Alcubierre drive research

[Guest]

You're all missing an important point: Alcubierre drive is not an FTL drive. It messes with spacetime curvature. In the example for Mars from the 1st page, you wouldn't actually be able to overtake your message, or any photons already underway. When you arrive at Mars, the message would still be in front of you. How's that possible? It's hard to imagine, but the most sensible approach to Alcubierre drive (the one actually being investigated) results in formation of a horizon-like structure in front of the warp bubble. It "scoops up" photons in front of the jumping ship, and then releases them all in a great big flash in front of the ship once the bubble is turned off. In our example with Mars, the flash would contain any messages sent within 4 minutes from departure (undoubtedly mangling them beyond recognition), ambient light, light reflected from Earth in the direction of Mars, and ship's headlight beams. All that energy would be accumulated in front of the ship and released all at once, so let's just say you need to be terrible careful where you point your warp ship.

You can never actually overtake any light, which should preserve causality, at least the way I understand it. Not many people, even scientists, can actually imagine spacetime itself bending like that, and it's understandable. However, papers are not based on imagination anyway (a fallible tool once you go out of "human comfort zone"), but on cold, hard math. A lot of near-unimaginable things are not only possible, but common in physics. This causes a lot of grief for anyone studying the matter (believe me, I'm doing just that ).

As for Ph.Ds., I'm going to just say that it really depends on which ones you believe. The scientific community is far from agreement on the Alcubierre drives, even in a much less far-fetched, STL version of the problem (interesting in it's own right, since it'd release you from the tyranny of Rocket Equation, being effectively a reactionless drive). Dr. Harold White definitely seems to think it's possible, he's got some theoretical physicists behind him, but there's a fair number of papers against the idea, too.

Edited October 14, 2014 by Guest

[ZetaX]

You're all missing an important point: Alcubierre drive is not an FTL drive.

If you are just using a "very fast but slower than c"-drive, then yes. But that's not what the OP was talking about, he needs to catch those photons.

In the example for Mars from the 1st page, you wouldn't actually be able to overtake your message, or any photons already underway.

Then the scenario in the OP would not even be possible. A true Alcubierre drive can go FTL, it's just not locally breaking c.

And if you can go faster than c, then there is no need to "mangle" the message. Just fly around it: go 10 dgeree to the "left", fly a bit faster than originally needed, and then correct at the end, then wait for the photons to arrive.

[N_las]

You're all missing an important point: Alcubierre drive is not an FTL drive.

As long as you reach a destination before a hypotetical light beam that was sent out at the same time, then you are travelling FTL. It doesn't matter if there is an actual light beam underway or not.

It messes with spacetime curvature. In the example for Mars from the 1st page, you wouldn't actually be able to overtake your message, or any photons already underway. When you arrive at Mars, the message would still be in front of you. How's that possible? It's hard to imagine, but the most sensible approach to Alcubierre drive (the one actually being investigated) results in formation of a horizon-like structure in front of the warp bubble. It "scoops up" photons in front of the jumping ship, and then releases them all in a great big flash in front of the ship once the bubble is turned off. In our example with Mars, the flash would contain any messages sent within 4 minutes from departure (undoubtedly mangling them beyond recognition), ambient light, light reflected from Earth in the direction of Mars, and ship's headlight beams. All that energy would be accumulated in front of the ship and released all at once, so let's just say you need to be terrible careful where you point your warp ship.

You can never actually overtake any light, which should preserve causality, at least the way I understand it.

What you are talking about is the explanation why the speed of light is constant c inside the bubble. It is the explanation why we can't "see" photons flying slower than us while using the drive. So it has to do whith local causality inside the bubble.

But is has nothing to do with the global preservence of causality, and as ZetaX described, it is possible to drive "around" a message, so you won't destoy it.

Edited October 14, 2014 by N_las

[Mr Shifty]

But is has nothing to do with the global preservence of causality, and as ZetaX described, it is possible to drive "around" a message, so you won't destoy it.

It's quite possible I'm missing something, but how could the A-drive, as described, be used to violate causality?

[N_las]

It's quite possible I'm missing something, but how could the A-drive, as described, be used to violate causality?

There is this locked thread: http://forum.kerbalspaceprogram.com/threads/93377-FTL-communication

We discussed how FTL communication would allow for communication in ones own past. It is a direct consequence of the fact, that vacuum light speed is constant for every observer.

And the A-drive as described could be used for FTL communication.

For a short explanation, look at this: http://en.wikipedia.org/wiki/Time_travel#Via_faster-than-light_.28FTL.29_travel

Or this: http://www.theculture.org/rich/sharpblue/archives/000089.html

Edited October 14, 2014 by N_las

[Guest]

As long as you reach a destination before a hypotetical light beam that was sent out at the same time, then you are travelling FTL. It doesn't matter if there is an actual light beam underway or not.

Here's the thing, though. You don't. A hypothetical light beam sent out at the same time, traveling through space with the same curvature function (remember, the curvature changes in time), would still be faster. An Alcubierre drive arrives faster only in comparison to a beam travelling through flat spacetime, or at least to unaltered spacetime. This is a fundamental difference. You're not moving, you're literally reshaping the universe to put yourself where you want to be!

Now, the thing with "driving around the impulse" is curious. Maybe you could indeed do this that way, but I'm not sure if that'll do. Weird things happen when you start messing with spacetime curvature. Certainly, it's possible to overtake some photons globally, but I'm not sure if there's a way of using the effect to actually "meet" photons you otherwise shouldn't. I'm not sure if "fly a bit faster than originally needed" applies, or rather, if ZetaX' example doesn't end with you being late/sweeping photons up anyway, due to some property of the curvature in question. Math involved is beyond me, but I know they can get weird.

Global issues with causality are an important point critics use. On the other hand, the very fact that the space is not flat (and in fact, curved incredibly strongly) around an Alcubierre drive means a lot of approximations we're familiar with stop to apply.

[Rakaydos]

Niven's Theory of Time Travel:

"In an infinite universe where changing the past is possible, the net result of all changes will be a universe where changing the past never happens."

For instance, in an infinite universe, someone might get the idea to prevent the invention of the time machine. The result is a universe where the time machine, while possible, is never invented. Or where the attempt results in the invention of the time machine in the first place.

In the event of a causual loop (I killed my grandfather, so I was never born to go back and kill my grandfather), quantum uncertianty is "rerolled" on each cycle, until you reach a consistant universe by sheer coinciddence. (As I go to kill my grandfather, the gun in Schrodinger cat-box in the next room goes off and sends me into a coma until after my father is born)

Edited October 14, 2014 by Rakaydos

[rtxoff]

We already discussed to death in another thread: FTL communication makes time travel possible. ANY FTL drive allows automatically for FTL communication, so ANY FTL drive will enable time travel.

Your remark about Ph.D.s lacking the ability to imagine the consequences of FTL travel is offensive in the highest degree, considering that you don't have one iota of understanding about relativity and physics in general.

And your remark about a sensless discussion, because there wouldn't be any experiments possible: The idea that FTL travel results in time travel can be easily derived with the axioms of special relativity. And we have unlimited experimental evidence confirming those axioms.

Well i am sorry that you feel like this but this is my opinion and as long as nobody actually builds an experimental alcubiere drive and delivers some facts my opinion about this won't change.

And you are still talking about relativistic FTL travel which never will be possible. And i am talking about non relativistic FTL travel without actually moving through space (or at least not very fast). This is the point where all your axioms of special relativity dissolve in a state of clueless. You are trying to reason here where there can't be any reason yet as long as we can't find a way to actually build such an drive.

You are still offending me by saying a have no clue about relativity where the exact opposite might be true. If you are really have a Ph.D. then please disclose who you are and share some of your work with us.

I am really anxious to see some of it.

[YNM]

Does bending the spacetime around you (like in a hypothetical alcubierre drive) then moving, along with the bubble WRT local outside-bubble spacetime allows apparent FTL ? ie. you'll arrive at the place where the world line from an event at your origin haven't reach that site ?

Maybe Alcubierre drive are useless ?

I have a thing to say through : due to Universe's expansion, it's expected that there'll be places where no light will approach us due to the spacetime there already moves faster than c wrt our local spacetime. Heard it from an astronomy mag and someone just about to complete master in astronomy. Is it true ? Can this fact be applied to the case in some way (maybe, if the universe stopped expanding) ?

[N_las]

Well i am sorry that you feel like this but this is my opinion and as long as nobody actually builds an experimental alcubiere drive and delivers some facts my opinion about this won't change.

And you are still talking about relativistic FTL travel which never will be possible. And i am talking about non relativistic FTL travel without actually moving through space (or at least not very fast). This is the point where all your axioms of special relativity dissolve in a state of clueless. You are trying to reason here where there can't be any reason yet as long as we can't find a way to actually build such an drive.

You are still offending me by saying a have no clue about relativity where the exact opposite might be true. If you are really have a Ph.D. then please disclose who you are and share some of your work with us.

I am really anxious to see some of it.

It doesn't matter if you actually move through space. The ship doesn't have to move fast. It could just jump to the targed position without any movement involved. The time travel isn't dependent on the method of FTL travel. I explained that in the communicaion thread, and you still don't understand anything of it. The effect doesn't arise from something having a velocity greater than c. It arises from information from point A reaching a point B outside of A's light cone. How the information got there is irrelevant. It doesn't matter if the A-drive ship actually moves or not, if it reaches a point outside of its origins light cone, then you can make time travel with it.

It is really obvious to anyone that you don't have any physics education beyond High School. So the claim that "the exact opposite might be true" is laughable.

You posted things like this, and you excpect anyone to think you would understand relativity?

Why should it? You just said the ping back is instant. Why should the velocity between sender and reciever matter? It will always be instant. However the controls of the rover will be more or less inert because for the rover time will run faster or slower.

If you could take a picture of the whole universe with an exposure time of the planck time you would have a moment in the universe that is absolutely same for all entities in it.

I don't have a Ph.D, only a B.Sc. in engineering physics.

But that doesn't matter, because the FTL time travel thing isn't "just my opinion". It is accepted by nearly every physicist in the world. It is something so fundamental that everybody who learned about relativity on an university level will agree.

[Rakaydos]

I don't have a Ph.D, only a B.Sc. in engineering physics.

But that doesn't matter, because the FTL time travel thing isn't "just my opinion". It is accepted by nearly every physicist in the world. It is something so fundamental that everybody who learned about relativity on an university level will agree.

Yet they always caviat it- they arnt saying FTL is impossible. They're saying "General Relativity, FTL, Global Causality... pick 2" and then pointing out how much evidence we have for General Relativity.

But as accurate and predictive as general relativity is, we KNOW it has blind spots- that's the reason Quantum mechanics exists. Hypothetically just as neutonian physics was valid and predictive short of near-lightspeed, there could be errors in General Relativity that require tachionic relative velocities to resolve.

Particularly as a warp drive brings it's frame of reference with it, includig warping it's lightwall in a narrow area to permit it.

But even if general relativity is perfectly true, it is still possible for FTL to exist. All you need to throw out is global causality. Local causality + predestination is just as valid as global causality.

Edited October 14, 2014 by Rakaydos

[N_las]

Yet they always caviat it- they arnt saying FTL is impossible. They're saying "General Relativity, FTL, Global Causality... pick 2" and then pointing out how much evidence we have for General Relativity.

But as accurate and predictive as general relativity is, we KNOW it has blind spots- that's the reason Quantum mechanics exists. Hypothetically just as neutonian physics was valid and predictive short of near-lightspeed, there could be errors in General Relativity that require tachionic relative velocities to resolve.

Particularly as a warp drive brings it's frame of reference with it, includig warping it's lightwall in a narrow area to permit it

We don't need general relatvity to explain FTL time travel. Special relativity can do the same, and it is perfectly compatible with quantum mechanics.

But even if general relativity is perfectly true, it is still possible for FTL to exist. All you need to throw out is global causality. Local causality + predestination is just as valid as global causality.

I agree. It could be that FTL exists, and global causality doesn't. I am just arguing for the point, that FTL will allow time travel.

Edited October 14, 2014 by N_las

[magnemoe]

It doesn't matter if you actually move through space. The ship doesn't have to move fast. It could just jump to the targed position without any movement involved. The time travel isn't dependent on the method of FTL travel. I explained that in the communicaion thread, and you still don't understand anything of it. The effect doesn't arise from something having a velocity greater than c. It arises from information from point A reaching a point B outside of A's light cone. How the information got there is irrelevant. It doesn't matter if the A-drive ship actually moves or not, if it reaches a point outside of its origins light cone, then you can make time travel with it.

It is really obvious to anyone that you don't have any physics education beyond High School. So the claim that "the exact opposite might be true" is laughable.

You posted things like this, and you excpect anyone to think you would understand relativity?

I don't have a Ph.D, only a B.Sc. in engineering physics.

But that doesn't matter, because the FTL time travel thing isn't "just my opinion". It is accepted by nearly every physicist in the world. It is something so fundamental that everybody who learned about relativity on an university level will agree.

Special relativity is not in general physics education even on university level, general relativity is.

Its pretty obvious you can not travel faster than light trough normal space as in accelerate to 2g. Any hypothetical FTL drive cheats.

Time travel as in traveling back in time so you can give an time machine to your younger self opens an major can of worms, travel forward in time, back again and then change the future is just surrealistic

Physicist know this, so why will any of them touch FTL with an ten foot stick? Stuff like UFOs or cold fusion sounds far safer

And yes the Alcubierre drive does not looks practical to me either.

[rtxoff]

But that doesn't matter, because the FTL time travel thing isn't "just my opinion". It is accepted by nearly every physicist in the world. It is something so fundamental that everybody who learned about relativity on an university level will agree.

I never denied the time travel with an alcubiere drive, sure you will travel through time but also through space and because you are not travelling relativistic the travel back will bring you back to your spacetime and not to your past. The case where you travel back to kill you uncle is just not possible, that is the point. So an paradox won't be created with an hypothetical AD at least none which would brake your causality.

And the stuff i posted in the FTL comms thread makes perfect sense. The logic in it is undeniable however i am aware it might not be the truth because FTL comms might stay wishful thinking forever. But also i know there is no way to verify it yet by experiment because we don't have FTL comms and also we don't have an alcubiere drive.

Also attacking someone personally makes no sense at all, it is an irrational thing to do in a science thread. You could attack my arguments if you had something relevant to say, obviously you are attacking me because you see no way attacking my arguments.

I have learned some stuff here about the hypothetical alcubierre drive by people which invested more work in trying to think through how to make it work. Look for an example in this thread.

I was also sceptical about it at first, however by now i think there might be a way to make it happen but it won't work as long as people think that an travel with it could bring you back in your past spacetime because that is something that IMO never will be possible.

[ZetaX]

Special relativity is not in general physics education even on university level, general relativity is.

What. That makes no sense. We even do special relativity (at least some aspects of it) in high school, and it is taught pretty early in university. And any course on general relativity (which will exist at any serious university) will require or recall special relativity.

Do you even know what special relativity is¿

I never denied the time travel with an alcubiere drive, sure you will travel through time but also through space and because you are not travelling relativistic the travel back will bring you back to your spacetime and not to your past.

Yeah, ignore my post on symmetry of physical laws (which are very well tested by the way). Look up the Noether principle, if your claim is true you would not only break special and general relativity, but also conservation of momentum and energy.

[N_las]

I never denied the time travel with an alcubiere drive, sure you will travel through time but also through space and because you are not travelling relativistic the travel back will bring you back to your spacetime and not to your past. The case where you travel back to kill you uncle is just not possible, that is the point. So an paradox won't be created with an hypothetical AD at least none which would brake your causality.

This is wrong and I explained it a hundred times already. You just don't understand the topic. Even on Wikipedia, you can find an easy to understand explanation: http://en.wikipedia.org/wiki/Time_travel#Via_faster-than-light_.28FTL.29_travel

And the stuff i posted in the FTL comms thread makes perfect sense. The logic in it is undeniable however i am aware it might not be the truth because FTL comms might stay wishful thinking forever. But also i know there is no way to verify it yet by experiment because we don't have FTL comms and also we don't have an alcubiere drive.

It makes sense from an ignorant point of view. I don't claim you are stupid and don't make sense. With this quotes, I just wanted to showcase that you don't understand relativity.

Also attacking someone personally makes no sense at all, it is an irrational thing to do in a science thread. You could attack my arguments if you had something relevant to say, obviously you are attacking me because you see no way attacking my arguments.

Pointing out that you don't have understanding about a certain topic isn't a personal attack. I have refuted your arguments. I explained (on the other thread) in great detail how FTL travel allows time travel. You haven't shown how these explanations are wrong, but you have claimed they don't apply to A-Drive, beause it isn't actually moving. I refuted that argument.

I have learned some stuff here about the hypothetical alcubierre drive by people which invested more work in trying to think through how to make it work. Look for an example in this thread.

I was also sceptical about it at first, however by now i think there might be a way to make it happen but it won't work as long as people think that an travel with it could bring you back in your past spacetime because that is something that IMO never will be possible.

Even it your linked thread, K^2 explaines that A-drive would mean time travel. He specified that in flat spacetime a single A-drive couldn't be used for that, but thats why all my explanations in the old thread contained a third observer. Please provide an argument, why those explanaitions fail.

Edit: I am not arguing againt the A-drive, I only want you to understand, that it would mean real time travel. You can (at least) sent information back to your own past with it. I don't understand why you accept the information about the A-drive on your linked thread, but you are violently opposed to the information given to you in the FTL-communications thread. They aren't mutually exclusive.

Edited October 14, 2014 by N_las

[Mr Shifty]

Yeah, I'll concur; I was incorrect. I hadn't considered a third observer travelling at relativistic speeds. You'd see time travel even in Minkowski (flat) space-time and should be able to show this using just the equations from special relativity. I tried to come up with an example with numbers last night, but was too tired.

[-Velocity-]

One of the quotes from one of the papers I linked yesterday reflects a feeling I have as well. Regarding the speed of light limit-

So a strong formulation of it seems somehow encoded in natural laws. Can this

have a deeper meaning? Is it just a limitation to our possibility to travel and communicate or is it required by

consistency in the spacetime fabric?

Calling c the "speed of light" is actually an oversimplification; many other things travel at c (massless particles) so besides the electromagnetic force, gravity and the strong force also travel at the speed of light (supposedly, the force carriers for the weak force have a rest mass, and so strangely, the weak force supposedly does not travel at the speed of light).

Anyway, for some time I've gotten the feeling that the incredible lengths the universe goes through to prevent anything from violating c is telling us something fundamental about reality. I'm not sure exactly what, but I just get the feeling that the inviolability of c may be one of the most fundamental aspects of existence, and it may reflect something deeper about the universe that we still don't fully understand, or at least, I don't understand. I do know that inconsistent realities (two causally-connected observers can experience entirely different events, an absolute impossibility) can be created using FTL communication using a thought experiment I found.

[Vanamonde]

And again I say, please discuss the subject rather than each other.

[AngelLestat]

One of the quotes from one of the papers I linked yesterday reflects a feeling I have as well. Regarding the speed of light limit-

Calling c the "speed of light" is actually an oversimplification; many other things travel at c (massless particles) so besides the electromagnetic force, gravity and the strong force also travel at the speed of light (supposedly, the force carriers for the weak force have a rest mass, and so strangely, the weak force supposedly does not travel at the speed of light).

Anyway, for some time I've gotten the feeling that the incredible lengths the universe goes through to prevent anything from violating c is telling us something fundamental about reality. I'm not sure exactly what, but I just get the feeling that the inviolability of c may be one of the most fundamental aspects of existence, and it may reflect something deeper about the universe that we still don't fully understand, or at least, I don't understand. I do know that inconsistent realities (two causally-connected observers can experience entirely different events, an absolute impossibility) can be created using FTL communication using a thought experiment I found.

That always was my stance too.

In my opinion, understand why the speed of light is as it is; holds the greatest secret of the universe. Its true form.

[Technical Ben]

It's one of those things. When we know why the limits are, we know not just what they are, but where they come from.

My PC is limited by it's clock speed. Software is limited by calls to the OS/hardware. Spacetime might be limited by it's force carriers or by it's fundamental construction.

[Blackholedestroyer]

What if you bemt space time half way in a black hole?

[PB666]

It's one of those things. When we know why the limits are, we know not just what they are, but where they come from.

My PC is limited by it's clock speed. Software is limited by calls to the OS/hardware. Spacetime might be limited by it's force carriers or by it's fundamental construction.

Or its not a construction at all, its the appearance of what the averaged many do. Where have we identified quantum gravity, if you know the certainty of this, you the are much better seated to understand the fundamentals of inertia.

Just a point, we know with great precision the gravitational constant of the earth, we neither know its mass or the universal constant with great precision. The Earth’s gravitational constant is based on empirical not theoretical science. Attempts to measure actual gravitational constant with great precision have rather fuzzy results. There is a theory out now that says that quantum gravity acts through worm holes, if so then these wormholes open with all the certainty of quantum mechanics.

[K^2]

supposedly, the force carriers for the weak force have a rest mass, and so strangely, the weak force supposedly does not travel at the speed of light

Higgs. It's all Higgs. That's why they have mass and why they propagate at sub-luminal speeds. Not that it has any impact on the thread as a whole. Just thought I'd point it out for these curious.

[SciMan]

According to our current understanding of how the physics involved work, If an Alcubierre drive can be made to work, it will be limited to never exceeding the speed of light.

The great thing about an Alcubierre drive is not that it can go FTL (it can't), it's that you don't experience time dilation because the space around the ship is moved, instead of the ship itself. If you think of the space around the ship as a "container" for the ship itself, the Alcubierre drive is moving the container, not the ship.

This means that you won't be able to look into the past any further back than when you launched the ship with the Alcubierre drive on it.

Once the ship gets to 100LY out and stops (which would take AT LEAST 100 years from the Earth's frame of reference), if you wait a hundred years on the ship you can get news from "100 years in the past on Earth from the ship's frame of reference" like you imagine. This does not violate causality, nor does it cause a closed timelike curve, or any of that silliness.

By the time the signals you send about your observations of the signals detected from Earth get back to Earth, they will be observations of events that took place at least 200 years ago. Still not time travel.

No FTL travel = no FTL communication = no time travel.

Crack FTL travel and you've cracked FTL communication, because messages can be carried by FTL ships.

Crack FTL communication, and you "might" have cracked FTL travel, assuming some form of tele-operation.

Crack either FTL travel OR FTL communications, and you've created a time machine. In fact, as far as we know, getting something to go faster than light is the ONLY way to make a time machine.

However, some time machines don't break causality.

There is at least one way that I know of to create a time machine, that doesn't break causality.

A stable traversable wormhole with one end at Earth and the other end 100LY away would allow you to look back 100 years. However, due to the fact that you can't take the "normal" space route from one wormhole end to the other in LESS than 100 years, it doesn't break causality, even tho it's a time machine.

If that wormhole was taken out there by using something that can travel at high relativistic speeds, there will be a time difference between the ends. Travel further and/or faster, and the time difference increases.

If the ends of this wormhole are then brought back together, eventually it forms a causality breaking time machine. The theories I've read indicate that at that point the wormhole reverts to a pair of black holes. Those same theories state that this process of reverting to a black hole is extremely violent even on a stellar scale. By that, I mean it can vaporize STARS.

In other words, time machines are possible, but not causality breaking time machines. Once the future can effect the past, star systems start exploding.