Thought Experiments


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

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.

Go Deeper
Editor’s picks for further reading

NOVA: Fabric of the Cosmos: Quantum Leap
Explore entanglement and teleportation in this episode from the NOVA series Fabric of the Cosmos.

NOVA: Spooky Action at a Distance
In this essay, physicist Brian Greene explores the history of quantum entanglement, and explains why Einstein found it so “spooky.”

The Open University: The Grandfather Paradox – 60-Second Adventures in Thought
This this brief animated video, discover the history of the grandfather paradox of time travel.

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Seth Lloyd

    Seth Lloyd was the first person to develop a realizable model for quantum computation and is working with a variety of groups to construct and operate quantum computers and quantum communication systems. Dr. Lloyd is the author of over a hundred and fifty scientific papers, and of "Programming the Universe" (Knopf, 2004). He is currently professor of quantum-mechanical engineering at MIT.

    • calP94

      Very well explained. I agree with Art Toegmann that many don’t realize the consequences of messing with the fabric of time. Look at the butterfly effect, a butterfly flapping it’s wings on one side of the world can cause a hurricane on the other side of the world. In a deterministic nonlinear system a small change in one place can cause a larger difference in a later time, sensitive dependency on initial conditions. I just don’t see how anyone can successfully go back in time without causing a ripple in the present. One little change will cause a change in something else that will cause a change in something else which will in turn cause a change in another thing and it will never stop.

    • ProtectMyDNA

      Time Tra?vel: As universe expands does history do also so everything changes beneath & including our own noses? Do we shapeshift & not know, since truth cannot be bound by time & space & by limits of speed of light, & questions & answers they crystallize at the same time at whatever times they may? Might we cleanse the human race by that in 2025* we send our children off to reside in space where they do not age as fast as those left on earth, in which case they arrive back to an aged human kind where walk siblings greatgrandchildren to remove & then take the place of, they’re younger with a better chance of enduring the end so come specifically to knock generations of humans who’d since become older than them. & though their parents had died long ago, do we who remain after them qualify as being practically ancestors, fallen back further in the gene pool than them like an ancestor does as genes over generations continue to lose potency? Is there a mass genocide to kill as though as many humans than stars when we work out we can continuously kill ‘ancestors’, the ghosts of earths future one by one been born ahead of us not behind us as we might at the moment assume we would :). Wonder quantum isn’t limited to that past should only happen prior to future :)? Now the dreadful thing is, it just may seem at the time that if we accept our deaths, that the ‘youngest’ every say hundred years or so would ship out & ship in to get to repetitively replace human civilizations left behind so the race would succeed to get to someday live out beyond stars. But, when our children left was it the case that on their return our earth didn’t recognise them to allow they spontaneously generate to the new beginning of earth as new stock as may god intend, let alone go back to precursors of warnings & influences they could remember in their own lives once more. They work out that as we aged we’d done so by being able to go to & fro within our own lives, avoiding tragedy, shapeshifting without a clue, & sending thoughts back to the past to develope what we need before we need it. Oh no, indeed when our children found out how we were living life for eternity whereas they weren’t, they became ropable & sent us into a deep sleep instead of killing us, to try timetravel ‘by association’ which sadly didn’t work as well as they hoped because they’d jammed the system so They, try as they might, became the underdeveloped they’d assumed We were. They’d pulled human kind from it’s routs & couldn’t replant themselves from anything but seed again which they were out of. We were gone though we lay there imprisoned like shells for there was not to be a future ahead. The experiment to be eternally youthful had failed & those the earth knew no more began to rot as she became old. Only men who remained belonging to our planet, survived the future collapse of the universe to be processed, to live in sanctity which was always meant to be ours on the earth forever – ours earlier given us to life on forever? Was everything indeed once made in 6 days, & did we of soil rise with the static on an early radioactive earth as smallest organisms to over time slowly adapt, so by history expansion (creationism) we have come to be who we think we’ve been all our lives each day:), ..though natural selection aswell did obviously have a hand at evolving us – environment because of fear of environment, & who in fear of ourselves had created us consciousness? (Really people, it is time to look outside the box). And as history builds behind us on & on while we’re oblivious, have we more & more we remember which even though hadn’t previously happened, now had? And as history keeps building behind us, is everything now found in great numbers, which results in our sending our children away thinking they must absolutely be entitled to take the place of we their elders (..even though we were elders they’d been born previous to not after) ..& for Their children to be able to return to likewise rid the planet of as many expendable as they can find, to end a line of us so far only assumed destined to fail the species survivalwise? ..We who weren’t materialistic & neither had holes in our reasoning (which made for rooms where lies loved security & spoke out our mouths in our places from, looking like us), be we fit information too like truths the law of nature says must move & multiply, & each time faced with Armageddon were we placed once again to a familiar earlier-on rather born to Not have sent our children away.. till a little later each time ..not knowing we’re repeating the past, a past not before us but rather was ahead of us once – vanished once again without a trace & remaining yet-to-happen still.
      By that it is a self-planned inevitably that in our masses human kind shall be put through a mill, does each time the good come face to the mass massacre to happen, it be delayed, & delayed, & delayed, so never happen, but never were the ones who were taken by Armageddon seen again. They who were poor information just never were & were not known to miss or replace. They were suddenly not in the house. We the seen & known, who are to inherit the earth & be kept safe apon it for alltime being given what we need before we need it (if that it be fit info too of course), & without a trace time & time again are taken in peace from the future without who we now don’t recall as ever having been, as they’d come to be no longer, having vanished from all existence taken by revelations Armageddon as unfit-info. These too go peacefully eventually, though the elders are crushed & the young murderers memories relying on use & stability begin to deterioate. They all kept on believing that the ‘old’ should die & their deaths would save human kind. Do we continue to be purified until the very end of our existence here, the last of the good finally avoiding Armageddon all together, & will in unison we then ask “where do we go?” & “what should we have known?”, right & honest statements indeed, which causes us to spontaneously generate back again, but this time back to the very beginning, to one day again wonder “who are our missing links” & not know that Between we New sapiens & the primitive Previous ‘man species’, was a world of advanced men once on a future earth not to be touched by us till what time it can be considered ours, and ‘never’ men’s. Every last ancestor man & woman except for those initial on earth would be wiped without a trace they’d ever been, and Our not having lived from a beginning before, could not have been Unless we Had once. The late men late men of yesterdays future. ..Did Neanderthals aswell make it to an endtime once when the idea for man had not been totally developed? Were They the chosen rulers on an earth would have no men, we being not a whole thought yet until a Neanderthal future would be over? We would only later on a new earth exist. & when early men placed the Sphinx was it to represent to us to keep those down of who were from the underworld for example the neanderthal, the sabor tooth or mamoth, which should Only exist in the past which appeared to get smaller compared to our historys to become spread over a billion years? Do such old species from their precursors be now only to begin to die again? But do they not tell, they only lived that long on a once smaller earth to a smaller universe & history Anyway? Is everyone happy, considering they we’ll never be aquainted on the new earth to the those who weren’t, as they had just (just) never been? Say visit CrazyLa at Airg, & if you’ll read “Abiogenisis story” there you’ll find the more indepth & lengthy details about ·time travel, ·creationism, ·shapeshifting, ·eternal life of fit info, ·Housed mis-truthes possessively dwelling in voids they create or which are already available in ones mind (possession). I’m Miss Krystel Spicer, neice of Robert Czolij Quantum physics professor Sydney University (discovered 4 years ago). Neice of Eugene Czolyj President of The World Congress Ukraine (discovered a year ago), & most recently, of the family found to hold the rare R1b1b1 heliotype being that of the 1st people to enter Ancient Briton. We are the latest & last group of human on earth & the genetic footprint of most British walking today. I wonder what will always have been tomorrow then :)?.
      ..& I wonder that for every question a profit answers, the future be altered so we’v different questions we asked & other miseries, for ‘to ever see God & know all we would surely die” ..& the profit sits thousands of years before us writing different verses in place of what he won’t now, on the very same paper once again he’d never writen on? And for truth which in other words is all fit information sensible to make something of, is unbound by the limits of the speed of light and is able to go back to times where universe & history were still in early stages of expansion, say with what a mammoth needs before he needs it. ..or whatever it turns out to be now :).

    • Dan Morgan

      I, however, disagree. Just imagine the future correct.

    • Dan Morgan

      If you don’t know where or “when” you’re going. How then, will you successfully get there? That is the question.

    • Charles W8EIV

      Before time travel to the past could be realized, there would need to be a discriminator available to detect the photon containing the data from the future so that dictates that we would first need to build a discriminatoer or photon receiver and detector and then in a few years we would be able to conduct our firs test at sending a photon back to the year that we set up the receiver, guess that means that we would be able to communicate with the past any further than when we set up the receiver…..
      tnx for your interest Charles Nova W8EIV