Alcubierre drive research

[shynung]

So a warp drive alone won't enable time travel unless the ship carrying it has a really big dV capacity?

[Voyager55]

Ok, there seem to be some misconceptions going on here.

OP is asking if we could use an FTL drive (on the assumption that an FTL drive, any drive, possibly an Alcubierre drive, possibly unicorn powered magic) to jump away roughly a hundred light years inside the span of say a couple months. Once there, you'd deploy a massive radio receiver system and/or telescope to look at Earth and try to watch events as they happened roughly a hundred years ago.

To answer this question, yes this would work assuming you did have an FTL drive.

You would have outpaced the light containing the radio and light-images that you sought to receive and would now be able to receive it. As far as WHAT you could see, probably not a whole lot. For an imaging telescope to resolve something like a person over the distance of a hundred+ light years you would need a telescope of stupidly large size. And if you were hoping to watch a specific event, you better hope there were no clouds that day. Unfortunately, there is a rather large portion of human history that can never be recovered this way. Sure, you MIGHT be able to do some fancy games with IR telescopes to see people doing stuff through the clouds, but real color images would be basically lost.

Turns out of you have a telescope with one of those "Stupidly large" apertures, say several AU's in diameter you could make reasonable visible light images. This wouldn't be well out of technological range for a race that races across the galaxy using a device that warps space-time, consuming more energy than all of earths expenditure combined.

[problemecium]

^ Exactly what I was saying earlier ^^

All the physicists criticizing the warp drive for being a magical time machine sound very confident about it, but I have yet to hear any explanation of specifically HOW a warp drive performs time travel. But they must have some idea, or they wouldn't have a leg to stand on, right?

I, and I think most of us here, could really use a step-by-step description if anyone can provide one.

[PB666]

I confess, I've never believed that FTL travel had anything to do with time travel whatsoever.

I understand that atomic clocks show some time dilation in long-duration spaceflight, but that's not really the same thing, is it?

I suspect that this is a matter of people thinking of the 'time-horizon' of visible light spreading across the universe, and trying to apply it to relativity.

Thought Experiment: Jump in your FTL ship and travel 'infinitely fast' to a point 100 light-years from earth. Point a radio antenna in Earth's direction, and you will receive radio from 1915. You are not actually in the past, but Earth appears 100 years younger to you.

Somehow, I think that layman-understanding of relativity has led to people assuming that this is time-travel. And, I suppose, you could argue that earth is 'younger' than when you left, so far as your perception is concerned, and 'human perception is reality' and whatever other pseudo-knowledge you want to toss in there, sure. But you didn't travel through time.

Disclaimer: I have no idea whether this is right or not, it just sounds VERY fishy to me.

It has something to do with causality, lets say you observe what happens, then you travel faster than the speed of light to the point where there is an effect, you could prevent that effect from happening.

So if we reduce this to the microscopic something that observes an event and can prevent that event from happening where the event parameters travel at C is effectively going back in time. We are our effects of the causal past. Our electrons and many of the atomic processes are moving at SoL, so if we could change their fate, we have in essence gone back in the past. If an asteroid collides with earth, you can't FTL out 100 years and then notify earth to change the course of some unknown asteroid. However if you were sitting on the surface of the asteroid, asleep, and suddenly woke as the asteroid plunged into earths atmosphere theoretically you could move all that matter to a safe distance from earth with an FTL drive effectively alter all the effects from realizing on earth including blocking the light and radiant energies that had already occurred. From the observers point of view on the ground the asteroid approached earth and then disappeared. However, also from other points of view any attempt to reverse all those effects would cause even more dramatic and destructive effects (so it creates a paradox).

Just want to make a point. According to the grand synthesis of the standard model quantum gravity separated from the other forces first, noting that we have yet to observe this in nature, but if we consider the basis of that force to be bosons, then its particle is its own anti-particle, this means essentially if quantum gravity exists there is no anti-gravity or collective negative mass, and alcubierre cannot exist. In addition since boson are fields and propagate at SoL, then the warp field could never propagate faster, any attempt to reach that speed would effectively destroy space-time capable of transmitting the field. The metric consists of a number of very unlikely preconditions for the drive to be possible, and even if the drive were theoretically possible, it is more than likely practically impossible, from both points of view, creating a negative energy field of great intensity and impractically sharp definition behind the flat-spacetime sphere and propogating a gravity well equally sharply defined in front of the flat space-time. Not only does this require an immense amount of energy (and the opposite thereof) but it also requires an alcubierre transport to create the field in front of the causality limit of the ship, which means that you need another alcubierre drive to make any alcubierre drive work. Summing you have an unknown physics operating behind the craft and an ungovernable metric in front.

This is the ultimate if and if and if and if argument where many of the ifs are all but false.

[Robotengineer]

I think of it in a car analogy. If a car leaves Earth at light speed, and then you leave Earth in a car that goes FTL, you will see the car behind you. However if you go back to Earth, you aren't going to arrive at a time before the light-car left, you are simply going to pass the light-car heading the other direction, and end up on Earth at the time you left, or after you left.

[rtxoff]

Well the thing that is misunderstood here is that people for whatever reasons believe that FTL travel to some point in the universe will involve a time travel back in time. Let's for the moment assume we are a photon that is travelling to KIC 8462852 from Earth. From our perspective we will arrive there instantly. From the perspective of Earth we will be travelling 1400 years. From the perspective of the photon if we started our Journey from Earth in the very same moment as when for example the drop in light intensity occurred that we registered with our telescopes the photon will arrive there at this very same moment and will be able to see what exactly just happened there causing this drop in light. Correct me if i'm wrong but i believe that is current knowledge in science.

Ok, now let's try to think what would happen if there is a wormhole connecting Earth and KIC 8462852 together. Getting there is just a matter of stepping a few meters through this wormhole but this steps will take you 1400ly away. Someone is tempted here to assume that this will be a travel into the past of KIC8462852 but i'm pretty sure that wouldn't happen. Much more likely IMO is that you will do a travel into the future of KIC 8462852. You will arrive there 1400years later then a photon will arrive there doing the same trip. So you will be able to see how KIC8462852 will progress 1400years in the future from Earths perspective. The thing is when you go back through the wormhole you will be back in you own space time and only the time you have spent at KIC8462852 will have passed.

I may be completely wrong on this but as long as nobody builds an FTL drive or finds a wormhole nobody can prove me wrong on this. And this makes much more sense then all this back in time travel mumbo jumbo i was hearing until now.

Also if this is true there is now way to break causality. Cause no information will travel faster then light which btw is not possible any way as we all know.

[K^2]

Everybody keeps making this jump to going backwards in time.

If our photon just left earth, and we leave to KIC, we arrive 1400 years before the photon does. We can't see the photon for another 1400 years. If we step back through the wormhole, the photon still just left. Nothing has changed in the flow of time.

No, you can actually leave after photon left and come back before it left. That's the problem. It's a genuine CTC, and it's possible with ANY method of FTL travel. Several people have explained this in detail several times.

[Bill Phil]

Everybody keeps making this jump to going backwards in time.

If our photon just left earth, and we leave to KIC, we arrive 1400 years before the photon does. We can't see the photon for another 1400 years. If we step back through the wormhole, the photon still just left. Nothing has changed in the flow of time.

Yes, the photon emitted when we left. Let's call the current time "A". But at time "B", in the past, photons were also being emitted. You then meat up with those photons.

- - - Updated - - -

No, you can actually leave after photon left and come back before it left. That's the problem. It's a genuine CTC, and it's possible with ANY method of FTL travel. Several people have explained this in detail several times.

Could you elaborate some, I'm confused as to how one can come back before it left.

[rtxoff]

No, you can actually leave after photon left and come back before it left. That's the problem. It's a genuine CTC, and it's possible with ANY method of FTL travel. Several people have explained this in detail several times.

How can someone explain in detail something which was not observed yet. Nobody ever observed time going backwards, and also nobody ever observed something going faster then light. FTL travel is simply not possible. And the whole FTL naming is misleading cause it's not what would happen if we could actually build the Alcubierre drive.

[problemecium]

Nobody has explained this in detail at all though.

People keep repeating "FTL = CTC = time travel = impossible" but nobody ever explains the details about WHY. WHY is it possible to come back before the photon left? HOW would you do such a thing?

And please don't say "because FTL means a CTC." That's like saying electrolytes are what plants crave because Brawndo has electrolytes and Brawndo says it has what plants crave.

[K^2]

How can someone explain in detail something which was not observed yet. Nobody ever observed time going backwards, and also nobody ever observed something going faster then light. FTL travel is simply not possible. And the whole FTL naming is misleading cause it's not what would happen if we could actually build the Alcubierre drive.

So, absolutely every part of that is wrong, including observing things. (We have observations from expansion of the Universe.)

This is actually very, very simple. We only discuss means of traversing space-time that leave it asymptotically flat. Which means that we can entirely ignore the warp bubble, wormhole, subspace, or any other implement of getting from A to B faster than light would. And the only thing we care about is world lines (or their projections) in associated flat Minkowski Space. Easy, right? Any world line that puts you outside of the light cone from starting point is FTL travel. Regardless of whether it was FTL locally. If we want to get to a point 10ly away in less than 10 years, we have to take such a trajectory. And any such trajectory would also permit time travel.

More importantly, whether you think FTL is possible or not is absolutely irrelevant. We have established constraints on hypothetical modes of travel that are consistent with all known laws of physics. If you know some laws of nature not known to scientific community, I would appreciate you publishing them in a peer-reviewed journal before insisting we take them into consideration.

[K^2]

Why? This is what everyone is asking. There's no reason that time travel is implied by this, unless you're referring to the notion that you can turn around and see light from the distant past of your point of origin. You didn't travel through time, but you are getting a visual impression of the past. Are we all missing something here?

If you are moving FTL in one frame of reference, you are moving backwards in time in another, and change of local frame of reference is merely a matter of accelerating. More precisely, if you accelerate to high enough speed, then perform an FTL jump, then accelerate in the opposite direction, and jump back, the net effect is moving backwards in time. You arrive at your starting point before you depart. Such is the geometry of Minkowski space.

[PB666]

CTC = closed timelike curve. And object starts at a given point in space time, this proceeds outward as light cones reflecting luminal and subluminal travel but can never return to any starting point. Such FTL travel allow for objects to return to a given point on the curve. Speculative because it creates the grandfather paradox.

[problemecium]

So if I get in a spaceship and accelerate to 0.99999 c, then I flip on the warp drive, go somewhere, and then accelerate to 0.99999 c in the other direction, I end up in the past?

Okay so I'm on Earth and I want to troll the universe. I rev up to near light speed and travel for several hours to KIC 8462852. When I arrive, 1400 years have passed. Rather than slow down, I switch on the warp drive and zoom back to Earth in five minutes. When I shut off the warp drive, presumably my momentum is conserved (no idea how I managed to stay inside the bubble, but no matter), so I find myself shooting back toward KIC 8462852 at near light speed.

The current year on Earth is 3415. I had seen Earth's future history go by in slow motion the first time I left, so I'm surprised to find that when I look at it now, everything's all shiny and futuristic. I blame time dilation for this.

Enraptured by the future, I slow down so I can stay and watch it. In so doing I accelerate strongly, which to me is equivalent to putting myself in a strong gravitational field. Time thus slows down for me, and by the time I've stopped, a few weeks have gone by. I'm still in Year 3415 though.

After getting my fill, I accelerate toward Earth until I reach near light speed, then flip on the warp again and zoom over to KIC 8462852. My time spent accelerating was for me a few minutes, but for outside observers a few weeks. When I go back to the distant star, five more minutes go by. I arrive there and... as far as I can predict I don't run into any future or past versions of myself.

Or did I do it wrong?

[Rakaydos]

If you are moving FTL in one frame of reference, you are moving backwards in time in another, and change of local frame of reference is merely a matter of accelerating. More precisely, if you accelerate to high enough speed, then perform an FTL jump, then accelerate in the opposite direction, and jump back, the net effect is moving backwards in time. You arrive at your starting point before you depart. Such is the geometry of Minkowski space.

This feels like an artifact of a particular calculation technique.

What is the Dv costs of going back, say, 1 second? how much do you need to accelerate, and how long would you need to jumpp for?

[PB666]

This feels like an artifact of a particular calculation technique.

What is the Dv costs of going back, say, 1 second? how much do you need to accelerate, and how long would you need to jumpp for?

FTL Energy cost are infinite, you dont have to go there. CTC is better defined as conjecture times conjecture.

If you had more energy than is present in the known universe,

if you could control it

if you had negative energy

if you then FTL ..........Thats the hard part accelerating backwards at 100g for 10 secs would suffice to move you back 10E-3 seconds provided you promptly returned FTL.

[K^2]

This feels like an artifact of a particular calculation technique.

What is the Dv costs of going back, say, 1 second? how much do you need to accelerate, and how long would you need to jumpp for?

It's an artifact of the geometry. FTL = Time Travel is a frigin theorem.

And dV cost can be as low as you want, so long as your FTL is sufficiently "fast". But for anything remotely sane, you need to get close to speed of light. So yeah, nobody says it's practical time travel. Just the fact that feasibility of one implies feasibility of the other, and physics has to deal with it. If time travel is flat-out impossible, than so is FTL. And that's that. If you can go FTL, however, it means that there is at least a theoretical possibility of going backwards in time, even if you'll never be able to actually manage it.

[Silver_Swift]

The way it was explained to me is that at a fundamental level the order of events is not a real thing for two events that are separated enough that they cannot influence each other without FTL.

So suppose you have place A and place B that are separated by 5 light minutes and event A1 and B1 that happen at those places within 5 minutes from each other. Now also suppose that from the perspective of someone at A, B1 happens before A1, then from the perspective of a spaceship C travelling at some specific (non-ftl) speed and direction A1 will happen before B1.

From there, it is easy to construct a scenario where FTL communication between A and C can be used as time travel. The tricky thing to accept here is that on a fundamental level neither the order A sees not the one C sees is wrong, this is not a case of our observations not matching reality. There is no "real" order between two events that are not in each others light cones, that is simply not how the universe works.

This, of course, goes completely against our intuitions about how time and distance are supposed to work, but that is because our brain was designed to not get eaten by tigers, not to think about the deeper nature of the universe.

Like I said, this is just how it was explained to me, is it about correct?

[lugge]

FTL Energy cost are infinite, ....

When you say this, do you mean FTL with "usual" propulsion? Like rocket- or Ion-drives?

Because, as far as I understand this topic, using FTL-drives (if there were any) is not really FTL.

Sure, you can go a 10LY distance in, say, 5 years.

It seems you went FTL, but in fact you just took an abbreviation (wormhole, folding the universe...)

I understand there are two different kinds of FTL, the "usual" one, which is in fact impossible (due to infinite energy required) and the "clever" FTL (using a physical cheat like wormhole...).

Am I right on this?

[rtxoff]

So, absolutely every part of that is wrong, including observing things. (We have observations from expansion of the Universe.)

So the expansion of the universe shows us time flowing back?

This is actually very, very simple. We only discuss means of traversing space-time that leave it asymptotically flat. Which means that we can entirely ignore the warp bubble, wormhole, subspace, or any other implement of getting from A to B faster than light would. And the only thing we care about is world lines (or their projections) in associated flat Minkowski Space. Easy, right? Any world line that puts you outside of the light cone from starting point is FTL travel. Regardless of whether it was FTL locally. If we want to get to a point 10ly away in less than 10 years, we have to take such a trajectory. And any such trajectory would also permit time travel.

Well this is something which still has to be observed, my standing point is we can't persist onto such things only by drawing a 2D diagram. Causality is basically the most important law of nature i really can't understand why an scientist would claim anything that will break it. Without causality our universe wouldn't exist yet ppl screaming here that an FTL drive would brake it. If we really will be able at some point to construct it then CTC is certainly not going to happen. You all can say what you want but this is surely something you can't mess with.

More importantly, whether you think FTL is possible or not is absolutely irrelevant. We have established constraints on hypothetical modes of travel that are consistent with all known laws of physics. If you know some laws of nature not known to scientific community, I would appreciate you publishing them in a peer-reviewed journal before insisting we take them into consideration.

Well i know some laws people here seem to try to break here constantly like causality. First you guys hold onto the well known laws of nature, then we talk further.

[PB666]

When you say this, do you mean FTL with "usual" propulsion? Like rocket- or Ion-drives?

Because, as far as I understand this topic, using FTL-drives (if there were any) is not really FTL.

Sure, you can go a 10LY distance in, say, 5 years.

It seems you went FTL, but in fact you just took an abbreviation (wormhole, folding the universe...)

I understand there are two different kinds of FTL, the "usual" one, which is in fact impossible (due to infinite energy required) and the "clever" FTL (using a physical cheat like wormhole...).

Am I right on this?

I mean the warp drive. just to power the known warp, you have to confine gravitational mass-energy to E-32 meters, which requires an amount of energy that exceeds an amount of energy available in our local. Production an equal an opposite negative energy field that has anti-gravity qualities is current not possible at at level outside of quantum scales.

you can propose any level of energy you like since all would not suffice. To make a true FTL device you need one to create the forward field, which then makes the gravitational energy requirement infinite. This goes by the fact to have one you need one, and it then needs one, and so on. Alternatively to travel any distance you would have tonhave a warp rail, which means you have to have been there first and second, you have to build the rail, the payload could not anticipate the activity of the field in either case.

Wormholes exist only at the quantum level, you can send any matter you like, provided you send a ship with the entangled particles. The problem is that whatever you want to send has to be subdivided to the quantum scale and reconstituted. This is plausible but impractable.

[cicatrix]

It would in theory work, but there are many things you won't be able to find out. Seeing even large details like Saturn's rings from 1MLj away is somewhere between very hard and impossible.

Albert VDS: you did not understand the original post. Also, the drive does not magically teleport you to your target but actually moves you (and a spacetime "bubble") to the destination, at a pretty high speed.

Theoretically, *IF* Alcubierre's Drive ever works, we could go several light years away and 'catch up' with the light reflected from Earth in the past. Then we need to build a HUGE telescope (with aperture diameter of several hundreds if not thousands kilometers). Then we would be able to actually see the past on Earth.

And yes, from the point of view of external observer, Alcubierre's drive allows FTL travel.

[Silver_Swift]

Causality is basically the most important law of nature i really can't understand why an scientist would claim anything that will break it.

Most important to whom? The universe is rather famously indifferent to our intuitions.

Without causality our universe wouldn't exist.

All this stuff is way above my knowledge of physics, but I don't think this is as self-evident as you think it is (or, depending on your definition, it isn't self-evident that time travel breaks causality). There are certainly apparent paradoxes that can be constructed, but that is also the case for other laws of physics that we are pretty damn sure about (like the whole racecar on a train scenario with regards to lightspeed).

The laws of physics as we understand them theoretically (might) allow for FTL communication and thus timetravel. The question we have to answer is whether the paradoxes we can construct can be resolved or whether they are real paradoxes, in which case there is something (that we missed) explicitly preventing FTL communication from happening regardless of loopholes used.

If this question was easy, we would already have the answer. There are so many extremely smart people working on this stuff that any open problems are not going to have obvious answers.

[PB666]

[rtxoff]

Most important to whom? The universe is rather famously indifferent to our intuitions.

What does intuition has to do with causality? Even a farmer knows that the apple can't hit the ground until it falls off the tree.

All this stuff is way above my knowledge of physics, but I don't think this is as self-evident as you think it is (or, depending on your definition, it isn't self-evident that time travel breaks causality). There are certainly apparent paradoxes that can be constructed, but that is also the case for other laws of physics that we are pretty damn sure about (like the whole racecar on a train scenario with regards to lightspeed).

The laws of physics as we understand them theoretically (might) allow for FTL communication and thus timetravel. The question we have to answer is whether the paradoxes we can construct can be resolved or whether they are real paradoxes, in which case there is something (that we missed) explicitly preventing FTL communication from happening regardless of loopholes used.

If this question was easy, we would already have the answer. There are so many extremely smart people working on this stuff that any open problems are not going to have obvious answers.

No it's exactly self evident as it is, there are only 2 choices here to go, either warp drives are possible but their function will turn out to be different then we assume and they will not break causality.The other option is they are impossible to do. It's your choice just don't say they are possible but will brake causality cause i'm sure this is not possible.