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-   -   E=mc2 question. (http://www.physicsforums.com/showthread.php?t=386435)

alex.cordero Mar14-10 03:03 AM

e=mc2 question.
 
I'm 3/4 of the way through David Bodani's, E=mc2: A Biography of the World's Most Famous Equation book and I'm really enjoying it. Thanks to this book, I finally understand that "e=m" and also understand why "C^2", but I still can't understand why Einstein used the speed of light to connect the two?

Is it just a constant? Did he use "c" because there is nothing faster?

ansgar Mar14-10 04:10 AM

Re: e=mc2 question.
 
it follows from the derivation simply,

http://www.physicsforums.com/showthread.php?t=76010

and this is not quantum physics...

alex.cordero Mar14-10 04:20 AM

Re: e=mc2 question.
 
Quote:

Quote by ansgar (Post 2623322)
it follows from the derivation simply,

http://www.physicsforums.com/showthread.php?t=76010

and this is not quantum physics...

Thank you. Can you enlighten me as to where I should have posted this?

Frame Dragger Mar14-10 04:33 AM

Re: e=mc2 question.
 
Quote:

Quote by alex.cordero (Post 2623326)
Thank you. Can you enlighten me as to where I should have posted this?

Relativity

alex.cordero Mar14-10 04:37 AM

Re: e=mc2 question.
 
Quote:

Quote by Frame Dragger (Post 2623334)
Relativity

Perfect, thank you.

Frame Dragger Mar14-10 04:40 AM

Re: e=mc2 question.
 
Quote:

Quote by alex.cordero (Post 2623337)
Perfect, thank you.

No sweat, enjoy PF!

bcrowell Mar14-10 11:24 AM

Re: e=mc2 question.
 
FAQ: Where does E=mc2 come from?

Einstein found this result in a 1905 paper, titled "Does the inertia of a body depend upon its energy content?" This paper is very short and readable, and is available online. A summary of the argument is as follows. Define a frame of reference A, and let an object O, initially at rest in this frame, emit two flashes of light in opposite directions. Now define another frame of reference B, in motion relative to A along the same axis as the one along which the light was emitted. Then in order to preserve conservation of energy, we are forced to attribute a different inertial mass to O before and after it emits the light. The interpretation is that mass and energy are equivalent. By giving up a quantity of energy E, the object has reduced its mass by an amount E/c2.

Although Einstein's original derivation happens to involve the speed of light, E=mc2 can be derived without talking about light at all. One can derive the Lorentz transformations using a set of postulates that don't say anything about light (see, e.g., Rindler 1979). The constant c is then interpreted simply as the maximum speed of causality, not necessarily the speed of light. Constructing the mass-energy four-vector of a particle, we find that its norm, E2-p2c2, is frame-invariant, and can be interpreted as m2c4, where m is the particle's rest mass. In the case where the particle is at rest, p=0, and we recover E=mc2.

A. Einstein, Annalen der Physik. 18 (1905) 639, available online at http://www.fourmilab.ch/etexts/einstein/E_mc2/www/

Rindler, Essential Relativity: Special, General, and Cosmological, 1979, p. 51

alex.cordero Mar14-10 12:21 PM

Re: e=mc2 question.
 
Quote:

Quote by bcrowell (Post 2623714)
FAQ: Where does E=mc2 come from?
Einstein found this result in a 1905 paper, titled "Does the inertia of a body depend upon its energy content?" This paper is very short and readable, and is available online. A summary of the argument is as follows.

Thank you for your summary. It's more clear now.


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