BUILDING A TIME MACHINE
From the perspective of science, time travel was impossible in Newton’s universe, where time was seen as an arrow. Once released, it could never deviate from its past. One second on Earth was one second throughout the universe. This concept was overthrown by Einstein, who showed that time is more like a river that meanders across the universe, speeding up and slowing down as it snakes across stars and galaxies. So one second is not absolute; it varies as we move around the universe.
Physicists hope to teleport complex molecules in the coming years. After that, a DNA molecule or virus could be teleported within decades.
Once considered to be fringe science, time travel has suddenly become a playground for theoretical physicists. As Thorne has written: “Time travel was once solely the province of science fiction writers. How times have changed! One now finds scholarly analyses of time travel in serious scientific journals, written by eminent theoretical physicists…. Why the change? Because we physicists have realized that the nature of time is too important an issue to be left solely in the hands of science fiction writers.”
The reason for all the excitement is that Einstein’s equations allow for many kinds of time machines. The most promising design is based on a traversable wormhole. This wormhole is constructed with both of its ends initially located close together. Two clocks, one at each end, tick in synchronization. Now take one end of the wormhole and its clock and send them into space at near-light speed. Time slows at that end due to an effect in Einstein’s theory of special relativity known as time dilation: Relative to a stationary observer on the ground, time aboard a spacecraft appears to slow down; relative to the spacecraft, time for the observer on the ground seems to speed up.
Since the two clocks at the ends of the wormhole are no longer in sync, a wormhole traveler passing from one end to the other can move back and forth in time. There is a limit to how much time traveling he can do, though—he is able to go back in time only to the point at which the time machine was built.
When it comes to the potential for time travel, there is still fierce debate. In 1997 Bernard Kay and Marek Radzikowski of the University of York in England and Robert Wald of the University of Chicago showed that time travel was consistent with all the known laws of physics, except in one place—near the wormhole entrance. This is just where we would expect Einstein’s theory to break down and quantum effects, which work at the subatomic level, to take over. The problem is that when we try to calculate radiation effects as we enter a time machine, we have to come up with a theory that combines Einstein’s general relativity with the quantum theory of radiation. Yet when we naively try to do so, the resulting theory makes no sense. It yields a series of infinite answers, which are meaningless.
This is where a theory of everything takes over. All the problems of traveling through a wormhole that have bedeviled physicists (the stability of the wormhole, the radiation that might kill you, the closing of the wormhole as you enter it) are concentrated at the horizon, precisely where Einstein’s theory makes no sense.
Thus the key to understanding time travel is to understand the physics of the horizon, and only a theory of everything that unites Einstein’s relativity and the quantum realm can explain this. So the final resolution to whether all these science fiction devices are possible will have to wait until physicists can finally develop a theory of the universe that transcends even Einstein’s.
BEAM ME UP
Perhaps the most tangible of the far-out technologies suggested by Einstein’s theories is teleportation. The key lies in a celebrated 1935 paper by Einstein and his colleagues Boris Podolsky and Nathan Rosen. Ironically, in their paper they proposed an experiment—the so-called EPR experiment, named for the three authors—to kill off, once and for all, quantum theory’s introduction of probability into physics. Quantum theory requires probability because its formulas do not directly describe things like the precise position of particles. Instead, the formulas describe waves, known as Schrödinger waves. The amplitude of the waves at a particular location translates to the probability that a particle will be found at that point.
As the EPR experiment pointed out, according to quantum theory, if two particles—electrons, for example—are initially vibrating in unison (a state called coherence), they can remain in wavelike synchronization even if they are separated by a large distance. Two electrons may be trillions of miles apart, but there is still an invisible Schrödinger wave connecting them, like an umbilical cord. If something happens to one electron, some of that information is immediately transmitted to the other, faster than the speed of light. This concept—that particles vibrating in coherence have some kind of deep connection—is called quantum entanglement. Einstein derisively called this “spooky action at a distance,” and he took it to “prove” that quantum theory was wrong, since nothing can travel faster than the speed of light.
But in the 1980s, Alain Aspect and his colleagues in France performed the EPR experiment using entangled photons emitted from calcium atoms and two detectors placed 13 meters (40 feet) apart. The results agreed precisely with quantum theory. Was Einstein wrong, then, about the speed of light’s being the speed limit of the universe? Not really. In Aspect’s experiment, information did travel faster than the speed of light, but the information was random, hence useless.