It was time the Bell was heard
Alain Aspect, Anton Zeilinger and John Clauser were jointly awarded the 2022 Nobel Prize for their efforts to demonstrate that quantum entanglement exists and is non-local. John Clauser was the first to demonstrate this experimentally doing a Bell test in 1972. His result – entanglement is a non-local effect – was confirmed in 1982 by Alain Aspect, but there were still loopholes that could explain his results in a classical physics way. Then – 35 years later in 2017 – Anton Zeilinger conducts a test that definitively excludes all possible loopholes.
Quantum entanglement exists and is non-local, i.e. the relationship the particles have with each other is instantaneous and does not depend on the distance from each other and thus conflicts with Einstein’s laws of relativity because such a relationship would involve instantaneous communication between the particles.
To be accurate, a test that excluded all possible loopholes was already done in 2015 by the team of Hanson and Henson in Delft. However, I heartily grant these three guys their well-deserved Nobel Prize. Non-locality was still a hotly contested idea in 1972 and this kind of research was not really very beneficial for your scientific career at that time. Non-locality raised (too) big questions about the fundamental behavior of nature then. It just couldn’t be. Clauser and Aspect were thus putting their careers at risk by just posing the question. See this quote from the Nobel Prize article on Quanta Magazine under the headline “Who performed Bell’s experiment?”.
"Initially, physicists including Richard Feynman discouraged Clauser from pursuing the experiment, arguing that quantum mechanics needed no further experimental proof."
I will briefly explain what a so-called Bell test basically means, a more extensive description can be found in my book, chapter 5, “Bell’s theorem”.
John Stewart Bell (1928-1990) published in 1964 what is now called the Bell theorem. In principle, this theorem can be used to demonstrate experimentally whether or not local variables play a role in quantum phenomena. I won’t explain local variables here, but it means ultimately that – if local variables apply in quantum physics – particles exist permanently throughout their journey from source to detector – in the same way that we assume that arrows exist permanently throughout their trajectory from source to target, and even before that. Remember that. The experimental setup of a Bell experiment should be such that faster-than-light communication between entangled objects is excluded.
Most Bell tests have been performed with polarized light – that is, polarized pairs of photons. An EM wave consists of an electric and a magnetic field component. These oscillate perpendicular to each other and both oscillate perpendicular to the direction in which the light travels.
The direction of oscillation of the electrical component of the EM wave is called the polarization. The wave in the above figure is horizontally polarized. A polarizing filter, such as Polaroid glasses, only transmits light that oscillates – after its passage through the filter – in a direction that is determined by the orientation of the filter. If the light oscillates at an oblique angle to the orientation of the filter, light is only partially transmitted. The transmitted light oscillates only in the direction the filter has enforced. If the incident light oscillates exactly perpendicular to the direction of the filter, nothing is transmitted. Light is an EM wave, but from a quantum physics point of view, that wave consists of masses of photons that are each polarized. How we should imagine the polarization of a single photon is not clear, so we don’t do that.
No halved photons but probabilities
Photons that are not polarized exactly in the orientation of the polarizing filter, for example hit the filter at an angle of 30o, are transmitted for 50% but are not halved. Their frequency is not affected, but the probability of passing through the filter is 50%. The probability of transmission of a single photon depends on the angle its polarization makes with the orientation of the filter. So, if they are polarized exactly perpendicular to that orientation, the probability of passing through is zero. At an angle of 45o, according to quantum mechanics, the probability that they will pass through the filter is about 71%. The photons transmitted by the filter have not changed in energy, wavelength and frequency. They certainly haven’t halved. So, it’s all about probabilities.
Bell test with polarized photons
The photons are detected by D+ or D-. The coincidences (co-occurring detections) and the angle between A and B are recorded in the coincidences detector. According to the conservation laws of physics, the polarization directions of both photons should be identical when they were created as a pair. But this joint polarization is a quantum manifestation that becomes real when one of the photons is measured and is therefore completely random. This begs the question if the measurement is done by the detector.
Spooky action at a distance?
If the left photon appears to have a certain polarization upon detection, then the right photon must have at the same time the same polarization since they were created as a pair. And that’s strange when their polarization only becomes ‘real’ upon detection, as quantum mechanics seems to imply. So, that looks on first sight like mutual communication. But as soon as you assume that you also have to ask yourself how the communication between the two manifesting photons actually works: “Hello partner, I have been measured, now you must immediately show your polarization and it should be the same as the one I am showing at the moment”. That’s Einstein’s “Spooky action at a distance”. Do you see why Clauser was discouraged from investigating this experimentally?
Either classical permanent particles or materialization by observation
The Bell experiment is therefore concerned with whether it can be determined if the polarization of the photons already existed from the moment of their creation (classical permanent particles) or if they only ‘materialize’ at the moment of their detection (non-local quantum interpretation). According to non-local quantum theory, if the two polarizers are not equally oriented with respect to each other, the correlation between the polarizations of the photon pairs – the [D-/D-] or [D+/D+] coincidences – must be greater than the correlation predicted by the local permanent particle theory.
That angle-dependent correlation between the coincidences can be predicted for both theories, classical local or non-local quantum. The genius of Bell was that he realized that differences between classical local and quantum theory occurred if the polarizers (A and B) made different angles with each other than 0o, 90o, 180o or 270o. See figure below for the predictions of the correlations as calculated in both theories. For example, the figure shows that for an angle of 158o between the two polarizers, the classical local expectation for the correlation will be 0.75 (75%), but for the non-local quantum expectation it will be 0.85 (85%).
If the measured correlation of all coincidences at that angle of 158o is greater than 75%, then local hidden variables are falsified and has it been experimentally confirmed that the polarizations of both photons only ‘materialize’ at the moment they are measured in the D+ and D- detectors. When it can be shown that mutual communication at a speed that is at most that of light is excluded, then the hypothesis that particles only exist when detected is strongly confirmed. Therefore, in a Bell experiment it is required that communication between the photons with at maximum the speed of light is excluded.
In any case, it means that very high demands are made on Bell experiments. Two absolute requirements are:
- Communication with the speed of light (or below) must be excluded; this means that the mutual distance of the detectors on the left and right must be very large or the time difference between the coinciding detections on the left and right must be very small.
- All photons sent in the experiment should also be measured to prevent photon pairs that do not show coincidence of the same polarization from being excluded from the measurement and thus making the measured correlation appear larger.
- All photons must come from a source that precludes their creation from being dependent on the experimenters.
Anton Zeilinger’s experiment in 2017 fully met all these requirements. He used starlight photons.
What now? When does something exist?
Every Bell test – see the timeline on the Quanta Magazine article – has so far confirmed with increasing probative value that the quantum particles only ‘get’ their properties – such as polarization – upon detection. In other words, they do not materially exist until they are detected.
That is quite something. Especially when you consider that the quantum laws are by no means limited to the atomic domain, but also apply to objects in the order of magnitude that can be perceived with our own senses, or even much larger. There is not a single good argument why the quantum laws should not apply at the level of our daily experience. The moon only exists when it is detected. Period. Sorry, Professor Einstein.
Now you can think about this: if the polarization of a photon does not exist before detection, how is it possible that a polarization filter even works? I’ll let you ponder this question for now.
That’s why you have to ask yourself what detection and observation actually mean and what it means if you close the door of your house behind you and no one is left behind. The contents of your house do not materially exist as long as no one is detecting them. The probability that the content will materialize again on your returm, almost exactly as you left it is 99.999999999% (or even closer to 100% but never exactly). That’s reassuring to hear, of course. So, as long as we do not recognize the role of the observer, the interpretation of quantum physics remains an issue that urgently needs to be solved. That’s my opinion, and I’m certainly not alone. Many physicists are already convinced of the role of the observer in experiments, such as Carlo Rovelli almost does with his hypothesis that all properties of objects – just like velocities were already – are relative. If you’re not convinced yet, I propose that you read Bernardo Kastrup, he has some very convincing arguments showing that the permanence of matter is a wrong image of reality.