[imdoor]

Does an Alcubierre warp drive violate causality? From my understanding, it's no different than regular FTL travel, which implies time travel, which implies violation of causality.

To me that would indicate that an Alcubierre warp drive still shouldn't be possible despite the negative energy requirement being lifted.

[XorNot]

It did recently occur to me that it's not a hard requirement that FTL be impossible, just that causality not be violated. There may be solutions to FTL where naively they could violate causality, but in practice other effects ensure that no usage of them ever will.

[captn3m0]

The Atomic Rockets website[0] describes the issue clearly:

Relativity proves that FTL travel is identical to Time travel.

Time travel makes Causality impossible, since it can be used to create paradoxes. Note that to a physicist, it is not enough that time travel never happens to be used to make a paradox. The mere fact it is possible is enough to utterly destroy Causality.

1. So if you have Relativity and FTL, Causality is impossible

2. If you do not have Relativity, then FTL is not Time travel, so you can have Causality.

3. Or more mundanely you can have Relativity and Causality, but no FTL/Time travel

∴ Causality, Relativity, FTL travel: chose any two.

[0]: http://www.projectrho.com/public_html/rocket/fasterlight.php...

[codethief]

> The Atomic Rockets website[0] describes the issue clearly:

I don't find their description very clear. In particular, they don't even define

- what "faster-than-light" is supposed to mean in a non-relativistic setting (Newtonian mechanics) where there's no such thing as a universal velocity and you're dealing with Galilean velocity transformations.

- what causality is supposed to mean exactly outside the relativistic setting where you have a universal velocity which induces the causal structure.

All in all, their definitions are very inter-dependent, so the "trichotomy" they present as truth doesn't make much sense to begin with. From my POV, they're not even wrong[0].

Moreover, they seem to only consider Special Relativity and completely ignore General Relativity where the speed-of-light bound is only a local bound. Basically, even if what they say were true, it would still not apply to the General Relativistic setting we're looking at here.

> Time travel makes Causality impossible, since it can be used to create paradoxes

This is not so clear, either, compare e.g. https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...

[0]: https://en.wikipedia.org/wiki/Not_even_wrong

[XorNot]

This is missing the point though: it's not "optional" per se - it's that naively, if we simply go "what if FTL" then yeah, based on current physics we have a problem.

But the logic doesn't then mean "can't FTL" - it's still "can't have paradoxes". An FTL system, theoretically incapable of creating paradoxes, has no such problems.

An FTL system which prevents you from going faster then light in directions which allow violating causality would be totally fine, provided this was a theoretical restriction: i.e. your FTL engine just plain can't thrust in causality violating vectors, because it encounters some temporal restriction field or similar.

So the problem isn't "FTL is a causality violation", it's FTL without a mechanism to prevent causality violation is probably impossible.

The sci-fi conceit of it being difficult to plot efficient paths through hyperspace or whatever might well be quite a real thing, and simply related to charting causality-allowed courses through FTL-space.

[dragonwriter]

> Time travel makes Causality impossible

No, it makes the idea that causal sequence and temporal sequence are identical and that the former mapped against the latter can’t have cycles impossible.

Making causality more complex than we'd like to think it is isn't making it impossible.

[codethief]

Relevant link: https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...

[pdonis]

> Does an Alcubierre warp drive violate causality?

The original Alcubierre metric does not, but that's because it only describes a single "warp bubble" that travels in one direction, never starting or stopping and never turning around.

As soon as you have more than one "warp bubble", or you let a single "warp bubble" turn around, then you will have closed timelike curves, which violate causality.

[shepardrtc]

No, for the original Alcubierre, its warping spacetime around the information and moving that pocket. The ship or information or whatever is still traveling inside the pocket at its normal speed. So while it could be virtually instant (ignoring energy requirements), it wouldn't be time travel. However, I believe you could arrive to your destination and look back and see yourself before you warped - and maybe as you traveled - as the light catches up to you. At least that's my understanding.

[Filligree]

No, every FTL construction can be used to violate causality. There’s no way at all around that. Fundamentally it’s a geometric problem — relativity has a hard barrier at light speed, but if you can get past that, there’s nothing at all special about infinite speed. Is the same sort of 4-momentum as standing still, and you can accelerate in any direction from there.

The warp drive in the article is subliminal, though, so it doesn’t have that problem.

[bloopernova]

I've always had serious trouble understanding why FTL travel will break causality.

Do you know of any good layman-level explanations?

Like, if I FTL from point a to point b, 10 light years apart, in my super duper warp vessel. It takes me, for the sake of argument, 10 minutes to make that journey. Now say I set off a big comms laser at point a, sending a message to point b, before I left. I don't see that laser until 10 years later.

What am I missing? I know I'm missing something, but that seems straightforward to me. It's weird to butt up against that seemingly incomprehensible.

[weavejester]

Suppose you set off in a spaceship at 80% of lightspeed, or 0.8c, travelling away from Earth. At this speed, according to relativity, time is slowed to 60% of it's 'usual' value. So for every 10 hours that pass on Earth, only 6 will appear to pass on the spaceship.

However, this is only true from the perspective of someone on Earth. From the point of view of someone on the spaceship, the opposite is true. From their perspective, the spaceship is stationary, and Earth is travelling away from it at 0.8c. Therefore, for every 10 hours that pass on the spaceship, only 6 will appear to pass on Earth.

Suppose there was a way of instantaneously communicating between the two. On Earth, 10 hours into the mission, mission control sends a message to the spaceship. Because of time dilation, the spaceship receives the message only 6 hours into the mission, from their perspective. The spaceship then sends a message back, and due to the same time dilation effect, the message arrives on Earth 3 hours and 36 minutes into the mission (60% of 6 hours). In other words, the reply from the spaceship will arrive 6 hours and 24 minutes before mission control sends the original message.

[trhway]

>On Earth, 10 hours into the mission, mission control sends a message to the spaceship. Because of time dilation, the spaceship receives the message only 6 hours into the mission, from their perspective.

And from their perspective - "for every 10 hours that pass on the spaceship, only 6 will appear to pass on Earth." - the message was sent at 3:36 of the Earth time. They immediately send a response message which immediately arrives at 3:36 of the Earth time from their perspective. No paradox so far.

[AnimalMuppet]

There's a paradox if the message replies to the first message, because the reply arrives before the original message is sent.

[Sanguinaire]

I'm in the same boat as the person you replied to; breaking causality never made sense to me.

In the case of your explanation, what sticks out to me is the "Suppose their was a way of instantaneously communicating" part - it seems more intuitive to me that the warp bubble would not allow any communication across the threshold, effectively becoming a pocket universe.

[NoGravitas]

The ship you're in isn't what's traveling FTL, though; it's going at normal relativistic speeds. But say both you and Earth have an ansible (a faster-than-light communicator); then you get the problem in the previous comment.

[weavejester]

Instantaneous communication makes the numbers easier because you don't have to account for travel time, but causality can be broken with any superluminal form of communication.

If you have a ship with warp speed, then you have superluminal communication, because you can just carry a message on board. Even if you can't communicate inside the warp bubble, as long as you can exit the bubble at some point, then you can travel via warp, pop out, and transmit your message conventionally.

In my earlier example, if the ship and mission control had messenger drones capable of travelling many times faster than light, then the ship's response drone could arrive on Earth before mission control's messenger drone was launched.

[xucheng]

https://youtu.be/HUMGc8hEkpc

[spoiler]

Oh wow, this was fascinating! I was vaguely made aware of some of these concepts by a friend who studies quantum physics, but on he's never been able to explain it so "simply" (that's not doing the explanation any justice, it's not simple and some of it still went over my head, but "it clicked").

I'm eager to check out more of their channel!

[fennecfoxen]

Here's the low-math version.

We see that matter and energy and all their fields and stuff move through time and space continuously. They don't spontaneously relocate themselves a million light-years away, and they don't generally relocate themselves a million years into the past. They trace a continuous path through time and space. (Oh, sure, for individual particles there's some quantum weirdness about exactly where it is along the way, but I think we're dealing with larger systems here?)

Now, physics have told us that space and time are glued together into a four-dimensional spacetime. So pick any two given points in spacetime — in whatever reference frame you like, doesn't matter if you're on Earth or in a space or next to a black hole — and take measurements of the distance between them, and the time between them. Ask yourself, "can some matter or energy travel along a path from the one point in spacetime to the other?" This is what it means to say, "are these points casually connected?" Because causes and effects have physical carriers, and they must move through time and space.

We found experimentally that there is generally a special speed limit — the "speed of light", though other things also are affected by this limit. We also found that this speed is constant in all reference frames. Some reference frames will disagree on how much space and time there is (your usual twin paradoxes and the like) but it all works out so that all observers agree on how much spacetime is between the points. (The mathematical objects for representing spacetime are quaternions, and the space and time vectors are just particular projections of the underlying quaternions, onto a set of measurements for a reference frame. But I don't know how to operate quaternions, and the specifics don't matter here.)

Now here's the thing:

If you are able to go faster than the speed of light (as measured in any reference frame of your choosing) it is geometrically equivalent to the power to go back in time. It doesn't really matter how you go faster, how you connected those two points — teleport there, open wormholes, warp space — there is some reference frame where you have travelled back in time. If you perform your maneuvers right, and take the right path, that reference frame could be Earth.

There's no other barrier: the speed of light itself was the barrier, and you just broke it. And if you have done so successfully, that means that the way cause-and-effect actually happens is ... something weird we only understand poorly.

[NoGravitas]

This is the best explanation I've been able to find:

http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...

[QuadmasterXLII]

observers moving relative to each other agree on what “the speed of light” is, but disagree about what “2 x the speed of light” is. Specifically, they disagree about which superluminal trajectories are forward in time vs backwards in time. Therefore, if you have a device that accelerates you to 2x the speed of light, it must accelerate you to 2x the speed of light in your own reference frame. As a result, you can accelerate to .99 c relative to earth using conventional means, activate the device, and then be travelling at 2x the speed of light in the .99c frame, which is backwards in time in the earth frame. You could then stop, turn around, and repeat the procedure to arrive at earth before you left.

The Alcubierre solution avoids this by being symmetrical, so that observers don’t have to agree on which direction is the destination and which is the origin

[ben_w]

That’s something I’ve been meaning to ask about:

Does FTL really inevitably mean that, or is that a consequence of saying “no preferred frame of reference” and therefore having e.g. “100c” meaning different things to observers in relative motion?

[crooked-v]

"No preferred frame of reference" is how the universe fundamentally works - if you remove that, you're looking at a universe that functions differently than our own in a lot of basic ways.

[ben_w]

Follow up question: while no preferred frame is obviously the simplest model, is it really impossible to add a preferred frame? e.g. https://youtu.be/6MfJ59lkABY?t=413

[QuadmasterXLII]

You are correct, it requires “no preferred frame of reference”.

[codethief]

> No, every FTL construction can be used to violate causality.

Not necessarily, compare https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...

[imdoor]

Ah, i somehow read the subluminal as superluminal in the abstract and got all excited. But the subluminal restriction makes the paper's findings make more sense.

[mabbo]

> I believe you could arrive to your destination and look back and see yourself before you warped - and maybe as you traveled - as the light catches up to you

That does sound like traveling faster than light, but I think there's a reasonable view that makes it not.

Suppose you traveled at some very fast speed around a black hole. It bends space time in such a way that you might, at some point, be able to collect photons that could be reconstructed into an image of you earlier in time.

In short, when space time is flexible and bending, there are paths in which you can see your previous self. An alcubierre drive is a means to bend space time.

[vbezhenar]

Why do you think that violation of causality is not possible?

[imdoor]

I don't know enough to think that violation of causality is not possible. It just just seems very unlikely. I can't imagine how the world would look like if it wasn't the case because of the paradoxes it would imply.

You seem to imply that you think differently. Care to elaborate?

[vbezhenar]

I have no idea either, but I'm not aware of any fundamental reasons that violation of causality is forbidden.

And imagining world with all those spooky quantum effects is already almost impossible, at least for me :)

[randallsquared]

Not the GP, but both the many worlds interpretation and superdeterminism would avoid (the issues of) causality violation, would they not, even in the presence of time travel?

[threesongs]

Violation of causality is equivalent to travelling back in time. Lots of problems there, and no good model for such things.

[pugworthy]

  ...regular FTL travel...

It's kind of exciting that we see phrases like this now. I mean it wasn't that long ago that the idea of a 9 story first stage rocket coming back to land (and fly again) was fantasy.