Quantum uncertainty has to do with us, not reality
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Two of the key founders of quantum physics, Einstein and
Schrödinger, were deeply sceptical of its implications about uncertainty and
the nature of reality. Today, the orthodox reading is that uncertainty is
indeed an inherent feature of quantum systems, not a reflection of our own lack
of knowledge. But Oxford physicist Tim Palmer now argues that chaos theory
shows that quantum uncertainty is in fact down to our own ignorance, not
reality itself. This could have far-reaching consequences for our ability to
marry quantum mechanics with general relativity.
Everyone knows that long-range weather forecasts are
uncertain. It’s because of those pesky butterflies. Unobserved, they flap their
wings, causing unpredicted storms to appear weeks later. This is the metaphor
used to describe the unpredictability of chaotic systems:tiny uncertainties in
the initial conditions of a system grow and grow until they completely destroy
the accuracy of any forecast. In this metaphor, the butterflies themselves
aren’t uncertain about the state of their wings, it is us humans that are
uncertain about them. Philosophers call this “epistemological” uncertainty –
uncertainty to do with lack of knowledge.
But according to the orthodox view about quantum mechanics,
our most successfully tested theory of physics, uncertainty is not always of
this epistemological type. Quantum mechanics is usually described as a theory
of atoms and sub-atomic particles, but in truth it is believed to be a theory
that underpins everything in the world, including the weather and the galaxies
– all of reality. According to orthodox view, there is an inherent uncertainty
about what happens to a quantum system when we attempt to observe it. At the
moment of observation, the quantum state of a system collapses randomly from a
superposition of possible states to some definite outcome. According to this
account, randomness is incorporated into the basic equations of quantum
mechanics. This in turn implies that quantum uncertainty is not purely
epistemological, but is additionally “ontological”, meaning that reality is in
itself inherently uncertain. But this orthodox view rests on a rarely
questioned assumption. Chaos theory provides strong motivation for questioning
the assumption. Rejecting it means that the uncertainty of quantum mechanics
could, after all, be of the same epistemological type as that of chaos theory.
Standard discussions about Bell’s inequality end by
concluding that quantum uncertainty is not fundamentally due to our uncertainty
about the quantum world, but due to the way quantum reality itself is.
Quantum Uncertainty and Bell’s Theorem
Two of the founding fathers of quantum mechanics thought the
idea that reality was uncertain was ludicrous, and as a result refused to
believe that quantum mechanics, in its current state, was the final word on the
subject. Erwin Schrödinger devised his famous cat experiment to show that this
interpretation of quantum mechanics leads to nonsensical cats which are half
alive and half dead, and Albert Einstein famously remarked, in exasperation,
that surely God does not play dice. And yet, despite this, most physicists
today believe that quantum mechanics is the final word as far as quantum
physics is concerned and that there is therefore an element of inherent
ontological uncertainty about the world around us. Quantum uncertainty, these
physicists would argue, has nothing to do with the butterfly effect: quantum
uncertainty is much more radical.
So why does today’s consensus reject the concerns of
Einstein and Schrödinger? The most important reason stems from a quantum
phenomenon that Schrödinger himself named entanglement. Specifically, two
particles can be emitted from a source, such that the properties of the two
particles – e.g., their angular momenta (also known as spins) are correlated.
This itself is not necessarily strange. However, the Northern Irish physicist
John Bell showed that, under seemingly reasonable assumptions, these
correlations, suitably combined, are limited in size. This is called Bell’s
theorem. The 2022 Nobel Physics Prize was given to three physicists (Alain
Aspect, John Clauser and Anton Zeilinger) who showed that in practice, the
combined correlations can exceed this limit. Hence one or more of these
seemingly reasonable assumptions must be wrong.
The standard interpretation of this experimental result is
that it confirms that quantum uncertainty is ontological, not epistemological.
That is, uncertainty is a feature of reality itself, not a reflection of the
limits of our knowledge. Of course, this is such a startling conclusion that
physicists have looked for other ways to explain Bell’s theorem. There is
indeed an alternative interpretation, but it is too weird to be plausible. It
assumes that the settings for the apparatus that measures the spin of one of
the entangled particles somehow influence the measurement outcome for the other
particle. It is a weird explanation because it implies what Einstein called
“spooky action at a distance” – the idea that what happens to one particle can
instantaneously influence another, distant particle. Einstein didn’t like
spooky action at a distance, and neither do I, nor indeed most physicists I
know. So standard discussions about Bell’s inequality end by concluding that
quantum uncertainty is not fundamentally due to our uncertainty about the
quantum world, but due to the way quantum reality itself is.
However, it turns out that there is another assumption in
what is called Bell’s Theorem. It’s one that physicists intuitively think is
true and therefore don’t tend to question. However, they should. The assumption
concerns the validity of a way of thinking that is second nature to us:
counterfactual reasoning. Below I will show that chaos theory casts doubt on an
unquestioned belief in the validity of counterfactual reasoning.
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