If we could build an Alcubierre Drive what would we see?

[Speakpigeon]

I was looking at what an observer at Alpha Centauri would make of the ship approaching. (It is the observer that determines what is "reality") The relative motion of Earth would be irrelevant. The approaching ship would be observed at a half light year away before it would be observed one light year away. From the position of the observer at Alpha Centauri this would mean that either the ship was moving away from Alpha Centauri toward Earth or it was traveling backwards in time. Either that or we need to make some major amendments to the theory of relativity.

Well, he wouldn't observe it coming at all, because there wouldn't be photons hitting the ship and bouncing off it toward the observer, because the ship would move faster than the photons and sweep them up. There would be an optic boom analogous to a sonic boom, spreading sideways in a cone.

But that aside, if the observations worked the way you say, and if how it appears to the observer determines what is reality, then that would mean there's a causality violation every time somebody gets shot. First the victim would feel the bullet, then he'd hear the whistling of the bullet from ten feet away, then he'd hear the whistling from twenty feet away, and so on until he hears the gunshot, and therefore the bullet traveled backwards in time.

The thing is, an approaching ship is never observed at a half light year away regardless of its speed -- the photons are observed, and they are observed at Alpha Centauri, and the conclusion that the ship is half a light year away is an interpretation. It's a calculation, not an observation.

I don't follow any of that. We are talking about a, most likely, impossible idea. Even at relativistic relative velocities, common sense used to understand daily life can not be used because it does not apply. Relativistic relative velocities create very non-intuitive results (time dilation and lorentz contraction effects are not intuitive for example). Faster than sound is allowed in physics so we will hear a gunshot after the bullet strikes. What we are examining is beyond relativistic speeds but relativity is the closest physics we have to examine it. By the theory of relativity the observer would observe what I described.

If this drive were to actually be possible then, as I said, we will need to make a lot of major amendments to relativity to describe what observers will observe. We don't have a physics that covers FTL events but relativity is the closest. Surely FTL physics (which is not allowed in relativity) would be even much more non-intuitive than relativity.

This doesn't apply to Alcubierre.

Alcubierre doesn't require relativistic conditions. As I understand it, the ship using Alcubierre may just as well not move much to be able to use the "drive". Space would be doing nearly all of the moving, you see. Same as inflation, sort of. Result: no relativistic speed.

Still, maybe I'm wrong, so explain to me why Alcubierre would imply relativistic conditions.

Wait, I know, you won't. Obviously.
EB

 

[Speakpigeon]

Sorry dude but you are imagining a Newtonian universe and trying to apply how you think it works to a trans-relativistic universe. Even the laws of an Einsteinian universe wouldn't apply but should be much closer.

???

You'd have to explain to me how anyone could get from the principles of "a Newtonian universe" to my rather vivid and accurate description of what an Alcubierre trip would look like.

It was simple. Your "rather vivid" description of the trip was a Newtonian explanation. Newtonian physics fails at relativistic relative velocities. This is why we don't use newtonian physics in the design of something even as slow as our GPS system satellites.

I have to point out the obvious here: you're not explaining in any way how my "description of the trip was a Newtonian explanation".

Try again?
EB

 

[skepticalbip]

I don't follow any of that. We are talking about a, most likely, impossible idea. Even at relativistic relative velocities, common sense used to understand daily life can not be used because it does not apply. Relativistic relative velocities create very non-intuitive results (time dilation and lorentz contraction effects are not intuitive for example). Faster than sound is allowed in physics so we will hear a gunshot after the bullet strikes. What we are examining is beyond relativistic speeds but relativity is the closest physics we have to examine it. By the theory of relativity the observer would observe what I described.

If this drive were to actually be possible then, as I said, we will need to make a lot of major amendments to relativity to describe what observers will observe. We don't have a physics that covers FTL events but relativity is the closest. Surely FTL physics (which is not allowed in relativity) would be even much more non-intuitive than relativity.

Still, maybe I'm wrong, so explain to me why Alcubierre would imply relativistic conditions.
EB

Because the ship using that drive would not be the only thing in the universe. To the rest of the universe and its observers, there would certainly be "relativistic conditions" and also for the crew of the ship looking out at the rest of the universe.

 

[skepticalbip]

It was simple. Your "rather vivid" description of the trip was a Newtonian explanation. Newtonian physics fails at relativistic relative velocities. This is why we don't use newtonian physics in the design of something even as slow as our GPS system satellites.

I have to point out the obvious here: you're not explaining in any way how my "description of the trip was a Newtonian explanation".

Try again?
EB

Dude, you gotta do your own homework and learn a bit. It would be futile to try to explain Newtonian physics and the theory of relativity so you would understand the difference to someone who shows no interest in even considering anything that is contrary to their current beliefs.

 

[Bomb#20]

That's not entirely true. Any FTL ship (bubble or no bubble) which moves in globally more less flat space violates causality, period.

What do you mean by "violates causality"? If what you mean is "goes faster than light", what's your point? If what you mean is "involves an effect preceding its cause", show your work.

It means that in certain (moving) reference frames FTL ship will reach destination before departure.
This is an obvious result of Lorentz transformations.

I.e., when SR constructs calendars for both planets in those particular reference frames, the arrival event has a date on the destination planet's calendar that's a smaller number than the departure event's date on the origin planet's calendar. That's a labeling artifact. It's exactly the same as how until 1918 when Russia changed its calendar, it was possible for a telegram from Germany to Russia to leave Germany in November and arrive in Russia in October.

For an actual causality violation you need an effect preceding its cause at the same place. For example, if the ship set out from Earth, got to Alpha Centauri "before it left", and then turned around and did it again, and got back to Earth before it left in Earth time, that would violate causality.

That can be easily arranged.
We have stars A, B and C. B is between A and C. And Both B and C move together with relativistic speed relative to A.
A decides to send FTL ship to C. So in A reference frame it goes faster than light from A to B then to C.
But in BC reference frame it arrives to C before B.

Okay, so far so good. (Assuming B and C are moving away from A.)

Now if C decides to send their own FTL ship to B with the message that ship from C arrived it will get there before ship from A.

That doesn't follow. The second ship will get to B before a light-speed message from C would get there, but how long before? To get there before the ship from A, it would have to go not just faster than light, but quite a lot faster than light .

Your conclusion follows provided ships can be sent infinitely fast in all directions in all reference frames. But if there's any upper limit on how many times faster than light a ship can go -- even an upper limit in some reference frames, or in some directions -- then you have to work the specific numbers, for how fast B and C are moving and what the limits on FTL speed are, in order to find out if you can construct a round trip that's backwards in time in total.

Now B receives that message decides that they don't want no stinking ship from A and put a large bomb along the pass of the A-ship and blows it up. Causality is screwed.
Of course you can make an attempt at solving this problem but it would create other problems.

But a causality problem with FTL travel hasn't actually been shown yet, only a problem with infinite speed. So let's be specific. Star Trek was kind of vague about how fast warp speeds were, so let's go with Ringworld. Suppose a ship can go in any direction at up to 121 times c, in the Milky Way reference frame*. Can you show that this is fast enough for the C'ers to launch a ship back toward B and have it get there in time to let the B'ers know to blow up the A'er ship?

(* In the Ringworld universe FTL physics was intimately connected to gravity. So presumably for a ship in some other galaxy the limit would be 121 times c in that galaxy's reference frame.)

 

[Bomb#20]

"Hello, Houston, no problem! We can see you!"

In the specific case of a ship using the Alcubierre drive, the ship will be observed as arriving after any photon reflected by it during its journey. However, as Bomb#20 says, the ship will be fast behind the photons, so that the ship will arrive immediately after the photons, and indeed all the photons reflected towards the observer on Alpha, by the ship during the trip, will arrive within a very short interval immediately before the ship.

That's not what I said. Of course, maybe that's what I should have said. In the real world, as far as we can tell, nothing can ever catch up with a photon. So there's no definitive answer to what happens when an object catches up with a photon, and we're left to draw inferences from speculative premises. I was assuming the photon is absorbed by the object, on the grounds it can't very well bounce off the front, since that would involve light going faster than light. But you appear to be assuming it's not the object itself that catches up with the photon, but rather the bubble of warped spacetime carrying the object. The bubble overtakes the photon, engulfs it, and smoothly changes it from moving at c in normal space to moving at c in the curved-space transition zone at the front of the bubble. That's probably a better assumption.

 

[Jokodo]

"Hello, Houston, no problem! We can see you!"

In the specific case of a ship using the Alcubierre drive, the ship will be observed as arriving after any photon reflected by it during its journey. However, as Bomb#20 says, the ship will be fast behind the photons, so that the ship will arrive immediately after the photons, and indeed all the photons reflected towards the observer on Alpha, by the ship during the trip, will arrive within a very short interval immediately before the ship.

That's not what I said. Of course, maybe that's what I should have said. In the real world, as far as we can tell, nothing can ever catch up with a photon. So there's no definitive answer to what happens when an object catches up with a photon, and we're left to draw inferences from speculative premises. I was assuming the photon is absorbed by the object, on the grounds it can't very well bounce off the front, since that would involve light going faster than light. But you appear to be assuming it's not the object itself that catches up with the photon, but rather the bubble of warped spacetime carrying the object. The bubble overtakes the photon, engulfs it, and smoothly changes it from moving at c in normal space to moving at c in the curved-space transition zone at the front of the bubble. That's probably a better assumption.

Either way, the hypothetical observer would be fried into oblivion before he could file his observations by the entire trip's worth of photons arriving all in one instant.

 

[Bomb#20]

I was looking at what an observer at Alpha Centauri would make of the ship approaching. (It is the observer that determines what is "reality") The relative motion of Earth would be irrelevant. The approaching ship would be observed at a half light year away before it would be observed one light year away. From the position of the observer at Alpha Centauri this would mean that either the ship was moving away from Alpha Centauri toward Earth or it was traveling backwards in time. Either that or we need to make some major amendments to the theory of relativity.

Well, he wouldn't observe it coming at all, because there wouldn't be photons hitting the ship and bouncing off it toward the observer, because the ship would move faster than the photons and sweep them up. There would be an optic boom analogous to a sonic boom, spreading sideways in a cone.

But that aside, if the observations worked the way you say, and if how it appears to the observer determines what is reality, then that would mean there's a causality violation every time somebody gets shot. First the victim would feel the bullet, then he'd hear the whistling of the bullet from ten feet away, then he'd hear the whistling from twenty feet away, and so on until he hears the gunshot, and therefore the bullet traveled backwards in time.

The thing is, an approaching ship is never observed at a half light year away regardless of its speed -- the photons are observed, and they are observed at Alpha Centauri, and the conclusion that the ship is half a light year away is an interpretation. It's a calculation, not an observation.

I don't follow any of that. We are talking about a, most likely, impossible idea. Even at relativistic relative velocities, common sense used to understand daily life can not be used because it does not apply. Relativistic relative velocities create very non-intuitive results (time dilation and lorentz contraction effects are not intuitive for example). Faster than sound is allowed in physics so we will hear a gunshot after the bullet strikes. What we are examining is beyond relativistic speeds but relativity is the closest physics we have to examine it. By the theory of relativity the observer would observe what I described.

Well, we don't have to settle that -- even if my or Sp's guess is correct, we can still fake your scenario. The warp bubble will presumably emit Cerenkov radiation sideways along the trip. So a line of beacons left in space ahead of time could relay "It's N light years away now" messages to Alpha Centauri. So we'll go with your observation sequence.

But here's the thing. In SR, determination of dates isn't based on "It is the observer that determines what is reality". That's metaphysics. SR dating is calculation: you measure when light arrives, measure the distance to the source, know that light travels at c, and do some arithmetic. For simplicity's sake, let's say the ship travels at 2c. It leaves Earth in 2518, passes the half-way beacon in 2519, and arrives at Alpha Centauri in 2520. The Centauris observe the ship arrive in 2520, observe the light from the half-way along ship and/or beacon in 2521, and see the ship leave Earth in 2522. In their frame of reference they know Earth is 4 light years away and the beacon is 2 light years away, so SR will tell them the ship left Earth in 2518 and passed the beacon in 2519. No backwards-in-time effect. To get a backwards-in-time effect, you need to have the observers in different reference frames, like in barbos's scenario where Alpha Centauri is speeding away from Earth at .5 c or so, in order to get a length contraction that allows the dating calculation to come out differently when the Centauris do it and when the Earthlings do it. (Hey, Jokodo's here. Yay! Hey Jokodo, correct me if I'm wrong. )

But be that all as it may, even if the observer at Alpha Centauri really does determine that the traveler really did travel backwards in time, that's not all it takes to get a causality violation. Suppose you really arrive at Alpha Centauri a year before you leave Earth. Whoop-de-do. What are you going to do about it? How are you going to use that timing advantage to shoot your grandfather^H^H^H^H starship mechanic and prevent the trip? She's four light years away. It's not enough to go back in time -- in order to violate causality you'll also need to get back home within a year and close the loop.

If this drive were to actually be possible then, as I said, we will need to make a lot of major amendments to relativity to describe what observers will observe. We don't have a physics that covers FTL events but relativity is the closest. Surely FTL physics (which is not allowed in relativity) would be even much more non-intuitive than relativity.

Undoubtedly right.

 

[barbos]

For an actual causality violation you need an effect preceding its cause at the same place. For example, if the ship set out from Earth, got to Alpha Centauri "before it left", and then turned around and did it again, and got back to Earth before it left in Earth time, that would violate causality.

That can be easily arranged.
We have stars A, B and C. B is between A and C. And Both B and C move together with relativistic speed relative to A.
A decides to send FTL ship to C. So in A reference frame it goes faster than light from A to B then to C.
But in BC reference frame it arrives to C before B. Now if C decides to send their own FTL ship to B with the message that ship from C A arrived it will get there before ship from A. Now B receives that message decides that they don't want no stinking ship from A and put a large bomb along the pass of the A-ship and blows it up. Causality is screwed.
Of course you can make an attempt at solving this problem but it would create other problems.

Fixed a typo.

 

[barbos]

It means that in certain (moving) reference frames FTL ship will reach destination before departure.
This is an obvious result of Lorentz transformations.

I.e., when SR constructs calendars for both planets in those particular reference frames, the arrival event has a date on the destination planet's calendar that's a smaller number than the departure event's date on the origin planet's calendar. That's a labeling artifact. It's exactly the same as how until 1918 when Russia changed its calendar, it was possible for a telegram from Germany to Russia to leave Germany in November and arrive in Russia in October.

For an actual causality violation you need an effect preceding its cause at the same place. For example, if the ship set out from Earth, got to Alpha Centauri "before it left", and then turned around and did it again, and got back to Earth before it left in Earth time, that would violate causality.

That can be easily arranged.
We have stars A, B and C. B is between A and C. And Both B and C move together with relativistic speed relative to A.
A decides to send FTL ship to C. So in A reference frame it goes faster than light from A to B then to C.
But in BC reference frame it arrives to C before B.

Okay, so far so good. (Assuming B and C are moving away from A.)

Now if C decides to send their own FTL ship to B with the message that ship from C arrived it will get there before ship from A.

That doesn't follow. The second ship will get to B before a light-speed message from C would get there, but how long before? To get there before the ship from A, it would have to go not just faster than light, but quite a lot faster than light .

Your conclusion follows provided ships can be sent infinitely fast in all directions in all reference frames.

Not infinitely fast, but for simplicity I assume infinite speed in the rest of frame of origin. It's really irrelevant because if ship moves FTL in one frame then in other frames it moves instantly or even back in time. But yes, I assume that SR holds and all reference frames are equal. You made a point that causality would have to be violated at one point in space and my setup with two FTL ships demonstrate just that.

But if there's any upper limit on how many times faster than light a ship can go -- even an upper limit in some reference frames, or in some directions -- then you have to work the specific numbers, for how fast B and C are moving and what the limits on FTL speed are, in order to find out if you can construct a round trip that's backwards in time in total.

Does not matter.

Now B receives that message decides that they don't want no stinking ship from A and put a large bomb along the pass of the A-ship and blows it up. Causality is screwed.
Of course you can make an attempt at solving this problem but it would create other problems.

But a causality problem with FTL travel hasn't actually been shown yet, only a problem with infinite speed.

Actually it has, I have shown simple scenario and if you go to wiki page you will see that author of the drive himself admit that.

So let's be specific. Star Trek was kind of vague about how fast warp speeds were, so let's go with Ringworld. Suppose a ship can go in any direction at up to 121 times c, in the Milky Way reference frame*. Can you show that this is fast enough for the C'ers to launch a ship back toward B and have it get there in time to let the B'ers know to blow up the A'er ship?

Does not matter, all you need is slightly higher then C. It only affects how fast BC system has to move.

(* In the Ringworld universe FTL physics was intimately connected to gravity. So presumably for a ship in some other galaxy the limit would be 121 times c in that galaxy's reference frame.)

 

[Speakpigeon]

"Hello, Houston, no problem! We can see you!"

In the specific case of a ship using the Alcubierre drive, the ship will be observed as arriving after any photon reflected by it during its journey. However, as Bomb#20 says, the ship will be fast behind the photons, so that the ship will arrive immediately after the photons, and indeed all the photons reflected towards the observer on Alpha, by the ship during the trip, will arrive within a very short interval immediately before the ship.

That's not what I said.

That's right, sorry for that. I was being unduly approximative.

Of course, maybe that's what I should have said.

Indeed.

In the real world, as far as we can tell, nothing can ever catch up with a photon.

That's my basic assumption here and I don't see any reason to discard it in the case the Alcubierre drive. It's not the ship which is moving faster than light, it's the space that does the moving. The situation seems very close to what does a moving walkway expanding at the start of the trip and contracting at the end (we tried this here in Paris for our longest walkway at Montparnasse; works well but unfortunately elderly people tended to trip and fall when stepping on it).

So there's no definitive answer to what happens when an object catches up with a photon, and we're left to draw inferences from speculative premises.

I was assuming the photon is absorbed by the object, on the grounds it can't very well bounce off the front, since that would involve light going faster than light.

You would have to explain why the ship would even catch up with photons in the case of the Alcubierre drive. The speed of ship would be well below that of light. The ship would catch up with all the photons in front but only up to a point. The photons would stay in front all the way, barely arriving before the ship at destination point.

But you appear to be assuming it's not the object itself that catches up with the photon, but rather the bubble of warped spacetime carrying the object.

I wasn't really assuming that much, only that I understood the Wiki article correctly and that it was what it said.

Further, I assumed the idea to be similar to inflation (between the ship and the departure point), further assuming it's possible to have the reverse of inflation (between the ship and the destination point). Highly speculative, obviously, but that's what we need assumptions for.

In fact, what is highly speculative is obviously not the possibility of inflation itself, not even that of the reverse of inflation but the possibility that engineering could somehow produce it.

The bubble overtakes the photon, engulfs it, and smoothly changes it from moving at c in normal space to moving at c in the curved-space transition zone at the front of the bubble. That's probably a better assumption.

In the case of a straightforward trip like the one to Alpha Centauri, I may be wrong but I wouldn't speak of a bubble myself but of space inflation behind the ship and perhaps space "compression" or "condensation" in front of the ship. Photons wouldn't be "overtaken", neither by the ship nor by the space around the ship. Instead, the photons in front of the ship, all the photons between the ship and the destination point, will be "condensed" with the space itself there, which has to be reduced to almost nothing, to, say, the few centimetres or kilometres that will be left for the ship to cover by itself. And condensation of the photons may well take the form of shorter wavelength, so there might be a safety issue there.
EB

EDIT
The walkway in Paris was called the "TRAX". So, you could speak of a TRAX effect rather than that of a bubble.

Trax

Another attempt at an accelerated walkway in the 1980s was the TRAX (Trottoir Roulant Accéléré), which was developed by Dassault and RATP and whose prototype was installed at Invalides station in Paris. The speed at entry and exit was 3 km/h (2 mph), while the maximum speed was 15 km/h (9 mph).

On Youtube:
https://www.youtube.com/watch?v=pBJN1X3LeJw

 

[Speakpigeon]

I don't follow any of that. We are talking about a, most likely, impossible idea. Even at relativistic relative velocities, common sense used to understand daily life can not be used because it does not apply. Relativistic relative velocities create very non-intuitive results (time dilation and lorentz contraction effects are not intuitive for example). Faster than sound is allowed in physics so we will hear a gunshot after the bullet strikes. What we are examining is beyond relativistic speeds but relativity is the closest physics we have to examine it. By the theory of relativity the observer would observe what I described.

If this drive were to actually be possible then, as I said, we will need to make a lot of major amendments to relativity to describe what observers will observe. We don't have a physics that covers FTL events but relativity is the closest. Surely FTL physics (which is not allowed in relativity) would be even much more non-intuitive than relativity.

Still, maybe I'm wrong, so explain to me why Alcubierre would imply relativistic conditions.
EB

Because the ship using that drive would not be the only thing in the universe. To the rest of the universe and its observers, there would certainly be "relativistic conditions" and also for the crew of the ship looking out at the rest of the universe.

Only in the sense that you could also insist on "relativist" conditions in the case of the effect of inflation on the photons moving in or through inflating areas, which I thing would be wrong.
EB

 

[Speakpigeon]

It was simple. Your "rather vivid" description of the trip was a Newtonian explanation. Newtonian physics fails at relativistic relative velocities. This is why we don't use newtonian physics in the design of something even as slow as our GPS system satellites.

I have to point out the obvious here: you're not explaining in any way how my "description of the trip was a Newtonian explanation".

Try again?
EB

Dude, you gotta do your own homework and learn a bit. It would be futile to try to explain Newtonian physics and the theory of relativity so you would understand the difference to someone who shows no interest in even considering anything that is contrary to their current beliefs.

Obfuscation. I obviously didn't ask you to explain Newtonian physics. I asked you to justify your assertion that my interpretation of the Alcubierre drive was based on Newtonian physics.

Should be easy to do if only you knew what you're talking about.

If you're utterly unable to justify a claim, don't make it to begin with.

Try again?
EB

 

[Speakpigeon]

"Hello, Houston, no problem! We can see you!"

In the specific case of a ship using the Alcubierre drive, the ship will be observed as arriving after any photon reflected by it during its journey. However, as Bomb#20 says, the ship will be fast behind the photons, so that the ship will arrive immediately after the photons, and indeed all the photons reflected towards the observer on Alpha, by the ship during the trip, will arrive within a very short interval immediately before the ship.

That's not what I said. Of course, maybe that's what I should have said. In the real world, as far as we can tell, nothing can ever catch up with a photon. So there's no definitive answer to what happens when an object catches up with a photon, and we're left to draw inferences from speculative premises. I was assuming the photon is absorbed by the object, on the grounds it can't very well bounce off the front, since that would involve light going faster than light. But you appear to be assuming it's not the object itself that catches up with the photon, but rather the bubble of warped spacetime carrying the object. The bubble overtakes the photon, engulfs it, and smoothly changes it from moving at c in normal space to moving at c in the curved-space transition zone at the front of the bubble. That's probably a better assumption.

Either way, the hypothetical observer would be fried into oblivion before he could file his observations by the entire trip's worth of photons arriving all in one instant.

That's right, but you would just need to manage the trip to spread the energy pulse over a long enough period. The trip may have to be made to last a few days or a few weeks, whatever would be needed.
EB

 

[Jokodo]

Either way, the hypothetical observer would be fried into oblivion before he could file his observations by the entire trip's worth of photons arriving all in one instant.

That's right, but you would just need to manage the trip to spread the energy pulse over a long enough period. The trip may have to be made to last a few days or a few weeks, whatever would be needed.
EB

That may save the travelers on board the ship, but it won't save the observer at the destination.

 

[Bomb#20]

Your conclusion follows provided ships can be sent infinitely fast in all directions in all reference frames.

Not infinitely fast, but for simplicity I assume infinite speed in the rest of frame of origin. It's really irrelevant because if ship moves FTL in one frame then in other frames it moves instantly or even back in time. But yes, I assume that SR holds and all reference frames are equal. You made a point that causality would have to be violated at one point in space and my setup with two FTL ships demonstrate just that.

I.e., (1) the ship moves FTL in one frame; therefore (2) there exists a different frame where it's going infinitely fast; (3) all reference frames are equal; (4) therefore it can go infinitely fast in all directions in all reference frames. I.e., you're simply deleting the scenario in which there's an upper limit on how much faster than light a ship can go. Surely you can see that's not a persuasive argument.

Perhaps we should turn the argument around the other way. We already know SR implies nothing can be accelerated past c, so if we postulate an Alcubierre drive, we're already committed to assuming there's something in SR that doesn't hold. What doesn't hold? (1) the ship moves FTL in one frame; therefore (2) there exists a different frame where it's going infinitely fast; (3) going infinitely fast in all directions in all reference frames leads to paradox; therefore (4) not all reference frames or directions are equal. So that's what no longer holds.

But a causality problem with FTL travel hasn't actually been shown yet, only a problem with infinite speed.

Actually it has, I have shown simple scenario and if you go to wiki page you will see that author of the drive himself admit that.

He says "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)". In principle there's no limit on the speed an Alcubierre drive can reach; but I've read arguments for why in practice an Alcubierre drive can't get over 10c. (Or over 1c, or over 1m/s, depending on the argument.) In principle there's no difference between principle and practice, but in practice there's a great deal of difference.

Does not matter, all you need is slightly higher then C. It only affects how fast BC system has to move.

Only if you pull out the 1.1=infinity argument.

Or consider this scenario. Suppose all reference frames are equal and you can send a ship from A to C infinitely fast in any reference frame. But suppose space is anisotropic -- physics only lets you go FTL if you're going in the galactic north direction. Anything headed toward galactic south is light-speed limited. How does C get a ship back to B in time to stop A? C can only send an FTL ship further north to D.

 

[Bomb#20]

Either way, the hypothetical observer would be fried into oblivion before he could file his observations by the entire trip's worth of photons arriving all in one instant.

That's right, but you would just need to manage the trip to spread the energy pulse over a long enough period. The trip may have to be made to last a few days or a few weeks, whatever would be needed.
EB

That may save the travelers on board the ship, but it won't save the observer at the destination.

Piece of cake -- don't aim for the destination. Aim your ship to come out of warp drive a light hour or so to the side of Alpha Centauri. All those piled up photons shoot off harmlessly into deep space, and then you fly your ship to the target planet at a nice safe speed like .1 c.

 

[barbos]

I.e., (1) the ship moves FTL in one frame; therefore (2) there exists a different frame where it's going infinitely fast; (3) all reference frames are equal; (4) therefore it can go infinitely fast in all directions in all reference frames. I.e., you're simply deleting the scenario in which there's an upper limit on how much faster than light a ship can go. Surely you can see that's not a persuasive argument.

No I can't see that. If we assume SR holds then causality is 100% screwed. But you are free to make a case where SR is not true, but if it holds true then it is what I said it is - FTL means going back in time.

Perhaps we should turn the argument around the other way. We already know SR implies nothing can be accelerated past c, so if we postulate an Alcubierre drive, we're already committed to assuming there's something in SR that doesn't hold.

You can postulate anything you want. I don't see how it helps you.

What doesn't hold? (1) the ship moves FTL in one frame; therefore (2) there exists a different frame where it's going infinitely fast; (3) going infinitely fast in all directions in all reference frames leads to paradox; therefore (4) not all reference frames or directions are equal. So that's what no longer holds.

But a causality problem with FTL travel hasn't actually been shown yet, only a problem with infinite speed.

Actually it has, I have shown simple scenario and if you go to wiki page you will see that author of the drive himself admit that.

He says "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)". In principle there's no limit on the speed an Alcubierre drive can reach; but I've read arguments for why in practice an Alcubierre drive can't get over 10c. (Or over 1c, or over 1m/s, depending on the argument.) In principle there's no difference between principle and practice, but in practice there's a great deal of difference.

Does not matter, all you need is slightly higher then C. It only affects how fast BC system has to move.

Only if you pull out the 1.1=infinity argument.

Yeah, that's a simple mathematical fact FTL+SR=any speed is possible.

Or consider this scenario. Suppose all reference frames are equal and you can send a ship from A to C infinitely fast in any reference frame. But suppose space is anisotropic -- physics only lets you go FTL if you're going in the galactic north direction. Anything headed toward galactic south is light-speed limited. How does C get a ship back to B in time to stop A? C can only send an FTL ship further north to D.

 

[Speakpigeon]

That may save the travelers on board the ship, but it won't save the observer at the destination.

Piece of cake -- don't aim for the destination. Aim your ship to come out of warp drive a light hour or so to the side of Alpha Centauri. All those piled up photons shoot off harmlessly into deep space, and then you fly your ship to the target planet at a nice safe speed like .1 c.

You could also clean up a corridor in front of the ship up to Alpha by sweeping all the photons there on either side. You might even be able to channel them so as to charge up the heating system in old people's homes on whatever planet there may be in Alpha. Or you could do all the actual moving of the ship within such a tiny region of space around the ship that you could move all the photons there around and behind the ship as it moves along. As if the ship was going through a gas.
A bit more tricky to achieve, though. But maybe we could ask other people how to do it.
EB

 

[barbos]

You could also clean up a corridor in front of the ship up to Alpha by sweeping all the photons there on either side.

With tachyon broom no doubt.