Closing the ‘free will’ loophole
MIT researchers propose using distant quasars to test Bell’s theorem.
Jennifer Chu / February 20, 2014
In a paper published this week in the journal Physical Review Letters, MIT researchers propose an experiment that may close the last major loophole of Bell’s inequality — a 50-year-old theorem that, if violated by experiments, would mean that our universe is based not on the textbook laws of classical physics, but on the less-tangible probabilities of quantum mechanics. Such a quantum view would allow for seemingly counterintuitive phenomena such as entanglement, in which the measurement of one particle instantly affects another, even if those entangled particles are at opposite ends of the universe. Among other things, entanglement — a quantum feature Albert Einstein skeptically referred to as “spooky action at a distance”— seems to suggest that entangled particles can affect each other instantly, faster than the speed of light.
In 1964, physicist John Bell took on this seeming disparity between classical physics and quantum mechanics, stating that if the universe is based on classical physics, the measurement of one entangled particle should not affect the measurement of the other — a theory, known as locality, in which there is a limit to how correlated two particles can be. Bell devised a mathematical formula for locality, and presented scenarios that violated this formula, instead following predictions of quantum mechanics. Since then, physicists have tested Bell’s theorem by measuring the properties of entangled quantum particles in the laboratory. Essentially all of these experiments have shown that such particles are correlated more strongly than would be expected under the laws of classical physics — findings that support quantum mechanics.
However, scientists have also identified several major loopholes in Bell’s theorem. These suggest that while the outcomes of such experiments may appear to support the predictions of quantum mechanics, they may actually reflect unknown “hidden variables” that give the illusion of a quantum outcome, but can still be explained in classical terms. Though two major loopholes have since been closed, a third remains; physicists refer to it as “setting independence,” or more provocatively, “free will.” This loophole proposes that a particle detector’s settings may “conspire” with events in the shared causal past of the detectors themselves to determine which properties of the particle to measure — a scenario that, however far-fetched, implies that a physicist running the experiment does not have complete free will in choosing each detector’s setting. Such a scenario would result in biased measurements, suggesting that two particles are correlated more than they actually are, and giving more weight to quantum mechanics than classical physics. “It sounds creepy, but people realized that’s a logical possibility that hasn’t been closed yet,” says MIT’s David Kaiser, the Germeshausen Professor of the History of Science and senior lecturer in the Department of Physics. “Before we make the leap to say the equations of quantum theory tell us the world is inescapably crazy and bizarre, have we closed every conceivable logical loophole, even if they may not seem plausible in the world we know today?”
Now Kaiser, along with MIT postdoc Andrew Friedman and Jason Gallicchio of the University of Chicago, have proposed an experiment to close this third loophole by determining a particle detector’s settings using some of the oldest light in the universe: distant quasars, or galactic nuclei, which formed billions of years ago. The idea, essentially, is that if two quasars on opposite sides of the sky are sufficiently distant from each other, they would have been out of causal contact since the Big Bang some 14 billion years ago, with no possible means of any third party communicating with both of them since the beginning of the universe — an ideal scenario for determining each particle detector’s settings. As Kaiser explains it, an experiment would go something like this: A laboratory setup would consist of a particle generator, such as a radioactive atom that spits out pairs of entangled particles. One detector measures a property of particle A, while another detector does the same for particle B. A split second after the particles are generated, but just before the detectors are set, scientists would use telescopic observations of distant quasars to determine which properties each detector will measure of a respective particle. In other words, quasar A determines the settings to detect particle A, and quasar B sets the detector for particle B.
The researchers reason that since each detector’s setting is determined by sources that have had no communication or shared history since the beginning of the universe, it would be virtually impossible for these detectors to “conspire” with anything in their shared past to give a biased measurement; the experimental setup could therefore close the “free will” loophole. If, after multiple measurements with this experimental setup, scientists found that the measurements of the particles were correlated more than predicted by the laws of classical physics, Kaiser says, then the universe as we see it must be based instead on quantum mechanics. “I think it’s fair to say this [loophole] is the final frontier, logically speaking, that stands between this enormously impressive accumulated experimental evidence and the interpretation of that evidence saying the world is governed by quantum mechanics,” Kaiser says.
Now that the researchers have put forth an experimental approach, they hope that others will perform actual experiments, using observations of distant quasars. Physicist Michael Hall says that while the idea of using light from distant sources like quasars is not a new one, the group’s paper illustrates the first detailed analysis of how such an experiment could be carried out in practice, using current technology. “It is therefore a big step to closing the loophole once and for all,” says Hall, a research fellow in the Centre for Quantum Dynamics at Griffith University in Australia. “I am sure there will be strong interest in conducting such an experiment, which combines cosmic distances with microscopic quantum effects — and most likely involving an unusual collaboration between quantum physicists and astronomers.”
“At first, we didn’t know if our setup would require constellations of futuristic space satellites, or 1,000-meter telescopes on the dark side of the moon,” Friedman says. “So we were naturally delighted when we discovered, much to our surprise, that our experiment was both feasible in the real world with present technology, and interesting enough to our experimentalist collaborators who actually want to make it happen in the next few years.” Adds Kaiser, “We’ve said, ‘Let’s go for broke — let’s use the history of the cosmos since the Big Bang, darn it.’ And it is very exciting that it’s actually feasible.”
ENTANGLED BLACK HOLES
MIT physicist finds the creation of entanglement simultaneously gives rise to a wormhole
by Jennifer Chu / December 5, 2013
Quantum entanglement is one of the more bizarre theories to come out of the study of quantum mechanics — so strange, in fact, that Albert Einstein famously referred to it as “spooky action at a distance.” Essentially, entanglement involves two particles, each occupying multiple states at once — a condition referred to as superposition. For example, both particles may simultaneously spin clockwise and counterclockwise. But neither has a definite state until one is measured, causing the other particle to instantly assume a corresponding state. The resulting correlations between the particles are preserved, even if they reside on opposite ends of the universe. But what enables particles to communicate instantaneously — and seemingly faster than the speed of light — over such vast distances? Earlier this year, physicists proposed an answer in the form of “wormholes,” or gravitational tunnels. The group showed that by creating two entangled black holes, then pulling them apart, they formed a wormhole — essentially a “shortcut” through the universe — connecting the distant black holes.
Now an MIT physicist has found that, looked at through the lens of string theory, the creation of two entangled quarks — the building blocks of matter — simultaneously gives rise to a wormhole connecting the pair. The theoretical results bolster the relatively new and exciting idea that the laws of gravity holding together the universe may not be fundamental, but arise from something else: quantum entanglement. Julian Sonner, a senior postdoc in MIT’s Laboratory for Nuclear Science and Center for Theoretical Physics, has published his results in the journal Physical Review Letters, where it appears together with a related paper by Kristan Jensen of the University of Victoria and Andreas Karch of the University of Washington.
The tangled web that is gravity
Ever since quantum mechanics was first proposed more than a century ago, the main challenge for physicists in the field has been to explain gravity in quantum-mechanical terms. While quantum mechanics works extremely well in describing interactions at a microscopic level, it fails to explain gravity — a fundamental concept of relativity, a theory proposed by Einstein to describe the macroscopic world. Thus, there appears to be a major barrier to reconciling quantum mechanics and general relativity; for years, physicists have tried to come up with a theory of quantum gravity to marry the two fields. “There are some hard questions of quantum gravity we still don’t understand, and we’ve been banging our heads against these problems for a long time,” Sonner says. “We need to find the right inroads to understanding these questions.”
A theory of quantum gravity would suggest that classical gravity is not a fundamental concept, as Einstein first proposed, but rather emerges from a more basic, quantum-based phenomenon. In a macroscopic context, this would mean that the universe is shaped by something more fundamental than the forces of gravity. This is where quantum entanglement could play a role. It might appear that the concept of entanglement — one of the most fundamental in quantum mechanics — is in direct conflict with general relativity: Two entangled particles, “communicating” across vast distances, would have to do so at speeds faster than that of light — a violation of the laws of physics, according to Einstein. It may therefore come as a surprise that using the concept of entanglement in order to build up space-time may be a major step toward reconciling the laws of quantum mechanics and general relativity.
Tunneling to the fifth dimension
In July, physicists Juan Maldacena of the Institute for Advanced Study and Leonard Susskind of Stanford University proposed a theoretical solution in the form of two entangled black holes. When the black holes were entangled, then pulled apart, the theorists found that what emerged was a wormhole — a tunnel through space-time that is thought to be held together by gravity. The idea seemed to suggest that, in the case of wormholes, gravity emerges from the more fundamental phenomenon of entangled black holes. Following up on work by Jensen and Karch, Sonner has sought to tackle this idea at the level of quarks — subatomic building blocks of matter. To see what emerges from two entangled quarks, he first generated quarks using the Schwinger effect — a concept in quantum theory that enables one to create particles out of nothing. More precisely, the effect, also called “pair creation,” allows two particles to emerge from a vacuum, or soup of transient particles. Under an electric field, one can, as Sonner puts it, “catch a pair of particles” before they disappear back into the vacuum. Once extracted, these particles are considered entangled.
Sonner mapped the entangled quarks onto a four-dimensional space, considered a representation of space-time. In contrast, gravity is thought to exist in the next dimension as, according to Einstein’s laws, it acts to “bend” and shape space-time, thereby existing in the fifth dimension. To see what geometry may emerge in the fifth dimension from entangled quarks in the fourth, Sonner employed holographic duality, a concept in string theory. While a hologram is a two-dimensional object, it contains all the information necessary to represent a three-dimensional view. Essentially, holographic duality is a way to derive a more complex dimension from the next lowest dimension.
Using holographic duality, Sonner derived the entangled quarks, and found that what emerged was a wormhole connecting the two, implying that the creation of quarks simultaneously creates a wormhole. More fundamentally, the results suggest that gravity may, in fact, emerge from entanglement. What’s more, the geometry, or bending, of the universe as described by classical gravity, may be a consequence of entanglement, such as that between pairs of particles strung together by tunneling wormholes. “It’s the most basic representation yet that we have where entanglement gives rise to some sort of geometry,” Sonner says. “What happens if some of this entanglement is lost, and what happens to the geometry? There are many roads that can be pursued, and in that sense, this work can turn out to be very helpful.”