Quantum physics is known for its weirdness. But much of this exists only in mathematical equations, so it’s difficult to know which concepts are actually real. Some physicists, though, hope to change all that.
“If we tell the public that quantum theory is weird, we better go out and test that’s actually true,” physicist Owen Maroney told Nature. “Otherwise we’re not doing science, we’re just explaining some funny squiggles on a blackboard.”
The proof that Maroney is hoping for has found life in a new series of experiments aimed at deciphering the true nature of the wave function — a concept that gives quantum physics much of its weirdness.
On paper, the wave function is a mathematical tool invented by Austrian physicist Erwin Schrödinger that describes the close connection between particles and waves. This link can be seen in one particular classical experiment: electrons (quantum particles) shot at a barrier with two closely-spaced slits pass through the same way that a light wave does — they create a banded interference pattern on the other side.
But two camps of physicists have gathered on either side of the wave function: those that think that the wave function represents our ignorance (known in philosophy as a psi-epistemic model); and those that believe that the wave function is a real entity with a life beyond the blackboard (the psi-ontic model).
To put these two models into the context of quantum physics, it’s useful to look at Schrödinger’s cat — the feline in the box that is both dead or alive until you open the box and observe the system.
According to a psi-epistemic model, we don’t know if the cat is alive or dead, simply because the box is closed (our ignorance). But a psi-ontic model, which sees the wave function as a real entity, says that the cat exists in two states until the box is opened. But which of these models is correct?
In one experiment, physicists attempted to tackle this question by measuring several features — such as polarization — of a beam of photons. They found a certain amount of overlap between these measurements, more than would be explained by the wave function-as-ignorance model.
Another approach that physicists have taken to understanding whether the wave function is real is by examining the many-worlds picture of quantum physics. This concept says that whenever we make a quantum measurement — by opening the box with Schrödinger’s cat in it — the universe splits into two copies (such as one with a dead cat and another with a live cat). This splitting is driven by the wave function.
To explore this further, physicists from Griffith University in Brisbane, Australia, created a multiverse model that could be tested. In their framework, though, they left out the wave function entirely. Instead, they set up quantum particles so they obeyed the rules of classical physics — such as Newton’s laws of motion.
After running many computer simulations, the researchers found that quantum effects — like the banded interference pattern — occurred as a result of the multiple universes interacting. And as the number of universes increased, their results more closely resembled standard quantum theory. Based on this experiment, there is no wave function. Reality is grounded firmly in classical physics.
These experiments are far from a final answer on the nature of the wave function and reality. But they have moved the weird world of quantum physics in a more concrete — and testable — direction. One day, physicists may be able to develop a test for objective reality itself.