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Matter Exceeding the Speed of Light

  1. Jan 24, 2004 #1
    I’m not sure if this is correct, but apparently there is an event horizon in space which causes the light from an object to never be able to cross over. This is a result of the distance between two objects reaching a speed that exceeds that of light.
    I was just wondering, if the distance between two objects exceeds that of light, does the influence between two objects become zero?
    The old saying that everything has an influence on everything else…
  2. jcsd
  3. Jan 24, 2004 #2


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    The idea that things moving away from each other at >c due to cosmic expansion makes them invisible to one another has been discussed somewhat in other threads. It has been stated that superluminal recession alone does not make things beyond each other's influence, because the stretching of spacetime allows light to make up the difference, so recessional speeds must be well in excess of c, many multiples in fact. Not sure I understood the whole idea, and not sure I buy it, so I hope one of the folks who spoke at length about it can come into this Topic and give you a better explanation.

    However, I think the main thrust of your question was, can a thing have any effect on some other thing if even light cannot reach from one to the other, correct? It would seem that it cannot. Two objects outside of each other's "light cone" (are you familliar with that term?) should have no way of influencing one another.

    Unless, of course, quantum entanglement can be shown to carry information (which theory disclaims).
  4. Jan 25, 2004 #3
    Yes that's what I was trying to determine, and your right it would seem that it cannot.
    Things that do pass outside of each other's light cone, have been said to fade and not turn invisible.

    But I'm not sure what quantum entanglement is. It reminds me of an experiment done that showed (I think) a particle being broken into it's two pieces, then sent thousands of kilometers apart, showing that information about the other particle somehow knew where the other was. So this would seem to say that when two things are seperated, perhaps there is some connection still between them.
  5. Jan 25, 2004 #4


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    Yes, this topic has come up several times, in a number of threads. I know I've promised to write something up on this, and I intend to do so (truly, really, cross my heart and hope to die!). marcus posted a link to an excellent article or three by Lineweaver et al; unfortunately I don't have that to hand. :frown:
  6. Jan 25, 2004 #5


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    Yes, that's the phenomenon to which I was referring. I have read statements from several different individuals, all authorities on the subject, that information cannot be transmitted in this way. So we are left with the old rule, no event can have an effect on anything outside its own light cone.
  7. Jan 26, 2004 #6
    This still seems strange to me. Not by the fact that if a galaxy for example is outside of another galaxies light cone, will have virtually no influence on each other, but the indirect gravitational pull it would still have on things outside of its light cone.

    Indirect meaning there is no direct gravitaional influence, but it's postion still directed by surrounding structures.
  8. Jan 26, 2004 #7


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    The galaxies are not moving through space FTL, and their light cones are completely causal. Instead more space is being added in between, so the distance is increasing so fast that they are observed by each other as receding FTL.
  9. Jan 26, 2004 #8


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    This problem may come from the usual science fiction depiction of flt spacecraft: from a stationary camera behind the craft, you see it accelerate and when it hits the "warp barrier" it disappears in a flash of light. But what has really changed from right before it crossed that threshold?

    Sure if its being illuminated by a nearby star or light source, that soucre is no longer hitting it, but what if it is emitting its own light? Well, the instant after the craft hits C, its not a whole lot further away (from your frame of reference) than right before it hit C. So the light it emits will still get to you, it only takes slightly longer because its slightly further away.

    If a spaceship passes you at C+1m/s the light that left it when it was 1 light second away gets to you 2 seconds after the ship passed you. When the ship is 2 light seconds away, the light gets to you 4 seconds after the ship passed you. 3->6, 4->8.

    Not a whole lot has changed, the light cone (if I'm using the term correctly) has just gotten a lot narrower.

    Time dilation creates an added complexity for the spaceship analogy (we obviously have to ignore it), but its not really an issue for expansion of space, as the proper motion of the galaxies is relatively low and insignificant really, compared to the recession rate.
    Last edited: Jan 26, 2004
  10. Apr 3, 2004 #9
    absolute relatives and relative absolutes

    Has it been suggested that the original Big Bang and much of the subsequent inflation of space moved matter apart faster than the speed of light, but that such expansion did not “really” violate Special Relativity because it did not "really" involve a movement of matter, but only an expansion of space?

    If that is true, then I am wondering this: over what period of “time” would the (movement?) expansion of space take place? If space is really expanding, must space be expanding over a time that must have some kind of absolute reality in relation to some kind of thing, even if only metaphysical?

    In other words, does a notion of inflating space necessarily assume that time has more than mere relative reality? Put another way, does relative time absolutely exist?
  11. Apr 5, 2004 #10


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    An archived thread has the Lineweaver references:


    There is a portion of the universe outside of our light cone, and it is possible that there will always be a portion of the universe we will never have any causal contact with.
    Last edited: Apr 5, 2004
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