This year was the Nobel Prize in Physics granted “For experiments with entangled photons, demonstrating the violation of Bell’s inequality and pioneering quantum information science”.
To understand what this means, and why this work is important, we need to understand how these experiments settled a long-running debate among physicists. The main player in this debate was the Irish physicist named John Bell.
In the 1960s, Bell discovered how to translate a philosophical question about the nature of reality into a physical question that science could answer—and along the way broke the distinction between What we know About the world and how the world Really is.
We know that quantum objects have properties that we would not normally attribute to objects from our ordinary lives. Sometimes light is a wave, sometimes it is a particle. Our refrigerator never does this.
When trying to explain this kind of unusual behavior, there are two broad types of explanation we can imagine. One possibility is that we perceive the quantum world clearly, just as it is, and it just so happens to be unusual. Another possibility is that the quantum world is just like the normal world we know and love, but our view of it is distorted, so we can’t see quantum reality clearly, as it is.
In the early decades of the 20th century, physicists were divided over the correct interpretation. Among those who believed that the quantum world was extraordinary were characters like Werner Heisenberg And the Niels Borg. Among those who believed that the quantum world should be just like the ordinary world, and our view of it simply blurry, were Albert Einstein And the Erwin Schrödinger.
At the heart of this division is an extraordinary prediction of quantum theory. According to the theory, the properties of some quantum systems that interact remain dependent on each other – even when the systems are moved a great distance.
In 1935, the same year he invented famous thought experiment Including a cat trapped in a box, Schrödinger coined the term “entanglement” for this phenomenon. It was ridiculous to believe that the world worked this way, he said.
If entangled quantum systems truly remain connected even when separated by great distances, they seem to somehow communicate with each other instantly. But this kind of contact is not allowed, according to Einstein’s theory of relativity. Einstein called this idea “spooky work at a distance.”
Back in 1935, Einstein and two colleagues created thought experiment which showed that quantum mechanics cannot give us the full story about entanglement. They believed there must be something more to the world that we can’t see yet.
But over time, the question of how to interpret quantum theory became an academic footnote. The question sounded very philosophical, and in the 1940s many of the brightest minds in quantum physics were busy using the theory for a very practical project: building the atomic bomb.
It wasn’t until the 1960s, when Irish physicist John Bell turned his opinion to the problem of entanglement, that the scientific community realized that this seemingly philosophical question could have a concrete answer.
Using a simple interlocking system, Bill they expanded Einstein’s 1935 thought experiment. He showed that there was no way a quantum description could be incomplete while outlawing “spooky action at a distance” and still matched the predictions of quantum theory.
Not good news for Einstein, it seems. But this was not an instant win for his opponents.
This is because it was not clear in the 1960s whether the predictions of quantum theory were actually correct. To really prove Bell’s point, someone had to put this philosophical argument about reality, which has turned into a real physical system, to an empirical test.
This, of course, is where two of this year’s Nobel laureates enter the story. Or not John Closerand then Alan AspectExperiments with the proposed Bell system eventually showed that the predictions of quantum mechanics are accurate. As a result, unless we accept ‘frightening action at a distance’, there is no other explanation for entangled quantum systems that can describe the observed quantum world.
So, Einstein was wrong?
It may come as a surprise, but these advances in quantum theory seem to have shown Einstein wrong on this point. That is, we seem to have no fuzzy view of a quantum world quite like our normal world.
But the idea that we clearly perceive an inherently unusual quantum world is also an oversimplification. This provides one of the main philosophical lessons of this episode in quantum physics.
It is no longer clear that we can reasonably talk about the quantum world beyond its scientific description—that is, beyond Information We have it up.
As this year’s third Nobel Prize laureate, Anton Zeilingerput it:
It is not possible to distinguish between reality and our knowledge of reality, between reality and information. There is no way to refer to reality without using the information we have about it.
This distinction, which we usually assume underpins our normal picture of the world, is now irreversibly blurred. And we have John Bell to thank.