4 ms·

Wouldn't an Alcubierre drive just always take you into the past? If you leave Earth using this drive and get to some distant star at FTL using this mechanism, m

by leokun 13y ago

Wouldn't an Alcubierre drive just always take you into the past? If you leave Earth using this drive and get to some distant star at FTL using this mechanism, maybe you escape relativistic effects, but you'll still get there before light from our star arrives there. And then when you go back wouldn't you arrive before the light left? It doesn't seem to make sense to me. Maybe someone can explain away my misconception about how that works?

crayola 13y ago
The light doesn't go back.
- leokun 13y ago
  I don't see anywhere in my post where I suggest that it did. I think though you could use this device to at least outrun light and then use a telescope to look into the past.
  - crayola 13y ago
    I mean – unless you have in mind some complicated relativistic effect, in which case my answer may be a bit naive, it feels a bit like you are overlooking the fact that, as you go back, you will kind of receive double your share of photons from the planet and play fast-forward on everything you missed on your way out. As I said, I suspect this is not how it would end up happening, as I remember reading somewhere some weird stuff about how the speed of photons hitting you was always the same regardless of your speed.. But that gets too weird for me to comprehend.
mratzloff 13y ago
Say we are having a swimming race and both leave the same starting side at the same moment. I get to the other side of the pool and return to the starting position before you reach the opposite side of the pool. Just because I lap you doesn't mean I travel back in time.
- leokun 13y ago
  Ok, that makes sense, but what happens to the light being emitted from your internal ship lighting. Are you traveling faster than that? How do you look around?
  - bradenb 13y ago
    It's relativity; internal lighting will look the same because light in your ship will still be traveling at the speed of light relative to you (if I'm not mistaken).
  - dlhavema 13y ago
    the wikipedia article linked above talks about this, saying that the "...This method of transport does not involve objects in motion at speeds faster than light with respect to the contents of the warp bubble; that is, a light beam within the warp bubble would still always move faster than the ship. As objects within the bubble are not moving (locally) faster than light..."
cwe 13y ago
You're arriving before the light arrives, not before it leaves. The light takes a certain amount of time to travel that distance: you take less time, not negative time.
mattstocum 13y ago
Why would you arrive before the light left? You could arrive at your destination before light from your source got there, so it would make phoning ahead to let them know you're on your way very difficult, but you wouldn't truly be in the past.
gus_massa 13y ago
Essentially yes. Faster than Light travel is essentially equivalent to traveling to the pass changing the reference system. That's why I'm very skeptic. But you need a more complex setup. If the two stars are static (or the relative velocity is 0) then in both trips you arrive later than the departure time, so there is no easy to spot paradox. But let's suppose that the Sun is static and the other star is moving very fast (with a speed <c). In the first part of the travel, form the Sun point of view you are traveling forward in time with a speed than is greater than c (in spite allegedly you are in a "bubble" so you locally are traveling wit a speed smaller than c. The bubble and everything that is in it is traveling with speed >c.). From the other moving star point of view you (or the bubble) are traveling backwards in time. You reach there and then they see you approaching there. We can compare this to sound of a supersonic plane, where the plane arrives first and then the sound arrives, but if you do the calculations the sound was emitted before the plane arrival time. But if you travel fast enough and the star is moving fast enough then the effect is different, with the correct standard calculations the light arrives later than the space ship, but the light "was" emitted after the ship arrival time. From their point of view the space ship was traveling backwards in time. Once you are there, you only have to recalibrate all the systems to use the other star reference frame. All the inertial reference frames are equivalent, so this is easy. And now you start the engines and travel forward in time with a speed > c in the other star reference frame. They don't see anything too special, only that you go too fast but they are unable to tell you that. But form the Sun's point of view you are now traveling backwards in time, so you arrive before you departure the other star! So, let's peek the Sun's reference frame. You travel first forward in time and then backwards in time. Is the sum positive? If the other star is static then the sum is always positive an there are no surprises. But if the other star moves fast enough then the sum is negative and you arrive before you departure. If you don't find a fast enough star then you can use a fast space station or a fast space ship as the other extreme in your journey. Or even use an abstract point traveling fast enough in the middle of the space.
fleitz 13y ago
Lets say you can travel 10 times faster than light in a warp bubble. You leave for alpha centauri and arrive in .437 years, you immediately return and arrive back on earth in another .437 years. A radio broadcast in that time has traveled .837 light years.
yk 13y ago
No, you are confusing special and general relativity. In a flat spacetime your only chance at getting faster than light travel is by violating local Lorentz invariance. In general realtivity, you can essentially go faster than light in two different ways, either you win an honest race or you find a shortcut. The Alcubierre drive is of the second variety, so you do not get to create closed timelike curves ( the technical term for time travel), but you can still arrive at a distant point faster than a 'naive' measurement of the distance would suggest.

by leokun 13y ago

crayola 13y ago

The light doesn't go back.

leokun 13y ago
I don't see anywhere in my post where I suggest that it did. I think though you could use this device to at least outrun light and then use a telescope to look into the past.
- crayola 13y ago
  I mean – unless you have in mind some complicated relativistic effect, in which case my answer may be a bit naive, it feels a bit like you are overlooking the fact that, as you go back, you will kind of receive double your share of photons from the planet and play fast-forward on everything you missed on your way out. As I said, I suspect this is not how it would end up happening, as I remember reading somewhere some weird stuff about how the speed of photons hitting you was always the same regardless of your speed.. But that gets too weird for me to comprehend.

mratzloff 13y ago

Say we are having a swimming race and both leave the same starting side at the same moment. I get to the other side of the pool and return to the starting position before you reach the opposite side of the pool. Just because I lap you doesn't mean I travel back in time.

leokun 13y ago
Ok, that makes sense, but what happens to the light being emitted from your internal ship lighting. Are you traveling faster than that? How do you look around?
- bradenb 13y ago
  It's relativity; internal lighting will look the same because light in your ship will still be traveling at the speed of light relative to you (if I'm not mistaken).
- dlhavema 13y ago
  the wikipedia article linked above talks about this, saying that the "...This method of transport does not involve objects in motion at speeds faster than light with respect to the contents of the warp bubble; that is, a light beam within the warp bubble would still always move faster than the ship. As objects within the bubble are not moving (locally) faster than light..."

cwe 13y ago

You're arriving before the light arrives, not before it leaves. The light takes a certain amount of time to travel that distance: you take less time, not negative time.

mattstocum 13y ago

Why would you arrive before the light left? You could arrive at your destination before light from your source got there, so it would make phoning ahead to let them know you're on your way very difficult, but you wouldn't truly be in the past.

gus_massa 13y ago

Essentially yes. Faster than Light travel is essentially equivalent to traveling to the pass changing the reference system. That's why I'm very skeptic. But you need a more complex setup. If the two stars are static (or the relative velocity is 0) then in both trips you arrive later than the departure time, so there is no easy to spot paradox. But let's suppose that the Sun is static and the other star is moving very fast (with a speed <c). In the first part of the travel, form the Sun point of view you are traveling forward in time with a speed than is greater than c (in spite allegedly you are in a "bubble" so you locally are traveling wit a speed smaller than c. The bubble and everything that is in it is traveling with speed >c.). From the other moving star point of view you (or the bubble) are traveling backwards in time. You reach there and then they see you approaching there. We can compare this to sound of a supersonic plane, where the plane arrives first and then the sound arrives, but if you do the calculations the sound was emitted before the plane arrival time. But if you travel fast enough and the star is moving fast enough then the effect is different, with the correct standard calculations the light arrives later than the space ship, but the light "was" emitted after the ship arrival time. From their point of view the space ship was traveling backwards in time. Once you are there, you only have to recalibrate all the systems to use the other star reference frame. All the inertial reference frames are equivalent, so this is easy. And now you start the engines and travel forward in time with a speed > c in the other star reference frame. They don't see anything too special, only that you go too fast but they are unable to tell you that. But form the Sun's point of view you are now traveling backwards in time, so you arrive before you departure the other star! So, let's peek the Sun's reference frame. You travel first forward in time and then backwards in time. Is the sum positive? If the other star is static then the sum is always positive an there are no surprises. But if the other star moves fast enough then the sum is negative and you arrive before you departure. If you don't find a fast enough star then you can use a fast space station or a fast space ship as the other extreme in your journey. Or even use an abstract point traveling fast enough in the middle of the space.

fleitz 13y ago

Lets say you can travel 10 times faster than light in a warp bubble. You leave for alpha centauri and arrive in .437 years, you immediately return and arrive back on earth in another .437 years. A radio broadcast in that time has traveled .837 light years.

yk 13y ago

No, you are confusing special and general relativity. In a flat spacetime your only chance at getting faster than light travel is by violating local Lorentz invariance. In general realtivity, you can essentially go faster than light in two different ways, either you win an honest race or you find a shortcut. The Alcubierre drive is of the second variety, so you do not get to create closed timelike curves ( the technical term for time travel), but you can still arrive at a distant point faster than a 'naive' measurement of the distance would suggest.