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I have a dim understanding that modern physics believes that faster-than-light travel is not possible, full stop. I don't quite understand the equivalence, but FTL is the equivalent of time travel, and since we believe that time travel would violate causality, we believe that FTL is impossible no matter what mechanism you propose (teleportation, hyperspace, whatever).

Even with the pretty diagrams I'm not sure I get it.

http://www.theculture.org/rich/sharpblue/archives/000089.html

I've also read that FTL shouldn't be impossible if the whole universe had a common frame of reference, but according to the theory of relativity, there is no such common frame of reference in the universe. But I've read a couple of discussions that say that maybe FTL would be possible if "hyperspace" or "subspace" travel imposes a common frame of reference. Again I don't really understand this.

http://www.calormen.com/star_trek/FAQs/warp-faq.htm

I'd love it if someone with physics understanding could explain it in a way that my poor grasp of physics can understand, using car analogies or whatever.

You could not travel somewhere and return before you left, therefore there is no time travel.

No, you are incorrect. See the first answer here, or a more thorough explanation here. When reading the latter, keep in mind that "FTL signal" and "FTL spacecraft" are equivalent for these purposes.

Causality doesn't say that it was time travel to appear to leave after you arrived at a location.

Special Relativity equates FTL to time travel, but that would be a violation of causality in any case. Causality means that all causes must precede their effects. Special relativity says that the ordering of events is dependent on the observer. The combination of the two means that in order to preserve causality, all causes must be observed to precede their effects in all reference frames. Since the destination in your example would observe the travel of the ship in reverse order, that would violate causality. Visiting your own past is also possible under SR, but I'll defer to the linked explanations there.

I may not have been clear but what I mean is that an ansible can allow any two other observers (one on each side) to communicate FTL in a flat space-time and these observers do not need to be at rest (even if the endpoints of your ansible must be at rest) and this is all that's needed to run into problems. Also, the speed of the observers does not necessarily need to be close to the speed of light and the farther away the two observers are the lower the minimum necessary speed for causing violations. A more concrete example:

Let's say observers A and B are at point (0,0,0) and C and D are at (1000ly,0,0). A and C are at rest relative to each other and have an ansible connection between them that will break if they are not at rest anymore. B and D are not in anyway restricted so they can travel and any slower-than-light speed that they want. So, B starts going at 0.5c relative to A and to the left of A and and D starts going at 0.5c relative to C and to the right of C. Thus B and D are going at a significant speed in each other's frame. A sends a radio message to B, which immediately relays it through the ansible to C, which sends it via radio message to D. Then it goes the other way from D to C by radio, through the ansible to B and by radio from B to A. So, what you have is A and D communicating much faster than light, while also moving thus easily allowing them to send messages into their past. In this case the fact that the ansible only works if the two endpoints are at rest relative to each other does not help you unless you also make up something that prevents A and C from communicating with nearby observers.

Another example with a somewhat different setup but still exploiting the same problem is shown here, with pretty space-time diagrams: http://www.theculture.org/rich/sharpblue/archives/000089.html. In this case you have A and B with an ansible and C and D with a separate ansible. A and B are at rest relative to each other. And so are C and D. However A and B are moving relative to C and D and are able to communicate by regular methods, which violates causality.

they all hinge on the principle that for something to exist it has to be observed. To me that's as stupid as a man claiming the sun doesn't exist when he can't see it in the sky.

No, it has nothing to do with that (which is indeed garbage). You actually do influence your past. For example you can send your FTL message to a satellite and have it send it back to you and depending on the speeds of the message and the satellite you can receive the message before you sent it. This can create a paradox because you can setup your experiment so that you only send the message if you haven't received it at time t. But after you bounce it off the satellite it can arrive at time t-1, which would cause you not to send it, but if you didn't send it, then how did you get it in the first place?

FTL communication resulting in being able to send a message to your past follows from special relativity. Here are a few resources that show the details of how it works:
http://www.theculture.org/rich/sharpblue/archives/000089.html
https://en.wikipedia.org/wiki/Tachyonic_antitelephone#Two-way_example
http://www.physicsguy.com/ftl/html/FTL_part4.html (unlike the others this also covers the case of non-flat spacetime, such as warp drives or wormholes)

This is better: http://www.theculture.org/rich/sharpblue/archives/000089.html

I'm going to skim over the irrelevant stuff about the speed of sound: the bullet arrives before the sound does, but not before the sound is made; you have to shoot the bullet faster than light to do that.

  It's all to do with relativity and time dilation effects. All movement causes your local notion of spacetime to bend; it's not noticeable however, unless you get up to a decent fraction of c. This bend causes time dilation, which means that time will pass more slowly for you than for someone travelling at a lower speed. (This is not only proven, it's used everyday in the GPS system) The faster you go, the greater the effect. If you could manage, by some miraulous event, to attain 1.0c, time will stop for you, your field of spacetime will have flattened into a two dimensional plane, and if you can figure out how to stop when you have no time in which to hit the brakes, you may find that many millenia have passed in less than the blink of an eye for you.

  This is all just fine and dandy from the universe's perspective. It doesn't start to get really weird until you try to go faster than light. At this point the rate at which time passes for you would drop below zero and go backwards, only to straighten out as you decelerated. What this would mean is the sort of thing I wouldn't try to think about without a good bong-load first. Whatever it's like on board, this bending of spacetime frames seriously snarls up the order of events in the universe. Below the speed of light, two people in different reference frames might disagree about the order in which two unrelated events happened, but they'll both agree that the causes preceded their effects; add FTL into the mix, and that's out the window.
  Add in the fact that there is no universal time reference to keep things straight, and your time's all out of whack.

  Now here's the gist of it, and most SF tries to handwave this away to keep Pandora's Box of Plot Problems closed: If you leave point A and then arrive at point B faster than the light you gave off at A, you will arrive before you left according to some frames of reference. (This hangs up a lot of people into thinking it's just a "trick," as if it were some kind of illusion, but wait!) Now, if you turn around, and go back to point A in the same FTL manner, you will arrive before you left according to all reference frames everywhere, including your own. You'd be able to wave goodbye to yourself as you left. What would happen if you told yourself not to go, and succeeded? It makes my head hurt.

Here's a nice webpage with pretty graphs showing how all this stuff works.

I can't think of it but I'd love to know it.

Really, it always makes my head hurt, but here you go. It's not that hard to understand. If you have trouble just read it a few more times.

Here's one such explanation.

Notice that in this example, the actual method of FTL travel/communication is irrelevant. They use the "ansible", an instantaneous communication device, but it could be warp ships or transporters or wizards for all it matters. The point is what the inertial observers in sub-luminal reference frames see -- and they'd see causality violated.

I'd love to see your explanation of how a ship jumping around the universe would arrive back before it left.

Here's one with pretty graphs, but just googling it would show that this is correct.

Time dilation and other such effects would not happen. Its why wormholes do NOT violate relativity.

Ah, but given FTL travel/communication, and regular sub-luminal reference frames which do experience time dilation, you can use the FTL to send a message and a response that will be received before the original message was sent. Paradox.

Note that in the explanation of how this occurs, it does not matter how the FTL is achieved. All that matters is that the sub-luminal observers see information pass from A to B faster than light.

The key word there is "before I saw you leave." , not "before you left." Human vision and radio receivers are not the arbiters of time.

But the speed of light in a vacuum is. You're thinking of time as if there is a universal clock by which the timing and thus ordering of events can be measured irrespective of observation from . This is pre-Relativity thinking. In Relativity the passage of time is relative, all observer's clocks are valid, and the speed of light determines the earliest that an event is capable of affecting you (even if you don't actually "see" the event).

Two observers in different reference frames will disagree on the relative timing of two events depending on their velocities with respect to the source of the event. However for all events with a "time-like" separation, meaning the time and distance is such that a signal traveling at the speed of light could go from one event to the other in that time, they will agree on the order. These are the events that can possibly have a causal relationship.

For events that have a "space-like" interval (ones that do not obey d <= t*c), however, observers in different frames will disagree on the ordering of events. This is okay as long as there is no causal relationship, but if there is then it causes problems.

While it may not be obvious how being able to observe someone arriving at their destination before they leave violates causality ("all you're doing is seeing it, that doesn't mean that's what happened"), the timing of events is defined in relativity by when you are capable of observing them. And this has real meaning because with the help of a couple Warp Drives, and a couple "regular" space ships traveling at high but sub-luminal velocity, you can take advantage of time dilation and create a closed causal loop -- sending yourself a message before you've written it. An obvious violation of causality.

Here is an explanation for how you accomplish that. Or you can just google "speed of light causality" for many other explanations. Suffice to say the link between causality violation and FTL travel is well known.

Here is a relatively understandable explanation of why beating a photon to its destination implies time travel, even if you don't locally travel faster than light: http://www.theculture.org/rich/sharpblue/archives/000089.html [theculture.org].

This reasoning is only valid for flat (Minkowski) space-time, and therefore does not apply to the warp drive proposals, which aim to create particular curved (i.e., non-Minkowski) spacetime configurations.

A very intuitive counterexample: suppose you couple a photon into a very long fiber that is wound many times, with the end being at your neighbors desk. Clearly, you can walk over to him and have a chat with him about everything you like before the photon arrives. No laws of physics were harmed in the making of this experiment.

So what if a radio signal goes slower than a spaceship? That doesn't mean anything.

If the speed of light had no relationship to spacetime, if this was a Newtonian universe, then yeah, what would it matter? But this is not the case, and the speed of light is intimately related to the nature of space and time and as a consequence the ordering of events and causality.

You can google "speed of light causality" to find that every source agrees. I like this page because it has graphs.

Strange how real physicists have no trouble with this stuff

Ha! And you think that's because they agree with you that there's no relationship between c and causality?

No, physicists have "no trouble" with this because they know when they're looking at things outside the bounds of known physics. The Alcubierre drive is a valid solution for the equations of General Relativity, but that doesn't mean you can actually create one in this universe. In particular, the Alcubierre drive requires that the warp in space already exists, and there is no way to enter or exit it. Also, there are the implications with causality if you did exit it (if you didn't then you never interact with the rest of the universe so it's no problem). And that "exotic matter" you tossed out isn't just some engineering problem. Negative mass is also something not known in current physics.

I'm not saying it's impossible. I'm saying it goes against the known laws of physics. Many people, especially physicists, hope that those laws are wrong and in such a way that FTL is possible.

Here's a good description.

Note that the method of FTL communication does not matter. The author uses an "ansible", what is essentially a magical instant communication device from Sci-fi. It can be assumed that the ansible itself violates no laws of physics.

All that matters is how the various sub-luminal reference frames see the ordering of events as a result.

I understand how this solution to GR allows one to avoid going FTL locally. I do not see how this gets around the larger perspective of how the sequence of events appears to other refererence frames. The whole point of Relativity is that the laws of physics must appear to hold according to everyone.

Maybe you should try some different physics classes. It's just special relativity. If you have a faster than light travel/signalling then there will be an interial frame of referance in which that travel/signal went back in time. Since the first postulate or relativity says that the laws of physics are the same in all inertial reference frames that means backwards in time travel/signalling is possible in all inertial frames of reference and we have killed causality. Or alternatively the postulates of special relativity (and hence the theory) are wrong.

http://www.theculture.org/rich/sharpblue/archives/000089.html has some reasonable spacetime diagrams for an example.

Here is a relatively understandable explanation of why beating a photon to its destination implies time travel, even if you don't locally travel faster than light: http://www.theculture.org/rich/sharpblue/archives/000089.html. Basically, if you can pass stuff along at FTL between people at sub-light speed, and those people are moving relative to one another, you can send stuff into the past.

There *are* workarounds. A fairly comprehensive list is here:

http://www.physicsguy.com/ftl/html/FTL_part4.html#subsec:specialframe

They're all kind of about relativity being wrong, and there's no evidence any of these are true. Mostly wishful thinking on the part of people that want to believe we can have an interstellar civilization but can't quite let go of causality. Briefly:

1. FTL takes you to a parallel universe. So if you try to kill your past self, there's no paradox and you keep living because it was actually your counterpart in a different universe.
2. There's some unknown physics that would prevent using FTL to violate causality. So even though there's technically time travel in some sense, it has no practical use and therefore you could say it isn't "really" time travel.
3. A specific case of the above: perhaps the act of travelling FTL prevents any other FTL travel within a certain spacetime "radius".
4. Violate relativity by having a "true" frame of reference with a "true" sequence of events. All FTL takes place in that context and is theoretically unlimited in speed. Within any other frame of reference, it looks like a speed limit, but still possibly faster than light speed.

I might be wrong of course but in my mind FTL doesn't mean time has to go backwards. FTL just means more of space-time is accessible from any other point in spacetime in the forward direction. It still takes positive time to get somewhere FTL it is just when us slower than light guys look at it afterwards and assume it must be slower than light too and back project the cause further back in time than it actually was.

That applies only if you disregard relativity and assume a single global reference frame. There is a good explanatory article with diagrams (somewhat graspable even for non-physicists like me) that explains how FTL signal propagation allows for an actual violation of causality (sending something and receiving a response before you sent it) merely by introducing instantaneous transmission and reference frames moving at high speeds relative to each other.

This post explains it nicely: http://www.theculture.org/rich/sharpblue/archives/000089.html

Yeah, I understand all of that, but (after a little more research) it doesn't make any difference when all parties are in the same inertial reference frame. It makes a difference when you add another inertial reference frame. I like the explanation at http://www.theculture.org/rich/sharpblue/archives/000089.html, which begins with the situation I proposed with Alice and Bob transmitting with an ansible (which would be equivalent to instantaneous transmission via some mechanism like quantum teleportation).

By itself, this single use of the ansible doesn’t create a causality violation. If Bob transmits a signal back towards Alice using a conventional light-speed transmitter, she receives it a later time than when she signalled to Bob. Even if Bob re-transmits with his ansible, Alice receives the reply just a little after she sent out her signal. The problems arise when we bring another inertial frame into play. Let’s suppose that we have another pair of inertial observers, Carol and Dave, who are moving with respect to Alice and Bob, and who have a pair of ansibles of their own. As Carol flies past Bob at event Q, Bob gives her the message from Alice and she transmits it to Dave as soon in the diagram . . .

Now causality is in real trouble, as we can see if we consider the pair of transmissions (from Alice to Bob, then from Carol to Dave) . . .

Notice that we’ve arranged for Dave to receive the signal from Carol as he’s flying past Alice. Notice too that he receives it before Alice has sent her first signal! This means that Alice can transmit information into her own past by way of Bob, Carol, Dave, some spaceships, and two pairs of ansibles. And that’s why faster than light travel or communication, special relativity and causality cannot coexist.

If they succeed in recreating the measurements, doesn't it just mean that c was set at too low a value, and that the true speed to light in a vacuum is slightly faster than originally thought?

No, probably not. Einstein came up with relativity after a thought experiment concerning what a light wave would look like if you were traveling at its velocity. Electro-magnetisim does not allow for a stationary vacuum solution, so he figured out that the way out was to have time stopped at the speed of light. If the speed of light isn't the speed of light, this problem reoccurs. Now, you could postulate a material (let's call it the... ether), so that light is traveling slow, while neutrino's bound on ahead, but that also would disagree with various experiments.

One way out is to have the neutrinos be tachyons, traveling faster than light, but that does allow for causality violations. (Read the link.) That is based on pretty basic stuff, so it's hard to escape it. That would trouble a lot of people, but it would allow for neutrino oscillations (changes from one type to another). You can't do that at the speed of light, as time is frozen there. (As oscillations have been observed, that is additional strong evidence that the neutrino velocity is not the new "speed of light.")

And, there is also the Supernova 1987a results, which conflict with these results (as the 1987A neutrinos do travel near c). Maybe there are oscillations between tachyonic neutrinos and non-tachyonic ones, which would be mind-blowing all by itself.

I think that a bunch of theorists will spin their wheels until this is better constrained experimentally.

If you have faster than light travel, you can have causality violations. (In other words, you could prevent your own birth, change history, things like that.) True, it might require sending neutrino detectors off at a substantial fraction of the speed of light, but what is that compared to messing around with the course of history, not to mention the stock market and the pool on the Super Bowl ?

Well, it's a bit complicated to explain, especially without being able to draw out the diagrams, but what it amounts to is if combine a faster than light signal with relativistic speeds person A can send a message to person B and receive a response back from B before they even sent the message that started the chain. If you don't mind learning how to read space-time diagrams (not really that hard) check out this page for a full explanation.

To rephrase my point: "[Y]ou can pick at most two members of the set {special relativity, causality, FTL}" (from comment by Rich) Now, general relativity is a really successful theory (it explains a lot of the observations it is supposed to explain), so guessing that that is the one to go is not a good bet.

Newton's laws also did a great job for a long time, in fact they still do a great job today; that doesn't mean they aren't superseded by the more recent QM for certain sets of information. I'm not saying that FTL is possible, the brute energy requirements alone would make it basically impossible to achieve. What I am saying is that FTL doesn't imply time travel as far as I can see, there's just no logical basis for it. The equations mean that relatively, time goes slower for the traveller approaching lightspeed, one week inside his spaceship might be one year outside. Past lightspeed, time doesn't go negative in a linear progression, it goes imaginary, which is meaningless. This I feel is the source of much of the confusion on the matter.

To rephrase my point: "[Y]ou can pick at most two members of the set {special relativity, causality, FTL}" (from comment by Rich) Now, general relativity is a really successful theory (it explains a lot of the observations it is supposed to explain), so guessing that that is the one to go is not a good bet.

I'm not sure what you mean by "a frame of reference outside your lightcone". If it makes my OP any easier to read, please substitute "frame of reference" for "inertial frame".

I've had the same question before. The best explanation that I can find is at http://www.theculture.org/rich/sharpblue/archives/000089.html

Basically, it has to do with the fact that the speed of light in a vacuum is constant, regardless of your own reference frame

I believe (and hope) it may be possible for information to travel faster than light, but information traveling backwards in time really messes up the universe.

Bad news, friend. Special Relativity demonstrates that if the former is possible, so is the latter.

http://www.theculture.org/rich/sharpblue/archives/000089.html

No, FTL is a direct violation of causality, and allows for time travel.
See here for a fairly good explanation of why this is.

And if time travel is possible, then (to paraphrase Fermi), where are all the time travelers?

The simplest solution is that time travel isn't possible, ergo superluminal travel is impossible.

But it doesn't allow stationary backwards time travel. You can "travel back in time" in the strict physics sense regarding special relativity, but you can't do it and end up where you were before you left.

Actually, if special relativity is correct - and it pretty much has to be for various reasons - FTL travel means you can travel back in time and end up where you were before you left.

I've never seen a decent prrof of the claim that an ansible necessitates a violation of causality.

It doesn't necessitate, it enables in what are from the point at which you have an ansible trivial ways. In other words, not every ansible usage violates causality, but there are ansible usages that do.

Here's the simplest and clearest explanation I've found explaining how, and that the mechanism doesn't matter.

but if you create an ansible by changing the topolgy of space such that two points are closer together, that doesn't seem to break relativity.

If that occurs only for the duration of the communication, then the space-time diagrams in the example apply. So even though you haven't technically violated the SR restriction against FTL, you have violated causality.

I wish I could whip up some spacetime diagrams to show you, as that's really the clearest way, but I lack the time and hosting. Here is a similar, though different, example with some pretty well made diagrams.

What it really boils down to is, with one event outside of your light cone, not everybody agrees on the order of events. You may say that A happens before B, but if A is farther from B in space than in time, there's always some other frame that says B happened before A. This is trouble when we connect A and B with some cause, like FTL travel or communication.

The bridge example - isn't this example of "reversed causality" just a matter of observations made by the FTL traveler appearing to occur in reverse because of the normal lightspeed limit on how fast the information can reach them?

This is what I thought too at first, but strange things happen at c and above. If you travel at the speed of light, your time and space axes merge into one, which is odd enough. Faster than c and they switch places. Meaning that you can always find a frame where the two events switch places in time, as long as they are not in the same place. This is true even when you extrapolate when they actually occurred, correcting for light lag. (And, in this case with FTL travel, this is true for any two events, even those where both are within your light cone.)

As for why causality is important, imagine if we fire two bullets at a barn, and set the doors to close when the first bullet hits the back wall with an FTL link. You can set it up so there's a frame where bullet 1 is outside the barn when 2 hits the back (and the doors close), and another where bullet 1 is inside. (Again, diagrams make this easier to see.) Of course, that means in one frame the bullet hits the door, and in the other it hits the back wall, and the two frames have mutually incompatible histories.

Sorry if I'm not too helpful here... I love relativity and it's so frustrating without being able to draw diagrams.

Oops, my second link to the page with nice graphs was hidden in a period. The explanation on that page uses instantaneous communication as its example for clarity and simplicity, but all you really need to do is break out of the light cone and you can potentially break causality with time travel.

Having an FTL drive doesn't mean it's a time machine. The actual method of travel is important here.

Yes actually FTL does mean you have a time machine, and the method of travel doesn't really matter. It's not like a Back to the Future time where you can arbitrarily go backwards and forwards as far as you want, it's limited to past-only and by how far and fast you can actually travel and how fast your non-superluminal spaceships can travel. But from some observer's reference frame you will have traveled back in time and broken causality by arriving at your destination before you left, simply by moving faster than c relative to them.

And if you incorporate a second FTL journey, it's actually possible to arrive at your starting point before you left according to all reference frames.

Here's an explanation. There's a nice explanation with graphs and everything .

Note that it does not depend on Lorentz Transformation of the super-luminal traveler/communication. The mechanism isn't important. That observers in normal, relativistic reference frames see you traveling faster than c is what is important. If you can do that, you can go back in time.

Whether or not time flows differently for the travelers (relative to the galactic frame of reference) depends entirely on the details of this technology that we do not yet have access to.

Time may pass differently for the travelers relative to some reference frame, but remember there are no privileged reference frames in Relativity. You can break causality if you go FTL relative to any reference frame, and if you aren't traveling FTL with respect to any reference frame, then you can't really be said to be traveling FTL can you?

If I punch a hole through space via a wormhole and travel at 45mph on a moped through the wormhole to travel a normal space distance of 600 light-years... I just traveled FTL. and THAT does not violate causality.

Except that I have been told repeatedly that it does violate causality. Here are some references:

http://www.theculture.org/rich/sharpblue/archives/000089.html

http://stason.org/TULARC/education-books/startrek-relativity-FTL/8-2-How-FTL-Travel-Implies-Violation-of-Causality.html

http://sheol.org/throopw/tachyon-pistols.html

But I still don't grok it. Probably a good thing I'm not a physicist.

steveha

special relativity and FTL communication together violate causality. If you ignore special relativity then there's no problem, but throwing out relativity is a rather serious step - it does get a lot of stuff right... using classical physics there's no issue at all with FTL, it's just information/objects moving as normal (well a bit faster than normal).

http://www.theculture.org/rich/sharpblue/archives/000089.html is a reasonable example of causality being violated, it won't make any sense if you don't have at least a flimsy grasp on special relativity.

But that's what confuses me. Much like light and sound travel at different speeds, but nobody claims a causality violation when you see something before you hear it (a distant explosion, say) why would my example violate causality?

Now imagine that you were waiting for this distant explosion because you were supposed to set off one of your own as soon as you saw the first one. And you did so faithfully. And then, when you radioed to your distant partner in pointless pyrotechnics, he was irate because he claimed that you had set off your explosive first! Either one of you is wrong/lying, or causality is broken.

One of the key aspects of relativity is that there are no privileged reference frames. Which means that for causality to mean anything, cause must precede effect according to all reference points that can observe them. Yet relativity also says that time can pass differently for different reference points based on speed/mass, and thus can have different views on when events happened. It is this combined with faster-than-light communication which would make it possible to effectively send information into the past.

Your example by itself would not result in a causality violation. But once you involve another reference frame that itself has instantaneous communication through gravity waves, then it becomes possible to create a chain in which the sender of one message receives a response before sending the message. I don't have the technical knowledge of relativity to show you, but here's a website with pretty pictures.

The Speed of Light limitation is in regards to Matter, i.e. something with Mass.

Actually, no. Relativity limits the speed of information transmission, too. At least if you want to keep causality. If you can transmit information faster than the speed of light, you can in theory violate causality.

Causality, relativity, FTL - pick two.

It sucks, but it handily doesn't violate causality. I'm not sure I'm willing to give that up just yet, though it might make for some interesting computer architectures :)

It doesn't matter *how* you get information or matter from point A to point B at a speed faster than light. As soon as you can do this, you violate causality, and you have all the negative effects of time travel whether or not you get to have the fun. In a world where FTL is possible, effect can cause cause rather than the other way around.

A reasonable description of this effect can be found at: http://www.theculture.org/rich/sharpblue/archives/ 000089.html