The Multiverse Can Only Kill Physics by Becoming Physics

I’m not a fan of the multiverse. I think it’s over-hyped, way beyond its current scientific support.

But I don’t think it’s going to kill physics.

By “the multiverse” I’m referring to a group of related ideas. There’s the idea that we live in a vast, varied universe, with different physical laws in different regions. Relatedly, there’s the idea that the properties of our region aren’t typical of the universe as a whole, just typical of places where life can exist. It may be that in most of the universe the cosmological constant is enormous, but if life can only exist in places where it is tiny then a tiny cosmological constant is what we’ll see. That sort of logic is called anthropic reasoning. If it seems strange, think about a smaller scale: there are many planets in the universe, but only a small number of them can support life. Still, we shouldn’t be surprised that we live on a planet that can support life: if it couldn’t, we wouldn’t live here!

If we really do live in a multiverse, though, some of what we think of as laws of physics are just due to random chance. Maybe the quarks have the masses they do not for some important reason, but just because they happened to end up that way in our patch of the universe.

This seems to have depressing implications. If the laws of physics are random, or just consequences of where life can exist, then what’s left to discover? Why do experiments at all?

Well, why not ask the geoscientists?


These guys

We might live in one universe among many, but we definitely live on one planet among many. And somehow, this realization hasn’t killed geoscience.

That’s because knowing we live on a random planet doesn’t actually tell us very much.

Now, I’m not saying you can’t do anthropic reasoning about the Earth. For example, it looks like an active system of plate tectonics is a necessary ingredient for life. Even if plate tectonics is rare, we shouldn’t be surprised to live on a planet that has it.

Ok, so imagine it’s 1900, before Wegener proposed continental drift. Scientists believe there are many planets in the universe, that we live in a “multiplanet”. Could you predict plate tectonics?

Even knowing that we live on one of the few planets that can support life, you don’t know how it supports life. Even living in a “multiplanet”, geoscience isn’t dead. The specifics of our Earth are still going to teach you something important about how planets work.

Physical laws work the same way. I’ve said that the masses of the quarks could be random, but it’s not quite that simple. The underlying reasons why the masses of the quarks are what they are could be random: the specifics of how six extra dimensions happened to curl up in our region of the universe, for example. But there’s important physics in between: the physics of how those random curlings of space give rise to quark masses. There’s a mechanism there, and we can’t just pick one out of a hat or work backwards to it anthropically. We have to actually go out and discover the answer.

Similarly, we don’t know automatically which phenomena are “random”, which are “anthropic”, and which are required by some deep physical principle. Even in a multiverse, we can’t assume that everything comes down to chance, we only know that some things will, much as the geoscientists don’t know what’s unique to Earth and what’s true of every planet without actually going out and checking.

You can even find a notion of “naturalness” here, if you squint. In physics, we find phenomena like the mass of the Higgs “unnatural”, they’re “fine-tuned” in a way that cries out for an explanation. Normally, we think of this in terms of a hypothetical “theory of everything”: the more “fine-tuned” something appears, the harder it would be to explain it in a final theory. In a multiverse, it looks like we’d have to give up on this, because even the most unlikely-looking circumstance would happen somewhere, especially if it’s needed for life.

Once again, though, imagine you’re a geoscientist. Someone suggests a ridiculously fine-tuned explanation for something: perhaps volcanoes only work if they have exactly the right amount of moisture. Even though we live on one planet in a vast universe, you’re still going to look for simpler explanations before you move on to more complicated ones. It’s human nature, and by and large it’s the only way we’re capable of doing science. As physicists, we’ve papered this over with technical definitions of naturalness, but at the end of the day even in a multiverse we’ll still start with less fine-tuned-looking explanations and only accept the fine-tuned ones when the evidence forces us to. It’s just what people do.

The only way for anthropic reasoning to get around this, to really make physics pointless once and for all, is if it actually starts making predictions. If anthropic reasoning in physics can be made much stronger than anthropic reasoning in geoscience (which, as mentioned, didn’t predict tectonic plates until a century after their discovery) then maybe we can imagine getting to a point where it tells us what particles we should expect to discover, and what masses they should have.

At that point, though, anthropic reasoning won’t have made physics pointless: it will have become physics.

If anthropic reasoning is really good enough to make reliable, falsifiable predictions, then we should be ecstatic! I don’t think we’re anywhere near that point, though some people are earnestly trying to get there. But if it really works out, then we’d have a powerful new method to make predictions about the universe.


Ok, so with all of this said, there is one other worry.

Karl Popper criticized Marxism and Freudianism for being unfalsifiable. In both disciplines, there was a tendency to tell what were essentially “just-so stories”. They could “explain” any phenomenon by setting it in their framework and explaining how it came to be “just so”. These explanations didn’t make new predictions, and different people often ended up coming up with different explanations with no way to distinguish between them. They were stories, not scientific hypotheses. In more recent times, the same criticism has been made of evolutionary psychology. In each case the field is accused of being able to justify anything and everything in terms of its overly ambiguous principles, whether dialectical materialism, the unconscious mind, or the ancestral environment.


Or an elephant’s ‘satiable curtiosity

You’re probably worried that this could happen to physics. With anthropic reasoning and the multiverse, what’s to stop physicists from just proposing some “anthropic” just-so-story for any evidence we happen to find, no matter what it is? Surely anything could be “required for life” given a vague enough argument.

You’re also probably a bit annoyed that I saved this objection for last. I know that for many people, this is precisely what you mean when you say the multiverse will “kill physics”.

I’ve saved this for last for a reason though. It’s because I want to point out something important: this outcome, that our field degenerates into just-so-stories, isn’t required by the physics of the multiverse. Rather, it’s a matter of sociology.

If we hold anthropic reasoning to the same standards as the rest of physics, then there’s no problem: if an anthropic explanation doesn’t make falsifiable predictions then we ignore it. The problem comes if we start loosening our criteria, start letting people publish just-so-stories instead of real science.

This is a real risk! I don’t want to diminish that. It’s harder than it looks for a productive academic field to fall into bullshit, but just-so-stories are a proven way to get there.

What I want to emphasize is that we’re all together in this. We all want to make sure that physics remains scientific. We all need to be vigilant, to prevent a culture of just-so-stories from growing. Regardless of whether the multiverse is the right picture, and regardless of how many annoying TV specials they make about it in the meantime, that’s the key: keeping physics itself honest. If we can manage that, nothing we discover can kill our field.


15 thoughts on “The Multiverse Can Only Kill Physics by Becoming Physics

  1. Peter Woit

    “We all want to make sure that physics remains scientific.”
    This is true, but the problem with this heart-warming thought is that some parts of the theoretical physics community want to change the definition of what is “scientific”. This is what others need to push back on, and unfortunately I don’t see that happening much at all. The list of prominent theorists pushing the anthropic landscape as science is a very long one now, starting alphabetically with Arkani-Hamed. Very few seem to be willing to publicly challenge this (among prominent theorists, hard to come up with names besides David Gross). Yes, you’re right that keeping theoretical physics intellectually honest is key. This means engaging in a difficult process of challenging smart people who are fooling themselves. I hope we’ll see that happen, but the current situation is rather bleak.


    1. 4gravitonsandagradstudent Post author

      So, this response also addresses ohwilleke’s comment:

      I think we have to draw a distinction between silly things people say in public and what they actually publish. As I said in this post, I don’t think it’s inherently impossible to do science with anthropic reasoning. I do think it’s potentially very easy to trick yourself into thinking you’re doing science with anthropic reasoning when you’re not.

      The remedy for this isn’t to call out every silly TV special or every wild comment made to a journalist. It’s to focus on the actual science, paper by paper, on whether or not that specific paper is a just-so-story or whether it’s actually predicting something falsifiable.

      ohwilleke asked, how do we stop them when others hold the purse strings? We stop them paper by paper. If a paper is unscientific, we reject it in peer review. If it gets through, we decline to cite it. If someone spends their career doing it, we don’t hire them and we don’t give them tenure. We may not “hold the purse strings”, but everything governments use to direct their money is something we can influence. (Private foundations are admittedly another matter, but while growing they’re still not the bulk of funding in the field by a long shot.)


      1. Peter Woit

        It’s not so simple. The problem is that if you stopped allowing the publication of papers that don’t predict something falsifiable, the theoretical physics literature would be very small indeed. Most of what people work on is trying to better understand poorly understood ideas, not having falsifiable predictions, but hoping that with better understanding these ideas may someday lead to falsifiable predictions. You can’t cleanly and easily distinguish science/non-science with the falsifiability criterion. In the case of the multiverse, people working in this area will assure you that they are, just like everyone else, hoping to find falsifiable predictions. You have to look into exactly what they are doing and evaluate whether they have a plausible case or whether they are fooling themselves.

        On the whole, I don’t think the scientific community takes seriously most of those who appear on TV specials and what they say there. University administrations are sometimes a different story.

        A significant aspect of the pro-multiverse campaign is not the outreach to the public, but the pursuit of this campaign through colloquium talks, talks at conferences, review articles, etc., all aimed at the physics community. Often this campaign involves specifically “answering the criticism” of multiverse research, although these answers are usually to straw man arguments, not the serious ones. I mentioned Arkani-Hamed because he’s a main offender: he’s very smart and very convincing as a speaker, gives a huge number of talks to large groups of physicists, and has a lot of impact.

        One of the main places that the community is supposed to hold the line against pseudo-science is by the peer-review panels at the granting agencies. A major goal of the pro-multiverse campaign is to win these over and I think this campaign is having a lot of success. Try searching for the term “multiverse” in NSF grant abstracts and I think you’ll find that before a few years ago it was unheard of, with peer-review panels very likely to shoot down any proposal invoking something widely believed to be pseudo-science. In the past few years this has changed and you’ll find successful proposals invoking the “multiverse”. The problem is very real and with us right now.


        1. 4gravitonsandagradstudent Post author

          Yes, falsifiability alone is the wrong criterion, for the reasons you describe. To clarify, I was thinking of applying it in cases where the paper is already claiming to be doing phenomenology, and not just in the “exploring possibilities” sense. Most theoretical work doesn’t look like that, but I don’t really see that as a problem: what we’d really want to stop is not anyone working on the multiverse at all, but specifically people claiming to have figured out why some situation or another holds when they haven’t actually made a falsifiable prediction about it. Just-so-stories, essentially. I get the impression that you’d disapprove of a much broader range of multiverse-related work than I would.

          At the outset I would have been surprised if there were all that many “multiverse” grants since there are so few “multiverse” papers. I also hadn’t realized NSF grant abstracts were searchable.

          Looking at it now, there are 15 grants with “multiverse” somewhere in the abstract or title. One is actually “multiversion” so we can ignore it, so 14. Of these 7 are currently active. To give a scale, searching for “string theory” gives 1184 grants, of which 186 are currently active. Just for fun I also checked “scattering amplitudes” which has 57 grants, 9 active (which on the surface sounds a bit depressing but many of the US people who do amplitudes are DOE-funded).

          The 14 grants are spread out over 9 principal investigators. You’re right in pointing out that all of them are “recent”, nothing earlier than 2009. Several are grants for a group of people where only one of them is actually working on the multiverse, but there are a few targeted to specific multiverse projects.

          What I do notice, though, is that a lot of these people have been working on something multiverse-related for a long time. Vilenkin, for example, is practically synonymous with eternal inflation, and the people working on flux compactifications in these grants have mostly been working on it from early on. So while these people only started mentioning the multiverse on their grant applications in 2009, I don’t think that means they’re suddenly getting away with doing research on the taxpayer’s dime that they weren’t before. Vilenkin’s 2004 abstract describes basically the same subject, it just doesn’t use the word “multiverse”. It doesn’t hide it either, it still discusses eternal inflation and dark energy varying from region to region. If anything it looks less like multiverse research had suddenly become acceptable and more like they suddenly could count on grant committees recognizing the word “multiverse”.

          This makes me wonder, was there some watershed event in multiverse popularity in 2008? Some landmark popularization piece? I wasn’t really following the field at the time. Do you remember something that might have triggered it?

          Anyway, this is all a bit of a digression. As I said, I think the core of our disagreement is that there’s a lot of multiverse research that I don’t think of as actively toxic, just unproductive. There’s a uniquely toxic dynamic with “just-so-story” fueled fields where everything gets cited as “evidence” of the field’s core assumptions and every observation gets given a pulled-from-thin-air “explanation” that gets accepted essentially without question. The current state of multiverse research (as opposed to popularization) is so much more tentative about this kind of thing, everything labeled as preliminary and buried in caveats, that it doesn’t seem like it’s anywhere near the toxic stage yet. But it’s possible I just haven’t seen the worst papers, is there one you’d point out as particularly dangerous?


  2. ohwilleke

    “If we hold anthropic reasoning to the same standards as the rest of physics, then there’s no problem: if an anthropic explanation doesn’t make falsifiable predictions then we ignore it. The problem comes if we start loosening our criteria, start letting people publish just-so-stories instead of real science.”

    I think that the clear and present danger is that this mindset will start driving how scarce physics research funds are allocated and undermine more fruitful pursuits. It is all good and well for physicists to ignore work that isn’t up to their standards, but physicists don’t hold the purse strings. The only way for physicists to prevent science that isn’t up to standards (and the multiverse is only one of many things in science today that isn’t) is to do more than be silent and try to ignore what isn’t satisfactory. If physicists don’t unite to make clear what isn’t making falsifiable predictions they will lose their funding to charlatans who are pushing that agenda.

    Liked by 1 person

  3. Giotis

    You must make the distinction between the laws of Physics which are incorporated in QFT and String theory and the space of solutions of these theories.

    The solutions are just that, solutions and they are numerous; the underlying laws/theories on the other hand are unique.

    You can do Physics either by further developing the theory and its foundations or by solving it and exploring landscape of the solutions.

    This is what people working in Theoretical Physics did in the past, are doing now and will continue to do in the future; nothing is peculiar or strange under the present circumstances.

    Now, did I mention the word “Multiverse” anywhere in the above?

    So all is good, irrelevant people should stop talking and let the people who can actually do Physics to continue their work.


  4. Dmitri

    “The only way for anthropic reasoning to get around this, to really make physics pointless once and for all, is if it actually starts making predictions. […] At that point, though, anthropic reasoning won’t have made physics pointless: it will have become physics.”

    I would be curious to get your take on Weinberg’s anthropic prediction of a small positive value for the cosmological constant. Does that meet the criterion stated above?


    1. 4gravitonsandagradstudent Post author

      I think it’s a decent suggestion that these methods can do something, but not really strong enough to justify anthropic reasoning on its own.

      Essentially, we already knew the cosmological constant was either zero or very small. Weinberg’s prediction was that the cosmological constant would be very small, but not zero. Most of that is retrodiction, only the specific claim that it should be nonzero was a prediction. And it’s not really clear that that’s a terribly unique prediction, it isn’t that often in physics that something turns out to be exactly zero.

      One way to look at it: if instead of through anthropic reasoning, someone predicted that the cosmological constant would be nonzero through some complicated phenomenological model, would that be viewed as good evidence for the model? Or would it be viewed as an argument, but not a fully convincing one? I think Weinberg’s cc prediction fails that criterion.


  5. Tom Andersen

    Continental drift is dual to physics.

    Continental drift was only accepted in the 1960s. Six decades passed from the correct theory being put forward – a simple obvious theory in hindsight. Physics has been in a hole for about 30 to 50 years. Dark Energy, Dark Matter, high Tc superconductivity, neutrino mass, etc – all failed to be predicted by physics. The multiverse is mainstream, as are dimensions > 4, the idea that 99% of particles have not been detected, the idea that it’s ok to have hundreds or more input variables. No experimental evidence for all those things, while the Astronomers keep shaming physics with new unpredicted discoveries.

    What’s staring the physics community in the face right now? I guess if someone can figure that out then progress can be made.


  6. Lubos Motl

    Lots of stuff written as “anthropic reasoning” is wrong, vacuous, wishful thinking, just bad. But some version or flavor extracted from this kind of reasoning may be needed in science of the future. Nobody knows for certain.

    It’s completely wrong to try to “ban” certain kinds of thinking because it’s insufficiently “scientific”, especially if some complete non-scientists, clueless men such as some of the commenters in this thread, would like to do this “work”. Scientists simply cannot be threatened by laymen, by politically or ideologically driven blackmailing etc. – “Aryan Physics” hasn’t achieved and couldn’t have achieved anything of lasting value. Even cornerstones of modern science such as quantum mechanics have been called unscientific by some people – including rather achieved people – just because they disagreed with their philosophical prejudices.

    The right attitude is to leave the contest to scientific meritocracy. Scientists, when they’re selected as capable of mastering the verified established science as we have known it and creative to do new one, simply naturally choose the most promising directions of research. They clump around it and focus their research on what seems most attractive, promising, or justifiable.


  7. Wyrd Smythe

    FWLIW (in such illustrious company) “anthropic reasoning” seems a bit of an oxymoron.

    The situation, as my simple mind sees it, is that a lottery company has only ever issued a single ticket (that we know about), and that ticket, against seemingly very high odds, is a big winner. This beggars the imagination, so we assume the lottery company must have issued billions of tickets we just don’t know about.

    As you put it: just-so story.

    If science is the study of what is, then we have plenty of is yet to study and observe, so I don’t have much fear of science ending, although areas of study may shift if roads finally peter out (I guess we’re still following what is now a dirt trail of supersymmetry).

    There’s nothing wrong with anyone, even scientists, speculating about what might be, but I think it’s a good idea to keep a firm grasp on what’s science and what’s science fiction. Multiverses are at least 95% science fiction.


    1. 4gravitonsandagradstudent Post author

      Yeah. I think the people who try to argue “unlikely parameters -> therefore multiverse” are jumping the gun in a big way. If there’s science there, it will be in using that logic to predict something unobserved…that is something people are at least trying to do, though I’m not optimistic about their chances.



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