In at least
one case, though, Becker himself is a bit too dismissive of a philosophical
line of argument. Becker discusses how
the notion of a multiverse has been
defended by many physicists on the basis of several independent considerations,
viz. Hugh Everett’s “many worlds” interpretation of quantum mechanics, inflationary
cosmology, and string theory. One
objection raised against the notion in any of these versions is that it is unfalsifiable– that is to say, that it
generates no predictions that could in principle be proven false by observation
and experiment, in which case it is empirically untestable.
Becker
rightly notes that falsifiability, a theme made famous by Karl Popper, is not
as straightforward a matter as popular presentations often suppose. For one thing, a scientific theory is never
tested in isolation, because it never generates predictions in isolation. Rather, its predictions follow from the
theory only together with various
further assumptions of a theoretical or empirical kind.
For example,
suppose researchers working for a soap company want to determine whether the
chemical ingredients in a new product they are developing really will, as they
suppose, kill certain kinds of bacteria.
They put samples of the bacteria on a slide, apply the soap, and see
what happens. If the bacteria are not
destroyed, has the theory been falsified?
Not necessarily. For in making and
testing the prediction that the soap will kill the bacteria, they are assuming
that dead bacteria will have such-and-such an appearance under a microscope, that
the slide on which they are put has been cleaned properly (and thus doesn’t have
some residue of chemicals that might counteract the effect of the soap they are
testing), that the standard theory about how microscopes work is correct, that
the particular microscope being used is not malfunctioning, and so on. And if the test does not come out as
predicted, it could be that one of
these background assumptions is false, rather than that the soap does not
really kill such bacteria.
Of course,
there may be very good reasons for judging that none of these assumptions is
false, so that the reasonable conclusion to draw is that the soap is not in
fact effective against the bacteria. The
point, though (famously emphasized by Pierre Duhem and W. V. Quine), is that testing
a scientific claim is not a matter of carrying out a “crucial experiment” that
might all by itself either falsify or vindicate the claim. There is often a certain amount of wiggle
room by which a theory might in
principle be upheld in the face of apparent counterevidence, even if actually continuing
to uphold it is not necessarily reasonable all things considered.
Becker
discusses a famous pair of examples from the history of science that illustrate
how complicated the matter of falsification actually is. Newtonian physics was in general
spectacularly successful in describing and predicting the observed motions of
bodies, but there were exceptions. The
motions of Uranus and Mercury did not conform to the predictions of Newton’s
laws, but for a very long time, this did not lead scientists to judge that Newton
had been refuted. After all, the theory
worked for most observations, and there was at first nothing better to put in
its place. So, they looked for
alternative explanations of the divergence between observation and theory. In the case of Uranus, it turned out that its
motion was being affected by the gravitational pull of another, heretofore
unknown planet, Neptune. That particular
problem for Newton’s theory was thus solved.
But the conflict with the observed motion of Mercury resisted any
similar solution, and it wasn’t until Einstein’s general theory of relativity
appeared – and explained the motion of Mercury along with all the observational
evidence that Newton could explain – that Newton’s theory was judged to have
been falsified, and Einstein’s adopted in its place.
So far so
good. But Becker then fallaciously draws
from these considerations the conclusion that “scientific theories don’t need
to be falsifiable” (p. 264) so that:
Claiming, then, that multiverse
theories are unscientific because they are unfalsifiable is to reject them simply
because they do not live up to an arbitrary standard that no scientific theory
of any kind has ever met. Claiming that
no data could ever force the rejection of a multiverse theory is merely stating
that a multiverse theory is just like any other theory. (p. 263)
This is not
true, and it certainly doesn’t follow.
To understand what is wrong with Becker’s position, we need to draw some
distinctions. First, Popper argued that
falsifiability comes in degrees. Some statements might have empirical
consequences independently of any others, other statements might have empirical
consequences only in conjunction with further statements, and yet other
statements might have no empirical consequences at all. The first sort of statement would be strongly falsifiable, the second weakly falsifiable, and the third
utterly unfalsifiable.
Now, even if
the considerations raised by Becker show that a scientific theory need not be strongly falsifiable, it doesn’t follow
that it can be altogether unfalsifiable. It may, for all Becker has shown, still need
to be at least weakly falsifiable. Now, the critic of multiverse theories might
argue that whereas Newton’s physics was weakly
falsifiable, multiverse theories are altogether unfalsifiable, so that the
parallel Becker wants to draw is bogus.
And in that case, Becker’s response does not suffice to save multiverse
theories from the objection in question.
He would have to show, either that multiverse theories are at least
weakly falsifiable, or that a scientific theory need not be even weakly
falsifiable. And he does not establish
either of these claims.
But even if
he were to take the second route and argue that scientific theories needn’t be
even weakly falsifiable, there is a further problem, as can be seen by drawing
some further distinctions. For there are
different ways in which a statement
might be empirically unfalsifiable, some of them unproblematic but some of them
problematic.
First, there
are statements that are unfalsifiable in
the way that mathematical and metaphysical truths can be. For example, that 2 + 2 = 4 and that the
fundamental constituents of reality are substances (as opposed to attributes, say,
or events) are, I would argue, not empirically falsifiable. That is not because they are less certain than empirical claims, but
because (as I would also argue) they are more
certain. They are bedrock truths that
pertain to any possible reality, to non-empirical
immaterial reality no less than to the empirical, material world.
Second,
there are statements that are unfalsifiable in
the way that truths of the philosophy of nature can be. These are claims that, unlike those of the first
category, do apply only to empirical reality yet are nevertheless certain. For example, take the claim that change occurs. We know this statement empirically, but it is
not empirically falsifiable for the
simple reason that to falsify something requires having a sequence of
experiences (as happens when we set up an experiment, carry it out, and then
record the results). And having a
sequence of experiences itself involves
change. To try empirically to falsify
the claim that change occurs would thus be self-defeating. That does not entail that the reality of
change is less certain than other empirical truths, but rather that it too is
more certain.
Third, there
are statements that are unfalsifiable in
the way that the most fundamental theses of modern empirical science arguably
are. For example, some have held
that the principle of the conservation of energy and the second law of
thermodynamics are unfalsifiable. This
is debatable, but it is certainly plausible to maintain that these ideas are so
central to modern science’s picture of the universe that they are treated in practice as unfalsifiable, even if
they are falsifiable in principle. The
idea is that giving them up would so radically undermine the rest of the modern
scientific edifice that, if there ever appeared to be evidence that conflicted
with them, scientists would judge that there must be something wrong with the
evidence or with other parts of science, rather than that these fundamental principles
themselves are false.
Fourth,
there are statements that are unfalsifiable in
the way that Popper famously took astrology, Marxism, and Freudianism to be. These are statements that purport to be
empirical rather than metaphysical, but are neither parts of the philosophy of
nature nor central to the modern scientific picture of the world. Because they do not fall into the first three
categories I’ve just described, the reason they are unfalsifiable is not that
they are necessary truths (the way mathematical and metaphysical truths are),
or because denying them would be self-defeating (the way denying my example of
a truth in the philosophy of nature would be), or because to deny them would
take down the whole edifice of science (as the examples in the third category
would). So, they do not have the certainty that truths in these other
categories have. The reason they are
unfalsifiable is instead that they make predictions that are too vague or
open-ended to be crisply testable.
Now, it is unfalsifiability
of this fourth kind that is the most
problematic, and that Popper took to be paradigmatic of pseudo-science. The first two kinds of unfalsifiable
statement are, I would argue, unproblematic, and the third kind is at least
arguably defensible. Suppose multiverse
theories are indeed unfalsifiable. Which
of these four classes would they fall in?
They don’t
fall into the first category, because their description of the world is not true
of metaphysical or arithmetical necessity. That is why even defenders of multiverse
theories typically allow that they might
be wrong, and at least try to come up
with ways of testing such theories empirically.
This would make no sense if the theories had the bedrock status that
truths of mathematics and metaphysics are traditionally claimed to have.
They also
don’t fall into the second category, because they aren’t fundamental truths
about what any possible empirical world must be like, which it would be
self-defeating to deny. Again, even
defenders of multiverse theories allow that they might be wrong, and certainly one can doubt such theories without
being led into incoherence (by contrast with the attempt to deny the reality of
change, which, I would argue, would
be incoherent).
Nor do multiverse
theories fall into the third category, because they are hardly fundamental to the modern scientific picture
of the world in the way that the conservation of energy and the second law of
thermodynamics are. This is obvious just
from the fact that they are highly controversial in a way that the fundamental
scientific principles mentioned are not.
So, if
multiverse theories really are not even weakly falsifiable, but altogether
unfalsifiable, it looks like they will fall into the fourth and most problematic
class of unfalsifiable theories, alongside astrology, Marxism, and
Freudianism. And in that case, Becker
will not have succeeded in defending multiverse theories from the objection in
question.
Successfully to defend them against that objection would require either (a) showing that unfalsifiable statements even of the fourth category are scientifically respectable, (b) showing that multiverse theories are, appearances notwithstanding, unfalsifiable in the way that statements in one of the other three categories are, or (c) showing that multiverse theories are in fact falsifiable and open to empirical testing. I don’t think that any of these routes is promising, but route (c) would certainly be the way to go if the defender of a multiverse theory wants to convince anyone that such theories are “scientific” in just the same sense that what Newton, Einstein, and the founders of quantum mechanics were up to was scientific. To do that, however, would not be to sidestep the objection from falsifiability (as Becker wants to do), but precisely to meet the objection head on.