Distant quasars to fill a loophole of Bell’s theorem

Scritto da Annalisa Arci il 02.03.2014

This article is also available in Italian / Leggi in italiano

In a paper published this week in Physical Review Letters, MIT researchers propose an experiment that could fill one of the most significant loopholes concerning Bell’s inequality. According to the scientists, the experiment could test a 50 year old theorem that, if violated by experiments, would mean that our universe is based on less tangible probabilities of quantum physics, and not upon the textbook laws of classical physics.

 This vision would confirm that quantum entangled particles can influence each other, instantaneously, at a speed much higher than the speed of light. Bell’s theorem shows that, if quantum mechanics is valid, then the measurements performed on the two particles are always correlated, regardless of the distance that separates them.

ULAS J1120+0641, a very distant quasar (Credit: ESO/M. Kornmesser).

ULAS J1120+0641, a very distant quasar (Credit: ESO/M. Kornmesser).

A magical correlation. Well, take two subatomic particles. They have a spin that rotates on their own axis just like the spinners or planets. Imagine that you have a system with two particles, very close to each other rotating in opposite directions: we commonly describe this situation by saying that the spin of a particle is up and the one of the other particle is down. But by measuring the spins of the particles after they have been substantially separated, we find that they have remained one up and the other down. Since spins behave like small magnets, it is possible to change their orientation by passing through magnetic fields: if we change the orientation of a particle so that, instead of rotating upward around a vertical axis it rotates to the left around a horizontal axis, we find that also the other particle rotates around a horizontal axis, but in the opposite direction, which will define the right.

These results were confirmed initially by two experiments, the first performed in 1972 by J. Clauser and S. Freeman in the United States, and the second by A. Aspect, P. Grangier and C. Roger at CERN in 1981. Consequently, although it may seem unusual, there is some form of instant communication between the two particles so that, the change in spin of one particle instantly implies the change in spin of the other. “Instantly” in physical terms means at superluminal speed, that is obviously greater 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 explained by classical physics, the measurement of an entangled particle entangled shall not affect the determination of the other particle (a theory known as the principle of locality, in which there is a limit to how correlated two particles can be). Bell has developed a mathematical formula for locality, and presented scenarios that violated this formula according to the predictions of quantum mechanics. This is a quantum feature Albert Einstein skeptically referred to as “spooky action at a distance”. Einstein would be horrified, but experiments confirm that if one of the two entangled particles is “altered” (a modification of state), then the other particle is also altered. Essentially all of these experiments have shown that such particles are correlated more strongly than it would be expected under the laws of classical physics.

The speed of light is an absolute value, a universal constant irrefutable that can not be denied: so how is it possible that a particle alters the status of the other when a communication between the two is, a priori, impossible? Physicists have repeatedly tested Bell’s theorem by measuring the properties of entangled quantum particles in the laboratory, showing that Bell is right: if some subatomic particles are entangled, they retain a permanent affinity that somehow seems to overstep the limitations of classical physics. Of course, there are scholars who highlighted the shortcomings of Bell’s work. Proponents of hidden variables leverage on this topic to show that the quantum explanation is just an illusion.  They 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. Making explicit the hidden variables in each explanation shows the explanatory classical counterpart.

Although many loopholes are now part of the history of physics, the “setting independence” problem, or more provocatively “free will” problem, remains: in an experimental context, the authors of the study that we present have imagined that there is a conditioning of experimental system that has an effect on the measurements in the laboratory.

This loophole proposes that the settings of particle detectors 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?” David Kaiser, Andrew Friedman (postdoc at MIT) and Jason Gallicchio (University of Chicago) have proposed an experiment to close this third loophole by setting  a particle detector through a part of the universe ‘s oldest light: that of distant quasars or active galactic nuclei, which was emitted billions of years ago.

The experiment with the quasar as photon detectors. If two quasars are located on opposite sides of the universe, the distance between them must be such as to make it independent of the causal chain that began with the Big Bang (about 14 billion years ago). This also means that, at the time, were not able to communicate with each other (an ideal scenario to determine the settings on each particle detector ). The description of the experiment is very complex and I will not summarize it (for details refer to the original article). I will instead provide the basic elements to understand the idea behind the experiment. Imagine two detectors and a particle generator, such as a radioactive atom that spits out pairs of entangled particles. Detector A must measure the first particle of the pair C and detector B measures the second particle. The settings of the two detectors are obtained through two distant quasars that – according to the authors of the article – have never had causal connections nor retain, of course, any memory. 

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 find that the measurements of the particles are 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 have some doubts on the immediate reproducibility (in many contexts and laboratories) of the experiment, but the authors of the paper argue that with current technology the experiment is feasible. All that remains is to wait for some laboratory to carry out the experiment and provide more details (practically speaking).