6 ms·

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

by 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.

by lainga 8y ago

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.