# The Naked Scientists Forum

### Author Topic: Relative Simultaneity  (Read 23973 times)

#### Bill S

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##### Relative Simultaneity
« on: 14/02/2015 20:29:23 »
Extract from the Haifa Lectures:

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?

#### 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.

#### 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.

#### 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.

#### 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.

#### 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)
Quote
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.

#### 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)
Quote
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.

Now that's just out there Pete. I will have to read it.

#### 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 »

#### 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 »

#### Toffo

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##### Re: Relative Simultaneity
« Reply #9 on: 17/02/2015 12:49:12 »
Extract from the Haifa Lectures:

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.

#### 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?

#### 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.

#### 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.

#### 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.

#### 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.

#### 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 »

#### 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.

#### 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 »

#### 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 »

#### 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 »

#### 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.

#### 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.

#### phyti39

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##### Re: Relative Simultaneity
« Reply #22 on: 19/02/2015 20:32:28 »
Extract from the Haifa Lectures:

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.

#### 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.

#### 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 #24 on: 19/02/2015 21:51:29 »