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Correct Energy Mass equation

  1. Jul 15, 2006 #1
    Dear All,

    whats the correct relation between Energy & Mass. I've read somewhere that E=MC2 is not complete and the nuclear labs use a similar, modified equation ? pls guide me and if someone can provide links to the proper derivation of this equation, it'll be great.
  2. jcsd
  3. Jul 15, 2006 #2


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    Are you perhaps looking for (m c^2)^2 = E^2 - (pc)^2?
  4. Jul 15, 2006 #3
  5. Jul 15, 2006 #4
    The only thing that I'm aware of is that there is a difference of opinion of what the m is in that equation. Some use it to refer to relativistic mass while others use it to refer to proper mass. That expression does fail when the system is not a closed system (e.g. a rod with forces acting to compress the rod).

  6. Jul 16, 2006 #5
    somehow.. i'm having a strange feeling, thought this equation may be correct, the correct explanation for the assumptions behind it is lacking. Why is the speed of light constant, irrespective of the observers motion ?
  7. Jul 16, 2006 #6
    Nobody knows why the speed of light is Lorentz invariant (i.e. remains unchanged by a Lorentz transformation, i.e. change from one inertial frame to another.

    A derivation of E = mc2 is found at



  8. Jul 16, 2006 #7


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    The constancy of the speed of light came from experiment (starting with the Michelson-Morley experiment, which eventually won the experimenters the Nobel prize), not from any theoretical assumptions. In fact, the result was quite a surprise to the experimenters.

    The constancy of the speed of light has since been confirmed by numerous different experiments, the latest of which are much more precise than the original experiments.

    Theories have to be made to fit the facts, not the other way around.
    Last edited: Jul 16, 2006
  9. Jul 16, 2006 #8
    Well one way to look at it (as Minkowski did almost 100 years ago!) is that lightspeed is not a velocity at all but simply the number of meters in one second! :smile:
  10. Jul 16, 2006 #9
    The answer is simple : because the Lorentz transform derivation is predicated on c=const. Look at Einstein's 1905 derivation of the Lorentz transforms.
    Last edited: Jul 16, 2006
  11. Jul 16, 2006 #10
    c = invariant cannot be proven. It is one of the postulates/laws of special relativity, i.e. nobody knjows why c = invariant.

    What you've stated here makes no sense to me. Why you refer to "the answer" above, what is the question to which you are answering? Is it "why is c invariant?" If your answer is "because the Lorentz transformation is predicated on c=invariant." then that is no answer at all. The question was "why" c = invariant. That the Lorentz transformation is predicated on c = invariant it does imply prove that c = invariant. It only assumes it.
    I've read it several times over years, thanks. I'd recommend that you yourself take another look at it. In the beginning section Einstein states the invariance of the speed of light as a postulate just as I explained above. To be exact Einstein wrote
    Nobody in the history of physics has ever derived the constancy of c and thus we don't know why it is constant. Postulates are those assertions which cannot be derived from other, more basic, postulates.

    Last edited: Jul 16, 2006
  12. Jul 16, 2006 #11
    Your question was (textually):

    "Why is c Lorentz invariant"
    My answer stands, what I answered is that your question made no sense because the Lorentz transforms are predicated on c=const.

    There is also ample experimental proof that light speed is invariant:
    -wrt the speed of the light source
    -wrt the speed of the observer

    I can give you the list of those experiments.
  13. Jul 17, 2006 #12
    Sorry clj4 but I never asked that question. I used the term "Lorentz invariant" to refer to those quantities (scalars) which do not change as you change from one set of spacetime coordinates S to another S' where both S and S' correspond to inertial frames of reference.
    My answer stands, what I answered is that your question made no sense because the Lorentz transforms are predicated on c=const.
    You can list them if you'd like to but I don't claim that the c=const. postulate is incorrect. In fact I hold it to be correct. One can't prove the second postulate by observing nature. One can only confirm predictions made by the postulate. I.e. you're speaking about experimental confirmation, not of proof.

  14. Jul 17, 2006 #13
    Your exact post, right? The answer I gave you stands.
  15. Jul 17, 2006 #14
    guys, pervect mentions that constancy of c was derived experimentally. well, then y did einstein mention it as a postulate while deriving E=MC2 ?
    and how far the photon theory of light is consistent.. I agree that the photo electric effect won the nobel.. but lets hit the core... A ray of light consists of photos ? if yes, what the distance between each of the photos, whats their properties like .. do they attract each other ? or stuff like that..
  16. Jul 17, 2006 #15
    Things like the constancy of light are never derived] by any type of experiment. They are merely observed or experimentally confirmed.
    The notion of a beam of light consisting of photons is consistent with observation. Observation is also consisent with light being a wave. That's where the particle-wave duality came into being. Whether photons really exist or not is hard to say. I read an article once by one of those Nobel Laureates (Willis Lamb) in which he asserted that photons don't exist.

  17. Jul 17, 2006 #16
    Your responded to
    with "The answer is simple : because the Lorentz transform derivation is predicated on c=const." You claimed that I asked "Why is c Lorentz invariant" when I never asked this question to anyone at anytime. I only asked you if Is it "why is c invariant?" was the question you were anwsering. You never responded to this question. You seemed to be objecting to my using "c is Lorentz invariant" as a statement of the second postulare of relativity. It appears to me that you think this is circular logic. It is not. The expression for the Lorentz transformation is derivced with the second postulate. When I used the term "Lorentz transformation" I was refering to "that change in coordinate such that c=constant."

    Long story short - The only thing that I was asking you was what was the question to which you posted an answer when you said "The answer is simple:" Complaints/objections about my use of the term "Lorentz transformation" detract from the users question. His question was "why is c=constant?" The answer is "Nobody knows why." I was trying to avoid the use of the phrase "c=constant" since I've never liked that phrase. It seems to me that some people would misread this law as "c does not change in time."

  18. Jul 17, 2006 #17
    Doesnt maxwells equations imply c=constant?
  19. Jul 17, 2006 #18
    There is a more general form of Maxwell's equations which has the value of the photon's proper mass explicitly in Maxwell's equations. If this value is different than zero then the speed of light is not constant. The corresponding Lagrangian is called the Proca Lagrangian.



  20. Jul 17, 2006 #19


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    Short answer: yes.
  21. Jul 17, 2006 #20
    That is incorrect. The most general form of Maxwell's equation accounts for the photon's proper mass. The constancy of the speed of light is tied to the proper mass of the photon. Postulating c=constant is equivalent to postulating that the photon's proper mass is zero. However it has never been proved that the photon's proper mass is zero. All that has been done was to reduce the photon proper mass to a small quantity. The expermimental evidence of photon proper mass does not allow us yet to assume that the value is zero. There is, however, an upper bound to it.

    Otherwise if you believe that the "short answer" is yes then please provide proof. So far in relativity today it is a postulate and thus cannot be proven.

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