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

Faster than the speed-of-light

  1. Nov 10, 2009 #1
    Einstein claims that is impossible for any object (with a mass) to reach the speed of light, however I have many questions that I would appreciate if were answered.
    To start with, the Largest Hadron Collider can accelerate an electron to a speed equivalent to 99.9999991% of the speed of light and the weight of that electron would be equal to the weight of a train moving at 80 km/h, which is huge (considering the initial weight of the electron), yet not infinitely huge. And it was only a tiny city on the border between Switzerland and France that was able to do it.
    Moreover, I once read that it would physically impossible for a space ship moving at the speed of light to reach the end of the universe, because universe is expanding at the speed higher than the speed of light. The only way something could move at the speed of light is if it had an infinite amount of energy, but then that doesn't make sense. For the universe to have an infinite amount of energy, the space it must be contained in must be infinite also, but since the universe is expanding, it can't be infinitely large, because otherwise it wouldn't have a value to its size.
    I also read somewhere else that in first second (up to 1^-37 seconds) after big-bang the laws of physics were different, in fact the 4 forces (electromagnetic, gravitational, weak and strong) didn't exist but there was only one force (Unified Force, where gravitational force was as strong as other forces) which later split into two different forces which after split again to become the 4 that we know now. So I thought that maybe in those first second, there was no limit to the speed, and it was that initial push that set us off on the speed faster than the speed of light, and then following the Newton's law of motion, the universe kept increasing at that speed because there were no external forces (assuming that there is nothing outside the universe) that would slow it down.
    And also wouldn't the particles during the "inflation" expand at a faster-than-the-speed-of-light velocity?
    Any opinion or answer will be appreciated,
  2. jcsd
  3. Nov 10, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    The universe can and does expand faster than light.
    Relativity prevents information (which in this case also means mass) going faster than light. There is nothing to stop two points moving apart faster than light - they just can't send information from one to the other.
  4. Nov 10, 2009 #3
    Well, if I'm not wrong in understanding the expanding universe, it's not really that two galaxy at the opposite ends of the universe are moving apart at a pace faster than light.
    Rather, there is a creation of space in which the galaxy and all the universe is immersed.
    Just like if on a ballon I draw two point, then I inflate the ballon. The point are not exactly moving, it's the baloon surface that is expanding.
    Please so. correct me if I'm wrong.
  5. Nov 10, 2009 #4
    If I may add something about travelling faster than light, let's say I make a trip 1 light year long.
    I begin the trip, I accelerate fast to 3/5 of c, travel 4/5 of a year, the I stop.
    My average speed has been v= s/t where space is 1 light year and time is 4/5 years.
    Can't I say I travelled at (5/4)c ?
    (Even if relative speed respect my original frame never exceeded c)
  6. Nov 10, 2009 #5


    User Avatar
    Science Advisor

    Are you talking about inertial coordinate systems in SR, or non-inertial coordinate systems in GR? If the former, then it's impossible for two objects with mass to have a relative velocity faster than c, so "there is nothing to stop two points moving apart faster than light" would be wrong (unless you're talking about the 'closing speed' between two objects as seen in the inertial rest frame of a third object, rather than the velocity of one of two object in the other's rest frame, which is how 'relative velocity' is normally defined in SR). If you're talking about non-inertial GR coordinate systems, the speed-of-light limit only applies in inertial frames, so it is quite possible for things (including photons) to move faster than c in a non-inertial coordinate systems and to pass information to each other. As mentioned on p. 4 of http://www.scientificamerican.com/article.cfm?id=misconceptions-about-the-2005-03 [Broken], we actually can observe galaxies that are receding from us "faster than light" in the standard cosmological coordinate system, so clearly we are getting information from these galaxies:
    Last edited by a moderator: May 4, 2017
  7. Nov 10, 2009 #6


    User Avatar
    Science Advisor

    To address the OP:
    But no matter how close you get, you can never actually get a particle with mass to reach the speed of light; that last 0.0000009% would increase the particle's weight by infinity (or to put it another way, it would take an infinite amount of energy to increase the particle's velocity from 99.9999991% to 100%).
    It would be physically impossible for a space ship to travel at the speed of light "locally", period. To understand what I mean by "locally" here, I'll quote something I wrote on another thread:
    A lot less than the first second--do you understand that 10^-37 seconds is a tiny fraction of a second? Also, gravity would only have been united with the other forces for about 10^-44 seconds, i.e. Planck time, after that it'd be separate, then after about 10^-36 seconds the strong force would become separate from the electroweak force, and finally the electromagnetic and weak force would become separate after about 10^-12 seconds (see the timeline here).
    I think all the mainstream theories of the grand unification era, when strong, weak and electromagnetic were united into one, would still be relativistic (Lorentz-symmetric) theories where c would be the top speed. As for the TOE era when gravity is united with the other three forces, there aren't really any complete theories to deal with this, but the attempts I know of like string theory are relativistic too.
    Not in the "local" sense I discussed above.
    Last edited by a moderator: May 4, 2017
  8. Nov 11, 2009 #7
    So if the universe expands faster than the speed of light and it is not through inflation, what else would expand the space in the universe? and since vacuum in the universe is not absolute vacuum, would a random generation of space defy one of the laws of physics? Matter can not be created nor destroyed?
  9. Nov 11, 2009 #8


    User Avatar
    Science Advisor

    Do you understand that nothing moves faster than light if you pick a local inertial coordinate system to analyze it? And do you also understand that when physicists say that nothing can move faster than light they are only talking about what is true in inertial coordinate systems, that there is no restriction in the laws of physics that says things can't move faster than c in a non-inertial coordinate system? This need not have anything to do with the "expansion of space", even in a flat SR spacetime you can pick a non-inertial coordinate system where things move faster than c, even though if the same objects were analyzed from the perspective of an inertial coordinate system we would find that none of them were moving faster than c. For example, I could define a coordinate system in my room in which my head was moving away from my computer monitor at 1000c...
    I don't understand what you mean by "a random generation of space". The expansion of space in general relativity is not random, it follows in a deterministic way from the motion and distribution of matter in the universe.
  10. Nov 11, 2009 #9
  11. Nov 11, 2009 #10
    I don't know... let's say you make a trip 100 miles long. You accelerate to 60mph, travel for an hour, and stop. Your average speed has been v=s/t where the distance is 100 miles and time is 1 hour. Can't you say you travelled at 100 mph? :smile:
  12. Nov 11, 2009 #11
    if by v you mean velocity, then shouldn't it be v=d/t (velocity = distance/time) as opposed to v=s/t (velocity = speed/time) as velocity is an s (speed) but with direction.
  13. Nov 11, 2009 #12
    I just copied what Quinzo used in #4 above. That post refered to "space" which is why s was used for distance.

    I suppose instead of just trying to make a joke, I should have pointed out the importance of making all of the measurements in the same frame of reference when you calculate velocity, but...
  14. Nov 11, 2009 #13


    User Avatar
    Staff Emeritus
    Science Advisor

    What you are describing is not a velocity - but it's a useful concept, it just needs another name. Some authors, see for instance http://arxiv.org/abs/physics/0608040 , call it a celerity.

    I'll take the liberty of quoting from the paper, since I suspect people won't read it otherwise. (the numbers are footnotes which are references in the original paper).

  15. Nov 11, 2009 #14
    Please show me the math for this.

    Thank you.
  16. Nov 11, 2009 #15
    A simple example is the reference in which I am currently at rest (earth's surface). The star Sirius is 8.6 light years away, and therefore moving at almost 20,000 times c relative to earth's surface.

    (2)(3.14)(8.6 ly)(365 rev/yr) = ~20,000 c = coordinate velocity of Sirius relative to earth's surface.
  17. Nov 11, 2009 #16


    User Avatar
    Homework Helper

    wait... what?
    I'm not understanding your logic in multiplying by the amount of days Earth has in a year. Firstly you have the circumference of the circle [itex]2\pi r[/itex] and then...?
  18. Nov 11, 2009 #17
    Use SR's multiple velocity equation.


    I want to see this in non-inertial frames.
  19. Nov 11, 2009 #18
    wasting your time
  20. Nov 11, 2009 #19
    Just to make the units work out. Alternatively, we could say that Sirius is (8.6)(365) light-days away. Either way, Sirius' velocity would be about 20,000 light-years/year or 20,000 light-days/day, or 20,000 c.
  21. Nov 11, 2009 #20
    Earth's surface is non-inertial. I was just giving a simple example of an object's coordinate velocity exceeding c in a non-inertial frame.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Faster than the speed-of-light