# Expansion of Space vs Movement in Space

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I asked this in a thread in the relativity forum and it got buried and never got answered.

Does the separation of matter caused by the expansion of space experience the same time dilation and length contraction as a similar distance change and speed from a rocket ride?

I am thinking that if two galaxies are at rest with the CMB but are separating at relativistic speeds due to a increase in distance from expansion, they will share the same time frame.

I am thinking that if the two twins blasted off from Earth experiencing equal acceleration but in opposite directions, They also would experience the same time frame. Despite there increasing separation speed.

Anyone have the answer and where am I going wrong?

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I asked this in a thread in the relativity forum and it got buried and never got answered.

Does the separation of matter caused by the expansion of space experience the same time dilation and length contraction as a similar distance change and speed from a rocket ride?

I think this is a great question.

I am thinking that if two galaxies are at rest with the CMB but are separating at relativistic speeds due to a increase in distance from expansion, they will share the same time frame.

I don't know. I think it would depend on how you define "time frame".

At rest with the CMB, or on the Big Bang Track is reference frame I like to use, but can it be considered an inertial frame? A continuum of inertial frames? Can a standard SR inertial frame extend cosmic distances and still give valid predictions?

I am thinking that if the two twins blasted off from Earth experiencing equal acceleration but in opposite directions, They also would experience the same time frame. Despite there increasing separation speed.

Earth would judge them to be in the same time frame. They each would each consider the Earth's time frame to have slowed the same amount, relative to their own, but they would not agree at all that they were in the same time frame.

At the same time (so to speak), wouldn't each be at rest with respect to some distant point on the Big Bang Track, at rest in an inertial frame that included a Big Bang Track "rest point"?

Anyone have the answer and where am I going wrong?

I don't, I have more questions than answers, but I think when you extrapolate local physics to cosmic distances and times, you must lose some degree of certainty.

Someone with a better understanding of Astronomy, Cosmology and General Relativity like Martin may be able to give you a more definitive answer.

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Earth would judge them to be in the same time frame. They each would each consider the Earth's time frame to have slowed the same amount, relative to their own, but they would not agree at all that they were in the same time frame.

How can all that be true? It sounds like a contradiction. I gave the two twins a master clock slaved to Earth's time frame and can not see how they would differ. And I thought I had those d@mned twins figured out.

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How can all that be true? It sounds like a contradiction. I gave the two twins a master clock slaved to Earth's time frame and can not see how they would differ. And I thought I had those d@mned twins figured out.

Let's say they stopped accelerating in a symmetrical predetermined way and started signaling each other as planned.

They would both agree that the master clock had slowed the same amount relative to their own.

But even after accounting for the time for transmission of the signals they would each think the other twin's clock had slowed, by more in fact than the master clock had slowed.

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They would both agree that the master clock had slowed the same amount relative to their own.

But even after accounting for the time for transmission of the signals ...

But that is why we have a master clock. So we do not have to wait for the time of transmission. Just because we see the lightning before we hear the thunder does not mean they were not simultaneous or synchronized.

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But that is why we have a master clock. So we do not have to wait for the time of transmission.

If you take the master clock with you on a trip it seizes to be a master clock...unless it worked on something outside of known physics.

Just because we see the lightning before we hear the thunder does not mean they were not simultaneous or synchronized.

So you do the math based on distance, the speed of light and speed of sound and happily are confident that they were simultaneous.

Do the same based on the distance and speed of light between the twins...and they each still think the other has slurred speech.

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If you take the master clock with you on a trip it seizes to be a master clock...unless it worked on something outside of known physics...

Our thought experiments have unlimited technology. The master clock is hooked to CMB speedometer and gravitonometer to stay synchronized to Earth time. The GPS satellites are synchronized to Earth clocks so it isn't impossible.

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Does the separation of matter caused by the expansion of space experience the same time dilation and length contraction as a similar distance change and speed from a rocket ride?

Nope.

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Nope.

Thank you!

I actually read that link before asking the question but limited terminology in physics let me pass over what I now think is the key.

"...distance is measured using the Lorentz interval...

I should have looked up:

Lorentz interval

"The Lorentz interval is a quantity that is used instead of distance when dealing with space-time geometry, because it is the only quantity that is the same to all observers regardless of how fast they are moving in spacetime, that is, in all reference frames. In other words, it is not affected by relative-velocity length contractions or time dilation."

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Nope.

Nice Link, but can you direct us to where it makes the time dilation distinction between expansion recession and recession due to movement otherwise?

If we are both on the CMB rest frame but very far apart and I send a probe that accelerates toward you, closes the gap, decelerates less than it accelerated originally to come to a halt in your lap, is it then in the same time frame it started? I don't know, and saw nothing in that link that would indicate either way when I scanned it.

Edit: I hadn't read NTWK's latest post. I will re-check and try and see if it makes sense to me.

Re-edit: Checked it rather quickly but not sure that's the key. Isn't the expansion of the Lorentz interval not equal in all directions if you are not at rest wrt the CMB? How can it be invariant for GR if that is the case? (I can see how it is invariant for SR)

Edited by J.C.MacSwell
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2nd and 3rd paragraphs:

While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, there is no such theoretical constraint when space itself is expanding. It is thus possible for two very distant objects to be moving away from each other at greater than the speed of light (meaning that one cannot be observed from the other). The size of the observable universe could thus be smaller than the entire universe.

It is also possible for a distance to exceed the speed of light times the age of the universe, which means that light from one part of space generated near the beginning of the Universe might still be arriving at distant locations (hence the cosmic microwave background radiation). These details are a frequent source of confusion among amateurs and even professional physicists

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2nd and 3rd paragraphs:

So...how does that differentiate time dilation due to expansion displacement vs movement displacement?

Not disagreeing or agreeing there's a difference, but I don't see it in those paragraphs.

Edited by J.C.MacSwell
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So...how does that differentiate time dilation due to expansion displacement vs movement displacement?

Not disagreeing or agreeing there's a difference, but I don't see it in those paragraphs.

I agree that I do not see the proof in those two paragraphs but it is possible that I do not see the relevance in the words.

Lorentz interval specifically said

http://en.wikipedia.org/wiki/Lorentz_interval

"it is not affected by relative-velocity length contractions or time dilation."

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I'm not sure if this is intimately related, but I suppose it is nominally. A common question on the high-school level of physics when learning about special relativity (especially velocity transforms) is given the following situation: two rockets traveling in opposite directions towards the earth at 0.9c relative to a stationary observer on the earth; what is the closing velocity of the rockets from the POV of the observer. The answer is, while students may find it uncomfortable to give 1.8c. There is no speed limit for the rate of shortening (or, as is relevant here, the rate of expanding) of space as there is for the velocities of specific objects. As such, since that integral part of SR fails to apply to lengths of space, I would dare say that length contraction and time dilation would not occur.

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I am thinking that if the two twins blasted off from Earth experiencing equal acceleration but in opposite directions, They also would experience the same time frame. Despite there increasing separation speed.

Anyone have the answer and where am I going wrong?

Have you considered the possibility that since you are in time-sync between objects at fixed expansion you cannot blast off in perfectly opposite directions at the same time because to remain in sync you find yourself "in the up" side of one object and the "at the down" side of the other?

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Have you considered the possibility that since you are in time-sync between objects at fixed expansion you cannot blast off in perfectly opposite directions at the same time because to remain in sync you find yourself "in the up" side of one object and the "at the down" side of the other?

Not sure what you are saying here but my point was that the two twins could blast off in any direction and their clocks would remain synchronized as long as their acceleration was equal. Both of their clocks would slow down, relative to Earth clocks, equally. Any course or speed change after blast off by either twin would unsyncronize their clocks.

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Not sure what you are saying here but my point was that the two twins could blast off in any direction and their clocks would remain synchronized as long as their acceleration was equal. Both of their clocks would slow down, relative to Earth clocks, equally. Any course or speed change after blast off by either twin would unsyncronize their clocks.

You began with a comparison to two galaxies, fixed in expansion with the reasonable assumption that time between the two is synchronized in a line drawn through their centers.

Unless your rocket ships blasting off in different directions are extremely special, the place from which they were launched can be only in one galaxy or the other or between them thus in neither--it cannot be in both galaxies.

You want to blast off opposite (or at least in different direction) rockets into a universe containing at least three galaxies only two of which are synchronous in time across space, yet you want to keep them bound to the same time reference existing at the point of launch.

What I am saying is:

Have you considered that even in this simplest of universes, the point of launch cannot be isolated in time relative to the remainder of the universe?

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You began with a comparison to two galaxies, fixed in expansion with the reasonable assumption that time between the two is synchronized in a line drawn through their centers...

...Have you considered that even in this simplest of universes, the point of launch cannot be isolated in time relative to the remainder of the universe?

To make things a little less abstract I would like to ignore GR in my scenario. The variable gravitational fields only complicate things when considering if two separating galaxies can share the same time frame. My only concern is if expansion of space causes time dilation and length contraction and at this point it seems it does not.

My twin example had both twins leaving Earth. I was just contemplating if it was possible for separating mass because of movement, not expansion, to share the same time frame. It seems it is possible but length contraction may be a different story.

As I am sure you can tell, I am very much a layman just trying to understand it all a little better. I do appreciate all participation as it is all food for thought.

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As I am sure you can tell, I am very much a layman just trying to understand it all a little better. I do appreciate all participation as it is all food for thought.

As I am sure you can tell by my current post regarding time shifting via astronomical alignment, I'm very much the same.

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2nd and 3rd paragraphs:

So...how does that differentiate time dilation due to expansion displacement vs movement displacement?

....

To make things a little less abstract I would like to ignore GR in my scenario. The variable gravitational fields ...

Grant (GDG) gave a good answer, I thought. MacSwell asked a question about how distance expansion differs from ordinary motion.

MacSwell, in ordinary motion you get somewhere, there is some destination you approach. But in standard cosmo espansion nobody gets nearer anything. It simply means that the geometry (the gravitational field) is changing. All (largescale) distances increase and you don't go anywhere. So naturally special rel time dilation does not apply! How could it? Nobody is moving.

NowThat, the 1915 lesson is that when you say gravitational field you mean geometry----the catalog of distances between things.

If you want to talk about recession rates (the rate of increase of distance between objects which are not moving) then that necessarily means talking about geometry.

Changing geometry is what you are talking about. General rel is our only handle on that (unless you count the newer quantum versions of GR too).

So it isn't practical to ignore GR in your scenario.

Recession is not ordinary motion (it doesnt go anywhere) and therefore it is not subject to Special Rel.

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Grant (GDG) gave a good answer, I thought. MacSwell asked a question about how distance expansion differs from ordinary motion.

MacSwell, in ordinary motion you get somewhere, there is some destination you approach. But in standard cosmo espansion nobody gets nearer anything. It simply means that the geometry (the gravitational field) is changing. All (largescale) distances increase and you don't go anywhere. So naturally special rel time dilation does not apply! How could it? Nobody is moving.

NowThat, the 1915 lesson is that when you say gravitational field you mean geometry----the catalog of distances between things.

If you want to talk about recession rates (the rate of increase of distance between objects which are not moving) then that necessarily means talking about geometry.

Changing geometry is what you are talking about. General rel is our only handle on that (unless you count the newer quantum versions of GR too).

So it isn't practical to ignore GR in your scenario.

Recession is not ordinary motion (it doesnt go anywhere) and therefore it is not subject to Special Rel.

Thanks Martin. So we can consider all points that measure cmb isotropy to be in the same time frame, no matter how far apart, and no matter their relative velocities due to the expansion alone.

Seems like a preferred reference frame.

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Thanks Martin. So we can consider all points that measure cmb isotropy to be in the same time frame, no matter how far apart, and no matter their relative velocities due to the expansion alone.

Well I certainly do not consider widely separated points to be part of the same reference frame. Reference frame is a technical idea belonging to special rel (which doesn't apply over large regions of curved spacetime!)

So you can toss the concept of reference frame in this context. But there is of course a criterion of rest, and of simultaneity, which cosmologists use all the time. Their Friedman model employs it, as does the Hubble Law.

Seems like a preferred reference frame.

What do you mean "frame", exactly?

There is definitely Friedman time, a preferred time slicing into (curved) spatial hypersurfaces. But I don't know of any frame, in the rectilinear sense of special rel, that is globally applicable. That wouldn't make sense, except perhaps in some special case like a universe without matter.

I have to go. Will check in later.

MacSwell, we may have a merely verbal misunderstanding here. Cosmologists have a preferred time, universe time, Friedman time, that enters into everything. But they do not have a preferred frame. Anyway I never heard of one.

Maybe you are confusing the ideas of global time with global frame (which doesnt occur in cosmology).

Edited by Martin
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...So it isn't practical to ignore GR in your scenario...

...Recession is not ordinary motion (it doesnt go anywhere) and therefore it is not subject to Special Rel.

Thanks for your reply. My question has been answered. As far as ignoring GR, I just didn't want local gravity having an impact on the clocks and complicating things.

Merged post follows:

Consecutive posts merged
Thanks Martin. So we can consider all points that measure cmb isotropy to be in the same time frame, no matter how far apart, and no matter their relative velocities due to the expansion alone.

Seems like a preferred reference frame.

My thoughts exactly. If nothing else, it would make sense for us to choose it as a "preferred reference frame". Then make adjustments for local gravity and we would have a universal GMT (Zulu) time.

Edit - Or master clock as swansont would prefer

Edited by NowThatWeKnow
Consecutive posts merged.
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...

My thoughts exactly. If nothing else, it would make sense for us to choose it as a "preferred reference frame". Then make adjustments for local gravity and we would have a universal GMT (Zulu) time.

Edit - Or master clock as swansont would prefer

There is something I'm missing here. A "preferred reference time" is fine, the professionals already have that and it is standard.

But how do you get a global frame from that?

What do you do for spatial coordinates? We don't know the largescale topology of space. One set of spatial coordinates might not fit the whole thing!

Like one flat map does not cover the whole earth. You can only make flat maps of a curved surface if you work in small local patches.

I don't see how anybody can talk about a preferred reference frame (except limited and local, for a given observer). What am I missing?

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I don't see how anybody can talk about a preferred reference frame (except limited and local, for a given observer). What am I missing?

Didn't you say that you could be anywhere in the universe and that the age of the universe would be the same when calculated as long as you had a constant CMB temperature in all directions at each location? Wouldn't that make it a common reference frame?

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