Why the speed of light?

[volvo179]

Can anyone explain why the constant used by Einstein in his e equals mc squared equation was the speed of light rather than any other large constant.  Because the relationship between energy and matter has now been prooved to be as set out by Einstein clearly c is the right constant but how did Einstein realise/work out that when he was starting from scratch?

[jake-the-peg]

Very good question!

This is not easy to answer simply and certainly not in less than 2000 characters that Answer Bank gives us.

Richard Feynmann wrote a series of brilliant lectures whilst he was at Caltech covering much of Physics. The one on Special Relativity does a good job of showing how this is derived but it was written for first year undergrads.

I found PDFs of all of them here:

ftp://80.233.150.49/pub/docu/FeynMan%20Physics%20Lectures%20V1,2,3/

Basically it falls out of what is called the Lorentz transformation, which introduced the term, initially with regards to Maxwell's equations for light.