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Faster than light travel proved sort off

  1. Mar 21, 2010 #1
    I was laying in bed and a thought came to me. Im sure this has been explored already so I wanted to know what the answer was as my physics is limited as I study biochemistry. I have a feeling this is related closely to general relativity, but again maybe someone could clear this up for me... thanks

    Ok, so its clear there are no fixed referance points in the universe, as in... movement is a meaningless concept unless related to another object. hopefully makes sense. I came to this conclusion because GPS satellites work by timing the time a radio wave bounces off the earth, so if we were 'moving' the speed of light would be different depending on what direction you was headed.... but of course 'direction' again is a meaningless concept. I did google this and it seems this is a consensus.

    So, there must be a point on the otherside of the universe (for example) where the stars are moving at faster the speed of light (in relation to the earth) perhaps across our path,,,, but so far away we may not even be able to detect it. I think this is a reasonbbale conclusion?

    So...... doesnt this mean that faster than light travel is possible.. because speed is a completely meaningless concept? right?

    So in theory (ignoring the fuel needs, and maybe fear of banging into objects floating around in the solar system) why cant we just get in a rocket, hit the accelerators and the speed will just increase theoretically into infinity (in relation to the earth of course). There is no limit to the speed we can reach because again.. speed is meaningless. I am aware that apparently nothing can travel faster than light, but how does this fact take into account what Im saying (that speed is meaningless as there are no referance points to go from)

    Im fascinated to know if this idea has been tested, and if there is any experimental evidence suppotying/disproving this idea.

    Thank you.
  2. jcsd
  3. Mar 21, 2010 #2


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    This is the issue Special Relativity deals with. Infact, the speed of light is CONSTANT in every reference frame. If you were on a train going half the speed of light and shined a flashlight forward, the light would travel at the speed of light. Similarly, someone who was simply watching the train pass by would also see the light traveling at the speed of light!

    That doesn't mean speed is an irrelevant concept. There is simply no preferred reference frame for there to be a preferred speed measurement. If said train in the previous example were to be traveling by me, my frame of reference saying that the train is traveling to the east, for example, is no more correct then the person on the train in his frame of reference saying that I'M traveling to the west at the same speed the train is going (only in the other direction...)!

    Also, when you look at the idea of just accelerating infinitely quickly, that's not possible. Velocities do not add up like we expect them to do out of every day experience when it comes to relativistic speeds. They add up (See the FAQ's) in a way that you never reach the speed of light.
  4. Mar 21, 2010 #3


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    Relativistic velocity addition is discussed here, and also see the http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken] page, which explains how a rocket with constant proper acceleration (constant G-force as felt on board the rocket, or equivalently constant rate of acceleration in the rocket's instantaneous rest frame at each moment) will never reach the speed of light since in any fixed inertial frame its coordinate acceleration will be decreasing.
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  5. Mar 21, 2010 #4
    Could he be thinking of blueshifting and recession velocity and be mixing them up, or is he just firing blanks in the dark?
  6. Mar 21, 2010 #5


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    Speed is not meaningless. It's only meaningless if you assign it to a particle with no frame of reference in mind. E.g. The statement "I'm traveling at 90mph" is technically (and rather pedantically) meaningless because the statement does not say what I'm traveling relative to; however, it is assumed I mean I'm traveling relative to the Earth's surface. If that is the case, then the statement "I'm traveling at 90mph (relative to Earth's surface)" IS a meaningful statement.

    Therefore, the maximum speed limit of light speed IS meaningful in that you can never travel faster than the speed of light in ANY reference frame.
  7. Mar 22, 2010 #6
    If the statement then commutes to: "I'm traveling at 90mph (realtive to the people screaming and pointing at my Toyota)"... and yes, I am just kidding.
  8. Mar 22, 2010 #7
    Thanks all for helping out with some things, but I still got few problems...

    There will surely be objects out there in relation to earth moving faster than light. Perhap a distant galaxy just floating about. Maybe even so far away we cant detect it and ever will. How does special relativity deal with this. Ive heard something along the lines that it may appear to be moving faster, but time will slow down so it is as if it were moving at the speed of light nd not faster.. is this correct?

    Also on acceleration... if speed only has meaning when releated to other bodies, how is there a limit to acceleration? Because it doent matter when you accelerate, you are not really moving anywhere at all, as there are no frames of referance in the universe, except relative ones.

    So...... does specil relativity mean that nothing can move faster than light relatively to objects, because if the try they literally slow down time from the point of view of an external observer? Is this right? and if so, is there any experiemntal or observational evidecne for any of these ideas?

  9. Mar 22, 2010 #8


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    Why do you say "surely"? In special relativity nothing would move faster than light from the perspective of an inertial frame, no matter how distant. In general relativity the question is more complicated, defining the "speed" of anything requires you pick a global coordinate system (so you can define distance/time), and there's no special class of global frames which are picked out as special by the laws of physics like inertial frames in special relativity. However, in GR the http://www.aei.mpg.de/einsteinOnline/en/spotlights/equivalence_principle/index.html [Broken] says that in any small region of curved spacetime, an free-falling observer will measure the same things that she would if she were moving inertially in flat spacetime, using a "locally inertial coordinate system". So, the speed of light as measured in a locally inertial coordinate system is still always c in GR, and all massive objects are moving slower than c in this local sense. In global non-inertial coordinate systems in curved spacetime the coordinate speed of light can be other than c, but then it's true even in special relativity that if you use a non-inertial coordinate system the coordinate speed of light may not be c.
    There is no limit on your rate of acceleration at any given moment, regardless of whether we're talking about proper acceleration or coordinate acceleration. However, you can't accelerate continuously in a way that would involve you passing the speed of light, because this would involve your energy going to infinity as seen in any inertial frame (and energy is conserved in any inertial frame, so this infinite energy would have to come from somewhere), and it would also require your proper acceleration to go to infinity.
    I'm not sure if that would actually explain why you can't accelerate to light speed...without looking at physical considerations like energy, it would be interesting to check whether it's possible to draw a curve in spacetime that reaches c in finite time as seen in some inertial frame, and also reaches c in finite proper time (time as measured by a clock moving along this curve, again ignoring issues like energy) or whether any curve that would reach c in finite coordinate time would also require infinite proper time.
    There's plenty of experimental evidence for the SR time dilation equation, like longer decay times for particles accelerated to a large fraction of c in particle accelerator experiments. See tests of time dilation from Experimental Basis of Special Relativity for papers on the subject, or the 'time dilation for particles' section near the bottom of this article from the page of the Stanford Linear Accelerator Center.
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  10. Feb 8, 2011 #9
    Again... I think im not making sense. Im starting to understand this theory, but I am having a fundamental problem.

    Forget relative points for a second. If I am in my ship and thrust in one direction, and than I do it again, and again, and again..... this is not really movement at all, because the universe has no fixed point in space. It becomes what we call 'movement' the second we put it in the context of a reference point, e.g. a planet.

    So I just don't understand why you would need infinite energy, or a advanced propulsion system to achieve this. You could put a ship the size of a petrol tanker in space, give it a crappy thruster, and just turn it on. If there is no resistance, or gravity acting on our craft than why a limit? I want to emphasise I mean from the perspective of the person/computer on the ship, not the space around them. If I pointed this ship at a star, and just kept the thrust going I could get there quicker than light.... correct? (I accept when I reach there, time would have gone forward)

    This is driving me crazy because people always reply I would need infinite energy to reach the speed of light but I keep repeating speed of light means nothing until it is related to another object so I am at a loss to how this makes sense. Right now we are all on earth moving faster than light relative to a point in the universe, perhaps billions of light years away... how does this work?
  11. Feb 8, 2011 #10


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    OK, suppose someone on a planet is watching you. If your thrusts are all identical from the perspective of the ship (i.e. same g-force, same period of thrust, etc.), then from the perspective of the observer on the planet, each thrust increases your speed by a smaller and smaller increment as you approach the speed of light (this follows from the velocity addition formula). So, no matter how many times you thrust, you never reach the speed of light from their perspective, nor from the perspective of any other inertial frame.
    Nope, from the perspective of any inertial frame you would always be going slower than light, and if an actual light signal were sent out from the starting point at the same moment you departed, the light signal would be guaranteed to beat you to the destination.
    You need to understand the concept of inertial frames of reference in relativity, which represent what would be measured by a grid of non-accelerating (inertial) rulers and clocks (as illustrated here). Each inertial frame measures the speed of any object in terms of distance on their own rulers and time on their own clocks, but it works out that if they each measure distance/time for a light beam, they will all conclude it is moving at c relative to themselves (for a numerical illustration of how ruler/clock systems moving at different speeds can both measure the same light beam to move at c relative to themselves, see [post=1561633]this post[/post]). This is why physicists talk about the impossibility of "moving faster than light" without needing to specify what the speed is relative to, because an object moving faster than light relative to one inertial frame would be moving faster than light relative to all inertial frames (and similarly an object moving slower than light in one frame is moving slower than light in all frames). Also, all inertial frames move slower than light relative to one another, so as long as we are talking specifically about speed as measured in an inertial frame, it's not correct to say "Right now we are all on earth moving faster than light relative to a point in the universe" (though in general relativity where spacetime is curved, you can't have an inertial frame on a large region of spacetime where the effects of curvature are significant...but it's best not to try to understand that until you have a good grasp on how things work in special relativity where there is no spacetime curvature).
  12. Feb 8, 2011 #11


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    You're contradicting yourself here, over and over again. How can you simultaneously be saying you can't measure your speed, yet at the same time think your speed is greater than the speed of light? You say speed has no meaning unless measured against a reference - yet you have a star to use as that reference! You can't have it both ways.

    The answer to your conundrum is this: if you truly have no sensors to detect things outside your spaceship (which in reality would be silly because you'd never know where you were going), then the only thing you have to go on it the detection of your own acceleration. Thus, the point at which you started becomes the other frame of reference from which you track your acceleration and you find your speed by tracking that acceleration alone.

    So now you have a choice: you can choose to ignore Relativity and just assume you're accelerating via f=ma or you can choose to track your acceleration using Relativity. If you use f=ma you may well get an answer bigger than C (in reality, no chemical rocket will get you anywhere close to C, but we'll let that go...).... So which is right? Or are they both right? The answer is that Relativity is right. If instead of just monitoring your acceleration you actually dropped-off a buoy before firing your engines and track it instead of the virtual starting point, you'd see that this is true.
  13. Feb 9, 2011 #12


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  14. Feb 10, 2011 #13
    I read more on this, and the FAQ and i gather that for the ship, it is possible to travel faster then light. But for outside observers, they would not be. The FAQ says it could take 12 years to get across the galaxy, from the point of view of the ship, but when they reach their destination thousands of years have passed.

    Have I read it correctly?
  15. Feb 10, 2011 #14


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    Although it's true the ship can cross the galaxy in a short time, you have to keep in mind that in the ship's inertial rest frame, the galaxy itself would be shrunk by a huge amount in the direction of travel (to less than 12 light-years in this example) due to length contraction! So, in this frame the ship does not actually need to travel faster than light to cross the galaxy in 12 years. As long as you stick to an inertial frame, the ship (and all other massive objects) will always travel slower than c.
  16. Feb 11, 2011 #15
    So I was right all along and you guys just confused me with your charts. ;)
  17. Feb 14, 2011 #16
    this is related to something i have been wondering it is that what is to prevent the crew of a spaceship from being the stationary observer and seeing the universe's gamma factor as increasing as not the ship approaching the speed of light but the universe approaching the speed of light. if neither agrees then maybe the two just do not see each other. the light spread between each is just redshifted to that point. if the fabric of spacetime is elastic couldn't it compensate for the fact that you are traveling faster than light? the spacetime in front may rupture. wow i wonder where you would go?
    wow i just look like a bumbling idiot on these posts.
    Last edited: Feb 14, 2011
  18. Feb 14, 2011 #17
    I want to see an experiment done by NASA where they accelerate a rocket to as 'fast' as possible in the solar system and just see what happens. From the external observer I gather the speed of light will never be reached, but for the rocket it is possible (in relation to Earth). Than, turn it around communicate with earth what it experienced, in terms of what happened to and perhaps a visual perspective of the surrounding universe and just some basic measurements.

    In my head this sounds perfectly possible, and quite easy... so why hasn't it been done?
  19. Feb 14, 2011 #18


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    Well, I imagine that if you put up all the money, NASA would be happy to do it! But they are not going to spend a huge amount of their own money to verify something they already know just because you don't.

    Force= mass* acceleration

    acceleration= Force/mass.

    If you just "turn on" your rocket as you suggested before, so that you are getting a constant force, you will have to take into account that the mass will increase- growing larger without bound as you go faster and faster. Since the mass (denominator of the fraction) is increasing while the force (numerator of the fraction) is constant, the acceleration will become less and less, eventually going to 0 as your speed approaches that of light.
  20. Feb 14, 2011 #19
    I dont believe the mass will increase from the point of view of the ship as the universe lacks frames of reference so the 'speed' is only increasing if viewed from external observers. I also believe that in the ship, acceleration is like going from 0 every time it is applied. The FAQ seems to agree as it clearly stated you could travel the length of the galaxy in a speed faster then light, but only from the perspective of the ship. So

    No need to be rude. Perhaps you are correct about Nasa not bothering, but I think it would be a fascinating experiment for people interested in Physics and would help to clarify a few of the inconsistencies of relativity, as some of the theory has not agreed with observations in space.
  21. Feb 14, 2011 #20

    Doc Al

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    What inconsistencies? Do you have references for this statement?
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