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Q & A: relativitistic invariants

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Most recent answer: 12/10/2012
Q:
Hi, I am starting to dig a little in the physics world and its puzzles. I want to know if what I understand is correct. Based on Einstein, every measurement is relative to the observer, well, everything but speed of light. Speed of light is the only constant in all systems whether the object is moving or static, everyone, the observer and the object will be able to get the same value of speed of light when measuring. is that correct? is there any other constant like this in his theory? thanks, I hope I was clear.
- Juan Nino (age 29)
Sydney, NSW, Australia
A:
Sure, relativity is filled with quantities that are the same in each good reference frame. These are called "invariants". Einstein at one point tried to rename the theory "invariants theory" but by then the name "relativity" had stuck.

Here's two examples, drawn from special relativity (I don't know general relativity well enough to give ones from it):
d2-c2t2 where d is the distance between two events, t is the time interval between them, and c is the speed of light.
E2-p2c2 , where E is the total energy of some stuff and p is its total momentum.

Note that the constant speed of light is just a special case of the first invariant, one where the invariant value happens to be zero.

Mike W.

(published on 12/10/2012)

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