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Faster then the speed of light

  1. Sep 22, 2008 #1
    Im a college student, first year of chemistry, but i just had a thought about light speed...
    lets say you have two particles, particle A and particle B. Particle A has a mass of 10 while B has a mass of 1. They are both traveling at the speed of light but are perpendicular to each other.
    If these particles were to hit then one particle would transfer more energy to the other (not sure which one, i think A has the net gain...) but basically, simple maths says that if A is traveling at the speed of light and then has a force applied perpendicular to it then the resultant force would be greater then the speed of light.

    This is only a simple thought and the situation is open to interpretation
  2. jcsd
  3. Sep 22, 2008 #2


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    Problem 1. Massive objects cannot move at the speed of light.
    Problem 2. What do you mean by "the resultant force would be greater then the speed of light" How can a force be greater then a velocity?
  4. Sep 22, 2008 #3


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    Welcome to Physicsforums, toxonic!

    The first comment is that "simple math" does not apply. That is the interesting thing about physics, a lot of key theory does not consist of simple (sometimes called classical) math. When it comes to velocity, .75c +.75c does not equal 1.5c (no matter how you interact particles, the result is always less than c). That was the amazing discovery of Einstein over 100 years ago! Please note that this "mere theory" of Einstein has been verified experimentally many many times. In fact all particle accelerators rely on his Special Relativity to work.

    When theory matches experiment, plus makes useful predictions regarding previously unknown phenomena (as Special Relativity has), then we call that a good theory. I hope this helps.
  5. Sep 22, 2008 #4


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    Now, when I put myself in your place, toxonic, I imagine that you have read these replies and thought, "Well then, just change the velocity of the two objects to just barely below c, and try the thought experiment again."

    First off, you are correct that the smaller object gets a much bigger acceleration force from the collission. So the smaller object would accelerate a lot more than the larger. However, this force would not be the infinite force required to accelerate to c, so the lighter object would accelerate to a velocity that is much closer to c than the heavier object, but still not not quite c.
  6. Sep 25, 2008 #5
    There's one thing I can imagine faster than light... wavefunction collapse! But even that's just theoretically since something, at least a massless photon has to be observed! :p
  7. Sep 26, 2008 #6


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    What do reason do you have for believing that "wavefunction collapse" is faster than light? And are you asserting that it involves a "massive object"?
  8. Sep 26, 2008 #7
    On the first count, the probability distribution for some given object is retained arbitrarily close to the actual observation (where, on finding the object someplace, the probability is made instantaneously unity there, even given uncertainties), it's wonderfully abstract for a physical theory like quantum mechanics.
    On the next, what I had in mind was that, given the massless photon defines the speed of light (our setting is a vaccum, say), and this constitutes a 'perceptual' barrier for any observation made, it becomes irrelavant that probability collapses are instantaneous (for we can never observe a sense of 'instantaneous' faster than light, and that suffices, for all practical purposes).
  9. Sep 28, 2008 #8
    In addition to all that's already been said, here's the formula:
    As you can see, even if, theoretically, you'd have a body A moving at speed c relative to a body B, which in turn is moving at speed c relative to a body D in the same direction, A would still be moving at speed c+c/1+(c^2)/(c^2)=2c/2=c relative to D.
  10. Sep 28, 2008 #9
    Can anyone explain the story behind 'tachyons'?

    I swear there was a post here...

    Anyway, I know they're hypothetical, but they're still 'allowed' by relativity, right? How exactly? (in simple terms hopefully...)
    Last edited: Sep 29, 2008
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