Science question.

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that guy
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Re: Science question.

Post by that guy »

Special relativity only holds under inertial frames of reference, i.e. frames of reference that are not accelerating. All the stories you hear about how one twin goes up in a spaceship and comes back younger than his other twin work because the twin in the space ship had to accelerate to near light speed. Special relativity doesn't work under acceleration and must be generalized to General Relativity (which I have never learned about).

One thing that often resolves issues with conflicting reference frames is that you don't just get time dilation, you also get length contraction. Since velocity is essentially length over time two observers can agree that light is traveling at c. One will just think it has traveled a shorter length while the other will thing it has taken a shorter time.

In that car example, if the car is traveling at 0.5c then the photons don't move away from it at a relative speed of 0.5c. They still move away at c from the cars perspective. The 'stationary' observer in that example wouldn't see the photons as having a relative velocity of c however since that would give them a velocity of 1.5c.

Special Relativity is a well proven theory. Some existing technology wouldn't work if it didn't hold (The only example I'm aware of is GPS).
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Jaedar
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Re: Science question.

Post by Jaedar »

I think you don't have to consider general relativity to much for the twin 'paradox', it works itself out if you do the math iirc(it was demonstrated at one of my lectures).
that guy wrote: Special Relativity is a well proven theory. Some existing technology wouldn't work if it didn't hold (The only example I'm aware of is GPS).
I think everything to do with satellites and space travel would stop working without relativity. I can't really think of anything more down to earth either.
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that guy
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Re: Science question.

Post by that guy »

With only two twins then the scenario from one twins perspective is a perfect mirror of the scenario from the other twins perspective. Whatever the velocity of twin A with respect to twin B, twin B has the exact opposite velocity with respect to twin A. Since special relativity doesn't care about the sign (the v component is squared in the Lorentz transform) the two scenarios are indistinguishable under special relativity alone. However special relativity doesn't apply for the whole experiment because the experiment requires that the twins start in the same frame of reference and thus one has to accelerate in order for them to have different velocities.
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