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

Changes in mass and length at high speed

  1. Nov 29, 2011 #1
    So, I've never been able to understand where the extra mass comes from when an object is traveling near the speed of light. The same for length, where does the excess length go? Also, why does the object increase in mass. I'm pretty weak in relativity.
    I understand the equations and that's usually good enough but somebody asked me why it gains mass and where does the mass come from and I don't know what to say.
    Any help? Thanks.
  2. jcsd
  3. Nov 30, 2011 #2
    In regards to your question about mass, you'll probably find your answer in this thread (read my post): https://www.physicsforums.com/showthread.php?t=553884

    In regards to length contraction, objects themselves don't actually change length - the objects isn't compressed or anything like that. A person that might appear to somebody as contracted in length wouldn't feel anything peculiar. Rather, different observers simply disagree on what an object's length actually is. It's a consequence of the relativity of simultaneity / the Lorentz transformations.
  4. Dec 1, 2011 #3
    Thanks for the reply. If I understand, length appears to decrease, time actually does slow down (does anybody have a link to an actual experiment where atomic clocks run slower?),and mass theoretically increases based on more KE added but it's not important. Is that about right?
  5. Dec 2, 2011 #4
    not really. To explain what "appears" you have to explain what observer, from what inertial frame, the observation is made.
    Let's say two observers pass at high speed: Each sees the other as "length contracted", each sees the other's clock ticking slower. So who is "right"??? They both are!!! Each sees his own local clock [and length] as normal.

    Only when two coincident observers separate with one accelerating away and then returning do clocks carried by each show a difference upon return. During the separation at high speed, each again measures the other's clock as running slower than their own, but it turns out the one who has accelerated will show the lesser elapsed time....

    This is not 'logical' with classical reasoning....neither time nor distance is fixed and immutable as Netwon assumed. Instead it is the speed of light that is constant locally.

    You can think of this as related to the "equivalence principle" of Einstein...it turns out both acceleration and gravitational potential are similar effects which can slow time between
    distant observers.
  6. Dec 3, 2011 #5
    The idea that the mass of an object increases with speed is a very outdated one and gives many misconceptions.

    Modern treatments of relativity use a definition of mass which is a constant for any speed. This modern definition of mass is used when giving the mass of an electron for instance.
  7. Dec 3, 2011 #6
    So how out dated is the mass increases idea? 5 years? 20 years? 50 years?
  8. Dec 4, 2011 #7
    I think you got some misunderstanding in this concept. The length is not "shortened", but contracted in the perspective of a rest observer. While the travelling observer is not likely to see a contraction in length him/herself.

    Increase in mass attributes to the fact that energy is not an invariant under transformations. In the perspective of the rest observer, the travelling one has kinetic energy which contributes to the increase in mass.
  9. Dec 4, 2011 #8


    User Avatar

    Staff: Mentor

    When I was in graduate school 30-35 years ago, working in high-energy particle physics (where we deal with ultra-relativistic particles all the time), none of us used "relativistic mass." If you asked anybody, "what's the mass of that electron?" you'd get the answer "511 keV/c^2" no matter how fast it was moving.
  10. Dec 4, 2011 #9
    Close to 50 years probably, not 5 of course. Although you can find this outdated idea in some 'recent' news using outdated references as a basis.

    You may find this outdated idea in original papers by Lorentz and Einstein, but not in modern papers published in Physical Review and similar journals.
  11. Dec 4, 2011 #10


    User Avatar
    Science Advisor

    How about:

  12. Dec 4, 2011 #11

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Just because the concept is outdated (and it is) doesn't mean everyone has abandoned it. Old paradigms die slowly. There still is a small cohort who adhere to the concept of relativistic mass. This group is ever-shrinking but is very adamant. I can envision the peer review for those papers. "Please replace relativistic mass with energy. It's an outdated concept." The response: "No."

    The problem is not so much that it is wrong as it that it is confusing and is not needed. (What's wrong with energy?) Because it isn't wrong, it's a bit tough to argue for rejecting a paper just because the author was hard over on using that term.
  13. Dec 4, 2011 #12


    User Avatar
    Science Advisor

    But here you are not arguing that it is an outdated concept. You are arguing that it is an outdated term for a still relevant concept.
  14. Dec 4, 2011 #13

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Energy is not an outdated concept. Relativistic mass is.
  15. Dec 4, 2011 #14


    User Avatar
    Science Advisor

    What's the difference?
  16. Dec 4, 2011 #15
    I get that length appears less. But does time really slow down or only appear to slow down to a stationary observer? What I'm read says many experiments have shown atomic clocks slow down at high speeds and in high gravitational fields.
    And most people agree we shouldn't teach school children mass increases.
  17. Dec 5, 2011 #16


    User Avatar
    Science Advisor

    The fundamental lesson is not whether we should name something X or Y. The fundamental lesson is we don't know what X or Y is unless we have a procedure - experiment and analysis, and a way to communicate that to other experimentalists - to define X.

    So if someone says "time is slowing down" or "time is not slowing down", you should ask what every word in that statement means operationally. Both statements can of course be true if the "time" and "slowing down" don't have the same operational meanings in each of them.
    Last edited: Dec 5, 2011
  18. Dec 5, 2011 #17
  19. Dec 5, 2011 #18


    User Avatar
    Staff Emeritus
    Science Advisor

    The word "mass" is getting overburdened and overloaded, with multiple meanings. For instance, ADM, Bondi, and Komar masses, not to mention quasi-local mass.

    Furthermore, an all-too-large segment of the lay population is under the misapprehension that you just replace "mass" with "relativistic mass" in order to go from Newton's laws to relativity. If you use the synonym for relativistic mass, energy, people won't make this mistake. A related, and similarly common, mistake is to replace the "relativistic mass" in Newton's force law F=GmM/r^2 and to think that has something to do with the gravity of a moving particle.

    For all of these reasons, I think "relativistic mass" deserves a fine funeral and a decent respectful burial. It will be one less mass to worry about in the clutter of different sorts of masses that is currently littering the playing field, and it will help avoid misunderstandings by reducing the temptation to replace "mass" with "relativistic mass" blindly and wrongly in Newtonian formulae, expecting them to become relativistic with one stroke of the pen.

    At the same time, we could also bury "transverse mass" and "longitudinal mass" in nearby plots, maybe with some nice flowers, getting rid of a couple of more of the current overload/overuse of the term mass.

    Of course, every time we try, it seems smoene cries "He's not dead yet! He's feeling much better". ((That's a joke, BTW - a Monty Python reference)).
  20. Dec 5, 2011 #19


    User Avatar
    Science Advisor

    That's just wrong. Please - it's Komar energy, ADM energy, Bondi energy;)

    This terminology discussion is silly, so let's play at https://www.physicsforums.com/showthread.php?t=557287 so as not to distract from serious work in this thread .
    Last edited: Dec 5, 2011
  21. Dec 5, 2011 #20


    User Avatar
    Science Advisor
    Gold Member

    When was the last time you saw someone try to use Komar mass in F=ma, or KE= 1/2 mv^2? What about the putting relativistic mass in these? (I see than mistake several times a year, at least, on these forums).

    In my mind, the fact that to get the relativistic mass concept to work with force requires you to invent transverse and longitudinal relativistic mass is the ultimate demonstration of its non-utility.

    Of course, just like for the LET, there will always be holdouts.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook