Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mass energy equivalence

  1. Jun 28, 2009 #1
    I obtained an analytical description of time dilation and length contraction which describes why these occur at high speeds. However there was no such analysis of Einstein's famous equation E=m(c squared).
    Instead, a derivation that I found in http://www.karlscalculus.org/einstein.html [Broken] starts off with

    m =

    √1 - v2/c2

    Then they derive the formula using calculus.

    Is an analytical treatment for E=m(c squared) possible? Is there any thought experiment which helps us to realise the applicability of the mass-energy equivalence?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jun 28, 2009 #2


    User Avatar
    Homework Helper

    Um... the web page you linked to does give an analytical derivation of [itex]E=mc^2[/itex] (although technically the equation is [itex]E^2 = m^2c^4 + p^2c^2[/itex]). I'm not sure what else you're looking for...
  4. Jun 28, 2009 #3


    Staff: Mentor

    I think the mass energy equivalence stems from the conservation of momentum and the fact that photons have momentum.
  5. Jun 29, 2009 #4
    I think DaleSpam provided me with what I wanted to know,though it would be better if it was elaborated a bit,(meaning how we can derive the formula from the very basic theory of conservation of momentum (applied to photons)).
  6. Jun 29, 2009 #5


    Staff: Mentor

    OK, it has been a while so I am a little sketchy on the details, but it is something like this:

    Suppose an object at rest of mass M emits a pair of photons each of energy E/2 and one to the left and the other to the right. Those photons carry equal and opposite momentum so the object remains at rest. Now, consider the same situation in a frame moving to the right, the right photon is blueshifted and the left photon is redshifted, so the photons carry some net momentum to the right, and we know the object's velocity, so by conservation of momentum the object's mass must have changed. When you work out the amount of the change you find that it changed by an amount m=E/c².

    I am sure that is missing some important details, but it is something to that effect.
  7. Jun 29, 2009 #6
    Okay,so I guess that through this very revolutionary experiment,which is based upon the doppler effect of electromagnetic waves and the law of conservation of momentum gave Einstein the whole idea.
    At the same time there seems to be no derivation as such for the formula which originates from the law of conservation of momentum --it was something that Einstein found to fit in perfectly with the other rules of nature,that's why we can arrive at it fom other sources like the one I found on that webpage I mentioned earlier,right?
  8. Jun 29, 2009 #7
  9. Jun 30, 2009 #8
    If true, this would be BIG news!. BIG<BIG news and pulitizer, emmy and Nobel worthy.

    Not to nitpick, but nobody knows why these occur, why space and time are dynamic; nobody even knows what space and time are. But we do have the Lorentz relationships.

    Interesting derivation at the Virgina website, Thanks!!!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook