Relative Simultaneity

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Offline Bill S

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Relative Simultaneity
« on: 14/02/2015 20:29:23 »
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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Offline David Cooper

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Re: Relative Simultaneity
« Reply #1 on: 14/02/2015 23:09:34 »
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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Offline jeffreyH

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Re: Relative Simultaneity
« Reply #2 on: 15/02/2015 01:24:32 »
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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Offline JohnDuffield

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Re: Relative Simultaneity
« Reply #3 on: 15/02/2015 15:15:21 »
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.

Quote from: Bill S
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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Offline David Cooper

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Re: Relative Simultaneity
« Reply #4 on: 15/02/2015 19:04:59 »
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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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #5 on: 16/02/2015 01:00:18 »
You might find this article interesting: Length Contraction Paradox by Wolfgang Rindler, Am. J. Phys. 29(6), Jun. (1961)
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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.
It can be downloaded at http://www.newenglandphysics.org/Science_Literature/Journal_Articles/Rindler_AJP_29_6.pdf

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Offline jeffreyH

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Re: Relative Simultaneity
« Reply #6 on: 16/02/2015 02:28:32 »
You might find this article interesting: Length Contraction Paradox by Wolfgang Rindler, Am. J. Phys. 29(6), Jun. (1961)
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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.
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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Offline David Cooper

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Re: Relative Simultaneity
« Reply #7 on: 16/02/2015 18:58:37 »
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.
« Last Edit: 16/02/2015 19:02:08 by David Cooper »

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Offline JohnDuffield

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Re: Relative Simultaneity
« Reply #8 on: 16/02/2015 22:06:36 »
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.
« Last Edit: 16/02/2015 22:11:17 by JohnDuffield »

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Offline Toffo

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Re: Relative Simultaneity
« Reply #9 on: 17/02/2015 12:49:12 »
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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Offline Toffo

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Re: Relative Simultaneity
« Reply #10 on: 17/02/2015 13:12:28 »
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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Offline JohnDuffield

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Re: Relative Simultaneity
« Reply #11 on: 17/02/2015 16:12:21 »
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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Offline David Cooper

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Re: Relative Simultaneity
« Reply #12 on: 17/02/2015 19:03:38 »
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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Offline Toffo

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Re: Relative Simultaneity
« Reply #13 on: 17/02/2015 20:39:04 »
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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Offline JohnDuffield

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Re: Relative Simultaneity
« Reply #14 on: 17/02/2015 21:16:20 »
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. If you think you have found one, do let me know.

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Offline yor_on

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Re: Relative Simultaneity
« Reply #15 on: 17/02/2015 22:07:31 »
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.
=

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.
« Last Edit: 17/02/2015 22:51:08 by yor_on »
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Offline jeffreyH

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Re: Relative Simultaneity
« Reply #16 on: 17/02/2015 23:04:47 »
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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Offline yor_on

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Re: Relative Simultaneity
« Reply #17 on: 17/02/2015 23:22:42 »
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'.
=

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 :)
« Last Edit: 17/02/2015 23:30:49 by yor_on »
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Offline JohnDuffield

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Re: Relative Simultaneity
« Reply #18 on: 18/02/2015 13:39:57 »
...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:



They're misleading, and they interfere with the message, which is that in atomic orbitals 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:


« Last Edit: 18/02/2015 13:49:04 by JohnDuffield »

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Offline David Cooper

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Re: Relative Simultaneity
« Reply #19 on: 18/02/2015 17:41:24 »
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.

Quote
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.

Quote
And wherever you look, you will not find a proper resolution to what is in essence the pole and the barn paradox. 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).]
« Last Edit: 18/02/2015 17:57:27 by David Cooper »

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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #20 on: 19/02/2015 17:00:18 »
Quote from: Toffo
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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Offline David Cooper

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Re: Relative Simultaneity
« Reply #21 on: 19/02/2015 18:18:17 »
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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Offline phyti39

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Re: Relative Simultaneity
« Reply #22 on: 19/02/2015 20:32:28 »
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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Offline PmbPhy

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Re: Relative Simultaneity
« Reply #23 on: 19/02/2015 20:45:45 »
Quote from: phyti39
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.

Quote from: phyti39
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.

Quote from: phyti39
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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Offline yor_on

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Re: Relative Simultaneity
« Reply #24 on: 19/02/2015 21:51:29 »
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.
=

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.
=


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.
« Last Edit: 20/02/2015 00:02:25 by yor_on »
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Re: Relative Simultaneity
« Reply #25 on: 20/02/2015 00:53:17 »
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.
=

I could possibly argue that locality to be defined need be 'at rest' with what it observes?
This one is tricky.
=

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.
« Last Edit: 20/02/2015 01:13:46 by yor_on »
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Offline jeffreyH

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Re: Relative Simultaneity
« Reply #26 on: 20/02/2015 01:08:51 »
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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Re: Relative Simultaneity
« Reply #27 on: 20/02/2015 01:15:14 »
I know Jeffrey, you can take this being 'at rest' into a micro management. But lets not go there :)
=

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?
« Last Edit: 20/02/2015 01:32:16 by yor_on »
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Re: Relative Simultaneity
« Reply #28 on: 20/02/2015 03:24:23 »
Quote from: jeffreyH
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!

Quote from: jeffreyH
Any point adjacent to it...
What is the "it" that you're talking to?

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Re: Relative Simultaneity
« Reply #29 on: 20/02/2015 11:21:39 »
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.
==

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.
=

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.
==

Yep, I think he's right.

==

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.
=

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.
==

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.
« Last Edit: 20/02/2015 13:41:58 by yor_on »
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Offline PhysBang

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Re: Relative Simultaneity
« Reply #30 on: 20/02/2015 13:18:42 »
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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Re: Relative Simultaneity
« Reply #31 on: 20/02/2015 14:25:43 »
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.
=

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.
« Last Edit: 20/02/2015 14:47:26 by yor_on »
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Offline Bill S

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Re: Relative Simultaneity
« Reply #32 on: 20/02/2015 16:03:51 »
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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Offline PhysBang

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Re: Relative Simultaneity
« Reply #33 on: 20/02/2015 16:20:54 »
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.)
Quote
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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Offline jeffreyH

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Re: Relative Simultaneity
« Reply #34 on: 20/02/2015 16:29:10 »
Quote from: jeffreyH
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!

Quote from: jeffreyH
Any point adjacent to it...
What is the "it" that you're talking to?

LOL probably

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Offline Bill S

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Re: Relative Simultaneity
« Reply #35 on: 20/02/2015 16:31:35 »
Quote
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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Offline PhysBang

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Re: Relative Simultaneity
« Reply #36 on: 20/02/2015 16:35:30 »
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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Offline Bill S

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Re: Relative Simultaneity
« Reply #37 on: 20/02/2015 16:41:26 »
Quote
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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Offline JohnDuffield

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Re: Relative Simultaneity
« Reply #38 on: 20/02/2015 16:54:46 »
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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Offline PhysBang

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Re: Relative Simultaneity
« Reply #39 on: 20/02/2015 17:04:13 »
Quote
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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Offline phyti39

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Re: Relative Simultaneity
« Reply #40 on: 20/02/2015 17:06:22 »
Quote
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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Offline yor_on

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Re: Relative Simultaneity
« Reply #41 on: 20/02/2015 17:19:30 »
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. 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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Offline PhysBang

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Re: Relative Simultaneity
« Reply #42 on: 20/02/2015 17:38:59 »
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.
Quote
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.
Quote
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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Offline PhysBang

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Re: Relative Simultaneity
« Reply #43 on: 20/02/2015 17:40:05 »
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. 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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Re: Relative Simultaneity
« Reply #44 on: 20/02/2015 17:48:34 »
:)

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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Offline Bill S

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Re: Relative Simultaneity
« Reply #45 on: 20/02/2015 17:53:14 »
(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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Offline yor_on

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Re: Relative Simultaneity
« Reply #46 on: 20/02/2015 18:06:51 »
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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Offline David Cooper

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Re: Relative Simultaneity
« Reply #47 on: 20/02/2015 18:13:07 »
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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Offline yor_on

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Re: Relative Simultaneity
« Reply #48 on: 20/02/2015 18:52:39 »
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. Then to that you can add special relativity (uniform motions), in which case it becomes more complicated, but hopefully still understandable  Relativity of Simultaneity.
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Offline phyti39

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Re: Relative Simultaneity
« Reply #49 on: 21/02/2015 17:45:35 »
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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.
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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?
« Last Edit: 21/02/2015 17:48:03 by phyti39 »