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I am really struggling to get my head round the issue of how you measure speed in an absolute way and furthermore between two referentials, e.g. in the case of the Langevin twins. I understand that time is relative to the referential you are in and depending on its velocity, but how do you define velocity (and the ratio to speed of light) if there is no "absolute referential". More specifically, in the case of the Langevin twins, how do you know who's traveling near the speed of light?

Thomas
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Well for one thing, people often ask why it is that if you send a pulse of light from point A to point B, not only do those who are at rest relative to points A & B measure the speed of the pulse of light as being 300,000 km/s, but so do other observers.

Meaning, other observers who could let's say be in the process of being in motion relative to points A & B, perhaps basically heading toward point A after leaving point B, or the other way around, they too will measure the speed of those very same bursts of light as being 300,000 km/s.

And so the question arises......how is this possible?

The only way that this can occur is if both the pulse of light and all the observers share something in common. What is common between them all is their "Absolute" magnitude of motion within an "Absolute" 4 dimensional environment known as Space-Time.

Light travels at 300,000 km/s across the vacuum of space of that Space-Time environment.

All the observers are also moving across that Space-Time environment with an exact equal magnitude of motion. The only difference is that there is always a percentage of the observers magnitude of motion being across the dimension of time. It is the remaining percentage of that magnitude of motion, that becomes motion across space.

Sean
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There is not a concept of absolute velocity in special relativity. Velocity is always relative to some referential. In the case of the twins paradox, The twin on earth is at rest in his own referential, while the travelling twin has a certain velocity (maybe near the speed of light) in respect to the referential of the earth twin. But in the referential of the travelling twin, his own velocity is zero. Hope it helps.

Patrick
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