Two realities can exist at the same time, at least in the quantum world, suggests a new study.
The existence of these multiple realities was first proposed in the 1960s in a thought experiment that showed that two people could observe the same photon, but end up with a different — but equally correct — reading of the photon’s state.
Now a team of researchers have replicated, and extended, this thought experiment in the laboratory using entangled photons and two pairs of “observers.” Their results were published February 13 in the preprint journal arXiv.
The original thought experiment — known as “Wigner’s Friend” — was proposed by Nobel Prize-winning physicist Eugene Wigner in 1961. It allowed two observers to experience different realities.
Wigner’s thought experiment goes like this: when you measure a polarized photon, it exists in one of two states — either vertically polarized or horizontally polarized. However, the laws of quantum mechanics state that before your measurement, the photon exists in both states simultaneously — what’s known as a superposition.
In Wigner’s experiment, he imagined that one observer, Alice’s Friend, measures the state of the photon in a laboratory and records the result. When Alice’s Friend measures the photon, it collapses into one of the two polarized states.
Alice is outside the laboratory, without access to the photon or to the friend’s measurement. So for her, the photon is still in a superposition — it exists in both polarized states at the same time. Not only that, but the measurement of the photon is in a superposition of all possible experimental outcomes.
Alice can also run an interference experiment to see if the photon is in a superposition. If the photon is in a superposition, it will act as a wave and create alternating dark and light bands.
Since Alice has not measured the photon, it still exists in a superposition — this is her reality. Alice’s Friend, though, has a different reality, one in which the measurement has been made. Although these realities conflict with each other, they are both equally valid, or real.
“This calls into question the objective status of the facts established by the two observers,” writes physicist Massimiliano Proietti, of Heriot-Watt University in Edinburgh, UK, and his colleagues, in their paper.
Wigner’s Friend remained a thought experiment until recently, when advances in quantum technologies allowed Proietti and his colleagues to put it to the test in a real laboratory. Basically, they wanted to see if different observers would experience different realities — in this case, the state of a photon, or particle of light.
The researchers extended Wigner’s thought experiment to include four observers and six entangled photons. Entangled particles are ones in which the properties of one particle affect the properties of its partner, no matter how far apart they are. The “observers” in this experiment were not real people, but represented observations and storing of the results.
The setup was the same as in Wigner’s thought experiment — Alice’s Friend was inside the laboratory with the photon and Alice was outside. There was also Bob’s Friend inside another laboratory with a separate photon, and Bob outside.
Even with two separate laboratories, the results lined up with Wigner’s thought experiment. The friends of Alice and Bob measured the polarization of the photon and stored the result. Alice and Bob then ran an interference experiment to see if their friend’s measurement and the photon were in a superposition.
The result? Alice and Alice’s Friend experienced separate realities, ones that are equally true. Likewise, Bob and Bob’s Friend have different realities that coexist, even though their outcomes conflict.
This is not just another strange quantum phenomena, but it has relevance to the real world, especially the real world inhabited by scientists. Measurement is an important part of science, but if results depend on the observer, how can scientists say “this” or “that” is an objective fact.
Of course, there could be a loophole in the experiment that gets us out of this dilemma. Until then, though, we will have to live with the uneasy feeling that we may all experience a slightly different version of the universe.