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Nothing can travel faster than light

  1. Aug 2, 2010 #1
    Is that a completely correct statement?

    I had a discussion with a co-worker the other day (while checking out a customer at a local grocery store lol) about the statement above. I firmly made the argument that nothing can travel faster than light with the exception of inflation and entanglement. I explained why using general relativity but he said i wasn't thinking of it the right way. My co-worker told me nothing can accelerate up to the speed of light however, nothing, as far as we know, can stop something from instantaneously traveling faster than light. I then asked him to name anything that is instantaneous. The conversation then ended.

    Oh and he also disagreed with me when I told him that as anything approaches the speed of light it becomes heavier, distances contract, and time slows down. This was unexpected.

    It was a nice discussion and I want to know what is right and which of the statements I've made are incorrect. I realize that I might be ignorant and that is okay. Because, it is only through discussions like this that I effectively learn. So please clear up everything that needs clearing up and Thank You.
  2. jcsd
  3. Aug 2, 2010 #2


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    It's possible in principle for a particle to be created with speed >c. Such particles are called tachyons. (Don't you guys watch Star Trek? :smile:). They're believed not to exist. Tachyons are pretty weird, but relativity doesn't forbid their existence. The mass m of a tachyon satisfies m2<0. They're easy to accelerate and hard to slow down. (It takes more work to decrease its speed by 1 m/s than to increase it by the same amount). They also have to have some other weird properties to avoid logical inconsistences, like Alice sends a message to Bob, who sends a reply that reaches Alice before she sent the original message and instructs her not to send it. See this post for details. (That post actually assumes that the speed of the "messages" is infinite in the rest frame of the transmitter, but that's just to simplify the problem a little bit. It doesn't change anything significant).
  4. Aug 3, 2010 #3
    Thank you Fredrik. I suppose next time I see him at work I will tell him he is partially correct. I'm sure he doesn't know what a tachyon is. And I can't believe i forgot about them.
  5. Aug 3, 2010 #4

    Like Fredrik has already pretty much said, relativity doesn't technically forbid the existence of faster than light objects. Your friend is right that what's impossible is accelerating something with mass up to the speed of light, since it would take an infinite amount of energy to do so (finite acceleration for eternity or infinite thrust over a finite time).

    As for the part about becoming "heavier", it depends what you mean. Rest mass never changes. If you were in a spaceship traveling 99.9% the speed of light, you wouldn't feel heavier. I'm surprised he disagreed about time slowing down, though; maybe he meant that time would remain the same from the perspective of the person traveling at that speed?
  6. Aug 3, 2010 #5
    I would drop the idea of masses becoming infinite. Masses are always their rest mass and what changes are the other parameters in the equation, speed being one. E.g. if an electron at SLAC (did they rename that yet?) moves at the speed of light, then if it's mass would be infinite - which means it would fill the universe. So instead of allowing that, we tell it, no v >= c, so that it's mass is the same, but it's energy get's really big.

    As for tachyons, only Data was awesome, Crusher was ok, and the rest of the crew to TNG could fly merely (ahem) into a giant supernova at warp speed 9. (Why was there never a warp speed 10?!?)

    Seriously though as for anything traveling faster than the speed of light there are two possibilities:
    1) causality is contradicted, time does not flow, the man is unbitten by the dog before the dog bites him, and you are Phillip J. Fry (your own Grandfather) or,
    2) if there are indeed tachyons, then we would need to rewrite E&M, and to maintain causality, which states that tachyons are the particle propagators of QED. But Feynman would never allow that... or maybe he would ... but only before he wouldn't.


    tachyons drull.

    Also, tell your co-worker to stop chatting and help that customer in Isle 3 find the Newtons. :)

    ...wait a minute, this is a joke, right?? Cause you guys must be from Vulcan b/c no workers in America discuss relativity at work - and in such an intelligent way. Sure they might discuss other important things like what's the latest tweet for Brittney. So confess, you're both students at the Spock Institute of Technology! ... come to think of it, are the people who use twitter called twits?
    Last edited by a moderator: Aug 3, 2010
  7. Aug 3, 2010 #6
    tachyon are hypothetical subatomic particles dude bt in this real world no such thng exits
  8. Aug 3, 2010 #7
    Take 10 balls, each with a rest mass of 10kg, spread out in space. Once gravity brings them together you will discover that the total mass < 100kg.
  9. Aug 3, 2010 #8
    I believe you are including the binding energy in that description.
    Last edited by a moderator: Aug 3, 2010
  10. Aug 3, 2010 #9


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    Yes, and excluding kinetic energy. Include both, and everything is ok.
  11. Aug 3, 2010 #10
    We are talking about rest mass being allegedly constant right?

    In GR there is no such thing, and even the term mass needs qualifications.
  12. Aug 3, 2010 #11


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    If there is no such thing, then why are you talking about it as an example of rest mass not being conserved? That doesn't make sense.
  13. Aug 3, 2010 #12

    Once the Higgs comes out of the ether we will know what mass is all about. So far the best definition is the phrase "hard stuff that hurts when my head hits it" (i.e. there is no good definition).

    ...hmmm... PF = PassionFlower = PhysicsForums = Phillp Fry ... are you another VULCAN!?! Stop teasing us Earthlings, we still don't have warp drive yet!
    Last edited by a moderator: Aug 3, 2010
  14. Aug 3, 2010 #13
    Thanks for the Posts everyone. I enjoy reading what you have to say. And I enjoy correcting myself.

    Living Dog: We actually do work at a local grocery store haha It's called HEB. Its kinda like kroger if you've ever seen that. I'll be majoring in physics beginning this year. I can hardly wait.
  15. Aug 3, 2010 #14
    I've seen them both (it's actually H-E-B) when the hoosers abducted me and tortured me for 6 flat out miserable years of my life. The only thing good about Indiana is ... that "ground zero" is another name for it.

    Ah ha! So you are a Vulcan! Since I tormented you with my posts I will apologize now and simply hand you a little gift which I think _EVERY_ future physicist should know about:


    Live long and prosper.
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