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How would the speed of light affect your mass?

  1. May 21, 2008 #1
    (This is probably answered somewhere, but I couldn't find it, I'm kinda new. >_>)

    So, say I was able to travel at the speed of light. My teacher told me once if you were say, running, you'd weigh a slight, unoticable amount more. (Or I just wasn't listening right. >_>)
    Anyways, would traveling at the speed of light raise your mass by like, 1000's of times? Or have I got this completely wrong? =/
     
  2. jcsd
  3. May 21, 2008 #2
    Your teacher is right. You are wrong (partially, right idea though :)). When traveling the speed of light, your mass becomes infinite and your size (in the direction of thrust) becomes infinitely small (0). If you want a more in depth solution/read, read up on Einstein's theory of relativity.
     
    Last edited: May 21, 2008
  4. May 21, 2008 #3

    berkeman

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    Staff: Mentor

    Read about the Lorentz Factor part-way down this intro page at wikipedia.org:

    http://en.wikipedia.org/wiki/Special_relativity

    As an object moves faster and faster, its mass increases. That's why it would take an infinite amount of energy to accelerate a material object to the speed of light (can't be done).
     
  5. May 21, 2008 #4
    Ahhh, yes. I've always wondered though, its not possible to have something reach the speed of light with mass, what about having a negative mass (entirely theoretical speaking)?
     
  6. May 21, 2008 #5

    berkeman

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    I don't think negative mass is a theoretical subject -- I believe mass is always zero or positive. And we don't discuss overly speculative concepts here on the PF, please keep that in mind.

    Photons are an example of an object that travels at the speed of light. It can do that because it has zero "rest mass". See the comments about photon mass part-way down this intro page for photons:

    http://en.wikipedia.org/wiki/Photon
     
  7. May 21, 2008 #6
    What about tachyons? With their imaginary mass? One of the things I have been trying to grasp lately is, what is an imaginary mass? is it negative? or what?
     
  8. May 21, 2008 #7

    berkeman

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    I dunno. Tachyons are hypothetical particles. Pretty fun to read about though...

    From http://en.wikipedia.org/wiki/Tachyon

     
  9. May 22, 2008 #8
    what about in space. what is speed compaired to in space? since there is nothing in space, just a vacuum, so nothing could slow you down, if you had enough fuel or whatever, wouldn't you keep accelerating? theres nothing to stop you from going faster, no resistance, so wouldn't you be able to get going the speed of light or faster? of course it would be incredibly inefficient, but would it theoretically work? prob not, but just curious
     
  10. May 22, 2008 #9
    No it would not theoretically work because the theory of general relativity states that as you approach relativistic speeds your mass eventually becomes infinite, this would mean that you would need an infinite amount of energy to move an infinite mass. Also, when you approach relativistic speeds your size in the direction of thrust becomes smaller and smaller, and when you reach the speed of light, it becomes 0, which is impossible. Two reasons why going the speed of light, no matter where you are is impossible. But keep dreaming up crazy schemes.
    (That was said by a guy I know) :D
     
  11. May 25, 2008 #10
    Negative mass has been discussed theoretically in the physics literature. A well known paper on this topic is

    Negative Mass in General Relativity, Herman Bondi, Rev. Mod. Phys, 29(3), July 1967

    Also its conceivable that there is negative mass in the universe which is causing its expansion to increase at an accelerating rate. Negative pressure or a positive cosmological constant will produce an antigravity affect. The effective mass density of a large enough negative pressure (i.e. tension) will produce a negative active gravitational mass. A vaccum domain wall produces a repulsive gravitational field. This is due to a negative active gravitational mass density.

    As far as negative inertial mass goes, its concievable that if you place enough tension in a rod then the rod's inertial mass will become negative. In practice I believe that the rod would break far before that amount of stress is induced.

    Pete

    ps - I can E-mail the Bondi article to anyone who sends me their e-mail address and a request to do so.
     
    Last edited: May 25, 2008
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