The multiverse as a scientific concept — part I

looking-for-life-in-the-multiverse_1by Coel Hellier

The multiverse concept is often derided as “unscientific” and an example of physicists indulging in metaphysical speculation of the sort they would usually deplore. For example, commenters here at Scientia Salon have said that the multiverse is “by definition not verifiable and thus outside the bounds of empirical science,’’ and that “advocates of multiverses seem to be in need of serious philosophical help” [1].

Critics thus claim that the multiverse amounts to a leap of faith akin to a religious belief. Indeed, the religious often accuse atheistic scientists of inventing the multiverse purely to rebut the “fine-tuning” argument that they say points to a creator god, though the fine-tuning argument is readily refuted in several other ways, and anyhow physicists really don’t care enough about theology these days to let that worry them; further, the concepts leading to a multiverse were developed well before theologians started taking note of the issue [2].

The purpose of this article is to argue that the multiverse is an entirely scientific hypothesis, arrived at for good scientific reasons and arising out of testable and tested cosmological models. To be clear, I am not asserting that the multiverse has been proven true, even on the balance of probability, but I am asserting that it is a serious scientific concept that will eventually be accepted or rejected on scientific grounds.

Several different concepts could be labelled a “multiverse”, but I am advocating one particular multiverse concept, that arising from what cosmologists call the “eternal inflation” version of Big Bang cosmology [3], which was developed to explain observations of our universe and predictions from this model have since been verified, putting it on a sound footing.

I am arguing that if a scientific theory predicts consequences A, B, C and D, and if we then verify that A, B and C are indeed the case, thus giving us confidence in the theory, then we have sound reasons for accepting D even if D cannot be directly verified. Indeed, we would be obliged to accept D unless we can construct another equally good explanation of A, B and C.

The Big Bang model

As long ago as Isaac Newton, people realized that a static universe doesn’t actually work. Stars and galaxies would fall together under gravity and thus they can no more be static than you could float a brick in mid air. But scientists ignored this — even Einstein rigged his equations to get a static universe — until Edwin Hubble produced empirical evidence that the galaxies are moving away from us. Observations tell us that our universe is expanding, and expanding in a uniform fashion as though space itself is expanding, carrying the galaxies along with it.

If one runs a uniform expansion backwards in time — say repeatedly halving all scale lengths and separations — eventually one arrives at a state with everything in the same place. However, we expect our understanding of physics to have broken down before any such “singularity,” and in particular we expect “quantum gravity” effects to dominate at a scale length of 10-35 meters, called the “Planck length’.’

We do not have a working theory of quantum gravity, but we know enough about quantum mechanics to suppose that our universe originated with a scale-length of 10-35 meters as a quantum fluctuation in a pre-existing ”state.” Understanding this process, and knowing about the pre-existing state in which the quantum fluctuation might have occurred, are not needed for the multiverse model that I expound below. However, where one quantum fluctuation can occur so can another, and thus it is natural to suppose that there might be many other such universes. In particular, we have no strong reason to suppose that the quantum fluctuation that originated our universe was the origin of all things or of time itself — though equally we lack arguments against those possibilities, and if you want to argue for them then go ahead [4].

The characteristic length of 10-35 meters leads to a characteristic time, being the time taken to cross that length at a speed limited by the speed of light, c, the fastest speed at which information can be transferred from one region to another. The value of c of 108 meters/second then gives a “Planck time” of 10-43 seconds. This in turn leads to the concept of an “observable horizon,” being the furthest distance (= ct) from which light/information can have traveled to us given an age, t, of the universe. Thus we conceive of our universe, having originated as a quantum fluctuation, obeying its natural tendency for length scales to expand (that being the solution of Einstein’s equations of General Relativity), and with the observable horizon also expanding as the universe ages.

Conventional cosmological models presume this universe to be spatially infinite in all directions (or, at least, so much larger than the observable horizon that we don’t have to worry about the observational consequences of any “edge”). Is there an alternative? One could argue that our original quantum fluctuation would have had some finite extent, and thus that our current universe would now have a much larger but still finite extent, and that somewhere beyond the observable horizon is a boundary to another “universe” originating in a separate quantum fluctuation, or to the “pre-universe” out of which our quantum fluctuation arose. Alternatively, one could have a “wrapped-around universe,” where space has a finite extent but one can travel forever in any direction, like ants walking on the surface of a sphere.

Observationally, the “curvature” of space in our universe is known to be very small (our universe is “flat” to 1% accuracy), and thus the scale of the wrap-around would have to be much bigger than our observable horizon. Still, if you wanted to argue that the original quantum-gravity fluctuation was self-contained, rather than being in a pre-existing state, then one could argue for a finite, wrapped-around universe along these lines [5].

The multiverse model is often criticized as “unscientific” for invoking universes that can never be seen and thus making claims that can never be verified. But this applies just as much to all cosmological models, which are usually presumed to extend to infinity. All of them are thus postulating that the universe stretches well beyond the observable horizon, from where (owing to the finite speed of light) we can never obtain information to verify any hypotheses. This feature does not make the model unscientific. If we are simply using principles of parsimony to postulate more-of-the-same, beyond where we humans can personally see it, then we’re being entirely scientific.

Is the above account supported by evidence? Yes, very much so. The expansion of space is observed in the red-shift of light from distant galaxies (caused by the expansion of photons’ wavelengths as they travel through expanding space). In addition, the Big Bang model predicts exactly what primordial elements were created in the expanding fireball of the first 1000 seconds. The abundances of Hydrogen and Helium-4, and of traces of Deuterium, Helium-3, and Lithium-7 are predicted to high accuracy — and when observers measured these abundances to test the Big Bang predictions they found an excellent match.

Further, the Big Bang model predicts the existence of “cosmic microwave background” (CMB) radiation, left over from when the ionized universe cooled enough to allow neutral atoms, 380,000 years after the Big Bang. After this prediction had been made the radiation was then found, exactly as predicted, stretched out into the microwave band by the subsequent expansion of space, leading to the first of three Nobel Prizes so far awarded for work on Big Bang cosmology [6].

The study of the CMB, a snapshot of how the universe was soon after the Big Bang, is now a major part of cosmology. Indeed, cosmology is on a sound empirical footing precisely because we can directly see what the early universe was like; since that time the universe has been transparent and thus we can observe photons that last interacted with matter in the early universe. Modelling the tiny temperature fluctuations in the nearly-smooth CMB produces strong constraints on cosmological models. As measurements have improved, from the COBE satellite to NASA’s WMAP and recently ESA’s Planck satellite, at each step better data could have produced results incompatible with the Big Bang models, but at each step the concordance between theory and observation has got better and more remarkable.

[next time: Inflation and the constancy of physical constants]

_____

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.

[1] See comments to this Scientia Salon article.

[2] See my blog post for why the “fine tuning” argument does not lead to any deity.

[3] Thus I am ignoring here the issue of a multi-dimensional multiverse possibly arising out of M-theory.

[4] See Lawrence Krauss’s book A Universe from Nothing for an example of this argument.

[5] When dealing with general relativity and quantum gravity the notions of time and space depend on the observer, so in discussing whether the universe has a finite extent we should specify where we’re observing from. A black hole could look finite from the outside but infinite from the inside.

[6] These being the 1978 prize to Penzias and Wilson for the discovery of the CMB, the 2006 prize to Mather and Smoot for the discovery of structure in the CMB, and the 2011 prize to Perlmutter, Schmidt and Reiss for the discovery of “dark energy” through the acceleration of distant supernovae. If the BICEP2 result holds up there is a likelihood of a fourth prize for the idea of inflation.

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98 thoughts on “The multiverse as a scientific concept — part I

  1. Hi Coel,

    Nice article and I think I mostly agree with you. However, I think there are a number of points on which we differ.

    I don’t think the idea of unobservable regions of space is what is dismissed as unscientific. Rather, what is dismissed as unscientific is the idea of regions of space where the constants or the very laws of physics are different. These are the ideas often raised to address fine-tuning, and these are the ideas which are unfalsifiable and not predicted by any scientific hypothesis which is supported by evidence. Yes, there are models of inflation which allow for the possibility that different bubble universes could have different physics, but there is currently no evidence to support this model over other models where all bubbles have the same physics or where there is only one eternally inflating universe, and there is little reason to suppose there will ever be such evidence.

    Even if inflation can be disproven, the idea of a multiverse will not. No matter what evidence we could ever find, there is always the possibility that there could be other universes. We cannot prove a negative, so the generic idea of a multiverse is genuinely unfalsifiable, even though specific models which allow specific kinds of multiverse may be falsified from time to time.

    So in a way I’m with the religious apologists. I think fine tuning is a problem which needs to be resolved, and I think that the multiverse, while a tantalising solution, is fundamentally an unscientific one.

    However, unlike the religious apologists, I don’t think this is much of a problem. I think the multiverse must be true for philosophical reasons, and the fact that it is unfalsifiable and unscientific does not bother me unduly. It would still resolve the fine tuning problem and this alone is grounds for rejecting fine tuning as a compelling argument for God.

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  2. When I first heard of the Big Bang I was about 8 and nobody told me that it was supposed to have only happened once, so I naturally assumed that what could happen once could happen many times.

    So the Multiverse has always seemed a perfectly natural idea to me.

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  3. It appears knowledge, provable statements about the future, advances based on better “microscopes” or tools for perceiving, mainly seeing, finer detail. We see this in brain research.

    Another reality seems to be that as tools uncover greater complexity the perceivable and imagined complexity, following rules of evidence laid out in this article, becomes truly daunting. it is quickly pushing the boundaries of human, comprehension, perception and certainly natural language and cultural forms.

    So, the natural processes, over millions of years, that produced the complexity inside of a cell, say microtuble transportation of neurotransmitters to cell membranes – instantly, also can presumed to have produced equal complexity in human brain > behavior mechanisms.

    Our minds, language and cultural institutions, of course, seek greater simplicity in the face of much, much great complexity and, thus, uncertainty. Uncertainty, in the face of hyper complex cosmological theories and brain theories, climate change, evolutionary theories, vaccines, etc cause a defensive fear response – fight or flight, freeze.

    As we see to the facts about the theory of multiverses from belief system like philosophy, religion, etc.

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  4. Hi DM,

    Rather, what is dismissed as unscientific is the idea of regions of space where the constants or the very laws of physics are different.

    This is discussed in Part 2, out Thursday, so I’ll defer a reply until then. Have a read of that and then let me know what you think.

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  5. The thing is, even without the new evidence available, a multiverse is technically no less parsimonious than a singleton universe because a singleton universe hypothesis would have to include a reason why whatever event began our universe could only happen once.

    . Cannot infer from our current state of knowledge “there is one universe”, unless you mean “there is at least one universe that we know about”.

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  6. Hi Robin,

    a multiverse is technically no less parsimonious than a singleton universe

    I would go further. I would say that a multiverse is more parsimonious than a singleton universe. That doesn’t make the multiverse a scientific hypothesis, but I guess that the assertion that the universe is a singleton is also unscientific.

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  7. Good post 🙂

    I don’t have any problems with multiverse but talking about whether or not it’s a scientific hypothesis would lead to talking about what exactly do you consider a scientific hypothesis versus a non-scientific hypothesis?

    It seems to me that cosmologists are using adductive logic (inference to the best explanation) to compare models of cosmology that best fit our current knowledge and that seems like pretty standard fare in science but certainly not exclusive to science.

    I guess another area that might be interesting is if no direct evidence is likely to come of any of the cosmological models and we can only infer from what we know, does it really make that big of a difference? Does the multiverse hypothesis guide research and create progress in physics in a different direction versus the other models?

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  8. I disagree. Science is a form of empiricism, and from an empirical standpoint the idea of a singleton is more acceptable, and certainly more scientific, than the idea of a multiverse, which is only deduced from theory. That doesn’t mean the idea of a multiverse is false, but no, I wouldn’t put the two on the same level.

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  9. Coel,

    “I am arguing that if a scientific theory predicts consequences A, B, C and D, and if we then verify that A, B and C are indeed the case, thus giving us confidence in the theory, then we have sound reasons for accepting D even if D cannot be directly verified. Indeed, we would be obliged to accept D unless we can construct another equally good explanation of A, B and C.”

    I really don’t think so. We are only obliged to seriously entertain D, not at all to accept it. Scientific theories are human constructs, and as such their epistemic reach is limited. We have plenty of examples from the past of successful theories that made initially untestable predictions, which eventually turned out to be false, leading scientists to reject or modify the theory.

    As Jim Baggott (in Farewell to Reality) argues, once science loses contact with the empirical it steps into metaphysics. This is fine as long as attempts are made to “step back behind the line” of empirical verification. If one stays permanently beyond such line, then one is engaging in scientifically informed metaphysics, as far as I’m concerned. (And I should add that I have no problem with that, as long as one admits that’s what one is doing.)

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  10. Hi Massimo,

    from an empirical standpoint the idea of a singleton is more acceptable, and certainly more scientific, than the idea of a multiverse, which is only deduced from theory.

    I cannot understand how it seems this way to you. The idea that the universe is unique is just as unfalsifiable and unsupported by evidence as is the idea that it is not. At least the idea of a an inflationary multiverse has theoretical support, whereas there is no reason at all, scientific or otherwise, to assume the universe is a singleton.

    Yes, not even Occam’s razor, which if applied properly would actually favour a multiverse in my view.

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  11. Hi Massimo,

    I really don’t think so. We are only obliged to seriously entertain D, not at all to accept it.

    You may be right in the specific case of a multiverse that the evidence is not sufficient to accept it, but I think you’re wrong on the principle here. I think we may indeed be obliged to accept D, depending on what A,B,C and D are and how much it would mess up our understanding of reality if D were false.

    For example we might have as our A,B,C every organism that has ever been described in biology, and our D as a new undescribed organism found in an ordinary place (i.e. not an extremophile or an alien). I boldly propose that D’s chemistry is carbon based and will have evolved by natural selection, as would be predicted by everything we know about terrestrial biology from studying A,B,C. I do not think that accepting this claim would be unscientific.

    Or, perhaps general relativity can deal with solar systems with up to eight or nine planets (the A,B,C of our own solar system) but falls apart when describing solar systems with many more planets.(a hypothetical D). Does that mean we are not justified in assuming that General Relativity does describe solar systems with thirty planets?

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  12. Dear Massimo,

    We are only obliged to seriously entertain D, not at all to accept it.

    I would argue for “accept”, but in the provisional sense that accepting any scientific statement is provisional and might be overturned by better data.

    … once science loses contact with the empirical it steps into metaphysics.

    Let’s take a specific example, we think that the central core of the Earth is mostly made of Iron and Nickel. But we have no sample of such material and cannot verify it directly. So is a claim about the composition of the Earth’s core beyond the empirical and thus metaphysics?

    What we do have is lots of theories about planet formation, seismology, the abundances of elements, etc, that predict all sorts of things that have been verified (the A, B, C etc). And these models also predict that the core of the Earth is made mostly of Iron and Nickel (this is the “D”).

    Thus all the evidence points to D. We should accept D (provisionally), unless we can produce some alternative model that does just as good a job on the planet-formation & seismology etc (the A, B, C, etc) but does not predict D. In that situation we can reasonably reject D.

    But if the only models that do a good job on all the other stuff all predict D, then we are obliged to accept D, even if we can’t verify it directly. It would be unreasonable and unscientific to reject the D (the idea that the core is Fe/Ni), just out of whim or personal aesthetic dislike of the idea.

    Indeed, this concept is pervasive in all of science, in nearly all parts of science we are using models to extrapolate to things that we cannot verify directly. But this sort of indirect verification is accepted as scientific. Indeed virtually all verification in science is indirect in exactly this sense, since primary sense data from immediate surroundings is the only “direct” empirical information.

    I thus argue that claims about a multiverse are entirely scientific in exactly the same way that claims about the composition of the Earth’s core are.

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  13. Hi DM,
    I agree. Let’s make the point by picking an extreme example. All 30,000,000 humans that have been tested (that number is a pure guess by the way) have DNA based on the same base pairs (A, C, T, G) and uses the same genetic code for translation into proteins.

    Now let’s consider Mrs Sarah Whoever, from Small-Town Wyoming, who has never had her DNA investigated. Should we “accept” the idea that Mrs Whoever’s DNA also uses the same base pairs and genetic code, or is that metaphysical speculation that goes beyond the empirical and is thus unscientific?

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  14. Does the logic still hold for something that is rather familiar, say existence of iron and nickel, versus something completely new like another universe (possibly with different set of physical laws)? At least to me it seems like we are inferring about the role of something that is very well verified in the first case and making a much bolder claim about something we can’t, even in principle, directly empirically test in the second.

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  15. I actually have no problem with speculation on multiverses, as long as the speculators are clear that it is speculation. Sophisticated mathematical speculation to be sure, but speculation just the same. Sure, scientists made predictions with the Big Bang, but the Big Bang itself’s relation to empirical evidence was much closer than that of most of the multiverse theories.

    As to whether or not it’s science, I think the demarcation between science and philosophy in these areas is an artificial one, and I wish physicists would lose their abhorrence of anything they do being labeled philosophical. Anyway, the more assumptions beyond observable evidence a theory has to make, the further it is from science and into logical speculation. Whether it makes sense to call it science will probably always be a matter of judgment, at least until it gets supporting empirical evidence, or some other way to graduate it to reliable knowledge.

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  16. Hi imzasirf,
    This is also something mentioned in Part 2 (which deals with possibly different laws of physics).

    The short answer is that if we are adding information content to a model without good empirical justification, then that is invalid and unscientific. However, if all one is doing is extrapolating from known information, then it is valid.

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  17. DM,

    “The idea that the universe is unique is just as unfalsifiable and unsupported by evidence as is the idea that it is not.”

    I didn’t claim that the universe is unique. I made the much more modest empiricist claim that all we see is one universe, so we need additional evidence in order to buy the quite extraordinary idea that there are others.

    “not even Occam’s razor, which if applied properly would actually favour a multiverse in my view.”

    Looks to me like you have an odd view of Occam’s razor. At any rate, the razor is just a heuristic, not a foundational principle of logic.

    “You may be right in the specific case of a multiverse that the evidence is not sufficient to accept it, but I think you’re wrong on the principle here”

    Your counterexamples (and Coen’s) are fair. I think it is better to say that acceptance of “D” solely as a theoretical consequence of ABC is warranted along a continuum that goes from the entirely uncontroversial (we have observed gazillions of carbon-based living organisms on earth, so the next one is likely to be carbon-based), to the reasonable but speculative (earth’s core is made of iron and nickel, to the ones about which judgment really ought to be suspended (multiverse).

    This is simply another version of Sagan’s (based on Hume’s) dictum: extraordinary claims and all that.

    Here is another way to see it: the first case above (carbon life forms) is a simple extrapolation of something we have observed many times; the second one (earth core) is actually indirectly verifiable (it makes predictions about geological parameters, planetary mass, etc.); the latter one invokes a class of things of which we have only experienced one, indeed zero if we are talking about universes with different laws.

    SelfAware,

    “I think the demarcation between science and philosophy in these areas is an artificial one, and I wish physicists would lose their abhorrence of anything they do being labeled philosophical.”

    Indeed. I see physics and metaphysics as continuous, just as I see science and philosophy as continuous — despite the fact that, as I’ve argued even in this forum — they do correspond to distinct “peaks” in the intellectual landscape. The peaks grade into valleys, and the valleys grade into each other. (Don’t ask me how I quantify the landscape, though…)

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  18. Hi Massimo,

    I didn’t claim that the universe is unique. I made the much more modest empiricist claim that all we see is one universe, so we need additional evidence in order to buy the quite extraordinary idea that there are others.

    You said that a singleton universe was more of a scientific concept than a multiverse. You didn’t claim that it was unique, but you seem to think that the claim that it is unique is in some sense more scientific than the claim that it is not. Neither claims are scientific in my view, but there is a symmetry there. If you think we need evidence to believe in a multiverse, I claim we need evidence to believe that there isn’t. I have no problem with your agnosticism, only with your apparent view of the relative scientific merit of the two claims.

    Looks to me like you have an odd view of Occam’s razor.

    Only if you confuse the razor to mean that we should prefer theories that posit lesser amounts of “stuff” as opposed to fewer, more elegant concepts.

    Which is more parsimonious, the claim that space extends beyond the visible universe or the claim that it doesn’t? The claim that it does posits more “stuff”, more particles and more space, but the claim that it doesn’t is less parsimonious because it makes an unfounded assumption that space stops even when there is no reason to believe it does. Occam’s razor favours elegant, simple theories, and a multiverse with billions of universes is more elegant than a solitary universe in the same way that a planet with billions of acorns is more elegant than a planet with just one.

    At any rate, the razor is just a heuristic, not a foundational principle of logic.

    That goes without saying. My point is that even Occam’s razor fails to provide a reason to prefer a solitary universe. Without Occam’s razor I can’t think what grounds you could possibly have to think that a solitary universe is more probable.

    to the ones about which judgment really ought to be suspended (multiverse).

    Then the two of us at least are on the same page. On inflation at least. I still think the MUH multiverse is almost a logical necessity, but the argument for that does depend on computationalism so there’s no point getting into it.

    This is simply another version of Sagan’s (based on Hume’s) dictum: extraordinary claims and all that.

    Why do you find the idea of a multiverse extraordinary? If you can imagine living hundreds of years ago, would you have found the idea of other worlds or suns to be extraordinary? It really seems just as banal to me by now, but maybe I have simply grown accustomed to thinking this way.

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  19. Hi SelfAwarePatterns,

    Anyway, the more assumptions beyond observable evidence a theory has to make, the further it is from science …

    I entirely agree. But now suppose that you are not making additional assumptions, but just extrapolating from models that you have already largely verified, isn’t that entirely scientific? The point about the multiverse is that cosmologists are not making *additional* assumptions to produce a multlverse, they are finding that the models that they need to adopt, in order to fit data, then go and generate a multiverse. Thus, to a great extent, we cannot model existing cosmological data *unless* we have a multiverse.

    On the point about philosophy: it is as much philosophers who insist that philosophy is something distinct from science. Personally I’d regard philosophy as a part of broadly defined science.

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  20. The A, B, C and D argument doesn’t sound convincing to me. You can easily have A, B, C without D. Classical mechanics is an example. The fact that a theory describes, explains and predicts A, B and C very accurately doesn’t mean that D is real. In general a theory doesn’t tell you always where its limits are. The limits are shown by experiments and observations, or by the incompatibility with other well-established theories.

    Also, I don’t think we know enough about inflation to attach belief to “predictions” by this particular theory. In 1850 classical mechanics was far better understood and tested than inflation now is. And still it turned out to be very wrong on important points.

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  21. Dear Massimo,

    I made the much more modest empiricist claim that all we see is one universe, so we need additional evidence in order to buy the quite extraordinary idea that there are others.

    I would not interpret empiricism that way. The empirical evidence does not validate specific entities so much as information and models about how the world works. If we observe one universe then we can conclude “there is a universe-generating mechanism”. Given that, there is no reason to suppose it operates only once as oppose to an infinite number of times. Indeed claiming that it does so once and then never again can be argued to be less parsimonious, since you have to explain the “never again”, whereas to explain an infinite number all you need is the initial “there is a universe-generating mechanism”.

    Looks to me like you have an odd view of Occam’s razor. At any rate, the razor is just a heuristic, not a foundational principle of logic.

    I’d argue that Occam’s razor is a sound principle that can be validated by probability theory. Under the razor the model requiring the least information is preferred, and the whole point about a multiverse is that it is the most parsimonious model in terms of specifying information (more on that in Part 2).

    This is simply another version of Sagan’s (based on Hume’s) dictum: extraordinary claims and all that.

    I agree with that dictum, but don’t regard a multiverse as an extraordinary claim, but more as a necessary consequence of the current cosmological models that work best. Cosmologists are not *adding* to those models to produce a multiverse, simply extrapolating them.

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  22. Hi Patrick,
    Firstly, science is always provisional. We take the best theories that we have that best model current data. You’re right that they may be superseded and shown to be wrong, but the scientific thing to do is to adopt the best that we have now.

    Thus, if the best theories that we have (theories that have made predictions which have been verified, thus giving us good reason to accept those theories) predict D then the scientific thing to do is to accept D.

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  23. Coel,
    I’d argue that Occam’s razor is a sound principle
    What makes it a sound principle rather than a heuristic?
    What is the principle?
    What do you mean by ‘sound’?

    I’d argue that Occam’s razor … validated by probability theory
    Really? How would you validate it?

    the model requiring the least information is preferred
    Preferred by you but are your preferences generally true?
    Why should it be preferred?
    Has this been demonstrated empirically to always be true?

    the whole point about a multiverse is that it is the most parsimonious model in terms of specifying information
    An infinite number of universes has less information than a single universe?
    That is a beyond extraordinary claim that needs more than extraordinary justification, not a bald statement. Can you give us some authority for this claim?

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  24. Coel,
    If we observe one universe then we can conclude “there is a universe-generating mechanism”. Given that, there is no reason to suppose it operates only once as oppose to an infinite number of times.

    There is no reason to suppose it only happened once.
    And equally there is no reason to suppose it happened an infinite number of times, or any number of other times.
    We simply don’t know and we should not elevate ignorance into certainty. We are talking about science and science is supposed to be anchored by empirical fact.
    Certainly, we may speculate, but then we must clearly label it as speculation and nothing else.

    There is an important cautionary principle here. When scientists state speculative hypothesis with an air of authority that seems to convey certainty then the lay world interpret it as a given fact. And why shouldn’t they? They know too little to discern between the many speculative hypotheses that are floated around. Science has an educational role and it must exercise that role with great care because the world at large has accorded them so much authority and recognition.

    I think more care is needed to make qualified and limited claims that recognise speculative hypotheses as being just that, highly speculative with very little empirical justification.

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  25. Coel,
    The multiverse model is often criticized as “unscientific” for invoking universes that can never be seen and thus making claims that can never be verified.
    That seems to be a rather good reason for claiming is is ‘unscientific’, if we understand science as a process of constructing models, testing the models or validating them by observation. After all, this is what happened with the Higgs boson. Shouldn’t we expect the same to be true for all science?

    But this applies just as much to all cosmological models, which are usually presumed to extend to infinity.
    That is a strange justification. All the cosmological models are highly speculative but that hardly makes the multiverse model more scientific or less speculative.

    Science usually prefaces new advances with speculative hypotheses. Some of the speculative hypotheses turn out to be good science but many others are discarded and quietly forgotten. An emotional commitment to a speculative hypothesis is a very dangerous thing that can severely distort the advance of science.

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  26. from Celebrating Stephen Hawking’s 70th Birthday
    http://www.ctc.cam.ac.uk/stephen70/

    “We can only live in those parts of the multiverse where the laws of mathematics and physics allow stable information processing and reliable predictions. That is why physics and mathematics are so efficient in our part of the multiverse.” — Andrei Linde

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  27. Coel,
    If we are simply using principles of parsimony to postulate more-of-the-same, beyond where we humans can personally see it, then we’re being entirely scientific.
    No, you are indulging in guesswork. I would never call that being scientific.

    This is what Sean Carroll said in his article ‘How Did the Universe Start’, 2007-04-27:
    Personally, I think that the looming flaw in all of these ideas is that they take the homogeneity and isotropy of our universe too seriously. Our observable patch of space is pretty uniform on large scales, it’s true. But to simply extrapolate that smoothness infinitely far beyond what we can observe is completely unwarranted by the data. It might be true, but it might equally well be hopelessly parochial. We should certainly entertain the possibility that our observable patch is dramatically unrepresentative of the entire universe, and see where that leads us.

    And Sean Carroll is one very shrewd physicist with some knowledge of philosophy. As he says, your observation is completely unwarranted by the data.

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  28. Coel wrote:

    I would not interpret empiricism that way. The empirical evidence does not validate specific entities so much as information and models about how the world works. If we observe one universe then we can conclude “there is a universe-generating mechanism”. Given that, there is no reason to suppose it operates only once as oppose to an infinite number of times. Indeed claiming that it does so once and then never again can be argued to be less parsimonious, since you have to explain the “never again”, whereas to explain an infinite number all you need is the initial “there is a universe-generating mechanism”.

    Yes. That it is “extraordinary” is simply how it strikes us. As I said before, I find the singleton universe idea more extraordinary just because of my initial assumptions on hearing about the Big Bang.

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  29. DM,
    I think the multiverse must be true for philosophical reasons,
    I have never heard that argument before. Can you expand on that and explain the philosophical reasons? As it happens, I think there are good theistic reasons for believing in the multiverse, so I would like to compare your philosophical reasons with my theistic reasons.

    Fine tuning is simply a problem of insufficient knowledge. In the presence of partial knowledge many things look genuinely puzzling.

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  30. I would go along with the A,B,C D thing just so far because there are predictions and predictions.

    There is a difference between saying that Relativity predicts that light bends in a gravitational field and saying (as Max Tegmark does) that Schrodinger’s wave equation predicts infinite parallel universes.

    This is really more of a conjectured consequence than a prediction since we could have no way of knowing if it exhausts the possibilities or even is the best option among possibilities.

    If ‘D’ is of the first type of prediction then, yes, we have good reason to accept it even before it is tested (or even if we can’t test it), but if it is of the second type, then no, we need more before we can accept it.

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  31. “I would argue for “accept”, but in the provisional sense that accepting any scientific statement is provisional and might be overturned by better data.”

    I’d use ‘entertain’ rather than ‘accept’ in the case of the multiverse because I think the strength of ‘scientific statements’ vary. In other words, I can see all ‘scientific statements’ as being provisional, but I think some are lot more provisional than others.

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  32. Hi Coel,
    From what I understand about multiverse theories, there are additional assumptions being made. You describe one in the post, that the universe is infinite. On the theories themselves, you’re also assuming that there are no unobserved variables, boundaries, or other complications. These may be good assumptions, or they may not be. Newtonian mathematics predicted the planet Neptune, but they also predicted the planet Vulcan.

    On science and philosophy, I’m not a professional scientist or philosopher, but I regard science as a philosophy, with a hazy boundary between other philosophies.

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  33. “We can only live in those parts of the multiverse where the laws of mathematics and physics allow stable information processing and reliable predictions. That is why physics and mathematics are so efficient in our part of the multiverse.” — Andrei Linde

    Just demonstrates my point that even brilliant physicists can founder when they venture into philosophy. That explain only how it is that we are able to do maths and physics in the first place, not why they are so efficient.

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  34. Robin,
    since we could have no way of knowing if it exhausts the possibilities or even is the best option among possibilities.
    Exactly so.
    An example of other possibilities is the cyclic universe hypothesis by Penrose, Turok and others.
    Sean Carroll once wrote that there were more than 1,600 papers on ArXiv proposing different cosmological mechanisms. He commented that it was time to stop generating ideas and start generating solutions.

    We are confronted by a profound mystery so it is right that we go through a preliminary and creative phase of generating many speculative ideas. This creative flowering of ideas increases the chances that we will find the right explanation.

    But then we must restrain our ideological enthusiasm and admit that these are no more than creative ideas. Dressing up creative ideas in dense mathematics is not a ‘proof’ of anything except mathematical skills.

    But more than this, there is an ideological elephant in the room. Why are a certain group of people with certain, strong ideological dispositions so eager to rush to judgement and prematurely endorse the multiverse hypothesis? I think some honesty about motivation is in order.

    There is a deeper problem here. Ever since the 1600s science has marched forward confidently, buoyed up by two great assumptions:
    1) the world is predictably orderly and lawful,
    2) all is observable, all is explainable.

    These were comforting assumptions that assured us that no matter what could not be understood today, it will be understood tomorrow. The scientific horizon was bright and beckoning, the scientific future seemed unlimited. Today there are storm clouds on the horizon. We are beginning to realise that some things are unknowable, that there can be boundaries to knowledge which science cannot penetrate. The eager rush to endorse certain speculative hypotheses seems to be an emotional reaction to this fact, a form of denialism, a nothing from nothing which explains nothing.

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  35. Hi Labnut

    An infinite number of universes has less information than a single universe?

    Absolutely. I can compress the contents of every possible sequence of characters between 0 and a million characters long into one short sentence. There, I just did it, and a computer program could reconstruct every sequence from such a description, though it might take a long time.

    However, to specify any one sequence of a million characters I will need, at a first approximation, a million characters (although if it is English text or some other non-random sequence it can likely be compressed somewhat by algorithms such as ZIP or RAR).

    More analogies abound. A block of marble has more stuff than a statue carved from that marble and could be used to produce an unlimited number of different types of 3D shapes, but a statue needs more information to describe it.

    A planet with a billion acorns is more likely than a planet with one acorn.

    It is simply true that a large (or infinite) set is often simpler than a single object.

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  36. Actually Sean Carroll’s point reinforces Coel. If the entire observable universe is not representative of all of reality, then there are other spaces far away which are very different. These can be referred to as other universes, and this is basically what the inflationary multiverse looks like. On inflation, our universe and other universes are spatially connected but different. They are causally disconnected because they are receding from each other much faster than light.

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  37. Of course, science is always provisional. But that doesn’t mean much if we’re trying to evaluate inflation. It may be the best theory, but how good is it really? We’re not talking about the standard model of particle physics here.

    I don’t think the mulitverse is “unscientific”. But in a certain sense it’s odd. We have a very succesful theory of particle physics (the standard model) but everybody agrees that it has too many problems to take it seriously as the ultimate description of nature. On the other hand we have a theory (inflation) that doesn’t look too bad, but that isn’t established like the standard model – far from it. And suddenly we should take predictions like the multiverse seriously? Shouldn’t we wait until we know more about inflation?

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  38. But of course you were arguing specifically against Coel’s view that the universe extends beyond the observable horizon.

    Carroll is not arguing against this point at all. Carroll would certainly agree that the universe continues much as it is, relatively homogenous for a long distance beyond the observable horizon, perhaps for an effectively infinite distance. However he also thinks that beyond this potentially infinite region (and yes, it is possible to be beyond an infinite amount of space on general relativity) there could be dissimilar regions.

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  39. Hi SelfAwarePatterns,

    From what I understand about multiverse theories, there are additional assumptions being made. You describe one in the post, that the universe is infinite.

    But that’s not an *additional* assumption, that is a common feature of non-multiverse cosmological models also. We’re trying to build a model of the universe using the fewest assumptions, and most in accord with observations. If you want to have a non-infinite universe then you have all sorts of awkward questions about a supposed “edge”, and you need to build extra information into your model to deal with that, and none of that is guided by observations. That is why standard cosmological models presume that they extend to infinity (or, as I say in the article: “so much larger than the observable horizon that we don’t have to worry about the observational consequences of any “edge””).

    On the theories themselves, you’re also assuming that there are no unobserved variables, boundaries, or other complications

    Again, those are not “additional” assumptions, they are just the standard practice of building the most parismonious model one can, and then adding complications if and when data demands it. This process is exactly what the scientific method is.

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  40. Hi labnut,

    Really? How would you validate [Occam’s razor]?

    My attempt at validating Occam’s razor is here. It’s not a claim for it being absolutely valid, but valid in the probabilistic sense of being overwhelmingly likely.

    An infinite number of universes has less information than a single universe?

    Yes! This is in Part 2, but in brief, the first of these takes far less information to specify:

    A) Take one universe; duplicate an infinite number of times; assign all fundamental constants at random.

    B) Take one universe; set G = 6.67E+11 m3·kg−1·s−2; set h = 6.626 E-34 J·s; set e = 1.602E-19 C, and so on for another twenty five or so constants.

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  41. Hi labnut,

    And equally there is no reason to suppose it happened an infinite number of times, or any number of other times.
    We simply don’t know and we should not elevate ignorance into certainty. We are talking about science and science is supposed to be anchored by empirical fact.

    First, we are not talking about “certainty”, but about what good scientific models imply. Second, this is indeed anchored in empirical fact.

    Let’s take a comparison. Suppose we find one fossil of a new type of animal, never seen before, that lived 200 million years ago. Do we take a very narrow view and say “we now have empirical evidence for ONE such animal having existed, but any suggestion that there was more than one at that time is unscientific speculation”?

    No, we don’t, we say that we know how animals arise because we understand evolution. And from that we know that it is *impossible* for there to have been only one member of that species. The only mechanism we know of for generating new species inevitably produces lots of members of that species.

    Therefore the scientific deduction is that, despite having found only one such fossil, there were many members of that species alive then.

    In the same way, suppose we develop a model for generating ONE universe, namely ours, and we look at that model and realise that it is impossible for that mechanism to produce only one universe, in the same way that it is impossible for evolution to produce a species with only one member. At that point the only scientific deduction is that there are many such universes.

    Certainly, we may speculate, but then we must clearly label it as speculation and nothing else.

    I agree with you in general, and some speculations are indeed just speculations, but the whole point I am arguing is that the multiverse is much more than that, and is nowadays supported by a lot of sound science and evidence.

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  42. Hi labnut,

    That seems to be a rather good reason for claiming it is ‘unscientific’, if we understand science as a process of constructing models, testing the models or validating them by observation. After all, this is what happened with the Higgs boson. Shouldn’t we expect the same to be true for all science?

    Yes, science is a process of “constructing models, testing the models or validating them by observation”. BUT, there is big difference between “can never be seen” and “can never be verified”. The point is that most verification in science is indirect. This is the whole point of my statement that if a model predicts A, B, C and D, and we *directly* verify A, B and C, then we have *indirectly* verified D (unless we have an alternative model that does just as good a job at explaining A, B, and C, but which implies not-D).

    [Indeed, even the verifications of A, B and C are going to be indirect, and rely on all sorts of other models that have been well-enough verified.]

    Indeed, this is exactly how it worked over the Higgs Boson. The Higgs particle can never be “seen” (it does not interact with photons), but the model that predicts the Higgs particle also predicts lots of other things that can be verified, and indeed have been verified, leading to confidence in the existence of the Higgs particle, despite the fact that we can never see it.

    The same can be said about lots of other aspects of physics (neutrinos, quarks, gluons, dark matter, etc), indeed virtually all of physics is of this nature, and the multiverse is no different in this regard.

    All the cosmological models are highly speculative …

    I don’t agree nowadays, after lots of advances over the last couple of decades they are now on a very sound evidence-supported footing.

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  43. Hi labnut,
    In reply to my:

    If we are simply using principles of parsimony to postulate more-of-the-same, beyond where we humans can personally see it, then we’re being entirely scientific.

    No, you are indulging in guesswork. I would never call that being scientific.

    You might not call that being scientific, but scientists would.

    This is what Sean Carroll said in his article ‘How Did the Universe Start’, 2007-04-27:

    If you read that snippet in context you see that Sean Carroll is arguing exactly what I am, the next sentences go on to argue for eternal inflation. Again, this is in line with parsimony of *information content*. (Interpreting things like parsimony and Occam’s razor in terms of information content is all-important here.)

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