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Faster than C

  1. Jul 29, 2010 #1
    I have recently been having a discussion with my father. We both know that it is impossible to go faster than the speed of the light. We were thinking what would happen if it was possible to exceed C. Would we be able to predict the future by seeing something happen on earth? Then go faster than C to another part of earth and tell your friend what just happened. Therefore predicting the future. Would you see time go backwards?
  2. jcsd
  3. Jul 29, 2010 #2


    Staff: Mentor

    You are free to write this science fiction story however you like.
  4. Jul 29, 2010 #3
    ...or just see absolute darkness? ... i dont think time is related to light in any way.
    And its possible, we just need to find out how to use the other 90% of our brains :P
  5. Jul 29, 2010 #4
    That is exactly what I say to my father. He just keeps bringing it up.
    Last edited: Jul 29, 2010
  6. Jul 29, 2010 #5
    ...anything is possible.
    I've always said:
    Impossibility is just reluctance to pursue.
  7. Jul 29, 2010 #6
    Nobody can answer this question because there is no proof since it is impossible to travel faster than light.
  8. Jul 29, 2010 #7
    Yea i know this. I stated it in my original post.
  9. Jul 29, 2010 #8
    Strictly speaking, that's true. In any case, SR states that a particle with rest mass cannot travel at at C because this would require infinite energy and such a particle would have infinite mass. However SR allows for superluminary particles, termed 'tachyons' which can only travel faster than C. If a such a particle existed, it would take infinite energy to slow the particle down to C. With less energy the particles would go faster. No one, afaik has found evidence for tachyons and mainstream physics does not, apparently, seriously consider they could exist. However, that's not the same as saying they are impossible.

    One philosophical problem with tachyons is time reversal and reverse causality (an event in the future causes an event in the past.) I never understood this argument since an imaginary application of tachyon "technology' would not violate causality. If wanted to to call a space craft one light hour away with my tachyon phone, I could make contact in less than one hour and get answer in less than two hours. The space craft couldn't respond to my question before I asked it. At best we would have near instantaneous transmission of messages, but not reverse causality from either observer's point of view.

    I don't think I'm wrong in this. If anyone believes I am, please point out my error.
  10. Jul 29, 2010 #9
    Think of it like this, what can you conclude from an impossible premise? Nothing. If you are going to break the laws to postulate something impossible then you can conclude whatever you want - you already broke the laws.

    It seems weird to people, because faster than c doesnt quite seem like an impossible premise. But I would liken it to this - what would happen if you had a square circle? What can you conclude from that? Nothing. Its an impossible premise to have a square circle, nothing can be concluded from that (you just have to make stuff up). Similarly nothing can be concluded from the impossible premise of faster than c (besides just making stuff up like the science fiction novel mentioned before).
  11. Jul 29, 2010 #10


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    Here's one example I've heard before. How would the Earth look if you're North of the North Pole?

    Impossible premise, how can any answer be justified?
  12. Jul 29, 2010 #11
    The OP does not pose a ridiculous question. To understand the possible, it's quite handy to examine the impossible.

    FTL travel does not necessarily mean backward time travel. First, consider that you are currently forward time traveling. If you left the Earth to travel close to c, when you returned you would be in the "future." (The more certain way to do this is to simply wait.) Comparatively more time would pass for those of us on Earth than that which passed for you.

    Forget about going faster than c for a moment. SR permits closed timelike curves. Wormholes might make backward time travel possible. Alcubierre designed a device that could be considered a "warp drive" that would allow backward time travel. (Even though no one knows how it might be built.) There's also the Tipler Cylinder (which also can't be built). I'm sure there are many others I don't know about.

    "Real" scientists think about time travel. Hawking has his chronology protection conjecture. Novikov has his self consistency principle. Carl Sagan asked Kip Thorne(sp?) about wormholes to explain FTL travel while writing Contact. Yes, I understand that it was a novel...

    As SW VandeCarr pointed, out there is speculation about tachyons. Quantum entanglement happens FTL, although it does not permit information to be transfered FTL. I found these interesting too.

    So my point is, why couldn't you just answer his question instead of trying to make him look stupid? We should feel free in science to ask "stupid" questions eg: Why did that apple fall?
  13. Jul 30, 2010 #12


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    No one is trying to make him look stupid. We are just pointing out what the question is similar to asking. It's trying to relate between two different mindsets. A physicist knows that the world stops at C, any speculation beyond that is just hearsay and conjecture. The layman doesn't appreciate just how strong of a condition the speed of light being a limit is. So the arguments may come off as sounding condescending or even childish, but they're really trying to convey the strength of the physical argument against FTL information travel.
  14. Jul 30, 2010 #13
    Simply & straight to the point:cool:
  15. Jul 30, 2010 #14
    OK Pengwuino, I can see your point. It must not be sinking in since I am a layman. I was under the impression that thinking outside of the box and questioning the dominant paradigms of traditional thought were how many major breakthroughs in science have occurred.

    c still might not be the absolute we currently think it is. Consider the Scharnhorst effect. Sure we can't test it, but it's still pretty exciting. Incidentally, it is neither hearsay nor conjecture, it's what one might call an educated guess. The Higgs boson, dark matter, dark energy, and others remain educated guesses. The truth is there is a lot we don't know.

    There are models created by legit physicists in which c is not the cosmic speed limit. I understand that they are not accepted by the science community at large, but they are conceived by bona fide physicists.

    Also please bear in mind that good science fiction has often been predictive rather than simply speculative.
  16. Jul 30, 2010 #15
    Hi there,

    Questioning is part of a good scientific process. But when a limit is set, not by the human minds, or by what we know now, but by the laws of nature, it might not bring anything more to try to go beyond.

    The speed of light is nothing like the biggest airplane built, knowing that it's just a matter of time to build a bigger one. The speed of light is a limit set by nature itself. In any frame of reference, the speed of light stays the same.

  17. Jul 30, 2010 #16
    Well said. I do believe it is unlikely that matter/information will ever travel FTL.

    I also believe it's important to understand why we are limited the way we are. Understanding that traveling faster than c means, an end to causality, infinite energy, infinite time (with finite acceleration), infinite acceleration (with finite time), and yada yada yada, is a valuable tool for understanding how the universe is put together. This understanding would never have come if the question hadn't been asked in the first place.

    I'm glad some one else understands the value of questioning.
    Last edited: Jul 30, 2010
  18. Jul 30, 2010 #17
    Hi there,

    It's funny. I was just talking about that with a collegue. Everyone agrees that matter and photons cannot travel faster than the speed of light. But information is nothing tangeable, therefore the sky is the limit.

  19. Jul 30, 2010 #18
    Although I agree that the sky is the limit...

    Perhaps we have different definitions of information. By information, I mean physical information. I'm talking about the state of a system. I'm talking about Claude Shannon, information theory information. I'm not talking about anything intangible.

    Both classical and quantum information are limited by the same constraints as matter in regards to c. They are not able to travel faster than it (afaik).

    What type of information has the potential to travel FTL?
  20. Jul 30, 2010 #19
    Hi there,

    Through this thread, I went to read a bit more on the web. I found this that could be interesting: http://en.wikipedia.org/wiki/Faster-than-light.

    If electrical pulse envelopes can be recreated at the end of cables faster than the speed of light, why not information that goes with it???

  21. Jul 30, 2010 #20
    fatra2, you're talking about group velocity right? In that case, only the leading part of a pulse envelope travels faster than c. The information contained in the pulse does not. The knowledge that information is coming arrives faster than c, but not the information itself.

    Having said that, I still keep an open mind. I think there are several possibilities that we have not investigated fully that might hold the potential for FTL. I like to think it's possible, just not very probable.
  22. Jul 30, 2010 #21
    Draw a space-time diagram. Add in lines for events happening in the normal world and see how that looks to the traveller's time axis.
  23. Jul 30, 2010 #22


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    If the other craft is travelling away from you at high speed, when it sends its tachyonic reply back to you (at the same faster-than-light speed relative to it as your message was relative to you), then, because of relativity of simultaneity, the reply is backwards relative to your own time even though it is forwards relative to the sender. So you get your answer before you send it.

    That's one reason for suspecting tachyons can't exist.

    See tachyonic antitelephone for more details.
  24. Jul 30, 2010 #23
    I agree that tachyons are "poison" to the Standard Model and that they probably don't exist. However that's not the same as saying that they are impossible. In my example I considered the space ship to be effectively stationary wrt the caller during the time period involved.
  25. Jul 31, 2010 #24


    Staff: Mentor

    Unless you can guarantee that all spaceships will always be stationary wrt the caller then that doesn't eliminate the problem with causality.
  26. Jul 31, 2010 #25
    True. We could just suppose that the tachyon link would be perceived to fail if it wasn't well contained within the same reference frame. Consider a link between an orbiting space colony in the Alpha-Centauri star system and the earth where local motion might be negligible compared to the distance. Any transmission that happens to violate causality would simply be perceived as cutting out or as a dropped call.

    We can discuss the details of some speculative superluminary communication. My point was that we can say it's inconsistent with the Standard Model, but saying it's impossible goes too far.
    Last edited: Jul 31, 2010
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