Multiverses and falsifiability

AdamBecker’s 2018 book Whatis Real? The Unfinished Quest for the Meaning of Quantum Physics is an excellent account of the longstanding and intractable controversy overhow to interpret quantum mechanics.  Oneof the main themes of the book is how much the direction of twentieth-centuryphysics was driven by personalities, political factors, career interests, and,not least, unexamined and woolly philosophical assumptions – something philosophersof science like Thomas Kuhn and Paul Feyerabend have shown has always been true of science historically.  The tendency of contemporary physicists,especially, to be both ignorant of and condescending toward philosophy comes infor special criticism.

In at leastone case, though, Becker himself is a bit too dismissive of a philosophicalline of argument.  Becker discusses howthe notion of a multiverse has beendefended by many physicists on the basis of several independent considerations,viz. Hugh Everett’s “many worlds” interpretation of quantum mechanics, inflationarycosmology, and string theory.  Oneobjection raised against the notion in any of these versions is that it is unfalsifiable – that is to say, that itgenerates no predictions that could in principle be proven false by observationand experiment, in which case it is empirically untestable. 

Beckerrightly notes that falsifiability, a theme made famous by Karl Popper, is notas straightforward a matter as popular presentations often suppose.  For one thing, a scientific theory is nevertested in isolation, because it never generates predictions in isolation.  Rather, its predictions follow from thetheory only together with variousfurther assumptions of a theoretical or empirical kind. 

For example,suppose researchers working for a soap company want to determine whether thechemical ingredients in a new product they are developing really will, as theysuppose, kill certain kinds of bacteria. They put samples of the bacteria on a slide, apply the soap, and seewhat happens.  If the bacteria are notdestroyed, has the theory been falsified? Not necessarily.  For in making andtesting the prediction that the soap will kill the bacteria, they are assumingthat dead bacteria will have such-and-such an appearance under a microscope, thatthe slide on which they are put has been cleaned properly (and thus doesn’t havesome residue of chemicals that might counteract the effect of the soap they aretesting), that the standard theory about how microscopes work is correct, thatthe particular microscope being used is not malfunctioning, and so on.  And if the test does not come out aspredicted, it could be that one ofthese background assumptions is false, rather than that the soap does notreally kill such bacteria.

Of course,there may be very good reasons for judging that none of these assumptions isfalse, so that the reasonable conclusion to draw is that the soap is not infact effective against the bacteria.  Thepoint, though (famously emphasized by Pierre Duhem and W. V. Quine), is that testinga scientific claim is not a matter of carrying out a “crucial experiment” thatmight all by itself either falsify or vindicate the claim.  There is often a certain amount of wiggleroom by which a theory might inprinciple be upheld in the face of apparent counterevidence, even if actually continuingto uphold it is not necessarily reasonable all things considered.

Beckerdiscusses a famous pair of examples from the history of science that illustratehow complicated the matter of falsification actually is.  Newtonian physics was in generalspectacularly successful in describing and predicting the observed motions ofbodies, but there were exceptions.  Themotions of Uranus and Mercury did not conform to the predictions of Newton’slaws, but for a very long time, this did not lead scientists to judge that Newtonhad been refuted.  After all, the theoryworked for most observations, and there was at first nothing better to put inits place.  So, they looked foralternative explanations of the divergence between observation and theory.  In the case of Uranus, it turned out that itsmotion was being affected by the gravitational pull of another, heretoforeunknown planet, Neptune.  That particularproblem for Newton’s theory was thus solved. But the conflict with the observed motion of Mercury resisted anysimilar solution, and it wasn’t until Einstein’s general theory of relativityappeared – and explained the motion of Mercury along with all the observationalevidence that Newton could explain – that Newton’s theory was judged to havebeen falsified, and Einstein’s adopted in its place.

So far sogood.  But Becker then fallaciously drawsfrom these considerations the conclusion that “scientific theories don’t needto be falsifiable” (p. 264) so that:

Claiming, then, that multiversetheories are unscientific because they are unfalsifiable is to reject them simplybecause they do not live up to an arbitrary standard that no scientific theoryof any kind has ever met.  Claiming thatno data could ever force the rejection of a multiverse theory is merely statingthat a multiverse theory is just like any other theory. (p. 263)

This is nottrue, and it certainly doesn’t follow. To understand what is wrong with Becker’s position, we need to draw somedistinctions.  First, Popper argued thatfalsifiability comes in degrees.  Some statements might have empiricalconsequences independently of any others, other statements might have empiricalconsequences only in conjunction with further statements, and yet otherstatements might have no empirical consequences at all.  The first sort of statement would be strongly falsifiable, the second weakly falsifiable, and the thirdutterly unfalsifiable. 

Now, even ifthe considerations raised by Becker show that a scientific theory need not be strongly falsifiable, it doesn’t followthat it can be altogether unfalsifiable.  It may, for all Becker has shown, still needto be at least weakly falsifiable.  Now, the critic of multiverse theories mightargue that whereas Newton’s physics was weaklyfalsifiable, multiverse theories are altogether unfalsifiable, so that theparallel Becker wants to draw is bogus. And in that case, Becker’s response does not suffice to save multiversetheories from the objection in question. He would have to show, either that multiverse theories are at leastweakly falsifiable, or that a scientific theory need not be even weaklyfalsifiable.  And he does not establisheither of these claims.

But even ifhe were to take the second route and argue that scientific theories needn’t beeven weakly falsifiable, there is a further problem, as can be seen by drawingsome further distinctions.  For there aredifferent ways in which a statementmight be empirically unfalsifiable, some of them unproblematic but some of themproblematic.

First, thereare statements that are unfalsifiable inthe way that mathematical and metaphysical truths can be.  For example, that 2 + 2 = 4 and that thefundamental 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, butbecause (as I would also argue) they are morecertain.  They are bedrock truths thatpertain to any possible reality, to non-empiricalimmaterial reality no less than to the empirical, material world. 

Second,there are statements that are unfalsifiable inthe way that truths of the philosophy of nature can be.  These are claims that, unlike those of the firstcategory, 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 isnot empirically falsifiable for thesimple reason that to falsify something requires having a sequence ofexperiences (as happens when we set up an experiment, carry it out, and thenrecord the results).  And having asequence of experiences itself involveschange.  To try empirically to falsifythe claim that change occurs would thus be self-defeating.  That does not entail that the reality ofchange is less certain than other empirical truths, but rather that it too ismore certain.

Third, thereare statements that are unfalsifiable inthe way that the most fundamental theses of modern empirical science arguablyare.  For example, some have heldthat the principle of the conservation of energy and the second law ofthermodynamics are unfalsifiable.  Thisis debatable, but it is certainly plausible to maintain that these ideas are socentral to modern science’s picture of the universe that they are treated in practice as unfalsifiable, even ifthey are falsifiable in principle.  Theidea is that giving them up would so radically undermine the rest of the modernscientific edifice that, if there ever appeared to be evidence that conflictedwith them, scientists would judge that there must be something wrong with theevidence or with other parts of science, rather than that these fundamental principlesthemselves are false.

Fourth,there are statements that are unfalsifiable inthe way that Popper famously took astrology, Marxism, and Freudianism to be.  These are statements that purport to beempirical rather than metaphysical, but are neither parts of the philosophy ofnature nor central to the modern scientific picture of the world.  Because they do not fall into the first threecategories I’ve just described, the reason they are unfalsifiable is not thatthey are necessary truths (the way mathematical and metaphysical truths are),or because denying them would be self-defeating (the way denying my example ofa truth in the philosophy of nature would be), or because to deny them wouldtake down the whole edifice of science (as the examples in the third categorywould).  So, they do not have the certainty that truths in these othercategories have.  The reason they areunfalsifiable is instead that they make predictions that are too vague oropen-ended to be crisply testable.

Now, it is unfalsifiabilityof this fourth kind that is the mostproblematic, and that Popper took to be paradigmatic of pseudo-science.  The first two kinds of unfalsifiablestatement are, I would argue, unproblematic, and the third kind is at leastarguably defensible.  Suppose multiversetheories are indeed unfalsifiable.  Whichof these four classes would they fall in?

They don’tfall into the first category, because their description of the world is not trueof metaphysical or arithmetical necessity.  That is why even defenders of multiversetheories typically allow that they mightbe wrong, and at least try to come upwith ways of testing such theories empirically. This would make no sense if the theories had the bedrock status thattruths of mathematics and metaphysics are traditionally claimed to have.

They alsodon’t fall into the second category, because they aren’t fundamental truthsabout what any possible empirical world must be like, which it would beself-defeating to deny.  Again, evendefenders of multiverse theories allow that they might be wrong, and certainly one can doubt such theories withoutbeing led into incoherence (by contrast with the attempt to deny the reality ofchange, which, I would argue, wouldbe incoherent).

Nor do multiversetheories fall into the third category, because they are hardly fundamental to the modern scientific pictureof the world in the way that the conservation of energy and the second law ofthermodynamics are.  This is obvious justfrom the fact that they are highly controversial in a way that the fundamentalscientific principles mentioned are not.

So, ifmultiverse theories really are not even weakly falsifiable, but altogetherunfalsifiable, it looks like they will fall into the fourth and most problematicclass of unfalsifiable theories, alongside astrology, Marxism, andFreudianism.   And in that case, Beckerwill not have succeeded in defending multiverse theories from the objection inquestion. 

Successfullyto defend them against that objection would require either (a) showing thatunfalsifiable statements even of the fourth category are scientificallyrespectable, (b) showing that multiverse theories are, appearances notwithstanding,unfalsifiable in the way that statements in one of the other three categoriesare, or (c) showing that multiverse theories are in fact falsifiable and opento empirical testing.  I don’t think thatany of these routes is promising, but route (c) would certainly be the way togo if the defender of a multiverse theory wants to convince anyone that suchtheories are “scientific” in just thesame sense that what Newton, Einstein, and the founders of quantummechanics were up to was scientific.  Todo that, however, would not be to sidestepthe objection from falsifiability (as Becker wants to do), but precisely tomeet the objection head on.