I can't understand time dilation in absence of absolute motion.

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Discussion Overview

The discussion centers on the concept of time dilation as described by the theory of relativity, particularly in scenarios where two observers, Alice and Bob, perceive each other as moving. The scope includes theoretical exploration of relativistic effects and the implications of relative motion on time measurement.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question whose time will be dilated when Alice and Bob are in relative motion, suggesting that the answer may depend on the frame of reference.
  • Others propose that both observers experience time dilation with respect to each other, raising questions about how this can be reconciled.
  • One participant notes that for Alice and Bob to compare their clocks directly, one must turn around, which introduces complexities related to the 'twin paradox'.
  • It is suggested that the relativistic effects, such as time dilation, are symmetric between inertial frames, challenging the notion of determining which clock is "really" dilated.
  • Another participant emphasizes that without absolute speed, one cannot have absolute time, leading to the conclusion that each observer sees the other's clock as running slower.
  • There is a reiteration that time dilation is a relative phenomenon, as mentioned by another participant.

Areas of Agreement / Disagreement

Participants express differing views on the nature of time dilation and whether it can be considered absolute or relative. The discussion remains unresolved, with multiple competing interpretations of the phenomenon.

Contextual Notes

The discussion highlights the complexities of comparing time measurements between observers in relative motion, including the implications of the 'twin paradox' and the symmetry of relativistic effects.

Soumya_M
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According to theory of relativity there is no absolute standard of rest. Now, if Alice and Bob are two observers floating in space, so that Alice thinks that she is at rest and Bob is moving away from her at a speed comparable to that of light and Bob thinks that he is at rest and Alice is moving away from him at a speed comparable to that of light, whose time will be dilated?
 
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Soumya_M said:
whose time will be dilated?
In which frame?
 
Soumya_M said:
whose time will be dilated?
The other person's.
 
DaleSpam said:
In which frame?

In both the frames. Could it be that both frames have time dilated with respect to each other? How does time dilation make sense that way?
 
Doc Al said:
The other person's.

What will happen if Alice and Bob compare their clocks? Alice's clock will be faster than Bob's or the other way? If both show the same time - where is the dilation?
 
Soumya_M said:
What will happen if Alice and Bob compare their clocks? Alice's clock will be faster than Bob's or the other way? If both show the same time - where is the dilation?
The only way for them to directly compare their clocks is for one or both of them to turn around and join the other. That changes everything. (Look up the 'twin paradox'.)

But the important point is that between inertial frames the relativistic effects (time dilation, length contraction) are completely symmetric. To ask which clock is really dilated is to miss the point.
 
You cannot have "absolute time" if you don't have "absolute speed". If you have relative speed then you must have relative time.

That is, Alice, observing Bob's clock as he goes whizzing past her, sees Bob's clock as running slower then hers. Bob, observing Alice's clock as she goes whizzing past him, sees her clock as running slower than his. That is what "time dilation" means.
 
Soumya_M said:
Could it be that both frames have time dilated with respect to each other?
Yes.

Soumya_M said:
How does time dilation make sense that way?
Because it is relative, as HallsOfIvy mentioned.
 

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