Recently, we ran a piece featuring the views of well-known science writer John Horgan who talked about a truly strange element of quantum physics confirmed by recent experiments — that it seems as if there is no knowledge apart from observers’ minds.
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.Sabine Hossenfelder, “Has quantum mechanics proved that reality does not exist?” at BackRe(Action) (February 19, 2022)
Hossenfelder acknowledges the experiments that Horgan discusses, beginning with Eugene Wigner’s thought experiment, “Wigner’s Friend,” which was confirmed in the lab (with photons as opposed to friends):
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.Sabine Hossenfelder, “Has quantum mechanics proved that reality does not exist?” at BackRe(Action) (February 19, 2022)
In Hossenfelder’s view, this should not be a big problem because in a real experiment, the measurement happens when Alice measures the particle hitting the screen. There is no superposition.
But here’s the problem that she goes on to identify:
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.Sabine Hossenfelder, “Has quantum mechanics proved that reality does not exist?” at BackRe(Action) (February 19, 2022)
She’s not happy with the outcome of the experiments, offering “If you claim that a single photon is an observer who make a measurement, that’s not just a fanciful interpretation, that’s nonsense.” She thinks that a new theory of quantum mechanics is needed:
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.Sabine Hossenfelder, “Has quantum mechanics proved that reality does not exist?” at BackRe(Action) (February 19, 2022)
Now, the interesting thing is that Hossenfelder is comfortable with how strange classical particle physics can be. Take neutrinos, for example”
First, they are the only particles that interact only with the weak nuclear force. All the other particles we know either interact with the electromagnetic force or the strong nuclear force or both. And the weak nuclear force is weak. Which is why neutrinos rarely interact with anything at all. They mostly just pass through matter without leaving a trace. This is why they are often called “ghostly”. While you’ve listened to this sentence about 10 to the fifteen neutrinos have passed through you.
This isn’t the only reason neutrinos are weird. What’s even weirder is that the three types of neutrino-flavors mix into each other. That means, if you start with, say, only electron-neutrinos, they’ll convert into muon-neutrinos as they travel. And then they’ll convert back into electron neutrinos. So, depending on what distance from a source you make a measurement, you’ll get more electron neutrinos or more muon neutrinos. Crazy! But it’s true.Sabine Hossenfelder, “The physics anomaly no one talks about: What’s up with those neutrinos?” at BackRe(Action) (September 18, 2021)
The neutrinos’ overall behavior, she tells us, is inconsistent with the Standard Model of physics. But that’s a “crazy” situation she finds easier to accept.
We might conclude that the universe is a stranger place than we have sometimes been led to suspect and that the amount and type of strangeness each of us can tolerate depends, to some extent, on prior commitments. But the universe is what it is anyway.
You may also wish to read:
Study: Science fiction not as strange as quantum physics fact. At least, that’s what we can assume from a failed effort to disprove physicist Eugene Wigner’s thought experiment. The research (and the QBism that resulted) eliminates the possibility that the mind is just an illusion. Apart from observers’ minds, there is no knowledge.
Some elements of our universe do not make scientific sense. Well-attested observations of neutrinos are not compatible with the Standard Model of our universe that most physicists accept. Theoretical physicist Sabine Hossenfelder walks us through the reasons that neutrinos, nearly massless particles with no charge, confound expectations.