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 replies

  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.

    • 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.

    • 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”.

    • 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.

    • 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.

    • 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.

    • 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.

    • Hi Labnut,

      I think the anthropic principle and fine tuning are enough to suspect that there must be a multiverse, but the MUH proves it if you accept naturalism, Platonism and computationalism, each of which I think have philosophical reasons for belief.

      The full argument is on my blog.

      http://disagreeableme.blogspot.co.uk/2013/12/the-universe-is-made-of-mathematics.html

  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.

    • Robin,

      Me too in a way. After I learned we lived in a galaxy that was just one of many galaxies in the universe, a multiverse containing many universes became conceptually possible, and so did a metamultiverse of multiple multiverses, and so on.

  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.

  4. But since we don’t seem to have reached the multiverse part yet, I will wait until then to comment further.

  5. 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?

  6. 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.)

    • 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?

    • 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?

    • 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.

    • 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.

    • 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.

    • “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.

    • I agree with A,B,C,D argument under one important condition: D must be a scientific statement, too. That is, there must be a way to test it directly, at least in principle. Composition of Earth’s core can be tested (theoretically): with neutrino rays, or just by blowing the hole Earth up. On the other hand, I am much less sure about multiverse,
      at least in the form it is usually presented.

  7. 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.

    • 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.

    • 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.

    • 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.

    • Coel,
      I think when you assume that there are no structural additions far enough beyond observations to make grand predictions about reality, you are making notable, essentially metaphysical assumptions. When such assumptions are not testable, not even in principle, then I think it’s fair to wonder if it still falls within the demarcation of science.

      BTW, the quilted multiverse (Tegmark’s Level I multiverse) is the only one I know of that “only” assumes infinity. From what I understand, the others, like bubble universes, are making additional assumptions.

      Again, as I said in my initial comment, I’m fine with speculation about multiverses. We might someday discover that one or more of the multiverse conceptions is reality. My complaint is that the people doing that speculation often act like what they’re doing is something fundamentally different than what many philosophers do.

  8. Reblogged this on Jordanfel's Blog and commented:
    This has yet to be read, but it will be!

  9. 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…)

    • 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.

    • 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.

    • 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?

    • 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.

    • 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.

    • 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.

    • 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.

    • 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.

  10. 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.

    • 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.

    • 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?

    • 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.

      I agree. It is still a tenative proposal, though the evidence for it keeps getting better. The next couple of decades (much better data on the CMB) should either confirm or refute it.

      And suddenly we should take predictions like the multiverse seriously? Shouldn’t we wait until we know more about inflation?

      Yes, we should take predictions from inflation seriously. Currently inflation is the only game in town in terms of explaining the origin of our universe. I don’t see what we gain by waiting, rather than pushing these ideas forward.

    • > I don’t see what we gain by waiting, rather than pushing these ideas forward.

      If you mean the multiverse with “these ideas”, I disagree. I can’t see what physics possibly could gain by pushing that particular idea forward. It’s much too early. Until we understand inflation, until we have chosen from the multitude of inflation theories the correct one and checked it thoroughly in that corner where its predictions are testable, the multiverse is physically speaking little more than a curious aspect of some theories that may or may not be correct.

      And even if inflation is correct in that testable corner, one should be very careful with the A, B, C and D reasoning. The electromagnetic aether was an interesting idea with some pretty spectacular properties. It seemed to be a necessary consequence of electrodynamics, a thoroughly tested theory. It was the only game in town, our best guess etc. But we all know what happened to the aether. Perhaps the multiverse is the aether of the twenty-first century. The difference is, of course, that we can test the effects of an electromagnetic aether. I don’t mind if people call the multiverse a scientific idea. But the important question is: “Is the multiverse a good scientific idea?” I’d rather answer “no”.

    • … one should be very careful with the A, B, C and D reasoning.

      We have no choice but to use such reasoning all the time in most areas of science, indeed many sciences are entirely dependent on it. All historical/observational sciences for a start.

  11. 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.

    • 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.

  12. 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

    • “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.

  13. 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.

    • 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.

    • 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.

    • 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.)

  14. 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.

    • 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.

  15. Coel,
    To clarify your ideas it helps to understand that there are many kinds of hypotheses with varying degrees of strength. The strongest level of hypothesis is good science while the weakest level hardly qualifies at all.

    To see the multiverse idea in context we need to find its place in the levels of hypotheses.

    1) strong hypothesis
    It is testable, logically derived from well established theory, has empirical foundations, has strong explanatory power and is the leading contender among alternative hypotheses.

    2) weak hypothesis
    As above but has weak explanatory power and may not be the leading contender.

    3) tentative hypothesis
    It is in principle testable, derived from established theory but more work needs to be done to develop the idea into a viable contender.

    4) speculative hypothesis
    An exploratory hypothesis, located in the empirical space, but with a weak theoretical and empirical foundation. It is intended to locate plausible ideas that may, with further work, become stronger, testable hypotheses.

    5) tenuous hypothesis
    An exploratory hypothesis that examines the edges of the possible. It may, just conceivably, be developed into a stronger hypothesis capable of being tested. Its’ links to existing theory are tenuous at best.

    6) faitheism
    A conceptual hypothesis that can not be tested. Its explanation lies outside the realms of the possible. It may be a useful intellectual exercise but has no practical bearing on empirical science. This is philosophy masquerading as science, often heavily disguised in abstruse mathematics.

    Seen in this way the multiverse hypothesis, given the most optimistic and kindest interpretation, can only be described as a level 5, tenuous hypothesis, that barely has any scientific pretensions. More realistically, it is an example of faitheism. What lends credence to this point of view, apart from complete inability to empirically test the concept, is the strong ideological commitment of its adherents.

    • What lends credence to this point of view, apart from complete inability to empirically test the concept, is the strong ideological commitment of its adherents.

      I have no ideological commitment to the inflationary multiverse since I am satisfied that the MUH is true. Nevertheless I would certainly not call it faitheism. It is supported by evidence because it does agree well with cosmological models which agree with observations.

      In any case, the multiverse is not a hypothesis at all. It is a result of most inflationary models. It is these models which are hypotheses, and these models taken collectively meet your criteria for strength.

    • DM,
      I am satisfied that the MUH is true
      Without any evidence? Now I see how you can make the next comment (below).

      It[the mulitverse] is supported by evidence
      Can you explain how it is supported by evidence?

      the multiverse is not a hypothesis at all
      That is a neat rhetorical dodge to escape my categories of hypothesis but it doesn’t do the job. You are trying to define the problem out of existence.
      But let’s take your statement at face value, implausible as that may seem. If it is not a hypothesis then what is it? What label do you apply to this kind of belief? Does anyone else agree with you?

      I will stick with calling it a faitheist hypothesis. This accurately describes the complete lack of evidence and tenuous link to reality.

    • DM,
      It[the mulitverse] is supported by evidence
      This of course raises the question of what standards you apply to evidence. What is supporting evidence? Is that evidence sufficient to sustain your assertions?

      For example, if I claim to be a big game hunter and you, critical empiricist that you are, challenge that assertion. I reply by stating that once upon as time, long ago, in my mis-spent teens, I hunted deer on our farm(quite true, by the way). This, I said, was supporting evidence for my claim that I am a big game hunter(a false claim). You would be sure to emphatically state this slight evidence was insufficient to sustain my extravagant claim and you would be right.

      As you can see, that vague statement, ‘supported by evidence’, covers a multitude of sins of misdirection, exaggeration and sometimes plain falsehood(as in my example). To have any credence you must say more than just that and you must show how the evidence is sufficient to sustain your assertion.

    • Hi labnut,

      This of course raises the question of what standards you apply to evidence. What is supporting evidence? Is that evidence sufficient to sustain your assertions?

      My position is not that the inflationary universe must be true. It is that it is not entirely without evidential support. I agree that there are different degrees of evidential support, and I am agnostic on the inflationary multiverse, although perhaps leaning ever so slightly in the direction of believing in it.

    • Hi labnut,

      Without any evidence? Now I see how you can make the next comment (below).

      Yes, on philosophical grounds. I don’t think the MUH is a scientific hypothesis, but I think it is logically necessary given some (to me) reasonable premises.

      Can you explain how it is supported by evidence?

      The **inflationary** multiverse is supported by evidence because it is predicted by models of inflation which are supported by evidence. I am not convinced that the evidential support is enough to consider the question settled, but it’s certainly not unsupported.

      If it is not a hypothesis then what is it?

      It is a prediction of a hypothesis. In the same way, it is not a hypothesis that my mug will fall if I drop it. This is a prediction of the law of gravitation (or of general relativity).

      Does anyone else agree with you?

      Yes. Tegmark does at least, and I imagine so would many other cosmologists.

      I will stick with calling it a faitheist hypothesis.

      Enjoy!

    • Hi labnut,

      Seen in this way the multiverse hypothesis, given the most optimistic and kindest interpretation, can only be described as a level 5, tenuous hypothesis, that barely has any scientific pretensions.

      I’d put the multiverse hypothesis at about 1 and a half on your scale.

    • “…often heavily disguised in abstruse mathematics.”

      A general description of what is meant here by “abstruse” and by “disguised” would be welcome, but would perhaps be asking too much.

      But at least an example or two of each could be given to help enlighten us.

      For example, would differential geometry be abstruse? Is there any way of detecting which researchers in pure mathematics are perhaps wasting their lives, immersed in a cesspool of abstruseness?

      And again, would those engaged in the mathematical calculations of quantum field theory be engaged in some cunning effort to disguise their work? Is this sort of thing somehow an effort to keep the ordinary working man, perhaps working in philosophy, from examining it carefully? It would really be helpful to know at least one example of a field of intellectual endeavour which is maintaining itself by this kind of subterfuge.

  16. DM,

    “If you think we need evidence to believe in a multiverse, I claim we need evidence to believe that there isn’t.”

    Absence of evidence is not evidence of absence? That sort of old theological chestnut? I am talking from an empiricist perspective: we see one universe, so we have pretty strong evidence for n=1. We don’ see (but some of our theories predict) other universes, so we have reasons to entertain, but not yet to accept, the idea that n>1.

    “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.”

    One of the reasons I’m skeptical of leisurely applications of Occam’s razor is because it is often not at all clear what counts as more or less parsimonious, and this is a good case in point. A the very least, if the multiverse is characterized by universes with different laws (is it?) that would seem to make it far less parsimonious, but I don’t know.

    “Without Occam’s razor I can’t think what grounds you could possibly have to think that a solitary universe is more probable.”

    Lack of empirical evidence, since we are talking science, after all. And I wouldn’t put it in terms of “less probable” (probabilities are another thing to handle with great care in these cases).

    “I still think the MUH multiverse is almost a logical necessity”

    Well, you know what I think of 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?”

    If you don’t find the idea of a multiverse extraordinary you’ve lost your sense of awe, my friend. At any rate, what I meant was that it is an extraordinary claim relative to our current knowledge (especially in terms of empirical evidence). Same went for other suns a few hundred years ago, of course. But some extraordinary ideas become accepted and ordinary, some don’t.

    Coel,

    “The empirical evidence does not validate specific entities so much as information and models about how the world works.”

    Ah, so you are an anti-realist about scientific theories? I’m not, so for me specific entities need empirical validation (direct or, more likely, indirect).

    “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.”

    But this is science, so I don’t need just reasons, I need evidence.

    “Indeed claiming that it does so once and then never again can be argued to be less parsimonious”

    See my comment to DM about Occam.

    “I’d argue that Occam’s razor is a sound principle that can be validated by probability theory.”

    I know, others have tried it as well. I think it is often so difficult to articulate precisely that it is close to useless for anything but the simplest cases. Moreover, Lee Smolin (in The Trouble with Physics) has a whole chapter about physical theories that were strongly preferred on ground of simplicity and elegance just during the 20th century. Many of them went the way of the Dodo…

    “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.”

    I would be very cautious about trusting so much a relatively recent model that comes in a variety of flavors, with each flavor making different predictions, as I understand it. And of course we still need confirmation of BICEP2 results. Moreover, as you know, there are still competitors to inflation, and there are a number of unresolved problems with the whole idea. It seems to me to be far less cut and dry than your presentation implies.

    • Hi Massimo,

      I am talking from an empiricist perspective: we see one universe, so we have pretty strong evidence for n=1

      That’s just not true. If I find one unfamiliar green spider in the garden, is that pretty strong evidence that the number of spiders of that species in existence is one?

      We have evidence for n≥1. We have absolutely no evidence whatsoever for n=1.

      so we have reasons to entertain, but not yet to accept, the idea that n>1.

      Sure. Agnosticism is fine by me. Just don’t go telling me we have reasons to believe n=1, because we don’t.

      A the very least, if the multiverse is characterized by universes with different laws (is it?) that would seem to make it far less parsimonious, but I don’t know.

      In my view, parsimony relates to how much information is needed to specify the important features of a described object.

      If there are finite numbers of those laws and they are entirely arbitrary, then yes, a lot of information is needed and this is not parsimonious. However if they are generated according to some more fundamental laws (e.g. inflation) then we need only to describe the fundamental law. If there are an infinite number of universes with all possible sets of laws (e.g. the MUH) then this sentence is more or less enough to specify the whole ensemble.

      If you don’t find the idea of a multiverse extraordinary you’ve lost your sense of awe, my friend.

      I guess I have in a way. That happens when you get used to an idea and recalibrate your intuitions. In now find the idea of evolution quite intuitive, for example, which I imagine you do too. It’s still kind of awesome though, isn’t it?

      I no longer find it extraordinary that there are other suns either. I guess I still feel awe sometimes when I look at the night sky, but I don’t find that these are extraordinary in the sense of requiring extraordinary evidence. The idea now makes perfect sense to me.

    • Dear Massimo,

      Ah, so you are an anti-realist about scientific theories? I’m not, so for me specific entities need empirical validation (direct or, more likely, indirect). [...] But this is science, so I don’t need just reasons, I need evidence.

      No, I’m not anti-realist about scientfic theories, but I don’t think that “specific entities” can be divorced from models of the universe.

      For example, if we ask what an electron is, then the answers are about how it interacts with other things, and those answers are part of a bigger model about how things work (e.g. about the electromagnetic force). That’s the sense in which I said that empirical evidence does not validate specific entities, it validates models of how things work.

      Thus, what cosmological data is actually doing is validating a model that explains why the universe around us looks as it does. If we then inspect the best model and find that it won’t just produce one universe but oodles of them, then we do have good *empirical* (but indirect) evidence for those other universes. And note that for most things in science we only have indirect evidence anyhow.

      I presume you would accept my comparison up-thread, that if we find one fossil of a hitherto-unknown species, then that is strong empirical evidence for the past existence of lots of members of that species. It is so because of our understanding of how such a fossil would have come to be, that the mechanism resulting in that fossil requires there to have been many members of that species. Few paleontologists would demand multiple fossils before accepting that there was a whole population of that species.

    • Coel,
      I presume you would accept my comparison up-thread, that if we find one fossil of a hitherto-unknown species, then that is strong empirical evidence for the past existence of lots of members of that species.

      No, that is a bad comparison. It only works because we have prior knowledge of fossils and species. We don’t have prior knowledge of other universes so we cannot possibly reason that one implies many. The implication, one implies many, is dependent on other knowledge which is completely absent in the case of multiverses. You have made a simple error in logic.

    • Hi Labnut,

      Yes, the conclusion about fossils works because we have prior knowledge of the processes that lead to fossils.

      What I am asserting is that, from observations of our universe, we now have understanding of the processes that led to our universe.

      Our understanding of those processes says that they won’t just produce one universe, but will automatically produce many of them.

    • Coel,
      In the case of fossils there was a great profusion of direct evidence for a multitude of fossils. In the case of multiverses we have absolutely no direct evidence of the kind that we have with fossils. Your comparison is stretching it I’m afraid.

      We have observed a huge number of fossils but we have never observed another universe and the theory suggest the we will never observe one.

    • Your comparison is stretching it I’m afraid.

      Yes, it is a “stretched” comparison to make a point, and the point is still valid: that if empirical data validates a model, and if that model predicts D, then we have empirical evidence for D.

      It is indirect empirical evidence, granted, but then nearly all evidence about everything in science is indirect in that sense.

    • Coel,
      your argument that the existence of one implies the existence of many is the strangest argument I have heard for a long time. There is no logical necessity that the existence of one thing implies that many exist. I invite you to show me the logical necessity of your argument.

      Your replies so far depend on the existence of other supporting evidence. That makes your one therefore many argument peculiarly dependent on the context and therefore not generally true.

    • Coel,
      if empirical data validates a model, and if that model predicts D, then we have empirical evidence for D.
      It seems to me you are making far stronger claims than is warranted.
      So let’s tighten up the argument and be more specific.
      1) What empirical data are you claiming supports your argument for a multiverse?
      2) What reasonable conclusions can be drawn from the data?
      3) What model has been validated by the empirical data? To the best of my knowledge only the Big Bang model has been validated. The Cyclic Universe and multiverse models are speculative extensions of the Big Bang model and they don’t necessarily follow from the Big Bang model.
      4) What experimental data validates the multiverse? How does it do so?

      And finally, you know that Penrose and Turok have constructed the cyclic universe model from the same data.
      5) How is it possible that such radically different models can be constructed from the same starting point?
      6) If we can construct such radically different models from the same starting point then surely we have a weak starting point. Why should we prefer one over the other?

    • Hi labnut,

      1) What empirical data are you claiming supports your argument for a multiverse?

      There’s more in Part 2, but mainly the power-spectrum of the CMB fluctuations (possibly also BICEP2, but that is unclear as yet).

      2) What reasonable conclusions can be drawn from the data?

      The CMB strongly points to inflation. Inflationary models almost inevitably generate a multiverse.

      3) What model has been validated by the empirical data? To the best of my knowledge only the Big Bang model has been validated. The Cyclic Universe and multiverse models are speculative extensions of the Big Bang model and they don’t necessarily follow from the Big Bang model.

      No, the multiverse model is not an “extension of” the Big Bang model. We build a model of how our universe arose in order to explain observations of our universe. We inspect that model and find that it doesn’t just produce one universe but produces oodles of them. In fact it is very hard to tune that model to produce only one universe, if you set the thing going it near-automatically produces “eternal” inflation that produces an infinite number of universes. That is the central point here. We currently cannot explain observations of our universe except with a multiverse-generating model.

      Can *you* suggest a model of the Big Bang that explains all of the observations and which does *not* produce a multiverse? (Turok’s cyclic model is also a multiverse model, by the way, so appealing to that won’t help you).

      4) What experimental data validates the multiverse? How does it do so?

      None, the relevant energy regime is too high to reproduce on earth.

      And finally, you know that Penrose and Turok have constructed the cyclic universe model from the same data.

      Turok’s cyclical model is vastly more speculative and vastly less supported by evidence and vastly less mainstream than the model I discuss in the articles. That does not mean it is wrong and the one I discuss right, but once again can I point out my central claim, which is not that the eternal-inflation multiverse has been proven true, but that it is solid and sensible science and supported by a lot of good evidence.

  17. “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.”

    Not being a predictivist and falsificationist, this is unconvincing. If you were to claim that we have experimentally confirmed the laws of nature to be thus, yet the inescapable conclusion of these laws is that there must be other universes, then I would grant your point. For demarcation purposes, either version does show I think that the multiverse approach is science, not metaphysics. But I think my version makes it a little easier to decide whether we consider it good science, (i.e., sound speculation, which is commonly essential to research,) or bad science.

    The fact that there are currently multiple models of the multiverse on tap does not in my view mean that the notion is more robust, but less. In your terms, there are more than one theory predicting D. I think that shows the multiverse is not a logically inescapable conclusion of the known laws of the universe. Further, the fundamental laws that would compel us to are somewhat disputed, or even, in the case of superstring theory, not yet formulated. We have so to speak, theories predicting A&B or B&C, while we have hopes of a theoy that really does predict A,B&C. Lastly, the actual infinite is not only full of conceptual difficulties but empirical ones. As I understand the literature, I conclude the multiverse approach is soundly rooted in known science but too speculative to compel assent..yet.

    A note on Occam’s razor and the universe generating mechanism. I think that Occam’s razor should be regarded as a foundational logical or ontological principle, rather than a heuristic. I think in context it was a proposition about Platonic Forms and similar metaphysical entities. I do think the notion that positing unnecessary entities without reason is unsound logic and raises an unanswerable question as to what it could mean to say they exist as causes when you cannot, even in principle, determine any effects. As a heuristic? Heuristics are for personal use, since they are not even arguments. The question here, first, is whether it is proper to posit the existence of a “universe-generating mechanism,” and separate fields of operation for its action?
    As noted, the laws of the universe as we know them now do allow the possibility. The question here, second, does the phrase “universe-generating mechanism” express this as if the UGM was some sort of Platonic Form? I think so. And I suspect there’s a little metaphysics smuggled in to try to strengthen the speculative science. I think it tends to weaken it.

    • Hi stevenjohnson,

      The fact that there are currently multiple models of the multiverse on tap does not in my view mean that the notion is more robust, but less. In your terms, there are more than one theory predicting D.

      Well no, “D” here is a specific prediction about a specific type of multiverse (the eternal-inflation multiverse arising out of current cosmological models). Any other proposals that might be labelled a “multiverse” are independent of that, and thus don’t affect the validity of that hypothesis.

      I think that shows the multiverse is not a logically inescapable conclusion of the known laws of the universe.

      That doesn’t follow. If “True Theory implies D” and also “False Theory implies D” then the latter does not detract from the former.

      Further, the fundamental laws that would compel us to are somewhat disputed, …

      True, but let’s remind ourselves what the article is about: “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”.

    • That doesn’t sound like much of an assertion to me. I assert that science will eventually accept or reject this assertion. I assert that the die, if square, in a crap game will total seven 1 in 6. I’d rather play craps.

    • Hi Thomas,

      That’s because you misunderstand Coel. He’s not saying the multiverse is true, he is saying it is a scientific idea, not idle speculation or metaphysics.

    • So, is it active (as opposed to idle) speculation? And what’s wrong with metaphysics, anyway?

    • Nothing is wrong with metaphysics in my view. But some physicists think there is, and Coel is one of them.

    • To put it in your terms, If theory I implies D, and theory II implies D, and theory III implies D and theory IV implies D, then it is a reasonable possibility that D is an implication of something all the theories have in common (conceptual troubles with infinity?) rather than a truly inevitable consequence of a thoroughly validated theory. Or worse, that we are mistakenly rediscovering a common bias in all these extrapolations. Having biases means it is more necessary to call for caution. Most importantly, although the implications of false theories are irrelevant, I don’t think that objection is relevant because we do not yet know which theories are false.

      But I hoped I was clear that I thought multiverse was largely scientific speculation, not metaphysics.

    • Well, since metaphysics is about deriving the logical implications of known facts, I fail to see a sharp line diving scientific speculation of this type and metaphysics…

    • I would have said that “deriving the logical implications of known facts” was physics (or other branches of science). Surely model-building in science is thinking about the implications of the known facts?

    • It depends on how far you go into speculative territory, I think.

    • Coel,
      speculation is an important preliminary tool for developing new solutions but it should never be passed off as good science. It becomes good science when it is verified empirically.

    • Hi labnut,

      speculation is an important preliminary tool for developing new solutions but it should never be passed off as good science. It becomes good science when it is verified empirically.

      I entirely agree, but you have way too narrow an understanding of what “verified empirically” actually means, it is vastly broader than seeing something with your own eyes. See my comment about Jurassic sauropods for more on that. Essentially X being “empirically verified” means we accumulate empirical evidence about a *model* that entails X to the extent that it would be perverse not to accept the model and thus X.

  18. > that if empirical data validates a model, and if that model predicts D, then we have empirical evidence for D. It is indirect empirical evidence, granted, but then nearly all evidence about everything in science is indirect in that sense.

    These statements are way to general to be of any use in physics. One doesn’t have empirical evidence for D, one has a prediction. A prediction that, when tested, may confirm the theory or break it. And if you can’t test it, it stays a prediction forever (and not a very interesting one, physically speaking).

    Some theories are so thoroughly tested that they can be used to make predictions about which we can say with a very high level of certainty that they are true. An engineer who designs a bridge is, in a certain sense, making a prediction. But she or he is making a prediction in a corner of a theory that’s thoroughly validated. I wouldn’t trust an engineer designing a bridge between the earth and the moon.

    There’s no comparable validation for inflation theories. It’s far from certain that such a validation is possible when it comes to the multiverse.

    And yes, nearly all the evidence in science is indirect. But in science, the “mediation” between observation and conclusion is done with thoroughly tested and validated techniques. Saying “inflation predicts the multiverse” is not a thoroughly tested and validated technique.

    • One doesn’t have empirical evidence for D, one has a prediction. A prediction that, when tested, may confirm the theory or break it. And if you can’t test it, it stays a prediction forever (and not a very interesting one, physically speaking).

      Again, I disagree, and stand by my stronger statement that we have empirical evidence of D. Let’s take the concept of giant sauropods roaming earth in the Jurassic. Do we have empirical evidence of it? Can we verify it? Well, what we do have are curious patterns in rock that we call “fossils”, and we have facts and theories about sedimentation and geology, and we have facts and theories about the physics of radioactivity — and what links all these together is a model of how things work. And that model explains/predicts many things, and many of these we can verify, such as the fossils. All of these are As, Bs and Cs.

      Another of the things that that model predicts is that there were giant sauropods roaming earth in the Jurassic. This is the “D”. Excellent, so let’s test this prediction, let’s hop on our time machine and go and see these dinosaurs with our own eyes. Only we can’t, and therefore “it stays a prediction forever”. Except that we can verify all sorts of other aspects of the model, and eventually we have such confidence in our overall model that we accept the predictions of the model (such as the roaming sauropods). At this point we say that we “have strong empirical evidence” for the sauropods. Which we do, even though we’ve never seen them. But this strong empirical evidence is actually entirely dependent on our models that connect everything together.

      Indeed, if we then think about it further, we realise that many of the As, Bs and Cs are really also Ds, things we can only deduce. Because, at the very basics, all we have are a stream of photons and phonons impinging on our sense organs. And everything that we think we know beyond that is a constructed model.

      We can never directly verify those dinosaurs because they lived in the past. But indirect verification — overall verification of models that predict dinosaurs — is sufficient, and it’s also sufficient in principle for the multiverse. To decline to accept that is anti-scientific in the same way that creationists and their “Were you there?” question is a wholesale rejection of science.

    • I genuinly don’t understand what dinosaurs are doing here. “Strong empirical evidence” simply doesn’t have the same meaning in physics and in paleontology.

    • > But indirect verification — overall verification of models that predict dinosaurs — is sufficient, and it’s also sufficient in principle for the multiverse. To decline to accept that is anti-scientific in the same way that creationists and their “Were you there?” question is a wholesale rejection of science.

      Ahem. That’s quite a remark. Physics goes forward exactly by not accepting that “overall verification” of A, B and C is sufficient for D. You really are rewriting the history of physics here.

    • It is sufficient for the *provisional* acceptance of D. Of course the next stage is then further development and testing of the models.

  19. DM,

    “That’s just not true. If I find one unfamiliar green spider in the garden, is that pretty strong evidence that the number of spiders of that species in existence is one?

    It really is beyond me how you can possibly equate one vs many spiders with one vs many universes. If not qualitatively, at the very least quantitatively we are talking entirely different situations, with the latter warranting much more epistemic caution than the former, no?

    Somehow this reminds me of a good joke about three philosophers traveling by train in New Zealand. Through the window they spot what appears to be a black sheep grazing in a pasture nearby. The first philosopher says: “Well, we now know that there are black sheep in New Zealand.” The second one says: “Actually, we now know that there is at the least one black sheep in New Zealand.” The last one says: “Technically, we now have evidence that one side of one sheep in New Zealand is black.” I know, to you I sound like the third philosopher…

    “parsimony relates to how much information is needed to specify the important features of a described object.”

    Maybe, but that notion is often surprisingly hard to cash out. Also, as Smolin has clearly demonstrated in The Trouble with Physics, there are plenty of instances in the history of physics where parsimony as a criterion of theory selection has failed.

    “If there are finite numbers of those laws and they are entirely arbitrary, then yes, a lot of information is needed and this is not parsimonious. However if they are generated according to some more fundamental laws (e.g. inflation) then we need only to describe the fundamental law.”

    Got any non speculative candidate for such fundamental explanatory principle? No, the MUH doesn’t count, I’m talking physics.

    “That happens when you get used to an idea and recalibrate your intuitions. In now find the idea of evolution quite intuitive, for example, which I imagine you do too.”

    Yes, but I don’t think the available science warrants accepting the idea of a multiverse as “intuitive.” And no, I don’t find evolution intuitive at all, I find the theory compelling, which is a whole different thing.

    “I no longer find it extraordinary that there are other suns either.”

    Of course the big difference is that you can see them.

    Coel,

    “I’m not anti-realist about scientfic theories, but I don’t think that “specific entities” can be divorced from models of the universe.”

    In a sense that’s correct, because of the interdependence of theory and what counts as data. But I still want empirical evidence before accepting the possibility of unobservables. In other theories we have it: we can’t observe electrons (to use your own example), but there is little doubt that they play a crucial role in well established physical theory. I simply doubt that the multiverse has reached anything like the status of electrons.

    “If we then inspect the best model and find that it won’t just produce one universe but oodles of them, then we do have good *empirical* (but indirect) evidence for those other universes.”

    I think that’s the crux of our disagreement: that’s not empirical evidence, direct or indirect. It is theoretical reason. We have indirect empirical evidence of electrons because we can measure some of their properties. We have no indirect evidence of the multiverse.

    “I presume you would accept my comparison up-thread, that if we find one fossil of a hitherto-unknown species, then that is strong empirical evidence for the past existence of lots of members of that species.”

    Yes, but I consider that a disanalogy with the case at hand, because we know of other biological species and we can thus reasonably extrapolate. We don’t know of any other universes outside our own.

    Well guys, let’s continue this discussion tomorrow on the new thread under part II of Coen’s essay. Also, heads up: next week I will be publishing a piece by Jim Baggott, author of Farewell to Reality: How Modern Physics Has Betrayed the Search for Scientific Truth, which will present a very different take from Coen’s on the status of contemporary physics.

  20. “… the pre-existing state in which the quantum fluctuation might have occurred …” The universe we now have is nearly flat. Thus, our universe might have originated from a pre-existing universe which was nearly flat. The main alternative to a quantum fluctuation causing the Big Bang might be a nearly instantaneous collapse of a previous expansion of a previous Big Bang.
    The main alternative to Linde-Guth type inflation might be some form of Milgrom inflation.
    the dark matter crisis

  21. Multiverse of Tegmark’s Type II has been proven experimentally first in 2005 from 10+ billion 1Hz gravity measurements taken by the (Canadian) superconducting gravimeter as the Earth’s most accurate instrument used for studying G, as well as mathematically by expressing G (and thus g) via c on both quantum and mechanist scales – as Einstein hinted in 1930s, and multi-physically that is w/o units of any universe as Newton attached units to G to close his own/our universe’s physics.

    http://lanl.arxiv.org/abs/physics/0608026

    http://hal.archives-ouvertes.fr/hal-00808674

    http://www.mynewsdesk.com/ba/pressreleases/as-big-bang-gets-downgraded-to-a-bang-the-first-scientific-proof-of-the-multiverse-claimed-975493

  22. [Sent to Scienta Salon 5/6/14]

    In the Middle Ages officials made theories about angels on a pinhead, now we make universes on a pinhead. With progress like that, who needs madness?

    Where in physics is it demonstrated that a “quantum fluctuation” can grow in a universe. What’s the law? What’s the experimental evidence? Is the so called “Big Bang” itself the evidence that proves the theory that gave rise to it?

    It may be time to cut the Gordian Knot. The Gordian Knot of unobservable universes. Perhaps, maybe, lets’ be honest, we just don’t need the Big Bang. All the computations about light elements may be rethought, in light of galactic core explosions and the like (those are assuredly very hot).

    All we have is the expansion, and we even have a mechanism for it: Dark Energy. If we just do with what we have, the universe is immensely old. Sorry for those who have universes in their head, though… Sounds like fun, while it lasts.

    Yet, science is about what can be checked, and making theories with what exist: the universe and Dark Energy exist, and have been checked. The multi-universes, with their inflation out of quantum fluctuation are just the stuff of bad science fiction, and when we get to zombie brains (another consequence of this weird derangement), you just will make guys in the street laugh. Hard.

    I have talked with highly educated people who, when their hear about these absurdities, want to cut money even to place that make excellent physics such as CERN. Madness has its drawbacks.

    http://patriceayme.wordpress.com/2013/12/19/universe-100-billion-years-old/

  23. Multiverse is the playground of the Atheist.
    Look, these guys entered the fields of Origins because unlike Theists–who mostly entered Medicine with their science scholarships–they cant accept God created the Universe . If you cant accept that… even though their own data has no confirmed this Universe is probabilistically impossible because of the overwhelming appearance of design in the Math…you are going to venture into the Absurd and thats precisely what they have done.

    This story is over and has been for 30 years. When a hypotheses–this is NOT a theory–is only believed by Atheists, it smacks of a pathological Bias. How does an atheist sleep at night when his data shows the universe is designed? Who accepts their own doom? Many have turned and accepted defeat but still many more have dug their heels in and embraced the stupidity of multiverse even when they know it destroys all probability and thus destroys scientific outcomes–universe wide. One nutjob even has proposed the universe must end soon so the probability of infinity can equal out–now that’s lunacy squared.

    All that is happening now is they are embarrassing themselves.

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