*[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]*

Physicists have shown that objective reality doesn’t exist. This is allegedly an insight derived from quantum mechanics. And not only this, it’s been experimentally confirmed. Really? How do you prove that reality doesn’t exist? Has it really been done? And do we have to stop saying “really” now? That’s what we’ll talk about today.

Many of you’ve asked me to comment on those headlines claiming that reality doesn’t exist. It’s a case in which physicists have outdone themselves in the attempt to make linear algebra sound mysterious. The outcome is patently absurd. In one article, Eric Calvacanti, a physicist who works in the foundations of quantum mechanics, writes: “If a tree falls in a forest and no one is there to hear it, does it make a sound? Perhaps not, some say.”

So what are those people talking about? The story begins in 1961 with the Hungarian physicist Eugene Wigner. Wigner was part of the second generation of quantum physicists. At his time, the mathematical basis of quantum mechanics had been settled and was experimentally confirmed. Now, physicists moved on to quantum field theory, which would eventually give rise to the standard model. But they still couldn’t make sense of what it means to make a measurement in quantum mechanics.

The way that quantum mechanics works is that everything is described by a wave-function, usually denoted psi. The change of the wave-function in time is given by the SchrÃ¶dinger equation. But the wave-function itself isn’t measurable. Instead, from the wave-function you just calculate probabilities for measurement outcomes.

Quantum mechanics might for example predict that a particle hits the left side or the right side of a screen with 50% probability each. Before the particle hits the screen, it is in a “superposition” of those two states, which means it’s neither here nor there, instead it’s in some sense both here and there . But once you have measured the particle, you know where it is with 100 percent probability. This means after a measurement, you have to update the wave-function. This update is also called the “reduction” or “collapse” of the wave-function. What is a measurement? Quantum mechanics doesn’t tell you. And that’s the problem.

Wigner illustrated this problem with a thought experiment now known as “Wigner’s friend.” Suppose Wigner’s friend Alice is in a laboratory and does an experiment like the one we just talked about. Wigner waits outside the door. Inside the lab, the particle hits the screen with 50% probability left or right. When Alice measures the particle, the wave-function collapses and it’s either left or right. She then opens the door and tells Wigner what she’s measured.

But how would Wigner describe the experiment? He only finds out whether the particle went left or right when his friend tells him. So, according to quantum mechanics, Wigner has to assume that before he knows what’s happened, Alice is in a superposition of two states. One in which the particle went left and she knows it went left. And one in which it went right and she knows it went right.

The problem is now that according to Alice, the outcome of her measurement never was in a superposition, whereas for Wigner it was. So they don’t agree on what happened. Reality seems to be subjective.

Now. It’s rather obvious what’s going on, namely that one needs to specify what physical process constitutes a measurement, otherwise the prediction is of course ambiguous. Once you have specified what you mean by measurement, Alice will either do a measurement in her laboratory, or not, but not both. And in a real experiment, rather than a thought experiment, the measurement happens when the particle hits the screen, and that’s that. Alice is of course never in a superposition, and she and Wigner agree on what’s objectively real.

If that’s so obvious then why did Wigner worry about it? Because in the standard interpretation of quantum mechanics the update of the wave-function isn’t a physical process. It’s just a mathematical update of your knowledge, which you do after you have learned something new about the system. It doesn’t come with any physical change. And if Alice didn’t physically change anything then, according to Wigner, she must indeed herself have been in a superposition.

Okay, so that was Wigner’s friend in the 1960s. You can’t experimentally test this, but in 2016 Daniela Frauchinger and Renato Renner proposed another thought experiment that moved physicists closer to experimental test. This has been dubbed the “Extended Wigner’s Friend Scenario.”

In this thought experiment you have two Wigners, each of whom has a friend. We will call these the Wigners and the Alices. The Alices each measure one of a pair of entangled particles. As a quick reminder, entangled particles share some property but you don’t know which particle has which share. You may know for example that the particles spins must add up to zero, but you don’t know whether the left particle has spin plus one and the right particle spin minus one, or the other way round.

So the Alices each measure an entangled particle. Now the thing with entangled particles is that if their measurements don’t collapse the wave-function, then now the two Alices are entangled. Either the left one thinks the spin was up and the right one thinks it’s down, or the other way round. And then there’s the two Wigners, each of which goes to ask their friend something about their measurement. Formally this “asking” just means they do another measurement. Frauchinger and Renner then show that there are combinations of measurements in which the two Alices cannot agree with the two Wigners on what the measurement outcomes were.

Again the obvious answer to what happens is that the Alices either measured the particles and collapsed the wave-function or they didn’t. If they did, then their measurement result settles what happens. If they didn’t do it, then it’s the Wigners’ results which settle the case. Or some combination thereof, depending on who measures what. Again, this is only problematic if you think that a measurement is not a physical process. Which is insanity, of course, but that’s indeed what the most widely held interpretation of quantum mechanics says.

And so, Frauchinger and Renner conclude in their paper that quantum mechanics “cannot consistently describe the use of itself” because you run into trouble if you’re one of the Wigner’s and try to apply quantum mechanics to understand how the Alice’s used quantum mechanics.

You may find this a rather academic argument, but don’t get fooled, this innocent sounding statement is a super-important result. Historically, internal inconsistencies have historically been THE major cause of theory-led breakthroughs in the foundations of physics. So we’re onto something here.

But the Frauchinger-Renner paper is somewhat philosophical because they go on a lot about what knowledge you can have about other people’s knowledge. But in 2018, Caslav Brukner looked at the problem from a somewhat different perspective and derived a “No go theorem for observer independent facts.”

His formulation allows one to use the measurement of certain correlations in measurement outcomes to demonstrate that the observers in the extended Wigner’s friend scenario actually had measurement results which disagree with each other. If that was so, there would in certain cases be no “observer independent facts”. This is the origin of all the talk about objective reality not existing and so on.

And yeah it’s really just linear algebra. There aren’t even differential equations to be solved. I assure you I’m not saying this to be condescending or anything, I just mean to say, this isn’t rocket science.

Finally, in 2019, a group from Edinburgh actually measured those correlations which Brukner calculated. That’s the experimental test which the headlines are referring to. Now you may wonder who in Edinburgh played the role of the two Alices and the two Wigners? How did the Alices feel while they were in a superposition? Did the Wigners see any wave-functions collapse?

Well, I am afraid I have to disappoint you because the two Alices were single photons and the two Wigners were photo-detectors. That’s okay, of course, I mean, some of my best friends are photons too. But of course an interaction with a single photon doesn’t constitute a measurement. We already know this experimentally. A measurement requires an apparatus big enough to cause decoherence. If you claim that a single photon is an observer who make a measurement, that’s not just a fanciful interpretation, that’s nonsense.

The alleged mystery of all those arguments and experiments disappears once you take into account that a measurement is an actual physical process. But since quantum mechanics does not require you to define just what this process is, you can make contradictory assumptions about it and then more contradictions follow from it. It’s like you have assumed that zero equals one, and then show that a lot of contradictions follow from it.

So to summarize, no one has proved that reality doesn’t exist and no experiment has confirmed this. What these headlines tell you instead is that physicists slowly come to see that quantum mechanics is internally inconsistent and must be replaced with a better theory, one that describes what physically happens in a measurement. And when they find that theory, that will be the breakthrough of the century.

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