1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Confusion with faster than light

  1. Aug 5, 2009 #1
    Confusion with "faster than light"

    Okay first off hi, this is my first post on these boards, let me clear a few things up here, I'm a bit of a laywoman when it comes to all of this so please go easy on me. I've been into Physics for a couple of years now as a bit of light reading to keep my mind active, I got into it cause of an ex-boyfriend, but the story isn't important.

    I've been talking with a couple of people, who seem to be quite scientifically minded, about faster than light travel and/or communication... the problem I'm having is, as i've read and learnt that nothing can travel faster than light, now I understand that in relative terms things can appear to, but they do not.

    Either way I've been saying this to them all morning and they refuse to acknowledge what I'm saying as correct and that my understanding of relativity is causing my blindness to this.

    What I want to know is, in an easy way to understand, and a way I can explain to them:

    Can anything travel faster than light other than light (photons and gluons I believe?) and if not, why?

    I always believed it was to do with mass etc... but i'm being told that relative speeds of faster than light still count as faster than light, even if in their own frame of reference they are not faster than light, this makes no sense to me, how can it be travelling faster than light when it's only relative? Surely that would make experiments confusing?

    To make things worse they are now talking about Quantum Mechanics and how I'm Einsteinian for not knowing the details of Quantum Entanglement, when I've never claimed to be an expert on any of this, just curious.

    So if you guys/gals could help me out here with my understanding that would be really appreciated.

  2. jcsd
  3. Aug 5, 2009 #2


    User Avatar

    Staff: Mentor

    Re: Confusion with "faster than light"

    A few points:

    1. "Motion" of an object is something that is measured relative to other objects. There is no absolute motion through space because....see previous sentence.

    2. No object with mass may travel at or faster than the speed of light. The faster the object moves, the more difficult it is to accelerate, when viewed from another refeence frame. This is why particle acclerators can give massive amounts of energy to tiny particles and never get them moving faster than C.

    3. If you use an inertial navigation system (one that detects acceleration and multiplies by time to get speed) it may record a speed of greater than C. That means it was programmed wrong. It was programmed using Newton's physics, not Einstein's.

    4.You're standing on earth and you see a star 4 light years away. You decide you'd like to visit and climb into your rocket ship, and quickly (it might take a year) accelerate up to just a little less than the speed of light. When you get there, your watch says that only two years have passed. Did you travel at twice the speed of light? No, I just said you traveled at just under the speed of light.

    5. I don't know what you mean by "relative speeds"....see #1.

    6. None of this has anything to do with quantum entanglement.
    Last edited: Aug 5, 2009
  4. Aug 5, 2009 #3
    Re: Confusion with "faster than light"

    Hi, wow thanks for such a quick reply.

    Well about the relative speeds, the argument is basically saying that as an accelerating observer objects appear to be travelling beyond c, which I can understand to a degree. But from all I've ever read is that nothing can travel faster than c, and the point I'm trying to get over is this. But every time I'm having my "lack of understanding relativity" thrown at me because of the apparent faster-than-light speeds observed by the accelerating observer.
    To me this isn't a valid way of claiming faster-than-light travel exists, it still remains that nothing with mass can travel faster than c, correct?

    I understand in accelerators that particles have been pushed around at over 99% c, but never c or above.

    The thing that is bugging me now is QM and Quantum Entanglement, this is a grey area for me and I honestly don't how how to reply to the point that when I say "Nothing can travel faster than c" they respond with "Stop being so Einsteinian and look at quantum entanglement" so now I'm reading up on this and getting confused by the 2 sides of an apparent argument of whether FTL travel/communication is possible with this...
    It appears that it is "apparently" possible, but it is not a valid argument because of randomness of some sort?

  5. Aug 5, 2009 #4


    User Avatar

    Staff: Mentor

    Re: Confusion with "faster than light"

    See late edits in my previous post....
  6. Aug 5, 2009 #5
    Re: Confusion with "faster than light"

    1. Okay, I understand this.

    2. So it is not possible to exceed c when measuring the particle in it's own reference frame, correct?

    3. The problem is it's not a calculation, it's saying that if an observer sees something exceeding c because the observer is moving that makes it look like the object is faster than c, and I understand that is a valid measurement for the accelerating observer but is not relevant in a FTL argument...

    4. To travel faster than the speed of light would this not imply time-travel?

    5. The observer can see object A moving >c because the observer themselves is accelerating. So the speed they see is relative? Is that more understandable?

    6. How so? I'm just reading a thread (https://www.physicsforums.com/showthread.php?t=317012 - and some others) and trying to get my head around it, it would be very much appreciated if you could maybe shed a little on why this has nothing to do with FTL travel/communication? As in my last post I mentioned the arguments about it to do with randomness etc?

    Again, thank you. It seems I've got a lot of reading to do before I can progress my argument that nothing can travel faster than c as they try to bring in irrelevant QM and theories to try justify their belief...

  7. Aug 5, 2009 #6
    Re: Confusion with "faster than light"

    Just to add a quote to show the point of view from the other side:

    "If something appears to be travelling faster than light, it is travelling faster than light"

    This is what I have to deal with :grumpy:

    Also when saying "nothing can travel faster than c" being told to look at quantum entanglement, which is a stalemate for me as I'm still reading up on it!

  8. Aug 5, 2009 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Re: Confusion with "faster than light"

    Read some more. There's nothing "travelling" in quantum entanglement when such a determination from a measurement is made. In fact, read any of the EPR-type experiment and show where they claim that SR has been violated.

  9. Aug 5, 2009 #8
    Re: Confusion with "faster than light"

    You'll have to excuse my need to be spoon-fed here, but could you elaborate on that a little, or provide a link to something that can help me get my head around it, I'm currently going through Quantum entanglement on Wikipedia and forum posts.

    It seems to be more about information communication and it seems that there is a collapse which allows FTL but the result is random? Or am I just totally lost and need to go back to step one?

    Apologies if I'm being a bit stupid here I think I'm maybe trying to take in too much at once.

  10. Aug 5, 2009 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Re: Confusion with "faster than light"

    Look up Eberhard's “no-signaling” theorem.
  11. Aug 5, 2009 #10


    User Avatar
    Homework Helper

    Re: Confusion with "faster than light"

    A wave within a beam of light composed of 2 frequencies can move faster than the speed of light (until it reaches the either end of the beam of light). An observer can see two beams of light approaching each other at faster than the speed of light (each beam moving at the speed of light). An obsever could calculate that the speed of an object as it reached the event horizon of a black hole is the speed of light, but could not actually observe the object once that object was at or within the event horizon.

    The question with quantum entanglement is if the two object always have had some potential but yet unobserved state, or if observing one of those objects actaully had an affect on that object and instantaneously affected the other regardless of distance.
    Last edited: Aug 5, 2009
  12. Aug 5, 2009 #11
    Re: Confusion with "faster than light"

    Thanks for all the brilliant replies and understanding of my poor knowledge in the area.
    I'll get reading up on all of this shortly!

    I'm starting with this thread:
    seems to be just what I need.

  13. Aug 5, 2009 #12
    Re: Confusion with "faster than light"

    Information cannot travel faster than light. That is the bottom line. You can measure some things to be speeds greater than c (for example, the difference in velocity between two space ships traveling in opposite directions), but never can you measure any "influence" faster than c.

    The reason is founded in special relativity. It has to do with simultaneity.

    In special relativity, we call points in spacetime events. Events are ordered. But the difference between Newton's time and Einstein's time is that events in SR are only PARTIALLY ordered. That means, given two events A and B, one of the following must be true:

    * A happens before B
    * B happens before A
    * A is B
    * The order of A and B varies by observer

    It's very clear to everyone that Newton was born before Einstein. But if two babies are born on distant planets within minutes of each other (given that the two planets are a lighthour away), it is no longer possible to prove which baby was born first. Two different observers moving at different speeds will see the opposite results. In fact, you can even have an observer who claims the children were born at the same instant.

    If you're into math, check out http://en.wikipedia.org/wiki/Partial_ordering" [Broken].

    The ordering is determined by the light cones of events. The light cone is the spacetime that could be reached if a wave of light was emitted during an event. The light cone has four parts: the past, the future, the present, and elsewhere (outside the cone).

    When stuff starts moving faster than light, our light cones get messed up. The whole purpose of the light cone is to order events, but if an event can influence another outside of its light cone, the light cone fails to serve its purpose and we need to scrap it and come up with a new theory.
    Last edited by a moderator: May 4, 2017
  14. Aug 5, 2009 #13
    Re: Confusion with "faster than light"

    The ability to travel faster than light also makes time travel possible. The current understanding of conditions around rotating black holes says that a trajectory through the ergosphere can pass close to a black hole--but not close enough to be trapped--and travel back in time.

    Is this the same as traveling faster than light? Up to you to decide. Certainly there are external observers who will conclude that the object traveled faster than the speed of light but did not travel in time.

    Is it possible to build a spaceship that can do this? You would have to spend hundreds of years to reach a black hole, and a lot of time (in your frame of reference) going around it, before returning to earth before you left. But current understanding of physics does not say it is impossible. Stephen Hawking recently raised what turns out to be a nasty complication to such travel. If a rotating black hole can be used for time travel there are also closed paths where a photon can return to its starting point--and time. Interrupting such a path would be equivalent to passing through an infinitely* powerful laser beam. Better polish your spaceship really well.

    I think that the only way past these closed timelike loops would be to drop enough mass into the black hole, precisely timed so that you would not encounter the loops. How do you get out? Not a problem, observe the past of the black hole to see when something enters. If you are traveling backwards in time, you should be able to see the light echoes off nearby clouds and objects from flares when something enters the black hole. Better pick a flare that is much brighter than your spaceship being annihilated, just in case.

    * Technically not infinitely powerful, but there is no better word. The actual intensity you encountered would be one choice from all the possible values between zero and infinity due to Heisenberg uncertainty.
  15. Aug 5, 2009 #14
    Re: Confusion with "faster than light"

    Wow those 2 replies were brilliant, what I wanted, and I actually understand them perfectly!
    Thank you so much :approve:

    Last edited by a moderator: May 4, 2017
  16. Aug 5, 2009 #15
    Re: Confusion with "faster than light"

    Is it not true that saying 'faster than light' is like saying 'a circle of more than 360 degrees circumference' or 'a rectangle with more than 4 sides' ?

    : The definition of what light is, in relativity, excludes the possibility of 'faster than light'.
  17. Aug 5, 2009 #16
    Re: Confusion with "faster than light"

    In a sense, this is true. It's still good to know why we have given it that definition.

    And speaking in unforgivable generality, whenever you generalize a concept, the potential arises that old definitions and theorems break down. In Euclidean geometry, the inner angles of a triangle sums to pi radians. But on a manifold, a "triangle" (obeying generalized ideas of what constitutes a Euclidean triangle) can sum to more or less than pi.

    It's not unreasonable that a consistent theory involving tachyons (faster-than-light) particles can be created. But such a theory probably doesn't match the real world.
  18. Aug 5, 2009 #17


    User Avatar
    Science Advisor

    Re: Confusion with "faster than light"

    You seem to be misunderstanding "reference frame". An object's speed, measured "in its own reference frame" is always 0! That's what "its own reference frame" means.

    I don't understand what you are saying here. What is say that?

    If you use certain formulas, the 'time' variable may become negative. That does NOT, by itself, imply time-travel. By the way, we are always "time-traveling"- just in one way and at one speed!0

    You were told before- all speed is relative. As for "moving > c because the oberver is accelerating"- I am not qualified to comment on that.

    Quantum entanglement, might, under some conditions, lead to the possiblity of communicating faster than light. I think the statement that "None of this has anything to do with quantum entanglement" was in regard to the other way around: "speed is less than c relative to any (non-accelerating) frame of reference" does not lead to "quantum entanglement".

    ?? Who do you mean by "they"?
  19. Aug 5, 2009 #18
    Re: Confusion with "faster than light"

    That's a fascinating answer Tic-Tacs. Thank you for that. I hope you don't mind my asking this further question: I see how the forms of euclidean geometry may not apply if the conventional 2 or 3 dimensional space is not applied, as in your manifold. And I know that there is non-euclidean geometry too, where a straight line is the longest distance between two points etc.
    But is not putting a triangle onto a manifold ( in horrible generality ) impossible with relativity? I mean that you cannot say ' what if ' with time and space, You have to accept what is. So taychons may be possible conceptually, but if they don't correspond to any observable reality, then they are no more than conceptual things.
    In other words... there is a point at which theory departs from observable reality, not because it is unobservable, but because it no longer corresponds to reality in any way.
    From where then does the notion of faster than light originate?
  20. Aug 5, 2009 #19
    Re: Confusion with "faster than light"

    Let me first clear up one thing here. The current standard model of (astro)physics says that during inflation (a very small fraction of a second after the Big Bang) the universe expanded in size much faster than the speed of light. In other words, two particles, well, let's make them very high energy photons, in the moments after the Big Bang could not communicate because space was expanding between them faster than light could traverse the distance.

    Ever since, more and more of the distant universe has become visible. One of those early photons may have traveled 14 billion years to get to a location (where Earth is now) that was one inch away just after the Big Bang.

    A corollary to this is that there is a huge amount of the universe which is outside the (14 billion light year radius) sphere we can see today. Some of that volume will eventually be visible from Earth. Other parts of the universe are moving away from the Earth at "faster than the speed of light" and will never be seen. There is nothing special about those volumes--that we know of. Just that the expansion of space between them and the local visible universe will prevent them from ever being seen.*

    So in that sense, most of the universe is moving faster than the speed of light with respect to the Earth, and will continue to do so.

    If faster than light (FTL) travel is possible, we can eventually visit those far places. But is FTL travel possible? A few years back, the American Physical Society came down on the yes side. Why? A concept called the Alcuberrie warp drive. (See http://en.wikipedia.org/wiki/Alcubierre_drive for more information.) The Alcuberrie drive does not violate special or general relativity. (Doesn't mean one can be built, just that it is consistent with SR and GR.) But the APS had another reason for voting that way. The joke at the time was, relativity, quantum mechanics, and causality: Choose two.** Work on a ToE (theory of everything) that includes both relativity and quantum mechanics at all scales is still going on, and causality is still often the odd man out. Most physicists today are willing to try to deal with "local" physics--black holes not welcome--and are willing to solve that problem before dealing with black holes. Unfortunately another joker just popped up. The remnant of an evaporating black hole may be identical to the fundamental strings in string theory. (Technically one variation of M-theory.) If so, GR cannot be ignored on any scale or at any energy level. :-(

    * To be complete, most current theories about the expanding universe say that some areas will never be visible. Obviously if the expansion of the universe slowed or stopped this would not be true. There are other solutions which have the universe continuing to expand, but at a slower and slower rate, so that the entire universe is eventually visible from anywhere inside.

    ** Causality is the rule that says that no event precedes its cause. Time travel and FTL are two obvious sources of global causality violations.
  21. Aug 6, 2009 #20
    Re: Confusion with "faster than light"


    I hope I am interpreting what you are saying correctly, but to my level of understanding "putting a triangle onto a manifold" is precisely what General Relativity does... (the manifold being the 4-dimensional space-time being bent by gravity/acceleration)

    There are many videos showing the 2-dimensional equivalent (which can be much more easily visualized) for this. It just so happens that these effects on our "close-to-euclidean" universe are pretty hard to show (though i believe have been consistent with experiment to this day).

    In fact, I think that Feynman dedicates an entire chapter about this in the second volume of his physics lectures... something i'm sure everyone here would strongly suggest reading if you have any interest in physics :)

    as for the discussion at hand:

    Hayley, you shouldn't be too hard on yourself... sounds to me like you have a pretty good understanding and are smart enough to not just take someone's word for it, and go out and try and understand it.

    though I think the answer to your initial question has been pretty-well answered, i will emphasize the following points:

    1. as pointed out before, a particle's velocity in it's own reference frame will by definition have a velocity of 0.

    2. in said particle's reference frame, any other particle (let's say excluding "tachyons") cannot travel faster than c in that particle's reference frame (in other words, relative to that particle).

    3. in said particle's reference frame, two other particles can be moving relative to one another, in that particle's reference frame, faster than the speed of light.

    4. quantum entanglement is related to FTL because (in laywoman's terms) it involves the possibility that by measuring one particle in a two particle entangled system, you can instantaneously effect the state of the other particle... now the tricky thing to ask yourself here is are you actually allowing information to travel faster than light, because in order to actually send bits of information you have to have information about both "sides" of the entangled system in order to actually extract the information... if that makes sense.

    i believe, though, that this is still up to some theoretical debate... debate I know nothing about. There are lots of "EPR and EPR-esque" thought-experiments that you can find all over the internet though that will go into greater detail about these entangled systems and how it relates to the integrity of SR and GR.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Confusion with faster than light
  1. Sound faster than light? (Replies: 16)

  2. Faster than light? (Replies: 1)

  3. Faster than light (Replies: 11)