Reference Material:
A
Primer on Determinism by John Earman (1986)
I’m
launching the blog with a series of posts on the philosophical doctrine of
determinism and its status in various theories in physics. My motivation for
this part of the project is to ultimately make some claims about the physical
world that are relevant to human action. My thinking along these lines is quite
immature, so I won’t expand on it here. Rather, I want to start from as solid a
foundation as possible, so I’m looking into our best current theories of the
physical world, and asking if determinism holds, and if not, in what way it
fails. The first theory in question is classical mechanics. Let’s dig in.
We
first need a working definition of determinism. Prior to reading A Primer on Determinism, I held
something close to what I’ll call the “standard” definition of determinism,
summed up nicely in the Stanford Encyclopedia of Philosophy article:
The
world is governed by determinism if and only if, given a specified way things are at a time t, the way
things go thereafter is fixed as a
matter of natural law.
The article goes on to attempt to clarify
conceptual issues surrounding the italicized concepts. The world must refer to the entire universe, basically because no
system can be sufficiently causally isolated to prevent outside influences from
interrupting a strict causal chain of events. The world is governed by natural laws. Do the natural laws do the governing?
That is, are they ontologically primary? I don’t claim to have a great answer,
but I’m also not convinced it matters much for my purposes here. The way things are at a time t is obviously
a fraught concept considering the lessons of special and general relativity,
but only classical mechanics will be treated in this post, so we will accept
this concept at face value for now. The way things are at a time t determines
the way things go thereafter, but
determinism, if it holds, generally holds bi-directionally. The future is of
particular concern because of any bearing determinism might have on the
question of free will. Placing an emphasis on futuristic determinism is not to
deny its time symmetry.
I
find the above definition a perfectly sensible one, and I will lean on it in
what follows. But I also want to describe an alternative definition that Earman
puts forward. The definition turns on the concepts of physical possibility, possible
worlds, and worlds generally. The actual world is “the collection of all events
that have ever happened, are now happening, or ever will happen”. Possible
worlds are different such collections of events that are physically possible,
that is, that are still consistent with the natural laws that apply in the
actual world. The set of all possible worlds is W. The world W is deterministic if:
for
any W’ in W, if W and W’ agree at any
time, then they agree for all times.
Like the first definition, this seems
intuitively plausible. Earman seems to think this definition is necessary to
ward off certain objections to the failure of determinism later down the line.
I saw no such necessity; perhaps I’m wrong. I will tentatively move forward
with the Stanford definition.
Consider
a world governed (or described, if you like) completely by classical mechanics.
Is such a world deterministic? Up until recently, I imagined the answer to be an
almost obvious yes. After delving into the reference material, my view is very
much changed. A few examples will serve to illustrate the ways in which
determinism fails in a classical mechanics framework.
The
first example involves so-called “space invaders”. Classical mechanics, unlike
its relativistic cousins, specifies no upper bound on the velocity of
particles. It is at least a conceptual possibility, then, that a particle be
continuously accelerated to ever greater velocity. In particular, it is
conceptually possible for a particle to be accelerated in such a way that its
velocity become arbitrarily high in a finite amount of time, prior to some time
t*. The asymptotic nature of such a particle’s worldline means that at t*, the
particle literally disappears from the world (the particle having a definite position
at t* would limit its velocity prior to t*, a contradiction). Now recall that
all laws within classical mechanics are time-reversible. The time reversal of
our disappearing particle is a high-velocity particle suddenly appearing,
seemingly out of nowhere, after t* – a so-called “space invader”. The
possibility of space invaders within classical mechanics is a flagrant
violation of determinism. To drive the point home, Earman cites a couple more
fleshed out physical systems, involving either infinite collisions among finite
particles or an infinite collection of particles, that produce behavior similar
or analogous to that of the space invader.
A
second example that seems more widely known is Norton’s Dome. The Dome is an
axisymmetric mound with a surface that bends away from the apex according to a
specific equation. A ball sitting exactly at the top of the dome should do just
that – sit at the top of the dome. Much to my surprise, there is actually
another solution to the motion of the ball, completely consistent with Newton’s
Laws, in which the ball spontaneously rolls down the dome in a single arbitrary
direction. It sounds absurd, but there is an intuitive way to understand that
this is possible; again, it’s a time reversal. Imagine rolling the ball from
the edge of the dome up towards the apex. Because of the precise shape of the
dome, it turns out that there is a velocity with which you can roll the ball
towards the top such that the ball will arrive there and stop. The time
reversal of this motion, is, of course, the completely counterintuitive
spontaneous rolling down the hill. Strange stuff.
There
are further examples. Collisions involving multiple point particles – classical
mechanics does not uniquely determine the absolute angle at which each particle
leaves the collision site, only that linear momentum be conserved. Tachyons
(hypothetical faster-than-light particles) can undermine uniqueness in
solutions to Maxwell’s equations. Instantaneous heat propagation can do the
same to solutions of the classical heat equation. I could go on.
Instead,
I want to highlight a thread of commonality running through all the examples I
encountered. When determinism fails to hold in classical mechanics, the failure
will manifest itself as randomness to the observer. Indeed, the failure will be
in some sense intrinsically random
and arbitrary. Take the space invader. An observer after time t* suddenly
experiences a particle appearing from spatial infinity. No information in the
universe at t* can enable this observer to predict the space invader’s visit.
It appears randomly, arbitrarily, and seemingly uncaused. Norton’s Dome behaves
similarly. The ball may slide down the dome at any moment. That it starts
sliding at a particular moment is acausal – no reason can be drawn from the
physical model of the motion for why the ball starts sliding at one moment and
not another. Again, the motion seems random and arbitrary. Three point
particles collide simultaneously, all leaving in different directions, with
linear momentum conservation satisfied. The directions with which they leave
cannot be explained – they seem random and arbitrary.
These observations
lead me to my take-away from this first post. All the events in a (hypothetical)
world governed by classical mechanics can be grouped into two classes: those
which are deterministic in their behavior, and those which aren’t. The
indeterministic events are random, arbitrary, and seemingly acausal.
As I write
the preceding two sentences, they seem almost tautological. Maybe time will
tell.
Additional Links: