One hallmark of quantum physics is that many of its concepts are very strange. For example, according to theories put forth over the years, particles and atoms are able to move in multiple directions at the same time, and two particles can remain connected even over large distances.
But some physicists are concerned that without experiments backing up these theories, it is impossible to know if these weird phenomena are actually real. And it’s also impossible to know what the true nature of reality itself is.
“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.”
This is all about to change. Physicists around the world have been embarking on a series of experiments to put quantum weirdness to the test. Many of these experiments are attempts to determine whether the wavefunction is real or simply a concept dreamed up by quantum physicists on a blackboard.
The wavefunction is a mathematical device invented by Austrian physicist Erwin Schrödinger that describes the close connection between particles and waves — such as the wave-like interference pattern that develops when electrons (quantum particles) pass through closely-spaced slits.
In one experiment, physicist Martin Ringbauer and colleagues, tested the reality of the wavefunction. When measuring the polarization and other features of a beam of photons, they found an overlap between these measurements. This was too large to be explained by our “ignorance” of the system. The test provided support for the real-world nature of the wavefunction, but only if objective reality exists.
In 2005, physicists Emmanuel Fort and Yves Couder investigated the belief that particles are guided by a ‘pilot wave’ that gives them the ability to act like particles and waves. In the test, they observed oil droplets falling into a vibrating tray filled with oil. At a certain frequency of vibration, ripples formed around the droplets. The result was a dual object — oil particles driven by their own waves. This provides a glimpse of how pilot waves might guide particles, although it doesn’t prove that they exist.
Another group carried out a computer simulation of parallel universes, one more strange phenomenon of quantum physics. In this test, physicists created a simulated universe where quantum particles obeyed the rules of classical physics, such as Newton’s law of motion. They found that quantum effects — like the banded interference pattern — arose when multiple universes were connected. The researchers see this as proof that the wavefunction is only a mathematical construct, and that the universe is really governed by the rules of classical physics.
Physicists in Australia also recently tested the nature of reality and found that, on a small scale, reality doesn’t exist until we measure it. This is a modern version of John Wheeler’s delayed-choice thought experiment, which allows a moving object to behave like a particle or wave. In the new version, researchers used helium atoms scattered by light. They found that the wave-like or particle-like behavior only came into existence when the atoms were measured at the end of their journey.
These experiments are all steps toward a greater understanding the true nature of reality and whether it really exists. Some, like the delayed-choice test, had to wait decades before technology was advanced enough for the thought experiment to be tested. But it’s a move in the right direction. The ultimate challenge for physicists — although right now no one knows how to do it — will be to test whether objective reality exists at all.
