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

[SLD]

Granted they’re extremely theoretical, but if we could build one and travel to Alpha Centauri in five minutes by the ship’s clock, what would an observer on earth see. Does the ship just completely disappear the moment it engages the drive? Obviously they can’t observe it at Alpha Centauri five minutes later since they are looking at it 4.3 years ago. Or would it appear to move there at exactly c?

If the ship returned five minutes later by its clock, what time would it be on earth? 10 minutes later? Does the Alcubierre drive preserve time then for both observers? But then how would that be observed from earth. Would there be two space ships? One moving away from earth at c and 10 light minutes away and one back from its trip?

SLD

 

[barbos]

Problem with any faster than speed of light drive is that causality is screwed, hence they can't exist.

 

[steve_bank]

There are FTL theories. but they all have problems. One creats a ST like bubble around the vehicle so all is normal om the ship. Theoretically it does not violate causality.Problem is it requires energy to jave a negative polarity, impossible in this universe.

 

[bilby]

There are FTL theories. but they all have problems. One creats a ST like bubble around the vehicle so all is normal om the ship. Theoretically it does not violate causality.Problem is it requires energy to jave a negative polarity, impossible in this universe.

"Not yet observed" is NOT the same as "impossible".

'Impossible' is a dangerous word to use without VERY good evidence to support it.

 

[Speakpigeon]

There are FTL theories. but they all have problems. One creats a ST like bubble around the vehicle so all is normal om the ship. Theoretically it does not violate causality.Problem is it requires energy to jave a negative polarity, impossible in this universe.

"Not yet observed" is NOT the same as "impossible".

'Impossible' is a dangerous word to use without VERY good evidence to support it.

Well, impossible to observe so far...

Maybe it's because we're not even looking.
EB

 

[Speakpigeon]

Granted they’re extremely theoretical, but if we could build one and travel to Alpha Centauri in five minutes by the ship’s clock, what would an observer on earth see. Does the ship just completely disappear the moment it engages the drive? Obviously they can’t observe it at Alpha Centauri five minutes later since they are looking at it 4.3 years ago. Or would it appear to move there at exactly c?

As I understand it from the Wiki article on it, I would assume that observers on Earth would stop seeing the actual ship, and this pretty much instantly. However, they wouldn't stop seeing something just yet. They would indeed see a ship. Just not the actual one.

The image of the ship received on Earth would make it look like the ship is going away very, very fast. From at least slightly above the speed of light to possibly much, much more than that. In the latter case, you'd need a slow motion showing of the film of the trip to see anything at all. In real time, that would look like the ship has just disappeared in the blink of an eye. But this effect could be mitigated to avoid distressing the family staying behind.

If the ship returned five minutes later by its clock, what time would it be on earth? 10 minutes later? Does the Alcubierre drive preserve time then for both observers? But then how would that be observed from earth. Would there be two space ships? One moving away from earth at c and 10 light minutes away and one back from its trip?

There would be just one spaceship throughout, but the Earth would only be able to observe the image of the ship, with at least some delay, from a few seconds to possibly an X-year delay for a trip X light-years away.

I think there's no relativist effect with Alcubierre. If one twin stays on Earth while the other one goes on an Alcubierre trip and comes back five minutes later, the two twins would still be of the same age after the trip, entirely as if the trip had been to the local greengrocer's.

Rather neat, this, I think. That would put the entire universe at a few minutes "drive" from the Earth and you could go and come back just as fast and as easily.

You would need first to find out how to do the Alcubierre thing, though. Tricky, that, I would guess.

Impossible? Who knows?
EB

 

[barbos]

There are FTL theories. but they all have problems. One creats a ST like bubble around the vehicle so all is normal om the ship. Theoretically it does not violate causality.Problem is it requires energy to jave a negative polarity, impossible in this universe.

That's not entirely true. Any FTL ship (bubble or no bubble) which moves in globally more less flat space violates causality, period.
These bubble tricks only work within, well, the bubble (region with funky curved space). So your bubble would have to occupy all the space you plan to travel. Basically you would have to screw whole lot of space-time to bring the star you plan to travel to closer to you. Sufficient to say that requires whole lot of energy plus ironically time.

 

[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.

 

[skepticalbip]

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.

Possibly, only a new definition is required. The current understanding is that the maximum speed of information transfer is c. Someone using an Alcubierre Drive traveling from Earth to meet another party at Alpha Centauri would arrive years before the information that they had left Earth arrived. Also according to the equations for time dilation in relativity, the observer at Alpha Centauri would determine that the traveller had to have traveled backwards in time.

Although I think that this is just a philosophical question to be pondered, not something that is possible. People do enjoy looking for loopholes in the theory of relativity and many have been postulated.

 

[Speakpigeon]

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.

Possibly, only a new definition is required. The current understanding is that the maximum speed of information transfer is c. Someone using an Alcubierre Drive traveling from Earth to meet another party at Alpha Centauri would arrive years before the information that they had left Earth arrived.

"Someone using an Alcubierre Drive traveling from Earth to meet another party at Alpha Centauri would arrive exactly at the same time as the information that they had left Earth arrived."

I fixed it.

You're welcome.
EB

 

[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.

Possibly, only a new definition is required. The current understanding is that the maximum speed of information transfer is c. Someone using an Alcubierre Drive traveling from Earth to meet another party at Alpha Centauri would arrive years before the information that they had left Earth arrived.

What Speakpigeon said.

Also according to the equations for time dilation in relativity, the observer at Alpha Centauri would determine that the traveller had to have traveled backwards in time.

I don't think that's correct. Alpha Centauri is roughly at rest relative to Earth. Based on my (limited) understanding of SR, Alpha Centauri would have to be moving at a good fraction of c for the equations to work out as you say.

Be that as it may, by "backwards in time" you're referring to the effect at Alpha Centauri preceding its cause at Earth. Any such determination is necessarily relative to the observer's choice of how to label events at a distant location with timestamps. SR proposes a procedure for doing so, and it's a perfectly sensible procedure in a universe with a maximum speed of information transfer of c, but it remains a labeling choice, not a falsifiable empirical observation. That sort of labeling can no more demonstrate a causality violation than a historical document proving the 1917 October Revolution in Russia was partly caused by what happened in November in Germany. 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.

Although I think that this is just a philosophical question to be pondered, not something that is possible. People do enjoy looking for loopholes in the theory of relativity and many have been postulated.

Agreed. I'd bet against an Alcubierre drive ever working.

 

[skepticalbip]

Possibly, only a new definition is required. The current understanding is that the maximum speed of information transfer is c. Someone using an Alcubierre Drive traveling from Earth to meet another party at Alpha Centauri would arrive years before the information that they had left Earth arrived.

What Speakpigeon said.

Also according to the equations for time dilation in relativity, the observer at Alpha Centauri would determine that the traveller had to have traveled backwards in time.

I don't think that's correct. Alpha Centauri is roughly at rest relative to Earth. Based on my (limited) understanding of SR, Alpha Centauri would have to be moving at a good fraction of c for the equations to work out as you say.

Be that as it may, by "backwards in time" you're referring to the effect at Alpha Centauri preceding its cause at Earth. Any such determination is necessarily relative to the observer's choice of how to label events at a distant location with timestamps. SR proposes a procedure for doing so, and it's a perfectly sensible procedure in a universe with a maximum speed of information transfer of c, but it remains a labeling choice, not a falsifiable empirical observation. That sort of labeling can no more demonstrate a causality violation than a historical document proving the 1917 October Revolution in Russia was partly caused by what happened in November in Germany. 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.

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.

 

[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.

 

[barbos]

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.

 

[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.

"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. Anybody looking with their own eyes would just see the ship appearing suddenly out of nowhere. The film of the approach of the ship taken by an impossibly high-speed camera would show, if displayed at very low speed, the ship travelling in the right direction from the Earth to Alpha, leaving the Earth first, then moving towards Alpha and then docking on Alpha, only all this "incredibly" fast, broadly in the blink of an eye if shown in real time. No problem with causality.

Depending on how it would be done precisely, you may also have a photon trailer behind the ship. Photons coming from the Earth this time, but swept behind the ship so that, from Alpha, you'd get to see the Earth as if it was just a few months or even a few days ago, and this at least for a few seconds or hours. Again, on film, it would all look in good "order", i.e. first a four-year old Earth, then a progressively younger Earth, until you'd see very nearly the Earth as it is on the moment, and then back again with a progressively older Earth, possibly very quickly, and back to the four-year old view of it. This only on film. Looking with your eyes, you would see nothing at all except the ship itself.

No only that, but the film would make the Earth looks as if it would be itself coming closer to Alpha, in proportion so to speak of it's apparent age. So, the Earth would seem as if it was coming to Alpha, somewhat before and behind the arrival of the ship. Immediately before and after the docking of the ship, you would have a view of the Earth at the time of the departure of the ship from the Earth, i.e. at very nearly that of its arrival on Alpha. No only that, but the Earth itself, or at least a good chunk of the departure area, would look as if it had also come to Alpha, if only for a very brief moment, and only visible on film.

Time to say, "Hello, Houston, we can see you!"

We could use this to communicate in real-time. Perhaps less expensive than moving actual ships.
EB

 

[skepticalbip]

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.

 

[skepticalbip]

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.

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

.............snip.............

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.

 

[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 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.

 

[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.

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

.............snip.............

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.

But, wait, I already know the answer, you won't.

See, I just guessed the answer even before you got to know you'd have to try one and decide against it. Faulty causality? No, experience. Never one to explain anything very much. Easier this way.
EB

 

[skepticalbip]

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.