Can Warp Speed Violate Causality? The Debate Among Physicists

[Dhruva Patil]

I have heard many physicists (ex:- Michio Kaku) saying "Warp speed" from Star Trek doesn't violate any known physical laws. But doesn't it violate causality?
Say, we make warp speed possible and get on it and travel towards Alpha Centauri (4.22 light years away) in warp speed and reach there, say, within a year (+/- few months) and blow it up. Now using Special Relativity, we could devise a frame of reference where in the observer would see the star blow up before we ever left planet Earth. Wouldn't that violate causality and make warp speeds unattainable?

 

[ghwellsjr]

I have heard many physicists (ex:- Michio Kaku) saying "Warp speed" from Star Trek doesn't violate any known physical laws. But doesn't it violate causality?
Say, we make warp speed possible and get on it and travel towards Alpha Centauri (4.22 light years away) in warp speed and reach there, say, within a year (+/- few months) and blow it up. Now using Special Relativity, we could devise a frame of reference where in the observer would see the star blow up before we ever left planet Earth. Wouldn't that violate causality and make warp speeds unattainable?

Here's where he discusses the issue (on the next page of the article):

We physicists used to laugh at Star Trek's warp factor. We don't laugh anymore. About 10 years ago, a Mexican relativist named Miguel Alcubierre was watching Star Trek, and he came up with a new solution to Einstein's [general relativity] equation. The loophole is negative matter -- Einstein never considered it. And Alcubierre got a solution that looked very similar to warp drive. The key is, you don't go to the stars, the stars come to you. Everybody assumes you have to go to the stars, which means you have to break the light barrier and violate the laws of physics. But you can compress the space like an accordion -- compress the space between you and the stars. It's like a wrinkle in space. There are some objections to this, of course. We don't have negative matter, for instance. But in principle, if you have, let's say, a meteorite made of negative matter, then it may be possible. Einstein never said that nothing can go faster than light. Empty space can contract or expand faster than the speed of light. That's the Big Bang. It's emptiness that expanded. It looks very similar to the rendition of warp drive in the movies -- you would see distortion of star light, stars would come at you very fast, but inside you feel nothing.

Sure we can do it, if only we had some negative matter, which we don't.

 

[yuiop]

So given some negative matter and a suitable folding of space we can have superluminal transport without violating any known physical laws ... as long as we don't count causality conservation as a physical law?

 

[Dhruva Patil]

Here's where he discusses the issue (on the next page of the article):



Sure we can do it, if only we had some negative matter, which we don't.

That is not the question. I know we need Negative matter/energy which we do not have or know how to create. But if we did, could we really be able to travel at Warp Speed? Wouldn't it violate causality?

 

[FactChecker]

I have heard many physicists (ex:- Michio Kaku) saying "Warp speed" from Star Trek doesn't violate any known physical laws. But doesn't it violate causality?
Say, we make warp speed possible and get on it and travel towards Alpha Centauri (4.22 light years away) in warp speed and reach there, say, within a year (+/- few months) and blow it up. Now using Special Relativity, we could devise a frame of reference where in the observer would see the star blow up before we ever left planet Earth. Wouldn't that violate causality and make warp speeds unattainable?

If you assume that we traveled faster than light, then it's hard to figure out how much of special relativity still applies. In general though, here is something to consider. Simultaneity of events at widely separated points is different for different reference frames. So the times that things happen depend on the reference frame. For events A on the Earth and B the star, it would be hard to agree if A and B were simultaneous, A before B, or B before A. The observer would agree that we blew up the star but he would not agree with us about when we left the earth.

 

[PeterDonis]

Say, we make warp speed possible and get on it and travel towards Alpha Centauri (4.22 light years away) in warp speed and reach there, say, within a year (+/- few months) and blow it up. Now using Special Relativity, we could devise a frame of reference where in the observer would see the star blow up before we ever left planet Earth. Wouldn't that violate causality and make warp speeds unattainable?

You can't use SR in this scenario. The process of creating the "warp bubble" and expanding and contracting space would also expand and contract time; you simply would not be able to construct an SR-style inertial frame, even an approximate one, that included all of the events in question.

 

[JesseM]

You can't use SR in this scenario. The process of creating the "warp bubble" and expanding and contracting space would also expand and contract time; you simply would not be able to construct an SR-style inertial frame, even an approximate one, that included all of the events in question.

Maybe it would depend on what's meant by "approximate one", but since the Alcubierre metric is asymptotically flat (as mentioned on the first page here), doesn't that mean that if you "zoom out" to sufficiently large scales of space and time, the warp bubble behaves like a small localized disturbance moving on a flat spacetime? If so, then if it moves faster than light, with multiple such warp bubbles moving in different directions it seems like you should be able to construct a causality paradox similar to the tachyonic antitelephone. And it has been shown that a system of multiple "warp bubbles" moving in different directions can give rise to closed timelike curves in general relativity, see the paper at http://exvacuo.free.fr/div/Sciences...tt - Warp drive and causality - prd950914.pdf.