The multiverse as a scientific concept — part II

multiverseby Coel Hellier

[The first part of this essay can be found here.]

Inflation

The comparison of cosmological models with high-quality and detailed observations of the early universe has led to the “inflationary” version of the Big Bang. This hypothesizes that, very early in the first second after the initial quantum fluctuation, only about 10-35 seconds later, our universe started an episode of exceptionally rapid exponential expansion, growing by a huge factor of about 1030 in only a tiny fraction of a second. This “inflation” was proposed in order to explain several otherwise puzzling features of the universe, including: (1) the similarity of the universe in opposite directions in the sky; (2) the exceptional smoothness of the cosmic microwave background (CMB); (3) the fact that the universe appears to have a very close balance between the total amount of matter and energy and the expansion rate, thus giving space a near-zero “curvature” on the largest scales; and (4) the absence of heavy particles such as magnetic monopoles that would otherwise be expected to be seen.

Thus inflation is motivated by strong empirical evidence [7]. Further, after inflation had been proposed, it was used to predict the spectrum of fluctuations expected in the CMB. These temperature fluctuations originate as quantum fluctuations in the inflating field, later to be frozen into the CMB, and the inflationary model gives specific predictions of their form and power spectrum. These predictions have now been compared to the better-and-better data from the WMAP and Planck satellites, and several ground-based experiments, and the result is again an exceptional agreement, giving strong support for inflation.

Further still, the inflationary model makes specific predictions about gravitational waves generated by the rapid inflationary episode. Recently the BICEP2 experiment has reported detecting these gravitational waves, imprinted on the CMB just as predicted, which would be an additional strong confirmation of inflation. These results are new and need confirming, but at a minimum they show that the inflationary model can be directly tested. Thus the inflationary Big Bang model is robust, mainstream cosmology. The mechanism driving inflation is, however, less understood.

Modern physics explains all known interactions in terms of four forces (gravity, electromagnetism, and the strong and weak nuclear forces), and at the exceptionally high temperatures in the very early Big Bang all four forces are expected to have existed in a “symmetric” state where they all acted similarly. As the universe expanded and cooled the symmetry was broken and the four forces developed the different characteristics that we see today. This process would have been analogous to the “phase transition” from a liquid (hot) state to a crystalline (cold) state. Such a phase transition has a “latent heat of crystallization” given out during the transition. It is, though, possible for material to get stuck in a “supercooled” state where it hasn’t yet made the transition, and thus has extra energy than expected for its temperature. It is this energy — thought to be associated with the phase transition that breaks up the strong and electroweak forces — that is thought to drive the ultra-rapid expansion of the inflationary era.

The tricky bit of inflationary models is then getting the universe to drop out of the “supercooled” inflationary state (rather than being stuck in that state forever), and thus give its energy into the hot Big Bang that then produces our universe. In order to get such models to work theorists have developed a scenario in which a quantum fluctuation can cause a limited region to drop out of the inflationary state, forming an expanding bubble of normal-state universe.

Thus, overall, we have our universe originating as a quantum fluctuation in the quantum-gravity era, at a scale of 10-43 seconds, leading to the exponentially expanding inflationary-state, followed by quantum fluctuations within the inflationary state, at a scale of 10-35 seconds, leading to bubbles of normal-state universe. However, in such a scenario, the inflationary-state stuff surrounding the bubble will be expanding vastly faster than the normal-state bubble, and thus the size of the inflationary-state regions continue to increase, even as bubbles continually drop out into the normal state.

The result is called “eternal inflation,” a “Swiss cheese” mixture in which bubbles of normal universe are continually forming out of a surrounding and exponentially expanding inflationary state. One of those bubbles would be our universe, and that bubble would now have expanded to vastly larger than our observable horizon. Thus, the only things we can now see are in our bubble, our normal-state universe.

This is a multiverse scenario. It says that somewhere beyond our observable horizon there is a transition, beyond which is supercooled, inflationary-state stuff. And in that rapidly expanding vastness are other bubbles, other universes, like ours, but now separated from us by unfathomable distances. If you don’t like the idea of a multiverse extending vastly beyond our observable horizon, or consider it to be unscientific, then realize that conventional cosmological models extend to infinity in much the same way. The stuff beyond our observable horizon is real, it is just a long distance away. There is no reason to declare such stuff not “real” just because of the finite value of the speed of light, which means that we humans can never receive information from those regions (especially since the location of that observable horizon depends entirely on where you are looking from, and it also continually recedes as you look at it).

The only sensible alternative to this multiverse idea is that our universe extends vastly beyond our observable horizon (to infinity?) and is normal-state all the way. Is that really preferable? If you do prefer normal-state all the way, then you have a problem in constructing an inflationary model that correctly makes the transition from the inflationary state to a normal state everywhere, especially given that any “transition front” would have a speed limited by c.

As it is, we have strong observational and theoretical arguments that lead us to an eternal-inflation model of the Big Bang, and that eternal-inflation produces a multiverse. It is currently rather difficult to produce a model of the Big Bang that works, and that explains the observations, and which does not automatically produce a multiverse. Admittedly we cannot observe those other universes, the other normal-state bubbles continually forming in the inflationary-state multiverse, but good, sensible and scientific reasons lead us to conclude that they likely exist. And, as stated early on, it is not necessary to empirically validate every prediction of a theory in order to have good confidence in that theory. To accept a theory and its implications all one needs is to have validated some of the predictions of the theory, and to have established that overall the theory does a better job than any alternative that we know of. As a comparison, no-one disputes the validity and scientific status of laws of gravity, that do an excellent job of predicting the times of future solar eclipses, just because we cannot verify those eclipse timings indefinitely into the future; and similarly it is not grounds to reject a model as unscientific just because we cannot verify its predictions indefinitely into the far distance.

Are the “physical constants” constant?

Now let’s ask a further question. Are those other bubble-universes in the multiverse just like ours? In what ways might they be different? How much scope is there for the bubbles to differ?

Above I used the analogy of a bubble dropping out of the inflationary state being akin to a liquid freezing. Consider a snowflake freezing in a high-up cloud. The freezing of each snowflake complies with the same underlying laws of physics, and yet each snowflake is different, with a related but distinct pattern. This tells us that some aspects of what we see are local “accidents,” variations allowed by the underlying laws but contingent on local circumstance.

The transition from inflationary state to normal state is thought to be due to the fundamental forces changing from a “symmetric” state, where they acted similarly, to a broken-symmetry state where they have different strengths. Further, physical “constants” such as the masses of particles and the values of electromagnetic charges are much the same thing as the strengths of the forces, since essentially they are all telling us how particles interact with each other. Thus we can ask whether the values of the masses and charges of particles and the strengths of forces are dictated by the fundamental physical laws, or whether they are local accidents, dependent on the local contingency of symmetry breaking early on in the Big Bang.

We don’t know the answer to that, but if it is the latter then we would expect each bubble universe to be different in the same way that each snowflake is different, and thus to have different physical constants. Note that — somewhat counter-intuitively — the latter suggestion is more parsimonious. The correct interpretation of Occam’s razor is in terms of the information content needed to specify a model [8]. If you have to explicitly specify the couple-of-dozen fundamental constants of the standard model of particle physics, that takes a lot of information. It takes less information to say that values for the constants are strewn around at random. After all, no-one claims that every snowflake having a different pattern is unlikely under Occam’s razor, since everyone accepts that the individual patterns are accidents, variations allowed by deeper-level rules.

As for falsifiability, it would be unscientific to add information to a model that had no observational motivation or consequences, and thus was unfalsifiable. However, if we are simply extrapolating from an observationally motivated model, or even reducing the information content of our model, while ensuring that the remaining information is observationally motivated, then that is scientific, even if not all of the implications of the model can be tested.

Considering this idea one quickly arrives at the obvious point that we human observers could only find ourselves in a bubble that had parameters suitable to have produced us, and it may be that the vast majority of such bubbles would be too alien to support or produce us. This is an entirely normal way of thinking. No-one nowadays supposes that there is some mechanism that ensures that an Earth-size planet gets placed in an orbit at the right distance from its star to allow it to have liquid water; we now know that extra-solar planets have a huge variety of orbits. We, of course, find ourselves on a planet suitable for us, but likely there are vast numbers of similar but uninhabited planets where the conditions are not right for life.

Similarly, no-one since Darwin would argue that we find polar bears in the Arctic and camels in deserts because that was carefully and deliberately arranged; rather, we understand that the local fauna is the contingent product of the local environment — a statement that applies just as much to the multiverse.

If you still find that line of reasoning unpalatable, consider that Steven Weinberg used it to predict that the value of the “dark energy” parameter in our universe would be small but non-zero. This prediction was made a decade before the observational detection of dark energy, at a time when most cosmologists assumed there was no dark energy. Then it was found, with a value in line with Weinberg’s prediction, but which is vastly smaller than given by attempts at calculating it from fundamental physics [9].

Weinberg’s verified prediction is currently the best explanation we have of the amount of dark energy in our universe. At this point, with an infinite extent of expanding inflationary-state stuff, dotted with island-universe bubbles, with each universe having different values for the physical constants, we have a full-blown multiverse of the sort to give critics a fit of the vapors. Yet, everything above is solid scientific reasoning, motivated and supported by observational evidence, and with already-demonstrated predictive power. That does not mean that it is true or proven, but it is a fully scientific concept and, regardless of the critics, the scenario is becoming increasingly accepted and mainstream among physicists.

A last remark: if the above is correct then in all likelihood we are near the middle of such a bubble and not near its edge. Yet there is some possibility that we are near an edge, and if we are then there would be nothing to stop us observing it if we looked far enough into the distant universe. In principle, then, the scenario is directly verifiable [10].

——

Coel Hellier is a Professor of Astrophysics at Keele University in the UK. In addition to teaching physics, astrophysics, and maths he searches for exoplanets. He currently runs the WASP-South transit search, finding planets by looking for small dips in the light of stars caused when a planet transits in front of the star. Earlier in his research career Coel studied binary stars that were exchanging material, leading up to his book about Cataclysmic Variable Stars.

[7] For an account of inflation and its observational motivations read this wiki page or this pdf of a chapter of Max Tegmark’s book Our Mathematical Universe.

[8] See this article of mine for a defense of Occam’s razor as a scientific concept.

[9] Naive calculations of the amount of dark energy expected give values about 10120 too big, perhaps the biggest error in the history of physics! Why is it that the actual value is 10120 times smaller than expected?

[10] And of course cosmologists are already looking for possible effects of “colliding bubbles”, which might be visible in the CMB. See this post for an example.

171 thoughts on “The multiverse as a scientific concept — part II

  1. Hi Massimo,

    I’m not being persistent to be troublesome, I am persistent because I share your frustration. If I am wrong, I am at least under the illusion that I am right, and your explanation doesn’t work to change my mind because it seems to me to miss the point Coel and I are making.

    when I see two thoughtful and smart people like you and DM simply unable to give up on something that is clearly not their domain I get depress.

    This is just how I feel when we discuss AI!

    I don’t think the basic idea of evolution is so difficult to grasp that our differences arise out of lack of expertise on my part or on Coel’s. Coel’s quote of Dawkins shows that at least one other evolution expert would take our side in this debate (and I see nothing anti-intellectual in that quote at all).

    It is perhaps not unlike the case where you tell String Theorists that String Theory is not science – you are making claims about a field that is not your domain because expertise in the domain is not really needed to understand the contentious issue.

    In your reply to me, you said that Gould was not talking about “survival of the fittest” but about the Theory of Evolution as a whole. This is partially true. The problem is that Gould equates the two. He actually says “Natural selection is defined by Spencer’s phrase ‘survival of the fittest'”. This is the problem. I maintain that survival of the fittest is indeed a tautology that inspires the overall ToE which is not a tautology. You say this tautology is entirely uninteresting, and perhaps you’re right as far as you’re concerned, but in my eyes this tautology is a profound insight that leads to the ToE as a whole.

    Tautologies are necessarily true, so they are sometimes accused of adding no new information. But sometimes these necessary truths are not appreciated. Tautologies can expose these truths and that makes them valuable. This is why Pythagoras’s theorem tells us something valuable and useful about the real world while being a tautology.

    So evolution is not a tautology, but survival of the fittest is. Gould’s essay is misguided because it answers Popper’s criticism in the wrong way. Instead of insisting that there is no tautology, he should have embraced the tautology and explained how the ToE builds on that to make a scientific theory which is not tautological. His efforts to redefine ‘fitness’ are therefore unnecessary and, in my view, incorrect.

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  2. Hi Robin,

    There are 3 ways to answer your question, not all of which are compatible.

    1) It could be eternal in the past. I believe this to be the most reasonable interpretation. The multiverse would have been inflating forever so there would be an infinite number of universes.

    2) You are applying the concepts of time and simultaneity as we perceive them within this universe to the multiverse as a whole. It is not clear to me that this is a legitimate move. You are wondering how many universes there are right *now*, but I’m not sure that *now* has a well defined meaning when applied outside of this universe. It may only make sense to ask how many universes there will ever be.

    3) As Tegmark says, a process continues forever will produce an unbounded number of universes. You can never stop counting all the universes that will ever be. Any possible universe you can ever imagine will eventually exist. So even if there are never an infinite number of universes at a specific point in time, there would still be an infinite number of universes created in the eternal lifetime of the multiverse.

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  3. Hi Robin,

    You say that the default model starts off as infinite, but none of the models start with infinitely many galaxies, as far as I know. It is this business of going from no galaxies to infinitely many galaxies that I don’t understand.

    The default model starts with an infinite extent of space, and that space contains an infinite amount of matter. There are no “galaxies” then simply because the matter is smoothed out, whereas galaxies are clumps of stuff, and because it was too hot, whereas for galaxies you need a cooler universe. So you start with an infinite extent of smooth matter, and that matter attracts other matter under gravity, and thus the stuff clumps together over time as everything cools, and those clumps are what we call galaxies and clusters of galaxies.

    So, since you start with an infinite extent of space and an infinite extent of matter, you produce an infinite number of galaxies. All of this applies regardless of whether you’re invoking an “eternal inflation” model or a multiverse model, since, as I said, the conventional default models also start with infinite space.

    A slightly more involved account is this. When the inflation process gets started, it causes exponential expansion which creates vast amounts of new space, and that vast amount of space contains vast amounts of “zero-point vacuum energy” (aka “inflaton field”). The creation of this energy is exactly cancelled by the negative gravitational potential energy of the stuff being produced, therefore the creation of all this vacuum energy is allowed, it does not violate conservation of energy.

    Then, at the end of the inflationary era, the universe drops out of the inflationary state, and all the energy tied up as “inflaton field” is then released as radiation. That makes the universe hugely hot, and produces vigorous creation of matter by pair production. Thus the energy goes into a vast number of particles and their anti-particles.

    As the universe then expands and cools, the particle, anti-particle pairs begin to annihilate with each other. For reasons that are not understood there appears to be an asymmetry in this process, such that you get a slight excess of matter over anti-matter, such that after everything is annihilated you are left with matter. That matter then, over time, clumps together under gravity and eventually forms galaxies.

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  4. Would any of you be willing to write a bit about the debate between Matt Dillahunty and Sye Ten Bruggencate? I just cannot understand if Sye is serious with his argument and perhaps if you clarify to him why he is wrong maybe he will understand. I also apologize for my comment not having much to do with the article on which I am commenting.

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  5. Why bring that into this discussion?
    I’m sure you have a point to make so why not simply explain that point to us? Comments should be reasonably self-explanatory.

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  6. Hi labnut,

    If we insist on empiricism we disqualify those presumed truths from serious consideration.

    To me that is too narrow an interpretation of “empiricism”, and Carroll’s article argues for a broader interpretation of it (which I’d agree with).

    Carroll is therefore claiming that some observable facts in our universe can only be explained by appealing to a hypothesis about the unobservable, outside our universe. […] Notice this is the exact reverse of Coel’s argument. Coel argues that data in our universe verifies the hypotheses outside our universe, while Carroll argues that the outside hypotheses explain the data internal to the universe.

    Really? It seems to me that what I’ve been saying is entirely in accord with Carroll’s article (which is very good, and with which I agree). We are both saying that in trying to explain facts about our observable universe we develop a model that applies generally to both our observable universe and what lies beyond it. We then validate the model by facts from our observable universe.

    I fail to see any difference between my stance here and Carroll’s article. Perhaps you are regarding them as different because you are overlooking that in both cases it is the same model that applies just the same to both observable regions and unobservable regions of the universe/multiverse.

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  7. Thanks for the ref! I think that’s the most useful info in this discussion so far.

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  8. Hi labnut,
    Just a couple of other points (since you bring atheism into this):

    activist atheists (like Sean Carroll and Lawrence Krauss) support the multiverse hypothesis with its something from nothing argument. This is essential to atheist belief.

    “Something from nothing” is not essential for an atheistic view. I don’t see anything wrong with past-eternal somethingness.

    Finally, there is a delicious irony when an activist atheist like Sean Carroll appeals to the unobservable to explain the observable.

    He doesn’t, he appeals to a model that is validated by data from our observable universe. The “unobservable” is simply an implication of the model.

    After all, this is exactly what he claims is the error of religion.

    The error of religion is that it is not a best-efforts attempt to explain data but an exercise in wishful thinking followed by post hoc rationalisation of the data.

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  9. Coel, you said:
    We then validate the model by facts from our observable universe.

    But Carroll says something very different, see this quote from him:
    We can’t (as far as we know) observe other parts of the multiverse directly. But their existence has a dramatic effect on how we account for the data in the part of the multiverse we do observe. It’s in that sense that the success or failure of the idea is ultimately empirical: its virtue is not that it’s a neat idea or fulfills some nebulous principle of reasoning, it’s that it helps us account for the data. Even if we will never visit those other universes.” (my emphasis)

    Let me help you out;
    You say: “We then validate the model by facts from our observable universe.
    Carroll says: “it’s that it[the model] helps us account for the data

    Or, in simpler terms:
    You say the facts validate the model
    Carroll says the model explains the data.

    You two have opposite view points. Nowhere does Carroll say the facts validate the model.
    After all, that is exactly why he wrote the article, to argue that we must relax the falsifiability criterion. If the facts were sufficient to validate the model he would have no need to make the argument we should relax the falsifiability criterion.

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  10. Coel,
    but an exercise in wishful thinking followed by post hoc rationalisation of the data.
    You have just accurately described the speculative multiverse hypothesis.

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  11. Coel,
    He doesn’t, he appeals to a model that is validated by data from our observable universe
    Wrong.
    Nowhere does he say that. As I pointed out above he very clearly says the model is necessary to explain the data.

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  12. Coel, perhaps you can make sense of Carroll’s concluding remarks in a way I cannot:

    “If string theory and multiverse theories help us understand the world, they will grow in acceptance. If they prove ultimately too nebulous, or better theories come along, they will be discarded. The process might be messy, but nature is the ultimate guide.”

    Two conditional statements followed by a statement that some “undefined,” seemingly ad hoc, process “might be messy,” along with an assurance that the ultimate guide will be nature. This sounds like a version of scientific fideism to me and makes a strong case for why we need philosophers to engage in the philosophy of science. In other words, “Trust us. After all, we’re scientists.”

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  13. Coel,
    let me show you how Carroll indulges in ‘ wishful thinking followed by post hoc rationalisation of the data.

    He has a problem, the vacuum energy is enormously lower than theory predicts it should be.

    Wishful thinking is indulged:
    Construct a multiverse hypothesis where some universes automagically have lower vacuum energy. That is the beauty of wishful thinking, you can skip over the tedious details of how that could be actually possible.

    Post hoc rationalisation follows:
    low and behold, it just so happens that we find ourselves in a low vacuum energy universe. Our imaginary hypothesis predicted that this is just what should happen.

    Case proven, or is it? We constructed a model to explain the data and then say, wow, the data predicts the model, so the model is validated! That is called post hoc rationalization.

    Anyone believing that kind of fiction should rather apply at the Institute of Cyclic Universes. Circular reasoning must be inherent in the cyclic universe theory.

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  14. Hi labnut,

    Or, in simpler terms:
    You say the facts validate the model
    Carroll says the model explains the data.

    These are the same, or rather are both aspects of the same thing. To validate a model we compare it with data. Does it explain the data? Does it make further predictions? Are these predictions verified? That process can go through multiple iterations.

    For example, take models of planetary motion and facts such as observations of where planets are in the sky and when eclipses occur. It is true to say that the “facts [locations of planets, times of eclipses] validate the model”. It is also true that the model explains the occurrence of those facts This is all the normal practice of developing a model in science by repeated and ongoing comparison with data

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  15. Hi Thomas,

    Coel, perhaps you can make sense of Carroll’s concluding remarks in a way I cannot:

    I’m happy to have a go, though it seems fairly straightforward to me. Here are my “translations”.

    “If string theory and multiverse theories help us understand the world, they will grow in acceptance.”

    If those theories prove to have explanatory power, and preferably predictive power also, when compared to observations of our world, they will grow in acceptance.

    “If they prove ultimately too nebulous, or better theories come along, they will be discarded.”

    If they prove either incapable of making predictions, or if the explanations they offer are so vague that they cannot be compared with data and verified, or if other theories come along that do a better job at these two, then they will be discarded.

    “The process might be messy, …”

    The process of science at the cutting edge, of developing models, of comparing them to data, might be messy ….

    ” … but nature is the ultimate guide.”

    But how well the models do at explaining and predicting actual observations of nature will be the ultimate guide.

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  16. Of course. How else do you “explain data” other than by a model? How else do you develop a model other than by attempting to explain data? How else do you validate a model than by how well it does at explaining/predicting data? These are all aspects of the same process.

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  17. Coel,
    First, you persistently ignore Carroll’s central contention, that we must relax the falsifiability criterion. After all this is exactly why he wrote the article. You should read and understand why he wrote the article. It was in answer to the question – what scientific idea is ready for retirement? Go back and look at the top of the article.

    He is saying that because we cannot validate the multiverse hypothesis. If we could there would be no need to relax the falsifiability criterion.

    And so, his basic starting point is that we cannot validate the multiverse hypothesis with the data at our disposal. You are saying the opposite, you are maintaining that we can.

    Once again, I ask you to understand why he wrote that article.

    Now let us use the example he quoted. Vacuum energy is lower than the prediction by some orders of magnitude. How does that fact validate the multiverse? Is it not possible that several other explanations are possible? Validate is a strong statement that there is a nearly inescapable link between the fact and the conclusion. That inescapable link is missing. This fact could be in accordance with many other hypotheses including the cyclic universe hypothesis.

    Note that I use the much weaker term ‘in accordance with’ instead of validate. What this points to is a semantic shell game that has crept into the conversation. The low vacuum is in accordance with the multiverse hypothesis. But that is a very long way from validating it.

    Let me stress again, saying a fact is in accordance with a model does not validate the model. To claim that is to claim a total fiction.

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  18. Hi labnut,

    Case proven, or is it?

    For the sixteenth time, no-one, not Carroll, not Weinberg, not me, is claiming that this stuff is “proven”.

    We constructed a model to explain the data and then say, wow, the data predicts the model, so the model is validated! That is called post hoc rationalization.

    No, you’re misunderstanding, Weinberg, in predicting the small but non-zero dark-energy, was NOT just trying to post-hoc explain data. At that time (for reasons we can go into if you wish), most cosmologists considered that there would be ZERO dark-energy. The standard mainstream models at that time had zero dark energy. There was no actual measurement of it at that time, though the observations would have ruled out a very large dark-energy.

    Wienberg then said “my model predicts NON-ZERO dark energy, contrary to what everyone is assuming. It would have to be fairly small, of course, but still non-zero.”

    Then, when the measurements were made, it was found to be non-zero (which was quite a surprise to many cosmologists). It was also in line with the value that Weinberg predicted. Thus there is nothing post-hoc about Weinberg’s prediction.

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  19. Coel,

    “On your questions on neutrino masses and other constants, we really don’t know what determines them or whether they vary in other bubbles. Again, I was making no claim that these things are known or certain, only that they are a sensible path to explore down.”

    But how do you intend this path to be explored? Other bubble universes are beyond observational reach, so there is absolutely no way to verify that the Standard Model constants take different values over there. I don’t see in what other way can the hypothesis of randomized physical constants be explored, either experimentally or theoretically. Moreover, our lack of understanding of the values of SM constants is sometimes spinned as a solution for itself — we don’t understand the values because they are environmental, and we postulate that the values are environmental because we don’t understand them. It’s claiming victory when faced with defeat. In your own words from one of the above comments:

    “One of the the reasons that theoretical physicists have started thinking about the constants being contingent accidents is that they were actually making rather little progress in explaining the values of the constants in terms of anything more fundamental.”

    The statement “the values of parameters are accidental” is not an answer — it is rather a lack thereof. In that sense, different constants in different bubbles is *not* a sensible path to explore down, and (lacking any possibility of observation) is not a scientific statement. Instead, shouldn’t we focus harder to try to find that elusive explanation for their values?

    Best, 🙂
    Marko

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  20. Coel,
    don’t you see how circular your reasoning is?
    You start with a fact that needs explaining(vacuum energy), construct a speculative hypothesis to explain the fact and than claim the model is validated because it has explained the fact.

    That is classic circular reasoning and we have even’t dealt with your misuse of the word ‘validate’. And you still haven’t accounted for why Carroll wrote that article in the first place.

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  21. Hi Marko,

    But how do you intend this path to be explored?

    By pursuing a wide program of trying to understand neutrinos and all the other particles in the standard model, and trying to understand what determines their masses. I don’t have any easy answers for how to do this. But advances often come from unexpected directions.

    In that sense, different constants in different bubbles is *not* a sensible path to explore down, …

    Exploring the idea that particle masses might be a contingent accident by pursuing observations within our observable universe is a sensible path to go down (and if evidence points to that idea then it would have the implication that they could well be different in other bubbles).

    … and (lacking any possibility of observation) is not a scientific statement.

    It is a scientific statement if it is pointed to by models that have been verified in our observable universe.

    Instead, shouldn’t we focus harder to try to find that elusive explanation for their values?

    That is not an “instead”, that is exactly the path one would take, in order to know whether they might be different elsewhere.

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  22. Which statement of mine in the original piece do you think was over-stated?

    What is over-state is the underlying idea that it is worth thinking, either for lay people or for scientists themselves, about a “multiverse” with varying measured physical constants to explain the values of the physical constants that we have already measured.

    For any such model that you can propose (whose Lagrangian has no free parameters, and instead the “vacuum” of the theory has large spatial variability), I can propose an alternative without varying measured physical constants (by inserting free parameters into the Lagrangian and removing the corresponding spatial variability from the “vacuum”, actually making it more vacuum-like). I would leave it to practitioners to decide which model would be simpler or more parsimonious. But I think that the choice will be obvious for anyone required to do any precision calculations with either model. There is no reason to fear or dislike free parameters in your model, especially if they make the life of practitioners easier.

    Of course, as soon as any evidence for spatial variability of measured physical constants appears, the game totally changes. But it is this evidence that is nowhere to be found.

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  23. Woops, sorry, I got a bit confused about what you were asking (although your question is perhaps a little confused too).

    If you are asking about infinite galaxies, then you should be asking about whether space is infinite. If you are asking about infinite universes then you should be asking about eternal inflation. Inflation has very little to do with whether space is infinite or whether there are infinite galaxies. Those questions are independent of inflation.

    My answer dealt with eternal inflation. Coel’s deals with infinite space/galaxies.

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  24. You just simply seem to be rewording here, The question here is whether he has in fact explained how this process works without some notion of falsifiability. For example, his statement, “help us understand the world,” would seem to mean “help us understand a model of the world.” I don’t see these as necessarily equivalent. Is perhaps the notion of the process of how to determine what’s falsifiable the question, given the nature of theoretical physics? In other words, he allows for “acceptance” and rejection (“discarded”), but doesn’t explain how it might unfold, other than to provide us the assurance that it will. Or is he saying that this process is perpetually ongoing or provisional of necessity?

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  25. Hi labnut,

    First, you persistently ignore Carroll’s central contention, that we must relax the falsifiability criterion.

    Not at all, he is saying exactly what I am saying. If we can validate a model, then we can legitimately accept predictions of that model, even if not all of those predictions are falsifiable. I agree with Carroll. If, for example, well-tested models of planetary motion predict a solar eclipse long before any humans were around to record it, and thus falsifying the prediction of that eclipse is not possible, then it is still scientific to accept the occurrence of that eclipse.

    You should read and understand why he wrote the article.

    I have done and do!

    He is saying that because we cannot validate the multiverse hypothesis.

    No, he is saying that we *can* validate the multiverse hypothesis, by using observations in our world that validate a model that entails the multiverse. Thus it is valid in the same sense that prediction of the pre-human eclipse is valid.

    If we could there would be no need to relax the falsifiability criterion.

    He is saying that not all aspects of a model need to be falsifiable, it is sufficient that some aspects are.

    And so, his basic starting point is that we cannot validate the multiverse hypothesis with the data at our disposal. You are saying the opposite, you are maintaining that we can.

    I have not said that *current* data is sufficient to validate the multiverse model, we need a lot more data before it would be secure. And that is what Carroll also says near the end of his article.

    Vacuum energy is lower than the prediction by some orders of magnitude. How does that fact validate the multiverse? Is it not possible that several other explanations are possible? Validate is a strong statement that there is a nearly inescapable link between the fact and the conclusion.

    OF COURSE there are other explanations possible! OF COURSE this one item does not “validate” the multiverse proposal! For the SEVENTEENTH time can I point out that no-one (not Carroll, not Weinberg, not me) is claiming any degree of certainty about this!

    But, at the cutting edge of science, where we do not have certainty, what we do is compare different tentative models and see which one is doing best. It is a simple fact that Weinberg’s PREDICTION of the non-zero and small value of dark-energy is so far the most successful explanation of the value of dark energy. Thus this multiverse model is currently doing well in comparison with competing models. That OF COURSE does not mean that it is right! But it does mean that it is in the game as a serious scientific proposal.

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  26. Guys, try to keep your cool and not shout (capital letters). At some point, discussions reach diminishing returns, and one ought to be content with having made one’s points and let the chips fall where they may.

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  27. Hi Thomas,

    You just simply seem to be rewording here,

    Yes, since I’m unsure what you’re taking issue with.

    The question here is whether he has in fact explained how this process works without some notion of falsifiability

    He is not doing away with falsifiability altogether, he is saying that not all aspects of a theory need be falsifiable. The validation process is still a comparison of the model with the data and seeing whether that verifies or falsifies the model.

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  28. Hi labnut,

    don’t you see how circular your reasoning is? You start with a fact that needs explaining(vacuum energy), …

    No, that’s not what Weinberg did. The crucial point here is the distinction between zero dark energy (which was the standard model at the time) and small dark energy. At the time there were no observations distinguishing between those two and thus he did not start from a “fact”. He went against the then-consensus for zero dark energy, and his prediction that it was not zero was later verified. He did not know it was non-zero at the time.

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  29. Coel: “He is not doing away with falsifiability altogether, he is saying that not all aspects of a theory need be falsifiable. The validation process is still a comparison of the model with the data and seeing whether that verifies or falsifies the model.”

    Yeah, I suppose I’m lost. You’re explanation of why Carroll thinks it’s time to do away with falsifiability sounds reasonable, a little like the Raven Paradox except without the hypothesis being even technically falsifiable. Kind of like assembling car parts from a junk yard without knowing in advance whether they work or will work together and repeating as necessary until they finally do, but not knowing for sure that the car will start. It either will or won’t.

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  30. On the”world of faith” and the “delicious irony when an activist atheist like Sean Carroll appeals to the unobservable to explain the observable” (labnut): The presence of physical unobservables in a physical model (A) is not the same as the addition of nonphysical unobservables to a physical model (B). One might call these two different kinds of faith, but in A the same physical vocabulary is used to model the physical unobservables as is used to model the observables in the overall model. In B, there is a completely different vocabulary needed to express the nonphysical unobservables that are added to a physical model. In B, the notion of a model that includes both physical observables and nonphysical unobservables is somewhat dissonant, whereas in A, there is one physical model expressed in the same physical vocabulary.

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  31. Hi, Coel.

    How about temporal variability of the inflaton field, is that a good starting point?

    Let me guess that you mean: Is that a good starting point for thinking about spatial/temporal variability of everything else that is not the inflaton field? Thought is cheap, evidence is not. The evidence for the latter hypothesis is still lacking, no matter how good the evidence for the temporal variability of the inflaton field, since the two things are independent.

    But I thought that we already covered this in previous exchanges. I’m not sure what there’s left to be said.

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  32. Coel, I suppose I’m beating a dead horse, but what surprised me about Carroll’s proposal for the retirement of the falsifiability criterion was the connection to theory as opposed to hypothesis. Throughout his proposal, he only mentions theory.

    As a lay person, I have always been told there is a difference between the two, most simply stated that the criteria for theory were greater than those for hypothesis, the difference being substantiation, with theory requiring testable data under more varied and broader circumstances. So far as I know, multiverses are still hypothetical. We discuss the MUH, not the MUT. And Einstein’s General Relativity is a theory, not a hypothesis.

    So is there any meaningful distinction between the two? And if so, what is it?

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  33. My final comment on this matter concerns the matter of scientific honesty.
    Underlying scientific method is a determined, unswerving dedication to rigorous intellectual honesty. This demands, among other things, that we clearly label our ideas honestly, to reflect the real state of affairs. Unverified, unverifiable, speculative hypotheses should be labelled for exactly what they are. Anything else is dishonest.

    In an earlier comment (http://bit.ly/1o1yJfg) I listed the five dangers of adopting Sean Carroll’s approach and retiring falsifiability as a criterion for good science. The sixth danger I should have listed is that of intellectual dishonesty.

    I know that we have entered the age of moral relativism but this is one principle we dare not sacrifice. Rigorous, careful honesty lies at the very bedrock of science, it is the foundation of science. We must label things for what they really are and not dress them up in what we hope or wish they could be.

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  34. Hi Thomas,
    Physicists don’t really make much of an issue of whether to use words like “hypothesis” or “theory”. The term “hypothesis” would tend to be used for something that can be stated in one sentence, whereas a “theory” would take more sentences to state. The word “theory” can be used for anything from something highly speculative and untested to something regarded as true beyond reasonable doubt.

    By the way, Carroll is not rejecting falsifiability entirely, he is saying that falsifiability and the need for empirical evidence should not be interpreted too narrowly, and that they can be very indirect.

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  35. Woops, sorry, I got a bit confused about what you were asking (although your question is perhaps a little confused too).

    I am certainly having trouble making myself understood.

    But the problem is conceptually simple. Tegmark is describing a situation where the number of galaxies is potentially infinite from the standpoint of the causal sequence, but actually infinite from the viewpoint of the observer.

    Coel’s description confirms that in the causal sequence it is a potential infinity. I you start with zero galaxies and start adding galaxies you always have finitely many galaxies even if this process continues forever.

    So all I am saying is that it seems to me that the reality of the situation is described by the causal sequence, ie a potential infinity, and not by the observer perception.

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  36. Hi Thomas,

    Yes, the standard distinction drawn between hypothesis and theory is that a hypothesis is tenuous and a theory is well-confirmed. This is what Massimo would tell you.

    Massimo and most other philosophers of science will disagree with what I’m about to say, but I don’t think that this distinction is quite right as it doesn’t really match usage, especially in theoretical physics. In my view, a theory is a well-developed model. String theory, theory of mind, number theory, musical theory, theory of evolution, phlogiston theory etc all match this definition. So in my view, phlogiston theory really is a scientific theory, but an obsolete one which has no place in contemporary science.

    On the other hand. a hypothesis is a claim that may or may not be true. So we could have the hypothesis that string theory is correct or a hypothesis that the theory of evolution is correct. Unfortunately, in this way of looking at it I don’t have a distinct word for theories that have been shown to correspond closely to reality. The best I could offer is corroborated theory or accepted theory.

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  37. Hi Robin,

    I don’t know which part of Tegmark you are referring to. Tegmark was probably talking about inflation, but the question you seem to be asking doesn’t have much to do with inflation.

    You seem to be confused about how we could start with no galaxies and end up with infinite galaxies. This ought not be too hard to understand. Imagine we have infinite apples, and zero rotten apples. Allow some time to pass. We now have infinite rotten apples.

    So if space is infinite, we start with no galaxies, but infinite amounts of energy which can later become particles which later condense into galaxies.

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  38. Mark, I don’t get the reference to Massimo. I’m sure he has his own thoughts on the matter. If you return to our conversation on google+, you will see my personal speculation as to Carroll’s motivation in nominating Falsifiability for retirement. It seems more like a case of special pleading where none is needed.

    There were nearly 200 contributions to the Annual Question at Edge, and of these the term appears 6 times, I think, mostly in Carroll’s piece.

    Much of Coel’s post seems to revolve around the question of whether the multiverse hypotheses are to be considered scientific or not. There have been many excellent comments regarding this question from some highly qualified to address it. I am not one of those.

    But I do sense that this question and Carroll’s proposal on the Edge are essentially about the demarcation problem and some special pleading by Carroll–circling the wagons, so to speak, against those pesky philosophers of science.

    As Rebecca Newberger Goldstein says in her proposal on Edge:

    “What’s wrong with this story? Well, for starters it’s internally incoherent. You can’t argue for science making philosophy obsolete without indulging in philosophical arguments. You’re going to need to argue, for example, for a clear criterion for distinguishing between scientific and non-scientific theories of the world.”

    And I haven’t seen that “clear criterion” yet.

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  39. Actually, I don’t think that “most” philosophers of science would subscribe to that view, and certainly I don’t. Yes, a theory is a well developed set of statements (which may be mathematically formulated) that describe a large chunk of reality – as in the case of the theory of evolution, or the theory of relativity. The term hypothesis, however, has different connotations. It can be a less well formulated, embryonic theory, but it more often then not it is a limited set of predictions, often based on a background theory. For instance, given the general theory of Newtonian mechanics and the anomalies in the position of the plant Uranus astronomers hypothesized the presence of an unknown additional planet, which turned out to be Neptune.

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  40. Hi Massimo,

    Actually, I don’t think that “most” philosophers of science would subscribe to that view, and certainly I don’t.

    I’m pretty sure I tried to make this point on a discussion on Rationally Speaking at some point, where you told me I was flatly wrong. There may have been some misunderstanding.

    However, your subsequent elaboration strikes me as reasonably similar to my summary of your views, where I described theories as well-confirmed and hypotheses as more tenuous. I accept and agree with the clarification that “hypothesis” is also less detailed, but the examples of theories you give (evolution, relativity) are well-confirmed, and you say that a theory is supposed to describe a large chunk of reality. This seems to answer in the negative the question of whether phlogiston theory or string theory are theories. I think they are, even though they may not describe reality. This is the essence of where I think my view differs from yours.

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  41. I’m not sure that Sean Carroll feels the need for a clear criterion. He is a supporter of philosophy and of the philosophy of science in particular, so it is not necessarily a problem for him if there is something of a gray area between science and metaphysics.

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  42. I don’t know which RS discussion you are referring to. But I should have said “attempt to describe a large chunk of reality.” String theory definitely qualifies as a theory.

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  43. There is no need to personalize this, DM. The point seems to be a matter of relative commitment to a position. It is not even clear whether the approach is one of logic or something else, etc, etc.

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  44. Hi Thomas,

    The point seems to be a matter of relative commitment to a position.

    Well, if you have been following what I was saying, it seems not to be this.

    Theory: Well developed model of something. May or may not be well supported by evidence.
    Hypothesis: Basic truth claim. Generally not yet well supported by evidence.

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  45. Hi Thomas,

    Yes, the link you gave explains the way this distinction is usually explained, and it is this explanation I take issue with because it does not provide the best account of how the word “Theory” is used in practice. I think this (in my view) faulty account of the difference arose mainly to defend the Theory of Evolution against the criticism that it was “just a theory”.

    String Theory is the best example of a problem with the definition of theory as something well-substantiated. In addition we have antiquated scientific theories such as phlogiston theory as well as the usage of the word theory in other fields such as psychology (theory of mind), mathematics (set theory) and music. In all of these cases, theory can be used to describe a well-developed model, though it is not always proven to be accurate.

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  46. Hi DM,

    I did not say that i could not imagine going from zero to infinity, I was speaking specifically of the process described by Tegmark. If it were as you describe then he would hardly need to invoke the trick with time and relativity.

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