By Teodor Teofilov
A quantum physics experiment has given evidence to the mind-bending idea that under the right set of conditions, two people observing the same events can see two different things happen and both be correct. This was previously in the realm of theory, but physicists from Heriot-Watt University have demonstrated for the first time how two people can experience different realities by recreating a quantum physics thought experiment (Wigner’s Friend).
Wigner’s Friend Thought Experiment
The thought experiment that the physicists of Heriot-Watt University demonstrated is called Wigner’s Friend. It was developed by Eugune Wigner in 1961 when he claimed that a quantum measurement requires a conscious observer, without which nothing ever happens in the universe.
Wigner, a Nobel Prize-winning physicist, outlined an experiment that demonstrated one of the less well known paradoxes of quantum mechanics. The experiment showed the strangeness of the universe, which allows two observers to experience different realities. He made quantum physics more subjective than had John von Neumann or even Erwin Schrödinger with his famous Cat Paradox.
Wigner’s experiment begins with a photon (a light particle). He purports that when an observer in an isolated lab measures it, they will find the polarization of the particle — its axis of spin — to be either vertical or horizontal. However, before the measurement, the particle is in all possible states at the same time until measured. This is because of the laws of quantum mechanics that state that the photon exists in a “superposition” of two possible states.
To visualize Wigner’s Friend, imagine there is a painting in an isolated room. It has two possible states – it is either white (vertical axis of spin) or black (horizontal). Before a person looks at the painting, it is both black and white at the same time.
As soon as the observer measures the photon, it assumes a fixed polarization. However, if another observer is outside of that lab and doesn’t know the result of the measurements, the unmeasured photon will still be in superposition. For this second person, their reality is different from the first one’s reality who had measured the photon. However, none of these conflicting measurements are wrong according to quantum mechanics.
In simpler terms, if John goes into the room with the aforementioned painting and observes it, the painting becomes either white or black. Let’s say that in John’s case he observes the painting to be white. Outside of the room is John’s friend, Chris, who doesn’t know what John has observed when he looked at the painting, so for him the painting is still both black and white at the same time. This means that both John and Chris have a different reality when it comes to the painting.
Wigner was asking, in essence, if there is an objective reality.
Wigner’s Friend has remained a thought experiment for decades, but important advances in physics in recent years have finally allowed experts to put it to the test. In February, 2019, a team of physicists from Heriot-Watt University in Edinburgh led by Massimiliano Proietti performed tested Wigner’s Friends paradox by entangling six photons. They proved that Wigner is right and that quantum reality is dependent on the observer.
Proietti and his colleagues tested the paradox with an even more thorough experiment, which doubled the scenario. They designated two labs where the experiments would take place and introduced two pairs of entangled photons.
This quantum entanglement entanglement happens when two particles become inextricably linked, and whatever happens to one immediately affects the other, regardless of the distance between them. The phenomenon is so strange, that Albert Einstein famously described it as ‘spooky action at a distance’. It is also the closest we are to Star Trek teleportation — we can only do it with information.
The experiment of the Heriot-Watt University team had real photons, but the people in the scenario — Alice, Bob and a friend of each — were not real, but instead represented the observers of the experiment. The two friends of Alice and Bob, who were inside each of the labs, each measured one photon in an entangled pair. This broke the entanglement and collapsed the superposition. This means that the photon they measured existed in a definite state of polarization. They recorded the results in quantum memory, meaning they copied in the polarization of the second photon.
Alice and Bob, who were outside, were given two choices for conducting their own observations. First, they could measure the results of their friends that were stored in quantum memory and arrive at the same conclusion. And second they could conduct their own experiment between the entangled photons.
The experiment, which is known as an interference experiment, shows that if the photons act like waves and are still in a superposition state, then the observer would see a characteristic pattern of light and dark fringes, where the peaks and valleys of the light waves add or cancel each other out. However, if the particles had become polarized, the observer would see a different pattern.
The study found that even in the double scenario, the conclusions of WIgner held. Alice and Bob and their friends could arrive at conclusions about the photons that are correct and provable, and that are different from each other. Both observations exists, even though they produce irreconcilable results.
This raises interesting questions that will force physicists to reconsider the nature of reality. The idea that observers can ultimately reconcile their measurements of whatever kind of reality is based on some assumptions, with the biggest being that universal facts actually exist and that observers can agree on them.
However, there are other assumptions too, such as that observers can make whatever observations they want and that the choices of one observer doesn’t have an influence on the choices of another.
These all hold if there is an objective reality that everyone can agree on.
But the result of the study suggests that there isn’t an objective reality. It means that one or more of the assumptions must be wrong. However, there is another way out of this — there could be some other loophole that might have been overlooked.
Nonetheless, the work will have important implications for the work of scientists.
“The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them,” say researchers in the study, but in the same paper, they undermine this idea
The next step will be to go even further, by constructing experiments that create increasingly odd alternate reality that can’t be reconciled. The end of this road is unknown, but one thing is for sure — Wigner and his friend won’t be surprised.