The Nobel Prize in physics this year has been awarded “for experiments with entangled photons, establishing the violation of Bell inequalities 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. And a key player in that debate was an Irish physicist
named John Bell.
In the 1960s, Bell figured out how to translate a
philosophical question about the nature of reality into a physical question
that could be answered by science – and along the way broke down the
distinction between what we know about the world and how the world really is.
Quantum entanglement
We know that quantum objects have properties we don’t
usually ascribe to the objects of our ordinary lives. Sometimes light is a
wave, sometimes it’s a particle. Our fridge never does this.
When attempting to explain this sort of unusual behaviour,
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 ordinary 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 about which explanation was right. Among those who thought the quantum
world just is unusual were figures such as Werner Heisenberg and Niels Bohr.
Among those who thought the quantum world must be just like the ordinary world,
and our view of it is simply foggy, were Albert Einstein and Erwin Schrödinger.
At the heart of this division is an unusual prediction of
quantum theory. According to the theory, the properties of certain quantum
systems that interact remain dependent on each other – even when the systems
have been moved a great distance apart.
In 1935, the same year he devised his famous thoughtexperiment involving a cat trapped in a box, Schrödinger coined the term
“entanglement” for this phenomenon. He argued it is absurd to believe the world
works this way.
The problem with entanglement
If entangled quantum systems really remain connected even
when they are separated by large distances, it would seem they are somehow
communicating with each other instantaneously. But this sort of connection is
not allowed, according to Einstein’s theory of relativity. Einstein called this
idea “spooky action at a distance”.
Niels Bohr (left) and Albert Einstein (right) argued for
many years over whether the world was really as fuzzy and strange as quantum
mechanics suggested. Paul Ehrenfest |
Again in 1935, Einstein, along with two colleagues, devised
a thought experiment that showed quantum mechanics can’t be giving us the whole
story on entanglement. They thought there must be something more to the world
that we can’t yet see.
But as time passed, the question of how to interpret quantum
theory became an academic footnote. The question seemed too 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 mind to the problem of entanglement, that the scientific community
realised this seemingly philosophical question could have a tangible answer.
Bell’s theorem
Using a simple entangled system, Bell extended Einstein’s
1935 thought experiment. He showed there was no way the quantum description
could be incomplete while prohibiting “spooky action at a distance” and still
matching the predictions of quantum theory.
John Bell in his office at CERN in Switzerland. CERN |
Not great news for Einstein, it seems. But this was not an
instant win for his opponents.
This is because it was not evident in the 1960s whether the
predictions of quantum theory were indeed correct. To really prove Bell’s
point, someone had to put this philosophical argument about reality,
transformed into a real physical system, to an experimental test.
And this, of course, is where two of this year’s Nobel
laureates enter the story. First John Clauser, and then Alain Aspect, performed
the experiments on Bell’s proposed system that ultimately showed the
predictions of quantum mechanics to be accurate. As a result, unless we accept
“spooky action at a distance”, there is no further account of entangled quantum
systems that can describe the observed quantum world.
So, Einstein was wrong?
It is perhaps a surprise, but these advances in quantum
theory appear to have shown Einstein to be wrong on this point. That is, it
seems we do not have a foggy view of a quantum world that is just like our
ordinary world.
But the idea that we perceive clearly an inherently unusual
quantum world is likewise too simplistic. And this provides one of the key
philosophical lessons of this episode in quantum physics.
It is no longer clear we can reasonably talk about the
quantum world beyond our scientific description of it – that is, beyond the
information we have about it.
As this year’s third Nobel laureate, Anton Zeilinger, put
it:
the distinction between reality and our knowledge of reality, between reality and information, cannot be made. There is no way to refer to reality without using the information we have about it.
This distinction, which we commonly assume to underpin our
ordinary picture of the world, is now irretrievably blurry. And we have John
Bell to thank.
Let's hear it for John Bell though. I mean Christ, really. It was thanks to him that quantum foundations became experimental physics and not philosophy. (Nothing against philosophy, but it's good to get answers sometimes.)
— Philip Ball (@philipcball) October 4, 2022
0 Comments