"Faster than Light Starflight"

Conducted by Geoffrey A. Landis

participants: Robert L. Forward, John G. Cramer, and Gregory Benford

This is a transcript of a "Science Forum" which I conducted for Science Fiction Age Magazine. Rather than have a science column, SF Age ran a feature called the "Science Forum", where we had a conversation with several scientists who are also science-fiction writers on a subject of interst to science fiction fans. In this particular forum, I was the interviewer, so for the most part you won't see me popping in with opinions; instead I pretty much tried to keep to the background, asking a new question every time the conversation seems to slow down. The interview was conducted after the NASA Workshop on Relativity, Quantum Mechanics and General Relativity, held in Pasadena, CA at the Jet Propulsion Laboratory

(Several months later, I also conducted a Science forum on Slower-than-light Interstellar flight, which can also be found on the web.).

Landis: Why is faster than light travel a constant theme in science fiction?

Cramer: Because it makes the plot more efficient. You have to get on with it, and if you have to spend years and years traveling light-years. then nothing much happens and it's a dull book. Therefore you need faster than light travel, in many cases, anyway, to make interesting plots.

Benford: Right. And SF is about very large horizons, and you can't get those without a quick way of getting there. Also, faster than light, with its typical mumbo-jumbo lingo, gives you a way of seeming scientific without having to do any homework-- or at least, that's true in a lot of books.

Forward: And the minute you mention faster than light you know you're reading science fiction.

Benford: Yes, except that, lately, you could be reading the Physical Review, because what struck me is that in the last decade faster than light has had a remarkable resurrection.

Cramer: Or put it another way, the standards of Physical Review Letters have slipped quite a bit. [laughter]

Forward: In fact, Matt [Visser] just said that the last Physical Review has five papers on time travel. Five papers on time travel in the latest Physical Review; and that's just one issue coming out every fifteen days.

Benford: Right. Don't try to buy this at your newsstand, but take our word for it, it's true, the magazine is just littered with this kind of stuff.

Cramer: You have to realize that Physical Review is divided up into various sections, and there's one ghetto section, called Physical Review D-15 which is only published on the fifteenth of every month, and all of the whacko ideas tend to go into Physical Review D-15.

Benford: Is that really true? I didn't realize that.

Cramer: Yes. And I have to admit that I subscribe to D-15 but not to D-1.

Benford: A millennium or so from now, this century is going to have two images, two men by which it will be known. And the odd thing is that they both came from the same culture, and they lived in the same city, but they they were arch enemies, and they never met each other. Einstein's the obvious one, and the other is Hitler. They both lived in Berlin for a period of time. Everything else is going to wash away, but Hitler and Einstein are going to last.

Landis: Why do we believe that Einstein was right about there being a light barrier? Or, was Einstein right?

Cramer: In high-energy physics one tests this lightspeed barrier all the time, and it's never been found to fail, so to first order, FTL is impossible. So, basically, what we have to do then is to look for second order effects which might provide loopholes by which you might be able to get around this basic problem that if you try to raise something to the speed of light you have to use all the energy in the universe to do it.

Benford: I became fascinated by this because of the theories which purported to give field solutions for faster than light particles, tachyons, so that, after the reported discovery of tachyons, in 1972, I began to mull over what would happen to the world, and the whole scientific enterprise, if tachyons proved to exist. I had at that time already written a paper, with David Book and Bill Newcomb, called "The Tachyonic Anti-telephone," which reduced to I think a shambles a previous discussion trying to reinterpret backwards in time transmissions as in fact forward in time transmission of an antiparticle. But still, it seemed to me that if tachyons could exist, then everything changes. Everything in physics. And I wanted to explore the expansion of that idea as a scientific paradigm, and to see the implications and also talk about the fact that time is the great ironic and dark element in the human consciousness, as we're the only animal that knows we're going to die, and we're the only animal that's nostalgic, and I wanted to explore that feeling of lost opportunity, and that's why we think about FTL and time travel. I should mention that FTL implies time travel in a lot of theories. I wanted to point out that they are the same thing; they are the desire to get beyond the bounds of the apparently possible and that's a very deep yen for the modern imagination. Most of modern times has been about exceeding the possible.

Forward: And the other thing about faster than light travel, as Greg said, is that any kind of faster than light phenomenon--practically any kind; it's hard to find any where this isn't true--end up with the result that Greg found in his paper, that just having a faster than light communication system allows you to violate causality. So that is the real reason for the speed laws, not that you can't exceed the energy or anything else; it's just that the minute you have any mechanism for going faster than the speed of light or even sending information faster than the speed of light you violate causality, which means-- in fact, everybody calls it the grandfather paradox but I think Kip Thorne and his latest book, which is a very heavy tome, reminds us that it's really a man's point of view; you really should think about killing your mother.

I just finished the book Timemaster--

Benford: Good book.

Forward: --In there, I use the latest theories, which were helped out by our other good guests here, and then I attempted to try and explore the causality problems, and that was the most difficult writing I've ever done, especially as you approach the Cauchy horizon, which I hope some of you know what I mean, but basically it's when you start being able to violate causality, and I tried to give examples of how you might be able to control that, how you might make it sensible, and it was very difficult to do, and I don't think I really did it as well as I ultimately could. What amazed me is that at the same time that the physicists were looking at this from a highly mathematical point of view, and they found that if you define your parameters right, you define your boundary conditions right, you can actually show that you can build time machines and violate causality but not violate any other laws of physics as we know them and have theoretical physicists that five years ago were proving time machines couldn't exist writing papers proving that time machines didn't violate any known laws of physics except causality, and then finding good reasons why nature will allow us to have these time machines because, things will arrange themselves so that no paradoxes are caused, that is what is amazing to me to find in the physics literature. That's one of the reasons I wrote Timemaster, to get that message out to people who couldn't read the Physical Review.

Benford: I quite agree. What I've tried to do is point out that essentially time is-- and therefore FTL is--one of the obvious problems in theoretical physics, which we're very confused about, and the level of confusion is rising, which suggests that we're coming to grips with the inadequacies of present physical theories. But problems to a scientist are not problems, in a sense: they are invitations; they're a challenge. And that's the state of the art at the moment.

Cramer: Last week I gave a talk about faster than light communications to a bunch of electrical engineers, and one of them at the end of the talk raised his hand and said that he'd just read a wonderful novel called Timemaster and wondered if it had been written by Kip Thorne [Note: Kip Thorne, a Professor of Physics at Cal Tech, is one of the leading gravitational theorists, the person most responsible for the recent interest in wormholes, chief theorist for LIGO, the NSF's new gravity wave detector, and co-author of Gravitation, the standard textbook in general relativity by Misner, Thorne, and Wheeler. He has written no SF, to our knowledge.]

As far as causality, I'd like to say that it is perhaps the most mysterious law of physics. We don't understand where it comes from. The literature on the problem of the arrow of time is very confusing. We know that there are a number of different reasons why time points in a particular direction: the universe is expanding, that defines a direction of time; we find retarded and not advanced solutions to the equations of electromagnetism, that defines an arrow of time; we find that in thermodynamics entropy is increasing and that defines an arrow of time; and in the decay of the K0 meson we find a difference between time running forward and time running backwards, which defines another arrow of time, and nobody really has a very good idea of how these things are connected together. And I've never seen a science fiction story that connects them together, either, but it would be nice to write one sometime. The science-fiction genre, however, has probably done more than anything but the recent physics literature to explore the problems implicit in time-travel paradoxes. How do you go about resolving what happens if you do go back in time and shoot your grandmother, or cause something to happen that did not happen in the past. And there are a number of solutions to this. One is that you simply branch off to another universe. The most innovative solution I've seen to this, perhaps, is embodied in some of Paul Preuss's works in which the universe is continually expanding and contracting, and what you really do when you travel in time is to travel to the next branch of a recycling universe where you have a clean slate and you can cause anything you want to happen.

Benford: It has higher entropy.

Cramer: It depends on who you believe on the question of the entropy of recycling universes. And I guess that the third possibility is that the universe is really deterministic and you can't change anything; you're locked in to whatever happens. Wheeler and Feynman, who did an extended paper on things involving advanced waves, investigated this and in 1949 published a paper in Reviews of Modern Physics in which they essentially said, although not precisely in these words, that if you go back in time with a ray-gun and attempt to shoot your grandmother, what you will do is just radiation damage her so that she will have offspring who are genetically pre-disposed to have bad aim and therefore don't shoot straight.

Landis: What are the physical possibilities we can see, using present-day knowledge, as to how we might be able to accomplish faster than light travel? Is it possible, and if so, how?

Cramer: The most obviously possibility is by using wormholes, which we have been extensively discussing in the last few days [at the NASA workshop on Quantum Theory and Relativity]. The basic idea is, that if you can find a spatial shortcut between point A and point B, without having to cover the intervening distance in real space, then as you go through the wormhole you're always traveling at a relatively low velocity, probably a lot less than the velocity of light, but in terms of the distance traveled in real space divided by the time it took, you're traveling faster than the speed of light. So if wormholes are a real possibility, then they represent a real opportunity for faster than light travel.

Another couple of things we talked about are Einstein-Podalsky-Rosen non-locality, which in the usual interpretation of quantum mechanics does not allow faster than light travel, but sort of indicates that nature is arranging correlations faster than the speed of light or even backwards in time, across spacelike or even negative timelike distances. If there is some possibility of finding some loophole in quantum mechanics, or a nonlinearity, one might be able to use that to at least communicate faster than light. The third possibility is that perhaps, on the basis of recent experimental evidence, the electron neutrino is actually a tachyon, which is a particle which always travels faster than the speed of light. If it's true that the electron neutrino is a tachyon, and you could use it for communication, which requires some engineering details which haven't really been worked out yet, then you may be able to at least communicate faster than the speed of light using tachyons. The last point which I would like to make is that there has been some recent discussion of a phenomenon which in the physics literature has been called teleportation, which basically allows you to transport quantum states from point A to point B by reproducing them someplace else. This always goes slower than the speed of light, in that there's no way of communicating the needed "code key" in a faster than light way, but on the other hand, if you had faster than light communication, and you had this kind of quantum teleportation available, you'd have faster than light travel as well.

Landis: Of course, teleportation looks faster than light to you, if you're the one teleported. It seems to you to take no time.

Cramer: Right.

Benford: I think that the most probable way of traveling about the galaxy at FTL, at the moment, must be the rather low probability of discovering wormholes. The point about wormholes is that they look, theoretically, obtainable, but they're extremely expensive. The cheapest was to get them is to ask God to have made them in the early universe and let them hang around until we find them. Well, if they've been made, in the first fraction of a second, and have hung around, then we should go look for them, because it's much cheaper to find one than to make one. They have few signatures, unfortunately. They could be unusual events in a particle physics experiment, and there are ways to look for that. The other way is to look for something odd in the fact that wormholes can refract light when seen against the distant stars, so if they're near the solar system you could see one. Personally, I'd prefer the laboratory solution, because I'd like to get my hands on one. I'd like to get one about the size of a football. Or, a little larger, so I could squeeze through. That's the kind that's of interest to us at the moment, and the fact that one hasn't seen them means that they're not only rare, but they may demand a certain energy in order to liberate. And the search for wormholes that happen to be left over from the early universe is probably the best way to advocate getting around the galaxy, at the moment.

Forward: Some of the other things that came out of our workshop is that there already exists a method of making a machine that allows for faster than light communication, and that is to take two pieces of aluminum foil, and bring them together, but not touching. Now, you don't have to put any charges on the pieces of aluminum foil, all you have to do is bring them very close together--about an atom's worth--and what happens is that the aluminum, being a good conductor, prevents electromagnetic waves from passing through it. So if there were any electromagnetic waves between the two pieces of aluminum foil, they would have to arrange themselves so that they have zero electric field in the aluminum. Now it turns out that, in quantum theory, empty space is full of half-photons, electromagnetic quantum fluctuations, and they have all kinds of phases and all kinds of frequencies. But the minute you take a piece of space and bound it by two pieces of aluminum foil, half of those modes are no longer allowed to exist. So what happens is that the space, which used to be thought empty, is really full of vacuum fluctuation, and if you put the aluminum foil on either side, that cuts the number of modes allowed, and so therefore you have a region of space that is emptier than the vacuum. So if it's emptier than the vacuum, it has less energy than the vacuum: it has negative energy. There is a paper in which somebody calculated the speed of light in this new form of vacuum, the emptier vacuum. In the direction from one aluminum plate to another, he calculated the speed of light, and found that it was greater than the speed of light in regular vacuum. So one of the things we could do is to think about doing an experiment and actually trying to measure this. Now, when you get down to the numbers, it's very difficult, but I think that's an example of a crack in the laws that way you can't go faster than the speed of light. You can do it between two very closely spaced pieces of aluminum foil.

I think that the other thing that didn't get brought up is that there are astrophysical objects called quasars that have jets on them, and the jets have blobs on them, and the blobs seem to be moving faster than the speed of light. Well, there's a good explanation for this. The explanation is that they are actually moving very close to the speed of light, but they're shooting right toward us, and so therefore if you took a naive picture of this, it looks like they're traveling five to eight times the speed of light, but if you do your mathematics properly, then everything works out okay, because they're coming right towards you. But then when you start looking at all the details, you find that there's too many of these jets coming straight at us, and if they were randomly selected, there wouldn't be as many jets as we do see. And I think that John Cramer can say a lot more on that subject.

Cramer: Yes. The problem is that these jets are seen on almost half of the quasars we know about. If you sort of rationally evaluate the probability that a given quasar would happen to be shooting a jet out in our direction, it seems a bit less likely than that.

These things are called super-luminal objects. One sees a ball of gas emerging from a quasar, and in about a year one sees it moving about eight light years. So they seem to be moving about eight times the velocity of light The conventional explanation of this is that they're heading almost towards us, so if you work out the relativistic effects, you can have that kind of apparent velocity, because of the way time dilation and space dilation work. However, the likelihood of all of these things moving directly at us from all of the quasars we see seems to be rather unlikely, so that if you look at all of the known quasars the statistics don't work out, which brings up the question of whether they aren't really moving faster than the speed of light and, if they are, and why we see so many of them.

Landis: We've talked about wormholes. What would it be like to travel through a wormhole? What would you see, and what would you feel, and what would the trip be like?

Benford: It would be like the New York subway, only worse [laughter]. There would be none of the homeless, and apparently very few graffiti, very narrow, and very quick. Surprisingly, getting from here to Alpha Centauri might involve scooting down a wormhole that might merely be the length of a football field. The trick about wormholes is to realize that they are shortcuts, that they are things with an enormous energy density, and therefore have an exorbitant price.

Cramer: Passage through the wormhole would be like human existence: short and brutal.

Benford: Well, that's what it's like to play football.

Forward: Actually, some of the wormholes we've been talking about, in order to construct them, you have to make two spheres, where the insides are connected by distances that are the order of an atom, and therefore it would be more like opening a door and walking through. And, if you design the thing properly, so that all of the exotic matter that forms the wormhole is safely ensconced away inside vacuum pipes, it will be just like opening a door and walking through it. So, the best wormholes we have, that is, the ones that are closest to being made, which I mean involves taking a couple of planets and converting them into ultradense matter, and making them into a frame, and making it so that it's quite usable--which is not something we're going to do next week, but is something we'll do before we turn from humans into something else probably, those are just basically doors, and can be very benign. Some of the other wormholes, of course, are a little tougher to get through.

There's one other aspect, and that is that there are very strange things going on in the small. An electron has many of the properties of a wormhole, mathematically, and it has some queer things about it that indicate that we really don't know what an electron is, but it conceivably could be that we could make a wormhole out of something like an electron, and send messages through it, even if we couldn't go through it, and that of course gets us back to the original paradox, which is that if you have any method of sending something faster than the speed of light, then you get into the causality problem.

We ended up with some ideas of things we ought to look at in ongoing experiments. There are many experiments where particles are being made, and conceivably some of those particles could be disappearing. Normally, the physicists who take this data, say, well, we didn't collect everything from that particular collision, so we're going to throw this out. And what we're saying is that we ought to take a look at those things where things are missing, and see if there wasn't an event that happened that indicated that a particle disappears from our space.

There's another example, where we have an experiment that's going on now, in which we have telescopes looking at lots and and lots lots of stars, all the stars that they possibly can, all the time--except when the sun's up, of course--and looking for the occasional time when some of them get a little brighter and then get dimmer, using a CCD camera, a regular television camera but super efficient. What that indicates is that something heavy moved near, or almost in front of the star, and the gravitational field of that object caused it to focus toward the Earth, making it brighter for a period of time ranging from minutes to hours-- is that right?

Benford: More like days.

Forward: Okay, days. And that particular rise and fall, if it happens to both the red and the blue, indicates that a heavy object passed between the star and the Earth--

Benford: --A "MACHO"--

Forward: --a massive compact halo object. One of the things that came out was that if wormholes existed, they might have negative mass, and they might be big enough to cause a de-lensing, or a decrease, in intensity.

Landis: Shall we call it a "NACHO"-- a negative compact halo object? [laughter]

Forward: Or a large astronomical negative mass object. And so, one of the things we're going to do is to suggest that they take the data from the MACHO search, and look for "NACHO"s-- or "LANMO"s.

Benford: So a NACHO is a chip off the old block?

Cramer: Why do these things have negative mass? Because if you have a wormhole, and mass passes through it, it sort of trails its gravitational field lines, and so on the way in it increases the mass of the wormhole mouth, and on the way out it decreases the mass of the wormhole mouth. This will be sort of dynamically unstable, because the the object gets on the positive gravitational end, the more mass it will attract and suck through, and therefore the more negative the other end will get. And so it seems to be a fairly good bet that, with very low odds admittedly, that one should look for these things as a way of detecting possible cosmological wormholes.

In the workshop, we discovered that the spherical wormholes that sort of started the whole ball rolling, the so-called Morris-Thorne wormholes, are not the only game in town, that there are other ways of constructing wormholes, particularly if you have negative mass cosmic string as a construction material, and it appears that in the early universe, if you had little loops of negative mass cosmic string around--and there's no reason why you shouldn't--then you might have ended up, at the end of the inflationary period, with wormholes surrounded by cosmic string, which connect one region of space with another in a very nifty way from the point of view of science fiction, since it provides an ideal stargate for going from one place to another; from one end of the universe, or even connecting one universe to another.

Landis: For the audience, could you tell us what a cosmic string is?

Cramer: A cosmic string is a disruption in space with a linear form, which has to do with the fact that space can have different phases, and therefore you can disrupt space by making walls, or solid objects, or planar objects, or even straight lines. A cosmic string is sort of a disruption in the geometry of space, which might have been created in the big bang, and which in certain cases might persist to this day. Positive mass cosmic strings are probably gone, because they vibrate and radiate their mass away in gravitational radiation. But negative mass cosmic strings don't have that option, because they can't radiate negative gravity waves away, and so they're sort of stuck with what they are. They might break up into smaller pieces, but that just means that the wormhole mouths that we have to deal with are smaller than the size of a galaxy, which is probably what we want.

Landis: Tachyons are a buzz-word often used as a tool in justifying FTL travel in science fiction. What are tachyons, and is there any possibility that they could be real?

Cramer: Tachyons are a cyclic phenomenon, let us say, in that they were very fashionable in the 1960's, they've gone out of fashion for a long period of time, and they have the potential for coming back into fashion recently, because measurements of the electron neutrino rest mass seem to indicate that there's a fighting chance at least that the electron neutrino has a negative mass squared, in other words, has an imaginary mass, meaning it's a tachyon. Tachyons are particles that always travel faster than the speed of light. If you give them a lot of kinetic energy, they kind of nuzzle up to the speed of light from the high side; if you take away their kinetic energy they go faster and faster, until when you take away all their kinetic energy they move with infinite velocity. Tachyons therefore provide a medium by which faster than light communication is possible. They also have some other interesting properties. They might be the way by which information is carried out of black holes, because black holes, for all their gravitational attraction, can't stop tachyons, because the more you pull on a tachyon, the faster it goes. So you can strip away all its energy, but you can't stop it.

Originally published in Science Fiction Age

Page by Geoffrey A. Landis
Copyright 1996, 1998