Beyond Star Trek

by Lawrence M. Krauss

  However, both these observers at least travel forward in time. As for traveling backward, Stephen Hawking made a telling argument for its impossibility: If it were possible, he said, we would be inundated by tourists from the future! I think this is a wonderful point (although I once countered it by suggesting that they all go back to the 1960s, where nobody would have noticed them).

  Shirley MacLaine, too, has a take on the subject of time travel: In Out on a Limb, she writes, “In déjà vu you are getting an overlap of a past-life experience, or you could be getting an overlap of a future-life experience.… That’s what Einstein said.” Well, not quite, but Shirley does lead us to an important issue. If clairvoyance, precognition, or even some forms of ESP (communications from distant alien civilizations, for instance) were to exist, they would require a radical rethinking of what we mean by space and time—a rethinking that would perforce violate our other experience of the physical world. Seeing into the future means that in a sense the future has already happened. Moreover, “seeing” into the future means that somehow a signal from the future has “leaked” back in time.

  While it isn’t stated very often, clairvoyance and precognition present the same kind of paradoxes that the more pedestrian version of time travel does. The standard brain-twister is the grandmother paradox: Suppose you go back in time and kill your grandmother several years before your mother is born? Then you couldn’t exist now. But if you don’t exist now, how could you have traveled back in time in the first place?

  Well, I can think of a version of this paradox which applies to clairvoyance and precognition. Say that you somehow intercept the future thoughts of your yet unborn great granddaughter, and that what you overhear alerts you not to marry the man you met on the bus today. Your future great granddaughter has apparently read, in some old family letters, that after he swept you off your feet and married you, he periodically beat you up. So, when he phones the next day to ask you out, you say no, and you never see him again. Therefore you have no offspring, or descendants, by him, so your great granddaughter by this man can’t exist. But if she can’t exist, how did you pick up her thoughts? The problems are as clear in the case of clairvoyance and precognition as they are in cases in which people are sent back in time from the future: If the Terminator had succeeded in his mission to assassinate Sarah Connor, then there would have been no need to have sent him back into the past in the first place; in Back to the Future, if McFly’s nemesis had become rich by betting on sporting events whose outcomes he brought back from the future, then he never would have been a bum hanging out in the future near the time-travel car that brought him back to the past to start his financial empire.

  Well, you say, that’s simple. She was your great granddaughter, but not by this man! OK, if that’s the case, then how does she know anything about his beating you? Uh-huh, so let’s try another tack. Say that in the future she is your great granddaughter, but that the minute you say no to him, she will no longer exist, because the future has now changed. But from the point of view of the future, your “today” happened a long time ago. Hence, there is no sense in which before you overhear her thoughts and after you overhear her thoughts have any one-to-one correspondence to the times before and after your great granddaughter learns that your husband beat you. The events that immediately preceded and followed your phone conversation with the gentleman happened long before your great granddaughter was born, so how could her existence in the future change because of something that happened today? It would be a creepy world if people popped in and out of existence today because of something that happened many years ago.

  We seem to feel that screwing up the future is more permissible than screwing up the past, but if you think about it the problems are essentially the same—especially if the future and past connect as they must in instances of precognition. In fact, seeing the future and traveling back in time are really equivalent, in a sense, since for you to see some event that happens in the future, some signal had to travel back in time.

  The real issue here is cause and effect. A sensible universe, describable by physical laws, is one in which causes always precede effects, and not vice versa. Thus, even as real-world physicists consider whether or not a universe involving time travel is physically possible, they are careful to check that cause and effect are maintained.

  For example, if time travel is possible (through space-time wormholes, say), then a round-trip is possible. This means that one can potentially relive scenes over and over again. There are some episodes of Star Trek in which this happens. The crucial question is, Are you the same person you were the first time you lived through the experience, or can you remember the previous cycles? If you can remember the previous cycles, then you can change your actions, which is what happens in the Next Generation episode “Cause and Effect,” when the Enterprise-D finally manages to escape collision with the Bozeman. However, this means that cause and effect go out the window, since you will have learned something from an event that has not yet happened to you. If cause and effect go out the window, then the laws of physics—every one of which, even quantum mechanics, is based on causality—would have to be modified. This is a lot to ask for a little precognition.

  Instead, one might imagine a world in which even if time travel were possible, causality still obtained and future events could not influence past ones. Just as in the case of those who do not learn from history and are thus doomed to repeat it, in such a universe a “closed timelike curve” continually rehearses the same events, over and over, so that you cannot change the past no matter how hard you try. For example, if you go back in time to try to kill Hitler before he became Fuhrer—when in fact he survived until shortly before the end of the Second World War—you will trip at the crucial moment, or the gun will misfire. In spite of its various psychedelic flights, the time-travel movie 12 Monkeys honors this sacred principle. At one point, the Bruce Willis character, who has traveled back in time, tells someone that there’s no need to worry about his presence in the time frame, because a time traveler cannot change history. Indeed, the Willis character’s death in the movie is a classic example of a closed timelike curve. (This is not to excuse the film’s incongruities, but I am not prepared to set to work on The Physics of 12 Monkeys.)

  In a world that contained such closed timelike curves, you could not change the past, and by the same token you could not change the future either. But this is a pretty boring view of time travel, and various individuals have tried to refute it in order to allow a more interesting and dramatic universe. (I like to imagine one of them as an expert witness in the O. J. Simpson trial, to bolster the defense that the real killers were time travelers, with Johnnie Cochran chanting, “If you can travel in time, there was no crime!”) Anyone who seriously believes in precognition has to face these same issues: (1) How does the informing signal travel back in time? and (2) How does one deal with cause and effect?

  Actually, I introduced the subject of time travel in the first place in order to apply a Hawking-like argument to the linked issues of ESP, clairvoyance, and precognition. It seems to me that the strongest constraint on such phenomena proceeds from the fact that Bill Gates is the richest man in America. I will now set out the argument, in case the connection isn’t obvious.

  Consider the following actual instance of a claimed positive ESP test result. In this experiment, a subject was able to correctly guess which one of 5 patterned cards was at the top of a facedown pile of such cards 947 out of 4050 times. Because there are 5 possible choices, one might expect a random guess to be right 1/5 of the time, leading to an expected number of correct choices of 4050/5 = 810. Probability theory tells you that this difference of 947-810 = 137 hits above the expected value would happen by chance only once in every 20 million repetitions of the experiment. Later on, the experiment was discredited, but that is not what I want to dwell on here; the point is that even ESP candidates don’t come up with the right answer 100 percent of the time.

 
But let us say that, on average, a candidate does succeed in getting things right 10 percent more frequently than the laws of chance say he or she should. In that case, the following experiment can be performed. Have the candidate designate the 100 stocks which have the best chance of going up the next day. Random chance suggests that if the market remains on the whole static, then on average 50 of the selected stocks will go up and 50 will go down. But a good ESP candidate may skew these odds to 55-45. Therefore, as long as the stock market as a whole doesn’t move downward, you are guaranteed to make money each day. Say that you increase your net worth by 1 percent each day in this way. In 5 years, if you started out with an initial investment of $100, then you will end up with $7,700,291,200.

  One can certainly quibble with the details in the above example, but the point is that even a slight ability to beat the odds in any human activity puts you at a tremendous advantage in life. The fact that we don’t often see instances of such exaggerated and apparently effortless gain is no more a proof against ESP and precognition than Hawking’s argument is against time travel to the past. But it strongly suggests that either (a) there is no such thing as ESP, clairvoyance, and precognition, or (b) all those people who possess these faculties are keeping it and their money a secret.

  CHAPTER TWELVE

  ALL GOOD THINGS…

  I’m strong to the finitsch, ’cause I eats me spinach.

  —Popeye the Sailor Man

  I very much enjoyed the recent John Travolta movie Phenomenon, until about two-thirds of the way through. In the movie, Travolta plays a likable small-town innocent who is suddenly catapulted onto a new plane of existence after seeing a flash in the sky. He finds his mental capacities increasing each day thereafter: he can learn foreign languages in hours, read books in minutes, and so on. His reactions, and the reactions of those around him, to these newfound powers are entertaining, and the mystery if why this has happened is intriguing. Eventually, he goes into the office of his country doctor to be examined. Needless to say, the doctor is incredulous until Travolta provides a demonstration. Pointing to a pen on the doctor’s desk, Travolta concentrates… until the pen slides across the desk into his hands!

  It seems that whenever we imagine super-intelligent beings, one of the first attributes we provide them with is mind over matter, the ability to control inanimate objects. Again, this seems a sensible extrapolation from our own experience: There is little doubt that our minds can control our own matter. By this, I don’t simply mean that our brain dictates the movements of our body, but that we can also apparently control even subtle aspects of our physiology—our heart rate, our blood pressure, our pain threshold, even sometimes our recovery rate from illness. Could not a superior intelligence therefore control matters outside the body in which it was confined?

  Of course, it is not always superior intelligences who are able to perform telekinesis. One of my favorite examples occurs in the original Star Trek episode “Plato’s Stepchildren.” The residents of the planet Platonius, an unpleasant society based on the teachings of Plato, developed telekinetic powers after ingesting native plants containing “kironide,” a rare and potent chemical compound—that is, all but Alexander, who, because of a pituitary hormone deficiency, was unable to absorb kironide. Since he has no telekinetic powers, Alexander is at the mercy of the other Platonians, and has been forced to act as the court jester. Kirk and the rest of the crew are lured to the planet by a distress call, and are likewise forced to perform for the Platonians. Whatever the chemical weakness of the plot, at least one point is well taken: kironide or no kironide, something has to provide power for the process of telekinesis.

  It may surprise you to find how much power is in fact needed. Say I want to do something simple, like lift a pen up from a table, or drag it across the table to my waiting hands. If the pen weighs, say, 4 ounces, or about 0.1 kilograms, then the energy required to lift it a few feet off the table or drag it toward you is about 1 joule. To expend this much energy in, say, 1 second means that 1 watt of power must be expended on the pen. Now, this doesn’t seem like a lot, but if the pen is pointed toward you, it presents a cross-sectional area of about 1 square centimeter. If it is located 1 meter away, then this target represents only about 1/100,000 of the surface area of a sphere with a radius of 1 meter. This means that if you were to radiate a signal that moves out uniformly in all directions, then in order for 1 watt of power to be expended on the pen, you would have to expend 100 kilowatts of power—more than the kilowatt output of most big-city radio stations!

  Of course, if you were able to beam your signal, like a laser, then the power requirements would be reduced, but given the geometry of your head (if that is the source of your signal), it is unlikely that you could reduce the spread of the signal by more than, say, a factor of 100 or 1,000. Thus at least 100 watts of power would still have to be generated and projected. This task would be, as we physicists like to say, nontrivial. One hundred watts, for example, is comparable to the total amount of energy generated in your body as you are performing your normal daily activity.

  However, once again science fiction points in a possible direction. When Obi Wan Kenobi exhorts Luke to “use the Force,” he is clearly implying that there exists some energy available in the ambient space and that you can learn how to tap into it. And Obi Wan is not alone. Many a sci-fi theme is based on tapping the resources of empty space. Now, this may sound ludicrous. How can empty space contain energy to be tapped? But in fact, the issue of whether or not empty space possesses energy is one of the most important in all of modern physics.

  A fascinating realization to come out of twentieth-century physics is that empty space—and by this I mean really empty space, devoid not only of matter but also of any radiation, such as radio waves and so forth—is not empty. The central law of quantum mechanics (which I argued in my last book makes Gene Roddenberry’s transporter impossible) is Heisenberg’s uncertainty principle. When coupled with the special theory of relativity, it implies that empty space is full of what one might call “virtual reality.” The uncertainty principle tells us that because of correlations between various sets of physical observables such as position and momentum, or energy and time—correlations that appear only at the quantum level and not in the macroscopic classical world—it is impossible ever to measure both members of these sets beyond a certain level of certainty. Thus, it is impossible to simultaneously measure both the position and momentum of a particle exactly. Likewise, if one measures a system over a certain time interval, one can never pin down its energy exactly: to do so would require measuring the system for an infinitely long time.

  Einstein’s special theory of relativity tells us that systems that can be measured must travel at speeds less than the speed of light. Since any given system’s clocks will slow down as the system approaches light speed, one can show mathematically that if a system exceeds light speed its clocks will travel backward. Even though there is no reason to believe in the existence of superluminal objects, they have a name—tachyons, which is liberally used on Star Trek (various beings emit them, and they are associated with cloaked Romulan vessels).

  If we combine Heisenberg’s uncertainty principle with relativity, the two together imply that empty space need not be empty. The reasoning—which, again, I borrow from Richard Feynman—is straightforward, if somewhat wild. First, the uncertainty principle tells us that over very short distances and times, since we cannot measure the momentum of a particle exactly, there is nothing that stops it from traveling momentarily faster than the speed of light. But if it does do so, then it must behave like it is traveling backward in time. But if it behaves like it is traveling backward in time, then it must pass by its former self traveling forward in time. If it then slows down and starts to travel forward in time again, it will pass its intermediate “backward-in-time” self. This means that if I start out with one particle, for a brief time three (almost) identical particles will coexist: (1) the original particle, (2) th
e original particle traveling back in time to get there, and (3) the original particle which, after traveling back in time, slows down and travels forward again.

  I insert the parenthetical “almost” because it turns out that if the original particle had an electric charge, then when it travels back in time it simply behaves like a particle with the opposite electric charge traveling forward in time. Thus, if one starts out with a proton, for a moment one will have two protons (the two particles traveling forward in time) and one antiproton. Indeed, it is precisely this reasoning that led some 70 years ago to the prediction that antiparticles must exist. After a while, you will end up with just one proton again. To an observer traveling forward in time (if you could observe the particles directly, which of course you can’t, because if you could then the particle couldn’t be traveling faster than the speed of light and thus backward in time in the first place), it would appear that you started with one proton and then momentarily a proton-antiproton pair appeared from nowhere only to disappear again shortly thereafter.

  Now, this whole scenario seems too fantastical to be true, but it is. “How do we know?” you may ask—since by definition this process is supposed to be invisible. Well, while we can’t see the three particles directly, we can indirectly detect their presence. For example, the electric field generated by three particles will be slightly different from that generated by one, even though the total charge will be the same. (The proton-antiproton pair has zero total charge.) So if the proton in question happens to be the nucleus of a hydrogen atom, the electron orbiting the proton will be influenced by a slightly different electric field than it would otherwise, and the energy levels of the orbiting electron will be altered in a calculable way. It was one of the great postwar successes of elementary-particle physics that this small shift could be both calculated and measured. Sure enough, the predicted effect of these particle-antiparticle pairs—called virtual particles, because they cannot be directly seen—is exactly in agreement, out to better than 9 decimal places, with the experimentally measured value.