# The Naked Scientists Forum

### Author Topic: Relative Simultaneity  (Read 23942 times)

#### 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.
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I could possibly argue that locality to be defined need be 'at rest' with what it observes?
This one is tricky.
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So okay, even though he defines a frame here as being the 'proper one', he doesn't mean it differently from Einsteins definitions, as I read him now. It's exchangeable frames, the helicopters frame becomes the 'proper one' if the cat wants to see what the helicopter saw, and he will need to make a Lorentz transformation too. Then we have causality left, and the ordering of events. Would it be as magic, not knowing relativity?  Does it break casuality for the guy in the helicopter, or not. From where do you define it, if so.
« Last Edit: 20/02/2015 01:13:46 by yor_on »

#### 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.

#### yor_on

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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 :)
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I've actually discussed that one extensively elsewhere if you're interested, but it will take us from the questions created by the idea of the cat disappearing in one frame of reference. To me it's about there being a causality, and how you then should define it? As presenting us locally understandable events, action and reaction, cause and effect, or not?
« Last Edit: 20/02/2015 01:32:16 by yor_on »

#### PmbPhy

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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
What is the "it" that you're talking to?

#### yor_on

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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.
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Is it about what logic defining a universe? If someone can think up a experiment validating Sachs idea here I would be much obliged. Let's put it this way. Assume everything is communication and information. And that the stuff doing it is 'c'. That is your reality, the 'force carriers' are 'c'. This is valid when watching the cat 'disappear' too.
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It's somewhat like the question of different uniform motions, and what they do to a universe, isn't it? Why is it just angular momentum that come close to 'c'? What would the universe be if we had found (distant) planets and suns moving relativistically relative us?
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I think it has to do with how to define locality to me. If I accept Sachs definition then I see it as a proof of frames of reference being disconnected from the laws and rules defining how it 'is'. 'What you see is what you get' in my definitions, but it places causality somewhere else. It's not the simple 'universe' we exist in any longer. The 'container' becomes a mathematical space, even though WYSWYG rules locally defined.
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Yep, I think he's right.

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It's a beautiful example he created, isn't it? Which is why I would like to see it experimentally validated. It fits my views on this universe being created out of rules and laws, properties and 'emergences'. It doesn't tell us what 'reality' is, unless you believe yourself to consist of mathematics too :) but it sure as h** tells us that it isn't the mechanical clock work we once expected it to be. It's much more than that.
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And finally, where it doesn't fit my ideas. That's the helicopter, not knowing relativity, watching the cat magically disappear. And the reason why we don't see it can be connected to my question on different relative motions in this universe, and why they're not relativistic. Unless they are we won't see anything like this. And being 'at rest' with Earth and our solar system, and galaxy, the only way to see it should be in a particle accelerator, well, possibly?

So that one has to do with preconceptions.
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Had to look him up after this, and it seems his ideas are somewhat incompatible to mine :) And with what I got out of this example you cited Bill. Because to me he do place causality inside a 'mathematical space' making the cat 'disappear' in one frame, not inside a 'physical space' as defined by matter alone, and so do any application of a Lorentz transformation thinking of it. A awesome thinker though.
« Last Edit: 20/02/2015 13:41:58 by yor_on »

#### 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.

#### yor_on

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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.
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to see the way I think we can use that 'muon example' in where we find complementary explanations. You can actually ignore those two explanations, instead concentrating on whether both frames are in constant communication with (observing) the other 'frame'. They are at all times 'communicating' as far as I can see, keeping causality intact in both frames. But when it comes to the cats 'instant disappearance' one frames information 'stops' there (helicopter), breaking cause and effect as observed from that frame, although continues following the type of causality we're used to, from the frame of the workman.
« Last Edit: 20/02/2015 14:47:26 by yor_on »

#### 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.

#### 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.)
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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.

#### 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
What is the "it" that you're talking to?

LOL probably

#### Bill S

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##### Re: Relative Simultaneity
« Reply #35 on: 20/02/2015 16:31:35 »
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I suspect that the author of the original piece spoke of the cat "disappearing" to simply mean that the cat fell away from view, not that the cat vanished in some instantaneous way.

Either that is just semantics, or you are saying that the cat fell into the hole in one F of R, but not in the other. Are we talking reality here?

#### 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.

#### 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.

#### 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.

#### PhysBang

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##### Re: Relative Simultaneity
« Reply #39 on: 20/02/2015 17:04:13 »
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According to relativity theory, if an event happens in one frame, it happens in every frame.

Yet at any given time, the cover could be open in one frame, but not in another.
No, because there is no "given time" between frames.

#### 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.

#### 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 :)

#### 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.
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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.
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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.

#### 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.

#### yor_on

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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 :)

#### 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?

#### 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?

#### 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.

#### 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.

#### 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 »

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##### Re: Relative Simultaneity
« Reply #49 on: 21/02/2015 17:45:35 »