That quantum mechanics is a successful theory is not in dispute. It makes astonishingly accurate predictions about the nature of the world at microscopic scales. What has been in dispute for nearly a century is just what it’s telling us about what exists, what is real. There are myriad interpretations that offer their own take on the question, each requiring us to buy into certain as-yet-unverified claims — hence assumptions — about the nature of reality.
Now, a new thought experiment is confronting these assumptions head-on and shaking the foundations of quantum physics. The experiment is decidedly strange. For example, it requires making measurements that can erase any memory of an event that was just observed. While this isn’t possible with humans, quantum computers could be used to carry out this weird experiment and potentially discriminate between the different interpretations of quantum physics.
“Every now and then you get a paper which gets everybody thinking and discussing, and this is one of those cases,” said Matthew Leifer, a quantum physicist at Chapman University in Orange, California. “[This] is a thought experiment which is going to be added to the canon of weird things we think about in quantum foundations.”
The experiment, designed by Daniela Frauchiger and Renato Renner, of the Swiss Federal Institute of Technology Zurich, involves a set of assumptions that on the face of it seem entirely reasonable. But the experiment leads to contradictions, suggesting that at least one of the assumptions is wrong. The choice of which assumption to give up has implications for our understanding of the quantum world and points to the possibility that quantum mechanics is not a universal theory, and so cannot be applied to complex systems such as humans.
Quantum physicists are notoriously divided when it comes to the correct interpretation of the equations that are used to describe quantum goings-on. But in the new thought experiment, no view of the quantum world comes through unscathed. Each one falls afoul of one or another assumption. Could something entirely new await us in our search for an uncontroversial description of reality?
Quantum theory works extremely well at the scale of photons, electrons, atoms, molecules, even macromolecules. But is it applicable to systems that are much, much larger than macromolecules? “We have not experimentally established the fact that quantum mechanics applies on larger scales, and larger means even something the size of a virus or a little cell,” Renner said. “In particular, we don’t know whether it extends to objects the size of humans and even lesser, [whether] it extends to objects the size of black holes.”
Despite this lack of empirical evidence, physicists think that quantum mechanics can be used to describe systems at all scales — meaning it’s universal. To test this assertion, Frauchiger and Renner came up with their thought experiment, which is an extension of something the physicist Eugene Wigner first dreamed up in the 1960s. The new experiment shows that, in a quantum world, two people can end up disagreeing about a seemingly irrefutable result, such as the outcome of a coin toss, suggesting something is amiss with the assumptions we make about quantum reality.
In standard quantum mechanics, a quantum system such as a subatomic particle is represented by a mathematical abstraction called the wave function. Physicists calculate how the particle’s wave function evolves with time.
Eugene Wigner, a Hungarian-American theoretical physicist, was one of the key minds behind the development of quantum theory. He was awarded the Nobel Prize in Physics in 1963. Oak Ridge National Laboratory, U.S. Dept. of Energy
But the wave function does not give us the exact value for any of the particle’s properties, such as its position. If we want to know where the particle is, the wave function’s value at any point in space and time only lets us calculate the probability of finding the particle at that point, should we choose to look. Before we look, the wave function is spread out, and it accords different probabilities for the particle being in different places. The particle is said to be in a quantum superposition of being in many places at once.
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