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Exceeding the speed of light

  1. Dec 20, 2006 #1
    If the outer galaxies are mobing at the speed of light, why aren`t they infinetly big? Also sub atomic particles which are accelerated to the speed of light are not infinitley big. Is this because sub-atomic particles behave differently to macro atomic objects?
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  3. Dec 20, 2006 #2


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    Could you perhaps point me to your source which suggests galaxies and sub atomic particles travel at C? I presume by 'infinitely big' you mean 'infinitely massive'.
  4. Dec 20, 2006 #3
    Private tuition from a Bio-Physicist and also by reading Stephen Hawking. And I am therefore of the understanding that this is the reason we cannot exceed the speed of light. And you are correct, I mean infinitely massive.
  5. Dec 20, 2006 #4


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    An object with a finite (non-zero) invariant mass cannot travel at C. A good lay explanation is given by Dr. John Simonetti of the Department of Physics at Virginia Tech;
    I do hope that your Bio-Physicist didn't say that it is possible for such particles to travel at C.
    Last edited: Dec 20, 2006
  6. Dec 20, 2006 #5


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    And as for the galaxies, they are not moving at relativistic speeds within the space that they occupy. The space itself is expanding away from us. That does not violate Relativity.
  7. Dec 20, 2006 #6


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    Particles in particle accelerators do travel at speeds that come quite close to the speed of light. As a result, their mass is thousands of times greater than when they are at rest.

    The detection chambers in these accelerators are specifically set up with this in mind, monitoring the space through which the particles will travel, given their much greater mass, as opposed to the space through which they would travel at their normal mass. The increase in mass that is observed in these particles is consistent with the predictions of GR, which also predict that their mass would become infinite if they reached light speed.
  8. Dec 20, 2006 #7


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    Cosmologists use coordinate systems that aren't compatible with SR. So while cosmologists do talk about speeds that area faster than light, these aren't the same sort of "speeds" that we talk about in special relativity. Saying that "space expands" is one way of trying to describe the difference - a possibly more accurate but abstract explanation requires a lot of discussion of coordinate systems and their properties, and why cosmologists use a coordinate system that is convenient for them but not compatible with special relativity.
  9. Jan 3, 2007 #8
    Could the infinitely massive factor when travelling at C be used to suggest omniprescence and allow a ship to reach different parts of the universe almost simultaneously?

    Does the factor of infinitely massive when travelling a C take into account a universe limited in size. For if space is still expanding then it is not yet infinite.
  10. Jan 3, 2007 #9
    With galaxies it is important to remember that the expansion of space is "metric expansion" i.e. the space-time itself is expanding. This allows objects to appear to move faster than c from our perspective. They do not exceed c, however, since the distance itself has expanded.

    If you imagine two spheres resting on an expanding sheet of rubber, they can both be at rest relative to the rubber, but appear to be moving apart at a rapid pace. If they are close enough together then they "share" a depression and the expansion needs to be strong enough to pull them apart, otherwise they remain in the same place.

    The only reason everything does not "explode" is because it is bound gravitationally, electrostatically, with nuclear forces etc..

    There are other relativistic situations where objects appear to exceed the speed of light relative to one observer, in all of these cases though these objects to not exceed the speed of light locally. This is due to the curvature of space.

    Also remember, objects travelling at c must be massless. Massive objects will be limited to < c and will always maintain a finite relativistic mass. Instead of considering relativistic mass you should be thinking in terms of invariant mass anyway. Relativistic mass is a concept which has caused as many problems as it solves. It is not "really real", i.e. it works for making some relativistic equations agree with classical theory, but not for all. Most importantly it breaks F = ma.

    I suggest you read this page: http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html
  11. Jan 3, 2007 #10
    If your question where "...when travelling almost at c..." then the answer would have been yes. You could travel for a few seconds and reach distant galaxies, for example. But, coming back on earth you would find millions or billions of years has passed there (maybe earth and solar system wouldn't exist anylonger).
  12. Jan 3, 2007 #11
    You just need to turn your head 180 degrees and you will see the stars turn around you at speed >> c with respect to your frame of reference.
  13. Jan 7, 2007 #12
    If there is an aspect of sound emanating from the big bang, as I have heard there is and it is universal then this medium could be used for travel and be combined with the factor of omnipresence or universiality of travelling at C. Also if time-space is curved then we could define these guages closer, analyse their behaviour; engage other gravitational factors and bend time so that no time discrepancy in travelling at C would take place and I did say previously the factor of travelling at C not the actuality.

    We would need to have a universal not individually progressive time measurement unit and do things like start ships
    from a past time energy field. This, of course, would need careful and precise calculation involving heat, time sound and gravity guages.

    I would suggest the ensuing crude equation to enable us to extricate a suitable applicable time measurement for throughout the universe:

    Infinite non-actual mass defined at a specific date, as the definition no longer applies at the present. Yet it is a constant. It
    is defined but can never be given or defined size-wise as an absolute
    Actual mass of ship at 30 mph

    equals a time definition.

    Being infinite cannot be defined, as what is infinite must change with the continuing accommodation of more space and time and what was infinite two hours ago is no longer infinite. However the factor infinite follows the expansion of the universe constantly but changes its guages phenominally quickly.

    Jheriko: please explain F=ma in longhand for me
    Last edited: Jan 7, 2007
  14. Jan 8, 2007 #13
    why? by m=m'*Y?? if that's so, that mass is relativistic right?? another question: is that mass "seems" normal?? or if we look at it, it has really increased??
  15. Jan 9, 2007 #14

    Chris Hillman

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    Hi, Glynis,

    Do I understand you to say that you think the Big Bang made an audible sound, as in sound waves? There would be several confusions there, including:

    1. the so-called "Big Bang theory" is a bit of a misnomer, since it really says that the universe was once much denser and hotter than it is now, and has been expanding and cooling ever since, somewhat like what might happen after a kind of explosion in deep space,

    2. while the idea of an explosion suggests the right idea in some respects, you shouldn't think of this as having occurred in some particular location,

    3. sound cannot propagate through the near vacuum of interstellar space.

    I'd suggest you put away the popular book by Stephen Hawking and try instead the popular book by Steven Weinberg which I recommend on this page: http://www.math.ucr.edu/home/baez/RelWWW/reading.html#pop
    This book directly addresses your confusion about the motion of "galaxies at the limits of observable universe", so to speak.
    Last edited: Jan 9, 2007
  16. Jan 9, 2007 #15
    Perhaps, not travel at "c" not greater than "c"--Einstein's theory doesn't object to this--objects could travel at speeds greater than "c" but not slower than "c".
  17. Jan 9, 2007 #16

    Chris Hillman

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    Tachyons again

    Hi, DocN,

    I think you're referring here to the notion of a tachyon, and we've just been through the reasons why this speculative notion appears to have no experimental support and is dubious for various theoretical reasons.

    The original question seemed to concern something completely different, a misconception about "Big Bang" cosmological models.
  18. Jan 9, 2007 #17
    Can you find some equation of Einstein that would prohibit some particle from always traveling faster than "c" (speed of light)?
  19. Jan 9, 2007 #18

    Chris Hillman

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    I think you misunderstood what I wrote. I know what you are talking about, and don't disagree with that, but there are further issues to consider in discussing whether or not we should seriously expect to ever observe any tachyons.
  20. Jan 9, 2007 #19
    I was not thinking of tachyons but any disagreements to Einstein's SRT equations. Although such a particle (s>c) would have to always travel greater than c, never lower.
  21. Jan 10, 2007 #20

    Chris Hillman

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    Which is the basic idea behing the tachyon proposal. I don't understand what the problem is here.
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