Multiverse News

Some items from around the multiverse:

  • Srednicki and Hartle have a new preprint on hep-th about Science in a Very Large Universe. Like many other multiverse papers, it doesn’t really have any equations in it, so it’s a bit hard to figure out what their argument is. Maybe readers can figure it out from the conclusion:

    It is no surprise that information about us is required to make predictions for our observations. Our data suggest that we are located some 13.7Gyr from a Big Bang. To make a reliable prediction from that information, we have to assume that it describes our physical situation. If the universe is rife with delusion, we must assume that we are atypical in order to have predictive and testable scientific theories. Indeed, it is only by making such assumptions that we are able to do science in a very large universe. We imagine that even Copernicus would have agreed that it was necessary to assume that Ptolemy was not deluded in his observations of the planets.

    The authors thank about a dozen or so other theorists for their help with this.

  • World Science Festival 2008 here in New York was a huge success, and I suspect that the 2009 version starting June 10 will be too, which is great. Of the many events, one where I might have a difference of opinion with some of the panelists will be a session on Infinite Worlds: A Journey through Parallel Universes, sponsored by the Templeton Foundation.
  • Seed magazine had a story a couple months ago about the theological implications of the multiverse.
  • Astrophysicist Jeffrey Zweerink has a book out called Who’s Afraid of the Multiverse?
  • Over at FQXI there’s a recent blog entry discussing the question of whether God might be “unsure as to whether He is really just a brain floating in a vat?”
  • Somehow I missed this one last year. Wheaton College held a research symposium on String Theory and the Multiverse: Philosophical and Theological Implications.
  • Update: For a more skeptical and philosophical take on the multiverse, there’s The Unique Universe, a piece by Lee Smolin that just came out in the latest Physics World.

    Update: The proceedings of the Wheaton College conference on string theory, the Multiverse and theology are available here. They include audio recordings of the discussions, and inform us that string theory

    implies that physical reality is far vaster and possesses greater grandeur than ever imagined.

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    37 Responses to Multiverse News

    1. perverse says:

      The universe IS rife with delusion … that it is one of a multiverse.

    2. Shantanu says:

      Peter are u planning to attend the 2009 Amaldi meeting ?
      2 weeks from now

    3. Peter Woit says:

      Thanks Shantanu, I didn’t know about that. I’ll be around those days, may try and catch some of the talks.

    4. Arun says:

      Re: Lee Smolin As we attempt to realize those principles, we seek a notion of law that cannot be applied to an imagined universe within a multiverse, and which cannot be imagined to hang around timelessly waiting for a universe to begin that it can then govern. Given that the universe only happens once, we must try to imagine a new kind of law that applies only that one time. Such a law need not — and should not — have any sense in which it exists outside of time. Nor could it be conceived of as apart from the universe it describes.

      Our universe began with low entropy – it applies to this universe, not imagined universes within a multiverse; it is not hanging around waiting for a universe to begin, it comes into being with the Big Bang. It cannot be conceived of as being apart from the universe it describes. Talk me down.

    5. GR says:

      Good lord. I may just be a beginning graduate student, but my bullshit detector reads off-scale high when I try to parse that. My best guess is that it means “in order to do science, our observations have to match up with reality”?

      Who knew Aristotle was the first string theorist. It’s sad that theoretical physics is rife with third rate philosophy hacks trying to come to grips with solipism.

    6. Pingback: Random stuff of the week. « Shores of the Dirac Sea

    7. Coin says:

      Why is the brain always in a vat? Is this really the most efficient and safe way to house a trapped brain? It seems like a neutrally buoyant brain floating in fluid might still tend to drift and hit the side of the vat, or something. And would a brain suspended in nutrient fluid actually even successfully be able to intake the fluid? In the human body the “nutrient fluid” has to keep continuously flowing via external force and if it ever goes stagnant we call this a stroke. Right? Why not just keep the arteries connected to the brain and pump fluid through those?

    8. Chris W. says:

      GR: I can guess, but would you mind being more specific about what triggered your bullshit detector?

    9. GR says:

      Well, to be fair, when I first wrote that I’d only read the excerpt. I’ve read the whole thing now and my opinion hasn’t really changed. Of course, I may just lack the sophistication needed to understand the paper, but it reads like an exercise in metaphysics, not physics.

      In any case, the sentence “To make a reliable prediction from that information, we have to assume that it describes our physical situation.” is what made me skeptical. I think they’re questioning the idea that the laws of physics are the same everywhere in the universe. Fine, but what really is the point? There’s not much use in doing cosmology if they aren’t.

      Again, from my perspective as a beginning graduate student, all of this wild speculation about multiverses just sounds like the string theory crowd making excuses for why their theory doesn’t come up with predictions…

    10. Chris W. says:

      Okay, so you were talking about the excerpt from Lee Smolin’s article in Arun’s comment (6/2, 10:42 am), and have since read the entire article.

      I get the impression that you’re fairly new to this discussion. Lee Smolin is not a string theorist, although he has done some work in the field. The whole point of his article is explain why certain assumptions implicit in our conventional understanding of quantum theory lead us to a multiverse in the context of quantum gravity and cosmology. He is trying to find a way past those assumptions, precisely because the idea of a multiverse severely undercuts or eliminates the possibility of making testable predictions.

      Is his article an exercise in metaphysics? Absolutely! In this context metaphysics is unavoidable. Modern physics inherited some key metaphysical ideas from the ancient world, most notably atomism, and found ways to give them empirical depth and substance, ie, to build testable theories around them. I think the same process must be repeated now, although the required ideas will almost certainly have a much more recent origin, and may have to be invented anew.

      Smolin is not questioning the importance of the universality of the laws of physics across space-time. What he is emphasizing is the potentially disastrous effects of separating the fundamental laws from effective laws in such a way that only specific effective laws can be tested, but are also assumed to be highly contingent on how the observable universe is situated in a hypothetical multiverse. The result is that a failure of any effective theory simply results in selecting another effective theory from a vast ensemble allowed by the fundamental theory. Potentially, then, the latter therefore can never be tested; it devolves into a generator of effective theories, with unlimited latitude for accommodating actual observations.

      Of course string theory (not to mention particle physics and cosmology), considered as a formal discipline, is a vast subject, and one can bury oneself in mastery of parts of it while ignoring these issues. Smolin and others are trying to resist this attitude. Mathematical formalism can be just as metaphysical in its relationship to nature as ideas expressed in prose, even if its logical structure is tighter and its definitions are more precisely stated.

    11. GR says:

      That quote was from the first bullet point in Peter’s post, not Smolin’s article, which I haven’t had time to read.

    12. Dude says:

      Peter, can you please consider using a widget in the sidebar that shows the titles of your posts for your archives? The same thing for your categories would also be nice.

      In the past, I used a widget called Wp-dTree. The page is here: Wp-dTree 3.5. One of the tabs near the top shows some screeshots.

      Here’s a blog that’s using Wp-dTree: here.

      You have posts that I may want to refer to in approximately 14.34 years, when I’ve achieved greatness of some sort, probably via American Idol, or Britain’s Got Talent, singing as fat woman. Or occasionally, I might like to surf through old posts of yours in a non-linear manner.

      I could link to your posts now to separate them from the myriad of posts on your blog, but this could be a waste of my time, considering there’s a possibility I’ll move to Sudan, or never move to Britain, and lose my chance of competing on American Idol or Britain’s Got Talent as a fat woman, and never achieve the greatness I desire, and therefore never have any real use for your posts.

    13. chris says:

      Chris W,

      i am not aware of even one single instance in modern physics where pondering about metaphysical questions resulted in progress of any sort. all examples that i am aware of show that progress is usually reached through detailed understanding of experimental data. metaphysics is usually produced in the aftermath of a breakthrough to digest what was learned or put it in some sort of perspective. this is very natural, but i seriously question its usefulness for any sort of scientific progress.

    14. A. says:

      The discussion above reminds me of the following exchange I overheard in the department, the other day:

      Senior string theorist: “The definition of “science” has nothing to with modern physics.”

      Postdoc physicist: “I’d say that modern physics has nothing to do with science.”

      Senior string theorist: looks like he has just been slapped.

      Sigh. Seriously.

    15. Peter Woit says:


      Thanks for the suggestion. I’ve implemented it for the archives, where it looks quite useful.

    16. Dude says:

      Peter, thank you very much. That’s the main thing I wanted, but here are two more requests/suggestions, although you may not care about implementing them.

      #1: Please increase the width of the sidebar as wide as can (but to where it still looks good). Wp-dTree truncates the post titles based on the width of the sidebar. This isn’t of ultimate importance since the complete title is shown if the mouse cursor is hovered over a truncated post title, but it would make viewing titles faster.

      #2: Include the sidebar in every view. For example, the sidebar is shown if I enter using your main blog URL, but if I click on a post, the sidebar isn’t shown. If I was surfing lots of old posts, I wouldn’t have to go back to your main page each time.

      Suggestion #2 would also be good for those who aren’t familiar with you, and who enter a page through a search engine. If the sidebar is there, a person may see something that interests them.

      To do the above, you’d have to edit the CSS, and I can’t tell you how to do that. The last time I did it, it took me a long time to figure out how to mess with CSS.

      Thanks again.

    17. Peter Woit says:


      I don’t think I’ll change anything more right now, have been trying to keep the changes from the default theme to a minimum, which makes upgrading the version easier.

    18. Mitch Miller says:

      A’s comment sort of reminds me of George W. Bush’s comment on the free market.

      “I’ve abandoned scientific principles to save the scientific system.”

    19. GR says:

      Chris W:

      Now that I’ve read Smolin’s article, I don’t have a problem with it. Mainly because it offers a concrete prediction: the universe maximises the production of black holes. I’ll be damned if I can ferret out any sort of prediction from Srednicki and Hartle.

      If string and multiverse theorists want to play the metaphysics game, they’re more than free to. But they shouldn’t call it physics. This whole probability game reminds me of that Feynman quote about the license plate…

    20. Chris W. says:

      — from ‘Chris’: “i am not aware of even one single instance in modern physics where pondering about metaphysical questions resulted in progress of any sort.”

      1) Since the 18th century, metaphysical considerations have remained in the background, and even in their intellectual biographies most scientists downplay or omit any mention of their influence. At most, they might acknowledge having studied philosophical writings in which metaphysical presuppositions are discussed. In addition, there is the question of what assumptions one considers to be metaphysical. I doubt that most working scientists spend much time thinking about the demarcation. They’re more concerned with distinguishing between ideas that might prove fruitful for the formulation (!!) and solution of scientific problems. In practice, “metaphysical” is often considered a synonym for “useless” or “sterile”. Indeed, most metaphysics, like much thinking in many areas, is of dubious value. (“Ninety percent of everything is crap.“)

      2) The metaphysical ideas that have most important to modern science (not just physics) so far have ancient roots, and are so embedded in our cultural and intellectual heritage that are rarely consciously considered, except by philosophers and historians.

      3) Avoidance of metaphysical assumptions, interpreted as an uncompromising operationalism, was very much part of the philosophical background of quantum mechanics. Einstein’s stated motivations for the development of special relativity provided considerable impetus for this, along with the influence of the Vienna Circle. (Einstein later repudiated this attitude.) This is deeply embedded in modern scientific culture, so that metaphysical assumptions, when employed, are rarely acknowledged or even recognized as such.

      4) Re your remark: “all examples that i am aware of show that progress is usually reached through detailed understanding of experimental data.” The question becomes, how is that understanding achieved? Perhaps it is considered to be in poor taste to discuss it publicly, but when the necessary ideas don’t exist, somebody has to wrestle with the obstacles to understanding, including the possible role of an unquestioned metaphysical framework, or the lack of such a framework. Of course, as soon as empirically testable assertions can be made, they take center stage, and most researchers not directly involved in developing the key ideas take little interest in their background.

      I hope these observations serve to indicate why consideration of metaphysical presuppositions is so routinely avoided, even when it could be illuminating, and even essential, both in formulating and in appreciating important scientific ideas.

    21. Observer says:

      …“The definition of “science” has nothing to with modern physics.”
      Are string theorists slipping their discipline into religious faith…? Now that sounds amazing .

    22. John Duffield says:

      Ouch. Is that what passes for HEP these days? The Srednicki/Hartle paper read like a spoof. It falls over in the first paragraph when it claims that “there is a quantum probability for these data to exist in any spacetime volume”. No, there isn’t. There is no evidence whatsoever for any kind of xerographic multiple copies of me sitting at this desk. There is no evidence whatsoever for an infinite number of Boltzmann brains magically popping into existence. These speculations do not serve as the basis for a typical/atypical computation or experiment dressed up with Bayesian respectability. This isn’t science. There are no testable predictions. These guys are riding the many-Worlds misinterpretation of quantum physics and they surely don’t even understand that the fine structure constant alpha α = e²/2ε0hc is a running constant or why. So when they say “It is only the combination of fundamental theory and an assumed xerographic distribution that is predictive and testable by observation” it’s just castles in the air.

    23. anon. says:

      they surely don’t even understand that the fine structure constant alpha α = e²/2ε0hc is a running constant or why

      It’s not as if one of them wrote a textbook on quantum field theory, after all! Oh, wait.

    24. Titanium Dragon says:

      Your extract has the scent of nonsense clinging to it, but it actually does make sense. It is an incredibly complicated restatement of a very basic principle. It could be summarized more simply as follows:

      In an infinitely large universe, any set of physically possible events will occur somewhere. Therefore, it is possible that all of our conclusions are the result of coincidence, so we can only be as confident in our data as probability allows.

      Or, in other words, it is possible (if vanishingly unlikely) that our observations of physical phenomena are coincidental. Of course, this is a worthless statement, as we all already knew this fact; its basic science that we are never absolutely certain of anything.

      It is also worth noting that this is not, in fact, limited to an infinite universe, as it can be true in even a finite universe, so the whole multiverse part is completely extraneous to this statement. We could simply say:

      We can only be as confident in our data as chance allows us to be.

      And be done with it.

    25. Titanium Dragon says:

      Incidentally, the linked-to article strikes me as pretty vapid itself, I’m afraid. Of course, so does the entire timelessness argument. He claims that we evoked chess into existence, as opposed to it always existing in some timeless state. Both strike me as completely meaningless, unscientific ideas. Indeed, both are equally falsifiable; we cannot confirm the existence of something until we do so by definition, so we cannot check if it was there “beforehand”; ergo both are meaningless.

      Really, the argument strikes me as a very metaphysical one which doesn’t make unique predictions about the universe. What in that article is really worth considering? That we cannot know the initial conditions of the universe? Isn’t that something which is generally suspected because of the massive amount of energy involved, as well as our inability to observe the universe as a whole and the fact that our laws of physics break down before we go back all the way to the big bang?

    26. jpd says:

      i think i agree with you ,but quick question

      re: “In an infinitely large universe, any set of physically possible events will occur somewhere”

      if the probability of a physically possible event is infinitely small, couldn’t it be ruled out in a (not sufficiently) infinitely large universe?

      in other words: a boltzmann brain is ridiculous.

    27. Shantanu says:

      Peter you found anything interesting at
      “New Prospects for Solving the Cosmological Constant Problem”
      conference at PI?
      It has a wide spectrum of talks from anthropic principle to LHC etc.

    28. Peter Woit says:


      I have the same problem with explanations of the CC that I have with quantum gravity in general. The only way that I can see that any purported explanation would be convincing would be if it also implied other non-trivial facts about observable physics. I haven’t seen anything like this, either from the anthropic people or from other attempted CC explanations.

    29. alex says:

      re: “In an infinitely large universe, any set of physically possible events will occur somewhere”,

      or as Hartle and Srednicki put it,

      “in the universes considered in contemporary cosmology the following often hold:

      1. The probability is near unity that our data D0 exist somewhere.

      2. The probability is near unity that our data D0 is exactly replicated elsewhere many times.”

      The reasoning behind this claim isn’t specified but it seems to be that given an infinite number of tries every possible outcome will occur. But his is not so. Even in an unbounded universe you have a countable number of planets, brains, desks or whatever else you include in “our data D0”. If the number of allowed states D0 is not countable, which would seem to be the case, then it is impossible for all of them to actually happen and the chance of there being another me sitting at an identical desk looking at an identical night sky somewhere out there beyond the observable universe is not close to unity, it is zero. At least if you consider D0 being “exactly replicated” as they state. There are some loopholes. You might think of a multiverse with uncountably many universes within it. Or better, you might consider states similar to but not exactly the same as D0. Maybe you’d want to consider the probability of states where the expectation values of all possible measurements are within epsilon of those for the state D0, and see if you can keep the probability of there being one at unity whilst epsilon tends to zero. Though you might then worry about chaotic systems and the amplification of arbitrarily small differences in initial conditions. I guess you might also consider that you are talking about some sort of reduced density matrix for each candidate subsystem and the implications of that. But they don’t.

    30. GR says:


      Have you seen the latest rehash of the string-theory-in-CM on slashdot?

    31. Peter Woit says:


      Hadn’t seen that. It’s the New Scientist article I wrote about here a while ago, which wasn’t bad, although Slashdot as usual manages to do a good job of garbling it with nonsense like:

      “His theory states that the known universe is only a 2D construct in anti-de-Sitter space, projected into 3 dimensions.”

    32. Claver says:

      re: “In an infinitely large universe, any set of physically possible events will occur somewhere”

      In the real and measurable universe only certain events happen. That is, doesn’t such a universe exhibit a bias? Such a bias permitting and excluding events.

      If the universe exhibits such a bias today, why shouldn’t an observer some where afar off not see the same bias? And, why should that bias change?

      I mean is the universe ‘compelled’ to pick different events tommorrow? Implying that the universe ‘remembers’ which events happened yesterday? (Which gives rise to an interesting observation: information from the past influences events of today.)

      Put in another way, if in an infinite amount of spacetime EVERY set of physically possible event will occur, then surely every event can be repeated, within the same infinite universe. In which case the probability of an event occurring in an infinite universe can be more than 1 because
      ‘who can measure how many times, in an infinite universe, a particular event is repeated?’Is there an observer? If not, then saying …

      “In an infinitely large universe, any set of physically possible events will occur somewhere”

      is saying something that cannot be physically proven. It is an unobservable and unmeasureable statement?

      i.e if ANY set of physically possible events WILL occur then there is a probability that ALL will occur.

      I thought that in physics (real and measurable) the notion of an infinitely large universe was purely a mathematical construct? A consequence of the inherent mathematical difficulty – in expressing ongoing repetitive structures – carried over into physics from mathematics.

      Or is this just being naive?

    33. Dr Lewney says:

      Hi Peter,

      Did you happen to have a look at the last page of the June issue of Physics World containing Lee’s excellent article? (I’m afraid it wasn’t picked for online publication.)



    34. Peter Woit says:

      Hi Mark,

      I’m afraid I haven’t seen a physical copy of the June issue of Physics World yet. What’s on the last page?

    35. Dr Lewney says:

      It’s a light-hearted article on my Schools Lecture Tour which mentions this very blog! Send me a private email and I can reply with the PDF.



    36. i am wondering if you had a chance to get to any of the World Science Festival this year? And if you do have any issues, i am curious about them. I thought that this year the events were rather brave in presenting opposing theories. I say brave, because there is a danger when presenting ground breaking and interesting science to a group most interested in the artistic interpretations of it. They might tend to misunderstand scientific discourse, relating it to differences in religion. In Religion the wrong party goes to hell. In science, they jump on board the proven theory and try to improve it. This is why scientific debate is always positive, and why religious debate always a waste of time.

    37. Peter Woit says:


      I was out of town for most of the World Science Festival this year, so just saw some reports in the newspaper, which indicated that, like last year it was a great success. The high level of public interest in this kind of thing is pretty amazing.

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