When I was a child, I was mesmerised by Isaac Asimov's Foundation books, which depicted the rise and fall of a galactic empire. They were set far in the future, when humanity has forgotten the planet it came from. Reading the novels, I wondered what technology might be like millennia from now. When people say something is "impossible," what they usually mean is that it is impossible with the technology of today or of the near future.
Now that I am a professor of theoretical physics, I realise that many "impossibilities" may actually become possible in the future; there is no law of physics preventing most of them from becoming a reality. All but a handful are just very difficult engineering problems. So I decided to rank these "impossibilities" into three categories:
Class I impossibilities do not violate any known law of physics, and may be possible within a few decades to a century. These include invisibility, teleportation, ray guns, anti-matter engines, telepathy, force fields, psychokinesis and star ships.
Class II impossibilities also do not seem to violate laws of physics, and might be possible on a scale of centuries to millennia. They include time travel, faster-than-light wormhole travel and interdimensional travel.
And finally, Class III impossibilities are those which do violate the laws of physics. For them to become possible would require a revision in all known physical laws. They include the violation of the conservation of matter and energy (perpetual motion machines) and precognition (seeing the future).
Take the Class I example of invisibility. I used to tell my optics students that invisibility was impossible, as a result of something called Snell's law. Light bends when it enters glass because the speed of light slows down. But for light to bend in a way that renders an object invisible, light would have to travel faster than the speed of light, which would violate Einstein's theory of relativity. End of story. Invisibility was impossible.
Or so I thought. Yet, in 2006, physicists at Duke University and Imperial College accomplished the "impossible": they made an object disappear (at least in the microwave range). They exploited a loophole to Snell's law, which assumes that the medium for light is uniform. They constructed a series of rings of copper and other materials and then put tiny impurities and micro-circuits inside the rings. These impurities deflected microwave radiation in a precise way, invalidating Snell's law. By adjusting these impurities, they were able to render an object within the rings invisible to microwave radiation. (The next step was taken last year by physicists at Caltech and the University of Karlsruhe in Germany. They were able to bend red and green laser light with a metamaterial.)
Given the speed of research in this area it might be possible within a decade to render an object totally invisible to a single colour. Within a few decades, it might be possible to render an object invisible to several colours, and even create a crude version of Harry Potter's invisibility cloak.
But who is interested in funding this exotic research? In this case it is the Pentagon. The military has spent billions trying to perfect stealth technology that can render airplanes and other military vehicles invisible to ground radar. Stealth technology is actually a hodge-podge of tricks used to scatter radar waves. Stealth bombers are created by using plastic resins instead of metals, by changing the shape of the wings and fuselage and by removing sharp edges, so that radar waves dissipate when they strike the plane. The resulting radar image is about the size of a large bird. So stealth technology is a far cry from full-blown invisibility.
Another "impossible" godlike power is teleportation, which is even mentioned in the Bible (Acts 8:36-40). Only the gods have the power to disappear and reappear at will. Until now.
In the new Hollywood movie Jumper, there is a race of people who can teleport at will. These jumpers are born with this ability. But in the laboratory, physicists accomplish this with lasers, at least at the atomic scale. In the last few years, physicists have learned to teleport individual photons of light and even atoms of beryllium and caesium. The world record is held by physicists at the University of Vienna, who teleported a photon 600 metres across the Danube. Within a decade, perhaps the first molecule will be teleported. Within a few decades, perhaps the first virus or fragments of DNA will be teleported.
Teleportation exploits a bizarre feature of quantum theory—that all matter vibrates at certain frequencies. If two particles vibrate in perfect unison, there is an invisible wave that connects them, even if they are separated by light years. Any disturbance to one particle is immediately transmitted to the other. So we are all, in some sense, connected to all the matter in the universe by a hidden web; by moving your hand you are, in some sense, influencing things on the other side of the galaxy.
Teleportation is a class I impossibility. (But teleporting an entire person is a class II impossibility.) But who funds this work? There is both commercial and government interest here, because the work is closely related to the quantum computer, which even the CIA wants to know about. By 2020, computer power, which for the past 50 years has been doubling every 18 months—the so-called Moore's law—will flatten out. Much of the world's economy has depended on this doubling. By 2020, the demise of Moore's law may even trigger a depression.
Why will Moore's law come to an end? In the future, silicon transistors will be so tiny, just a few atoms thick, that electrons will leak out of the wires, causing a short circuit. The laptop on my desk, for example, has a Pentium chip which has a layer about 20 atoms across. By 2020, it might be five atoms across, so thin that electrons leak out. In other words, the age of silicon could be drawing to a close.
To lay the groundwork for the coming post-silicon era, physicists are interested in creating quantum computers that can compute on individual atoms. (The CIA is interested because a powerful quantum computer could in principle break every code on earth.)
The physics behind quantum computing and teleportation are very similar, both relying on the esoteric properties of quantum theory (where electrons can be in two places at once, and cats are neither dead nor alive). As a result, progress in quantum teleportation is benefiting from the rapid progress in quantum computing.
There are some technologies which remain out of reach. The ultimate technological breakthrough would be a time machine, which would allow you to alter the past. There seems to be no law of physics preventing this. Einstein's equations have plenty of solutions in which, upon entering a "wormhole," one is instantly transported back into the past, so that you return from a voyage before you left.
A wormhole is a shortcut through space and time. Like the looking glass of Alice, a wormhole is a tunnel through which one can instantly traverse great distances in a blink of an eye. This technology is a class II impossibility, requiring a technology perhaps millennia more advanced than ours. Designs for time machines have appeared in physics journals and books (including mine), but the energy necessary to create a wormhole and stabilise it is truly staggering, stretching the limits of our knowledge of physics.
But perhaps this is just as well. Humanity does not have the maturity yet to be able to become masters of time and to tinker with destiny itself.
