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

Is this explanation of E=mc^2 correct or incorrect and why?

  1. Jul 31, 2009 #1

    I came across the following explanation of E-mc^2 on pbs.org:

    "So why would you have to multiply the mass of that walnut by the speed of light to determine how much energy is bound up inside it? The reason is that whenever you convert part of a walnut or any other piece of matter to pure energy, the resulting energy is by definition moving at the speed of light. Pure energy is electromagnetic radiation—whether light or X-rays or whatever—and electromagnetic radiation travels at a constant speed of roughly 670,000,000 miles per hour."

    Link: http://www.pbs.org/wgbh/nova/einstein/legacy.html" [Broken]

    I've never heard this explanation before and it seems suspicious to me. It's as if the author is implying that the Newtonian equation for kinetic energy, E=mv^2/2, applies to something that is moving at the speed of light, and substitutes c for v, but doesn't divide by 2. So does the given explanation constitute one of the correct ways of looking at the meaning of E=mc^2, or is it only partially correct, or incorrect entirely?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 31, 2009 #2
    It's a bit of a hand wavy explanation but its not totally wrong. If you think of time as a dimension then you could explain it as follows. Imagine that all matter and energy moves at a speed c but in different directions in space and in time. E=m c^2 applies only when the mass is stationary hence its only moving in the time direction at a speed c. A photon on the other hand moves only in the space direction and hence moves at c in space. To check this makes sense you can note that for a photon the time doesn't change due to time dilation. Matter that is not stationary will move in a direction that is partly spacelike and partly timelike(think of it at an angle to the time and space axes).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook