Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed of light exceeded

  1. Aug 28, 2005 #1
    Take 2 squares (sheets of paper) and slide them over each other at right angles. the intersection is a still point. But if you tilt 1 square (sheet of paper) and slide them over, the intersection is a moving point. If you tilt it to a very narrow angle the intersection will exceed the speed of light. Is this a known result ?
  2. jcsd
  3. Aug 28, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    Yes, but if you think about it there's nothing here that violates relativity. The 'point of intersection' is just an abstract concept. It is not a physical entity and there's no problem with it going faster than the speed of light.
    Likewise, if you take a laser pointer and you shine it at the moon, you can point from one side of the moon to the other with a flick of the wrist. The dot on the moon can travel faster than the speed of light. Here too, there's nothing violating relativity.
  4. Aug 28, 2005 #3
    In fact scientists have actually made light move faster than the speed of light. Well, sort of, they shot a wave packet of light and allowed it to stretch. The Center of the wave packet travelled at the speed of light, but since it stretched, the front of the packet moved faster than the speed of light, and the back moved slower. I think overall it just cancells the affect out, but none the less they can claim they made it go faster than the speed of light. Its all just trickery
  5. Aug 29, 2005 #4


    User Avatar
    Science Advisor
    Gold Member

    Nonsense, not a single experimental result has been offered. Don't mean to be picky, but, what example of 'superluminal' velocity do you have in mind dgoodpasture2005? Perhaps you are thinking of phase velocity - a common misperception.
    Last edited: Aug 30, 2005
  6. Aug 30, 2005 #5


    User Avatar
    Homework Helper

    In a mundane sense, Cerenkov radiation happens because a charged particle is moving through an environment faster than light moves along that path.

    They are not called "superluminal" because in these cases the speed of the objects is still *less than the speed of light that we measure in a vacuum*. (At least this is true for the ones that we're responsible for, anyway, given momentum by ElectroMagnetic Fields).

    IF you're willing to contemplate a theory for which "local gravity field" is well-defined, I think you have to admit that the speed of light in vacuum depends on the value of the local gravity field, being faster in weaker fields. FermiLab routinely makes protons travel faster than "the speed of light in interstellar space."
  7. Aug 30, 2005 #6


    User Avatar
    Science Advisor

    What do you mean by "local gravity field"? General relativity, the current best theory of gravity, says that a light beam will always be observed to travel at the same speed through a vacuum by observers in the same local region. And what's the basis for your claim that 'FermiLab routinely makes protons travel faster than "the speed of light in interstellar space"'? Do you have a link/reference for what you're talking about there?
  8. Aug 30, 2005 #7


    User Avatar
    Homework Helper

    As you imply, a local gravity field is not well-defined in GTR.

    This is really not the right thread, but you asked!

    If you don't remember, a gravity field is the what *used* to cause
    electrically neutral objects to change their momentum
    when they were near another electrically neutral object.
    Usually they were functions imposed upon flat space;
    strong near large masses, and weaker farther away
    (sort-of-like curvature). Having flat space meant that
    if you made an equiangular triangle in space, and light
    took a longer time to travel from angle A to angle B
    than it took to travel from B to C (or C to A), those
    watching would say that light went slower near the mass.
    (They didn't know that the excess time should be interpreted
    as a longer path distance with constant velocity.)

    Sorry, what I thought I had written was:
    ... faster than "the speed that light seems to have,
    through the strong fields near the Sun." In this interpretation,
    the "universal speed limit" would be the faster speed of light
    in the "weaker gravity field of interstellar space."
    Somehow I lost a couple of lines.
  9. Sep 4, 2005 #8
    What is the equation that defines change in speed versus strength of field? Thanks.
  10. Sep 16, 2005 #9
    I recall reading an article a while back where they shined a laser through a gas tube under water. To accelerate light. And they said the light shined through the other end a split second before they even shined the light.

    If anyone has this article please post it. I googled for it, and couldnt find it.
  11. Sep 20, 2005 #10

    f=ma perhaps
  12. Sep 20, 2005 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Sounds like nonsense. But I'd be interested to see if anyone can find such a reference.
  13. Sep 21, 2005 #12


    User Avatar
    Science Advisor

    Maybe you will find something when you google "Nimtz".
  14. Sep 26, 2005 #13
    this local gravity idea has sparked my attention. I think I had this idea myself, and I was trying to explain it in a couple other threads. The fact that length (practically in all directions) and rate of time are both contracted in your 'local space' the closer you get towards a gravity source tells you that even though to you in that 'local space' measure the speed of light at c, someone else further away from the same gravity source in a 'local space' that isn't contracted as much will measure a different distance between points in your 'local space' which has a higher strength 'local gravity' and they would conclude that the speed of light in your referance frame is lower than the speed of light in thier referance frame. But all that really matters is the strength of gravity in your local space, and the weaker the gravity strength, the less contracted space-time is, and the 'faster the speed of light' when measured from a frame with higher strength 'local gravity'. Although, in both stated referance frames (or 'local spaces'), the 'local' speed of light is the same. Therefore, light 'accelerates from high gravity potential to low gravity potential. does this make sense? I think you are able to extract the relation from GR.
    Last edited: Sep 26, 2005
  15. Sep 26, 2005 #14


    User Avatar
    Homework Helper

    I think that to "extract the relation from GR" would
    require an unambiguous (not vague) scenario so that
    the GR results can be interpreted in flat-space terms.
    That's why both the transmitter "A" and receiver "B"
    must be far from the star, in essentially flat space.
    Only paths directly through the star are undeflected,
    which is needed since the time-delay and deflection
    will be effects of same order.

    Anybody know how to do this convincingly?
    (theory only, for now; experimentally next year (;->)
    will still
  16. Sep 26, 2005 #15
    I'm bettin it can be extracted from the energy density intrinsic in the curvature of space at any point r from the mass. Of course the unit of measure that you use to define volume should be unaffected by the mass, ideally at infinity. Then find a relation between energy density at r and length contraction of a meter stick brought from infinity to r (contraction measured by the meter stick at infinity). Is this a good way to approach the problem, that all measurement be compared to the length of a meter stick that isn't in a gravity field? It seems to me that we're gonna need a universally constant unit of measure that sits outside the stretchability of space-time, which can be used in the knowledge of contraction factor caused by the local energy density of a gravity field to measure distances in that local space. za? :confused:
    Last edited: Sep 26, 2005
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook