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

Speeds greater than the speed of light

  1. Nov 5, 2008 #1
    Are there speeds greater than the speed of light? If so, then can we discard c to embrace the new faster speed as the universal constant to explain relativity?
  2. jcsd
  3. Nov 5, 2008 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

  4. Nov 5, 2008 #3
    Log of superluminal radio conversation.

    Bob: "Superluminal active"
    Alice: "Switch to the prototype superluminal radio channel and confirm you have done so by saying 'superluminal active'".
    Bob: "O.K"
    Alice: "I am going to ask you a series of simple test questions to test the new superluminal communicator, OK?"
    Bob: "Seven"
    Alice: "What is six plus one?"
    Bob: "Tuesday"
    Alice: "What is the next day after Monday?"
    Bob: "Yes, this is wierd"
    Alice: This is wierd isn't it?"
    Bob: "Yes, it means we have no free will"
    Alice: "Does this mean we have no free will?"
    Bob: "You do not have any choice"
    Alice: "What happens if I don't ask the question in the future that corresponds to the answer I received in the past?"
    Bob: "OK, I've switched back to normal speed comms channel"
    Alice: "I've had enough. Switch the dammed superluminal communicator back to the normal speed comms channel and tell me when you have done so"

    I won't give a link but you can google using keywords like "superluminal implies time"
    Last edited: Nov 5, 2008
  5. Nov 5, 2008 #4


    User Avatar

    Staff: Mentor

    Sure, I can easily calculate speeds greater than the speed of light. None of them achievable by an object with mass, in reality, though...
    You can't just discard a universal constant because you feel like it. It isn't an arbitrary thing.
  6. Nov 5, 2008 #5
    There are many examples of phenomena faster than the speed of light. One book about just such phenomena is FASTER THAN LIGHT, Superluminal Loopholes in Physics,Nick Herbert.

    The point is none can be used to transmit information... that is causality....none is faster than light.
    Last edited: Nov 5, 2008
  7. Nov 5, 2008 #6
    Try the thread here LIGHTHOUSE PARADOX REVISITED....I have not read it but the subject discussion may give you some insights ideas and perspectives.
  8. Nov 5, 2008 #7


    User Avatar
    Science Advisor
    Homework Helper

    >Log of superluminal radio conversation.
    Isn't really a satisfactory explanation. Imagine a species that was blind, it could develop a similair theory based on sound, you obviously can't have any speed faster than sound because then you would be able to get the answer before the question had reached you.

    A better explantion might be that relativity says there is a maximum speed of information and Maxwell's equation coincidentally say that the speed is 'c'.
  9. Nov 5, 2008 #8
    Nonsense, superluminal communications implies no such conversation.
    If the parties both rebroadcast the conversation over a luminal transmission both could pick up a echo (time delayed) of both sides of the same casually ordered conversation from the other at a later time on the luminal communications channel.

    They should also be able to figure out if they are on a preferred ref frame by simply checking over the superluminal communications channel to see if their local clocks are actually synchronized as the luminal communications channel implies.
  10. Nov 5, 2008 #9
    I don’t think Maxwell or Einstein would consider that a coincidence. Neither should we.
  11. Nov 6, 2008 #10
    The difference is that a theory based on sound requires a medium which provides an absolute reference frame for a sound based theory. With relativity and light there is no medium and no absolute reference frame and the laws of physics are required to be the same from the point of view of any inertial observer. Any meaningful communications at superluminal sppeds topples that house of cards.

    Nonsense is what follows once the notion of the speed of light as an absolute limit on the speed of communication is rejected.

    The attached time space diagrams clearly demonstrate that superluminal signaling predicts being able to signal backwards in time from the future to the past. (The space time diagram was generated by a nice program created by Mentz114 available here.)

    The first diagram is the reference frame of Alice whose worldline is shown in red. She sends a superluminal signal (green worldline) at twice the speed of light to Bob. Bob,whoes worldline is shown in blue is moving at 0.9c relative to Alice. In Bob's reference frame the signal is traveling backwards in time. If we accept the principle that the laws of physics are the same in any inertial reference frame (and that superluminal signalling is possible) then a signal can be sent backwards in time in any reference frame. On receiving the signal, Bob sends a superluminal signal (magenta worldline) back to Alice at 3c. It is easy to see that Alice receives the reply before she sends the question in both reference frames and this is true for any observer at any speed relative to Alice. With superluiminal signalling, effect precedes cause.

    We can take a more extreme example to show how the known laws of physics and rational logic all break down once superluminal signalling or superluminal transmission of particles is accepted.

    Imagine Alice has a superluminal weapon that fires antiparticles at 2c. Alice does not like the look of Bob and makes a pre-emptive strike against him using her superluminal partical weapon. Bob's craft is damaged and he is injured but he is intact enough to return fire with his 3c superluminal particle weapon and completely destroys Alice's craft and kills Alice. Bob claims it was self defence. However, his case that he killed Alice because she attacked him in the future is going to have a hard time in court. The only available evidence according to witnesses is that Bob killed Alice before she fired a shot and it looks like Bob made a completely unprovoked attack on Alice. Does Bob's craft still get damaged in the future by Alice and if so, how does that happen if Alice and her craft were destroyed before that happens? This is the sort of nonsense that follows if superluminal transmission is proposed.

    To accept superluminal communications requires that at least one of the following is rejected:

    1) Cause precedes effect in any reference frame.
    2) The laws of physics are the same in any inertial reference frame.
    3) There is no absolute reference frame.

    Which of these principles do you propose to reject?

    It has nothing to do with clock synchronization. Bob does not even have to have a clock. The sequence of events can be entirely determined by Alice's single clock. Alice is aware that she receives the reply to her questions before she transmits the questions using her single stationary clock.

    P.S. As I hinted at before, if we accept the notion that superluminal communication is possible, we probably have to reject the notion of free will and accept that the future is predetermined.

    Attached Files:

    Last edited: Nov 6, 2008
  12. Nov 6, 2008 #11


    User Avatar
    Science Advisor

  13. Nov 6, 2008 #12


    User Avatar
    Science Advisor

    There are theories in which Lorentz invariance is violated. Experiments constrain the violation to be very tiny. This article reviews the theories and experiments - the comments on "doubly special relativity" may be relevant to your question: Mattingly, Modern Tests of Lorentz Invariance http://relativity.livingreviews.org/Articles/lrr-2005-5/.

    I'm not fond of free will, but some people claim it's possible: Conway and Kochen, The Free Will Theorem, http://arxiv.org/abs/quant-ph/0604079.

    These guys claim we have free will and the future is determined: Aharonov and Tollaksen, New Insights on Time-Symmetry in Quantum Mechanics, http://arxiv.org/abs/0706.1232.
  14. Nov 6, 2008 #13
    What you are doing here is accepting an unfounded assumption to support what amounts to a Science Fiction story line.
    First I don’t accept superluminal communications as possible at all – you’re the one making unfounded conclusions about reality based on assumptions on what would happen if you built such a system, like the idea we have no free will.

    I do not need to reject a principle;
    I reject the notion that “3) There is no absolute reference frame.” is a principle.
    Certainly not based on Special Relativity and the Simultaneity Rule;
    It does not say you can arbitrarily pick any frame as the absolute reference frame. (and here you even graph and use two different frames and selectivly use both as absolute)

    It says although realism demands that separate events can happen simultaneously, you cannot determine if they are by any frame of synchronized clocks. Including any home frame you might prefer.
    Simultaneity says the only time you can know two events are actually simultaneous is when they happen physically spatially next to each other.
    Nonsense, it has everything to do with clock synchronization
    you cannot solve a problem by removing Bobs clock to reducing the amount of information available.
    If Alice and Bob have synchronized their clocks but Alice can read Bob’s clock through an instant superluminal communications as being sometime in the future, then clearly their clocks cannot be used as a standard of absolute.
    More importantly whatever the discrepancy is; Bob and Alice should be able to use that information to correctly establish what frame has synchronized clocks usable as coordinating when events are simultaneous.
    Only if we accept your notion that you can definitely establish that some future reading on Bob’s clock is absolutely simultaneous with the current time on Alice’s clock.
    Assuming so is a complete violation of the SR Simultaneity Principle,
    and until you legitimately update that principle you are dealing with Science Fiction here. [/QUOTE] Alice is aware that she receives the reply to her questions before she transmits the questions using her single stationary clock. [/QUOTE] Really when and under what conditions; prove it with an experiment designed convincing enough to update the Einstein Simultaneity Rule.

    If the established SR principles are followed, there is no refutation of free will here.
  15. Nov 6, 2008 #14


    User Avatar

    i think that communication (or the "transmission of information") requires a physical action of some sort to bear that information upon (in Electrical Engineering, we call that "modulation"). the way we normally do that now is that the transmitter of information pushes some electrical charge around and that motion of charge perturbs some other charge that the recipient has and his/her receiver senses that perturbation and, from that detection of the movement of charge, information is conveyed. perhaps, if the transmitter of information was big enough, he/she could wave around a planet or a star or something, and the disturbance due to changes in gravitational attraction to it of a planet that the recipient has could be detected and information could be conveyed upon that.

    trouble is, both physical actions (and the other fundamental actions, so i am told) propagate at the same speed of c. so, without having some physical action that is superluminal to carry your information, how can anything be communicated at a speed that is faster?

    superluminal communication
  16. Nov 6, 2008 #15
    We seem to be at cross purposes here and possibly I am to blame for that due to the slightly confrontational tone of my last post. You do not seem to have noticed that I do NOT accept the notion that superluminal communication is possible either. I was trying to demonstrate that IF such a notion is postulated then the consequences are ridiculous (and therefore the notion of superluminal communications should be rejected).

    You claim that I am "making unfounded conclusions about reality based on assumptions on what would happen if you built such a system". Clearly, in this quote below you are reaching conclusions about what would happen if such a system was built:

    Your response suggests you have a clear idea of what superluminal communications implies, but you have not made it clear what exactly you think it does imply.

    When I first read your response I thought you were suggesting that superluminal communications would not predict anything unusual and that it would shoehorn straight into the special theory of relativity without any modifications to the theory. I was trying to make the case that superluminal comms does not fit into an unmodified special relativity theory.

    It is often stated that there is no law of physics that forbids particles that move at greater than the speed of light as long as they as never slow down to subluminal speeds. It is easy to calculate that such particles have imaginary length, imaginary proper time and imaginary mass. In other words superluminal particles are purely imaginary in every way. These are the hypothetical tachyons. Personally I believe the laws of physics does forbid the existence of such superluminal particles and it almost certainly forbids the interaction of any such particles with normal matter making them useless for communications as they can not be detected.

    You reject the notion that “3) There is no absolute reference frame.” is a principle. Maybe I was wrong to call it a principle, but it is certainly an implication of "2) The laws of physics are the same in any inertial reference frame." In fact it just another way of stating principle 2 if we define an absolute reference frame as a special unique reference frame. The principle that the laws of physics are the same in all reference frames clearly implies there is no special, unique or absolute reference frame.

    I half expected I would be accused of selectively using reference frames because I was being lazy to keep the explantion brief and perhaps I could have been clearer. I will break down the explanation further here to try and make my reasoning clearer.

    Alice sends a superluminal signal to Bob at 2c in her reference frame.

    In Bob's reference frame, Alice is going away from him at 0.9c and from Bob's point of view, her signal appears to be going from the future to the past. This is a simple Lorentz transformation.

    If we accept the Lorentz transformation as valid then we see that Alice sending a 2c superluminal signal in her own reference frame is the same as sending a signal into the past in Bob's reference frame. Now this is the crucial part of the argument:

    IF Alice can send a superluminal signal to Bob in her own reference frame then Alice can send a signal backward in time in Bob's reference frame. IF Alice can send a signal backwards in time in Bob's reference frame then Bob can send a signal backwards in time in Alice's reference frame because the laws of physics are the same in all inertial reference frames.

    We can now do it all just in Alice's reference frame.

    Alice sends a superluminal signal to Bob (forward in time)
    Bob sends a reply to Alice backwards in time. (We have established that IF superluminal signalling is allowed by the laws of physics, THEN sending a signal back in time is also allowed by the laws of physics.)
    Bob's reply is received by Alice before she sends the initial query.

    Now to implications for free will:
    You will notice that I said "we probably have to reject the notion of free will and accept that the future is predetermined." The word "probably" means I am not sure. Certainly if superluminal signalling is allowed then are implications for the concept of free will that would have to be carefully evaluated. If we extend the idea of superluminal signalling to the idea of sending physical particles at superluminal velocities then there can be physical iinteractions in the past that are caused by events in the future. If those physical interactions (such as Bob killing Alice) prevent Alice from carrying out the actions in the futre that caused Bob to kill Alice in her past, then we have physical effects with no cause which is generally against the spirit of physics in general and not in agreement with actual observations.

    At this point I do not want to delve to deeply into free will as it touches on the very meaning of our existence and I don't want to go there. I think it would be depressing to find out that there is a proof that there is no free will as it makes life seem pointless. I do not claim to have presented a proof or refutation of free will. I merely suggested that there are implications for free will if superluminal signalling is allowed by the laws of physics. I hope I have made it clear that that I do not believe superluminal signalling is is allowed by the laws of physics. I have presented my arguments for why I think superluminal signalling is not allowed. I would be interested to see your arguments for why super luminal signalling is not allowed beyond the "it just isn't" argument.

    I agree. That is why I suggested only using Alice's clock. The events "sending query" and "receiving reply" happen in the same location and the order of those events is determined by one single stationary clock (Alice's) so that no ambiguity of the simultaneity of spatially separated events is involved.

    We both know how to synchronise clocks in different reference frames and how special relativity works and how to do a Lorentz boost so there is no need to establish all that again. All we have to do is accept the special theory of relativity and accept that the Lorentz transformation is a valid transformation and show that the Lorentz transformation of a superluminal signal or particle results in a signal or particle going backwards in time.
    Last edited: Nov 6, 2008
  17. Nov 7, 2008 #16
    Well the problem with our superluminal communication argument negating free will is contextual. That is you are using our intuition to disregard a flaw in your reasoning. That is the time it takes for a message to get from sender to receiver. That time can never be less than zero because there will always be a distance covered. So if it takes 10 seconds for a luminal signal to traverse from Bob to Alice, then a superluminal message sent at 2c will take half the time, not negative time. All you are doing is further dividing the distance by the rate which will give you ever decreasing time it takes for the message to reach it's destination, never giving you a negative number (going back in time). It is a funny conversation but unfortunately not an accurate portrayal of our universe.

    That being said, this isn't a endorsement of speeds greater than that of light, just that superluminal messaging isn't a solid argument against it.
  18. Nov 7, 2008 #17
    Hi Jake ( and welcome to PF )

    You are right that if it takes 10 seconds for a luminal signal to travel from Alice to Bob then a superluminal message sent at 2c will take half the time. You are wrong that "time it takes for the message to reach it's destination, never giving you a negative number (going back in time)." In a different reference frame a superluminal signal going forwards in time can become a signal going back in time. It is your intuition that is leading you to the wrong conclusion and intuition will nearly always let you down in relativity as it far removed from our everyday experience.

    Intuition is the romantic glow of a candle lit dinner. Maths is the cold light of day.

    Here is the maths:

    I am using coordinate notation such that (x,t) is (distance,time)

    Alice's frame (S)

    Event A: Alice sends a superluminal signal at (0,0)
    Event B: Bob receives the superluminal signal at (10,5)

    The signal travels 10 lightseconds in 5 seconds (2c) in Alice's frame.

    Bob's frame (S')

    Event A' = (0,0)
    Event B' = (x',t')

    Using the Lorentz transformation

    [tex]x^{\prime} = \gamma (x - vt)[/tex]

    [tex]t^{\prime} = \gamma \left( t - \frac{vx}{c^2} \right)[/tex]

    where [tex]\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}[/tex]

    Assume Bob is moving at 0.8c relative to Alice and units such that c=1.

    [tex]\gamma = \frac{1}{\sqrt{1 - \frac{0.8^2}{1^2}}} = 1.666[/tex]

    [tex]x^{\prime} = 1.666 ( 10 - 0.8*5) = 10 [/tex]

    [tex]t^{\prime} = 1.666 \left( 5 - \frac{0.8*10}{1^2} \right)= -5[/tex]

    There is the negative time interval: minus 5 seconds.

    I have shown that there is a reference frame where the superluminal signal travels backwards in time.
    As I mentioned before a basic principle of relativity is that the laws of physics are the same in all inertial reference frames. Therefore I only have to show that a superluminal signal can go backwards in time in one inertial reference frame to show that a superluminal signal can go backwards in time in any inertial reference frame, (if superluminal signals are allowed by the laws of physics).

    I have drawn the spacetime diagrams and done the maths. Everything else is handwaving.

    As soon as you admit superluminal signalling as a law of physics you allow signalling backwards in time as law of physics. That is an inescapable conclusion. (That does not mean I am endorsing superluminal signalling as a law of physics either.) The implications for free will are debateable.
    Last edited: Nov 8, 2008
  19. Nov 8, 2008 #18
  20. Nov 8, 2008 #19

    Jonathan Scott

    User Avatar
    Gold Member

    If superluminal messaging is only relative to some absolute "ether" frame, then it doesn't result in causality problems. However, if it were possible to send messages even slightly faster than light relative to any general moving frames, and Bob and Alice are moving relative to one another, then this would mean that messages moving at superluminal speeds relative to Bob could be travelling back in time relative to Alice and vice versa.
  21. Nov 8, 2008 #20
    I do not see how you can conclude Backwards or Forwards motion in time without make assumptions outside the rules of SR Simultaneity. You will need to be much more detailed to show how you do that.
    I made a conclusion not an implication.
    I concluded that by using SR rules (including causality) and adding new information from a fictitious FTL Communications Network we would be able to establish a Preferred frame.
    At least accurately enough to know that other frame clock synchronizations are well out of sync with realty and not useful for determining real Simultaneous events.
    A determination not possible within the realistic rules of SR.
    Since FTL communications does not exist in reality, SR alone implies that a preferred frame cannot be defined. But saying a preferred frame cannot be defined does not conclude that one may not exist, only that it cannot be defined.

    The problem I see is so many that think they know SR, somehow believe SR Simultaneity gives them permission to use any arbitrary frame as preferred.
    The Einstein SR Simultaneity Rule is clear, no frame of synchronization is to be trusted to define Simultaneous.
    Why would you only half expect that?
    Please explain how you established this using SR rules without arbitrarily picking a preferred frame and then using a frame that then gives the appearance of Backwards as also preferred and “real”.
    It requires violating the Simultaneity Rule twice, by using two frames as correct in defining "real" time. Simultaneity does not let you even pick one to do that.
    No, not without violating the Simultaneity Rule at least once maybe twice more.
    But you didn’t use just Alice’s clock you used other clocks as preferred to show events as moving forward or backwards with respect to Alice’s clock then switched back to Alice’s clock as preferred.
    Not within the rules of SR and completely unrealistic.

    So NO
    If you accept Lorentz Transformation and Simultaneity Rules as valid, you cannot imply anything about free will by violating Simultaneity three or four times.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?