It involves Eugene Wigner’s Paradox from sixty years ago:

He imagined a friend of his, sealed in a lab, measuring a particle such as an atom while Wigner stood outside. Quantum mechanics famously allows particles to occupy many locations at once—a so-called superposition—but the friend’s observation “collapses” the particle to just one spot. Yet for Wigner, the superposition remains: The collapse occurs only when he makes a measurement sometime later. Worse, Wigner also sees the friend in a superposition. Their experiences directly conflict.

Now, researchers in Australia and Taiwan offer perhaps the sharpest demonstration that Wigner’s paradox is real. In a study published this week in Nature Physics, they transform the thought experiment into a mathematical theorem that confirms the irreconcilable contradiction at the heart of the scenario. The team also tests the theorem with an experiment, using photons as proxies for the humans…

… the new study’s authors believe something just as fundamental is on thin ice: objectivity. It could mean there is no such thing as an absolute fact, one that is as true for me as it is for you.

“It’s a bit disconcerting,” says co-author Nora Tischler of Griffith University.

George Musser, “Quantum paradox points to shaky foundations of reality” atScience(August 17, 2020)

Paper. (*paywall*)

*Hat tip:* Philip Cunningham

One wonders what will happen to science when the people who hope for the end of objectivity meet up with the war on math, crowd.

Our physics color commentator Rob Sheldon comments,

The latest buzz in Foundations of QM is an implementation of the thought experiment posed by Nobel Prizewinner, Eugene Wigner.

QM says you can replace point-particles with waves, or more precisely, probability-waves. The outcome then becomes the square of the wave-amplitude (which makes it always positive). In the early days of Bohr versus Einstein, this fuzziness was attributed to big clumsy experiments trying to measure tiny atoms (the “instrumentalist” answer). Einstein (and Podolsky and Rosen) responded with the “entanglement” paradox, which didn’t depend on clumsy experiments, arguing that the atom knows what it is even if we don’t. Bohr’s response to the EPR-paradox, was to dismiss it as “tiny things are QM waves, big things are classical particles”. So Schroedinger made his classic “cat-in-a-box” argument that the tiny, wavy radioactive atom could influence behavior of the big, non-wavy cat, so that there was no separation between “QM regime and classical regime”. While a head-scratcher, the philosophical wiggle room was that it was really the observer’s information that was wavy, not the classical cat.

So Wigner came up with his “friend” version, where the observer of the cat-in-a-box is himself encased in a box and observed by Wigner. Now the question is whether the physicist measuring the observer-in-a-box sees a wavy observer, and whether the observer feels wavy about it. That is, after the experiment is over they compare notes, whether Wigner’s observation of the friend’s measurements differ from what the friend measured. If “big things are classical” then the two notebooks should agree, but if even scientists can waffle, there could be a disagreement.

That outcome remains a thought experiment, but what this new paper does relate is a test that replaces the observer with a detector, and the cat is replaced with an entangled photon pair. We already know the results of the entangled pair (cat-in-a-box) experiment, it supports the Bell inequality which negates classical “hidden variable” answers. What we don’t know is what happens when we now entangle the sub-experiments. Does the super-entanglement come out with a classical answer? Or does the super-entanglement show that even the measurements of the measurements are as entangled as the sub-experiments?

The answer came back that the two detectors do not agree, in agreement with QM that treats the whole thing as a giant wave function. Without answering the question whether Wigner’s big friend can be replaced with a tiny QM wavefunction, the experiment seems to suggest that QM allows independent observers to have differing observations of reality. But that isn’t the only interpretation (though the one currently fashionable in Post-Modernist circles). There are actually 3 assumptions, any one of which could be the culprit:

- Einstein locality (all physics has to be local, causes not travelling faster than the speed of light)
- Reality is what we measure
- The outcome is not known beforehand (also called free-will, or its violation, Super-Determinism).

My favorite of the three is #1, because it shows up numerous places in physics. But Sabine Hossenfelder opts for #3, and I think many physicists prefer #2. And of course, it could be #0 — tiny photons are not big observers like Wigner.

Is this outcome surprising?

No, actually the debate over “the meaning of QM” has been going on since 1935 when Einstein published his EPR paper. It is just that the wiggle-room is getting reduced as our straight-jacket is being cinched tighter.

Rob Sheldon is also the author of *Genesis: The Long Ascent* and *The Long Ascent, Volume II.*