Space Travel: SR Questions Explored

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Discussion Overview

The discussion revolves around questions related to Special Relativity (SR) and its implications for space travel, particularly concerning the visibility and communication between two travelers moving at relativistic speeds. Participants explore concepts such as relative velocity, information transfer, and the effects of cosmic expansion on these principles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether two space travelers moving away from each other at just over 1/2c would vanish from each other's view and if this situation violates SR.
  • Another participant clarifies that velocities do not add linearly in SR, providing a reference to the velocity addition formula.
  • A participant expresses surprise at the implications of velocity addition in SR.
  • One participant cites Lawrence Krauss, suggesting that due to Dark Energy, galaxies will eventually move away from each other at speeds greater than c, raising questions about the validity of SR in this context.
  • Another participant responds that SR does not apply to cosmology and that the relative velocity of distant cosmological objects is not well defined, emphasizing that only local velocities must remain

Areas of Agreement / Disagreement

Participants express differing views on the applicability of SR to cosmological scenarios, with some asserting that it does not apply while others reference popular interpretations that suggest otherwise. The discussion remains unresolved regarding the implications of cosmic expansion on the principles of SR.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about visibility and communication between travelers, as well as the definitions of velocity in different contexts (SR vs. cosmology). The implications of Dark Energy and its effects on the universe's expansion are also not fully explored.

PerenialQuest
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Hello All,

I'm trying to wrap my head around SR and I have a couple questions: If two space travelers (A) and (C) take off from a Planet (B) in opposite directions and each travels just over 1/2c, do they vanish from each others' view? I don't see why they wouldn't but

If yes, doesn't that violate SR which nothing can travel at v > c? Since they don't observe each other is that avoided?

Also does that mean that no information can ever travel from (A) to (C)? If yes, couldn't (B) just act as a medium of communication which is moving at < c in relation to both (A) and (C).

Also, let's say (A) and (C) turn around and travel back toward each other with a combined v > c, what would this look like to each space traveler?

Thanks for your time,
Rich
 
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huh! it really is! that's wild! thanks
 
Ok but Lawrence Krauss told me that because of Dark Energy, all the galaxies will eventually be traveling at > c with relation to each other and will disappear from each others' view. He also said that this doesn't violation SR. Explain that!
 
SR doesn't apply to cosmology. In GR, the relative velocity of cosmologically distant objects is not uniquely well defined. It's only locally that relative velocities are well defined and have to be <c.

The interpretation of cosmological horizons that you're quoting is an oversimplification: Davis and Lineweaver, Publications of the Astronomical Society of Australia, 21 (2004) 97, msowww.anu.edu.au/~charley/papers/DavisLineweaver04.pdf
 
ok thanks!
 

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