Alain Aspect | The man who proved Einstein wrong

Who, on this Earth, can boast of having faulted the great Albert Einstein?

This is the case of the French physicist Alain Aspect, at the origin of a now legendary experiment carried out in 1982 and which earned him the Nobel Prize in physics last year.

When I learned that Professor Aspect was visiting Sherbrooke at the end of last year, I reacted as my colleague Alexandre Sirois would if Taylor Swift announced a show at the Vieux Clocher in Magog two days before notice. (Yes, Alex is a Swiftie, he has publicly confessed to it⁠1.)

So it was to somewhat perplexed bosses that I announced that I absolutely had to disrupt my schedule to go meet the man whose work fascinated me from my first contacts with modern physics, at CEGEP.

I had the privilege of having some one-on-one time with this master of experimental physics. Then I attended the (masterful) conference he gave at the Quantum Institute of the University of Sherbrooke, before receiving an honorary doctorate.


Alain Aspect’s conference at the Quantum Institute of the University of Sherbrooke, before receiving an honorary doctorate

It emerges that Alain Aspect is not the type to be proud of having proven Einstein wrong.

If you listened carefully to my lecture, who is my hero? It’s Einstein. He was wrong, yes. But at the time he developed his position, he didn’t know he was wrong. His position was completely legitimate. So if I could meet Einstein, I would tell him: (…) this is the result of the experiments. So, what do you conclude from this, my dear Albert?

Alain Aspect, physicist

To understand Alain Aspect’s fundamental contribution to modern physics, we must go back to the 1920s and 1930s.

If you don’t understand everything that follows, it’s okay and that’s normal. The important thing is to realize that we are touching here on the very nature of the world – and the way in which humans can understand it.

At the time, quantum physics, the science which describes the behavior of objects in the infinitely small, was in full development.

Quantum mechanics is a strange, deeply counterintuitive discipline that I myself juggled with during my master’s degree in engineering physics without ever feeling like I understood it well.

This science is probabilistic, in the sense that it cannot predict almost anything with certainty. It describes objects that can be both waves and particles, and whose speed and position cannot be known simultaneously.

Einstein is known for relativity and his famous equation E = mc⁠2, but he is also one of the founders of quantum mechanics. At the time he developed it, however, he felt that the science was incomplete. He believes that its probabilistic aspect hides a misunderstanding.

“God doesn’t play dice,” he says in a phrase that has become famous.


Physicist Albert Einstein

In 1935, with physicists Boris Podolsky and Nathan Rosen, Einstein published an article that caused a stir. Physicists show that the equations of quantum physics lead to an extremely strange phenomenon: entanglement.

In a thought experiment, Einstein and his associates imagined two particles of light sent in opposite directions. They show that according to quantum mechanics, a measurement made on one particle automatically leads to the same value on the other, instantly.

What magical link can connect distant particles in this way? How can information travel at infinitely high speed between the two? Mystery. The case is called the “EPR paradox”, for Einstein-Podolsky-Rosen.

Einstein sees this as proof that quantum mechanics is incomplete. He believes that if the two particles can thus influence each other at a distance, it is because there are “hidden variables” which were present before their separation.

His point of view opposes that of his great friend Niels Bohr, a Danish physicist who argues on the contrary that we must accept the entanglement and probabilistic nature of quantum mechanics.

“But who are you, Albert Einstein, to tell God what to do? said Bohr to Einstein in response to his quip about dice.


Physicist Niels Bohr

This debate between titans, it is the man I have in front of me who will decide it.

Alain Aspect will take seven years to reproduce in the laboratory the thought experiment imagined by Einstein, Podolsky and Rosen.

You have two enormous physicists, Niels Bohr and Albert Einstein. The two disagreed. The debate was thought to be purely philosophical. And then we understand that we will be able to decide it through an experiment. I found it fascinating.

Alain Aspect, physicist

Thanks to an extremely clever mechanism and exceptional experimental manipulations, Professor Aspect proves beyond any doubt that Einstein’s “hidden variables” do not exist. He would be rewarded with the Nobel Prize in Physics 40 years later.

Did he know at the time that he would agree with Bohr rather than with Einstein?

“No, no, no,” he replied. The experimenter must be completely open. »

Another question burns on my lips. We know how counterintuitive quantum mechanics is. Does Alain Aspect have a mental image of what he has proven? Does he understand how two particles can exchange information instantly?

“Yes, I have a mental image,” he replies. In this image, I have the right to put something instantaneous between one side and the other. This image is questionable – I don’t know if, in the real world, it’s like that. But reasoning this way allows me to have intuitions that I can then control through calculation. »

Alain Aspect admits that he absolutely did not foresee that quantum entanglement would one day lead to technological developments. Today, the fact that two particles can be linked remotely is propelling work in quantum cryptography, a method which allows an encryption key to be transmitted between two interlocutors to secure their communications.

Quantum teleportation, of which Quebecer Gilles Brassard was one of the inventors, uses the principle of entanglement to move information from one place to another.

And finally there is quantum computing, very much alive in Sherbrooke, which promises to give birth to a new generation of computers much more powerful than those of today. Alain Aspect himself jumped into this boat by co-founding the company PASQAL.

Fortunately, this does not prevent the Nobel recipient from touring the world to speak to teachers and students.

“I love explaining,” he told me. I like to see, in the eyes of the people who listen to me, the glimmer that shows that they understood what I was telling them. »

There were several of us that day who came out of the University of Sherbrooke with lights in our eyes.


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