A conversation with Alain Aspect, Nobel Laureate and seminal experimenter in quantum physics

 You may or may not have heard of Alain Aspect (pronounced Ass-pay), but he’s a significant figure in the history of the development of quantum mechanics. Looking him up, I was surprised to learn he’s not much older than me. He was in his mid-thirties when he did his groundbreaking experiments: among the first to demonstrate Bell’s theorem in practice, not just in theory, and effectively proving that entanglement is non-local, meaning it breaks with special relativity.
 
This was almost 30 years after Einstein died, and effectively proved he was wrong regarding his views on entanglement. Having said that, it was Einstein who set the ball rolling with the famous 1935 paper titled, "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?" that he co-wrote with Podolsky and Rosen, so better known as the EPR paper. Aspect jointly won the Nobel prize with John Clauser and Anton Zellinger for his definitive experimental contribution to that topic.
 
So I was surprised and very pleased to come across a 55 min interview with him by Brian Greene on YouTube, as part of a series. I read an interview with Aspect decades ago in a book co-edited by P C W Davies and Julian R Brown titled The Ghost in the Atom. It included interviews with other luminaries in the field like John Bell, Eugene Wigner, John Wheeler, David Deutsch and David Bohm, plus more.
 
Aspect is French, but his English is excellent. It’s unusual to find interviews with experimental physicists as opposed to theoretical physicists, and I would call it refreshing, because he tends not to elaborate or speculate beyond what the evidence tells him. Having said that, Greene presses him on what his intuition tells him, and even that is informative, because he keeps it simple.
 
While I was watching, I made some notes. I did not know that he was the first to produce isolated photons. If you go to roughly the 16-17m mark, he explains how he ‘split’ the wavefunction of the photon into ‘2 half wave packets’ (along 2 separate paths using beam splitters), which seems impossible for an individual photon. He says that the only way he can explain it is with non-locality. In his own words, ‘if I measure the wave packet on the right, the other wave packet on the left instantaneously collapses to zero.’
 
I can still remember when I was studying physics at university in the 70s, writing that a single photon could travel down 2 separate paths and being marked down for it. I’ve no idea where I read it, but Alain Aspect proved it in the 1980s.
 
When asked specifically by Greene, ‘Does the photon travel down both paths?’ Aspect answers unequivocally, ‘Yes’. But then he says ‘if he takes a measurement, it only appears on one side’. Curiously, when Greene asks him about the well known ‘measurement problem’ and what his ‘intuition’ is on that, Aspect said he doesn’t have one: ‘it’s a great mystery’, but then says it’s ‘irreversible’. Aspect then says that if you ask a cosmologist, they will say there is a wavefunction for the whole universe, where there is no measuring apparatus. I think that’s the nub of the issue. Non-local means instantaneous, which is Aspect’s description, and by my simplistic reasoning, this means the entangled particles must occupy the same ‘now’ in time, though no one ever mentions that because it’s a heresy. And if you have a wavefunction for the entire universe, then arguably you have the same ‘now’ throughout the universe, which is even more heretical.
 
The best part of this video is that Aspect takes us through the entire history of entanglement, starting with Schrodinger who coined the term and famously said that 'entanglement was the defining characteristic of quantum mechanics separate from classical physics'. I think, along with superposition, it’s what led me to believe the Universe obeys 2 sets of rules: quantum and classical. QM rules before decoherence of the wavefunction and classical physics rules after.
 
Naturally, Greene asks him about the MWI (Many Worlds Interpretation), which some argue overcomes the measurement problem. Aspect responds that ‘it’s a logical solution, but it’s absolutely not palatable’ (to him), while acknowledging it’s popular with many cosmologists. Just as an aside, Mithuna Yoganathan (from the Looking Glass Universe YouTube channel) specifically eschews the idea that the Universe obeys 2 sets of rules and that alone makes MWI attractive to her.
 
Interestingly, Aspect makes an analogy with the second law of thermodynamics (~21m) by pointing out that it can’t be derived from Newtonian mechanics, where everything is time-reversible. I’d say the same applies to chaos theory. A lot of laypeople are unaware that Schrodinger’s equation is deterministic, meaning it’s time-reversible, but the ‘measurement’ makes it irreversible. Paul Davies has made this same point. Aspect doesn’t articulate this, but what he’s saying is that the second law of thermodynamics is just as ‘radical’ (my word, not his) as QM when it comes to confounding our expectations based on previously known physics.
 
Greene says, ‘[QM] has been unreasonably successful and unreasonably effective’ to which Aspect replies, ‘Yes.’ This introduces their discussion of the 1935 EPR paper (~22m), and is arguably the most erudite and stimulating part of the discussion, because it logically leads to a discussion on John Bell’s theorem in some detail, which is what led to Aspect’s now equally famous experiment.
 
Another aside: on Quora I met a physicist, Ian Miller, with whom I had some interesting and convivial conversations. He’s one of the few people I know who disputes Bell’s Theorem, or at least its consequences, and has argued he can refute it. I’ve always respected him, simply because he knows more than me, and I too have some heretical ideas, plus I agree with him that in SR, it’s the ruler that changes and not the space it’s purporting to measure. Much later, I learned that Kip Thorne, of all people, made the point that it's impossible to tell the difference (between the ruler and the space its measuring) from the mathematics alone. Regarding Bell’s Theorem, Miller contends it’s just mathematical not physical, yet Alain Aspect would beg to differ.
 
One of the aspects of Bell’s theorem that many people don’t know is that Bell wanted to prove Einstein right, but effectively proved him wrong. Others have contended that Bell’s conclusion to his own discovery was that the universe must be super-deterministic, but I know he didn’t say that in his interview in the book I cited earlier, and Aspect doesn’t mention it either. I can understand, however, if you believe that the entangled particles don’t experience the same 'now', then superdeterminism is a logical conclusion. Hossenfelder is a keen advocate for superdeterminism.
 
In fact, Aspect claims that Bell was a ‘realist’, which I understand means that he believed what Aspect believes: there is an independent reality (to the observer) and non-locality is a feature of the Universe. I remember reading an article in New Scientist, where it was argued you can have realism or ‘locality’, but not both.
 
One of Aspect’s salient points is that the famous arguments between Bohr and Einstein became epistemological, meaning they were philosophical differences rather than differences in reasoning, but only when Einstein introduced entanglement of more than one particle. According to Aspect, when they were arguing about one particle, Bohr’s arguments were based on pure logic. As Greene points out, the EPR paper introduces the concept of ‘hidden variables’ which, according to Einstein is what would make quantum mechanics ‘complete’. Aspect claims that Bohr’s response to the EPR paper was purely philosophical. In hindsight, we know we had to wait for Bell to give it a mathematical framework, which would ultimately make it testable, which is what Aspect achieved.
 
Just on that point, it illustrates the necessary relationship between mathematics and physics. There is an intrinsic relationship between a mathematical model and the need to measure physical attributes to determine, not only if the mathematical model is valid, but what its limitations are. This, in effect, is how the physical sciences have advanced since Newton. We have reached a point where some of our mathematical models can’t be measured using the technology currently available (string theory, anyone?).
 
Aspect says that Bell found ‘you cannot have locality in a hidden variable theory rendering all the predictions of quantum mechanics in the EPR situation.’ (~28m) I find this interesting because I’ve come across people on YouTube (Hossenfelder) who claim that Bell’s Theorem doesn’t disprove hidden variables. They could be right, because Aspect is not saying that non-locality rules out hidden variables and Greene doesn’t ask him. But Aspect’s conclusion certainly rules out Einstein’s hope that hidden variables would save locality. Aspect gives credit to David Bohm for reformulating the EPR thought-experiment in terms of a dichotomy – spin-up or spin-down – and not a variable of position and velocity as per Einstein.
 
Aspect goes into some detail concerning his development of his experiment, including the work of others, which took him 7 years. According to Aspect, John Bell followed his work and respected his result; even saying publicly, ‘I am sorry for the result, but I respect it.’ Which says a lot.
 
At 41m Greene brings up MWI again, saying that many argue it solves non-locality. To which Aspect responds that, for him, accepting MWI is ‘worse’ than accepting non-locality. And Greene agrees.
 
Greene also raises the issue of free will, and Aspect’s response is amusing and, in his own words, ‘Simple. If I don’t have free will to adjust the knob on my apparatus, I stop being a physicist.’ Green smiles, yet doesn’t give his views which I’ve written about elsewhere. Greene is a free will sceptic, if not denier (like Hossenfelder). Aspect elaborates, arguing that the contrary position is: ‘If it’s written in the Great Book, ever since the Big Bang, it’s an explanation for everything.’ So, not a believer in superdeterminism.
 
He spends some time explaining how non-locality doesn’t contradict SR (special relativity) in as much as you can’t use it to signal FTL (faster-than-light), though I do in my science fiction, which is why it’s called science fiction. He points out rather cleverly that it’s solely because of the random nature of QM that you can’t use it to send a signal, because the measurement outcome is completely unknown and can’t be forced. Because it’s random, neither party can know the outcome.
 
Towards the end, he explains how he has become an ambassador for science, which I imagine he’d do brilliantly. He says he is an ‘optimist’ despite attacks on science, especially under America’s current administration.