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Interesting Claim

  1. Oct 1, 2006 #1
    A friend recently made this statement:

    "An object cannot travel faster than c".

    I am unable to find any evidence supporting this statement.

  2. jcsd
  3. Oct 1, 2006 #2
    Special relativity enforces the rule. Moreover, nothing has been observed to violate this principle.
  4. Oct 1, 2006 #3
    If I made the claim "an object can travel faster than c" there would be no evidence supporting this statement either.
  5. Oct 1, 2006 #4
    I was under the impression that SR made the statement more along the lines of nothing with a positive rest mas can -accelerate- past (or to, for that matter) the velocity of light. Possibly splitting hairs here...Better listen to someone smarter than me for a final answer.
  6. Oct 1, 2006 #5

    Your friend's statement is widely accepted as a scientific fact by physicists.

    Any object with a non-zero rest mass can not travel at c, "objects" (like photons) with no rest mass ALWAYS travel at c, and objects with negative rest mass are purely imaginary. Objects made up of atoms always have positive rest mass and always travel with a speed less than c.

    This is an important concept in the Theory of Special Relativity, and as such, has been verified on countless occassions... even including each time you turn on your computer!

    The number of tests done to confirm the validity of this statement is overwhelming and if you search this forum, you will find specific examples of such tests.
  7. Oct 1, 2006 #6


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    If you run into instances that seem to condradict the statement, such as in Cherenkov radiation, remember that c is the speed of light in vacuum. The speed of light in a refractive medium can be exceeded by physical objects with mass.
  8. Oct 1, 2006 #7

    Andrew Mason

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    Just so we don't confuse people here, the speed of photons in a refractive medium is still c. It is just that the molecular structures in the medium continually absorb and emit the photons so the time it takes for a photon to emerge from the other side of the medium is greater than the thickness divided by c. There is a very good post by Zapperz on this in the Physics FAQ

    Last edited: Oct 1, 2006
  9. Oct 1, 2006 #8


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    My understanding is that SR does not explicitly forbid objects travelling faster than c, which is where the theoretical tachyon particle comes in. It is as impossible to decelerate a tachyon particle to c as it is to accelerate a regular particle to c.

    Note that no one as ever found any evidence that tachyons exist, nor is there any reason to think they do, the point being that SR does not forbid them.
  10. Oct 1, 2006 #9
    You are correct, Dave. My original post did not say anything about accelerating from <c to >c.
    That is why I did not post this question in the relativity forum.
  11. Oct 1, 2006 #10


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    Dave is, of course, correct -- relativity does not forbid velocities greater than the speed of light, but no object can ever cross the "boundary," from either side.

    If you knew the answer to the question, why did you ask?

    - Warren
  12. Oct 1, 2006 #11


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    Quite right. I was remiss in not mentioning that. Sorry. Thanks for clarifying it.
  13. Oct 3, 2006 #12

    Claude Bile

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    It depends how you define 'object'.

    Relativity technichally forbids the propagation of information faster than c. There are other 'objects' such as crests of a wave that may travel faster than c.

    You might have to ask your friend for clarification :smile:.

  14. Oct 3, 2006 #13


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    Maybe he wanted to know if there was anything explicitly prohibiting tachyons?
  15. Oct 4, 2006 #14
    when i learned about special relativity back in physics 11 i knew all the answers for this, but since i no longer remember the answer to this question........ try wikipedia!!!
    if you read the section on relativistic mass it should explain it
    i couldnt really explain it still after reading it to you, so i think it'd be best if you read it yourself
    the jist is that as an object with mass approaches c,
    the mass of the object gets bigger and bigger
    it gets bigger by it flattening (like a pancake)
    please refer to wikipedia as i forgot how to explain this, but....
    at c an object with mass will be infinitely massive
    that means no matter how big the universe is
    the object will completely fill the universe so that it can no longer go any where
    which means it can't accelerate and therefore cannot exceed c
    and since it fills up the universe it also stops moving basically
    again im not using the best terms and words to explain it
    i might not even be correct in what i said,
    but i no im right
    when it fills the universe it can't go any where
    so in thoery it is impossible to reach c with an object with mass
    just check it out
  16. Oct 4, 2006 #15


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    Welcome to PF. A word of advice: if the only contribution you can make to a thread is linking to a wikipedia page, perhaps you should refrain from contributing.

    Futhermore, most of your post is incorrect -- special relativity says nothing about an object's physical size growing to fill the entire universe as its velocity approaches c.

    - Warren
  17. Oct 4, 2006 #16
    OK, I'm curious. How exactly did my computer just verify that nothing travels faster than c? (I hope you're not just going to say that everything electromagnetic provides such a proof.. else I'm going to insist that my computer is as likely just at rest in the ether :smile:)
  18. Oct 7, 2006 #17
    well im sry if u think i am incorrect
    and maybe u r rite since its been 2 yrs since i studied this subject
    but here's a lil back up to my arguement


    "For non-relativistic objects Newton defined momentum, given the symbol p, as the product of mass and velocity -- p = m v. When speed becomes relativistic, we have to modify this definition -- p = gamma (mv)

    Notice that this equation tells you that for any particle with a non-zero mass, the momentum gets larger and larger as the speed gets closer to the speed of light. Such a particle would have infinite momentum if it could reach the speed of light. Since it would take an infinite amount of force (or a finite force acting over an infinite amount of time) to accelerate a particle to infinite momentum, we are forced to conclude that a massive particle always travels at speeds less than the speed of light.

    Some text books will introduce the definition m0 for the mass of an object at rest, calling this the "rest mass" and define the quantity (M = gamma m0) as the mass of the moving object. This makes Newton's definition of momentum still true provided you choose the correct mass. In particle physics, when we talk about mass we always mean mass of an object at rest and we write it as m and keep the factor of gamma explicit in the equations."

    in that section it talks about the momentum of the object get greater and greater as it approaches the speed of light,
    the part i might be wrong is the part where i talked bout the mass getting bigger
    but according to length contraction the object gets flatter and flatter as it approaches c except it doesnt lose and mass along the way
    which can only mean that the object stretches further and further across the universe
    thus eventually filling up the universe anys
    but if thats the wrong theory
    the part about momentum should tell u that c is not possible to reach with any object with mass
    cause where r u gonna get infinite unblanced force from?
    and if its not the velocity thats gettin bigger its not accelerating anymore and the momentum continues to grow then it could only mean that the mass is increasing, but then that wont work cause its not gettin any closer to c so that doesnt even matter
    trust me im rite
    i just forgot the specific arguements for it myself
    but i could ask my physics teacher for u if u like
  19. Oct 7, 2006 #18
    an object that has mass travelling at c has infinite amount of energy, since gamma is infinite. Also, for anything travelling greater than c, gamma is imaginary... so it wouldn't make much sense. Also, if something does travel faster than c, casuality is violated!...
    kinda hard to explain, basically it means that effects can happen before the cause in some frame of references (events may go "backward" in some reference frame).
  20. Oct 8, 2006 #19

    Claude Bile

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    Length contraction only applies to the direction of travel. Objects do not relativistically contract in one direction and expand in others.
    You cannot expect to be treated seriously if all you post are well-known conclusions from well-known theories (yours was even incorrect in places as has been pointed out by myself and others), with your only backup argument being 'trust me I'm right, I forgot why I'm right, but I can ask my teacher if you reaaaly want me to', and the posting of somewhat irrelevant links and quotes that you do not understand.

    That's not to mention the spelling, punctuation and grammar.

  21. Oct 8, 2006 #20


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    It's also more than possible that some of the people here taught your teacher. You're dealing with world-class experts here, not some guy with a teacher's degree and a high-school science education. If you're not willing to accept their opinions, you're in the wrong place. (I am not an expert myself, but others are and I appreciate everything that they have to say.)
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