8 ms·

There seems to be little recognition of the fact that FTL implies time travel, and just how unlikely that really is. I expect major advances in physics in the f

There seems to be little recognition of the fact that FTL implies time travel, and just how unlikely that really is. I expect major advances in physics in the far future, but I’m not sure that bypassing causality and FTL will ever be one of them.

lainga 8y ago
The "warp drive" they are discussing, and which was so hyped in 2010 or so, doesn't involve FTL, it involves contraction and expansion of spacetime as the ship moves in freefall. So no time travel.
- GW150914 8y ago
  It doesn’t matter, it still implies time travel. Use the wormhole acceleration-return thought experiment as an example. If you can leave point A and arrive at point B before light could, it doesn’t matter if you took a shortcut.
  - lainga 8y ago
    OK, touche - it's true that an Alcubierre drive could be used for time travel, but it's not necessarily implied, whereas it is for an object actually travelling beyond the speed of light. The discoverer of the method itself [0] (Miguel Alcubierre ) believed that chronology protection [1] would prevent time travel, as the spacetime bubble would destroy itself somehow. [0] https://en.wikipedia.org/wiki/Alcubierre_drive https://en.wikipedia.org/wiki/Alcubierre_drive [1] https://en.wikipedia.org/wiki/Chronology_protection_conjecture https://en.wikipedia.org/wiki/Chronology_protection_conjectu...
    - GW150914 8y ago
      It’s a cool idea, and admittedly would be a sort of loophole, but it might just mean that such a drive would require an arbitrarily high amount to work. Maybe Stephen Hawking was right and as soon as you build a machine capable of time travel, turning it on blows it up, because of QM effects. It does seem a little neat if how we intend to use the machine is the deciding factor in how it works.
      - AnimalMuppet 8y ago
        It also could blow up if it accidentally causes time travel...
      - sls 8y ago
        Back in his collection All the Myriad Ways, Larry Niven wrote a brief essay called "The Theory and Practice of Time Travel" that explored ideas like these. One excerpt: > GIVEN: That the universe of discourse permits both time travel and the changing of the past. > THEN: A time machine will not be invented in that universe. > For, if a time machine is invented in that universe, somebody will change the past of that universe. There is just too much future subsequent to the invention of a time machine: too many people with too many good motives for meddling with too many events occurring in too much of the past. thus > Niven's Law: If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.
    - jerf 8y ago
      I actually spent some time sketching out the math for an aborted science fiction world in which I had FTL that was prevented from time travel by a cosmic censor. You don't really have to "work it out" because the numbers are quite plain as day. It turns out that if you use any remotely sane numbers on the speed of an FTL craft, it is extremely difficult to travel in time. To get time travel, you have to accelerate on the other end to very significant fractions of the speed of light, which itself takes so much time that it's hard to recover it via the FTL drive again. You have to put in absurd numbers for the speed of the drive and your ability to accelerate (and your ability to carry energy for conventional acceleration!) before you can get time travel out of most FTL drives, because bending your light cone enough to put the past into your FTL drive's path takes a lot of energy. The only fictional drives that have serious time travel possibilities are the ones in universes where accelerating to significant fractions of the speed of light is easy-peasy, and the FTL drive is very, very fast (some are instantaneous) and can be arbitrarily steered. It's actually not that long of a list; a lot of FTL drives can hypothetically be used for time travel but the engineering in the universe still doesn't practically permit it. Just adding a cosmic censor post facto would preserve the story just fine. Many of the universes that meet those criteria are already so fantastical that they do in fact have time travel, so... those work out too. (There's also several video universes where it's nominally possible, but the video wildly contracts the documentation. Both Star Wars and Star Trek have documentation that their ships can get to pretty significant fractions of the speed of light in small amounts of time, with ludicrous amounts of acceleration available to them, but on screen we only see them moving with absurd levels of acceleration like 50 or 100G, but which still takes a long time to get to light speed as a result, and with refueling being necessary often enough that it's hard to believe they could actually make it.)
      - dragonwriter 8y ago
        > Both Star Wars and Star Trek have documentation that their ships can get to pretty significant fractions of the speed of light in small amounts of time, with ludicrous amounts of acceleration available to them, but on screen we only see them moving with absurd levels of acceleration like 50 or 100G, but which still takes a long time to get to light speed as a result, , and with refueling being necessary often enough that it's hard to believe they could actually make it.) I don't know about Wars, but Trek strongly implies that impulse drive reaches significant fractions of light speed with near-zero acceleration time; there is certainly nothing to indicate that the on-screen non-warp acceleration is limited to 50-100G, and there is little on-screen indication that refueling (as opposed to drive system maintenance, and replacing non-fuel consumables used onboard) is regularly needed.
    - SubiculumCode 8y ago
      Any progress on that idea (Alcubierre drive)? Always seems the most interesting. I do a Google Scholar search, but I don't know the physics journals, so its hard for me to know legit from crank abuse, without laboriously checking each result for journal impact factors.
      - TheOtherHobbes 8y ago
        The debate continues. E.g. https://www.hindawi.com/journals/isrn/2013/482734/ https://www.hindawi.com/journals/isrn/2013/482734/ I suspect using GR to design a warp drive is like using a spoon to build a freeway. Without a theory of quantum gravity - which appears to mean not just a theory of quantised space time, but also of the non-locality and complex distributed relationships that haunt QM - you're probably not going to get very far.
  - fooker 8y ago
    Why does this imply time travel? You could look backwards in time, sure, but we can already do that.
    - surge 8y ago
      https://www.youtube.com/watch?v=HUMGc8hEkpc https://www.youtube.com/watch?v=HUMGc8hEkpc
  - flatline 8y ago
    Doesn’t the wormhole scenario require bending the thing back on itself? I always thought that just because something could violate causality doesn’t necessarily mean it would, but IANAP and maybe that distinction doesn’t matter.
  - ummonk 8y ago
    That assumes that you can accelerate a wormhole mouth...
  - monochromatic 8y ago
    How does that imply time travel? Yes, you get there before the photons, but so what?
    - aristus 8y ago
      If you go from A to B before the photons can, from B's reference frame you are arriving from the future with perfect knowledge of events that have not yet "happened".
      - SimbaOnSteroids 8y ago
        Yes however if you were to pop back to A you wouldn't have any future knowledge at A (minus the 'future' knowledge of B).
      - monochromatic 8y ago
        But you can’t go back to A and change those events before they happen. That’s not time travel. You could get the same effect by just blocking the photons before they make it to B, and then traveling at conventional speed.
      - trentlott 8y ago
        That's always struck me as sort of inconsequential, even though light is the apparent "speed limit" of the universe. The same thing happens with sound regularly. A thing happens, and we receive light before we receive sound. Some third faster thing that gets to us before light would just be some other thing. The distance of travel requires time relative to both reference frames. If a star explodes at A and I travel to B at 2x the speed of light to tell folks at B, so what? Some time has elapsed and those two frames are still chugging along with causality. Maybe there's some physics I've not learned that makes this obvious, but I've never grasped exactly why violating light speed prevents entropy from continuing. Even if at two different rates. If we perfectly timestamped everything happening on two sides of the world and sent a telegram at twice lightspeed from A to B, it would still arrive after it is sent.
        8y ago
  - hinkley 8y ago
    The thing that always bugged me about time travel fiction is that the reference frame is just assumed to be the surface of the earth. How fast are we wizzing through the universe? Why wouldn’t I reappear somewhere out past Saturn when I travel forward or backward in time? Similarly, if wormholes are real, and you construct one in a gravity well, what happens to gravity? Especially if both ends are in a gravity well?
    - hossbeast 8y ago
      Does the influence of gravity reach through the wormhole?
      - hinkley 8y ago
        If a wormhole is a 'fold' in space, instead of a magic teleportation device, why wouldn't gravity and light and EM fields travel through the wormhole?
    - andrewflnr 8y ago
      It seems that the usual thing to do is put the wormhole in orbit around your planet or whatever. That's based more on science fiction than physics, albeit scifi by smart knowledgeable people. I'm not sure what to think about gravity "reaching through". Gravity and the wormhole are both just curved spacetime. Maybe the larger body distorts the wormhole throat in a way that looks like "reaching through"? Certainly, you need to have some effect that causes energy conservation when you make a perpetual water wheel by suspending one mouth over another and running water through it...
wuliwong 8y ago
Hah yea, good point. I guess if you were using FTL travel to get to Mars, you would set your speed by how far back in time you wanted to arrive. :)
TeMPOraL 8y ago
> FTL implies time travel I've heard that many times now, but I'm not sure I fully understand it. So, could someone ELI5 how, given a starship equipped with a warp drive (or in range of a series of cleverly put stable wormholes), how can I give myself winning lottery numbers, and meet my great-great-grandfather?
- GW150914 8y ago
  I don’t think there is a true ELI5 answer, at least, I can’t think of one. The basic concept involves general relativity, and while there are ways to simplify it (i.e. no math) it’s still complicated. This does an ok job: https://physics.stackexchange.com/questions/128075/how-would-wormhole-based-ftl-violate-causality https://physics.stackexchange.com/questions/128075/how-would... Better job, but less accessible: https://physics.stackexchange.com/questions/52249/how-does-faster-than-light-travel-violate-causality https://physics.stackexchange.com/questions/52249/how-does-f... The classic version involving FTL ships: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... A little math, but useful because it applies to the ability to send signals FTL, regardless of means: https://www.quora.com/Why-would-traveling-faster-than-light-make-me-go-back-in-time https://www.quora.com/Why-would-traveling-faster-than-light-...
- krohling 8y ago
  As you approach the speed of light, the amount of time you experience gets slower as compared to a stationary observer. This means that a photon, which travels at the speed of light, actually experiences no time at all. I like to think of time and space as two different directions you can go i.e. forward/backward (time) or left/right (space). A stationary observer travels only forward, i.e. through time. So they experience lots of time but very little or no space. As you get closer and closer to the speed of light you're moving less in time and more in space. Your vector gets closer and closer to pointing right (using this analogy). An observer traveling at light speed experiences only space, i.e. no time at all. Then, once you exceed the speed of light your arrow starts pointing a little bit backwards. You're still traveling mostly in space, very little in time but now the time you do travel in is actually negative. I forget where I read this vector explanation but it's one that makes the concept tractable in my mind :)
deleted 8y ago
[deleted]
carry_bit 8y ago
FTL doesn't imply time travel if it's restricted to a special frame.
- russdill 8y ago
  I've always had the view that FTL travel necessitates time travel in certain frames of reference. Special frames of reference are currently "unpopular". It sounds like you are referencing a special reference frame where FTL could prove such a special reference frame exists, and that such a frame would allow FTL to be possible without time travel. Can you explain a little more about this idea?
dqpb 8y ago
Isn't the idea here that you could just travel a shorter distance in less time than it takes light to travel a longer distance?
marcosdumay 8y ago
If the math says so, or if some preliminary experiments say so, it's the task of a scientist to go there and check. Not doing so because you don't like the consequence is incredibly arrogant and anti-scientific. That said, the usual disclaimers about confidence intervals apply. Also, not checking because you have better things to do is perfectly ok.

lainga 8y ago

The "warp drive" they are discussing, and which was so hyped in 2010 or so, doesn't involve FTL, it involves contraction and expansion of spacetime as the ship moves in freefall. So no time travel.

GW150914 8y ago
It doesn’t matter, it still implies time travel. Use the wormhole acceleration-return thought experiment as an example. If you can leave point A and arrive at point B before light could, it doesn’t matter if you took a shortcut.
- lainga 8y ago
  OK, touche - it's true that an Alcubierre drive could be used for time travel, but it's not necessarily implied, whereas it is for an object actually travelling beyond the speed of light. The discoverer of the method itself [0] (Miguel Alcubierre ) believed that chronology protection [1] would prevent time travel, as the spacetime bubble would destroy itself somehow. [0] https://en.wikipedia.org/wiki/Alcubierre_drive https://en.wikipedia.org/wiki/Alcubierre_drive [1] https://en.wikipedia.org/wiki/Chronology_protection_conjecture https://en.wikipedia.org/wiki/Chronology_protection_conjectu...
  - GW150914 8y ago
    It’s a cool idea, and admittedly would be a sort of loophole, but it might just mean that such a drive would require an arbitrarily high amount to work. Maybe Stephen Hawking was right and as soon as you build a machine capable of time travel, turning it on blows it up, because of QM effects. It does seem a little neat if how we intend to use the machine is the deciding factor in how it works.
    - AnimalMuppet 8y ago
      It also could blow up if it accidentally causes time travel...
    - sls 8y ago
      Back in his collection All the Myriad Ways, Larry Niven wrote a brief essay called "The Theory and Practice of Time Travel" that explored ideas like these. One excerpt: > GIVEN: That the universe of discourse permits both time travel and the changing of the past. > THEN: A time machine will not be invented in that universe. > For, if a time machine is invented in that universe, somebody will change the past of that universe. There is just too much future subsequent to the invention of a time machine: too many people with too many good motives for meddling with too many events occurring in too much of the past. thus > Niven's Law: If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.
  - jerf 8y ago
    I actually spent some time sketching out the math for an aborted science fiction world in which I had FTL that was prevented from time travel by a cosmic censor. You don't really have to "work it out" because the numbers are quite plain as day. It turns out that if you use any remotely sane numbers on the speed of an FTL craft, it is extremely difficult to travel in time. To get time travel, you have to accelerate on the other end to very significant fractions of the speed of light, which itself takes so much time that it's hard to recover it via the FTL drive again. You have to put in absurd numbers for the speed of the drive and your ability to accelerate (and your ability to carry energy for conventional acceleration!) before you can get time travel out of most FTL drives, because bending your light cone enough to put the past into your FTL drive's path takes a lot of energy. The only fictional drives that have serious time travel possibilities are the ones in universes where accelerating to significant fractions of the speed of light is easy-peasy, and the FTL drive is very, very fast (some are instantaneous) and can be arbitrarily steered. It's actually not that long of a list; a lot of FTL drives can hypothetically be used for time travel but the engineering in the universe still doesn't practically permit it. Just adding a cosmic censor post facto would preserve the story just fine. Many of the universes that meet those criteria are already so fantastical that they do in fact have time travel, so... those work out too. (There's also several video universes where it's nominally possible, but the video wildly contracts the documentation. Both Star Wars and Star Trek have documentation that their ships can get to pretty significant fractions of the speed of light in small amounts of time, with ludicrous amounts of acceleration available to them, but on screen we only see them moving with absurd levels of acceleration like 50 or 100G, but which still takes a long time to get to light speed as a result, and with refueling being necessary often enough that it's hard to believe they could actually make it.)
    - dragonwriter 8y ago
      > Both Star Wars and Star Trek have documentation that their ships can get to pretty significant fractions of the speed of light in small amounts of time, with ludicrous amounts of acceleration available to them, but on screen we only see them moving with absurd levels of acceleration like 50 or 100G, but which still takes a long time to get to light speed as a result, , and with refueling being necessary often enough that it's hard to believe they could actually make it.) I don't know about Wars, but Trek strongly implies that impulse drive reaches significant fractions of light speed with near-zero acceleration time; there is certainly nothing to indicate that the on-screen non-warp acceleration is limited to 50-100G, and there is little on-screen indication that refueling (as opposed to drive system maintenance, and replacing non-fuel consumables used onboard) is regularly needed.
  - SubiculumCode 8y ago
    Any progress on that idea (Alcubierre drive)? Always seems the most interesting. I do a Google Scholar search, but I don't know the physics journals, so its hard for me to know legit from crank abuse, without laboriously checking each result for journal impact factors.
    - TheOtherHobbes 8y ago
      The debate continues. E.g. https://www.hindawi.com/journals/isrn/2013/482734/ https://www.hindawi.com/journals/isrn/2013/482734/ I suspect using GR to design a warp drive is like using a spoon to build a freeway. Without a theory of quantum gravity - which appears to mean not just a theory of quantised space time, but also of the non-locality and complex distributed relationships that haunt QM - you're probably not going to get very far.
- fooker 8y ago
  Why does this imply time travel? You could look backwards in time, sure, but we can already do that.
  - surge 8y ago
    https://www.youtube.com/watch?v=HUMGc8hEkpc https://www.youtube.com/watch?v=HUMGc8hEkpc
- flatline 8y ago
  Doesn’t the wormhole scenario require bending the thing back on itself? I always thought that just because something could violate causality doesn’t necessarily mean it would, but IANAP and maybe that distinction doesn’t matter.
- ummonk 8y ago
  That assumes that you can accelerate a wormhole mouth...
- monochromatic 8y ago
  How does that imply time travel? Yes, you get there before the photons, but so what?
  - aristus 8y ago
    If you go from A to B before the photons can, from B's reference frame you are arriving from the future with perfect knowledge of events that have not yet "happened".
    - SimbaOnSteroids 8y ago
      Yes however if you were to pop back to A you wouldn't have any future knowledge at A (minus the 'future' knowledge of B).
    - monochromatic 8y ago
      But you can’t go back to A and change those events before they happen. That’s not time travel. You could get the same effect by just blocking the photons before they make it to B, and then traveling at conventional speed.
    - trentlott 8y ago
      That's always struck me as sort of inconsequential, even though light is the apparent "speed limit" of the universe. The same thing happens with sound regularly. A thing happens, and we receive light before we receive sound. Some third faster thing that gets to us before light would just be some other thing. The distance of travel requires time relative to both reference frames. If a star explodes at A and I travel to B at 2x the speed of light to tell folks at B, so what? Some time has elapsed and those two frames are still chugging along with causality. Maybe there's some physics I've not learned that makes this obvious, but I've never grasped exactly why violating light speed prevents entropy from continuing. Even if at two different rates. If we perfectly timestamped everything happening on two sides of the world and sent a telegram at twice lightspeed from A to B, it would still arrive after it is sent.
      - 8y ago
- hinkley 8y ago
  The thing that always bugged me about time travel fiction is that the reference frame is just assumed to be the surface of the earth. How fast are we wizzing through the universe? Why wouldn’t I reappear somewhere out past Saturn when I travel forward or backward in time? Similarly, if wormholes are real, and you construct one in a gravity well, what happens to gravity? Especially if both ends are in a gravity well?
  - hossbeast 8y ago
    Does the influence of gravity reach through the wormhole?
    - hinkley 8y ago
      If a wormhole is a 'fold' in space, instead of a magic teleportation device, why wouldn't gravity and light and EM fields travel through the wormhole?
  - andrewflnr 8y ago
    It seems that the usual thing to do is put the wormhole in orbit around your planet or whatever. That's based more on science fiction than physics, albeit scifi by smart knowledgeable people. I'm not sure what to think about gravity "reaching through". Gravity and the wormhole are both just curved spacetime. Maybe the larger body distorts the wormhole throat in a way that looks like "reaching through"? Certainly, you need to have some effect that causes energy conservation when you make a perpetual water wheel by suspending one mouth over another and running water through it...

wuliwong 8y ago

Hah yea, good point. I guess if you were using FTL travel to get to Mars, you would set your speed by how far back in time you wanted to arrive. :)

TeMPOraL 8y ago

> FTL implies time travel I've heard that many times now, but I'm not sure I fully understand it. So, could someone ELI5 how, given a starship equipped with a warp drive (or in range of a series of cleverly put stable wormholes), how can I give myself winning lottery numbers, and meet my great-great-grandfather?

GW150914 8y ago
I don’t think there is a true ELI5 answer, at least, I can’t think of one. The basic concept involves general relativity, and while there are ways to simplify it (i.e. no math) it’s still complicated. This does an ok job: https://physics.stackexchange.com/questions/128075/how-would-wormhole-based-ftl-violate-causality https://physics.stackexchange.com/questions/128075/how-would... Better job, but less accessible: https://physics.stackexchange.com/questions/52249/how-does-faster-than-light-travel-violate-causality https://physics.stackexchange.com/questions/52249/how-does-f... The classic version involving FTL ships: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... A little math, but useful because it applies to the ability to send signals FTL, regardless of means: https://www.quora.com/Why-would-traveling-faster-than-light-make-me-go-back-in-time https://www.quora.com/Why-would-traveling-faster-than-light-...
krohling 8y ago
As you approach the speed of light, the amount of time you experience gets slower as compared to a stationary observer. This means that a photon, which travels at the speed of light, actually experiences no time at all. I like to think of time and space as two different directions you can go i.e. forward/backward (time) or left/right (space). A stationary observer travels only forward, i.e. through time. So they experience lots of time but very little or no space. As you get closer and closer to the speed of light you're moving less in time and more in space. Your vector gets closer and closer to pointing right (using this analogy). An observer traveling at light speed experiences only space, i.e. no time at all. Then, once you exceed the speed of light your arrow starts pointing a little bit backwards. You're still traveling mostly in space, very little in time but now the time you do travel in is actually negative. I forget where I read this vector explanation but it's one that makes the concept tractable in my mind :)

deleted 8y ago

[deleted]

carry_bit 8y ago

FTL doesn't imply time travel if it's restricted to a special frame.

russdill 8y ago
I've always had the view that FTL travel necessitates time travel in certain frames of reference. Special frames of reference are currently "unpopular". It sounds like you are referencing a special reference frame where FTL could prove such a special reference frame exists, and that such a frame would allow FTL to be possible without time travel. Can you explain a little more about this idea?

dqpb 8y ago

Isn't the idea here that you could just travel a shorter distance in less time than it takes light to travel a longer distance?

marcosdumay 8y ago

If the math says so, or if some preliminary experiments say so, it's the task of a scientist to go there and check. Not doing so because you don't like the consequence is incredibly arrogant and anti-scientific. That said, the usual disclaimers about confidence intervals apply. Also, not checking because you have better things to do is perfectly ok.