Human beings have always been attracted by the possibility of predicting the future.
Sometimes this takes on a more mystical or magical flavor in fortune telling or astrology. But,
there is a limited way in which modern science has been seen as a tool for predicting the future,
at least in specific, well-controlled situations.
But, how deep does this predictability go? Are the regularities of nature the sorts of things that
actually allow future events to be predicted? Is there really a necessary connection between the
present and the future, where what we observe in nature is exactly determined by what came before?
This connection between scientific explanation and prediction has its roots in the beginning of modern science.
The first great success of classical physics was Newton’s Law of Universal Gravitation, which,
combined with his Three Laws of Motion, allowed physicists to predict the paths of
falling objects and the orbits of planets. In an ideal scenario, only one or two objects,
like the Earth and the Sun, would be considered. If, at some moment, we could know exactly where
the Sun and the Earth were and exactly how they were moving relative to one another,
we could, in theory at least, combine that with Newton’s Laws to fairly easily solve
for where the Earth would be, relative to the Sun, at every moment in the future.
Of course, in reality, there are more factors to take into account than just the Sun and the Earth.
Even just adding in the moon makes the math much more complicated!
But, in principle, with good enough initial data and a good enough computer,
the idea is that we could still predict the location of the Earth in perpetuity. Adding in
the other planets, comets, asteroids, and other small objects that slightly perturb its motion
increases the difficulty of the calculation, but the argument is that the principle still holds.
This was the gold standard for Classical physics, and most scientists presumed that
the physical world was ultimately deterministic. The assumption was still that, in principle,
if we could know the position and velocity of every molecule of gas in the atmosphere or every
chemical and cell in the body, we could exactly predict the future state of that system or,
taken to the extreme, exactly predict the future state of the universe.
While no one dreamed of actually succeeding in this task, the fact that it was possible
“in principle” strongly suggested that the future state of the universe was,
in fact, already fixed or “determined” by its current state and the “Ultimate Laws of Physics”.
The goal of science and scientific explanation was seen as getting closer and closer to this
ideal of perfect prediction. To explain the results of a scientific experiment
was to provide the initial conditions and the laws that would have predicted
the observed outcome, and that would predict similar outcomes in the future.
This confidence in a deterministic view of the world and the hope in steady progress towards more
perfect predictability was dashed by the rise of Quantum Mechanics in the early twentieth century.
In short, built into Quantum Mechanics, in the famous Heisenberg Uncertainty principle,
is a limit on how well the actual conditions of any object – such as its position and momentum,
for example – could be known. This was not simply a technical limitation that could
be overcome by better tools, but an inherent limit on how well the position and the momentum
of an object could be known at the same time. On the scale of planetary orbits, this limit is
miniscule and negligible, but in experiments on small collections of atoms, and subatomic
particles, it is a major barrier. This barrier to detailed knowledge of
the exact state of particles and systems of particles prevents the possibility of exact
prediction of results in most experiments involving quantum mechanics. Instead,
our best theories can only provide the probability that particular outcomes might come about.
On the surface, this might seem to be a physical barrier to how much we can know,
one that might destroy the possibility of explaining physical phenomena.
In one sense, this is true, but in a deeper sense, by giving up on the hope of exact predictability,
we actually have a more thorough and solid understanding of the physical world.
We’re able to provide explanations for strange new phenomena, as well as some simple phenomena that
previous generations of physicists simply took for granted--like exactly how it is that light diffracts through glass, or why,
when we combine one cup of water with another, we get two cups of water of roughly the same temperature.
Everyone who takes Quantum Mechanics seriously agrees that the Heisenberg Uncertainty principle
limits our ability to know every detail about the state of natural objects and to predict
the exact outcomes of most experiments. That said, there is a further question
of whether these are just limitations in what we can know about nature or whether
there is something inherently undetermined, inherently probabilistic, about nature itself.
For various reasons, some physicists and philosophers of physics
find such an indeterministic world unsatisfying and have gravitated towards interpretations
of Quantum Mechanics that are fundamentally deterministic, even if we could never observe it.
They argue that, for every experiment, there is actually one and only one possible outcome, but
we lack some information to make the right prediction. There are either hidden initial
conditions that we could never observe, or parts of the outcome that we could never actually experience.
These interpretations of Quantum Mechanics can be crafted to make roughly the same
probabilistic predictions as “standard Quantum Mechanics” does, although there are arguments
about how successfully they reproduce all of the experimental results we have found.
As such, for now, it seems the “choice” between a deterministic and an indeterministic world is
not explicitly testable--that it is not a purely scientific question, but more of a philosophical one.
Rather than trying to adjudicate such a complicated scientific and
philosophical argument here, I want to simply suggest why we should not be so
surprised if the world is not so deterministic. I think that part of the hope, the desire, for a
deterministic physical world among physicists--one I’ve felt myself at times--is a somewhat misplaced hope
that the natural world is ultimately just a really complicated math problem--that the sort of
exact solutions that we are able to find in mathematics, the precision and detail, the necessary connection we
find between premises and conclusions, is perfectly mirrored in the physical world.
It is definitely true that, before the scientific revolution,
no one, including ancient and medieval thinkers like Aristotle and Aquinas, completely recognized the power and
complexity with which mathematics can describe the physical world.
But, it is also the case that many physicists and philosophers overestimate the power of
mathematics as well. In fact, there are recent arguments, independent of any questions about
Quantum Mechanics, that suggest that the sort of near, if not actually infinite,
precision necessary for a deterministic world like that seemingly described by Classical physics
could never actually be physically instantiated. The universe is not an ideal mathematical object,
but a complicated combination of physical objects that can be described with mathematics in detail and subtlety
far beyond anything we could have imagined even a few centuries ago.
That said, it is still the case that our best explanations about nature
are not perfect predictions. Nature is messy and contingent, and while we continue to learn
more and more, our understanding of nature will always be, as Aristotle argued,
not perfect and necessary but contingent and only explain what happens “for the most part.”
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