Universe Expansion: Does Speed of Motion Matter?

In summary: There is no unique answer, as different observers in different frames of reference would see the clocks at the distant galaxy ticking at different rates. In summary, it is not yet known/understood whether time dilation affects clocks at a distant galaxy.
  • #1
DuckAmuck
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TL;DR Summary
How does universe expansion relate to time dilation?
So the universe is expanding, and galaxies are getting farther apart from one another on average. Does this motion count the same as ordinary motion, in that if a galaxy is being expanded away from us at 0.5c, that clocks in that galaxy would appear to tick slower at 0.866 the rate of clocks here?
 
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  • #2
No, they keep perfect time for the inhabitants.
 
  • #3
DuckAmuck said:
Does this motion count the same as ordinary motion, in that if a galaxy is being expanded away from us at 0.5c, that clocks in that galaxy would appear to tick slower at 0.866 the rate of clocks here?

This question cannot be answered as you ask it because time dilation is frame dependent. There are choices of frame for which the distant galaxy's clocks tick slower relative to ours, and there are choices of frame in which the distant galaxy's clocks tick at the same rate as ours. There is no unique answer.
 
  • #4
PeterDonis said:
This question cannot be answered as you ask it because time dilation is frame dependent. There are choices of frame for which the distant galaxy's clocks tick slower relative to ours, and there are choices of frame in which the distant galaxy's clocks tick at the same rate as ours. There is no unique answer.
So it is not yet known/understood?
 
  • #5
DuckAmuck said:
So it is not yet known/understood?

No, that's not what I said. Read my post again.
 
  • #6
@PeterDonis, I think we can make the OP's question explicit by asking what we would see if we pointed a telescope at a galaxy receding from us at 0.5c and we could make out one of their clocks. I think we would in fact see the clock ticking at a slower rate, for the same reason that we see light redshifted. Do you agree?
 
  • #7
phyzguy said:
I think we can make the OP's question explicit by asking what we would see if we pointed a telescope at a galaxy receding from us at 0.5c and we could make out one of their clocks. I think we would in fact see the clock ticking at a slower rate, for the same reason that we see light redshifted. Do you agree?

Sure, but the redshift factor is not the same as the time dilation factor. The observed redshift is an invariant; it's not frame dependent. The time dilation factor is frame dependent. The OP was asking about the time dilation factor.
 
  • #8
Yes. But for those who are close enough to see the clock, it will appear to tick at exacty the same rate as our clocks appear to tick to us... :smile:

I considered answering with two letters only to nudge ducky to ask further...

[edit] ah, I see there is an anamnesis here...
and actually ducky is expected to be able to phrase the question better ...
##\ \ ##
 
  • #9
BvU said:
for those who are close enough to see the clock, it will appear to tick at exacty the same rate as our clocks appear to tick to us...

Not if it's receding at 0.5 c. The redshift factor for that is ##1 / \sqrt{3}##, which is easily detectable.
 
  • #10
Ah, I will have to learn about GR o:) -- wrongly restricted my thinking to SR
 
  • #11
DuckAmuck said:
So it is not yet known/understood?
No unique answer doesn’t mean that the answer is not known/understood. It means that the question was not formulated in a way that provides a unique answer.

As a simple analogy, I can ask you to solve the equation ##x^2=4##. The equation and its solution is completely understood, but the answer is not unique. Both ##x=2## and ##x=-2## are valid solutions.

Here you have asked a question with an infinite number of possible solutions.
 
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FAQ: Universe Expansion: Does Speed of Motion Matter?

What is universe expansion?

Universe expansion refers to the theory that the universe is constantly growing and expanding, with galaxies and other celestial bodies moving away from each other.

How do we know that the universe is expanding?

Scientists have observed that galaxies and other objects in the universe are moving away from each other, and this movement is accelerating. This supports the theory of universe expansion.

Does the speed of motion matter in universe expansion?

Yes, the speed of motion does matter in universe expansion. The faster an object is moving, the more it will be affected by the expansion of the universe.

What is the role of dark energy in universe expansion?

Dark energy is a mysterious force that is believed to be responsible for the acceleration of universe expansion. It is thought to make up about 70% of the total energy in the universe.

Will the universe continue to expand forever?

Based on current observations and theories, it is believed that the universe will continue to expand forever. However, the rate of expansion may change in the future due to the influence of dark energy and other factors.

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