Einstein always loses in the quantum realm.
On Tuesday, the 2022 Nobel Prize in Physics was awarded to
three researchers: Alain Aspect, John F. Clauser and Anton Zeilinger. These
scientists’ work opened up new frontiers in quantum weirdness to study. What
their findings also showed is that the most philosophically challenging aspects
of quantum mechanics are also its most essential. Those challenges mean that
anyone taking quantum mechanics seriously is faced with strange choices in
thinking about the nature of reality and our place in it. That is what I want
to focus on today.
Where Einstein always loses
To be explicit, the three physicists share their prize for
their studies of quantum entanglement. When particles are entangled, they can
no longer be thought of as having separate properties. Imagine I have two
particles with properties that I cannot know before I take measurements of
them. But if the particles are entangled, then a measurement of just one out of
the pair instantly establishes what a measurement on the other would produce.
This is true even if the particles are separated by a distance so large that
there would be no chance for them to communicate in the time it would take to
measure one and then the other. In this way, entangled particles seem to form a
coherent whole across space and time.
Entanglement is exactly the kind of “spooky action at a
distance” that Einstein was famously concerned about in quantum mechanics. It’s
why he felt quantum theory was somehow incomplete, meaning there must be
something about it we have yet to understand.
What Einstein wanted was a physics that returned us to a
classical view of reality — a view where things have their own distinct
properties, regardless of whether a measurement of those properties was made or
not. In 1964 Irish physicist John Stewart Bell proposed a way to clearly
differentiate Einstein’s vision of reality from the spookier quantum version.
Measuring entanglement was the key. It took a few decades, but eventually
measurements of separate entangled particles became commonplace, and in every
experiment, Einstein lost. Reality really is spooky.
But what exactly is that spookiness telling us? The answer
is that no one knows. Unlike classical physics, quantum mechanics always
requires an interpretation to be pinned on top of mathematical formalism.
Whereas Newtonian physicists could easily imagine their laws of motion
governing atoms that acted just like tiny billiard balls, quantum physicists
never had any such assurance. The heart of the dilemma comes with the role of
measurement. Quantum mechanics is famous for its wave-particle duality, where
an electron, for example, will behave as a wave or a particle depending on
which kind of experiment you perform. It’s the choice of measurement — of a
wave kind or a particle kind — that seems to determine the result.
Reality is as strange as its measurement
So, is the electron a wave spread out through space, or is
it a particle holding just a single position at any one time? And why should
the choice made by a measurer have any effect? What is a measurement anyway,
and what is a measurer? Is it always a person — an observer — or does any
interaction with any kind of “thing” count? The answers to these questions
cannot be found in the mathematical theory — at least not yet. That leaves
people to interpret the mathematics according to the features of the reality
they think the mathematics must express. But the problem is that no one agrees
on which interpretation is correct, and the interpretations can vary wildly.
And the spookiness of quantum cannot be made to go away — every interpretation
is forced to accept something about reality that seems really, really weird.
For example, the Many Worlds Interpretation of quantum
mechanics holds that there is still a reality out there independent of
measurers, but there is a price paid for this view. Every measurement — in
other words, every interaction with anything — forces the Universe to split
into a near infinity of copies. Each of these many worlds holds one of the
possible measurement results.
In Quantum Bayesianism, on the other hand, the measurements
of quantum mechanics never reveal the world in itself, but our interactions
with the world. QBism has no problem explaining the importance of measurements,
but it gives up on the dream (or fantasy) of a perfectly objective view of reality.
As you can see, the Many Worlds interpretation is very different from Quantum
Bayesianism. But each shows the kinds of choices you must make when you try to
ask what quantum mechanics tells us about reality. If someone could tell us
which choice we simply have to make, well, that would be worth another Nobel
Prize.
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