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My little theory: what would happen?

  1. Apr 6, 2009 #1
    First post ever! Long time forum browser, but only registered yesterday! =D
    I'm in my final year of high school, and have therefore only properly been doing Physics for two years, but it's something I'd like to pursue in university. Sorry if my knowledge of relativity is lacking a little...

    I had this possibly stupid notion a while back... It's all hypothetical obviously:
    We, happily standing in our inert reference frame, see someone start to run. They get faster and faster... in fact, they soon approach the speed of light.
    Am I right in saying that we will observe them to gain mass?
    BUT... in their reference frame, they are stationary. So will they be aware they have gained mass? If they were to measure their own mass, would they find it to have increased?

    If anyone has any idea about this, please tell! Not sure if I'm wrong!
  2. jcsd
  3. Apr 6, 2009 #2


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    Staff: Mentor

    No, on both counts. Nor will they notice themselves as being length-contracted, nor will they notice their clocks running slower.

    To see why this has to be true, note that relativistic effects work both ways. The runner also observes you as being length-contracted, time-dilated, etc.

    Now imagine that you are being observed by two runners, running at different speeds relative to you. The two runners will observe you being affected by different amounts of length contraction, time dilation and relativistic mass increase. But you obviously cannot observe yourself as simultaneously having masses of 70 kg and 100 kg!
  4. Apr 6, 2009 #3
  5. Apr 6, 2009 #4


    Staff: Mentor

    Welcome to PF! Or at least welcome to the posting side of PF :smile:
    There is more than one concept of mass in SR, and some ongoing debate about which is the best mass to use. We will observe them to gain http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html" [Broken]. They will not gain rest mass because it is invariant.
    In their own frame they will obviously only measure their rest mass, which is relativistically invariant and will not change.
    Last edited by a moderator: May 4, 2017
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