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

Relativistic Mass vs. Invariant Mass.

  1. Sep 19, 2011 #1
    Hey all, I'm quite confused on this and am curious to be put straight. Now I understand the basic principles of relativity, this one just bugs me.

    Now I have always been taught that the famous E=MC^2 formula was proof that mass would reach toward infinity as it neared the speed of light. Stephen Hawking even says "Because of the equivalence of energy and mass, the energy which an object has due to its motion will add to its mass." in his book A Brief History of Time.

    Now I was informed by some other members that this isn't really the case, and that the whole notion of a massive object approaching the speed of light would not collapse into a black hole, etc. etc. and I believe them to be correct in their assessment.

    So what is going on here, and why am I reading two very different views on a similar subject, or are they both correct and it's just the Mr and Mo terminology that is off and screwing everything up
  2. jcsd
  3. Sep 20, 2011 #2


    User Avatar
    Science Advisor

    The statement by Hawking presumably refers to the relativistic mass in special relativity, which does not include gravity, nor black holes.

    In general relativity, there are several notions of mass. One notion is the stress-energy tensor, which determines the curvature of spacetime. A counterpart of the relativistic mass forms only one component of the stress-energy tensor, so "moving" fast does not necessarily lead to the formation of a black hole. The sorts of motion that lead to collapse can be studied beginning with the http://www.ias.ac.in/pramana/v69/p15/fulltext.pdf".
    Last edited by a moderator: Apr 26, 2017
  4. Sep 20, 2011 #3


    User Avatar
    Science Advisor
    Gold Member

    Also, note that the invariant mass of a system of particles does include a contribution from the relative motion of the particles. Thus the kinetic energy of gas contributes mass towards 'becoming a black hole', but overall motion of a ball of gas relative to something else does not.
  5. Sep 20, 2011 #4
    Wait. Several? Are there more than two?
    Last edited by a moderator: Apr 26, 2017
  6. Sep 20, 2011 #5


    User Avatar
    Science Advisor
    Gold Member

    See, for example: http://relativity.livingreviews.org/Articles/lrr-2009-4/ [Broken]

    Among others: ADM, Komar, Bondi, Bartnik, Hawking
    Last edited by a moderator: May 5, 2017
  7. Sep 20, 2011 #6
    Your formula is wrong. I can't really type it as I want on this post but it looks like the following E=(MC^2)/(1-v^2/C^2)^.5. As velocity v gets larger, the total energy of mass M gets larger. But this is a relative mass - a guy riding on M will not see any change in the value of M no matter how big v is.

    Think of it this way. Guy 1 is on mass M and guy 2 is on another mass stationary relative to mass M. Now Guy 2 and his mass accelerate to high v. Well, nothing has happened to guy 1 and mass m, but guy 2 still sees a relative velocity of v. So guy two can do experiments that will show that mass M is now larger by the formula I gave you above. But nothing has actually happened to guy 1 and mass M. That's why Einstein called it Relativity Theory.
  8. Sep 21, 2011 #7
    Thanks. I'd forgotten.
    Last edited by a moderator: May 5, 2017
  9. Sep 21, 2011 #8


    User Avatar

    Staff: Mentor

    In the early days of relativity people sometimes used "transverse mass" and "longitudinal mass" in connection with the effects of forces perpendicular and parallel to an object's motion, in F = ma.
  10. Sep 22, 2011 #9
    Indeed it's mainly a matter of terminology: nowadays it has become most popular to mean with "mass", "rest mass". And it should be clear from the principle of relativity that an object can only collapse into a black hole if it has a great rest mass.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Relativistic Mass Invariant Date
B Special Relativity & Relativistic Mass Nov 23, 2017
B All mass is relativistic? Oct 6, 2017
B Newb question about relativistic mass Sep 26, 2017
B False predictions of relativistic mass Sep 4, 2017