Who Needs Non-Empirical Confirmation?

I’ve figured out what was bugging me about Dawid’s workshop on non-empirical theory confirmation.

It’s not the concept itself that bothers me. While you might think of science as entirely based on observations of the real world, in practice we can’t test everything. Inevitably, we have to add in other sorts of evidence: judgments based on precedent, philosophical considerations, or sociological factors.

It’s Dawid’s examples that annoy me: string theory, inflation, and the multiverse. Misleading popularizations aside, none of these ideas involve non-empirical confirmation. In particular, string theory doesn’t need non-empirical confirmation, inflation doesn’t want it, and the multiverse, as of yet, doesn’t merit it.

In order for non-empirical confirmation to matter, it needs to affect how people do science. Public statements aren’t very relevant from a philosophy of science perspective; they ebb and flow based on how people promote themselves. Rather, we should care about what scientists assume in the course of their work. If people are basing new work on assumptions that haven’t been established experimentally, then we need to make sure their confidence isn’t misplaced.

String theory hasn’t been established experimentally…but it fails the other side of this test: almost no-one is assuming string theory is true.

I’ve talked before about theorists who study theories that aren’t true. String theory isn’t quite in that category, it’s still quite possible that it describes the real world. Nonetheless, for most string theorists, the distinction is irrelevant: string theory is a way to relate different quantum field theories together, and to formulate novel ones with interesting properties. That sort of research doesn’t rely on string theory being true, often it doesn’t directly involve strings at all. Rather, it relies on string theory’s mathematical abundance, its versatility and power as a lens to look at the world.

There are string theorists who are more directly interested in describing the world with string theory, though they’re a minority. They’re called String Phenomenologists. By itself, “phenomenologist” refers to particle physicists who try to propose theories that can be tested in the real world. “String phenomenology” is actually a bit misleading, since most string phenomenologists aren’t actually in the business of creating new testable theories. Rather, they try to reproduce some of the more common proposals of phenomenologists, like the MSSM, from within the framework of string theory. While string theory can reproduce many possible descriptions of the world (10^500 by some estimates), that doesn’t mean it covers every possible theory; making sure it can cover realistic options is an important, ongoing technical challenge. Beyond that, a minority within a minority of string phenomenologists actually try to make testable predictions, though often these are controversial.

None of these people need non-empirical confirmation. For the majority of string theorists, string theory doesn’t need to be “confirmed” at all. And for the minority who work on string phenomenology, empirical confirmation is still the order of the day, either directly from experiment or indirectly from the particle phenomenologists struggling to describe it.

What about inflation?

Cosmic inflation was proposed to solve an empirical problem, the surprising uniformity of the observed universe. Look through a few papers in the field, and you’ll notice that most are dedicated to finding empirical confirmation: they’re proposing observable effects on the cosmic microwave background, or on the distribution of large-scale structures in the universe. Cosmologists who study inflation aren’t claiming to be certain, and they aren’t rejecting experiment: overall, they don’t actually want non-empirical confirmation.

To be honest, though, I’m being a little unfair to Dawid here. The reason that string theory and inflation are in the name of his workshop aren’t because he thinks they independently use non-empirical confirmation. Rather, it’s because, if you view both as confirmed (and make a few other assumptions), then you’ve got a multiverse.

In this case, it’s again important to compare what people are doing in their actual work to what they’re saying in public. While a lot of people have made public claims about the existence of a multiverse, very few of them actually work on it. In fact, the two sets of people seem to be almost entirely disjoint.

People who make public statements about the multiverse tend to be older prominent physicists, often ones who’ve worked on supersymmetry as a solution to the naturalness problem. For them, the multiverse is essentially an excuse. Naturalness predicted new particles, we didn’t find new particles, so we need an excuse to have an “unnatural” universe, and for many people the multiverse is that excuse. As I’ve argued before, though, this excuse doesn’t have much of an impact on research. These people aren’t discouraged from coming up with new ideas because they believe in the multiverse, rather, they’re talking about the multiverse because they’re currently out of new ideas. Nima Arkani-Hamed is a pretty clear case of someone who has supported the multiverse in pieces like Particle Fever, but who also gets thoroughly excited about new ideas to rescue naturalness.

By contrast, there are many fewer people who actually work on the multiverse itself, and they’re usually less prominent. For the most part, they actually seem concerned with empirical confirmation, trying to hone tricks like anthropic reasoning to the point where they can actually make predictions about future experiments. It’s unclear whether this tiny group of people are on the right track…but what they’re doing definitely doesn’t seem like something that merits non-empirical confirmation, at least at this point.

It’s a shame that Dawid chose the focus he did for his workshop. Non-empirical theory confirmation is an interesting idea (albeit one almost certainly known to philosophy long before Dawid), and there are plenty of places in physics where it could use some examination. We seem to have come to our current interpretation of renormalization non-empirically, and while string theory itself doesn’t rely on non-empirical conformation many of its arguments with loop quantum gravity seem to rely on non-empirical considerations, in particular arguments about what is actually required for a proper theory of quantum gravity. But string theory, inflation, and the multiverse aren’t the examples he’s looking for.

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9 thoughts on “Who Needs Non-Empirical Confirmation?

  1. Dilaton

    Nice (and much needed) to see another voice of reason apart from Lumo, who gives the overreaching incompetent anti-science ranters well-reasoned contra!

    BTW Happy New Year and all the best to you 😉

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  2. Василий Комаров

    Falsifiable theory does not require non-empirical evidence. If it touches any objects that are currently impossible to feel empirically, but it well describes the rest of the phenomena with the possibility of prediction in a certain range, so there is no reason to consider it falsified. If an essential condition for the existence of such theory is the Multiverse and the anthropic principle (for example), it means they do not need to challenge within the theory. The condition for refutation in this case may be a falsification of entire theory in any way.

    Every hypothesis requires a theory which sends it into falsifiable category. Mankind close to the time it is created for Multiverse, anthropic principle and something more simultaneously, imho.

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  3. Giotis Mth

    I feel there is a broken link between String theory and String phenomenology; the developments at the pure string theoretic front are not trickled down (to the extent they can be) to phenomenology.

    BTW congrats for proving that 5d SYM is (as was expected) perturbatively non renormilizable.

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    1. 4gravitonsandagradstudent Post author

      Could you elaborate? It seems plausible that some useful ideas aren’t filtering through, but I can’t really think of obvious examples, and the people I’m aware of in string pheno seem reasonably aware of the wider state of string theory.

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      1. Giotis Mth

        Well indeed it’s very hard to pinpoint the relevant research explicitly at this point; it’s not an easy task by all means.

        I was thinking basically how developments around dualities, non perturbative definitions of the theory and exploitation of high energy symmetries can be used to restrict (or maybe understand better) the acceptable solutions/vacua and thus the landscape of the theory in a top down approach.

        For example the expected high energy symmetries (also in relation to Vasiliev’s Higher Spin algebra), efforts for a non perturbative defition via String field theory (e.g. around Ashoke Sen’s conjectures in open string field theory); or what about insights relevant to 4d SUSY theories obtained via e.g. AGT 4d/2d correspondence, Gaiotto dualities and 6d (2,0) SCFT and what these can teach us?

        I’m not saying that there are ready answers of course but on the other hand I see very important and sometimes ground breaking developments on the theoretical front but no substantial efforts to understand their implications (if any) from phenomenological point of view.

        What I see only are efforts to adjust the same low energy perturbative soloultions (with the same few non perturbative elements) to SM + SUSY +dS or inflation from a strict 4d bottom up point of view.

        But maybe I’m wrong this is just a feeling I have.

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        1. 4gravitonsandagradstudent Post author

          Yeah, I get the same impression, string pheno doesn’t seem to make much use of more modern methods.

          On the other hand, there are people involved in string pheno who are certainly aware of this stuff, Shamit Kachru for instance. So it may be that there just aren’t any useful applications of most of it in a string pheno context.

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  4. ohwilleke

    It is worth noting that the flip side of non-empirical confirmation is non-empirical rejection of theories. Why? Because lots of empirically driven theories that we use on a day to day basis can either be ruled out as true theories of Nature, or at least can’t be rigorously shown to meet basic non-empirical tests (e.g. mathematical consistency).

    For example, we know that the Standard Model can be rejected as a true theory of Nature because its rules reflect special but not general relativity. The mathematics of general relativity is full of infinities that probably reflect flaws in its classical description of a true quantum gravity theory, and some of those infinities, like black holes and the Big Bang, are not mere technicalities that arise far from the areas where the theory is applicable. Needless to say, the Standard Model and General Relativity viewed combined as our best working model of a TOE are mutually inconsistent with each other. From a non-empirical basis, the fact that the path integral in the photon propogator in QED including amplitudes for paths at photon speeds other than “c” looks deeply problematic even though the empirically established formula does just that.

    Feynman famously suspected that renormalization, in addition to not being rigorously proven to be a mathematically valid tool at the time he played a central role in developing it, was actually invalid at a rigorous mathematical level. I don’t know if anyone has subsequently proven that in fact it is mathematically valid in a rigorous way, but it was a technique widely used at a time when it could have been shown invalid by non-empirical means.

    Now, admittedly, most of these non-empirical rejections can be tempered if the theories in question are offered up with a formally stated domain of applicability and standard of precision. Then one merely need to show that any non-empirical rejections only apply outside the stated domain of applicability and standards of precision. But, it is still illuminative to be aware that non-empirical methods can conceivably produce false negatives as well as false positives.

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    1. 4gravitonsandagradstudent Post author

      Regarding renormalization, the impression I had is that defining it rigorously is one of the main obstructions to the program of Constructive Quantum Field Theory. So in a sense, that is a potential area where non-empirical theory falsification could yet play a role.

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  5. Pingback: The act of renormalization | Physics Forums - The Fusion of Science and Community

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