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wormholes, part II: Kip Thorne

I started my review of wormholes by reading Kip Thorne's famous paper on them from 1989. Thorne is the T in "MTW" a book by Misner, Thorne, and Wheeler called Gravitation, written in 1973 and still one of the most widely used textbooks on general relativity.

I'm not actually sure whether Kip Thorne believes that wormholes are possible--I assume he would at least lean towards "no" but I have no idea. You might think that because he has written important papers on them and because he consulted on a movie that depicts one, he believes they are. But that doesn't follow, because theoretical physicists often explore ideas that they don't think will work out, to see where they will lead and find the limits of existing theories and uncover new questions or problems with them. I didn't search for comments from him so I don't know what his present take on them is or if it has changed any, but in his 1989 paper he doesn't say they are possible, he just outlines what the conditions would have to be in order for them to be possible.

In his paper, he does three main things. The first is to construct a simple example of a stable traversable wormhole geometrically. In other words, he describes what the shape of space and time would have to be, and what distribution of energy would be needed in order to create this shape. (Remember, the basic idea of general relativity is that matter and energy warp space and time; given any distribution of energy you get a well defined shape of space and time.) Unfortunately he finds that the distribution would have to be quite "exotic", meaning it would require a lot of negative energy, a substance which is very different from ordinary matter and energy. The main question is: could such a substance exist or be created somehow, and if so could it exist in large enough quantities to make a wormhole? At the time, little was known about the answer to this question but a lot more work has been done since which is the topic I focused most of the rest of my reading on.

The second important thing he does in his 1989 paper is to show that if it is possible to create even the simplest kind of wormhole that just connects point A to point B in space, then it is also possible to build a time machine out of the wormhole, that could be used for traveling backwards in time.

So while the fact that a prominent very respectable physicist was even discussing the possibility of wormholes must have been very exciting to the sci fi community, what they may not have realized is that both of these results make wormholes less likely, not more likely. The first because he demonstrated that they depend on a substance not known to exist. And the second because time travel has a whole set of causality and consistency problems that come with it. If it were possible to build a wormhole that couldn't be made into a time machine, that would be much more believable than a wormhole that could be made into a time machine. But sadly, it doesn't seem like the first scenario is possible, at least according to Kip Thorne's 1989 results. However, there is some encouraging news here: in 1992 Stephen Hawking conjectured that there may be weird as of yet unknown effects in physics which act to protect against time travel. (He called this the "Chronology Protection Conjecture".) It seems like pure speculation to me, but if Hawking's suggestion is right then there might plausibly be some mechanism that prevents someone traveling through a wormhole if they plan to travel backwards in time. Like, maybe the wormhole suddenly closes up or becomes unstable. However, I don't think he has much reason to believe this is true other than wishful thinking--it would be nice if some kind of wormhole were possible, without having to face all of the obviously troublesome inconsistencies that time travel brings (grandfather paradox, etc.) So he tried to think of any way in which it could be. This is one way of avoiding that problem, but seems unlikely and doesn't do anything to solve the main problem which is a lack of negative energy.

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