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Are Parallel Universes Unscientific Nonsense? Insider Tips for Criticizing the Multiverse

The views expressed are those of the author and are not necessarily those of Scientific American.

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Although we don’t know whether parallel universes exist, we know something else about them with certainty: many people instinctively dislike them, and whenever a physicist writes a book about them, the Web erupts with claims that they are unscientific nonsense.

My new book Our Mathematical Universe proved to be no exception. “Is this still science?” the biologist Mark Buchanan wondered on the pages of New Scientist, “Or has inflationary cosmology veered towards something akin to religion?” The physicist Peter Woit dismissed it as “grandiose nonsense”.

Baby pictures of our universe 400,000 years after the Big Bang taken by the Planck Satellite match the predictions of the theory of inflation. / Max Tegmark

If you’re a multiverse skeptic, you should know that there are many potential weaknesses in the case for parallel universes, and I hope you’ll find my cataloging of these weaknesses below useful. To identify these weaknesses in the pro-multiverse arguments, we first need to review what the arguments are. Many physicists have explored various types of parallel universes in recent books, including Sean CarrollDavid DeutschBrian Greene, Michio Kaku, Martin ReesLeonard Susskind and Alexander Vilenkin. Interestingly, not a single one of these books (my own included) makes any outright claims that parallel universes exist. Instead, all their arguments involve what logicians know as “modus ponens”: that if X implies Y and X is true, then Y must also be true. Specifically, they argue that if some scientific theory X has enough experimental support for us to take it seriously, then we must take seriously also all its predictions Y, even if these predictions are themselves untestable (involving parallel universes, for example).

As a warm-up example, let’s consider Einstein’s theory of General Relativity. It’s widely considered a scientific theory worthy of taking seriously, because it has made countless correct predictions – from the gravitational bending of light to the time dilation measured by our GPS phones. This means that we must also take seriously its prediction for what happens inside black holes, even though this is something we can never observe and report on in Scientific American. If someone doesn’t like these black hole predictions, they can’t simply opt out of them and dismiss them as unscientific: instead, they need to come up with a different mathematical theory that matches every single successful prediction that general relativity has made – yet doesn’t give the disagreeable black hole predictions. This has proven a remarkably difficult task, eluding many brilliant scientists for about a century. In other words, for a theory to be testable (and hence scientific), we don’t have to be able to test all its predictions, merely one of its predictions.

So are there parallel universes, or is the universe we observe (the spherical region of space from which light has had time to reach us during the 13.8 billion years since our Big Bang) all that exists? We don’t know. The interesting claim that these books collectively make is that various theories imply that various types of parallel universes exist (see table), so that by modus ponens, if we take any of these theories seriously, we’re forced to take seriously also some parallel universes. Conversely, if we can experimentally rule out any of these theories based on their other predictions, we’ve destroyed the evidence for the corresponding parallel universes.

For example, Alan Guth, Andrei Linde and Alexander Vilenkin have argued that the cosmological theory of inflation generically predicts the Level I multiverse: a single space so large that it contains many universe-sized regions. Inflation may or may not turn out to be correct, but the recent confirmation of many of its predictions by cosmic microwave background experiments etc. have caused it to emerge as the most popular scientific theory for what happened early on, and ongoing experiments may provide additional tests. A second argument is that if we add to inflation the separate assumption that the correct theory of quantum gravity (say string theory, loop quantum gravity or some competitor) has more than one homogeneous solution (just as the equations for water have three solutions corresponding to ice, steam and gas), then this implies the Level II multiverse: a single space containing universe-sized regions with each kind of space. A third argument, first made by Hugh Everett III, is that the bare-bones theory of quantum mechanics free from so-called wave-function collapse implies a third type of multiverse. A fourth argument, made in my book, is that if there’s an external reality completely independent of us humans, then there’s a fourth type of multiverse realizing all mathematically possible universes.

The most persistent and acrimonious debates often occur when the debating parties misunderstand each other, so it’s important that people on both sides of the multiverse debate be as explicit as they can about what they’re claiming. The fact that parallel universes aren’t a theory, but predictions of certain theories, means that there are three (and only three) logically possible lines of attack on parallel universes, corresponding to three types of claims:

  • One or more of these “X implies Y” arguments is incorrect (inflation doesn’t predict Level I, say).
  • One or more of the theories X predicting multiverses are incorrect (inflation, say).
  • Parallel universe are indeed predicted by scientific theories, but scientists shouldn’t waste their time thinking about such topics.

Since C is a matter of personal opinion, let’s focus in more detail on A and B, which are straightforward scientific claims that can hopefully one day be settled by calculation and observation. We’ll see that there’s no shortage of such scientific lines of attack. Before delving into them, however, it’s worth noting that the best way to weaken a strong case is to overstate it, so multiverse skeptics should avoid undermining their case by going beyond A, B and C with vague and unscientific claims of “fantasy,” “nonsense,” etc.

A type-A attack on Level I would show that inflation doesn’t produce a Level I multiverse. Although Guth, Linde and Vilenkin have shown that almost any inflation model produces an infinite space, the “almost” allows a line of attack: there are still some models which don’t, and even though they’ve been criticized as contrived, they remain a logical possibility. A type-B attack on Level I should weaken the case for inflation. This could happen either through theoretical progress (for example, proof that competing theories such as the ekpyrotic universe or string gas cosmology are free from the obstacles currently limiting their popularity) or through new experimental results disagreeing with generic inflation predictions (for example, detection of a small but non-zero curvature of space, or growing evidence that the claimed anomalies in the cosmic microwave background images from the Planck satellite need to be taken seriously).

A more radical and potentially devastating type-B attack is to question the assumption that space can be stretched out indefinitely. Although it’s a standard assumption in physics that physical space is continuous, with even the smallest volume containing infinitely many points, it’s an Achilles heal in the sense that we have no experimental evidence for anything truly continuous or infinite in nature. Contrariwise, we suspect that our intuitive picture of space breaks down on tiny scales. Killing the continuum could kill eternal inflation, resulting in a Level I multiverse that is merely large but not infinite, potentially eliminating the prediction that there are near-identical copies of you far out in space.

Any of the above-mentioned Level I attacks could torpedo Level II as well. A second line of attack against Level II is to challenge the other assumption upon which it rests: that the correct theory of quantum gravity has more than one homogeneous solution. If further work on quantum gravity leads to a theory with a unique solution that matches what we experimentally observe, Level II will have had the rug pulled from under it. A third line of attack is to give a compelling explanation for the observed fine-tuning of physical constants that doesn’t rely on a Level II multiverse.

Since the Level III multiverse is implied by the (collapse-free) Schrödinger equation of quantum mechanics, it can be demolished with a type-B attack: an experimental demonstration of a violation of the Schrödinger equation. For example, if the current multi-million dollar attempts to build quantum computers fail and the cause is determined to be that the Schrödinger equation is violated by some form of wavefunction collapse process, then there are no Level III parallel universes.

The Level IV multiverse is also vulnerable to a type-B attack: we can simply reject the notion that there’s an external reality completely independent of us humans, for example in the spirit of Niels Bohr’s famous dictum, “no reality without observation”. A second type-B attack option is to falsify the mathematical universe hypothesis by demonstrating that there’s some physical phenomenon that has no mathematical description.

In summary, there is no shortage of potential weaknesses in the arguments for parallel universes. Attacking all these weaknesses involves doing interesting experimental and theoretical physics research. If any of the attacks succeed, the corresponding multiverse evidence is discredited. Conversely, if all the attacks fail, then we’ll be forced to take parallel universes more seriously whether we like them or not – such are the rules of science. In this way, parallel universes are no different from any other scientific idea.

Max Tegmark About the Author: Known as “Mad Max” for his unorthodox ideas and passion for adventure, Max Tegmark's scientific interests range from precision cosmology to the ultimate nature of reality, all explored in his new popular book, “Our Mathematical Universe." He is an MIT physics professor with more than 200 technical papers credit, and he has been featured in dozens of science documentaries. His work with the SDSS collaboration on galaxy clustering shared the first prize in Science magazine’s “Breakthrough of the Year: 2003.”

The views expressed are those of the author and are not necessarily those of Scientific American.

Comments 58 Comments

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  1. 1. rloldershaw 6:35 pm 02/4/2014

    Less than 100 years ago scientists discovered that most of what we could observe was contained within one galaxy, and there were at least 100 billion other galaxies at larger distances.

    The fundamental physics in each of these galaxies appeared to be exactly the same, and that hypothesis has been validated over the subsequent decades.

    Is it so unthinkable that the same type of discovery process could be repeated? Perhaps, once again, what we observe is all part of one metagalactic object, and there are countless numbers of these objects, and the fundamental physics is exactly the same in each one.

    There’s a “multiverse” without tooth fairies.

    Moreover the recent Planck mission results confirming a fundamental anisotropy (directionality) on the largest observable scales are consistent with the Metagalaxy model.

    If our Metagalactic system is undergoing an analogue of a type-II supernova event, and the observable universe is a tiny volume deep inside the event, there’s your Big Bang without an acausal “birth” of the whole Universe, and there’s an explanation for the initial relatively low entropy.

    A discrete fractal paradigm for the cosmos offers a more natural and highly testable understanding of nature.

    You ask: Where would this hierarchy end? I answer: Why would you assume that there must be an “end”?

    Robert L. Oldershaw
    Discrete Scale Relativity/Fractal Cosmology

    Link to this
  2. 2. tegmark 7:15 pm 02/4/2014

    Hi Robert: what you’re describing sounds a lot like the Level I multiverse: many universe-sized regions sharing the same physical laws. I plan to check out your link after dinner.

    Link to this
  3. 3. rloldershaw 7:43 pm 02/4/2014

    Hi Max,

    Thanks for your interest in Discrete Scale Relativity.

    By the way, here are 15 definitive predictions of DSR and the observational evidence that support them.

    And may the Boston Bruins prevail tonight.

    Link to this
  4. 4. Dr. Strangelove 3:38 am 02/5/2014

    “Conversely, if all the attacks fail, then we’ll be forced to take parallel universes more seriously whether we like them or not – such are the rules of science. In this way, parallel universes are no different from any other scientific idea.”

    We cannot take parallel universes seriously if they are unobservable even if they are predicted by a theory whose other predictions are observable. That’s why Occam invented the Razor long before the scientific method became commonplace. Anything unobservable and unphysical must be taken out of a scientific theory because it is metaphysics. In the same category as gods and angels. They may well exist but they are beyond science and indistinguishable from imaginations. Parallel universes are no different from any other metaphysical entity.

    Link to this
  5. 5. tegmark 7:01 am 02/5/2014

    Hi Dr. Strangelove,
    Thanks for this interesting perspective! What you propose is an extremely controversial minority interpretation of Occam’s Razor that I’ve never heard a single one of my science colleagues argue for. This viewpoint sounds logically inconsistent to me: A is true and A implies B, but you still don’t accept B. Are you arguing against logicians’ use of modus ponens? Does this mean that you view black holes as unscientific as well? Would you agree that, as opposed to black holes, gods and angels are *not* predicted by general relativity or any other mathematical physics theory, and that this is makes a crucial difference?

    Link to this
  6. 6. David Cummings 7:14 am 02/5/2014

    Thanks for posting this excellent blog entry which points out that hypotheses of multiple universes are falsifiable and thus the subject of science.

    Link to this
  7. 7. mpc755 8:44 am 02/5/2014

    Multiverse, many worlds, the Big Bang are all nonsense.

    Aether has mass which physically occupies three dimensional space. Aether is physically displaced by particles of matter.

    There is no such thing as non-baryonic dark matter anchored to matter. Matter moves through and displaces the aether.

    Displaced aether pushing back and exerting inward pressure toward matter is gravity.

    More correctly, the state of displacement of the aether is gravity.

    A moving particle has an associated aether displacement wave. In a double slit experiment the particle travels through a single slit and the associated wave in the aether passes through both.

    Our Universe is a larger version of a black hole polar jet.

    It’s not the Big Bang; it’s the Big Ongoing.

    Dark energy is evaporated matter (i.e. aether) continuously emitted into the Universal jet.

    Link to this
  8. 8. hamidsadeghipour 10:21 am 02/5/2014

    Let’s have a look to Schroedinger equation. It has a solution with e to power of -ixyzt for example. i as a complex number is anjother dimension. for a+ib and i-ib we can have a coordinates an points on a surface. The space is continious. The universes are side by side as points. But atoms are not continious. there are big distances between atoms and electyrons like the Sun and planets. How far is the next univers? There should be like plank constant some standard distance. Is it next block? In reality are we living the multiunivers already?

    Link to this
  9. 9. Marc Levesque 10:41 am 02/5/2014

    Hi Max,

    “if X implies Y and X is true, then Y must also be true. Specifically, they argue that if some scientific theory X has enough experimental support for us to take it seriously, then we must take seriously also all its predictions Y ”

    Looking at your table on theory and prediction, do you mean a level 4 multiverse should not be taken seriously because it is not based on a scientific theory but on a hypothesis, namely the external reality hypothesis (which, at least the versions I found, seemed very poorly defined), or does a Level 4 multiverse have other experimental support for us to take it, as you say, seriously.

    Link to this
  10. 10. rloldershaw 11:26 am 02/5/2014

    If Max had a chance to look at
    I hope that he noticed the following properties of Discrete Scale Relativity.

    1. DSR was not derived from abstract theoretical studies. Rather it was derived entirely from studying nature observationally and looking for fundamental patterns in those observations.

    2. Once the scaling equations were derived empirically roughly 35 retrodictions of fundamental observations from subatomic particle to galactic supercluster scales were possible, and were successful.

    3.In post #3 I gave a link to 15 definitive predictions by which DSR can be unambiguously tested observationally, and right now. Some have already been verified.

    4. Discrete Scale Relativity proposes one set of principles and laws for all of nature. That means one unified physics for the entire cosmos.

    5. Using General Relativity, classical Electromagnetism, some basic Quantum Mechanics, and the discrete global conformal symmetry of the discrete fractal paradigm, we can understand and model all of nature in a testable way, and without smoke, mirrors, tooth fairies, strings, sparticles, Boltzmann brains, 10^500 “universes” with random physics,…, i.e., without pseudo-physics.

    Why has such a beautiful, natural and unified paradigm almost totally ignored by the physics community for 37 years?

    Discrete Scale Relativity/Fractal Cosmology

    [Note: Occam's Razor roughly says that given an extremely complicated (especially a conceptually complicated) theory and a much simpler theory with fewer untestable assumptions and epicycles, the latter is usually the better theory.]

    Link to this
  11. 11. tuned 12:02 pm 02/5/2014

    Again, it doesn’t seem to change my day any.
    Have you cured cancer yet?

    Link to this
  12. 12. racer79 12:12 pm 02/5/2014

    “Would you agree that, as opposed to black holes, gods and angels are *not* predicted by general relativity or any other mathematical physics theory, and that this is makes a crucial difference?”

    I would agree that this WOULD make a crucial difference between the two, however as the author of the article states, general releativity tends to point towards multiple universes existing, and if a Type II multiverse exists it is logical (as with Schodinger’s cat) that since we have not directly observed these/this other universe/s that it/they would/could exist in a superposition in which it/they is/are simultaneously only one other mathematically possible universe and every other mathematically possible universe which in essence would be a Type IV multiverse,
    (@Marc Levesque, “or does a Level 4 multiverse have other experimental support for us to take it, as you say, seriously(?*)” there ya go buddy)
    and if in fact we do exist in a Type IV multiverse then indeed general relativity does predict “gods and angels” (due to the nature of infinite mathematical possibilities) albeit not quite as mathematically directly as it does black holes.
    My understanding of this subject on the whole is quite limited, so if anyone knows better I would very much appreciate the criticism.


    Link to this
  13. 13. Spironis 12:19 pm 02/5/2014

    equations for water have three solutions corresponding to ice, steam and gas” (solid, liquid, gas, fluid re critical point?) Add the Mpemba effect, arXiv:1312.1014, 1310.6514, 1101.2684 Hermetically contained parallel universes are one/observer. Given leakage, one suspects gravitation leaks. The Hubble deep field denies universal gravitational lensing distortions.

    Physics applies exquisite vacuum boson photon symmetries to fermionic matter (hadrons): parity violations, symmetry breakings, chiral anomalies, Chern-Simons repair of Einstein-Hilbert action. Euclid is incomplete given cartography. At least five classes of experiment expose physics’ weak postulate. Heretical results do not contradict any prior observation, but do remove the curve-fittings. Is there a baby oil Mpemba effect?

    Link to this
  14. 14. tegmark 12:24 pm 02/5/2014

    Thanks Mark for your interesting question!
    The way you’ve phrased it, the shortest path to the Level IV multiverse is to start instead with X=the mathematical universe hypothesis (MUH),
    which makes the testable prediction that our cosmos has no non-mathematical properties. So if you can prove that some aspect of our cosmos can *not* be described by math, then you’ve falsified the MUH.
    You can then argue against the Level IV multiverse either with a type-B attack (arguing against the MUH) or with a type-A attack (identifying a logical flaw in the MUH=>Level IV argument I give in the book).

    Link to this
  15. 15. M Tucker 1:08 pm 02/5/2014

    “… let’s consider Einstein’s theory of General Relativity. It’s widely considered a scientific theory worthy of taking seriously, because it has made countless correct predictions – from the gravitational bending of light to the time dilation measured by our GPS phones. This means that we must also take seriously its prediction for what happens inside black holes…”

    And yet each of the predictions and implications of general relativity do need to be verified and many brilliant physicists have spent a lot of time and energy trying to do just that. If we could just accept the predictions about black holes, without evidence, then we could have ignored the predicted geodetic effect and the so called frame-dragging effect. We would not have needed the decades long, extraordinarily expensive and problematic Gravity Probe B mission; we could have just accepted the predictions. Without the experiments all we have are untested predictions and that is how black holes will remain until we have a way to test them.

    “for a theory to be testable (and hence scientific), we don’t have to be able to test all its predictions, merely one of its predictions.”

    Are you saying that correctly describing the advancing perihelion of Mercury was a sufficient test of general relativity? So we did not need all those expeditions to view solar eclipses? We could have just taken it as valid. No, because it was such a hugely transformative theory with so many unusual perditions about our universe and so many implications for our understanding of space and time each prediction needed to be verified.

    And so too with inflation theory and so too with the mulitverse predition. The multiverse predition is very interesting but without proof it is just another unverified prediction to be added to the list that includes black holes.

    Link to this
  16. 16. andelang 2:09 pm 02/5/2014

    @ tegmark

    1. “… that our cosmos has no non-mathematical properties …” =
    1.1 “all properties of our cosmos *are* mathematical” (my emphasis)

    2. “… that some aspect of our cosmos can *not* be described by math …” =
    2.1 “all aspects (= properties) of our cosmos *are describable by* math” (my emphasis)

    When reading these statements I got the impression that they contain a kind of “sliding referent” problem, i.e. they refer to different phenomena, at the same time giving the impression that the author is talking about the same thing. Whatever a “mathematical property” of the cosmos might be, I don’t think it is equivalent to a “property which can be *described* by means of mathematics”. A mathematical description of a property is not, IM(H)O, identical to the property itself (I should perhaps add “if one believes in the existence of a reality independent of our perceptions and descriptions”, which I, naively of course, do). Describing a property using math does not transform the ontological status of that property. Please note than I am neither a physicist nor a mathematician, just a psychologist and social scientist terribly interested in cosmology.

    By the way, what William of Ockham really wrote was ”Plurality is not to be assumed without necessity”
    and ”What can be done with fewer assumptions is done in vain with more”.

    A personal note in Swedish to Max Tegmark:
    Hej, it’s a small world! En nära väninna till oss som är professor i slaviska språk vid SU har haft din bror Per som student.

    Link to this
  17. 17. Mikers 3:13 pm 02/5/2014

    Parallel universes or Multiverses do exist. But one needs to be clear what one means by this. As a mathematician, we are involved daily with such universes. The set of the Reals is a universe. Again, the set of the Reals is another, and parallel universe to the first set of the Reals. Any set is a universe, axiomatically defined or not. Some of these universes contain interesting structures and some dont. For instance, Conway’s Game of Life is based on a few simple axioms; It is proven that this universe produces emergent structures that “travel”, reproduce, heal, compute, and even the emergence of “matter” (what I have called the “materialization of math”).

    Perhaps our “physical universe” is simply a mathematical universe of which intelligence has emerged, or perhaps not. Nonetheless, before claiming that (parallel) universes exist or not, a clear definition of what is meant is in order.

    Now, are such contemplations “useful”? Will it permit us to model *our* universe and to make predictions?
    No harm in trying it out and see where it leads…

    Link to this
  18. 18. peterwoit 3:47 pm 02/5/2014

    So, according to you, if I’m not happy with your various multiverses, basically my only option to argue against them is
    Level II: Find a unified theory of quantum gravity with only one solution that matches reality.
    Level III: Show that the Schrodinger equation is violated.
    Level IV: Find a physical phenomenon that can’t be described mathematically.

    About Level III, no one seriously thinks that’s going to happen and I certainly don’t. I also think the issues behind how classical physics emerges from the Schrodinger equation are what is interesting, and this particular “multiverse” tells you nothing interesting. But, for those who enjoy grandiose things with little content, enjoy.

    About Level II, I think you’re safe and can happily continue your publicity campaign and write books about this for quite a while. To the extent your campaign succeeds, you will do a good job of effectively discouraging young people from making progress on fundamental physics problems, by convincing them it is hopeless to try.

    About Level IV: What you are doing here is kind of like a monotheistic believer telling me that his hypothesis of the existence of a certain kind of deity is scientifically testable, because if a different kind of deity appears before us tomorrorw with golden tablets, that will falsify his hypothesis.

    It’s this Level IV, your specific contribution to this, which I continue to claim is grandiose nonsense. You are making strings of meaningless statements, and absurd claims about scientific testability. And I think this is the consensus opinion of your colleagues. I have never met any scientist who takes your views on this seriously. Can you point to even one prominent physicist who agrees with you about this?

    Link to this
  19. 19. tegmark 4:12 pm 02/5/2014

    Hi M. Tucker,
    From what you write, it doesn’t sound like we disagree on anything at all: no matter how many tests GR (or some other theory) successfully passes, we should never become 100% convinced that it’s true, and therefore keep making whatever experiments we can to test it further. If it passes lots of tests and we lack a more compelling theory passing the same tests, then we take seriously its other predictions, whatever they may be – while remembering that the whole theory may still ultimately prove to be incorrect, meeting the same fate as Newton’s theory of gravity did after two hundred years of successes.

    I think you’ll agree that we should apply the same standards to inflation. It’s clearly nowhere near GR in terms of validation so far, but definitely a scientific theory worth taking seriously, and it will IMHO be extremely interesting to see if ongoing CMB polarization experiments can detect the inflationary gravitational waves that are widely viewed as smoking-gun evidence for inflation. Same thing for unitary quantum mechanics: it’s a well-defined scientific theory that makes the firm testable prediction that all isolated systems obey the Schrödinger equation, and ongoing experiments with quantum computers etc. will provide more stringent tests of this prediction than have ever been made before, so we should work hard to make these experiments and not take for granted that they won’t rule the theory out.

    Link to this
  20. 20. tegmark 6:13 pm 02/5/2014

    Hi Peter (Woit),
    In my opinion, science shouldn’t be about emotions and sociology, but about logic and experimental facts. So why are you bringing emotions into the discussion (“if I’m not happy”, “enjoy”, “happily continue”)?
    Do you really feel that scientific ideas should be judged by how “prominent” their advocates are rather than by the quality of the arguments and evidence? This and your talk of a “publicity campaign” seems to assume a scientific paradigm where you and I have strong emotional preferences for what we want to discover and are guided by these rather than by logic and facts. I’m sorry, but I strongly disagree with this approach. I feel that my job as a scientist isn’t to try to reach conclusions that I find emotionally pleasing or sociologically popular, but to follow the trail of evidence wherever it leads.

    If you have specific scientific critique of any of the 8 types I listed above (IB, IIA, IIIB, etc), please write about it and develop better physics, but let’s keep emotion and sociology out of our discussions from now on.

    Link to this
  21. 21. rloldershaw 7:03 pm 02/5/2014

    The levels of the multiverse sound a bit like the levels of hell in Dante’s “Inferno”.

    If one needs 10 or 11 dimensions, most of which are unobservable, to model the cosmos then I think you have a BIG problem. Especially when a far simpler new paradigm, which requires only the usual 4 S-T dimensions and global discrete conformal symmetry, has been readily available for decades.

    Why the proverbial ostrich strategy?

    Robert L. Oldershaw
    Discrete Scale Relativity/Fractal Cosmology

    Link to this
  22. 22. peterwoit 7:05 pm 02/5/2014

    I’ve made detailed arguments about exactly what the problem is with your Level IV multiverse in the WSJ review, on my blog, and here. You’ve completely ignored them, in favor of personal attacks on me as “unscientific” and “emotional”, driven by “hate”.

    Again, for about the fifth or sixth time. If your Level IV multiverse is a meaningful scientific statement, it should have testable (at least in principle) implications. The only one you have provided is that it is science because it could be shown to be wrong by finding “that there’s some physical phenomenon that has no mathematical description.” This is just absurd and I don’t think you can identify any serious scientist who would take it seriously. It’s completely unclear what “having no mathematical description” means, as well as absurdly implausible on its face. You are saying “my theory is falsifiable because if something absurdly implausible were to happen it would be wrong”. Why is this different than my theory that the world is controlled by a huge Green Goblin, which is a falsifiable theory, since a huge Purple Goblin may make an appearance and show he can control the world?

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  23. 23. M Tucker 7:30 pm 02/5/2014

    Thanks for your comments Dr Tegmark. All your writings give me a great deal to think about and delight in.

    My reservations regarding inflation come from what Paul Steinhardt wrote in a 2011 SciAm article. He presented some very interesting arguments for and against inflation and he concluded by saying that finding the gravitational wave would lend support for the pro camp. I am willing to keep an open mind. I was very persuaded by the arguments against and it is interesting how the theory has evolved since it was first introduced.

    I am also willing to accept the properties of a black hole as described by GR but an untested or untestable prediction will always leave me feeling unsatisfied. And black holes seem present a lot of questions that are answered by more untested or untestable theories.

    The science is fascinating (and a little disturbing) and that makes it fun. I am not looking for an end to discovery. What I am looking for and hoping for and what I enjoy most of all is the hard work and perseverance that goes into making the experiments that demonstrate the validity of the theory.

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  24. 24. maddogatc 9:00 pm 02/5/2014

    When you mentioned having equations for water, aren’t the solutions? Ice, liquid and gas? And in actuality, with enough temperature and pressure, plasma?

    Link to this
  25. 25. stevenlmeyer 10:01 pm 02/5/2014

    >>Killing the continuum could kill eternal inflation, resulting in a Level I multiverse that is merely large but not infinite, potentially eliminating the prediction that there are near-identical copies of you far out in space.>>

    But in fact we are seeing growing evidence that space-time is “grainy.” For a summary of the state of play see this New Scientist piece:

    Breaking relativity: Celestial signals defy Einstein

    >>In May, he and his colleagues circulated a paper claiming to see a time lag of hundreds of seconds between the burst’s lower- and higher-energy gamma rays ( “The numbers work out remarkably well. This is the first time there is robust evidence of this feature,” says Amelino-Camelia.>>

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  26. 26. Mr. Mxyzptlk III 10:10 pm 02/5/2014

    If it is intrinsically impossible to design an empirical experiment to test any given hypothesis about the structure of reality, then what is being done is not science but speculative metaphysics.

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  27. 27. Dr. Strangelove 10:13 pm 02/5/2014

    Let me answer your points.

    “What you propose is an extremely controversial minority interpretation of Occam’s Razor that I’ve never heard a single one of my science colleagues argue for.”

    I suspect your colleagues are mathematicians and mathematical physicists like you who believe 10-dimensional space must be real because they can compute it. Otherwise you would hear about Occam’s Razor since almost all scientists embrace it.

    “This viewpoint sounds logically inconsistent to me: A is true and A implies B, but you still don’t accept B. Are you arguing against logicians’ use of modus ponens?”

    So you believe science is logic. Do you also believe the logical proofs of God’s existence by St. Anselm and other philosophers? Do you reject quantum mechanics since it defies logic?

    “Does this mean that you view black holes as unscientific as well?”

    Have you ever observed the parallel universes? Ask the astronomers for observational evidence of black holes. Ask them about Cygnus X-1 and V404 Cygni and the supermassive black hole at the center of our galaxy.

    “Would you agree that, as opposed to black holes, gods and angels are *not* predicted by general relativity or any other mathematical physics theory, and that this is makes a crucial difference?”

    Does general relativity or any other mathematical physics theory predict that gods and angels cannot possibly exist? An omnipotent god can in principle create all the laws of physics that you observe and yet be outside the physical universe and therefore unobservable to us. What’s the crucial difference between this god and your parallel universes? If Einstein rejects gods and parallel universes, will that invalidate general relativity?

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  28. 28. hamidsadeghipour 1:36 am 02/6/2014

    Tuned says: how about cancer?
    when the radioactivity was discovered resulted in x-ray discovery and radiology. Nobody knows the advent of science. It is like a seed of wheat in a wheat farm.If you prefer an apple, you have too.

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  29. 29. Dr. Strangelove 2:06 am 02/6/2014

    Which theory is more scientific?

    A. Standard Model – Confirmed by numerous particle physics experiments; accurately predicts specific particle interactions; have not been falsified by experiment and observation

    B. String theory, M-theory and the like – Untested and practically untestable; incomplete and lacks predictive power; do not offer unique solutions due to huge number of solutions; requires bizarre metaphysical assumptions like parallel universes (would it be less believable if it were angels and unicorns?)

    Most scientists and sensible people will choose A. Max and mathematical physicists choose B. Why? Perhaps because A is “ugly” with many empirically derived parameters. B is mathematically elegant and derived purely from first principles, no messy experiments and observations.

    I wouldn’t say their search for beauty is futile. After all, Maxwell predicted the electromagnetic waves because he thought his equations would look more elegant with this assumption. However, the world accepted electromagnetic waves not because of Maxwell’s aesthetic sense but because Hertz actually produced radio waves in his lab and Marconi used it for communication.

    The present generation of mathematical physicists are like Maxwell without Hertz and Marconi. Well actually Maxwell was better off. At least he had a complete theory. Our physicists have incomplete theories plus a lot of hand waving.

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  30. 30. deSitter 7:52 am 02/6/2014

    None of this has anything to do with science. You just ignore transparently obvious arguments. One wonders if you are even capable of understanding a scientific argument itself, and the evidence above and elsewhere is that you cannot.

    Having read your lecture notes, I am not surprised.


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  31. 31. Marc Levesque 9:28 am 02/6/2014

    Hi Max (12:24 pm 02/5/2014)

    “if X implies Y and X is true, then Y must also be true. Specifically, they argue that if some scientific theory X has enough experimental support for us to take it seriously, then we must take seriously also all its predictions Y ”

    “the shortest path to the Level IV multiverse is to start instead with X=the mathematical universe hypothesis (MUH)”

    Interesting, but it seems that like in the case of the external reality hypothesis the mathematical universe hypothesis does not qualify as a “scientific theory with enough experimental support for us to take it seriously” so it doesn’t follow that the Level 4 Multiverse prediction should be taken seriously.

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  32. 32. tegmark 10:08 am 02/6/2014

    Thanks M Tucker for your encouraging words! I agree with you:
    we certainly don’t know for sure whether inflation is correct, and the most interesting thing right now is to continue working on those new experimental tests of it – just as we should focus on continuing to test every scientific theory.

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  33. 33. tegmark 10:14 am 02/6/2014

    Thanks maddogatc for bringing up plasmas. You’re quite right that many known substances have more than three phases, so the idea that homogeneous space have have more than one phase isn’t particularly radical if one accepts the idea that space too should be thought of as a kind of substance.

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  34. 34. tegmark 10:22 am 02/6/2014

    Thanks stevenlmeyer for linking to this interesting paper about possible spacetime “graininess”
    Although the statistical significance isn’t very high (see figures 2 & 3 in, I think this type of experimental studies are fascinating, and very much look forward to seeing what nature tells us as the data sets get larger.

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  35. 35. curiouswavefunction 10:39 am 02/6/2014

    #29 Dr. Strangelove: Some may claim that string theory is ugly because of 10^500 potential solutions and no simple equation tying it all together. An elegant mathematical theory like general relativity or the Dirac equation usually explains and predicts several experimental facts using a set of short, (relatively) simple equations.

    I must also say however that I don’t put the ugliness or the beauty of any physical theory on a pedestal; in the end the only thing that matters – as Feynman memorably put it – is agreement with experiment. In fact it’s worth noting that even QED is in a sense mathematically ugly because of the infinities and the renormalization (which Dirac pretty much hated) and yet we take it seriously because of spectacular agreement with experiment.

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  36. 36. tegmark 10:45 am 02/6/2014

    Thanks Mark for bringing us to fun questions that philosophers have written about throughout the ages! I agree with you that you can never prove the external reality hypothesis experimentally, because you can’t rule out the solipsism theory that nobody but you exists and the external reality doesn’t exist. On the other hand, as many epistemological philosophers have pointed out, we can never prove *anything* in science. All we ever do is disprove theories or try and fail to disprove them, and if we keep failing, we gradually take them more seriously – but “take seriously” never equals 100% certainly.
    I suspect that you’d still bet your money the same way that I would: that the existence of the Andromeda galaxy has nothing to do with us humans.

    It was to avoid the philosophical issues to do with the external reality hypothesis in my earlier reply to you that you focus on the shorter logical step “mathematical universe hypothesis” => “Level IV multiverse” instead.

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  37. 37. tegmark 12:02 pm 02/6/2014

    Hi Peter,

    Hi Peter (Woit),
    You keep making very strong claims without backing them up.
    > “that there’s some physical phenomenon that has no mathematical
    > description.” This is just absurd and I don’t think you can identify any > serious scientist who would take it seriously.”
    Why is this absurd, when the literature is full of scientists who think that consciousness can never be understood using mathematical equations?
    If you want to claim that these scientists somehow aren’t as eminent as you and therefore shouldn’t count, that’s your prerogative.
    You seem similarly dismissive of many of my science colleagues: do your assertions that not a single scientist other than me finds the Level IV multiverse idea worth taking seriously mean that you count the scientists who endorsed my book as so far beneath you in eminence that they don’t even deserve to be called scientists? I think our personal opinions of other scientists are completely irrelevant to the topic of this blog.

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  38. 38. tegmark 12:57 pm 02/6/2014

    Hi again Peter (Woit),
    Thinking more about our spirited multiverse discussions in recent weeks, I think we’ve now reached the point where they’re generating more heat than light. I’ve argued my case in my book and you’ve argued your case in your book review and blog posts, so others out there interested in these topics have all the information they need to make up their own minds. In terms of you and me making up our own minds or perhaps changing our minds, I’m happy to agree with you to disagree the issues for now and to continue out discussions in person at some point. If you’d like to continue our conversation, I’m happy to take you out for lunch sometime in New York or Cambridge – just let me know.

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  39. 39. peterwoit 2:07 pm 02/6/2014

    I’m no expert on consciousness, but I would have thought that those people who claimed it had no mathematical description were essentially the same who thought it was not a physical phenomenon.

    This has nothing to do with personal opinions of other scientists. I’m just trying to find out if you have a realistic understanding of what your colleagues think about your Level IV multiverse business. If you’re going by what blurbs people are willing to write, that’s not likely to lead to such understanding. Actually, I suspect Witten was making himself clear there (“something to disagree with”).

    Sure, I’d be happy to have lunch sometime, if you’re in New York with some free time, let me know. I’ll likely next be up at MIT in May (for the conference in honor of David Vogan, who I think you know), may then see if you’re around.

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  40. 40. Dr. Strangelove 3:05 am 02/7/2014


    Max could not distinguish between science and metaphysics. His argument for the existence of the metaphysical parallel universes is logic. This is a throwback to the logical proofs of God’s existence by medieval philosophers. Humanity has already advanced from logic to science in the last 500 years and Max is still in the Medieval era.

    Why should we trust logic? It cannot even determine if simple statements like “I am lying” is true or false. After 2,300 years of logic and logicians, they still could not resolve such paradox without contradiction. Finally Godel concluded that arithmetic itself contains the Liar’s paradox that it must be inconsistent or incomplete and its statements will eventually lead to a contradiction. Sure but ordinary people ignore formal logic and use arithmetic to count things without contradiction since time immemorial. Using logic, Zeno argued that Achilles cannot win the race against a tortoise. Of course such nonsense is easily dismissed with observation. Max’s logical proof of the metaphysical parallel universes is just as absurd and comical.

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  41. 41. Marc Levesque 12:47 pm 02/7/2014


    “I agree with you that you can never prove the external reality hypothesis experimentally…”

    That is interesting, but I’m not sure I’m following you. I think the ERH hypothesis is too ill defined for us to be able to speak about its proof or disproof.

    “It was to avoid the philosophical issues to do with the external reality hypothesis in my earlier reply to you that you focus on the shorter logical step “mathematical universe hypothesis” => “Level IV multiverse” instead”

    I was commenting on the scientific level of support around the “MUH => Level 4″ but I think I may not have been expressing myself very well. In a nut shell, I think the level of ‘seriousness’ with which one can consider a prediction is dependent on the level of seriousness of the theory which is making the prediction –and the level of seriousness of the theory which is making the prediction is dependent on the level of experimental, or empirical, support the theory receives.

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  42. 42. GnarlyKat 4:29 am 02/8/2014

    Interesting topic, and an interesting book, Max.

    So far in all the various books that bring up multiple universes, the one point of commonality seems to be how these various universe are separated from each other. Whether created from the same event, say eternal inflation, or completely separate structures such as your level IV multiverse, there is no present day connection possible between them.

    I’d like to bring up another possibility (a level V universe?) that hasn’t been examined before. The possibility exists that our universe is a subset of a larger universe, and that this larger universe is actively supporting the existence of our universe. I guess you could call this the Matryoshka doll hypothesis .

    I’ve recently put a paper online dealing with this very subject. That I may be onto something is that, like string theory, my model requires nine dimensions. However, unlike any other multiple dimensions model which requires some form of dimensional compactification to get rid of the ‘pesky’ extra dimensions, my model completely eliminates the need for compactification, which has always seemed to be an ad hoc hypothesis tacked on to make the models work. Anyway, you may want to check out my idea:

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  43. 43. christinaak 12:30 pm 02/11/2014

    Max, I know that inflationary cosmology was developed to address problems with Big Bang cosmology and the apparent flatness, homogeneity, and isotropic appearance of the universe. However, if a quantum theory of gravity is developed which eliminates the notion that the universe expanded from a singularity as predicted by General Relativity, then inflationary theory may not be adequate to solve the horizon problem. Without knowledge of the size and structure of the universe at the moment of the Big Bang one can not make definitive predictions about initial conditions, and how they impact the causal connection and development of the expanding universe.
    An Evolutionary Cyclic Model eliminates the Horizon problem by suggesting that the currently observed flatness, homogeneity and isotropic structure of our universe has an evolutionary origin. The observed complexity of our current universe (including the ‘fine-tuning’ of cosmic parameters) is the product of incremental evolutionary changes (adaptations) that have arisen over the course of a multitude of cosmic cycles.
    The 2nd Law of Thermodynamics has been the bane of Cyclic models of the past (Richard Tolman). There is no question that the 2nd Law applies to the universe during the expansion phase of the cyclic universe. However, it is not at all certain (keeping Arthur Eddington’s admonition in mind) that the 2nd Law must have a transcyclic impact. If the universe is able to recycle its energy at the onset of each cosmic cycle then the 2nd Law does not apply. Our present universe began in a low entropy state, and if a cyclic model is the correct model than some modification of the Laws of Thermodynamics must take place.

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  44. 44. rloldershaw 6:05 pm 02/11/2014

    There is an interesting discussion today of the multiverse situation at Peter Woit’s blog: Not Even Wrong. The title is: The Multiverse, Evidence and Theology.

    Quo Vadis!?


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  45. 45. Dr. Strangelove 9:56 pm 02/11/2014

    To quote Peter Voit:

    “Rubenstein ends the section about what physicists have to say with Tegmark, seen as having reached the final endpoint of the “Ultimate Multiverse”:

    “So some worlds will be linear, and some will be cyclical; some will be singular, and some will be plural; some will be infinite, and some will be finite; some will branch forward, and some will branch back. Some worlds will be manufactured, and some will be simulated; some designers will be kind and some will be cruel, some capable and some all but incompetent.”

    “And, presumably, some of the set of all possible world will have a creator-god who breathes over primordial waters, who separates the sea from dry land.
    How on earth did we get back here?”

    “I take Tegmark’s vision as empty, so a good thing to ignore, but Rubenstein sees this as an opening for theologians to get back into the mainstream cosmology business…”

    As I said previously, a throwback to the metaphysics of medieval theologians.

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  46. 46. tegmark 10:20 pm 02/11/2014

    Hi Robert & Dr. Strangelove,
    This “critique” by Peter Woit is neither of type A or type B. If some theologian misunderstands the multiverse predictions of inflation etc., this doesn’t constitute a scientifically valid critique of inflation. I very much doubt that all theologians imagine a deity that strictly obeys mathematical equations, and can therefore equivalently be described as a purely physical entity.

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  47. 47. Dr. Strangelove 10:48 pm 02/12/2014

    Let’s assume Voit’s criticism is Type C.

    Parallel universe are indeed predicted by scientific theories, but scientists shouldn’t waste their time thinking about such topics.

    Why is it not valid? Let me change that statement slightly.

    Work-energy polygons are indeed predicted by the PV diagrams of thermodynamics, but scientists shouldn’t waste their time thinking about such topics.

    This is indeed the case. Why? Because these polygons are geometric representations of work done by cyclic heat engines. They are not real world objects. Heat, work and energy are not polygons. Scientists and engineers can distinguish between reality and idea. But mathematicians and mathematical physicists tend to believe mathematical abstractions are real world objects. Some are, some are not. The final arbiter of scientific truth is observation. Logic is also an abstraction, mathematical or otherwise. Something that is unobservable in principle is the same.

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  48. 48. dchobday 1:02 am 02/14/2014

    Dear Max,
    This must be a very mundane objection but it goes in parallel with the above suggestion that a picture of a heat engine isn’t a heat engine. Doesn’t our universe dividing into two universes every time someone makes an observation defy thermodynamics? How can the amount of energy in a certain piece of the multiverse be doubled so often? Where does the heat come from? Why aren’t we frozen?

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  49. 49. GingerGiselle 5:35 pm 02/17/2014

    Dear Max, The question that seems most important to me is not whether there are multiple universes predicted by inflation, for instance. As an engineer, and therefore a non-physicist, I trust the physicists to decide upon such things. It is whether those proposed, perhaps even proven different universes represent different physical schema. The H2O analogy for different possible homogenous solutions is a good one for something that is likely. Nonetheless, each state is just one phase of an overall equally homogenous solution. Well, why can’t I just ask my question clearly, eh? What I think is important is whether any differing solution would truly indicate an entirely different solution as opposed to just differing manifestations engendered by the same overall rules. Nothing is really provocative about different universes unless that were the case. Different universes with entirely different physical systems who be profound. Yours, Ginger

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  50. 50. GingerGiselle 5:38 pm 02/17/2014

    Ah, forgive my misspelling of homogeneous. Perhaps my subconsciousl thought we were talking of milk, rather that water. Or, more likely, I am a bad speller.

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  51. 51. DavidJohnson 11:09 am 03/22/2014

    Hello Max Tegmark, perhaps you can answer this more general point. Even if one part of inflation theory turns out to be right, it doesn’t follow that all other parts of it are automatically right. In the past, theories have often been right in places but not in others. This is true not only of Newton’s work – general relativity and quantum mechanics are incompatible, so on it probably goes. Theories are often partly wrong or incomplete, that’s why we test things with experiments – and with the multiverse you can’t. Any experimental result that seems to support the idea of a multiverse could always be caused by something else.

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  52. 52. jtdwyer 8:49 am 03/26/2014

    What is the motivation for physicists to propose universes and dimensions that generally lay outside our ability to confirm their existence?

    I’ve not made a study of this question, but the most obvious answer is that it enables the consideration further proposals that predict effects that are not observed within our own universe.

    For example, one of the favorite uses of such hiding places is in explanation for the seeming ‘weakness’ of gravitation in comparison to ‘the other forces of matter’. It’s often suggested that the ‘missing’ portion of gravitation’s effects are imparted to other dimensions. If this were the case, shouldn’t we also expect to see unexplained effects produced in in these other dimensions appearing in our own?

    Of course, the popular illustration of gravitation’s relative weakness is the ability of a magnet to pick up a paper clip from the Earth’s surface – illustrating that a small magnet can overcome the gravitational ‘force’ of the entire Earth! Don’t try to explain that the Earthbound magnet cannot pick up a paper clip from the Moon…

    The idea that gravitation is a material force at all was originated in Newton’s rather successful mathematical description of it’s effects – and is further supported by particle physicists who are strongly motivated to fit gravitation into the infrastructure of quantum theory.

    This, however, ignores that general relativity best describes the effects of gravitation as an interaction between the potential mass-energy of matter and kinetic effects produced by some property of spacetime corresponding to its dimensional coordinates.

    In this regard, any quantum gravitational interaction may more closely related to the kinetic extraction of potential mass-energy from dimensional spacetime than any strictly material localized energy field.

    In this case there’d simply be no comparing gravitation to the ‘other forces of matter’, because it’s _not_ properly a force of matter! Therefore – no need for any place to ‘hide’ the perceived ‘missing’ effects of gravity.


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  53. 53. DavidJohnson 6:10 am 04/4/2014

    I agree with some of these points. Other universes are sometimes a very efficient carpet to sweep things under – no-one’s going to look there. As you say, unexplained facts like the weakness of gravity in relation to the other forces are sometimes put down to the missing effects being in other universes. This is far from being good testable science.

    In other areas such as philosophy, the idea of a multiverse is sometimes more respectable than that, but in physics, there is above all a lack of falsifiability in these ideas. Newton’s scientific method of testing by experiment, as we see it nowadays, includes the need for an idea to be falsifiable, otherwise it’s not a scientific theory. Some will say we can falsify a type X multiverse. But then you will always be able to dream up a type Y multiverse, and keep adjusting your imaginary world. It has an ability to explain anything, and that’s the other thing that’s so unscientific. It gets a bit like religion in that way.

    In relation to inflation, as I said on John Horgan’s blog , we haven’t pinned inflation down anything like enough to speculate about those kind of consequences.

    So to Max Tegmark’s question in the title here “Are Parallel Universes Unscientific Nonsense?” Yes.

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  54. 54. hemanth 3:42 pm 04/29/2014

    According to realism theory from my findings multi universe were there and there were so far to the dark matter of the universe in the space.Firstly we have to come out from foolishness that proton has self power to attract.There is magic in world by realism.which everything done has relation with other unlike magic. According to my findings our universe was first an dust of particles of dark matter .

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  55. 55. hemanth 3:54 pm 04/29/2014

    we doesn’t need to go-out of our universe and search for other, we just need to verify our universe clearly in the dark matter, according to my findings if we found dark matter unsuitability or total density of our universe to actual density of universe should have,we could find the possibilities,also there are chances to find the dark matter walls in the units representing our universe doesn’t formed without the help of others.

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  56. 56. PhilipM 7:10 am 04/30/2014

    Should parallel universes exist with me in them would not my consciousness be shared amongst all my bodies? I mean, a copy of me wouldn’t actually be me, would it? With a potentially infinite number of me’s my consciousness would be spread pretty thinly throughout the multiverse. I am no physicist and I don’t care much whether the sums add up, it just seems highly unlikely to me and can never be proved. The speculation will continue for as long as the multiverse continues to exist or not exist.

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  57. 57. Brain1 7:21 pm 09/7/2014

    When atheists enter the fields of origins to prove the world only “appears” designed and discover the hidden Math is even more designed than appearance we have corresponding proof of design.
    When all of mankind throughout history believes in God in one form or another AND science discovers this independent fine tuning–something no one even factored into their belief in God in the first place..we have a pretty good reason in questioning, not only the motive, but the long prose about how we no longer need any proof for scientific theories.

    Funny how” If X Than Y ” didnt add up to Design but can somehow prove a trillion universes !!! To defeat the FT with something you couldnt possibly observe nor prove was not Fine Tuned itself.

    Now, There is a Max T that is an exact copy til this very second and the next second he realizes Design in the best explanation for the FT, making his opinion completely arbitrarily. We all see how absurd MW are, so there must be bias involved in promoting something that make the world into a joke. The lists of lunacy are too high a price to pay. We simply wouldnt be able to do science if it was true and we can so its not.

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  58. 58. HansKuijper 3:04 pm 09/23/2014

    Being a social scientist by profession, I would like to raise three questions:

    1) How does culture fit into nature?

    2) If culture (i.e. the realm of the mind) belongs to the ‘mathematical universe’, how is the mathematician’s mind to be described mathematically?

    3) If culture does not belong to the ‘mathematical universe’, what does this mean for ‘the ultimate nature of reality’?

    Immanuel Kant (1724-1804) considered human beings, like gardens, to be places where nature/matter and culture/mind mingle.

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