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Positive Energy Warp Drive from Hidden Geometric Structures

arxiv.org

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Re: Positive Energy Warp Drive from Hidden Geometric Structures

#31

Post by mikewarot »

gary_0 wrote: post #9

What do the authors of these warp drive papers think will happen causality-wise when the proposed vehicle effectively moves faster than c? You can't do that without doing nasty things to your light cone[0] and effectively traveling into your own past. Is a prerequisite of entertaining the idea of a warp drive that we disregard causality? As far as I know, there aren't any plausible loopholes in General Relativity tha…

We're all traveling through spacetime at the speed of light, most of our velocity is in the time dimension, the faster you translate in the x,y,z dimensions, the less is left over for the time dimension, and thus time seems to slow down. (That's how this non-physicist things about it, anyway)

In a warp field, you're just somehow shrinking X,Y,Z while not doing the same to time. I see no reason to think you'd travel backwards in time.

Re: Positive Energy Warp Drive from Hidden Geometric Structures

#33

Post by jxf »

canjobear wrote: post #32

Previous work seemed to conclude pretty strongly that superluminal warp travel required negative energy. What's the insight here that gets around that?

The goal of warp drive is to produce a modified region of spacetime that makes it feasible to travel faster than light. There are an infinitude of different approaches. The authors in this paper find that if you impose specific constraints on the way the modified region of spacetime is shaped and evolves over time, you don't need the negative energy anymore.

However, you do need a very high energy density, strong enough to create a black hole. And there is, of course, no guarantee that such an energy source is feasible or even possible. But you no longer need exotic matter — you just need enough regular matter in one spot.

Re: Positive Energy Warp Drive from Hidden Geometric Structures

#34

Post by perihelions »

gary_0 wrote: post #9

What do the authors of these warp drive papers think will happen causality-wise when the proposed vehicle effectively moves faster than c? You can't do that without doing nasty things to your light cone[0] and effectively traveling into your own past. Is a prerequisite of entertaining the idea of a warp drive that we disregard causality? As far as I know, there aren't any plausible loopholes in General Relativity tha…

"What do the authors of these warp drive papers think will happen causality-wise"

According to §3.3, they aren't sure. If I parsed it correctly, they haven't mathematically resolved whether closed timelike curves are necessary, or precluded, or neither.

Re: Positive Energy Warp Drive from Hidden Geometric Structures

#35

Post by mcswell »

XorNot wrote: post #10
beervirus wrote: post #7

Earlier quoted context omitted.

> 4 orders of magnitude less than the sun's mass does sound like a positive development - still a lot but less than before, right? That’s something like 10^43 joules. Still not achievable anytime soon.

Progress in warp field research though has resulted in significant energy reductions though - everytime a more efficient formulation is found its a step on the way to a possible practical system. After all, if it was easy then everyone would be doing it.

"if it was easy then everyone would be doing it": And, I suppose, we'd see them. Little green men, I mean.

Re: Positive Energy Warp Drive from Hidden Geometric Structures

#36

Post by vanchor3 »

gary_0 wrote: post #25
TheDudeMan wrote: post #16

Earlier quoted context omitted.

"effectively traveling into your own past" Can you explain how traveling faster than light leads to that? Perhaps you meant "observing your own past?"

No, not "observing your own past". Instead, your past self could observe your future self. And then your future self could turn their FTL ship around and run you over, killing their past self and creating a paradox. https://old.reddit.com/r/askscience/comments/l9sba/why_does_...

Sorry if I don't fully understand, I've read through link a bit but it just leaves me with one question in particular: Wouldn't your relativity have to be both different and the same simultaneously for this to work?

In the tachyonic antitelephone example, Bob and Alice send messages that travel at 2.4c, but neither observers are at that speed. Wouldn't the limits of their observation and time dilation cause the messages to appear in order?

If you were to theoretically travel into a black hole, and then somehow escape out of it very quickly, you would see yourself still in the black hole, but you wouldn't see yourself coming out of it, because that light was further from the black hole and thus moving at a different speed?

Re: Positive Energy Warp Drive from Hidden Geometric Structures

#37

Post by jlokier »

javajosh wrote: post #29
a1369209993 wrote: post #28

Earlier quoted context omitted.

> SR is 2 assertions, really: first, that the laws of physics are invariant in all reference frames, and second (and far stranger), that the speed of light is invariant in all reference frames. Technically that's only one assertion (that is, the second is a strict subset of the first).

No, it isn't. Light is unique in this way.

No it isn't. The speed of light in SR is not really about light. It's just a good way to explain things.

You can calculate the speed of light from other physical, measurable constants without using light (the permittivity and permeability of free space). Those you can measure with inductors and capacitors. So because those measurable physical behaviours are invariant in all reference frames under SR, it follows that the speed of light is also invariant.

Re: Positive Energy Warp Drive from Hidden Geometric Structures

#38

Post by nine_k »

sigmoid10 wrote: post #15
gary_0 wrote: post #9

What do the authors of these warp drive papers think will happen causality-wise when the proposed vehicle effectively moves faster than c? You can't do that without doing nasty things to your light cone[0] and effectively traveling into your own past. Is a prerequisite of entertaining the idea of a warp drive that we disregard causality? As far as I know, there aren't any plausible loopholes in General Relativity tha…

General relativity actually tells us nothing about whether causality is absolute. You can easily make up spacetime configurations that violate causality and still conform to general relativity. It's just that most people believe that causality should be protected, because... well it would be really weird if it wasn't. But it's just a conjecture and not like a theorem or anything.

AFAICT, GR is symmetric in relation of the direction of time, as are most other important physical theories.

What may add the arrow of time is the second law of thermodynamics: a statistical evolution of a system is not symmetric.

I wonder now how a violation of causality could either circumvent of contradict the second law.

Re: Positive Energy Warp Drive from Hidden Geometric Structures

#39

Post by cletus »

The lesson we should really take here is the power of wishful thinking. People dearly want to believe in FTL travel. Without it, the Universe just seems so vast, empty and distant. The truth is it is and it isn't. It's only really that on our current time scales.

Not to be a broken record (there's some new variation on some old idea like every week it seems), but I'm firmly in the camp that believes FTL isn't possible. There are two prongs to that.

1. Theoretical: a lot of these theories rely on people not understanding the domain of a function (eg plugging in negative mass or energy into an equation; just because a function can operate over real numbers doesn't mean real numbers are the domain of that function); and

2. The macro evidence in that if FTL were possible it makes it that much more unlikely that we haven't detected starfaring life. Specifically, the potential volume of spacetime goes from a line cone of several million light years to the entire Universe (not just the observable Universe). We don't know how big that is of course but it's may orders of magnitude larger.

So this paper tries to "solve" the negative energy/mass problem. I honestly can't speak to the soundness of this idea. I am however skeptical.

Re: Positive Energy Warp Drive from Hidden Geometric Structures

#40

Post by AnimalMuppet »

sigmoid10 wrote: post #15
gary_0 wrote: post #9

What do the authors of these warp drive papers think will happen causality-wise when the proposed vehicle effectively moves faster than c? You can't do that without doing nasty things to your light cone[0] and effectively traveling into your own past. Is a prerequisite of entertaining the idea of a warp drive that we disregard causality? As far as I know, there aren't any plausible loopholes in General Relativity tha…

General relativity actually tells us nothing about whether causality is absolute. You can easily make up spacetime configurations that violate causality and still conform to general relativity. It's just that most people believe that causality should be protected, because... well it would be really weird if it wasn't. But it's just a conjecture and not like a theorem or anything.

That conjecture is one of the two foundational principles of relativity - that the laws of physics are the same in all valid frames of reference. "Violating causality" means that in some valid frame of reference A, event X caused event Y, and in some other valid frame of reference B, event Y caused event X. The laws of physics cannot be the same in A and B for both of those to be true.
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