E=mc2 question.

  1. 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?
  2. jcsd
  3. Relativity
  4. Perfect, thank you.
  5. No sweat, enjoy PF!
  6. bcrowell

    bcrowell 6,383
    Staff Emeritus
    Science Advisor
    Gold Member

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