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    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeMar 24th 2014
    • (edited Mar 24th 2014)

    I kept being annoyed about the nature of discussion of the “multiverse” (the one in cosmology, not the one in set theory). Now I thought instead of steadily being annoyed, I should start an nnLab entry that does it better. So I did now (or tried to), at multiverse.

    • CommentRowNumber2.
    • CommentAuthorspitters
    • CommentTimeMar 24th 2014

    You avoid the many worlds interpretation. I guess that this is intensional?

    There is a recent topos theorectic treatment of Deutch multiverses by Guts. Unfortunately, I have not had the time to look at either.

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeMar 25th 2014
    • (edited Mar 25th 2014)

    Regarding the “Many Worlds Interpretation” (MWI): that this even exists as a meme in the academic world, together with the meta-meme that it is related to the “multiverse”-meme is one of the things that I find so annoying about the multiverse discussion. Together with “Boltzmann brains” and their ilk, I regard this as the kind of stories that maybe high school kids would entertain themselves with at campfire before they learn how to think more professionally and scientifically.

    For what is MWI? It’s just this: once upon a time Richard Feynman famously realized that given a mechanical system with configuration space PP, so with space of histories the path space P IP^I, then the quantum time evolution of the system may be regarded as a path integral which assigns to each point in P IP^I (a “history”) a probability amplitude, adds these all up (that’s subtle, but not the issue we are discussing here) and declares that the result is the total probability amplitude for the system to evolve from some time instant to the next.

    Now to look at this path space P IP^I and exclaim “these are MANY WORLDS!” seems to me to be a reaction of the kind only people unfamiliar with mathematical abstraction would show.

    In fact we might instead have a stochastic system and instead of path-integrating complex probability amplitudes we would just be adding up probability densities and produce the expectation value for the evolution of the system. Mathematically this is almost exactly the same as before.

    There is a silly old joke about the nature of quantum mechanics which goes as follows. Silly and old as it is, it seems to me to be at least as clever as the many worlds interpretation. The “joke” goes like this:

    Classical mechanics says that the physical world is intrinsically lazy: it always takes the path of least action.

    Quantum mechanics adds to this that the physical world is also intrinsically stupid: it tries every other path first.

    Finally regarding that article which you point to: this is plain nonsense built on an elemenry misunderstanding of the concept of SDG. Check it out…

    • CommentRowNumber4.
    • CommentAuthorTodd_Trimble
    • CommentTimeMar 25th 2014

    Wow, Urs. I don’t follow this type of interpretive discussion much, but it sounds like you are saying that proponents of MWI (DeWitt, Deutsch, Tegmark, maybe also Hawking, and obviously Hugh Everett) are just being idiotic here, or are being at the least unprofessional and unscientific. That sounds pretty strong!

    My own guess would have been that MWI is purely speculative and doesn’t lead to any interesting testable predictions, but that it is mainly a proposed solution to the problem of wavefunction collapse. Is that a correct understanding?

    • CommentRowNumber5.
    • CommentAuthorUrs
    • CommentTimeMar 25th 2014

    I wish argument by authority in such matters would still work these days.

    Since Bas asked, I stated my opinion. We should not discuss this matter further, though, because that’s just how this virus works.

    • CommentRowNumber6.
    • CommentAuthorTodd_Trimble
    • CommentTimeMar 25th 2014

    Urs, as if there there could possibly any doubt, I regard you as an authority on quantum theory (otherwise I wouldn’t ask you follow-up questions). Or are you the authority you are referring to?

    I just didn’t know if you really meant for it to sound as harsh as it did. We don’t have to discuss it here, if it makes you uncomfortable. Should we chance to meet in person in the future, I can ask you in a private conversation.

    • CommentRowNumber7.
    • CommentAuthorUrs
    • CommentTimeMar 25th 2014
    • (edited Mar 25th 2014)

    Hi Todd,

    sorry for being unclear. What I meant was that unfortunately the list of names which you mentioned (presumeably regarding them as authorities) doesn’t help me seeing how MWI is not a discussion counting angels on pins.

    (I just checked on Wikipedia here and learned that apart from an occurence in writings of Thomas Aquina there is no indication that medieval scholastics actually debated issues of this sort. So my feeling that some contemporary physicists are trying to get us back to the middle ages is actually unfair… to the middle ages. ;-)

    But let’s leave it at that. If you or anyone else here has interest in discussing MWI and maybe writing about it on the nnLab, then I won’t interfere. Above I just tried to explain myself why I didn’t mention it. And also I don’t want to indirectly be the cause of more discussion of MWI, as I think it is just not worthy of serious discussion, just as for instance most conspiracy theories are not. These are just meme-viruses rotating in people’s brains and keeping them from having substantial thoughts.

    • CommentRowNumber8.
    • CommentAuthorspitters
    • CommentTimeMar 25th 2014

    Hi Urs, thanks for confirming my suspicion! I followed the wikipedia article on “multiverse” and was a little surprised to find the MWI linked to what you were writing about.

    • CommentRowNumber9.
    • CommentAuthorTodd_Trimble
    • CommentTimeMar 25th 2014

    doesn’t help me seeing how MWI is not a discussion counting angels on pins.

    This makes me smile, and expresses well my gut instinct about the matter. So, no worries.

    This is sort of à propos: a while back I was reading a nice post by Andrej Bauer on intuitionistic mathematics and physics (I see he also mentions David Deutsch! and some of the wrong ideas he has). Particularly, what counts as evidence of truth, and what the difference is between a proposition and its double negation for an intuitionist:

    That is a bit complicated. In essence, it says that ¬¬ϕ\neg\neg\phi is accepted when there is no evidence against it. In other words, ¬¬ϕ\neg\neg\phi means something like “ϕ\phi cannot be falsified” or “ϕ\phi is potentially true”. For example, if someone says

    “There is a particle which does not interact with anything in the universe.”

    that would be a statement which is not accepted as true, for how would you ever present positive evidence? But it is accepted as potentially true, for how would you ever falsify it?

    Similarly, MWI strikes me as one of these things that is unfalsifiable – but what could count as showing that it’s actually “true”?

    • CommentRowNumber10.
    • CommentAuthorUrs
    • CommentTimeMar 25th 2014
    • (edited Mar 25th 2014)

    This makes me smile, and expresses well my gut instinct about the matter.

    Thanks, Todd. Sometimes in this context I have an eery feeling of loneliness.

    This is a phenomenom of our days (starting pretty much with the turn of the millenium, actually) which one doesn’t read much about in the newspapers, and which is maybe unwise for non-tenured people to go on about too much, but it is nevertheless true: among the perceived big-shots in the fundamental physics community these days there is a striking amount of fundamental nonsense.

    With MWI I don’t even feel it’s unfalsifiable. For me, it doesn’t even parse.

    This is a shame, because on the other hand it is true that there are deep interesting problems to think about if the fundamental theory of nature is intrinsically probabilistic. I just wish this discussion would be had in a substantial and interesting manner.

    Where are the professional philosophers of physics who would help poor Hawking with his philosophy-is-dead-attitude and only naive QFT-lore to replace it with?

    In fact I asked this question over on Philosophy.SE, too. :-)

    • CommentRowNumber11.
    • CommentAuthorDavid_Corfield
    • CommentTimeMar 25th 2014

    My understanding of the “Angels on head of pin” issue was that it was part of the campaign of anti-scholasticism of the 17th century to ridicule mediaeval ’Schools’ philosophy/theorlogy. I think Aquinas did wonder whether angels could coexist in the same space, not a crazy question if you believe in angels. Mind you, he did claim to know about the 9 ranks of angels,

    • 1st hierarchy: Seraphim, Cherubim, Thrones.
    • 2nd hierarchy: Dominions, Virtues, Powers.
    • 3rd hierarchy: Principalities, Archangels, Angels.

    Descartes mocks him for speaking with such certainty on the subject – it was as though Aquinas lived among them.

    • CommentRowNumber12.
    • CommentAuthortonyjones
    • CommentTimeMar 25th 2014
    • (edited Mar 25th 2014)
    David Wallace is a philosopher of physics who has written a book called the 'Emergent Multiverse'.
    P.S. Note that Wallace has a Phd in both physics and philosophy
    • CommentRowNumber13.
    • CommentAuthorDavidRoberts
    • CommentTimeMar 25th 2014
    • (edited Mar 25th 2014)

    Is the general-physics arXiv category (wherein lies the Guts article) like math.GM?

    • CommentRowNumber14.
    • CommentAuthorDavid_Corfield
    • CommentTimeMar 26th 2014

    Re #12, is there a way of disambiguating multiverse in the sense we have it from its use by proponents of MWI, without giving much airspace to MWI?

    Like Urs, I have always thought that interpretation to be nonsensical.

    • CommentRowNumber15.
    • CommentAuthorUrs
    • CommentTimeMar 26th 2014
    • (edited Mar 26th 2014)

    Yeah, the step from the word “universe” to “multiverse” in cosmology is already unfortunate (that’s what I tried to sort out a bit in the entry multiverse), but okay. But when did it happen that the term was then also used for the space of all trajectories in configuraton space? That’s just not sensible.

    I wonder if those interested in “many worlds” realize that the relevant space, the one which I denoted P IP^I above contains not just the beloved “infintely many copies of oneself, leading different lives” which MWI discussion likes to be about. Instead it is dominated by entirely random “worlds” with no laws or regularity of any sort.

    Particle physics text usually do warn that the mathematical device of “virtual particles” is really just that, a mathematical device. This is a small aspect of the “many worlds” issue, and this warning carries over to the general situation all the more.

    • CommentRowNumber16.
    • CommentAuthorDavid_Corfield
    • CommentTimeMar 26th 2014

    We’re reading Carnap’s Empiricism, Semantics, and Ontology in a postgrad seminar today. Interesting to read it through HoTT spectacles. His main thought is that just because something is expressible in your linguistic system doesn’t mean that you’re committed to the real existence of entities designated by terms of your system. I think he’d accuse MWI people of making this mistake.

    When it comes to what he calls ’external questions’, these are not about statements of fact, but are about the pragmatic benefits of adopting this or that system. E.g., just because I include arithmetic in my system, and have a term ’five’, doesn’t make it sensible to ask outside the system “But does ’five’ designate something that really exists?”

    • CommentRowNumber17.
    • CommentAuthorUrs
    • CommentTimeMar 26th 2014
    • (edited Mar 26th 2014)

    Thanks, David, yes. And the main question is maybe: what does “real existence” actually mean; and will we be able to decide by talking about it in informal English; and if we do so, why does this end up on the physics arXiv??

    It is a common place in linguistics etc. that the language one speaks shapes the perception that one has of the world. Why then do we assume that everyday language is going to help us with speaking about highly mathematical physical theories. I suspect that if one instead adopts a suitable formal language for these (some flavor of linear logic…) then the apparent problems will just disappear and instead useful and substantial statements will appear.

    • CommentRowNumber18.
    • CommentAuthorUrs
    • CommentTimeMar 26th 2014

    I have added to “multiverse” under a new section headline “Critique” the following quotes form

    • George Ellis, Does the multiverse really exist?, Scientific American, August 2011 (web):

    Similar claims [about a multiverse] have been made since antiquity by many cultures. What is new is the assertion that the multiverse is a scientific theory, with all that implies about being mathematically rigorous and experimentally testable. I am skeptical about this claim. I do not believe the existence of those other universes has been proved—or ever could be. Proponents of the multiverse, as well as greatly enlarging our conception of physical reality, are implicitly redefining what is meant by ‘science.’—pg 39

    For a cosmologist, the basic problem with all multiverse proposals is the presence of a cosmic visual horizon. The horizon is the limit to how far away we can see, because signals traveling toward us at the speed of light (which is finite) have not had time since the beginning of the universe to reach us from farther out. All the parallel universes lie outside our horizon and remain beyond our capacity to see, now or ever, no matter how technology evolves. In fact, they are to far away to have had any influence on our universe whatsoever. That is why none of the claims made by multiverse enthusiasts can be directly substantiated. —pg 40-41

    A remarkable fact about our universe is that physical constants have just the right values needed to allow for complex structures, including living things. Steven Weinberg, Martin Rees, Leonard Susskind and others contend that an exotic multiverse provides a tidy explanation for this apparent coincidence: if all possible values occur in a large enough collection of universes, then viable ones for life will surely be found somewhere. This reasoning has been applied, in particular, to explanation the density of the dark energy that is speeding up the expansion of the universe today. I agree that the multiverse is a possible valid explanation for the value of this density; arguably, it is the only scientifically based option we have right now. But we have no hope of testing it observationally. —pg 42

    All in all, the case for the multiverse is inconclusive. The basic reason is the extreme flexibility of the proposal: it is more a concept than well-defined theory. Most proposals involve a patchwork of different ideas rather than a coherent whole. The basic mechanism for eternal inflation does not itself cause physics to be different in each domain in a multiverse; for that, it needs to be coupled to another speculative theory. Although they can be fitted together, there is nothing inevitable about it. … Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is. —pg 43

    As skeptical as I am, I think the contemplation of the multiverse is an excellent opportunity to reflect on the nature of science and on the ultimate nature of existence: why we are here… In looking at this concept, we need an open mind, though not too open. It is a delicate path to tread. Parallel universes may or may not exist; the case is unproved. We are going to have to live with that uncertainty. Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is.

    • CommentRowNumber19.
    • CommentAuthorUrs
    • CommentTimeApr 14th 2019
    • (edited Apr 14th 2019)

    As I was checking cross-links from here to set-theoretic multiverse, I was reminded of lack of content here. On a whim, I ended up writing the following:


    In physics, the term “multiverse” refers to certain picture of cosmology. The intended meaning tends to differ between authors and/or remain vague (see below) but broadly what goes with it is the idea that, just as the configuration of matter and energy changes from place to place within our observable universe, clearly, it could be that beyond the observable universe (for immediate or not so immediate notions of “beyond”) eventually even the laws of physics might change from place to place, possibly altogether, but at least as concerns the values of coupling constants or even field-content.

    The idea remains hypothetical, naturally, and might ultimately be empty as an idea about physics as opposed to philosophy, but it is arguably in historical continuation with a sequence of famously confirmed insights about cosmology, which led from an archaic picture of a cosmos centered all around the world directly experienced by humans (the continent a disk inside an ocean hung in the center of the heavenly spheres), via the Copernican revolution, then the confirmation of distant galaxies etc., to the realization that the observable universe is immense and varied and our physical place and whereabouts inside it just a random result of chance, subject only to the tautological condition that it allows us to be here at all – known as the “anthropic principle” in these discussions.

    Part of the motivation of contemplating a multiverse even in the face of its ontological elusiveness comes from attempts of formulating fundamental theories of physics such as quantum gravity or theories of everything, since the motivation and success of such theories depends to some extent on what one thinks they can or should explain or predict, and how much room for chance and chaos there may remain in a truly fundamental theory of physics.

    For example, if a fundamental theory of everything were to predict, say, the generations of fundamental particles and the values of the Yukawa couplings in the standard model of particle physics, then these would be fixed – by pure logic, ultimately – to be what they are, and would be the same “everywhere” in the universe, making obsolete the idea of a multiverse.

    Part of the interest in the idea of the multiverse derives from the feeling that this scenario of the standard model of particle physics, with its particular field content and coupling constants, being fixed by pure logic, is no more likely than Johannes Kepler’s attempt to derive the distances in the solar system from the Platonic solids (the old Music of the Spheres), which today we understand is a confusion of what is fundamental law and what is random initial conditions.

    diff, v10, current

    • CommentRowNumber20.
    • CommentAuthorUrs
    • CommentTimeApr 14th 2019

    should have added one more line (done now):

    Of course, at this point, it might as well be the other way around, see also at universal exceptionalism.

    diff, v11, current

    • CommentRowNumber21.
    • CommentAuthorTodd_Trimble
    • CommentTimeApr 14th 2019

    These articles are really nicely written; thanks.

    • CommentRowNumber22.
    • CommentAuthorDavid_Corfield
    • CommentTimeApr 14th 2019
    • (edited Apr 14th 2019)

    Perhaps worth distinguishing ’multiverse’ from the ’many worlds’ interpretation of QM, associated with Hugh Everett. I’ve never thought the latter idea at all plausible, but there are proponents. See at Quanta and SEP.

    I see we mention ’many worlds’ at quantum operation.

    [Edit: I see we’ve already spoken of this above.]

    • CommentRowNumber23.
    • CommentAuthorDavid_Corfield
    • CommentTimeApr 14th 2019

    More interestingly, say the universal exceptionalist line is right (maybe through a cohomotopic M-theory), how then to think about what satisfied some physicists about the multiverse, that is resolves the fine-tuning problem, that the constants are just right for life to be possible, etc.?

    • CommentRowNumber24.
    • CommentAuthorDavid_Corfield
    • CommentTimeApr 15th 2019

    I added a paragraph on fine-tuning.

    diff, v13, current

    • CommentRowNumber25.
    • CommentAuthorUrs
    • CommentTimeApr 15th 2019

    Thanks Todd, thanks David.

    David, I moved your paragraph up to right after the mentioning of the anthropic principle that I had there, prefixed it by “Accordingly, one motivation for…”, and then made my previously following paragraph start out with “Another motivation for…”. This way it seems to fit the flow of the logic better.

    Eventually I might want to bring in the following thought:

    In hindsight we know that

    a) Giordano Bruno was completely correct with his multi-solar-system hypothesis

    (and strikingly so: as my exoplanet science colleagues here emphasize to me, the observations of the last years strongly suggest that the fraction of stars with at least one planet is pretty much unity. Because, so far pretty much every single star they aimed their detectors at eventually revealed a planet orbiting it.)

    but

    b) no amount of debate at Bruno’s time could have decided this question either way. What they needed for progress on the matter were… better theory and better observations.

    Today we are in exactly the same situation with the multiverse, just a few steps further down the road.

    The notorious debates about fine-tuning/naturalness suffer from not being rooted in an actual theory, see at naturalness – Dependency on a renomelization scheme, much like Bruno’s speculations were not rooted in any theory.

    It’s thought provoking that of all combatants on the matter out there, it is Gordon Kane to highlight this point here:

    The alternative to naturalness, often neglected as an alternative, is having a theory.

    For instance, I used to say that string theory comes with no intrinsic preference for the famous class of KK-compactifications to N=1N =1 in D=4D = 4, i.e. for F-theory on Spin(7)-manifolds, M-theory on G2-manifolds and string theory on CY3-manifolds. Which matters for the multiverse/landscape discussion, because it is only after assuming these types of compactification in the first place that the arguments for a discrete set of string vacua apply.

    And I still think that it’s true: plain string theory as known today has no logic/mechanism to single out these compactifications. But then we realized that Cohomotopy does single out precisely these. More on this tonight. :-)

    • CommentRowNumber26.
    • CommentAuthorDavidRoberts
    • CommentTimeApr 15th 2019

    More on this tonight. :-)

    Ooh! I can’t wait…

    • CommentRowNumber27.
    • CommentAuthorDavid_Corfield
    • CommentTimeApr 15th 2019

    …of all combatants on the matter out there, it is Gordon Kane…

    What do I need to know about Gordon Kane to make sense of this? Is he unusual in some respect relative to other “combatants”?

    • CommentRowNumber28.
    • CommentAuthorDavid_Corfield
    • CommentTimeApr 15th 2019

    Perhaps it’s because he works more at the phenomenological end of things rather than the theoretical?

    • CommentRowNumber29.
    • CommentAuthorUrs
    • CommentTimeApr 15th 2019
    • (edited Apr 15th 2019)

    He has drawn flak for iteratively claiming definite predictions for superpartner masses motivated from the G2-MSSM, which makes it easy for combatants not so inclined to systematic top-down theoretical argument to dismiss – or mainly: ignore – his actual point (see e.g. here). This is a little tragic, because even if his numbers end up experimentally falsified, they are more interesting than most numbers thrown around in these discussions for their perceived “natural” value, as his do come from hypothesizing a coherent theoretical framework and then trying to extract consequences from there.

    • CommentRowNumber30.
    • CommentAuthorDavid_Corfield
    • CommentTimeApr 15th 2019
    • (edited Apr 15th 2019)

    I see, thanks.

    Re #25, so you may be about to fill in:

    Some compactifications, such as M-theory compactified on a ‘G 2G_2 manifold’, or some Calabi–Yau compactifications, automatically give the 4D supersymmetric world naturally.

    One could hope that the string theories somehow automatically lead to that projection, and to the correct one. No one has yet figured out how the theory might do that… (String theory and the real world, 2-1)

    • CommentRowNumber31.
    • CommentAuthorUrs
    • CommentTimeApr 15th 2019
    • (edited Apr 15th 2019)

    How do you find reading the book? I thought it is the most clear-headed while up-to-date non-technical account available.

    • CommentRowNumber32.
    • CommentAuthorDavid_Corfield
    • CommentTimeApr 17th 2019

    Yes, it’s the clearest account I’ve read of how M-theory/string theory can be expected to make contact with acquirable evidence.

    Nature has played a cruel trick on physicists, and on journalists and bloggers. The Higgs sector data look like a Higgs sector would look if it were a pure Standard Model Higgs, with one complex doublet. But it is not that, and could not be that because of the hierarchy problem, and also because of the stability of the vacuum. (6-8)

    Shouldn’t physicists know not to be so tricked?

    I come across ’sector’ talk quite a lot, as here:

    Another new way in which the physics of compactified theories enters is by the presence of ‘hidden sectors’, which are very important. One can think of them as arising as follows. When the string/M-theory is compactified, it does not give just one 4D theory that describes our world, it gives many possible ones. We do live on one of them, which we call the visible sector. (4-5)

    I’m not sure I could quite say what a sector is. We have a stub for charge sector, which appears in any case to be a synonym for instanton sector. Then there’s hidden sector. A page such as string theory FAQ has ’Higgs boson sector’, ’topological sector’ and ’two special sectors of first quantized string theory’.

    So a sector arises from some kind of splitting into parts?

    • CommentRowNumber33.
    • CommentAuthorDavid_Corfield
    • CommentTimeApr 17th 2019

    So are ’topological sector’ and ’instanton sector’ used to refer to the images of certain (epimorphic) maps? Then I guess one particular such sector is still a subpart of a whole, one equivalence class amongst all such classes. But still ’the topological sector of a field’ is used more in an ’underlying’ sense?

    • CommentRowNumber34.
    • CommentAuthorUrs
    • CommentTimeMay 17th 2019

    In re-reading the entry, I made a few minor changes of wording, added more hyperlinks, and then ended up adding a paragraph of warning (right before the section “Critique”) about the “landscape of string theory vacua” not being well understood.

    diff, v16, current

    • CommentRowNumber35.
    • CommentAuthorUrs
    • CommentTimeMay 19th 2019

    expanded the last sentence if the lead-in paragraphs as follows:

    In this sense, the idea of the multiverse is a form of empiricism and in contrast to theoretical aestheticism such as universal exceptionalism.

    diff, v17, current

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