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Is c the fastest known velocity?

  1. Nov 8, 2005 #1
    Is "c" the fastest known velocity?

    Hi all,

    Newbie here wants to know, after a discussion over dinner with some friends, if the speed of light is the fastest known rate in the universe.

    Specifically, I want to know if the universe can expand at a rate > "c".
    I think not, because if the the universe does expand at a rate > "c", than our Hubble and other land based telescope won't be able to catch lights from distant galaxies.

    I know I should run a search in the forum first :biggrin: , but I figure I might get a quicker answer this way. If I do not get any replies, I'll then run a search.

    Thank you all, and I will actively join in in the discussions of this forum.

    :biggrin: :biggrin: :biggrin:
  2. jcsd
  3. Nov 8, 2005 #2
    This is by no means my area of expertise (not that I really have any), so someone correct me if I'm wrong:

    Inflationary theory holds that space-time itself expanded at a super-luminal velocity. This has to be the case in order for regions in the universe that are separated by a distance greater than ct to be in thermal equilibrium.

    In other words, inflation was introduced to allow the big bang theory to predict the observed homogeneity of the background radiation.

    This might seem to contradict special relativity, that states c is the speed limit, but in fact, AFAIK, special relativity doesn't say anything about space-time itself (in that respect).

    And yes, if the universe were to accelerate at a rate greater than c we would be unable to observe distant galaxies.
  4. Nov 8, 2005 #3
    Thanks for the quick reply.

    So is there a value for which the universe is expanding? ie expanding at "c" or slighly less than "c".


    :biggrin: :biggrin: :biggrin:
  5. Nov 8, 2005 #4


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    Not my area of expertise either, but a slight clarification:
    The "speed" (put in quotes because expansion rate isn't really a speed) alone is not what determines whether or not we can see something. Ie, if an object on earth suddenly zipped away at 2C, we'd see it 2 light minutes away after 1 minute, 4 light minutes away after 2 minutes, etc.

    What matters is the speed of light overcoming the expansion, and there is a horizon that can be calculated there, though I don't know quite how to do it.
  6. Nov 8, 2005 #5


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    Every cosmological model that expands according to the Hubble law has superluminal expansion velocities. This is because v = H d is valid everywhere and if the universe is big enough one can find distances for which v > c. By the way, the distance d = c / H is called Hubble distance. In the current cosmological model this distance is located more or less at redshift z = 1.5 (you might want to try some numbers here or here). Recession speed is only apparent due to the expansion of space; the objects do not move in space. This means that cosmological redshift is not a Doppler effect and the velocities are actually not in contradiction with special relativity. Further on, in the current cosmological model the Hubble sphere is nearly equal to the event horizon. The event horizon is the limit for objects whose light will never reach us in future due to the strong expansion of space between us and them. We might be able to observe this objects now receiving the light emitted by them in past, but the light they are emitting now will never reach us in future.
  7. Nov 8, 2005 #6
    If we put d=c/H in the v equation, we'll get v=c. :surprised

    Can this be true? If not how large does the universe have to be for v > c?:biggrin:

    Just trying to enlarge my understanding for the expansion of the universe.

    Thank you, and if I didn't have to eat, I'll go back to school and get Physics degree. :biggrin:

  8. Nov 8, 2005 #7


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    The Hubble law is v = H d. This means that the recession velocity increases with distance proportional to the Hubble parameter which is equal to 71 km/s for each Megaparsec (one parsec are 3.26 lightyears). If you want to find out the distance at which the recession velocity is equal to c, then put c = H d and you will get d = c / H. You can calculate d = c / H = (300000 km/s) / (71 km/s Mpc) = 4225.35 Mpc = 13774 Mega-lightyears. This is the distance you would measure on a surface of constant time, this means, putting a rod from here to there (it is different from the light-travel distance). You may try also for example the first link I gave an put redshift = 1.4, you will find out that recession speed = c and d = 13729 Mega-lightyears. Thus, objects farther away than ~ 13800 Mly are receding now at speeds greater than c.
    Last edited: Nov 8, 2005
  9. Nov 8, 2005 #8
    I don't know why but so far i never heard any physisist say "oh that galaxy is moving with v>c, einstien was worng". even never seen an eqation reading so.......somebody might want to help??
    Or maybe its mere a figment of ur imagination........
  10. Nov 8, 2005 #9
    Its complicated. galaxies at the universe are receding "faster than c", but that is as space itself expands. Its a General Relativity problem, not a special relativity problem. Unfortunately I that's the best explanation I can give, as "its not my field of research."
  11. Nov 9, 2005 #10
    You can find a discussion of the misconceptions related to superluminal velocities in the expansion of the universe in the work of Lineweaver and Davis, such as the Scientific American article: Misconceptions about the big bang , and the paper: http://xxx.arxiv.cornell.edu/abs/astro-ph/0310808 [Broken].
    I'll mention here that I disagree with the idea of expanding space put forward in the above, and have written about this on my website: Stretchy space? and Cosmology, Special Relativity and the Milne Universe
    Last edited by a moderator: May 2, 2017
  12. Nov 10, 2005 #11
    General Relativity extends the principle of relativity to accelerated frames of reference, in which objects routinely travel faster than light.

    For example, the earth is an accelerated reference frame due to its rotation, and all of the objects in the sky move at v > c every night (millions of light years every night!).

    The General Theory of Relativity tells us that the universe can "expand faster then light", and cosmologist claim that this possibly may be or have been the case. This is because the expansion of the universe is a local phenomenon, it happens all around us bit by bit (the expansion of our living room is slower then the speed of light, but imagine an infinitude of such living rooms all expanding at an infinitesimal rate contributing to something which has v > c).

    Since there does not exist an inertial (non accelerated) reference frame in which one can see the universe expanding at v > c (there isn't even an accelerated one, clearly), special relativity isn't even close to being violated.

    File this one away with a lot of people standing and doing "the wave" at the same time (like at sports games). They could make it look like "the wave" is traveling faster then light, but nothing is actually moving at v > c.
  13. Nov 10, 2005 #12
    I thought the universe was estimated to be around 150 billion light years in diameter, which, when you remember that it is only abount 13.7 billion years old, would suggest it has expanded pretty fast.

    Also I remember hearing that the limit on the velocity was restricted to particles and objects in space such as photons and electrons, and not space itself. If it is expanding at a rate faster than light (say the distance between our galaxy and the next is increasing at v>c) then will we ever be able to reach that galaxy? It could be like running on a treadmill, where the speed of the treadmill is faster than you can run, so you'll never reach the front of it.
  14. Nov 10, 2005 #13
    Michio Kaku states the universe is expanding at 10^50 times per second. In other words, this is much faster than light, this does not violate Einstein's theory because empty space is not matter, this space may accelearate but does not contain anything with mass.
    What I do find confusing is that my science teacher claims gamma rays move faster than 'light', an example he gave is that visible light can not pass through your hand, however gamma and x-rays could, thus his conclusion is that these 2 along with infrared rays are 'faster than light'.
    But is c is the velocity of visible light? If not, c is the velocity of which type of ray on the electromagnetic chart?
  15. Nov 10, 2005 #14
    Velocity in two reference frames cannot be directly compared due to the fact that the two reference frames may well be non-inertial. In the universe, two galaxies are almost certainly non-inertial to each other, therefore the velocity of one galaxy is not really a concept which makes sense in another galaxy unless properly transformed with the tensor transformation law. Rest assured, however, in local frames nothing moves faster than c.
  16. Nov 10, 2005 #15
    I've never heard anything like that. I don't want to say it's wrong because he is a teacher, but I'll say that it contradicts everything I've been taught upto this point.

    As I understand it, all em waves travel at c in all inertial frames. What does the fact that gamma rays can travel through skin tell you about the velocity?
  17. Nov 10, 2005 #16
    I believe what he meant was that the shorter the wave length the faster the speed. It would mean that visible light's velocity would be half way between radiowave's and gamma ray's. My teacher's explanation does sort of makes sense, but I misunderstood a major part, what he meant as speed of light, he actually meant visible light, not C. C is the velocity of light in a vacuum, which is what I believe to be its 'fastest state'. If radiowaves were slower, it would either be absorbed by an object or it'll bounce back, which is exactl what happens.
    If this explanation is not true, it would mean at different wave lengths, the photon has a different mass.
  18. Nov 11, 2005 #17
    Still not following... Visible light does travel at c, all electromagnetic waves do, and all phtons have no rest mass. I would say trying to introduce the concept of different wavelengths moving at different overall speeds through certain mediums is very confusing when trying to explain relativity.

    In a medium, the speed of the photon when travelling between particles is still the same, only some time elapses between absorption and emission of light by electrons, I think.
  19. Nov 11, 2005 #18
    I don't know what is so hard to understand the man just said 10^50 times per second, and u r confussing its with 10^50 mps. it is the rate of expansion not its velocity.
    And moreover universe is a just frame and it is not bound by any law or anything, c is bound to be the ultimate speed within that frame. And u can not measure how fast a frame is expanding because then you need another frame of reference so that u can calculate the velocity. I believe you all understand what a frame or a frame of reference means, don't ya!!
    Last edited: Nov 11, 2005
  20. Nov 11, 2005 #19


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    Chronon's references for Lineweaver and Davis address this issue. Any of their papers should put you on track, they have published several in the same general area.

    We do in fact see light from galaxies that have ALWAYS been receding from us at a velocity >c. This seems impossible, but it must be remembered that the light is traveling to us through space which is not static in the ordinary sense (that is why it is not an inertial frame relative to us). It is true that during part of their journey towards us, photons are actually receding from us rather than getting closer. But they move into space-time regions which are themselves receding from us at relatively lesser velocities. Over time, the photons move into a region of space-time which is receding from us at a velocity less than c. At that point, the photons will start approaching us - although at a speed less than c. From that point, it is a matter of time before they reach us - 13 billion years later...

    These photons arrive with a red shift indicating the source galaxy is moving away from us at several times the speed of light. Pretty amazing stuff.
  21. Nov 11, 2005 #20
    Mmmm.. neither do I...
    my teacher is weird sometimes, he claims that photons have next to no mass, so it has the ability to exceed C. He said if white light cant pass through your body, and X-rays can, x-ray/gamma rays must be moving faster than white light. Also, something accelerates, its length decreases, maybe corresponding with his claim that shorter wave lengths= faster photons.

    10^50x per second is from Michio Kaku's Paralle worlds.
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