Author Topic: Trap Doors in Time and Space: Teleportation, Time Travel, and Escape from Black  (Read 4371 times)

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Offline mayya

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Trap Doors in Time and Space: Teleportation, Time Travel, and Escape from Black Holes

By Seth Lloyd
 on December 16, 2013
11:53 AM



Do you feel stuck in space and time? Do you want to fast forward to the future, replay the past—be anywhere but here and anywhen but now? Does your work situation seem like a black hole, sucking you remorselessly towards a point where your consciousness is squished into nothingness along with all the known laws of physics? I'm with you on all counts, if only because this article was due last week!

Credit: Floriana/iStock

Luckily for you and me, these are good days for teleporting out of here, traveling through time, and escaping from black holes. Hard as it is to bend our minds about such non-standard forms of transportation, the laws of physics allow and even encourage them. The secret lies in a feature of quantum mechanics called entanglement. Two quantum systems are said to be entangled when they have more information about each other than they can have classically: they "know" more about each other than they have any right to know. Entangled systems possess a kind of quantum intimacy that goes beyond anything that is allowed by the classical laws of physics.

Entanglement gives rise to what Einstein called "spooky action at a distance" (spukhafte Fernwirkung), where an action performed on one quantum system seems to have an instantaneous effect on an entangled system. Under normal circumstances, quantum spooky action at a distance doesn't allow one to communicate instantaneously, but it does allow the quantum effect known as teleportation. Teleportation is a quantum version of the process familiar from Star Trek: the person/molecule/atom to be teleported dematerializes here, and rematerializes over there. For a long time, scientists thought that quantum mechanics didn't allow teleportation, because if you make measurements on the system to be teleported, quantum mechanics guarantees that those measurements are both destructive and incomplete: They destroy features of the state of the measured system, and they cannot reveal the full quantum state of the system to be teleported. As a result, physicists reasoned, accurate teleportation of any quantum system was impossible.

In 1993, however, a group of quantum physicists realized that entanglement allows one to teleport a quantum system even though measurement is destructive and incomplete. The protocol works as follows. Alice wants to teleport an electron to Bob. Suppose that in addition to the electron that Alice is going to teleport, Alice and Bob share a pair of entangled electrons. Alice makes a measurement on the electron she wants to teleport, as well as her half of the entangled pair. She sends the result of the measurement to Bob. Now, even though Alice's measurement is completely destructive and, taken on its own, reveals no information about the state of the electron that she wishes to teleport, the result of the measurement contains exactly the information that Bob needs to recreate the original electron from his half of the

entangled pair. (A more detailed description of how quantum teleportation
works can be found here.)
Shortly after it was proposed, experimentalists made quantum teleportation a reality, teleporting particles of light, or photons, and even larger stuff, like electrons or atoms, over distances which now range beyond a hundred kilometers. Really large quantum systems, like human beings, have proved harder to teleport.

Escape from black holes
Teleportation may be an unorthodox mode of traveling, but in one sense, it is quite conventional: It does not allow one to go faster than the speed of light, because Alice has to send the results of her measurement to Bob in order for him to recreate the original system. Under ordinary conditions, that information can travel no faster than the speed of light. The interior of a black hole does not represent ordinary conditions, however. At the center of the black hole is a singularity, a place where all the known laws of physics break down, and infalling matter and energy are squished to nothingness. Because of the breakdown of known physics at the singularity, we don't know what happens there. While some might regard such ignorance as an impediment, to a scientist, it represents an opportunity: Since we don't know what happens at the singularity of a black hole, we are free to postulate any dynamics that we like. That is exactly what theorists Gary Horowitz and Juan Maldacena did to construct a theory of escape from black holes based on quantum teleportation.

Without quantum mechanics, when you fall into a black hole you are doomed: You will be sucked into the singularity in a time no longer than twice the radius of the black hole divided by the speed of light. Before you hit the singularity, you will be ripped apart from tidal forces. Bummer. You might think that a desperate blast on your rockets might at least slow down the inexorable descent, but it turns out that the theory of relativity implies that fighting to blast your way out of a black hole is counterproductive: trying to accelerate away from the singularity actually means it takes you less time to arrive there. Black holes are like quicksand: If you fall in, don't struggle.

With quantum mechanics, however, there is a faint hope. In 1974, Steven Hawking showed that black holes emit a faint, ethereal form of radiation. Just maybe, the Hawking radiation could contain the bits of information required to assemble any lost travelers who ventured too close to the hole.

How could that be? Hawking radiation consists of entangled pairs of particles, a negative energy particle that falls into the hole, thereby reducing the hole's mass, and a positive energy particle that escapes off to infinity. So if a would-be teleporter inside the hole were able to make a measurement on you together with the infalling Hawking radiation, and could send the results of that measurement outside of the hole, then that information could be used to recreate you out of the outgoing Hawking radiation. Unfortunately, to send the results of the measurement out of the hole requires faster-than-light communication, so you are still stuck. But there is a way out. Horowitz and Maldacena invite us to consider that the process of being smooshed into nothingness at the singularity is effectively such a measurement, but in contrast to the measurements made in teleportation, which probabilistically yield different outcomes, in the Horowitz-Maldacena model, the measurement made by the singularity always yields the same outcome. Your rescuers outside the hole can therefore recreate you by the same process as in teleportation. You are saved! Although you are still likely to feel a bit smooshed.

Time travel
To escape from a black hole, you must effectively travel faster than the speed of light. But as everyone knows, if you can travel faster than the speed of light, you can also go backwards in time. Althoughit sounds like science fiction, time travel is actually allowed in Einstein's theory of general relativity: space times can possess closed timelike curves which you can enter in the future and exit in the past. In 2009, my colleagues and I showed that the quantum mechanics of closed timelike curves was essentially the same as that of teleportation and escape from black holes. In addition to providing a novel theory of quantum time travel, we performed an experiment that was the moral equivalent of the famous grandfather paradox of time travel: we sent a photon a few billionths of a second back in time and had it try to kill its former self. What happened? Well, let's just say that our experiment was not like one of the movies where they say at the end, "No animals were harmed during the making of this movie." Gajillions of photons died. Luckily there is no society for prevention of cruelty to photons—yet. Ironically, however, the one photon we sent back to perform auto-homocide failed to off its former self.

How exactly did we send the photons back in time? Like escape from a black hole, travel through a closed timelike curve is based on teleportation—in this case, teleportation from the future to the past. Recall that in the Horowitz-Maldacena model, the singularity effectively performs a measurement on you and the infalling Hawking radiation just as you are being smooshed into nothingness. A similar effect occurs at the future entrance to a closed timelike curve: As you enter the curve, you disappear from view of observers in the "normal" spacetime outside the entrance. So far as they are concerned, you are disappearing into nothingness. In our model, as you disappear, the closed timelike curve effectively performs a measurement on you along with curve's analog of Hawking radiation, called horizon radiation. Just as the effect of the measurement at the singularity of a black hole causes you to reappear outside the hole, the effect of the measurement at the entrance to the closed timelike curve causes you to reappear at the exit to the curve in the past.

Actually teleporting something to the past with certainty would require a true closed timelike curve. (Or maybe a pair of black holes!) Unfortunately, the department of workplace safety at MIT wouldn't let us construct such an object in the laboratory. At University of Toronto, however, Aephraim Steinberg's group was able to do the next best thing: perform a teleportation experiment using entangled photons in which some fraction of the time one of the entangled photons in the past was identical to the time traveling photon in the future. The photon returning from the future was tasked with trying to prevent its former self from entering the teleportation device using a device called a photon gun, which was pointed closer and closer at the photon in the past. But the photon from the future couldn't prevent the photon from the past from performing the teleportation, no matter how directly the photon gun was pointed at its past self. That is, no matter how hard it tried, the photon couldn't kill its former self. The closer it got to killing itself, the less and less likely the teleporation was to succeed. For a detailed account of the quantum theory of time travel and the results of our experiment, see here.

Quantum mechanics is famously weird, and quantum weirdness opens up opportunities for funky methods for getting from point A to point B, even if point A is inside a black hole, and point B is outside, or point A lies in the present, and point B is in the past. To date, only teeny things have been teleported and effectivelysent backwards in time. Since big things are made of teeny things, they too obey the laws of quantum mechanics, and are candidates for funky quantum transportation. In the not so distant future, we too will be quantum commuters.