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Why does E=MC2?

  1. Mar 20, 2003 #1
    I know how scientists first came to realize that E = MC2, but I do not know why it does. If it didn't, would the matter and energy in the Universe act any differently?
  2. jcsd
  3. Mar 20, 2003 #2
    (m0*v^2)/2 -> kinetic energy;(Newton)

    m*c^2=E - > total energy;...
  4. Mar 20, 2003 #3
    Maybe I am in the wrong section to be asking this question but why the 'c^2' bit. Is this just because it is a large number or does it have a larger significance to the relationship between mass and energy[?]
  5. Mar 20, 2003 #4
    Scientists do not simply make a certain equation because they want it so.
    all Physicists do is that they find the rules of nature, and it was found (as a rule of nature) that E=mc2.

    But there is a little 'thingy' that might make you feel better.
    For an equation to be valid, the dimensions on each side should be the same.
    the dimension of m is [ M ] (m stands for mass)
    the dimenson of c (which is a speed) is [ L ]/[ T ] (where L is length, and t is Time).
    Therefore the dimension of c2 is [ L ]2/[ T ]2
    So the dimension of mc2 is [ M ][ L ]2/[ T ]2 which is the same as the dimension of the energy.

    If c2 was not there, then the dimensions on both sides will not be the same, so the equation will be wrong.

    But still, this is not why c2 is there, it is only an idea that i wanted to say.
  6. Mar 20, 2003 #5
    Don't forget about a constant like E=hf;f->requency;
    h->planck const;
    you could have E=m*k*c;where, for example k=5m/s;
  7. Mar 20, 2003 #6
    "c" seems to be the universe's conversion factor between distance and time.... when you work it out, c^2 is the natural conversion factor for mass and energy. Massless particles always travel at c; light, being massless, is one of these.

    Why this is so.... I can't think of any deeper reason now than "it just is."
  8. Mar 20, 2003 #7


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    Yeah, this is maybe more of a philosophy or religion question than a scientific one.
  9. Mar 21, 2003 #8
    I disagree. this is either mathmatical or physics, NOT philosophy or religion. how can c^2 be discovered and understood by religion? please explain.
  10. Mar 21, 2003 #9
    Do a google search on special relativity.

    Question solved.
  11. Mar 21, 2003 #10


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    Having read Einstein's Relativity(special and general) I can tell you that it is basically a logical deduction.
    Einstein started by redefining coordinate systems. Then based on the work of Maxwell and others who determined that the speed of light is ALWAYS the same, no matter what your reference coordinate.

    The next step was to realize that Lorentz transformations applied to reference coordinate systems. From there he realized the speed of light is an absolute limit. Other deductions led to
    E^2 = M^2C^4 + P^2C^2
    Objects with mass have rest mass M and therefore contain an enermous amount of energy.
  12. Mar 21, 2003 #11
    Well, the E=mc^2 comes out of the basic postulates of special relativity, where c is the maximum speed, or conversion factor for space and time.

    The philosophy/religion part comes when you ask "well why does it work like that?" That's kind of like "why is there a gravitational force that attracts things?" Well... uh... it just is. That's one of those questions you'll have to ask God about, cause we just don't know. :)
  13. Mar 22, 2003 #12


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    I agree with this. I just wanted to add somenthing.

    There are many questions that can be answered in terms of simpler "elements" or components of a system, but this is often just a different description that may give more accurate predictions, but not an "explanation". For instance...

    Why things fall?
    "Because of gravity" (this is not an answer; it is just a name)
    "Because there is an attractive force towards this planet" (gives more info, but you can argue that it doesn't answer "why")
    "Because every object attracts any other" (even more info, still no answer to "why do we live in a universe that has such attraction?")
    "Because there is a spin 2 quantum field between any pair of massive objects" (this actually is going backwards: we cannot help but use such kind of field for gravity because it is attractive; the latter is an observation that constraints the theory,... which then can barely be used as an "explanation" of the phenomenon)

    The point is that, in the end, the universe behaves in some way and we can only describe it.
  14. Mar 25, 2003 #13


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    You miss my point. Discovering that it exists doesn't explain WHY it exists. A religious person might say it exists because God created it that way. An athiest might say it exists because if it didn't, we wouldn't be here to dind it. That kind of question and those answers are generally outside the scope of science. Hence, a religious question.
    Yes, thats exactly what I was getting at. "Why" isn't as simple of a question as it seems.

    Did you ever get into a "why?" loop with your parents when you were a kid?

    Why is the sky blue? Because sunlight scattered by....
    Why does nitrogen scatter blue light? Because the electron shell...
    Why do electrons absorb and emit photons? Because...
    Why, why, why, why, why?

    Eventually, the answer becomes (becaus you don't feel like going any further) "God made it that way."
  15. Mar 26, 2003 #14
    I'm curious to know, what's with the electron shell?

    Yeah. Why to?
  16. Mar 27, 2003 #15


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    Bubonic, it just IS.
  17. Mar 28, 2003 #16
    [to parsons:]

    if it didn't, matter could travel faster than light, which RT does not allow.
  18. Mar 30, 2003 #17
    Well, i don't whether it just is, or whether there is a reason behind it, so i might as well ask, who knows, i might learn something new.
  19. Mar 31, 2003 #18


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    You can always argue why something is or isn't by way of consistency. In other words, you could say E has to equal mc2, because, if it didn't, relativity wouldn't be self-consistent.

    We generally assume that the universe is self-consistent, so we cannot accept physical theories that are not self-consistent. (The universe's self-consistency is really something of an axiom of rational thought.)

    In this example, we can argue that E must equal mc2 for the sake of consistency. We can also state that relativity is strongly supported by experiment.

    We cannot, however, argue why it is that relativity is strongly supported by experiment; the answer to that one really has no better answer that 'it just is.' I don't really think the anthropic arguments are much stronger. ;)

    - Warren
  20. Mar 31, 2003 #19
    Why Pythagorean theorem, why local Euclidean metric, why photonic carrier of constant velocity, why absolute observer and object, why conserved energy as integral of conserved momentum?
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