Space Travel: SR Questions Explored

[PerenialQuest]

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

 

[bcrowell]

Velocities don't add linearly in SR. A's velocity relative to C is 0.8c.

http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_theory_of_relativity

 

[PerenialQuest]

huh! it really is! that's wild! thanks

 

[PerenialQuest]

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!

 

[bcrowell]

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

 

[PerenialQuest]

ok thanks!