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The FTL hornet's nest

  1. Jan 22, 2017 #1
    Okay, against my better judgment, I'm going to go ahead and kick the hornet's nest. Superluminals. FTL. Whatever you want to call it, moving faster than the speed of light.

    One of the things a lot of physicists have a conniption over is that FTL would mean causality violations. Special relativity goes screaming to mommy Physics that you can't do that because it puts you at odds with the Future Light Cone and that since you've gone and broken the speed limit, there would be reference frames where the object sent arrives before it actually gets sent. Then the whole universe breaks down and implodes and there's an enormous cosmic blue screen of death. Universe.exe has stopped responding.

    Okay, not that last part. (If the universe actually ran on DOS, it would've crashed way before now.)

    But the reason I'm nailing this post to the wall is to ask a basic question: why? To be honest, the whole idea sounds, well, ridiculous. Common sense logic would indicate that the fastest anything could travel would put it at its destination instantly, not before it left. Which means your travel time is effectively greater than or equal to zero. Never less than.

    What I'm getting at without attempting to be judgmental or annoying to anyone is that isn't it entirely possible that FTL travel is entirely possible, even reasonable, but with our very limited understanding of such things it only seems impossible? After all, once upon a time, it was thought that the sound barrier was insurmountable, too. It just seems like this is an over-reliance on mathematical models created only on suppositions rather than something that's been completely thought out.

    Please, no flames, no equations...just an analysis? I'd like to see where the holes in this logic are.
  2. jcsd
  3. Jan 22, 2017 #2


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    Human common sense develops on Earth where everything is slow. You cannot use it to understand special relativity.

    Something that is instantly (or FTL in general) for one observer is going to the past for other observers.

    FTL without time travel means physics has to be different for different observers. This is a mathematical statement - you can prove it, it is as unbreakable as 2+2=4. That scenario is not completely impossible, but no measurement ever found a hint of that.

    We have many threads discussing this already.
  4. Jan 22, 2017 #3


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    "Instantly" in one frame is "back in time" in another frame.
  5. Jan 22, 2017 #4
    In terms of your proper time, you can "in theory" approach zero travel time but not attain it. That is the best you can do. I think that answers your question ;)
  6. Jan 22, 2017 #5
    The sound barrier was never unsurmountable. Surviving it was.
  7. Jan 22, 2017 #6
    OK, here's an analysis. You haven't bothered to learn the first thing about what you are criticising.
  8. Jan 22, 2017 #7


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    Here's a causality violation scenario:
    You are traveling at 0.866 c relative to the Earth. After 1 year, by your clock, you sent an instantaneous message to another space ship which according to you, is .866 light years behind you and right next to the Earth. We will also assume that you and the Earth synced your clocks when you passed Earth. Since, according to you, the Earth clock has been time dilated and will read 0.5 year. The space ship you send the message to hands it off to the Earth as he passes. Thus the Earth gets the message when its clock reads 0.5 years.
    The Earth, in turn, sends an answer to the message to a space buoy that is 0.433 light years away from the Earth, which where you will be after 0.5 years on the Earth clock traveling at 0.866 c. And due to time dilation, according to both the Earth and the Buoy, your clock reads 0.25 years. Not only that but according to your own clock you passed the buoy, which due to length contraction, you measure to be 0.2165 light years from the Earth when your clock read 0.25 yrs.

    But this means you receive the answer before you sent the message.

    As far as the sound barrier goes, it was actually broken before it was broken. There were already a multitude of examples of objects that traveled faster than sound before the sound barrier was "broken", and their was no law that forbid it. The "sound barrier" problem was a matter of aerodynamics and had to do with designing an airfoil-control surface combination that would allow for controlled flight at supersonic speeds.
  9. Jan 22, 2017 #8


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    A word you can't use in relativity without qualifiers.

    You might want to check out the sticky thread at the top of this forum with a link to experimental evidence for special relativity. We don't just make thos stuff up...
  10. Jan 22, 2017 #9

    Mister T

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    What common sense dictates is not relevant. What's relevant is how Nature behaves.

    The fallacy in your logic is this: The fastest anything can travel would put it at its destination after it left, not instantly.

    Consider two events, ##E_1## and ##E_2##. These events can be separated in such a way that a light beam leaving ##E_1## arrives at ##E_2##. In such a case we can definitely say that ##E_1## occurred before ##E_2##. (We would say the two events have a light like separation.) Likewise if a signal travels slower than light between them. (We would say the events have a time like separation.)

    However, if a signal would have to travel faster than light to get from ##E_1## to ##E_2## then we cannot say for sure which event happened first. (We would say that the events have a space like separation.) For some observers the events are simultaneous, for others ##E_1## happens first, and for others ##E_2## happens first.

    So the thing that bothers us is when ##E_1## is the cause of ##E_2##. We cannot make sense of a universe where an effect precedes its cause. Can you?
  11. Jan 22, 2017 #10
    I know the models aren't made up; they're best explanations for what we see and know. And, for what it's worth, I'm not just hurling this stuff out blindly, either. It's...well, call it a curious mind attempting to make sense of what appears to be a plot hole. Experimental results can be unintentionally skewed to support the wrong answer; being unaware of a particular variable or contaminant, or even just a failing of imagination. Stepping back far enough to see a bigger picture. Please understand, I'm not trying to be obtuse or combative, and I'm not trying to step on anyone's toes here.

    The problem as I see it is that's a case of violating causality at all. I would say it's reasonable to state that when something is observed to happen makes no difference whatsoever to what actually happened. Just because we spot a supernova next week that happened a billion years ago doesn't change a thing; it happened when it happened. Likewise, had we received an FTL signal of the event the microsecond it happened, it doesn't send us back in time -- it just means that we knew the instant it occurred.

    What every single argument I've read hinges on is observers. Since a signal from A to B moved faster than light, then to observer X, it seemed to arrive after it left, to Y it happened at the same time, but to Z it arrived before it left. So what? This seems to be as off-base as the "if a tree falls in the woods and nobody is around does it makes a sound?" Of course it does. What occurs in the universe has absolutely no care at all if there's an observer there to notice it.

    Let's say I can send a signal at infinite speed. Instantaneous transmission, regardless of distance. I push a button. That signal goes a light year instantly to a relay, which takes no time to route it on a second leg of another light year, to another instant relay, back to me via the same path. Four light years' worth of distance instantly. To me, though, I see the same thing: the light bulb pops on as soon as I push the button. It doesn't go on before I come into the room, or even a second before I push the button. It's at the instant I trigger the circuit. In a similar vein, let's say I teleport myself to Sydney. Just because it takes me zero time to cover the range doesn't mean that there would be two of me for any span of time; it means I vanish here and appear there in the same instant.

    I would agree without reservation that it's impossible to move anything, particle or wave, faster than instantly, because then you really are violating causality. But instantaneous transit - transit without transit time - seems entirely within the bounds of the rules. Of course, it could be practically impossible because of energy requirements to do so, but that's another can of worms entirely. Again, I'm really, truly not trying to get anyone riled up or offend anyone. I'm just trying to make sense of something in our universe.
  12. Jan 22, 2017 #11


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    Our universe doesn't work that way. "when it happened" depends on the observer. You are stuck in a Newtonian world view and then you try to use it in special relativity. That cannot work. You'll have to learn about special relativity if you want to understand it. You cannot "see a bigger picture" if you don't even know where the painting is.

    You got some good explanations here in the thread. If those don't help, or if you ignore them, there is no point in continuing this thread.
  13. Jan 22, 2017 #12


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    You are misunderstanding the arguments. The observers are only there because they're a convenient way of describing the physical phenomena in question without using the mathematical language of coordinate systems. To understand what's going on, I would recommend Taylor and Wheeler's "Spacetime Physics".
    The "same room" part is fine. But for your scenario to work it also necessary that the first, second, and third relays all trigger at the same time as well. Before you say that that is possible, you must propose a definition of "at the same time" that works no matter what speed your arrangement of relays, button, and bulb is at rest or moving at.

    An essential requirement for this definition of "at the same time" to work is: When it is New Years Day 2017 you push the button and send a flash of light as well as the instaneous signal to the first relay. We verify that the instaneous signal arrives at the first relay at New Years Day 2017. One year later, New Years day 2018, you push the button again. This time, the instantaneous signal must reach the relay at the same time as the flash of light from the first button push - it had one lightyear to travel and one year to do the traveling.

    If you try it (and I do mean "try it", not attempt a proof by emphatic assertion) you will find that it is not possible.
  14. Jan 22, 2017 #13


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    Well, it isn't like it hasn't been tested. If you apply a really large force to a particle for a little while, do you know how fast it can be accelerated to? Do you know why? If you are in a spaceship going 1 ft/sec slower than C and walk forward in it at 2 ft/sec, do you know what happens? I don't really like the causality violation argument either, but do you even know how real objects/forces/interactions behave at really high speeds?
  15. Jan 22, 2017 #14


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    Edit: Oops. I see that the thread was closed while I was composing a reply
    You may want to re read your original post in this thread. Words like "conniption" and phrases like "screaming to mommy" are combative. You ask us not to flame you, but you are the one who brought in the inflammatory language.

    That is correct, but the fact that you think this is an argument indicates that you don't understand the issues at all.

    Again, instantaneous in one reference frame is backwards in time in another reference frame.

    I think this is the beginning of your misunderstanding. What every argument hinges on is reference frames and the laws of physics. The term "observer" is just shorthand for "the inertial reference frame in which some object is at rest".

    The "so what" is that the laws of physics are equally valid in Z's reference frame. Therefore, if it arrives before it left in Z's frame then the laws of physics permit causal influences to go backwards in time.

    Again, instantaneous in one reference frame is backwards in time in another frame. And if the laws of physics in that other frame allow signals to go backwards in time then those same laws of physics allow signals to go backwards in time in your frame.

    For I think the fourth time, instantaneous in one frame is backwards in time in another frame. In that frame there are two of you for a span of time, and the laws of physics are valid in that frame.
    Last edited: Jan 22, 2017
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