**Einstein's fundamental assumption of Special Relativity**

You're on a train and I'm standing on the side of the track. As your train passes I measure your speed.. it's 60 mph.

So I tell everyone you are travelling at 60 mph.

But one person totally disagrees with me.

They say they started driving their car next to the train until they reached 60 mph and then measured your speed. The answer they got was 0 mph. So for them you are not moving.

Now, replace the train with a beam of light, which travels fast but still at a finite speed (186,000 mps).

I measure the speed while standing next to the track and I get 186,000 mps. No problem, I get in a rocket ship and start travelling next to the beam until I reach 186,000 mps. Then I measure the speed of the beam again.

And I get 0 mps. Nn I don't.. I get.. drum roll please.. 186,000 mps.

That's the fundamental cornerstone of Relativity, or at least Special Relativity. That the speed of light is a fundamental constant independent of any reference frame.

Einstein's genius was not in seeing this (it was already known), his genius was in making it the fundamental assumption of a new theory and then deriving the consequences.

Content written and posted by Ken Abbott abbottsystems@gmail.com

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