An Oxford Physicist Discovered Particles Doing the Impossible. It Could Finally Solve the Quantum Gravity Mystery.

6 min readHere’s what you’ll learn when you read this story:A single quantum object could share information with itself across different moments in time, not just with another entangled object in space. This is what the author coined as “entanglement in time.”This idea may help address the black hole information paradox, especially the problem that black holes seem to violate a key quantum mechanics rule.A lab experiment with entangled photons showed that temporal entanglement can evade this rule, which may suggest a possible path toward understanding quantum gravity.This story starts way back in 1995 when I began my PhD in physics at the Imperial College in London. I wanted to measure a particular type of relationship between particles in quantum systems, whose behavior is governed by quantum mechanics, the physics of the tiniest components in the universe. These quantum particles become entangled, which means they’re so connected that even when they are in distant points of space, measuring the properties of one system tells you about the other, entangled one, too.What I realized—after completing my doctorate degree—is that entanglement need not exist only between two or more quantum systems. Such behavior can also exist in one and the same quantum object between two or more instances of time. I introduced this concept and gave it its name (“entanglement in time”), in distinction to entanglement in space, which describes entanglement between different quantum systems, or how quantum information is shared between different systems. Less intuitive is the idea of entanglement in time, which captures how an object at one time shares quantum information with itself at a later time.Little did I know at that time that my work would later lead to a potential unification of quantum physics and gravity, and that black holes could give us the strongest evidence for such unification. With hindsight, this is not surprising. That’s because our best current account of how gravity behaves is thanks to Einstein’s theory of general relativity, which tells us that gravity consists of ripples in spacetime. One of its most famous predictions is the existence of black holes, objects so massive and compact that their gravitational forces prevent anything that has fallen into them from escaping. If an apple or a pear or a beam of light falls into a black hole, the result is always the same: any information about the origin or composition of the stuff falling in gets erased.But is that really what happens?This loss of information inside black holes doesn’t play well with our other, equally successful theory of the universe—quantum physics, which demands that information is always preserved. Because the laws of quantum physics are reversible, no information can ever be lost. Anything that has happened in one direction can, in principle, be undone. Light can be converted into matter, such as when photons stimulate the creation of an electron and a positron, but the electron and positron can also annihilate to give us back light.This tension between quantum physics and general relativity is frequently cited as one of the main obstacles to their unification. Indeed, how can we unify quantum theory and gravity if, according to one theory, information is always conserved, while in the other it can sometimes be lost? This unification requires that gravity acquires quantum features; for instance, having a superposition of two different gravitational strengths at one and the same point. However, because of their information loss, it seems that black holes could either be quantum mechanical objects or gravitational objects, but never both at the same time.Stephen Hawking, who originated the concept of the black hole information loss paradox, subsequently brought us closer to its resolution. Hawking showed that black holes are not “entirely” black. They must radiate according to the laws of quantum mechanics. This means that stuff must also come out of a black hole, ultimately leading to its evaporation. So, black holes do demonstrate quantum features, because whatever initially falls into a black hole could ultimately be recovered after the black hole has completely evaporated.But, as frequently happens in physics, the resolution of one paradox invariably leads to the creation of another. According to Hawking’s calculation, when matter leaves a black hole, it creates entanglement between the interior and exterior of the black hole. As a reminder, entanglement is the quantum state in which two objects share their information so fully that they perfectly mimic each other’s behavior. After half the matter has evaporated, we reach a state of maximum entanglement between the interior and exterior of the black hole.However, Hawking’s logic suggests that as evaporation continues beyond this point, still further entanglement must be created. And this presents a problem for quantum physics because entanglement is “monogamous.”Monogamy of entanglement is a core property of quantum information. It means that when two things (like the interior and exterior of a black hole) are maximally entangled, they cannot then also be entangled with anything else. So, by treating black holes quantum mechanically in order to solve the information loss paradox, we have actually made black holes violate this property.It’s at this point that I became involved with this problem because entanglement is my specialty. I discovered a key property of entanglement in time: unlike entanglement in space, it need not be monogamous. This gave me an idea for how to resolve the black hole monogamy violation. I was very excited because this might finally get us to the theory of quantum gravity. Black holes evaporate by creating pairs of entangled particles, one of which leaves the black hole while the other falls inside it. Now, as far as the particle on the inside is concerned, space and time have swapped their roles. As the outside particle must invariably evolve forward in time, the inside particle must invariably evolve forward in space toward the center of the black hole; this is what Einstein’s equations tell us. So black hole evaporation might be thought of as the conversion of spatial into temporal entanglement.Now, it would be great to test this idea with real black holes. However, our present technology does not allow us to do anything like that. So, after several discussions with my colleague Chiara Marletto, DPhil, we thought we could instead simulate this process in the lab. We approached our colleague Marco Genovese, PhD, who is a wizard with photons. He routinely creates pairs of entangled photons and performs different kinds of information processing with them. We suggested that he create a pair of entangled photons, but measure one of the photons twice in time. This is tantamount to having one spatially entangled pair of photons and one temporally entangled pair. And when Marco and his group tested both these entanglements, they found that both could be maximal. This corroborated the fact that temporal entanglement need not be monogamous. And if particles in the lab can violate monogamy in space and time, then perhaps the same thing happens in black holes.No one at present knows whether my idea could lead to the ultimate resolution of the information loss paradox. It’s a possibility, but we won’t know until we’ve tested real black holes. However, there is something equally interesting here that also deserves our attention. When two particles are entangled in space, we can send information from one to the other through the protocol known as quantum teleportation. Could it be that quantum teleportation is also possible using temporal entanglement? And, more fascinatingly, could temporal entanglement then allow us to send information from the future back to the past?Stay tuned to find out.Vlatko Vedral is a professor of physics at the University of Oxford, known for both his theoretical and experimental work on quantum information, including developing a novel way of quantifying entanglement and applying it to macroscopic physical systems. When not studying the fundamental nature of reality, Vlatko enjoys drawing, wakeboarding, and playing his electric guitar "up to 11." He is the author of the 2010 book Decoding Reality, as well as the latest "Portals to A New Reality." Born in Serbia, he now lives in Oxford.