Time Dilation: Away or Toward Observer?

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Discussion Overview

The discussion revolves around the concept of time dilation in special relativity, particularly focusing on whether the direction of travel (away from or toward an observer) affects the perception of time between a moving observer and a stationary one. Participants explore the implications of this concept as presented in a video by Brian Greene.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that a traveler's proper time always passes more slowly than a stationary observer's clock, regardless of the traveler's direction.
  • One participant references a video where an alien riding a bicycle away from and then toward Earth appears to have time behaving differently in each case, leading to confusion about the nature of time dilation.
  • Another participant challenges the interpretation of the video, suggesting that the observed differences in clock rates are due to the Doppler effect rather than time dilation itself.
  • It is proposed that when accounting for light travel time, both observers would calculate that the other's clock is running slower than their own, which relates to relativistic time dilation and the relativity of simultaneity.
  • A participant emphasizes that the difference in perceived time for the alien going away versus toward Earth is related to how clocks are synchronized in different reference frames, rather than a change in the rate at which time passes.

Areas of Agreement / Disagreement

Participants express differing interpretations of time dilation and its relationship to the direction of travel. There is no consensus on the implications of the video or the correct understanding of the phenomena discussed.

Contextual Notes

Participants highlight potential misunderstandings regarding the effects of the Doppler effect and the synchronization of clocks in different reference frames, indicating that these factors complicate the discussion of time dilation.

Involute
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I thought the answer was no. I.e. the traveler's proper time always passes more slowly than a stationary observer's clock regardless of the traveler's direction with respect to the observer. I was watching Brian Greene's NOVA episode on time, however, and, at 23:15, he has a demonstration involving an alien 10 billion lightyears from Earth riding a bicycle away from, and then towards, us. When he's riding away, time slows down for him (with respect to us). When he's riding towards, it speeds up. This seems contradictory, but maybe I'm missing a detail, or misunderstood SR to begin with (or both). Thanks for any tips.
 
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Involute said:
I thought the answer was no. I.e. the traveler's proper time always passes more slowly than a stationary observer's clock regardless of the traveler's direction with respect to the observer. I was watching Brian Greene's NOVA episode on time, however, and, at 23:15, he has a demonstration involving an alien 10 billion lightyears from Earth riding a bicycle away from, and then towards, us. When he's riding away, time slows down for him (with respect to us). When he's riding towards, it speeds up. This seems contradictory, but maybe I'm missing a detail, or misunderstood SR to begin with (or both). Thanks for any tips.
That all makes no sense. I notice the title of the video is:
The Fabric of the Cosmos: The Illusion of Time

I doubt you can learn any serious science from a video like that. It's not surprising you got some nonsense about an alien on a bicycle!
 
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@Involute If A and B are moving relative to one another and watching each other’s clocks through telescopes... if they are moving towards one another they will both see the other’s clock running fast, and if they are moving away from one another they will both see the other’s clock running slow. This has nothing to do with relativity and time dilation; it’s just the Doppler effect, caused by the light travel time changing as the distance between them changes.

However, when they allow for the light travel time - that is, the light they see in their telescopes was emitted earlier and spent some time getting to them - and calculate the clock rates that would produce what they actually see then they both calculate that the other’s clock is running slower than their own. That is relativistic time dilation, and it is best understood as a manifestation of the relativity of simultaneity, not as “time slowing down”.
 
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You are misinterpreting that part of the video. The difference between the two cases of the alien going away versus toward Earth is not about how fast the clock ticks. It is about how the clocks at different positions are synchronized to define simultaneous time ("now") in the reference frame of the alien as he goes away versus as he goes toward the earth.
 
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