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. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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!

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. “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.

  17. 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”.

  18. 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.

  19. 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.

  20. 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?

  21. 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.

  22. > 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”.

  23. 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.

  24. > 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.

  25. … 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.

  26. 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.

  27. 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…

  28. 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.

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

  30. 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?

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

  32. 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.

  33. 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.

  34. 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.

  35. 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.

  36. 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.

  37. “…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.

  38. When I have heard the sheep joke it was a psychologist, a biologist and a physicist. Guess which plays which part.

  39. 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.

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

  41. > 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.

  42. “… 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

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

  44. 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

  45. [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/

  46. 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.

  47. 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.

  48. How does Multiverse make atheists “accept their own doom” if Infinity replaces any need for fairy-tales such as the Genesis? It’s the other way around, actually, or, as Linde put it vividly in his audio-interview to the Financial TImes: bye bye God, we don’t need you any more (http://www.ft.com/cms/s/2/9a306276-bf03-11e3-8683-00144feabdc0.html). It appears you theists are now resorting to the last weapon: confuse the enemy into believing they’re attacking their own tranches. Then “friendly fire” takes care of the enemy’s own soldiers. How clever!

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