Why Trust a Theory? — part II

Munich[Notes: what follows are only lightly edited notes taken while the meeting was in progress, so they are more likely than usual to contain typos and odd phrasing; also, apologies to my readers, but I may not be able to participate much to the hopefully interesting discussion at PlatoFootnote about this series of posts, I’m trying to also get some sleep here in Munich… Below and throughout, comments in brackets are mine.]

Continuing my coverage of the workshop on the current status of fundamental physical theory, organized by philosopher Richard Dawid at the Center for Mathematical Philosophy based at the Ludwig-Maximilians University in Munich, Germany, a conference that was prompted by a high profile, controversial position paper published in Nature by George Ellis and Joe Silk (who are co-organizers of the workshop), entitled “Defend the integrity of physics.”

The first speaker of the day was Bjorn Malte Schafer on “Dark gravity, dark matter, and dark statistics” Physical cosmology has three main building blocks: gravity, fluid mechanics, and statistics. Gravity is the most important force at cosmological scales. Fluid mechanics is important because of the concern with the formation of structures that are then described statistically. Gravity is “given” by general relativity.

Schafer pointed out that it is impossible to investigate different aspects of cosmology — different parameters describing physical models of cosmological structures — independently, because measurements usually combine them all. This is very different from what happens in particle physics.

Observations in cosmology are along the past light cone, focusing on the differential acceleration of objects moving along a geodesic relative to our geodesic, with special relativity determining the dynamics.

Known things: the Hubble constant, curvature, acceleration, isotropy of the metric, thermal history of the universe. Not so well known things: homogeneity. Interpreted things: matter density and cosmological constant, the latter “preferred” over dark energy [yeah, not sure what that means either…].

Structures are believed to be generated by inflation in the early universe. What we know is that the Gaussian assumption is well found and that the inflationary model “works.” Not so well known are the direct individual measurement of slow-roll parameters and the amplitude of gravitational waves. The effective statistical description of nonlinear structures is completely unknown.

We do know a number of things about cosmological structure, including the universality of haloes and the scaling laws for simulation, as well as the influence of dark energy. We are not as confident about the stability of structures and the relation between luminous and dark matter.

All measurements in cosmology are statistical, simple models are preferred, and aesthetics plays an important role. Apparently, Bayesian approaches are all the rage. Even so, without making assumptions about the functional form of the parameters to be estimated, the relevant quantities are difficult to evaluate. Two different models are compared by looking at the ratio between their likelihoods [which is standard also in other disciplines, such as quantitative ecology]. Simple models end up being preferred because they have more “likelihood within the prior.” As a result, of course, there is a tradeoff between models that are better at describing the data, but are more complicated, and models that are simpler, but also less good at describing the data.

There are statistical limits to cosmological modeling. Algorithms need to be able to deal with very large quantities of data, and the available sampling methods work best with unimodal distributions, but we know that we are actually dealing with multimodal ones. Indeed, for a variety of reasons, it is increasingly difficult to develop accurate models, because of the presence of many second-order effects. The author claimed that within a few years we will have observed pretty much everything there is to observe within the light cone, which means that cosmologists will literally run out of new data! [That’s going to be interesting: the end of cosmology?]

The next talk was by Chris Smeenk on “Gaining Access.” [A bit of a cryptic title, isn’t it?] The topic was the use of theory to gain access to inaccessible quantities, through proxy data. The author began by reminding us that geologists seem to have a pretty good idea of the structure of the interior of the earth, even though we obviously do not have direct access to it. [Well, similarly, we don’t see electrons, and yet we have very good reasons to think they exist.]

There are different senses of the word “theory”: frameworks specifying the kinematics and structures for state space; qualitative hypotheses that provide general explanations of phenomena; mathematical equations expressing law-like relationships among variables. He focused on the latter for the remainder of the talk.

Theories in this sense amplify knowledge based on relatively accessible data, and they are necessary to interpret data; also, the best evidence for a theory only develops based on ongoing use of theory itself.

Theoreticians often face the circularity challenge: a given theory is being tested and, at the same time, used to interpret the data. It could be, for instance, that the theory is so flexible that the whole exercise reduces to curve fitting. From a logical perspective, how is it that further use of a theory lends support to the initial use of that theory?

There are historical examples of effective responses to the circularity challenge. However, does the inflationary paradigm in cosmology meet the challenge?

A good historical example concerns the atomic theory (“atomism”) and its relation to Avogadro’s number. The kinetic theory of molecules — and the measurements derived from it — both agreed with the atomic theory and put strict constraints on the estimates of Avogadro’s number. The independence of a number of pertinent measurements made it unlikely that the agreement with the theory was due to some inherent flexibility of the latter.

The author presented a second “good oldie” example, based on the measurement of the orbit of Uranus. Both cases featured robust physical sources for theories and data, which were independently confirmed. The response to the circularity challenge in these cases is that we can either accept the theory or admit to an enormously implausible coincidence. Such theories then become permanently accepted, on penalty of having to unravel the whole edifice of reasoning that led to them.

What about inflation? How do we distinguish between the theory merely accommodating data vs identifying correct cosmological features? Smeenk at this point proceeded with providing a quick reminder of what inflation theory is about and how it works.

The idea is that there are both observational constraints and (somewhat weaker) constraints coming from particle physics. There are inflationary models meeting all of the constraints from most recent data. Still, unlike the historical cases discussed before, Smeenk claimed that there is much less independence of the measurements when inflation is concerned. There is also a problem caused by the lack of a canonical model that provides us with an independent feature of the universe against which to test inflation (something like, say, the observational discovery of Neptune, which validated theories about the anomalous orbit of Uranus).

An additional issue specific to inflation is that we do not know how to best reason about initial conditions, in this case pre-inflationary conditions (again, in contrast to the “good oldie” cases): what was the state of the inflaton field prior to inflation? One response has been the invoking of “eternal” inflation, which leads to the formation of “pocket” universes with different low energy physics. This depends on very speculative ideas, which are meant to respond to worries about the so-called fine tuning problem. The author suggested that this approach is compatible with many other explanations other than inflation, and so that eternal inflation — ironically — undermines the case for inflation in our universe. Eternal inflation is not an exact / mathematical theory, at best it is a framework.

After coffee break the first talk was by Gordon Kane on “String/M-Theories about our world are testable in the traditional physics way.” [Note that a number of attendees had expressed skepticism about Kane’s talk even before it was delivered, since apparently he has a habit of shifting the goal posts for what counts as testable predictions. For more, see here.]

Kane began by saying that in order to test a 10D theory in a 4D world you must “compactify” your experimental approaches as well. String/M theory is a very powerful framework to get at a comprehensive underlying theory that incorporates the Standard Model and goes beyond it.

He made the point that one doesn’t need to “be there” in order to test theories — for example no one witnessed the Big Bang or the extinction of the dinosaurs. [Yes, that’s a basic anti-creationist argument, I don’t think it’s needed with this audience.]

All “superpartner” predictions made in the past were based on “naturalness,” not on a theory, so it is not surprising they failed. Additionally, many string theorists don’t know how to “compactify” their approaches for 4D testing, so one shouldn’t be paying too much attention to them. Much of what is written by people like Woit and Smolin is misleading and string theorists don’t give it much thought. [Well, I think some people would beg to differ here, seems like far too cavalier a dismissal.] String/M theory is too important to be left to string theorists, since apparently they are not much bothered by the issue of testability [this was a rather cryptic remark, which was not made any clearer by the remainder of the talk].

According to Kane there is a well established procedure to compactify string theory, but the results do not yet allow to make precise calculations. However, all is needed is one falsifiable prediction to make a theory testable. [Well, no, that’s only if you are more Popperian than Popper and believe in crucial experiments, an idea that Duhem should have put to rest, like, a century ago.]

A compactified string theory is analogous to the Lagrangian of a system. Now, all tests of theories in physics have depended on assumptions, from Galileo on [yes, but I don’t think anyone has been questioning that] but, interestingly, the author said a general test of string theory — of the kind you have for field theory — is unlikely, yet insisted that the compactified version is testable. Gravity, for instance, provides pertinent evidence. [??]

String/M theory gets us not only a quantum theory of gravity, but an entirely new physics, for instance about “moduli fields,” which describe the sizes, shapes and metrics of small manifolds. Moduli dominated the energy density of the universe after inflation ends, and they decayed before nucleosynthesis into dark matter.

Kane provided an example of his compactification approach, on G2 manifolds. Work on this began in 1995 and it has led to the establishment of a “powerful, rather complete” framework. Compactified M-theory on a manifold with G2 holonomy in fluxless sector is well motivated and technically robust. [Okay, okay, I have no idea what this means…] This led to calculating the supersymmetry soft-breaking Lagrangian, which in turn led to the anticipation of the mass of the Higgs boson. [Boy, I would love some independent opinion on these claims.]

[During the talk itself Kane was strongly challenged on the extraordinary assertion that he was able to predict the mass of the Higgs on the basis of M-theory. Even by David Gross, who is strongly sympathetic to string theory.]

Kane resumed his talk by saying that predictions of the mass of the gluino, wino, and bino are generic but robust, as is “clear to any knowledgeable person who goes through the derivation.” These should not have been seen at the LHC run 1, but should be observed in run 2. [I guess we’ll see soon, won’t we?]

Kane admitted that there are many many solutions to string theory [the infamous 10^500 landscape], but argued that many are not “populated,” which means that the landscape issue might not be an obstacle to explain the existence of our world on the basis of the theory.

The talk ended with a list of (alleged) issues in fundamental physics addressed by M-theory. The interesting bit was a list of those that aren’t, which include why we have three large dimensions, and why there is a universe to begin with. [Oh well, I guess we can leave those for another time…]

The Kane presentation generated quite a bit of controversy at the conference and online. I reproduce here, with permission, a bit from Peter Woit (a well known critic of string theory) for context:

“From reports I’ve heard about the conference today, it seems that Gordon Kane’s claims of a string theory prediction of the gluino mass around 1.5 GeV were not accompanied by any acknowledgement that he has been making exactly this kind of claim for years. For example, back in 1997 the gluino mass was about 250 GeV (see here) and before LHC Run 1 data arrived it was “well below a TeV” (see here and here). I’ve documented (here) the edits made to Kane’s 2000 popular supersymmetry book to remove all falsified predictions and replace them with new ones, with no acknowledgement this was done.”

Next: Joseph Silk (one of the co-authors of the provocative Nature article that prompted the workshop), on “The Limits of Cosmology, Post-Planck.” Some theories are intrinsically untestable, some generically untestable but testable “if the dice roll favorably,” and yet others are generically testable in the remote future.

The theory of inflation dates to 1980. According to Andre Linde, a co-founder of the theory, the inflationary multiverse helps us understand all sorts of facts about why our universe is the way it is — including why the universe does not rotate, or why parallel lines do not converge. [Wow.]

There are several experiments ongoing and proposed over the next few years that will be looking for signatures of inflation. The problem is, there are reasons to believe that signals based on gravity waves are likely to be so confounded by other factors (like interstellar dust scattering) as to be forever undetectable.

What Silk thinks might actually be testable are predictions that inflation makes about the deep structure of the cosmos. The famous 3K background radiation is completely uniform, and with the Planck satellite we can see the mildly granular universe at a scale of 1 in 10^5. If we want to test inflation we need to do much better.

The only robust prediction of inflation is non-gaussianity, which however is very small. We are talking about a precision of the order of 2×10^-6, right now we have 0.01. Can we get there? Yes, if we focus on the 21cm hyperfine transition of neutral hydrogen from the so-called “dark ages” of the universe (before structures starting forming). This corresponds to a frequency of 30MHz, which means that in order to do unimpeded experiments about this we would have to go to the far side of the Moon. [Damn interference from cell phones!] Silk actually laid out some details of the envisioned experiment, which would include 10^6 dipoles scattered on the lunar surface. He thinks that could be done by 2050.

Next: do we need the multiverse? The multiverse is motivated by the need to explain why dark energy is so small. There are strong opinions in opposite camps here, and we are reminded of what Lev Landau famously said of cosmologists: “Often in error, never in doubt.”

Silk thinks there are both astrophysical and particle physical explanations of dark energy which do not require the invocation of an inflationary multiverse. The first have to do with interpretations of supernova data that invoke spatial heterogeneities and do not need dark energy; the latter would rely on string theory been able to narrow down the populated space of its landscape to a small number of alternatives [which, at the moment, doesn’t appear likely].

The afternoon session resumed with Fernando Quevedo on “Achievements and Challenges for String Phenomenology/Cosmology.” [Yet another string theorist. I must say, there does seem to be a stacking of them at this conference, and no experimentalists have been invited either]. The talk began with a quick summary of some of the major implications of relativity and quantum mechanics, which set the stage for string theory. In order to get specific, quantitative (as opposed to generic, qualitative) predictions you have to move from a framework to a specific theory, like the Standard Model.

The SM is arguably the greatest theoretical achievement of the past 70 years. It is simple, and it describes several phases of gauge theories. However, it is not complete, as it does not include baryogenesis, dark matter and gravity. It is “ugly,” since it comprises a lot of particles and relations that are unaccounted for. This means that there are a number of open questions concerning gravity, some 20 parameters to be explained, the number of dimensions, and so on.

The long term plan of string “phenomenology” (as opposed to “noumenology” [ah!]) is to find a scenario that satisfies all particle physics and cosmological observations and hopefully lead to measurable predictions. Meanwhile, the theory makes generic “predictions,” including extra dimensions, supersymmetry (not necessarily low-energy), branes, and so forth. Oh, string theory also predicts gravity! The theory further makes generic predictions in its 4D projected version (compactified), including antisymmetric tensors (like brains) and no global symmetries.

So, even according to Quevedo the string “framework” makes very few predictions. To make progress one has to construct realistic models and try to extract properties of classes of models. This would not test the theory itself, but only specific scenarios within the theory.

The challenges for string models include: gauge and matter structure of the SM, hierarchy of scales and masses, hierarchy of gauge couplings, stable proton and baryogenesis, dark matter, dark radiation, and dark energy, among others. Failing even one of these would doom the theory.

The “landscape” is a problem, which according to Quevedo should  to be used to solve physical issues like the fine tuning one. And so far the theory has not been able to deal with the empirically demonstrated existence of a non-zero cosmological constant. The author concluded that string theory has been making “continuous, a-cumulative” progress [interesting notion, right?]. People complain about there being too many string models, but he thinks there are too few, since none of them so far is sufficiently realistic with respect to the actual universe.

Next we had Chris Wüthrich on “Considering the role of information theory in fundamental physics.” There is something that seems to be universally agreed upon within the string wars: there are black holes and they have certain thermodynamic characteristics (following the Bekenstein-Hawking formula). As a philosopher, Wüthrich is skeptical of universal agreement on anything…

The robustness of this consensus is surprising since there is no empirical confirmation of Bekenstein entropy. So do people have convincing non-empirical reasons to accept Bekenstein’s formula about black holes as thermodynamic objects?

The original motivation that spurred Beckenstein to develop a thermodynamics of black holes is that otherwise these objects would violate the second law, so he was looking for a generalized version of the law: “The introduction of a black hole entropy is necessitated by this process,” Beckenstein put it in 1973.

Now, the second law is not itself exceptionless, since it is a statistical principle. Still, a few years before Beckenstein’s work, Hawkins et al. had demonstrated the area theorem, stating that under certain conditions the surface area of the future event horizon never decreases with time. Notice that this is not a statistical statement.

Beckenstein developed an analogy between the surface of a black hole and entropy, which means that Hawking’s area theorem could be deployed to give an account of black hole thermodynamics.

It also turns out that the general formula for entropy (Gibbs’) has the same structure as Shannon’s formula for information/entropy. However, Callender has pointed out that the two have different interpretations, because in Shannon entropy the probabilities are based on our degrees of belief, whereas in statistical mechanics the probabilities are based on objective features of the physics.

Nonetheless, Beckenstein explicitly discussed black hole physics from the point of view of information theory. He then introduced the concept of black hole entropy as the measure of the inaccessibility of information inside the black hole.

But now we are conflating epistemology and ontology. Information — according to Shannon himself — is an inadmissible concept outside of a communication system (which doesn’t have to be between people, but it does entail a sufficiently complex system). So we now have a category mistake!

If these worries are correct, than the analogy between the area of a black hole and its entropy fails, and it turns out that one thing that all participants to the string wars agree on, they really should’t. [I don’t know, of course, whether the author’s argument is correct, though it smells that way. Regardless, his presentation was a delightful example of philosophical reasoning, and an instance of how philosophy can contribute to science, to boot!]

The conclusion shouldn’t be that black holes are not thermodynamic objects. But it does suggest that the original analogical argument advanced by Beckenstein does not hold. There are thought experiments proposed subsequently that do support the conclusion that black holes are thermodynamic objects. But remember: so far, no empirical confirmation. And at the end of the day, it is only the latter that will settle the issue.

Speaking of the end of the day, the last talk was by Viatcheslav Mukhanov on “Is the Quantum Origin of Galaxies falsifiable?” I wish I could report on it, but an awful combination of the author’s unbelievably fix Russian accent and handwritten slides made it close to incomprehensible for me. Oh well, good night and good luck! Last installment of this series, hopefully tomorrow.

Advertisements


Categories: Philosophy of Science

52 replies

  1. Hi Coel,

    > To have an infinite spatial extent, and only one Big Bang, simply doesn’t work.

    Why not? If all of infinite space originated in the same Big Bang (as is the case if the curvature of space space is open or negative) then having infinite space is perfectly consistent with having only one Big Bang.

    Like

  2. Coel wrote: “Any sensible attempt to produce a Big Bang starts with quantum fluctuations in a pre-existing “state”.

    That is because we’re relying on our conventional notion of time that there must have been a state prior to the Big Bang. Suppose you were an ant on the surface of a ball, seeking its origin – would you ever find it? Likewise as we extrapolate the universe backwards to when it was on the Planck scale, how do we know that time continues to work the same way, that we can find a “before the Big Bang” and a “pre-existing state”?

    Anyway, scientifically speaking, this is a fruitless discussion.

    Like

  3. Hi DM,

    Why not? If all of infinite space originated in the same Big Bang …

    Since the time since the Big Bang is finite, you cannot get to infinite space out of a Big Bang unless you start with infinite space (finite space * finite expansion rate does not give you infinity).

    But, if you *start* with infinite space (some pre-existing state in which a Big Bang happened), then any (remotely plausible) physical process cannot produce a Big Bang everywhere at once in that infinite space, since that’s a violation of causality.

    Any quantum-fluctuation-like origin of the Big Bang is going to be localised compared to infinity. At that point my argument kicks in: if in one location, why not in others?

    Thus any attempt to find a physical process that produces a Big Bang tends to produce lots of them. Of course the BB could be something utterly unlike existing physics, but that’s a weird thing to default to.

    Hi Arun,

    That is because we’re relying on our conventional notion of time that there must have been a state prior to the Big Bang.

    Yes, I’m assuming that the Big Bang had some naturalistic “cause”, and that requires some preceding state that “caused” it. That’s the natural thing to do. **On** that assumption, then the expectation is multiple Big Bangs rather than one (for reasons already given). You are entirely right that maybe the “origin” of the Big Bang was something utterly unlike known physics.

    But one would surely not apply a high Bayesian prior to the suggestion of something utterly unlike known physics. My whole argument here is then that the naturalistic expectation is of a multiverse, and that we should prefer it to the claim of “… and only once”. It is that “and only once” that is weird, and requires something utterly unlike known physics.

    Like

  4. Well, thanks, for the response, Coel. I still don’t understand why you are so attached to this viewpoint. Or, for that matter, why not speculate that multiple big bangs have already occurred and continue to occur, but that we exist in only one of these many and that the process itself precludes an other than speculative or imaginative understanding? The question seems to revolve around the question of whether such speculation is indicative of science or of science fiction.

    Like

  5. Hi Coel,

    > Since the time since the Big Bang is finite, you cannot get to infinite space out of a Big Bang unless you start with infinite space (finite space * finite expansion rate does not give you infinity).

    See, I don’t think you’ve got this right. Check with a cosmologist, but my understanding (I don’t have a deep understanding of the science or the mathematics, but the upshot I get from following the likes of Krauss and Tegmark and Hawking and Carroll) is that standard cosmology does allow for infinite space right from the outset of the Big Bang, and for there to be one Big Bang.

    The key is to think not so much of the Big Bang as expanding space outwards but as a dramatic decrease in density of the infinite space. As you go back in time, the infinite space doesn’t get smaller (that wouldn’t make sense), but rather it gets denser. The Big Bang is a point of infinite density but not necessarily of infinitesimal volume.

    > But, if you *start* with infinite space (some pre-existing state in which a Big Bang happened), then any (remotely plausible) physical process cannot produce a Big Bang everywhere at once in that infinite space, since that’s a violation of causality.

    No, you’re not necessarily starting with a pre-existing state in which a Big Bang happened. If time was created in the Big Bang, there was no pre-existing state. It’s just that the *first* state was a state of infinite density everywhere from which the universe expanded. Causality doesn’t apply because there isn’t necessarily a time before.

    > Any quantum-fluctuation-like origin of the Big Bang is going to be localised compared to infinity.

    Again, I think this is a little out of touch with cosmology. As far as I understand it (on inflation anyway), a Big Bang can correspond to an expanding bubble of spacetime which is infinite on the inside but finite on the outside. The space in which the Big Bang happens is not the same as the space the Big Bang creates, I guess.

    Like

  6. Hi again Coel,

    Just thinking about this a little more, and it seems to me that your conception of what it would mean for spacetime to be flat or negatively curved (infinite in extent) is a little naive.

    You accept that space may be infinite, but you do not appear to accept that our Big Bang can be responsible for this infinite space.

    If I try to extrapolate from your apparent beliefs of what an infinite space would entail, you seem to be suggesting that space may be infinite but the extent of stuff ejected from the Big Bang is not. There is perhaps somewhere out there a wave front, beyond which there is nothing. Effectively, though space is infinite, there is an edge of the universe of stars and galaxies. Correspondingly, there is a centre.

    If you think there are many Big Bangs, then presumably if you go far enough out into the void you will find another expanding island universe. Presumably it is possible for these universes to expand into each other, which would mean, I suppose, that stars and galaxies would be streaming past each other in opposite directions at many times the speed of light (since the edges of the observable universe are expanding away from each other at many times the speed of light).

    Hopefully you see that this is not what cosmologists believe. They don’t think there is an edge and they don’t think there is a centre. If space is infinite, it is stars and galaxies all the way out, and all the stars and galaxies in our infinite spacetime have their origin in our Big Bang.

    Like

  7. Hi DM,

    It’s just that the *first* state was a state of infinite density everywhere from which the universe expanded.

    But then you need an explanation for why it happened everywhere (infinite distances apart) at once. That’s the unphysical thing.

    Hi Thomas,

    Or, for that matter, why not speculate that multiple big bangs have already occurred and continue to occur, but that we exist in only one of these many and that the process itself precludes an other than speculative or imaginative understanding?

    That may well be the case. The correct conclusion might be: we might very well be in a multiverse; and it may be that we can never prove it.

    The question seems to revolve around the question of whether such speculation is indicative of science or of science fiction.

    Science, at the root, is the process of trying ones best to understand the world around us (an emphasis on empirical data, for example, is then the *product* of that attitude, rather than being the “definition” of science).

    Thus being “unscientific” is a matter of going beyond available evidence. Whether the multiverse is “unscientific” then depends on what statement about it one is considering.

    The statement “we are in a multiverse” is unscientific since it goes beyond available evidence. The statement: “we may very well be in multiverse even though we may never be able to prove it” is not unscientific — it is entirely in line with the available evidence.

    Note that the statement: “there is/was only ever one Big Bang” is also unscientific, just as much so as “we are in a multiverse”, since both go beyond the available evidence!

    Like

  8. Hi Coel,

    > But then you need an explanation for why it happened everywhere (infinite distances apart) at once. That’s the unphysical thing.

    Yeah, but it seems to me this is a case of you not understanding it. I can’t explain it very well because I don’t understand it well either, but this is what cosmologists are telling us.

    Again, I’ve tried to point to a couple of approaches. One is that this is just the first state. It didn’t “happen” in the sense that it had a cause, because it’s just a brute fact that this was the first state of the universe and we don’t know why it was so. It’s not a case of some event rippling out through a pre-existing spacetime, because there was no pre-existing spacetime. This is just the first state of the spacetime we have and it was like this throughout the whole thing. Might as well have started out infinitely dense as empty, right? And perhaps it is in the nature of infinitely dense space to inflate? There need be no communication at a distance if this is the case.

    The other explanation, on inflation, is that a bubble formed and expanded, and that this bubble was infinite in extent on the inside though finite and local from an external perspective.

    Like

  9. Coel,

    Thank you for the reply and excuse me if I do treat you as a punching bag on occasion. Yes, I am someone out on the fringe and that entitles me to the crackpot designation. I shouldn’t be questioning the experts. But I do.

    Among other issues, I very much see “spacetime” as a mathematical universe construct, in which measures of distance(space) and measures of duration(action in that space) are correlated, with such fantasies as wormholes, block time, etc emerging from it. Then having it used as one of the original patches for Big Bang theory.

    You have given me the opportunity to debate these issues in front of an audience, a bit a small one, but it is safe to say that by this point, while there hasn’t been any great move to defend your authority, conversely there has been little imprint caused by my arguments on others thinking. No one else seems to have found my views particularly intriguing and since I do have the conceit that I am somewhat of a realist, I have to admit it has been an unsuccessful effort on my part.

    Now don’t get your hopes up that I will simply sulk quietly away, because I do still think I have a valid argument and so, from my perspective, it will be a matter of sitting back and watching the pros get ever more entangled in their wild speculations. Which while it may be somewhat frustrating, will also be highly entertaining.

    In fact, I will give you a speculative possibility, that while you will reject, coming from me, I will wager some small amount of money, will enter the debate at some point;

    That we could travel/send satellites, through wormholes, back in time to the Big Bang and find a way to get to other universes that way.

    Regards and much luck, at least with the paycheck,

    brodix

    Like

  10. Coel, it’s when you say things like “The statement: ‘we may very well be in multiverse even though we may never be able to prove it’ is not unscientific — it is entirely in line with the available evidence,” that you’ll note others gradually vacate the room before it collapses around them.

    Still and all, I confess to a grudging admiration for your tenacity and scientism in much the way I value a long time friend of mine who will end a conversation on a moral subject with “Whatever the Pope says.” And then I say, “Yes, embrace the dogma.” And we both laugh.

    Like

  11. Hi DM,

    One is that this is just the first state. It didn’t “happen” in the sense that it had a cause, because it’s just a brute fact that this was the first state of the universe and we don’t know why it was so.

    Yes, that’s possible (I guess), but that isn’t any sort of “process” producing a Big Bang, it amounts to just starting with the Big Bang underway, coupled with a by-fiat declaration that there is only one of them.

    My argument is about any sort of natural *process* that produces a Big Bang, that is anything like in line with known physics.

    The other explanation, on inflation, is that a bubble formed and expanded, and that this bubble was infinite in extent on the inside though finite and local from an external perspective.

    In that scenario, the inflationary bubble is surrounded by the pre-inflationary state. The inflationary bubble is then expanding exponentially and getting very (very) large. Within that bubble there is a probability that a region will quantum-tunnel to the non-inflationary state (i.e. to the universe as we see it today) — that, indeed, being the whole point of the inflationary model. It cannot do that quantum transition everywhere simultaneously, it has to do it locally within that inflationary bubble. But if it can do it somewhere within the bubble then it can do it other places within the bubble (again, quantum mechanics is probabilistic). Thus, your scenario will inevitably produce oodles of universes like ours.

    Essentially that process of dropping out of the inflationary state is what we “see” as the Big Bang. The many different droppings-out are multiple Big Bangs. This is why, whenever anyone tries to make an inflationary model work, it makes multiple Big Bangs.

    Hi Thomas,

    Still and all, I confess to a grudging admiration for your tenacity and scientism …

    Thanks! 🙂 , though this multiverse stuff I don’t see as particularly scientistic, it’s just science. (Others will differ, no doubt 🙂 ). One thing about this conference is that it seems not to have had any defenders of a multiverse from a *cosmological* perspective (which really is a different matter from a string-theory multiverse; many-worlds QM is different again).

    Like

  12. Hi Coel,

    To be clear I don’t have a problem with your main argument. Only with your apparent contention that an infinite space is logically inconsistent with a singular Big Bang (it isn’t).

    > It cannot do that quantum transition everywhere simultaneously, it has to do it locally within that inflationary bubble.

    I don’t think that’s right. I think the bubble more or less is the transitioning out of the pre-existing state, which is inflationary. The space between universes is expanding faster than the universes. The bubbles are the bits which stop expanding, I think, and as far as I understand the space within the bubbles can be infinite on the interior.

    I don’t understand it well enough to explain it, but I think the picture you’re painting is not that painted by cosmologists so I think you have it a little wrong.

    The book that most shaped my understanding on this is the chapter on inflation in Tegmark’s Our Mathematical Universe (worth a read for the cosmology alone — the MUH stuff is only about a quarter of the book or less, I’d say).

    > if it can do it somewhere within the bubble then it can do it other places within the bubble

    If you are suggesting that somewhere within our infinite spacetime there may be other Big Bangs (a finite distance away) then I think you are wrong. Inflation does predict other universes, but they correspond to different bubbles, not different local transitions occurring within the same bubble.

    Like

  13. Hi DM,

    Only with your apparent contention that an infinite space is logically inconsistent with a singular Big Bang (it isn’t).

    I’m not saying that those are *logically* inconsistent, I’m saying they are inconsistent given any sort of normal physical process, where causation is limited by light-travel time. Having a transition either to or from an inflationary state happen simultaneously over infinite distances is unphysical.

    Inflation does predict other universes, but they correspond to different bubbles, not different local transitions occurring within the same bubble.

    Inflation depends on two transitions: the transition to the inflationary state, followed by the transition to the normal state. One can use the term “bubble” for either an inflationary state “bubble” forming within the pre-existing stuff, or for a normal-state “bubble” forming within the inflationary state.

    It is the inflationary state regions that are vastly (vastly, vastly) bigger. Within that inflationary state, bubbles of normal-state drop out (each one of those transitions being essentially a “Big Bang”); we are presumably in one of those.

    If you want an inflationary model and only a single BB, you need some mechanism to transition the entire *vast* inflationary state region all at once. That’s the thing that is unphysical.

    Like

  14. Hi Coel,

    > I’m saying they are inconsistent given any sort of normal physical process,

    I think you’re making an assumption that there is a process which produces Big Bangs. There may not be. If this is the only universe, and it originates with the Big Bang, then having an initial state where everything is maximally dense is just part of what makes the universe what it is, like the laws of physics. If you appeal to a process, you have to ask where that process came from, ad infinitum. You have to hit bedrock somewhere. At some point that’s just how things are. And the possibility remains open that the Big Bang is it.

    Besides, I’m not seeing what infinite space has to do with anything. If space is finite much the same arguments would apply.

    So, given that we can’t legitimately assume that the Big Bang is the result of a *process*, your argument that infinite space is inconsistent with a singular Big Bang doesn’t appear to me to hold water. Leave infinite space out of it and just say that since it happened once it seems reasonable that it could have happened more than once. Even if space is finite, space could just be a property of this universe and that doesn’t rule out there being other universes and other spaces outside it.

    > Inflation depends on two transitions: the transition to the inflationary state,

    I’m thinking more of eternal inflation, in which if I understand it correctly there is no transition to the inflationary state. The inflationary state is the default. A bubble forms when a local region of the “inflationary medium” condenses and stops inflating. Space can be infinite within that bubble.

    Now, you are of course right, that in such a model it seems unreasonable to suppose that such a bubble could form only once. Again, I’m just saying that space being infinite has little do do with the possibility of there being a multiverse, because whether the space within those bubbles is infinite or not is irrelevant to the question of how many times these bubbles could form.

    The inflationary medium itself being infinite is another issue, I guess, but that’s not normally what people are talking about when they discuss *our* space being infinite (which I was clearly referring to when I discussed zero or negative curvature)

    Like

  15. I keep reading these comments and now can’t shake this poem by Philip Whalen. It’s like an earworm that Socratic alluded to some posts back (apologies–Wordpress doesn’t allow for formatting in this case)

    Metaphysical Insomnia Jazz Mumonkan xxix”, inspired by the Original Face-koan

    Of

    Course I could go to sleep right here
    With all the lights on & the radio going

    (April is behind the refrigerator)

    Far from the wicked city
    Far from the virtuous town
    I met my fragile Kitty
    In her greeny silken gown

    fairly near the summit of Nanga Parbat & back again, the wind
    flapping the prayer-flags

    “IT IS THE WIND MOVING.”
    “IT IS THE FLAG MOVING.”

    Hypnotized by the windshield swipes, Mr. Harold Wood:
    “Back & forth; back & forth.”

    We walked beside the moony lake
    Eating dried apricots
    Lemons bananas & bright wedding cake
    & benefits forgot

    “IT IS THE MIND MOVING.”

    & now I’m in my bed alone
    Wide awake as any stone

    Liked by 2 people

  16. Then again, like biology, instead of a tree of universes, it could be a network of universes and we could ride the wormholes directly.

    Like

  17. Hi DM,

    I think you’re making an assumption that there is a process which produces Big Bangs.

    Yes, my whole argument rests on the assumption that the Big Bang is natural process.

    I’m thinking more of eternal inflation, in which if I understand it correctly there is no transition to the inflationary state. The inflationary state is the default.

    Normal inflationary models do have a transition *to* the inflationary state, since there are proposals for the physics that sends it into the inflationary state. One can, though, scheme up past-eternal inflation models also.

    But, the bubbles that drop *out* of the inflationary state are finite (starting with finite spatial extent, and expanding at a finite rate for a finite time).

    Like

  18. Hi Coel,

    > Yes, my whole argument rests on the assumption that the Big Bang is natural process.

    Right, but the multiverse-deniers are probably in the camp of thinking the Big Bang is just a feature of the universe and not the result of a process, for precisely the reasons you present in your argument.

    > But, the bubbles that drop *out* of the inflationary state are finite

    I don’t think that is necessarily so. Damn, wish we had a cosmologist here to settle it. It’s a hard one to Google. I was left with the impression that they could be infinite (from the point of view of interior observers at least) from reading Tegmark’s book on the subject. Do you want me to try to find a quote to support that? I may not have time for a little while.

    Like

  19. Hi DM,

    Right, but the multiverse-deniers are probably in the camp of thinking the Big Bang is just a feature of the universe and not the result of a process, for precisely the reasons you present in your argument.

    Yes, exactly. So the single-BB advocates are just making a by-fiat declaration “that’s the way it is”, without making any attempt to justify that physically (or logically or philosophically).

    Whereas the multiverse advocates are saying that any physical process that produces a Big Bang and is at all like physical processes that we’re aware of, will naturally lead to a multiverse.

    And yet the ones in the former group are accusing the multiverse advocates of being “unscientific”. I’m just pointing out that it’s actually the reverse! (I like doing that sort of thing 🙂 )

    Like

  20. But, as I said, you’re going to have to hit “that’s just the way it is” eventually, right? I mean, any process that produces Big Bangs is going to be based on some set of physical laws that could logically have been otherwise. At some point you’re just going to have to accept that that’s the way nature is, aren’t you? What’s wrong with postulating that the Big Bang is the point where you do that?

    Like

  21. Hi DM,

    What’s wrong with postulating that the Big Bang is the point where you do that?

    At various points in Earth’s history, people would have thought that the planet Earth was unique, or that the Sun was unique, or that our galaxy was unique. At each point they could have explained that with: “well, that’s just how it is, that’s the starting point”.

    Or they could (as they indeed did) develop an understanding of the physical processes that led to planets, stars, galaxies, etc. So why not try the same with Big Bangs? Isn’t that the natural and scientific thing to do?

    Yes, it might not succeed, it might indeed be that the BB was “the starting point, just the way it is”, but that could also have turned out to be the case regarding planets, stars and galaxies.

    There is no **reason** to suppose that the BB is any different from planets, stars and galaxies in that regard. The natural and scientific thing to do is to look for the physical process that produces Big Bangs. It’s the people who don’t to do that and who want to just accept a single BB by fiat who are being unscientific.

    Like

  22. Yes, I “need” the multiverse. I am addicted to it.

    Like

%d bloggers like this: