone, and the lower half, the past light cone.
One event can influence another, or be influenced by another, only if both events lie within the light cones. How does this influence synchronicity and causality?
Richard Wolfson, “Simply Einstein”, says: “….. the time order of events may depend on one’s frame of reference.” He continues: “Because relativity gives every uniformly moving reference frame equal status, this reversal of time order isn’t just some illusion. It’s really true that I can observe event A to occur before B, that you can observe B before A, and that we’re both right. But how can that be? Doesn’t it wreak havoc with causality?”
Wolfson stresses the fact that we cannot meaningfully talk of a universal past or present. However we can talk of something happening before or after a particular event. This event could be the present in our frame of reference, or it need not be. Wolfson states categorically that “There are, in fact, events that are unambiguously in the past – meaning that they occurred at a time before the present event.” He continues: “So there is no such thing as a universal ‘present’. There is, for me, the present event – namely, whatever is occurring here and now. ‘Now’ isn’t enough: I have to indicate ‘here’ as well – and that means I’m talking about an event, not just a time.”
Wolfson’s explanation is worth quoting in full. “What are some events that are truly in the past, meaning they unambiguously occurred before your present event, that is the event of your reading these words? For one, your birth. There are no observers, in any state of motion, who would judge that event to occur after you’re here and now (although different observers will disagree about the amount of time between those events). We don’t have to restrict ourselves to events in relation to the here and now. We can also ask, for example, whether the event of the Titanic hitting the iceberg preceded the event of the great ship’s sinking. The answer is an unambiguous yes. Again, one event is clearly in the other’s past. Consider also that in 1987 astronomers observed a supernova – an exploding star – in a neighbor galaxy some 160,000 light-years away. Clearly the supernova event itself occurred before the astronomers observed it, since it took light from the supernova 160,000 years to reach the astronomers’ telescopes.
What do these pairs of events we’ve just considered have in common? They’re all causally related. Your birth is a necessary cause of your reading these words. Had the first event not occurred, the second could not have occurred either. Had the Titanic not hit the iceberg, it would not have sunk. Had the supernova explosion not occurred, the astronomers would not have observed it. In each case, the earlier event was capable of influencing the later one and, in fact, did influence it. That provides a more robust definition of the past: The past of a given event consists of all those events that are capable of influencing the given event. Similarly, the future of the given event consists of all those events that the given event can influence. Note that I’m talking about past and future in relation to a specific event; in a Universe in which simultaneity is relative, there’s simply no such thing as a universal past and a universal future. But when one event is in another’s past, that relationship is not ambiguous. All observers will agree about which event came first (although, again, they may disagree on the amount of time between the events).”
In the scenario we were considering, this must mean that the pulling of the lever is unambiguously in the past of the cat falling into the hole. How, then, can there be a F of R in which this sequence does not hold?
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You can change it into information carriers too Bill. What defines the universe for each one of us are information, and that information has a speed. Doing so you can ignore comparisons (time dilations), concentrating on why this flow of information would stop for one, but not for the other? Relativity, as I read it, doesn't state that 'time stops' anywhere just as 'c' doesn't 'stop'. It's a constant speed of information, so presuming you could watch that cat it shouldn't matter from which frame you does it. But? If it did? :) Well, that would be about causality, wouldn't it?
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I answered this whole cat thing in the second post of this thread - from the helicopter it is not the case that the cat is seen to vanish before the lever is seen being pulled. The light that comes from the location of the lever cannot fall behind the signal sent from there to the manhole cover. It is a really badly thought out thought experiment.
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The concept behind Sachs description should be due to the relativity of simultaneity as a guess. This one is rather clear on it, using just light signals. Simultaneity Ain't what It Used to Be. (https://www.fourmilab.ch/documents/RelativityOfSimultaneity/) Then to that you can add special relativity (uniform motions), in which case it becomes more complicated, but hopefully still understandable Relativity of Simultaneity. (http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_rel_sim/)
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Abstract - A certain man walks very fast-so fast that the relativistic length contraction makes him very thin. In the street he has to pass over a grid. A man standing at the grid fully expects the fast thin man to fall into the grid. Yet to the fast man the grid is much narrower even to the stationary man, and he certainly does not expect to fall in. Which is correct? The answer hinges on the relativity of rigidity.
Given the grid opening of 1 meter, g=10m/sec, and a slow v=.1c:
The vertical fall distance over an opening is on the order of 10-14 m, the dimensions of a nucleus.
How could he fall in?
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PmbPhy #23
There's no such frame as "The frame where the event occurred." An event exists independent of a frame of reference. For example: consider the event: "Fire cracker exploded." There is no unique frame which can be said that this event occurred in that frame. The event occurred in "all" frames of reference, inertial or otherwise.
The "cat falling into the manhole' occurred on earth. It did not occur on the moon, or Jupiter, or on a body orbiting Betelgeuse. All those objects are moving relative to earth, thus the "event" did not occur at any of those locations (in their frame of reference). The "event" can be perceived at those locations, after occurring on earth. Until detecting the images of the "event", a viewer at those locations will have no awareness of the "event".
You're not speaking of a single event here but two events since its only between two events that a spatial distance or a temporal separation exists between them.
In the last post to Bill S, the cat and cover moving simultaneously can be considered as one composite event.
Not at all. No physicist would ever say that the synchronization procedure outlined by Einstein was misleading in any way. And it's quite reasonable to assign a time to any event regardless of where its located. You may have made such an assertion but you never said anything to support it.
Let's see what the author of SR says.
On the Electrodynamics of Moving Bodies', Albert Einstein, 1905:
part 1, par. 1
"But it is not possible without further assumption to compare, in respect of time, an event at A with an event at B. We have so far defined only an ``A time'' and a ``B time.'' We have not defined a common ``time'' for A and B, for the latter cannot be defined at all unless we establish by definition that the ``time'' required by light to travel from A to B equals the ``time'' it requires to travel from B to A."
"The "time" of an event is that which is given simultaneously with the event by a stationary clock located at the place of the event, this clock being synchronous, and indeed synchronous for all time determinations, with a specified stationary clock."
Relativity, Crown Publishers, 1961, pg 23
"That light requires the same time from A to M as from B to M (M being the midpoint of A to B), is in reality neither a supposition nor a hypothesis about the physical nature of light, but a stipulation which I can make of my own free will in order to arrive at a definition of simultaneity."
There is no known method of measuring the speed of light relative to a moving observer, and consequently knowing the time and position of a remote reflection. By definition it is calculated to be a constant.
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The "cat falling into the manhole' occurred on earth. It did not occur on the moon, or Jupiter, or on a body orbiting Betelgeuse. All those objects are moving relative to earth, thus the "event" did not occur at any of those locations (in their frame of reference). The "event" can be perceived at those locations, after occurring on earth. Until detecting the images of the "event", a viewer at those locations will have no awareness of the "event".
And all this is irrelevant to the fact that every event is in every frame of reference (that is well-formed). It's similar to saying that everything measured in meters also has a measurement in feet, even if nobody does the measurement.
In the last post to Bill S, the cat and cover moving simultaneously can be considered as one composite event.
One could do that, but then one would be abandoning contemporary relativity theory. Thens that happen separated by spatial distance are separate events. They may be simulataneous in some frames, but cannot be simultaneous in all frames.
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There is no spatial distance between two items here, so the action where the cat falls through the manhole is simultaneous in all frames of reference. What will vary in different frames will be the delay between the light travelling from the cat to the man with the lever and the signal from the lever getting back to the manhole, and that delay will be shortest when measured in the frame of reference in which the street is stationary. All frames in which the street is moving will perceive events as being slowed down, but at no point would any observer think the cat can walk across the manhole without falling in.
The other case with the fast moving man and the grid involves such a high speed of travel that there's no way the man could fall into it even if it wasn't length contracted, though he could perhaps trip up on it.
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The "cat falling into the manhole' occurred on earth. It did not occur on the moon, or Jupiter, or on a body orbiting Betelgeuse. All those objects are moving relative to earth, thus the "event" did not occur at any of those locations (in their frame of reference). The "event" can be perceived at those locations, after occurring on earth. Until detecting the images of the "event", a viewer at those locations will have no awareness of the "event".
You made the mistake of thinking that a frame of reference is synonymous with a location. It isn't. A frame of reference typically has an infinite extent. So one doesn't speak of "Their frame of reference" as being the same as a location. That is contrary to what a frame of reference is and how its defined. Please read the definition of it at: http://en.wikipedia.org/wiki/Frame_of_reference
There is no known method of measuring the speed of light relative to a moving observer, and consequently knowing the time and position of a remote reflection. By definition it is calculated to be a constant.
Prove it.
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You made the mistake of thinking that a frame of reference is synonymous with a location. It isn't. A frame of reference typically has an infinite extent. So one doesn't speak of "Their frame of reference" as being the same as a location. That is contrary to what a frame of reference is and how its defined. Please read the definition of it at: http://en.wikipedia.org/wiki/Frame_of_reference
in physics, a frame of reference (or reference frame) may refer to a coordinate system……
In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point….
How does the position of a point differ from the location of a point?
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Bill S #45
Wolfson article
Your birth is a necessary cause of your reading these words.
The birth of Bob did not cause Bob to read any particular text. His birth allowed the possibility of Bob to get an education, acquiring the skill of reading. He then willfully chooses to read some online text.
Had the first event not occurred, the second could not have occurred either.
Had Bob not won the lottery, he could not have spent the prize money.
Does this not read like a tautology?
Had the Titanic not hit the iceberg, it would not have sunk.
All boats that hit icebergs, do not sink.
Some boats have multiple encounters with icebergs.
If the Titanic had been built to better standards, it may not have sunk.
The writer, in his attempt to form a chain of causality, concludes, there is only one outcome.
Contrary to his thinking:
The world is not totally deterministic. There are multiple possible outcomes for many events. If there weren't there would be no variety. Introduce human free will, and it becomes more random.
Following the Wolfson logic:
Vinnie gets drunk drinking too much beer. While driving home, he hits a pedestrian, who dies. In court, Vinnie's lawyer says: "If the brewery hadn't made and sold the beer, my client would not have gotten drunk drinking it. The brewery is to blame for the pedestrian death.
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How does the position of a point differ from the location of a point?
There is none. The terms are synonyms.
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Phyti, whilst what you say is correct, I suspect you are reading more into Wolfson’s reasoning than he probably intended.
I think his point was that if Bob had not been born he would not have been reading those words, or any others. Thus there is a causative relationship between the two events, but it does not oblige him to read any specific text.
The statement: “Had the Titanic not hit the iceberg, it would not have sunk” says nothing about there being a causal relationship between the Titanic hitting the iceberg and the sinking, or otherwise, of any other craft.
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There is none. The terms are synonyms.
Following the reasoning in #54, does your answer not indicate that frame of reference and locating can be the same thing?
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The writer, in his attempt to form a chain of causality, concludes, there is only one outcome.
Contrary to his thinking:
The world is not totally deterministic. There are multiple possible outcomes for many events. If there weren't there would be no variety. Introduce human free will, and it becomes more random.
There is no free will, so it doesn't become more random. Furthermore, the only room for non-deterministic events is in the quantum realm, and even then it may have a fully deterministic mechanism lurking behind it which we cannot access, so there may only be one path which the future can follow. To assert that there isn't a single path is at least as wrong as to assert that there is - we currently don't know. The whole universe we see around us could be a simulation in which there is no room for anything non-deterministic to happen at all (at any level).
Following the Wolfson logic:
Vinnie gets drunk drinking too much beer. While driving home, he hits a pedestrian, who dies. In court, Vinnie's lawyer says: "If the brewery hadn't made and sold the beer, my client would not have gotten drunk drinking it. The brewery is to blame for the pedestrian death.
No one is to blame for anything because no one has free will. However, we still calculate that by punishing people we can modify their future behaviour and give people better lives on average as a result, so "blame" really relates to those causes which involve calculation within thinking systems like the brain, and especially where rules have not been followed correctly.
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PhysBang #51
And all this is irrelevant to the fact that every event is in every frame of reference (that is well-formed). It's similar to saying that everything measured in meters also has a measurement in feet, even if nobody does the measurement.
An event E happens once, at a specific location, at a time noted by a local (at that location) clock. There can be many perceptions of E at locations distant from E. These perceptions consist of detecting images of E, and recording the time of detection on a local clock. If the distant observer knows the approximate distance to E, they can assign a local time to E. The perception of an event is not the same as the occurrence of the event. A photo of a person is not the same as the person. Seeing the event is another "figure of speech".
The system of rods and clocks is a fictional and overly simplistic configuration of a frame of reference, and is logistically impossible. The GPS system of clocks requires periodic corrections, and that is local. A frame of ref. is merely a location common to a set of measurements, just as coordinates to an origin. It's not magic, it's a system of measurement requiring a reference and a unit of measure or standard.
One could do that, but then one would be abandoning contemporary relativity theory. Thens that happen separated by spatial distance are separate events. They may be simulataneous in some frames, but cannot be simultaneous in all frames.
Nothing is abandoned.
The example specifies the cat being over the hole when the cover drops away. The person activating the cover could be replaced with a photocell detector at the hole, and is irrelevant to the setup. The desired effect is similar to a gallows, where the criminal drops simultaneously with the trap door. The cat and cover drop simultaneously, and therefore are a single event. There is no space or time
difference to manipulate via motion of a passing observer. All who view the event perceive a single cat+cover motion.
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PmbPhy #53
You made the mistake of thinking that a frame of reference is synonymous with a location. It isn't. A frame of reference typically has an infinite extent. So one doesn't speak of "Their frame of reference" as being the same as a location. That is contrary to what a frame of reference is and how its defined. Please read the definition of it at: http://en.wikipedia.org/wiki/Frame_of_reference
Wikipedia 2006Relativity theory depends on "reference frames". A reference frame is a point in space at rest, or in uniform motion, from which a position can be measured along 3 spatial axes. In addition, a reference frame has a clock moving with the reference frame, allowing the measurement of the time of events.
"Frame of reference" existed millenia ago, and is not a development unique to Relativity. The definition above may not agree with the one you saw, since it depends on who is editing the articles, and when you look. I'll just repeat the part posted to #51.
A frame of ref. is merely a location common to a set of measurements, just as coordinates to an origin. It's not magic, it's a system of measurement requiring a reference and a unit of measure or standard.
Spatial measurements are between two objects, which implies two locations.
From "Relativity", Crown Publishers, 1961,
A.E. describes "a system of coordinates rigidly attached to a body of reference".
Since points in space are abstract and inaccessible, an object is required for a measurement, eg. the molecules at each end of the rod.
Prove it.
This will get a separate response.
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Wikipedia 2006
Relativity theory depends on "reference frames". A reference frame is a point in space at rest, or in uniform motion, from which a position can be measured along 3 spatial axes. In addition, a reference frame has a clock moving with the reference frame, allowing the measurement of the time of events.
You didn't provide a link to this statement. In any case it's wrong. What I'm telling you is a fact. It's what all mainstream physicists use as the definition of "frame of reference". Do yourself a service and contact any relativist that you can find (e.g. Sean Carroll, Edwin F. Taylor, Hans C. Ohanian, etc) and ask them what the definition of a frame of reference is, or read their textbooks. In no sense of the term does the phrase "frame of reference" apply only to a finite region of space. That's all there is too it. You double check me by picking up ****Any Text in Special or General Relativity ****. Please stop trying to pass off this misinformation as fact.
You can't simple make a claim that what you stated was the definition of a frame of reference. That definition is already given, has been defined for over a hundred years in relativity, and will not change in the foreseeable future. You can't take what's on the internet as being factual. Wiki is reliable and the definition I linked to is correct. That's why I used it. But I know from experience that the countless books and journal articles out there that use the term use it as I defined it for you. I have about 15 relativity textbooks and they all define the term that way.
"Frame of reference" existed millenia ago, and is not a development unique to Relativity.
So what? The topic of this thread is relativity, nothing else.
I'm not about to keep arguing about a well-known definition from relativity. It's a waste of everyone's time. Either you wish to learn or you don't.
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A frame of reference includes everything being 'at rest' with you. So if you're on a spaceship in uniform motion then this 'rest frame' should include all objects in a infinite universe that could be defined as being 'at rest' with you, and in reality we can't state how many objects that might be. There is always a locality to the definition in that you need to define what you consider as your object of interest (frame of reference) defining this 'at rest' from, in this case we use the space ship. Using scaling, and especially QM, it becomes a lot more tricky. What is a 'electron cloud' 'at rest' with? The nucleus? And when is it 'at rest', if so? Before the outcome, in the outcome? Never? I think never but that's just my thought.
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Everything becomes weird down there, doesn't it? Can a electron cloud be defined as being 'at rest' with the nucleus before a measurement? Actually, just as you can split an acceleration into infinitesimally small 'bits' and so find a 'flat space', or in this case 'instants' of motion, so you should be able to do with a whole universe, all included. If you do so, presuming it correct, then motion should cease to exist. and that is one of the weirdest arguments still :)
Easiest to see if you think of splitting a circle into those bits. As you go down in scale each bit will be 'straighter and straighter' measuring it. At some scale it should be a line, although as I think, still containing the 'property' of a circle, as we know where we started. Property's are really weird, and so is scaling. Another reason why I like Planck scale btw, at least it puts a limit to our measurements.
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And all this is irrelevant to the fact that every event is in every frame of reference (that is well-formed).
PhysBang - Will you do me a favor and please explain this fact to phyti39? He has a crazy notion of what a frame of reference is. Thanks.
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There is none. The terms are synonyms.
Following the reasoning in #54, does your answer not indicate that frame of reference and locating can be the same thing?
I don't understand your line of reasoning. Will you please explain/clarify it for me? Thanks.
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An event E happens once, at a specific location, at a time noted by a local (at that location) clock. There can be many perceptions of E at locations distant from E. These perceptions consist of detecting images of E, and recording the time of detection on a local clock. If the distant observer knows the approximate distance to E, they can assign a local time to E. The perception of an event is not the same as the occurrence of the event. A photo of a person is not the same as the person. Seeing the event is another "figure of speech".
You didn't have to say all of this. PhysBang is quite aware of all of this as is any physicist. I don't understand your need to state something so obvious, unless you thought that he was extremely ignorant on such an obvious fact? Please explain. Thank you.
The system of rods and clocks is a fictional and overly simplistic configuration of a frame of reference, and is logistically impossible.
Yep. We're all aware of that. A coordinate system is an artificially imposed grid that you place on a problem in order to make quantitative measurements. When one actually looks at experiments and how measurements are really made it becomes clear that there is a definite reality to the coordinate system. It's just too difficult to explain or put in a simple definition.
...A frame of ref. is merely a location common to a set of measurements, ...
Both very wrong and very confusing.
It's not magic, ...
Did you think that someone here actually though it was really magic?
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PmbPhy #62
You didn't provide a link to this statement. In any case it's wrong. What I'm telling you is a fact. It's what all mainstream physicists use as the definition of "frame of reference". Do yourself a service and contact any relativist that you can find (e.g. Sean Carroll, Edwin F. Taylor, Hans C. Ohanian, etc) and ask them what the definition of a frame of reference is, or read their textbooks. In no sense of the term does the phrase "frame of reference" apply only to a finite region of space. That's all there is too it. You double check me by picking up ****Any Text in Special or General Relativity ****. Please stop trying to pass off this misinformation as fact.
The link was (Wikipedia-Special Relativity-section on reference frame) as edited in 2006. When compared to the Einstein definition, the condition of rigid body was not included. It is usually understood that the "editor" knew that "point" implies a physical object, else how would you find it. The extent of the"frame" wasn't mentioned in the Wiki article or in the book cited. How would you perform an experiment involving an object at an "infinite" distance? Einstein required the measurement system to be anchored/attached to a rigid body, eg. the train car or the embankment. An anaut floating in space, with a laser and a clock is a frame of reference, with an extent depending on the range of the laser.
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You didn't have to say all of this. PhysBang is quite aware of all of this as is any physicist. I don't understand your need to state something so obvious, unless you thought that he was extremely ignorant on such an obvious fact? Please explain. Thank you.
Yep. We're all aware of that. A coordinate system is an artificially imposed grid that you place on a problem in order to make quantitative measurements. When one actually looks at experiments and how measurements are really made it becomes clear that there is a definite reality to the coordinate system. It's just too difficult to explain or put in a simple definition.
If we both know these things, then we agree on something.
...A frame of ref. is merely a location common to a set of measurements, ...
Both very wrong and very confusing.
To you, but are you speaking for everyone?
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I'm like to think of myself as a guy who "roots for relativity". But I also think that when it comes to special relativity, a reference frame is little more than a state of motion. Look at the Wikipedia article (http://en.wikipedia.org/wiki/Frame_of_reference), and state of motion is mentioned 9 times. For example you start off motionless with respect to the Earth, then you accelerate to some speed relative to the Earth, and you say you've changed your reference frame. But you haven't changed your reference frame like it's some library book. All you've really done is changed your speed.
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But I also think that when it comes to special relativity, a reference frame is little more than a state of motion.
It's clearly far different from a state of motion, since reference frames are the standard against which we determine motion.
It's like you said that numbers are little more than bananas.
Look at the Wikipedia article (http://en.wikipedia.org/wiki/Frame_of_reference), and state of motion is mentioned 9 times.
Yes, mostly to point out the difference between choice of reference frame and the state of motion of a given observer. That's an important thing not to miss.
For example you start off motionless with respect to the Earth, then you accelerate to some speed relative to the Earth, and you say you've changed your reference frame.
No, you don't. You can say that you are currently co-moving with a different inertial frame of reference than the one you were in before. Or you can identify a system of coordinates in which you were at rest. Or you can identify a system of coordinates in which you moved along only one axis. There is a great deal of freedom of choice.
But you haven't changed your reference frame like it's some library book. All you've really done is changed your speed.
If an object, observer or not, is accelerated, then it accelerates in every inertial reference frame. If one allows any system of coordinates, like general relativity does, then whether something moves or not is dependent on the system of coordinates (the reference frame) chosen. (Even in Newtonian physics, one can use reference frames that have a parallel acceleration relative to other frames without problem, so some accelerations there are dependent on the reference frame chosen.)
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No, you don't.
I strongly disagree. To change one's frame of reference from one inertial frame to another one, one simply changes which frame of reference they are at rest with respect to. That requires accelerating from one frame to another.
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No, you don't.
I strongly disagree. To change one's frame of reference from one inertial frame to another one, one simply changes which frame of reference they are at rest with respect to. That requires accelerating from one frame to another.
I do not have a frame. I am currently at rest with respect to many different frames. I am free to use many different frames of references to make measurements and to refer to positions, speeds, and momenta.
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I do not have a frame. I am currently at rest with respect to many different frames. I am free to use many different frames of references to make measurements and to refer to positions, speeds, and momenta.
You are only at rest in one frame of reference. In every other frame you are regarded as moving. You can certainly use any frame of reference to calculate things, but all but one of them will apply numbers to you that claim you're moving.
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I do not have a frame. I am currently at rest with respect to many different frames.
I disagree here too. Your "frame" is the system in which you are at rest as are the devices and coordinate systems used to measure observables. The only sense that you can legitimately say that you're at rest with respect to many different frames is to distinguish frames that are rotated and translated with respect to each other. However in "your frame" you measure one and only one value of the kinetic energy, momentum, etc.
May I inquire as to where you got your definition of "frame of reference" from? This is essentially the one I'm using at the present time: http://en.wikipedia.org/wiki/Frame_of_reference
In physics, a frame of reference (or reference frame) may refer to a coordinate system used to represent and measure properties of objects, such as their position and orientation, at different moments of time. It may also refer to a set of axes used for such representation. In a weaker sense, a reference frame does not specify coordinates, but only defines the same 3-dimensional space for all moments of time such that the frame can distinguish objects at rest from those that are moving.
The underline is mine. It refers to the important aspects of the frame of reference.
I'll get back later with a survey from a sampling of my mechanics and relativity text books.
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The use of "observer" talk has lead to a lot of mistakes in the way that people think about reference frames. Even here, with the definition right in front of PmbPhy, he would rather ignore the definition and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.
Arguably the most common measurement of velocity is that of the speed of a moving vehicle. This measurement is almost never done from the reference frame in which the vehicle is at rest, nor from the reference frame of some observer within the vehicle. Yet we do not think that these measurements are without merit.
When we speak of a reference frame (in the context of this discussion), we speak of a system of coordinates. I can use kilometers as my metric or I can use miles. I can use hours or I can use some other measurement of time. I can use one set of axes or I can use another. I can use Cartesian coordinates or polar or spherical...
So even considering those reference frames in which I am at rest, there are an infinite number of such frames.
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.. he would rather ignore the definition ...
Why are you saying that and why are you saying it in that way (i.e. quite rude)? You're not a mind reader you know. I didn't ignore "the" definition? In fact I used the actual correct definition that is defined and used by the relativity community. As for you claim, what you believe to be a definition doesn't merely become one just because you want it to be and then pronounce that you're correct you know.
and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.
Hmmm. I didn't realize that you were that rude of a poster when someone corrects you or you disagree with a member. Since this doesn't seem to be something that's going to change I will no longer respond to any of your comments to come since I'm not in the mood for rude people/comments.
Goodbye!!
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Dude, if you are going to post a definition that you then ignore, then fine by me if you're going to ignore me too.
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When speaking about frames of reference, one needs to remember the difference between identifying local coordinates of space and time and referring to Einsteinian relativity. Einsteinian frames of reference apply to observational references between a moving observer and the phenomenon or phenomena under observation.
I think what Pete is referring to are Einsteinian frames of reference and not just reference frames associated with coordinates of local space and time.
The following text is directly from Wikipedia:
In Einsteinian relativity, reference frames are used to specify the relationship between a moving observer and the phenomenon or phenomena under observation. In this context, the phrase often becomes "observational frame of reference" (or "observational reference frame"), which implies that the observer is at rest in the frame, although not necessarily located at its origin. A relativistic reference frame includes (or implies) the coordinate time, which does not correspond across different frames moving relatively to each other.
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The use of "observer" talk has lead to a lot of mistakes in the way that people think about reference frames. Even here, with the definition right in front of PmbPhy, he would rather ignore the definition and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.
Arguably the most common measurement of velocity is that of the speed of a moving vehicle. This measurement is almost never done from the reference frame in which the vehicle is at rest, nor from the reference frame of some observer within the vehicle. Yet we do not think that these measurements are without merit.
When we speak of a reference frame (in the context of this discussion), we speak of a system of coordinates. I can use kilometers as my metric or I can use miles. I can use hours or I can use some other measurement of time. I can use one set of axes or I can use another. I can use Cartesian coordinates or polar or spherical...
So even considering those reference frames in which I am at rest, there are an infinite number of such frames.
I have reviewed Pete's posts and never once saw him tie an observer to a reference frame.
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..., if you are going to post a definition that you then ignore,....
Wrong. False accusations and attempts at mind reading like these are why I'm ignoring you.
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PmbPhy is actually not using reference frames as special relativity used them, even though Einstein introduced much of the use of talking about observers that ended up being misleading. And even though the wikipedia article uses the phrase "observational frame of reference", it does not do so in the manner in which one identifies one and only one reference frame for an observer. The term is defined as a short-hand way to speak of a reference frame in which an observer is at rest (hopefully when this term is used both the observer and the frame are defined before using the term). The article also suggests that the term can be used to speak of "an entire family of coordinate systems", i.e., that one can identify a set of reference frames in which the observer identified is at rest.
As I said, and as the definition that PmbPhy pasted from wikipedia says, a reference frames is "a coordinate system used to represent and measure properties of objects, such as their position and orientation, at different moments of time." This is the sense in which reference frame is used in special relativity, which seems to be the context of this thread.
As such, we can, for any object--whether the object can make observations or not--identify an infinite number of reference frames for which that object is at rest, merely by arbitrary shifts along any axis or by rotation of axes, or by choice of alternate form of axes (e.g. Cartesian vs. polar).
Where I definitely part ways with the wikipedia article is in holding that one can describe observations only about the properties of one's own frame. This is simply not the case. One can make observations only of events that one can observe, but one can use those events to describe how those observations appear in any frame. Every day, millions of people determine their own speed as non-zero relative to a reference frame tangential to the surface of the Earth when they drive a car and they look at their spedometer. The core of special relativity (and GR as well) is that one can use what one observes to determine the series of events in any reference frame.
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This is just another row about language and meaning, so we're arguing at cross purposes as usual. I can now see why PhysBang says there is an infinite number of frames of reference in which the same object is at rest, but I've always thought of them all being the same frame (because the coordinate system imposed on it is arbitrary and not a physical reality). Both approaches are valid.
Instead of arguing about which usages of terms are right or wrong, it's better either to agree on specific definitions to be used within a particular conversation or to assume different definitions are being used by different individuals and always interpret them the way those people do when you are reading their words rather than misunderstanding them by imposing your own definition on what they're saying. That way, the conversation can focus on the actual meat of the issue instead of being diverted into pointless wars over which definitions are officially correct. There are too many people falling out over such trivial matters. Different people acquire their knowledge from different sources and end up using the same or similar terms while understanding them quite differently from each other. Ideally everyone would use the same definitions, but standard definitions don't appear to have been established strongly enough to be universal, and that means we all have to work hard to make sure we are in sync with each other before we write people off as stupid and start being rude to them. I'm not aiming this at any particular person, but at all of us, and myself included. We can all do better.
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We can all do better.
I agree completely Dave. These disagreements are becoming very tiresome and I suggest we all just take a deep breath and try to place our efforts toward congeniality instead of personal ego.
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Pete, I owe you an apology! In #65 you asked me a question which I have not answered. I'll rectify that now.
In #53 you made the point that a frame of reference is not synonymous with a location. You also linked to Wiki. It was following that link that led me to wonder:
If, “in physics, a frame of reference (or reference frame) may refer to a coordinate system……”, and if: “In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point….”; could the following argument be made.
1. A frame of reference refers to a coordinate system.
2. A coordinate system determines a position.
3. A position is synonymous with a location.
4. A frame of reference may, therefore, be synonymous with a location, at least in certain circumstances.
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1. A frame of reference refers to a coordinate system.
2. A coordinate system determines a position.
3. A position is synonymous with a location.
4. A frame of reference may, therefore, be synonymous with a location, at least in certain circumstances.
No. You're missing the fact that a coordinate system must have clock in order to be a frame of reference. Recall the Wikipedia article and note that part where it says "at different moments of time".
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You can think of the location of either an observer or an object being observed but not the frame, but only if they are at rest in the frame under consideration. I made the remark that a frame of reference can be thought of as an infinitesimal point in spacetime as any point adjacent to it could be different by some infinitesimal amount. Pete rightly jumped on that and corrected me. I was just showing how absurdly detailed you can be if pedantic enough. We are mainly considering either particles or macroscopic objects when using frames of reference. A moving object has no definite position over time.
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There's no such frame as "The frame where the event occurred." An event exists independent of a frame of reference. For example: consider the event: "Fire cracker exploded." There is no unique frame which can be said that this event occurred in that frame. The event occurred in "all" frames of reference, inertial or otherwise.
I'd like to clarify something. This comment is referring to globally inertial frames. Not frames of reference in GR where the spacetime is curved. When I said that there is no unique frame I had something specific in mind.
When one speaking of problems in special relativity (SR) the one is talking about spacetimes which are flat (i.e. zero spacetime curvature). So when someone is working an SR problem and mentions two or more inertial frames, i.e. S, S', S", etc. which may or may not be in relative motion then its assumed that they're speaking about the same region of spacetime, not two totally disconnected regions of spacetime.
I should have made that clear. But this is what I had in mind when I said what I did, especially since the context in which you were speaking indicated that those inertial frames were in the same spacetime.
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PmbPhy is actually not using reference frames as special relativity used them, ...
See? More nonsense/bogus claims. In the first case nowhere did I post anything regarding frames of reference which makes your claim true. Also regarding the definition of "frame of reference" I listed the Wikipedia site giving the definition since that encyclopedia since it has a high track record of being correct. I know what an inertial frame of reference is since I've been a relativist for 15 to 20 years and know what I'm talking about. Look in Exploring Black Holes by Edwin F. Taylor and John A. Wheeler and read the glossary of terms and you'll see that I wrote it, and it contains that term (which in that text is called "free-float frame"). So you have absolutely NO justification for making such an accusation.
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No. You're missing the fact that a coordinate system must have clock in order to be a frame of reference. Recall the Wikipedia article and note that part where it says "at different moments of time".
Thanks Pete, that's clearer.
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No. You're missing the fact that a coordinate system must have clock in order to be a frame of reference. Recall the Wikipedia article and note that part where it says "at different moments of time".
Thanks Pete, that's clearer.
Hi Bill,
I just found a definition that I really like. It's from A First Course in Special Relativity - Second Edition by Bernard Schutz, page 2.
It is important to realize that an 'observer' is in fact a huge information-gathering system, not simply one man with binoculars. In fact, we shall remove the human element entirely from our definition, and say that an inertial observer is simply a coordinate system for spacetime, which makes an observation simply by recording the location (x, y, z) and time (t) of an event. This coordinate system must satisfy the following three properties to be called inertial:
(1) The distance between point P1 (coordinates x1, y1, z1) and P2 (coordinates x2, y2, z2).
(2) The clocks that sit at every event point ticking off ticking off the time coordinate t are synchronized and all run at the same rate.
(3) The geometry of space at any constant time t is Euclidean.
Notice that this definition does not mention whether the observer accelerates or not. ... It will turn out that only an unaccelerated observer can keep his clocks synchronized.
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To you, but are you speaking for everyone?
Not simply to me. This is how it is to the entire physics community. I've been a relativist for some 15-20 years and know how my colleagues think and what they mean when they speak or when I read their private communications to me or their publications.
Keep in mind that the context from what you're talking about means that you're talking about special relativity and in special relativity what I say is correct. And I've been tutoring physics long enough to know what is and what isn't confusing. Where did you get the impression that
A frame of ref. is merely a location common to a set of measurements...
is true or meaningful?
The term "location" is defined as
http://www.merriam-webster.com/dictionary/location
: a place or position
: a place outside a studio where a movie is filmed
: the act of finding where something or someone is : the act of locating something or someone
From the context of that post you used it as the first meaning, i.e. "a place or position". A coordinate system is a key part of the definition of a frame of reference. A coordinate system is not a place or position since for something to fit that definition it must be finite in size, which a coordinate system is not.
If you go on to study special relativity then part of that is the example of the twin paradox. Part of that example to have two planets which are many light years apart. In that example the two planets are said to be in the same frame of reference. Its implicit in the physics and the mathematics.
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PmbPhy is actually not using reference frames as special relativity used them, ...
See? More nonsense/bogus claims. In the first case nowhere did I post anything regarding frames of reference which makes your claim true.
RiiiiIIIIiiiight.
So first you "stongly disagree" with the idea that there is not one single reference frame for every observer. Then you cling to this idea despite pasting a definition that indicates that this is false.
Also regarding the definition of "frame of reference" I listed the Wikipedia site giving the definition since that encyclopedia since it has a high track record of being correct.
Yes, but you didn't really use the definition you cut and pasted from that.
I know what an inertial frame of reference is since I've been a relativist for 15 to 20 years and know what I'm talking about. Look in Exploring Black Holes by Edwin F. Taylor and John A. Wheeler and read the glossary of terms and you'll see that I wrote it, and it contains that term (which in that text is called "free-float frame").
First, you can't say that a text contains a term and then say that it uses a different set of words for that term. Second, there are frames of reference and then there are a sub-class of all frames of reference where we identify that the object of which we are interested is freely falling or floating. So that is a significant difference in definition.
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No. You're missing the fact that a coordinate system must have clock in order to be a frame of reference. Recall the Wikipedia article and note that part where it says "at different moments of time".
Thanks Pete, that's clearer.
Hi Bill,
I just found a definition that I really like. It's from A First Course in Special Relativity - Second Edition by Bernard Schutz, page 2.
It is important to realize that an 'observer' is in fact a huge information-gathering system, not simply one man with binoculars. In fact, we shall remove the human element entirely from our definition, and say that an inertial observer is simply a coordinate system for spacetime, which makes an observation simply by recording the location (x, y, z) and time (t) of an event. This coordinate system must satisfy the following three properties to be called inertial:
(1) The distance between point P1 (coordinates x1, y1, z1) and P2 (coordinates x2, y2, z2).
(2) The clocks that sit at every event point ticking off ticking off the time coordinate t are synchronized and all run at the same rate.
(3) The geometry of space at any constant time t is Euclidean.
Notice that this definition does not mention whether the observer accelerates or not. ... It will turn out that only an unaccelerated observer can keep his clocks synchronized.
This is exactly the point I was getting at. It is misleading to cling to talk of observers or claim that observers are wedded to reference frames; there is a much better way to think of them and use them.
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PhysBang #75
The use of "observer" talk has lead to a lot of mistakes in the way that people think about reference frames. Even here, with the definition right in front of PmbPhy, he would rather ignore the definition and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.
A proxy such as a video device can be used to collect image data (space probe, telescope, etc.), but ultimately the data only has meaning to the human observer/s, who designed and implemented the experiment.
Einstein's definition of a 3-axis Cartesian coordinate system and a clock, as a system of measurement, works just as well today as in 1905, when he published it. His later examples of the train car and the embankment demonstrated that either location served as a reliable reference frame, for measurements, but when there was relative motion, the observers would record different values for distance and time.
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PmbPhy #91
Not simply to me. This is how it is to the entire physics community. I've been a relativist for some 15-20 years and know how my colleagues think and what they mean when they speak or when I read their private communications to me or their publications.
In reading books and papers by various authors involved in physics, I don't find a unanimous agreement about everything. Some consider time dilation and length contraction real physical effects, while others consider them as measurement effects, Einstein being one of the latter.
Keep in mind that the context from what you're talking about means that you're talking about special relativity and in special relativity what I say is correct. And I've been tutoring physics long enough to know what is and what isn't confusing.
It's good of you to educate those interested, in contrast to forums that can't or won't attempt to explain in terms they can understand.
"A frame of ref. is merely a location common to a set of measurements..."
is true or meaningful?
A coordinate system has an origin. The origin has a location relative to a "body of reference". There is a clock at the origin. These are the necessary elements for a reference frame. The array of rods and clocks is optional. If you are an anaut in a space capsule, in space, you have no array, but you can still make measurements using a laser and a watch. All your measurements are relative to your position, which you have assumed to be static. The "infinite extent" is superfluous, since no one can make measurements of that type. Even radar type measurements are only practical over distances that are "short" in astronomical terms. There are other means of determining distances.
I regularly consult a dictionary for word meaning and origin, and would recommend it as the primary reference for any field of knowledge. It would eliminate some of the arguing over semantics.
If you go on to study special relativity then part of that is the example of the twin paradox. Part of that example to have two planets which are many light years apart. In that example the two planets are said to be in the same frame of reference. Its implicit in the physics and the mathematics.
What makes you think I have not studied SR?
The "twin paradox" doesn't require two planets. It involves the accumulated time for each twin on separate paths between positions A and B. The example could be simplified to two clocks, in separate capsules, initially at rest, i.e. having the same velocity.
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Not really Phyti, he defined different clocks and rulers as a result of local measurements, those comparing other frames of reference relative ones local clock and ruler. When it came to which 'frame of reference' that was more 'correct' he gave every frame a equal (locally defined) importance. He tried to avoid the discussion of whether you should blame it on 'illusionary effects', relative 'real effects'. It was just 'frame dependent', and that was it.
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PhysBang #75
The use of "observer" talk has lead to a lot of mistakes in the way that people think about reference frames. Even here, with the definition right in front of PmbPhy, he would rather ignore the definition and stick to his notion of reference frame as wedded to some sort of observer that has only existed in the most cartoonish of examples.
A proxy such as a video device can be used to collect image data (space probe, telescope, etc.), but ultimately the data only has meaning to the human observer/s, who designed and implemented the experiment.
Einstein's definition of a 3-axis Cartesian coordinate system and a clock, as a system of measurement, works just as well today as in 1905, when he published it. His later examples of the train car and the embankment demonstrated that either location served as a reliable reference frame, for measurements, but when there was relative motion, the observers would record different values for distance and time.
I agree that Einstein's 1905 work is pretty good. However, none of that work relies on the idea of an observer at the origin. It is far better to simply consider reference frames as the best, in principle, that can be done to assign coordinates to an event, whether or not there is an observer that is at the origin of that reference frame and whether or not there is an observer that is at rest in that reference frame. The only role for and observer in "On the electrodynamics of moving bodies" is to establish a best, in principle procedure that everyone agrees, given the starting assumptions, should allow for the synchronization of clocks at a distance--this gives the idea of synchronization at a distance something to mean for physics.
When Einstein writes of what an observer would measure, he does not actually write about what someone would see, he writes about what events would be simultaneous or synchronous in a given system of coordinates.
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One of the problems for the layman in understanding physics is the level of abstraction. The absence of an observer is such an abstraction. Much of the mathematics of physics is opaque to the layman. This is not a bad thing as abstraction is sometimes a necessity in simplifying things. This forum however is not, to my knowledge, a place for mainly professional physicists. Sometimes, in order to demonstrate something of interest, the abstraction could be minimized.
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In reading books and papers by various authors involved in physics, I don't find a unanimous agreement about everything. Some consider time dilation and length contraction real physical effects, while others consider them as measurement effects, Einstein being one of the latter.
Well said phyti. Like PmbPhy I consider myself a "relativist", but I don't agree with everything he says. And I certainly don't agree with everything by Wheeler or Taylor or Schutz.
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Abstractions is what will define the experiment, then there will be mathematics supporting it. If the experiment fails the mathematics was wrong, here. It doesn't discuss whether those mathematics will fit 'somewhere else' though. you can look at it at least two ways. One in where whenever a mathematical proof becomes true, it also has a possibility of existing 'somewhere', even if not here. The other is experimental physics, sometimes the mathematics won't exist, so you will have to invent them, to fit the experiment, or logic.
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Actually this is even more true when it comes to theoretical mathematics. As I gather Perelman had to invent a lot of new ways to describe it, and as I remember, so it was with Einstein.