Quantum reality is either strangely different than it collapses

Quantum mechanics, when properly researched, raises deep questions about reality. These questions often take the form of thought experiments, which are later (usually much later) followed by real experiments. One of the most difficult and profound of these is a thought experiment proposed by Eugene Wigner in the 1960s, called “Wigner’s Friend” (you do not want to be Wigner’s friend). Now, much later, Wigner and his friend have been formalized and extended. The result presents us with a contradiction: either reality is much stranger and less real at the level of quantum mechanics, or quantum states cannot possibly exist on a large scale.

Do not be Wigner’s friend

To understand why Wigner should not have friends, we must first go into some details of quantum mechanics. Imagine that you are measuring the spin of a single electron. Spin has an orientation in space, but it is not possible to measure that orientation. Instead, we must choose an orientation and measure the spin along that orientation. That we may ask an electron if its spin is vertically upwards or downwards. The result (all else being equal) will either be up or down by 50 percent.

Let’s say we measure the spin and find that it is up. All subsequent measurements will also confirm that it is up. The measurement has defined the vertical spin component (a process often called wave function collapse). But it says nothing about the horizontal component of the spin – the horizontal component will remain in a superposition of left and right spins. This means that if we rotate our device so that we measure spin to the left and right, then the result will be random – the electron will be either spin-left or spin-right with 50 percent chance.

This is a relatively simple experiment to do, so Wigner grabs his friend, Alice, and places her in a sealed lab. Alice measures the spin of a stream of electrons being prepared in a superposition state. Wigner is located outside the lab and will measure the entire lab. Alice determines before proceeding that there is an electron spin-up. But Wigner has not made any measurement that he sees Alice in a superposition of having measure spin-up as spin-down. If Wigner makes his measurement, hypothetically, he could end up with a result where Alice measures spin-down when in fact they measure spin-up.

Two “facts” against each other, but both are based on reality. Wigner’s solution to this problem was that the quantum state could not exist at the level of the observer: the superposition state must collapse before it can occur.

Expand Wigner

The newly published extended version combines Wigner’s original thought experiment with John Bell’s thought experiment (Bell’s thought experiment is old enough that it is now routinely tested in experimental physics). Bell’s experiment is a little more complicated than Wigner’s friend experiment, and combines the idea of ​​superposition and entanglement. Let’s assume I have two electrons sitting together. This means that they have a combined quantum state, one that is defined only by the two electrons together. As an example, the filming process could result in two electrons with a total spin of zero.

That means they are in a superposition of up-down and down-up. When I measure one of those electrons to spin after, the other automatically spins down. If performed correctly, the Bell experiment shows that it is impossible to predict the spin of the electrons in advance. They must be in a superposition; when measuring, they must randomly choose an orientation.

Bell experiments are performed with large differences between where the particles (mostly photons) are measured, making information impossible to travel between the two ends of the experiments without moving faster than light. Indeed, the last twenty years have seen a systematic closure of possible holes in Bell experiments.

In Wigner’s extended Wigner experiment, the two friends perform Bell’s experiment on a scattered pair of particles in separate closed laboratories. Two Wigners (otherwise called superobservators) then measure the laboratories and compare results. The measurement results show the correlations between the scattered pair, filtered by the wave function collapse of the two observers.

So what is this reality thing anyway?

What Wigner and Bell tell us about reality requires some thinking. Physicists generally describe reality through a set of mathematically defined conditions. Causality tells us, for example, that an effect on time must be preceded by a cause. Locality says that propagation causes at the speed of light: if a photon cannot travel between the location of the cause to the location of the effect before the effect occurs, then it is in conflict with location (and potential causality).

Now we also need to consider things like measurements. The researchers define the absoluteness of observed events, which means that what I observe is real and does not depend on anything else. They assume there is no super-determinism (we make free choices) and location is still operational. The researchers refer to this trifecta as local friendliness.

The researchers used mathematical definitions of these statements to calculate the boundaries of local friendliness. They show that under the exact circumstances correlations that transcend these boundaries will be observed in Wigner’s extended friendship experiment. Their laboratory experiments confirmed that these violations actually occur. Local friendliness is not how the Universe works.

If we reject local friendliness, then we need to make some decisions. We must accept some of the following possibilities: the observations of an observer are not necessarily real, the reality is not local, super-determinism is real, as quantum mechanics ceases to function anywhere before macroscopic observers participate.

Huh?

To expand on that last paragraph: Wigner’s friend’s researchers’ version was not a macroscopic observer – it was a photon, which is an inherently quantitative object. It is possible that the entire thought experiment with macroscopic observers in a quantum state was simply forbidden by some unknown law of physics. On the other hand, it is not beyond the realm of possibility (according to the researchers) that an artificial intelligence on a quantum computer can be considered as a macroscopic observer that is still in a quantum state, which means that we are testing this in the future can expand.

The bigger point, however, is that reality can depend on quantum effects that wash out before they become too large. The alternatives are that we have much less free choice than we think we do, which is a bit unpalatable. In addition, we need to reject localization and perhaps causality, which may be even more painful for physicists.

Natural Physics, 2020, DOI: 10.1038 / s41567-020-0990-x (About DOIs)