Warp Drive May Be More Feasible Than Thought, Scientists Say • Hacker News Board

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#171

Post by Dylan16807 » Wed, Sep 19, 2012, 5:15 PM UTC

lmm wrote: post #161

Dylan16807 wrote: post #148

Earlier quoted context omitted.

Well, either one can send a single message. It's the reply from a different frame that causes trouble. In fact you can have unlimited FTL messages sent in a single reference frame. It's only when you combine multiple FTL reference frames that things get wonky.

But if B can't send an FTL-message to A because he's going to receive an FTL-message from A in what to him is the future, that's your causality violation right there.

Right, I should be more precise with the scenario. A cancelled-message is still a transfer of information. To avoid problems you have to set up the scenario such that only one ship would send a message no matter what. But it can be either ship, and you don't have to know which ahead of time.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#172

Post by nessus42 » Thu, Sep 20, 2012, 3:25 AM UTC

cygx wrote: post #160

cygx wrote: post #132

Earlier quoted context omitted.

Tachyons do not necessarily violate causality if you actually take the time to derive the laws of tachyonic kinematics consistent with special relativity. Sure, tachyons will be strange (different observes won't agree on direction of travel or charge of a tachyonic particle - if we are even able to localize the particle at all; there will be frames of reference where a tachyon won't have energy but will still have mo…

This answer doesn't deserve its downvote: The problems with tachyons were largely resolved in the 80s, based on work by Stückelberg and Feynman, Schwartz, Recami and others. The community just lost interest in them because our current models of particle physics prevents bare tachyons: QFT does predict real tachyonic particles. However, they are solutions of unstable vacua, which transit to a tachyon-free phase via sy…

But you can't use them to transmit information at FTL speeds without violating causality.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#173

Post by nessus42 » Thu, Sep 20, 2012, 4:02 AM UTC

cygx wrote: post #129

nessus42 wrote: post #100

Earlier quoted context omitted.

I'm not sure whether the Alcubierre drive contradicts causality, but if it does, I highly doubt that it is because of the arguments presented on this forum. The Alcubierre drive most certainly does violate causality, and this follows from just the most basic amount of Special Relativity that you might learn if only you'd spend your time learning real things, rather than spreading disinformation.

Your reasoning is flawed. The argument against the Alcubierre drive cannot stem from special relativity as it would only apply if there were a violation of local causality. The problem is that using an Alcubierre drive, one could construct closed timelike curves, which are impossible in a 3+1-dimensional, asymptotically flat and singularity-free spacetime where the weak energy condition holds. Also keep in mind that…

My reasoning is not flawed. And the paper you pointed me on re tachyons, specifically states that we cannot use tachyons to transmit information via them.

I never stated that nothing can travel faster than light. Only that WE cannot travel faster than light, nor can we transmit information faster than light, without violating causality.

On the other hand, maybe we can violate causality.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#174

Post by nessus42 » Thu, Sep 20, 2012, 4:31 AM UTC

stryker wrote: post #165

nessus42 wrote: post #152

Earlier quoted context omitted.

Wormholes most certainly do allow you to violate causality! How about I just point you at the FTL FAQ? http://www.physicsguy.com/ftl/html/FTL_part4.html

Can I talk to you offline? How can I message you on this forum? I feel like I'm missing something. The FAQ defines FTL as "getting to a location faster than light" but what if light is taking a suboptimal path? Assuming wormholes exist, depending on your frame of reference, it would take light different times to get to the destination. The FAQ says to consider the case of someone shooting a bullet through a wormhole…

Special Relativity only applies if the space between A and B is relativity flat. As long as space is flat in the vicinity, then the predictions of Special Relativity hold true. It doesn't matter if you made some weird topological wormhole that provides a shorter path. If Special Relativity has shown you how to, for instance, make a closed causal curve in the flat part of the space, using the events that occur in the flat part of the space, then SR will be correct in that prediction. The fact that there are non-flat parts of space elsewhere that were used to facilitate the aforementioned events in the flat part, is irrelevant to SR.

If, on the other hand, you are talking about cosmological wormholes that might let you travel some place that is far, far away--e.g., outside our Hubble sphere--then Special Relativity might not apply. For instance if the reason we can have a wormhole is because space ever-so-slowly curves back onto itself, and there ends up being a short little bridge between two pieces of the universe that turn out to be very close to each other in the space that encloses our 3D space, but far away when measured as an ant crawls within our space, then Special Relativity won't be of use because the space between A and B is not flat. But in that case, GR lets you violate causality with such wormholes, even if you can no longer analyze the issue with SR:

http://en.wikipedia.org/wiki/Worm_holes#Time_travel

You can write to me at doug at alum.mit.edu, but I can't provide much deep insight to GR, as my knowledge of it isn't much more than that of a knowledgeable layperson. I only really understand SR and when it can be applied.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#175

Post by lmm » Thu, Sep 20, 2012, 9:45 AM UTC

SolarNet wrote: post #163

lmm wrote: post #122

Earlier quoted context omitted.

Under SR, for two events (an event is a point in 4D space, i.e. a space and a time) with spacelike separation (i.e. outside each other's light-cones), neither comes objectively before or after the other. Different observers travelling at different speeds have different "surfaces of simultaneity", which say which events happen simultaneously. It's a theorem (or intuitively obvious) that for any two events with spaceli…

But is it really important that it looks like you arrived after you left? Can't we just know that the light from one thing took longer to reach us than the other so what we are seeing is just an artifact of FTL. Can that prevent causality? Or does it prevent relativity?

All observers are equally valid under SR; it would be very arbitrary to say that some of them are seeing "artifacts". Physics is based on observation, and the natural way to interpret our observations is to say that what we saw really happened. (Obviously this isn't true if some observable effect is distorting our perception, but that's not what's happening here - the observer who sees the person arrive before they left is a perfectly normal observer using perfectly normal equipment). Now if you want to say physics works differently for observers in different frames, that's fine, but it goes against relativity and all known physics.

Now assuming we accept that FTL-traveller really did arrive before they left, if that's all they do it's a matter of language as to whether they have violated causality by so doing. But it would take some very perverse physics (again, basically discarding relativity) to say they couldn't simply jump back, again going backwards in time, and arrive back at their starting point before they left it, at which you definitely have causality violation.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#176

Post by lmm » Thu, Sep 20, 2012, 9:54 AM UTC

SolarNet wrote: post #162

lmm wrote: post #121

Earlier quoted context omitted.

We're not applying SR to an observer who warps, just to two observers in inertial frames of reference. Suppose we have guy A and guy B who have clocks. They're on spaceships which are both drifting away from each other (i.e. provide inertial frames) at speed c/root 2; at time t=0 they were in the same place. From SR it should be clear that in A's frame, his clock reads 100 at the same time as B's frame reads 50, whil…

So This may not be an FTL drive, but the way I always sort of pictured it is that the FTL moves you forward in time to the relative observer (so if you move distance x you move (c/x)^-1 forward in time). So if you send an FTL courier (assuming travel time of 2) from point A with times tA=102 tB=51 it would arrive at point B at times tA=52 tB=104 (A time 53 units in the future from tA), if it got sent back it would ar…

So as far as B's concerned, when he sends the FTL courier to A that courier will goes slower than a light-speed message? He could send a message at tB=53,tA=26.5 and it would reach A well before tA=106.

This only works if you assume the FTL drive is fixed to some sort of absolute reference frame (which happens to be the same as A's frame). At which point you've given up on relativity, and I'll bet it's possible to come up with some cute paradoxes

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#177

Post by nessus42 » Thu, Sep 20, 2012, 12:46 PM UTC

Dylan16807 wrote: post #170

nessus42 wrote: post #169

Earlier quoted context omitted.

> I know that 'causality' is a bad term for me to use here; I'm open to suggestions. I think that you are looking for the term "closed causal curve" or "closed causal loop". If you use FTL to achieve this, this is often called a "closed spacelike curve". If you use GR without FTL to achieve this, it is often called a "closed timelike curve". I'm not convinced that other sorts of violations of causality are any better…

Well the important part here is that you can pretty simply have a self-consistent universe that has mildly-limited FTL (and highly-limited time travel) if nobody makes a loop. And I'll try to use something like 'causal loop' in the future.

And what happens if you try to make a loop? Do you think it sensible that the universe provide you with the ability to make causal loops and then somehow forbids you from actually making them, even though once you've been given the ability, they are very easy to generate?

Additionally, if the universe is going to keep stepping in your way of what it has given you the ability to do, there's no reason for it to forbid you from making causal loops. All it has to do is forbid you from generating a paradox via such a loop.

I.e., you can have a self-consistent universe that has unlimited FTL and unlimited time travel as long as everyone plays nice. In fact, you can have unlimited time travel even is everybody is trying to kill their grandfather. They just won't ever succeed.

Never did I assert anywhere in this entire thread that a universe where causality is violated--or even one with closed causal loops--is impossible. They do have serious issues, however. And for anyone to propose FTL travel as a serious possibility and then ignore the causality issues as non-serious, either indicates some amount of either ignorance, disingenuousness, or rationalization.

If, on the other hand, a proponent of FTL travel plainly states that they have no problem with a universe that always steps in somehow and prevents you from succeeding in killing your own grandfather, then at least they've made their position clear, and everyone can make up their own mind as to whether that's a likely way for the universe to be.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#178

Post by nessus42 » Thu, Sep 20, 2012, 11:20 PM UTC

teamonkey wrote: post #168

nessus42 wrote: post #149

Earlier quoted context omitted.

All Special Relativity requires is that space be more or less flat between point A and point B. It doesn't matter if the information travels from A to B through a wormhole and then into another universe and then out the butt of Dumbo, if it ends up at B before light would, then causality has been violated. If, on the other hand, your FTL drive bends up space all around us to be like a giant hyperspace pretzel, (1) we…

Causality is a consequence of the physical laws you are currently using, not a law in themselves. It just paints a logical boundary condition around what is possible using the current set of laws that you are using. Compare Newtonian causality where there is only one timeframe everywhere to SR causality with its local timeframes to GR which supports warping of space and time. All different and not interchangable. Wor…

Special Relativity is not invalidated by GR: Special Relativity is a special case of GR that tells us what to expect in inertial frames in a flat space. As long as points A and B are in a flat enough space, then the predictions of Special Relativity about the causal nature of information transmitted between A and B BY ANY MEANS will still be true, even if there are nearby wormholes. If Special Relativity tells us that we can make a closed causal loop via the FTL signals that make it through a wormhole, then GR will not overturn this, no matter how the information made it from A to B.

I agree that if the space Between A and B is distorted enough so that the light that travels in open space between A and B is seriously affected by a curvature that extends from A to B, then it is true that you cannot use SR to make your predictions, as the premises of SR are violated. But then GR will predict that causality can be violated anyway. (It's generally easier to find ways to violate causality in GR that SR anyway, since it doesn't require FTL transmissions of information in GR to create causal loops.)

I refer you to the FTL faq, since it seems that I am not able to talk sense into people on my own:

http://www.physicsguy.com/ftl/html/FTL_part4.html

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#179

Post by Dylan16807 » Sat, Sep 22, 2012, 4:59 AM UTC

nessus42 wrote: post #177

Dylan16807 wrote: post #170

Earlier quoted context omitted.

Well the important part here is that you can pretty simply have a self-consistent universe that has mildly-limited FTL (and highly-limited time travel) if nobody makes a loop. And I'll try to use something like 'causal loop' in the future.

And what happens if you try to make a loop? Do you think it sensible that the universe provide you with the ability to make causal loops and then somehow forbids you from actually making them, even though once you've been given the ability, they are very easy to generate? Additionally, if the universe is going to keep stepping in your way of what it has given you the ability to do, there's no reason for it to forbid…

There are other solutions, too. It might simply be infeasible to accelerate an FTL device to sufficiently relativistic speeds. Or what if there were fields coming off of FTL devices that interfered with the speed when they criss-crossed. That one would stop any sort of loop and it wouldn't require the universe to somehow specifically forbid paradoxes.

Re: Warp Drive May Be More Feasible Than Thought, Scientists Say

#180

Post by nessus42 » Sun, Sep 23, 2012, 4:12 AM UTC

Dylan16807 wrote: post #179

nessus42 wrote: post #177

Earlier quoted context omitted.

And what happens if you try to make a loop? Do you think it sensible that the universe provide you with the ability to make causal loops and then somehow forbids you from actually making them, even though once you've been given the ability, they are very easy to generate? Additionally, if the universe is going to keep stepping in your way of what it has given you the ability to do, there's no reason for it to forbid…

There are other solutions, too. It might simply be infeasible to accelerate an FTL device to sufficiently relativistic speeds. Or what if there were fields coming off of FTL devices that interfered with the speed when they criss-crossed. That one would stop any sort of loop and it wouldn't require the universe to somehow specifically forbid paradoxes.

FTL devices don't have to "criss-cross" for you to be able to create causal loops. And even if you don't have any causal loops, other violations of causality violate thermodynamics. It's not as if if you've solved the problem of causal loops--which as far as I'm aware, no one has done without postulating a preferred frame of reference, which is basically rejecting Relativity outright--then you're off the hook. All violations of causality are going to make most scientists shake their heads. And, if it is infeasible to accelerate an FTL drive up to speed, then the point is moot for space travel or communication.