Science
The meaning and history of the Bell Test
A Bell test is an experiment to decide whether the world is really as strange as quantum mechanics says it is. Our ordinary experience tells us that inanimate matter has a reality independent of ourselves. For example, we believe the moon was there, orbiting the earth, long before there were any people to admire it. Quantum physics seems to say something different: that the act of observing the world can change it. Niels Bohr, a towering figure in quantum mechanics, even claimed that observables such as “the position of the atom” have no meaning until someone measures them. If this is true, the act of observation at least alters and maybe creates the world, quite the opposite of an independent reality. Physicists and philosophers have debated Bohr”s interpretation of quantum mechanics since it was published in 1927.
Albert Einstein was the most vocal opponent of Bohr”s interpretation. In a several-year-long debate with his friend Bohr, Einstein articulated two principles he saw as fundamental: Realism says that objects have well-defined properties even when we are not looking at them and locality says that objects can only be influenced by causes in their immediate vicinity, not by “action at a distance.” Locality is, among other things, important to Einstein”s theory of relativity, in which time and space depend on the observer. In relativity, locality ensures that effects come after, and not before, their causes.In 1935, Einstein and colleagues Podolsky and Rosen published a strong attack on Bohr”s interpretation, now called the “EPR paradox.” The paradox uses the mathematics of quantum mechanics to describe a pair of entangled particles in different locations. According to Bohr”s interpretation, when one particle is measured, the other particle would change instantly, even though it might be very far from the first particle. Einstein considered this communication between the particles, which he called “spooky action at a distance,” to be so implausible that it disproved Bohr”s claims that measurement influences the world. The EPR article also suggested that quantum mechanics should be replaced by a more complete theory, one that specifies what happens also when we are not watching. Einstein did not, however, have such a theory to offer.
In 1964, John Bell, a physicist at CERN, translated Einstein”s philosophical positions of locality and realism into a precise mathematical description, now called “local realism.” With this mathematical description, he proved that Einstein”s local realistic world-view is incompatible with quantum mechanics. That means that there are experiments for which quantum mechanics gives one prediction, and any theory Einstein would have approved of gives a different prediction. Bell”s work thus made it possible to test in the laboratory what had previously been a philosophical question.
Bell tests: does Nature agree with Einstein or with Bohr?
There remains the question of whether Nature itself agrees with Einstein or with Bohr. This requires an experiment, a Bell test. This experiment looks a lot like the EPR scenario: the experimenter produces a pair of entangled particles (entanglement means that their properties are strongly correlated; for example if one spins left, the other must spin left, too) and sends them to two separated measurement stations, traditionally called “Alice” and “Bob.” Alice and Bob make simultaneous, unpredictable measurements on the particles. Quantum mechanics says that the measurement Alice makes will instantly influence Bob”s particle, with the effect that the measurement results agree. In local realism, this influence cannot happen, and Bob and Alice”s measurement results will often disagree. This agreement or disagreement, called correlation, is the signal that allows an experiment to decide about local realism.The first experimental tests were performed in the early 1970s, but were very difficult, and the various experiments obtained contradictory results. By 1982, however, a new generation of experiments had clearly shown correlations too strong to explain by local realism. It looked like quantum mechanics had won the debate at last. But new doubts emerged, in the form of so-called loopholes.
The 2015 loophole-free Bell tests
In 2015 three extraordinarily advanced experiments, performed at TU Delft (the Netherlands), IQOQI Vienna (Austria), and NIST Boulder (USA), resolved the few remaining weaknesses (the so-called “loopholes”) of previous tests, including giving strong physical arguments for the unpredictability of their measurements. They used physical random number generators to turn unpredictable physical events like spontaneous emission (which Einstein also studied) into measurement choices. The results were clear: they saw correlations too strong to come from local realism. The New York Times summarized the situation: “Sorry, Einstein. Quantum Study Suggests “Spooky Action” Is Real.”
The BIG Bell Test of 2016
The BIG Bell Test (BBT) is a worldwide project to bring human randomness to cutting-edge quantum physics experiments. On November 30th, many people contributed randomly chosen bits. These were distributed in real-time to experimental groups around the world (see map) for use in quantum physics experiments, including the first Bell test(s) with human-generated randomness. In addition to testing fundamental physical principles like non-locality, human randomness is useful in important applications such as secure communications, and also as a “seed” for the generation of even more randomness.
Why quantum physics with people?
Bell test experiments must be performed under strict conditions in order to be convincing. One such condition is using unpredictable and independent input to decide which measurements to perform on quantum objects like atoms and photons. There are many ways to guarantee independence; the BIG Bell Test will use the Bellsters, free human minds independent of each other, to control the measurements on quantum particles through their decisions. Unlike electrons or protons or the Higgs boson, which are perfectly interchangeable particles that behave similarly under the same conditions, every human-being acts genuinely on his/her own, and this is very valuable for the Bell test requirements. The BIG Bell Test aims to show for the first time that human choices can contribute to fundamental science, and at the same time to perform a suite of never-before-attempted experiments.
Why so many people?
As in any scientific experiment, we want to be sure of the precision our result, to know that the effect we are observing is really a consequence of the properties of the physical world. A common way to reduce the uncertainty on the result of an experiment is to repeat it many times and then check if the results are statistically significant. It is like trying to know if a coin is biased or not: if we toss it just a couple of times we cannot be really sure, but as the number of tosses increases, we obtain a more and more precise estimation of how often we obtain tails against head.Every random number the Bellster community contributes allows the scientists to perform another run of the experiment, and to reach a more precise result. Moreover, the more different individuals are participating, the more we are assuring the statistical independence that is so important for this kind of experiments. This is really the case to say: the more, the merrier!
How did it work?
The participants send their contributed bits through this website. There are two ways to participate: (1) using a plain interface in which you introduce zeros and ones as fast as you can, and (2) a video game interface. All those bits were sent directly to the experiments, to choose the measurements. Each human-generated bit used in the experiment was thus the result of a unique and conscious decision process.The day of the experiment, ICFO distributed in real time the human-generated randomness to many top-notch Bell tests (and related experiments) all around the world (see map). In parallel, we also distributed random numbers from a physical random number generator designed and built at ICFO. A cloud-based server collected all the random numbers and distributed them to the experimental teams via Internet. All the human-generated bits before and during the experiment will also be archived for later study.If you”d like to know more you can visit The BIG Bell Test Museum
What happened in each lab?
The experimental teams received three streams of random bits: one real-time from participants, one real-time from ICFO”s physical RNG, and one stream of archived bits entered earlier by participants.Each experimental group contributed by performing the most interesting experiment(s) with human-generated randomness. Would you like to know more? You can visit the results page with info from each lab.
