Use the Moment Map, not Noether’s Theorem

For a fourth provocative slogan about quantum mechanics I’ve chosen:

Use the moment map, not Noether’s Theorem.

Pretty much every physics textbook these days explains the way symmetry principles work as:

  • Start with an action functional, invariant under a Lie group G.
  • Use Noether’s theorem to get a conserved charge (for each element of the Lie algebra of G).

There’s a short (slightly mystifying) calculation always given to derive this. I’d like to argue that this is really not the best way to think about the implications of having a Lie group act on a physical system, that for this it’s better to take the Hamiltonian point of view. There the way symmetry principles work is:

  • For a function on phase space (or on a general symplectic manifold) you get a vector field. This is just Hamilton’s equations, giving the vector field for time evolution corresponding to any Hamiltonian function.
  • The infinitesimal action of G on phase space gives a vector field for each element of the Lie algebra of G. The moment map takes an element of the Lie algebra to a function on phase space (the one corresponding to the vector field).

I’m ignoring some subtleties here having to do with the relation between vector fields and functions not being quite one-to-one.

All of the basic examples of conservation laws in physics come about this way. The action of time translation gives the Hamiltonian function, space translation the momentum, rotations give the angular momentum, and phase transformations give charge. You can get these either as moment maps, or using Noether’s theorem.

The moment map however gives you much more, with phase space providing structure that is not visible just from the action. A simple example is the harmonic oscillator in 3 variables. SO(3) rotations act on the configuration variables, preserving the action, so Noether’s theorem gives you 3 conserved quantities, the angular momentum variables. The moment map point of view however gives you much more. The phase space is 6 dimensional (3 positions + 3 momenta) and the Lie group Sp(6,R) of linear symplectic transformations acts on it, with a subgroup U(3) preserving the Hamiltonian. The U(3) includes the SO(3) rotations as a subgroup, but it is much larger (9 dimensions vs. 3), so the moment map gives you many more conserved quantities. After quantization, you learn that energy eigenstates are U(3) representations, telling you much more about them than what angular momentum tells you.

The moment map point of view also gives you quantities corresponding to the directions in Sp(6,R) that are not in U(3). In the quantum theory these act on the full state space (not preserving energy eigenstates) and your state space is a representation of (a double cover of) this group.

For the simplest possible harmonic oscillator, in one-dimension, Noether’s theorem doesn’t really tell you anything. The moment map point of view says that there is an Sp(2,R) acting on phase space, with a U(1) subgroup preserving the Hamiltonian. The moment map is just the Hamiltonian itself. In the quantum theory you find that the harmonic oscillator state space is a representation of (a double cover of) Sp(2,R), with the U(1) action on states characterized by integers, which correspond to the energy. This integrality is the essence of the “quantum” in “quantum mechanics”, and it’s quite invisible to Noether’s theorem, but a basic fact of the moment map point of view.

In some sense this is an argument for the Hamiltonian vs. Lagrangian point of view in general. The relation between the two is that, given a Lagrangian, one constructs a symplectic structure on the space of solutions of the variational problem, and thus a Hamiltonian formalism. Noether’s conserved quantities are then examples of moment maps. The problem is that typically this requires the use of constraints and the quite tricky constrained Hamiltonian formalism.

The positive argument for the Lagrangian point of view is that it comes into its own in the relativistic setting, making Lorentz invariance easy to handle by the Noether’s theorem method. This is quite true, with the standard version of the Hamiltonian formalism distinguishing the time direction and breaking Lorentz invariance. There is however a less well-known “covariant phase space” point of view, where one tries to work with the space of solutions of the equations of motion as one’s phase space. Only if one identifies a solution with its initial data at a fixed time does one distinguish the time direction. I’ve recently enjoyed reading Igor Khavkine’s review article, which in particular does a great job of explaining the history of this line of thinking.

The Lagrangian also comes with the extremely seductive point of view on quantization of the path integral. This point of view works very well for dealing with Yang-Mills theory, and I spent much of my early career convinced that all there was to quantization was figuring out how to make sense of integrating over the exponential of the action. I’m now much more aware of the advantages of the Hamiltonian point of view, especially in terms of understanding quantum theory as representation theory. In some sense what one really wants is to understand quantization in a way that takes advantage of both points of view, but the relationship between them is quite non-trivial.

The discussion here has been far too wordy for most people to make sense of. If you want to understand any of this, you need equations. Luckily, I’ve provided lots of them and many details here, see chapters 12 and 13 for the moment map, chapter 19-22 for the harmonic oscillator.

Posted in Quantum Mechanics | 25 Comments

Quick Links

Now back from the West Coast, here’s a list of things I’ve run across that may be of interest:

  • One piece of news from Berkeley is that Peter Scholze will be there this fall, giving a course describing new techniques for dealing with Langlands conjectures for number fields in an analogous manner to ones successfully used in the function field case. The course announcement (described by David Ben-Zvi as “shockingly futuristic”) is here.

    Since it’s never too early for Fields Medal predictions, I’m predicting an award for Scholze at the next ICM, in Rio, August 2018.

  • It also wouldn’t surprise me if Scholze gets one of the $3 million Milner/Zuckerberg breakthrough prizes in mathematics before then. I’m pleased to see that last week there was an announcement that some of the winners of this year’s math prize have banded together to fund graduate math fellowships in developing countries.
  • On the physics front, rumor from Peter Coles is that:

    I have it on very good authority that Planck’s analysis of the Galactic foregrounds in the BICEP2 region will be published (on the arXiv) on or around September 1st 2014.

    so next week we may (or may not…) find out if BICEP2 was seeing primordial gravitational waves or just dust.

  • In the meantime, this week there’s COSMO 2014, a cosmology conference going on in Chicago. The conference organizers have decided to have their public event mix artists and scientists, brought together around the topic of Multiverse: Fact, Fictions and Fantasies.
  • Also in the Chicago area, Fermilab last week hosted a Nature Guiding Theory workshop, considering the question of what to make of the failure of the LHC to find SUSY or other physics predicted by the “naturalness” paradigm. Some of the discussion was of the conventional SUSY or multiverse variety. For instance, see here, or Raman Sundrum’s Super-Natural vs. Other-Worldly in Fundamental Physics, which ends up arguing that:

    Naturalness, anthropic selection, Multiverse are Meta-theories. The collection of naturalness-related experiments – LHC, flavor, axion searches, tests of Inflation (e.g. BICEP2, …), Dark Matter search, form a Meta-experiment.

    I’m not sure what a “Meta-experiment” or “Meta-theory” is, other than that it’s not the conventional sort of science where you have the usual notions of how to make scientific progress.

    Some talks dealt with dumping the “naturalness” argument in favor of ideas about fundamental conformal invariance that I mentioned here. See for example here and here. By the way, Natalie Wolchover’s excellent article on these ideas has been given by Wired the title Radical New Theory Could Kill the Multiverse Hypothesis.

  • Going on at the same time as the cosmology conference in the Chicago area this week is a Fermilab workshop on Next Steps in the Energy Frontier. Today there were talks on future plans from Fermilab, CERN and the Chinese. The Chinese now have a Center for Future High Energy Physics directed by Nima Arkani-Hamed and are talking about a huge new machine that would start off as an electron-positron Higgs factory in 2028, then become a 50-90 TeV proton-proton collider in 2042. CERN is discussing similar plans, although they will have the high-luminosity LHC keeping them busy through 2035.

    On Thursday Arkani-Hamed will end the conference with a talk at 12:15 entitled “Go Big or Go Home…” (the US I guess already has decided to go home, with no plans for a big new machine). He really is ubiquitous at fundamental physics conferences in a variety of areas, since at 9-9:45 that morning he’ll be addressing COSMO 2014 up in Chicago on the topic of “Cosmological Collider Physics”. This Tuesday night he’ll be at the other public event there, a showing of the movie Particle Fever, which stars him discussing the multiverse.

  • There seems to be little scientific news anymore about string theory, but it is everywhere in popular culture. This past weekend it made Dilbert, it’s playing a big role in DC superhero comics, and a few weeks ago some peculiar British TV show designed to torment young “child geniuses” had string theorist Brian Wecht bringing an 11-year old to tears with questions on the subject.
  • John Horgan has put online an interesting interview with Carlo Rovelli, following up on one with George Ellis. Both Ellis and Rovelli criticize physicists for knocking philosophy. On the whole I’m quite sympathetic to what both Ellis and Rovelli have to say, although I think the problem with string theory is not really the “philosophical superficiality” that Rovelli sees as the problem. Jerry Coyne has a blog posting criticizing Rovelli as an “accomodationist”. Note that if you want to argue about religion, please do it at Coyne’s blog, not here.
  • For those interested in the metaphysical end of philosophy as applied to physics, Oxford is hosting a conference in October on The Metaphysics of Quantum Mechanics. It’s organized by the Power Structuralism in Ancient Ontologies and Metaphysics of Entanglement Projects. The first of these is funded by the European Research Council, the second by the Templeton World Charity Foundation (which I’ve never heard of before, and wonder about its relationship to the Templeton Foundation). Entanglement is the hot topic in fundamental physics these days. The Metaphysics of Entanglement project promises to bring quantum mechanics and theology together bu (Jerry Coyne is going to hate this…)

    bringing the research results of the above investigation to bear on our understanding of questions regarding the metaphysics of the incarnation and of the Trinity in philosophy of religion.

  • Anyone interested in making some easy money might want to contact Gordon Kane. He has a letter to the editor in the latest Scientific American arguing that string/M-theory predicts superpartners visible at the LHC and complains that:

    Predictions based on such theories should be taken seriously. I would like to bet that some superpartners will be found at the LHC, but I have trouble finding people who will bet against that prediction.

Update: One more. The New York Times today has a very good story about IMPA, the math institute in Rio de Janeiro.

Posted in Uncategorized | 11 Comments

Nature is Fundamentally Conformally Invariant

For a third slogan I’ve chosen:

Nature is fundamentally conformally invariant.

Note the weasel-word “fundamentally”. We know that nature is not conformally invariant, but the kind of thing I have in mind is pure QCD, where the underlying classical theory is conformally invariant, with quantization dynamically breaking conformal invariance in a specific way.

The group of conformal symmetries, its representations, and what this has to do with physics are topics I haven’t written about in the notes I’ve been working on. This is because I suspect we still haven’t gotten to the bottom of these topics, and properly dealing with what is known would require a separate volume. Part of the story is the twistor geometry of four dimensions, which Roger Penrose pioneered the study of, and which recently has found important applications in the calculation of scattering amplitudes.

As a more advanced topic, this slogan would normally have been put off until later, but I wanted to point to a new article by Natalie Wolchover in Quanta magazine which deals with exactly this topic. It describes several different efforts by physicists to rethink the usual story about the hierarchy problem, taking a conformally invariant model as fundamental. For the latest example along these lines, see this arXiv preprint. The whole article is well-worth reading, and it includes a quote from Michael Dine (whose work I’ve been critical of in the past) that I found heart-warming:

“We’re not in a position where we can afford to be particularly arrogant about our understanding of what the laws of nature must look like,” said Michael Dine, a professor of physics at the University of California, Santa Cruz, who has been following the new work on scale symmetry. “Things that I might have been skeptical about before, I’m willing to entertain.”

Perhaps particle theorists are beginning to realize that the landscape is just a dead-end, and what is needed is a re-examination of the conventional wisdom that led to it.

Posted in Quantum Mechanics | 33 Comments

Grand Unification of Mathematics and Physics

For a second slogan about quantum mechanics I’ve chosen:

Quantum mechanics is evidence of a grand unification of mathematics and physics.

I’m not sure whether this slogan is likely to annoy physicists or mathematicians more, but in any case Edward Frenkel deserves some of the blame for this, since he describes (see here) the Langlands program as a Grand Unified Theory of mathematics, which further is unified with gauge field theories similar to the Standard Model.

This week I’m in Berkeley and have been attending some talks at an MSRI workshop on New Geometric Methods in Number Theory and Automorphic forms. Number theory is normally thought of as a part of mathematics about as far away from physics as you can get, but I’m struck by the way the same mathematical structures appear in the representation theory point of view on quantum mechanics and in the modern point of view on number theory. For example, the lectures on Shimura varieties have taken as fundamental example the so-called Siegel upper-half space, which is the space Sp(2n,R)/U(n). Exactly the same space occurs in the quantization of the harmonic oscillator (see chapters 21 and 22 of my notes), where it parametrizes possible ground states. Different aspects of the structure play central roles in the math and the physics. In the simplest physics examples one works at a fixed point in this space, with Bogoliubov transformations taking one to other points, something which becomes significant in condensed matter applications. In number theory, one is interested not just in this space, but in the action of certain arithmetic groups on it, with the quotient by the arithmetic group giving the object of fundamental interest in the theory.

The workshop is the kick-off to a semester long program on this topic. It will run simultaneously with another program with deep connections to physics, on the topic of Geometric Representation Theory. This second program will deal with a range of topics relating quantum field theory and representation theory, with the geometric Langlands program a major part of the story, one that provides connections to the number theoretical Langlands program topics of this week’s workshop. I’ve got to be in New York teaching this semester, so I’m jealous of those who will get to participate in the two related MSRI programs here in Berkeley. Few physicists seem to be involved in the programs, but these are topics with deep relations to physics. I do think there is a grand unified theory of some kind going on here, although of course one needs to remember that grand unified theories in physics so far haven’t worked out very well. Maybe the problem is just that one hasn’t been been ambitious enough, that one needs to unify not just the interactions of the standard model, but number theory as well…

Posted in Quantum Mechanics, Uncategorized | 7 Comments

Quantum Theory is Representation Theory

For a first slogan (see here for slogan zero) I’ve chosen:

Quantum theory is representation theory.

One aspect of what I’m referring to is explained in detail in chapter 14 of these notes. Whenever you have a classical phase space (symplectic manifold to mathematicians), functions on the phase space give an infinite dimensional Lie algebra, with Poisson bracket the Lie bracket. Dirac’s basic insight about quantization (“Poisson bracket goes to commutators”) was just that a quantum theory is supposed to be a unitary representation of this Lie algebra.

For a general symplectic manifold, how to produce such a representation is a complicated story (see the theory of “geometric quantization”). For a finite-dimensional linear phase space, the story is given in detail in the notes: it turns out that there’s only one interesting irreducible representation (Stone-von Neumann theorem), it’s determined by how you quantize linear functions, and you can’t extend it to functions beyond quadratic ones (Groenewold-van Hove no-go theorem). This is the basic story of canonical quantization.

For the infinite-dimensional linear phase spaces of quantum field theory, Stone-von Neumann is no longer true, and the fact that knowing the operator commutation relations no longer determines the state space is one source of the much greater complexity of QFT.

Something that isn’t covered in the notes is how to go the other way: given a unitary representation, how do you get a symplectic manifold? This is part of the still somewhat mysterious “orbit method” story, which associates co-adjoint orbits to representations. The center of the universal enveloping algebra (the Casimir operators) acts as specific scalars on an irreducible representation. Going from the universal enveloping algebra to the polynomial algebra on the Lie algebra, fixing these scalars fixes the orbit.

Note that slogan one is somewhat in tension with slogan zero, since it claims that classical physics is basically about a Lie algebra (with quantum physics a representation of the Lie algebra). From the slogan zero point of view of classical physics as hard to understand emergent behavior from quantum physics, there seems no reason for the tight link between classical phase spaces and representations given by the orbit method.

For me, one aspect of the significance of this slogan is that it makes me suspicious of all attempts to derive quantum mechanics from some other supposedly more fundamental theory (see for instance here). In our modern understanding of mathematics, Lie groups and their representations are unifying, fundamental objects that occur throughout different parts of the subject. Absent some dramatic experimental evidence, the claim that quantum mechanics needs to be understood in terms of some very different objects and concepts seems to me plausible only if such concepts are as mathematically deep and powerful as Lie groups and their representations.

For more about this, wait for the next slogan, which I’ll try and write about next week, when I’ll be visiting the Bay area, partly on vacation, but partly to learn some more mathematics.

Posted in Quantum Mechanics | 43 Comments

Smashing Physics

I recently finally found a copy of Jon Butterworth’s Smashing Physics, which came out in the UK a few months ago, but still hasn’t made it to the US. As far as I know it’s the first book about the Higgs written by someone actually involved in the LHC experiments. While there are several books already out there that do a good job of explaining the Higgs story (see for instance here) this one is a great read, giving a lively picture of what it was like to be involved in one of the experiments that made the discovery.

The book is structured as a bit of a travelogue, traveling through space to CERN and to various conferences, through time as the ATLAS LHC experiment unfolded, with physics explanations interspersed. A reviewer at New Scientist didn’t like this, but I think the personal and idiosyncratic approach works well. We’re given here not the highly processed take of a professional science writer, but a picture of what this sort of professional life is actually like for one specific scientist, from what happens in collaboration meetings, to an overnight spree on the Reeperbahn.

The perspective is definitely British (a lot of drinking goes on, with a scornful observation of American couples at a French bistro “drinking water”), and includes a fair amount of material about recent science funding problems in Britain. Butterworth’s comments are often to the point, if sometimes impolitic. For instance, about the “God Particle” business, there’s a footnote:

Yes, I know Lederman claims he wanted to call it The Goddamn Particle and blames his publishers for the change. But my publishers wanted to call this book something really silly, and I managed to stop them.

For readers who know nothing about the physics involved, this book may not be the right place to start, with the well-known scientific story not getting a detailed treatment, and little in the way of graphics besides some Feynman diagrams. On the other hand, if you’ve read one of the other books about the Higgs, Butterworth takes you a lot deeper into the subject of LHC physics, including some extensive material on his work on boosted objects and jet substructure, which may lead to important results in future LHC analyses. If you like your science non-abstract and human, this is a great place to learn about the Higgs discovery story.

There’s a quite positive review in the Guardian by Graham Farmelo, which describes the book well. That review though contains (like another review and like his wonderful book on Dirac) some odd material about string theory, in this case a long paragraph defending the theory, and telling us that “he [Butterworth] and his fellow sceptics will be proved wrong in the long term.” Actually there’s very little about string theory in the book other than some sensible comments about being more interested in things experimentally testable. Like Tom Siegfried, it seems some science journalists are likely to always be unwilling to admit that they were sold goods that didn’t turn out to work as advertised, and uncomprehending that most physicists, like Butterworth, never were buyers.

I gather the book may appear here in the US early next year, hope it gets some attention then.

Posted in Book Reviews | 5 Comments

2014 Fields Medals

I thought this wasn’t supposed to be announced until late this evening New York time, but the Fields Medal announcement is now online. The winners are:

  • Artur Avila
  • Manjul Bhargava
  • Martin Hairer
  • Maryam Mirzakhani

Mirzakhani is the first woman to win a Fields medal. Congratulations to all four.

I’m not at all knowledgeable about the work of this year’s medalists, for this you can consult the press releases on the ICM page.

Update: Quanta magazine has profiles of the winners. Avila, Bhargava, Hairer, Mirzakhani.


Update
: For ICM blogging, clearly the place to go is the blog of a Fields Medalist.

Update: According to Tim Gowers, the Fields Medal Committee was: Daubechies, Ambrosio, Eisenbud, Fukaya, Ghys, Dick Gross, Kirwan, Kollar, Kontsevich, Struwe, Zeitouni and Günter Ziegler.

Update: For two very different sorts of blog posts about the Fields Medal, see Terry Tao and Mathbabe.

Posted in Uncategorized | 53 Comments

What’s Hard to Understand is Classical Mechanics, Not Quantum Mechanics

For a zeroth slogan about quantum mechanics, I’ve chosen

What’s hard to understand is classical mechanics, not quantum mechanics.

The slogan is labeled by zero because it’s preliminary to what I’ve been writing about here. It explains why I don’t intend to cover part of the standard story about quantum mechanics: it’s too hard, too poorly understood, and I’m not expert enough to do it justice.

While there’s a simple, beautiful and very deep mathematical structure for fundamental quantum mechanics, things get much more complicated when you try and use it to extract predictions for experiments involving macroscopic components. This is the subject of “measurement theory”, which gives probabilistic predictions about observables, with the basic statement the “Born rule”. This says that what one can observe are eigenvalues of certain operators, with probability of observation proportional to the norm-squared of the eigenvector. How this behavior of a macroscopic experimental apparatus described in classical terms emerges from the fundamental QM formalism is what is hard to understand, not the fundamental formalism itself. This is what the slogan is trying to point to.

When I first started studying quantum mechanics, I spent a lot of time reading about the “philosophy” of QM and about interpretational issues (e.g., what happens to Schrodinger’s famous cat?). After many years of this I finally lost interest, because these discussions never seemed to go anywhere, getting lost in a haze of complex attempts to relate the QM formalism to natural language and our intuitions about everyday physics. To this day, this is an active field, but one that to a large extent has been left by the way-side as a whole new area of physics has emerged that grapples with the real issues in a more concrete way.

The problem though is that I’m just knowledgeable enough about this area of physics to know that I’ve far too little expertise to do it justice. Instead of attempting this, let me just provide a random list of things to read that give some idea of what I’m trying to refer to.

Other suggestions of where to learn more from those better informed than me are welcome.

I don’t think the point of view I take about this is at all unusual, maybe it’s even the mainstream view in physics. The state of a system is given by a vector in Hilbert space, evolving according to the Schrodinger equation. This remains true when you consider the system you are observing together with the experimental apparatus. But a typical macroscopic experimental apparatus is an absurdly complicated quantum system, making the analysis of what happens and how classical behavior emerges a very difficult problem. As our technology improves and we have better and better ways to create larger coherent quantum systems, thinking about such systems I suspect will lead to better insight into the old “interpretational” issues.

From what I can see of this though, the question of the fundamental mathematical formalism of QM decouples from these hard issues. I know others see things quite differently, but I personally just don’t see evidence that the problem of better understanding the fundamental formalism (how do you quantize the metric degrees of freedom? how do these unify with the degrees of freedom of the SM?) has anything to do with the difficult issues described above. So, for now I’m trying to understand the simple problem, and leave the hard one to others.

Update
: There’s a relevant conference going on this week.

Update
: I’ve been pointed to another article that addresses in detail the issues referred to here, the recent Physics Reports Understanding quantum measurement from the solution of dynamical models, by Allahverdyan, Balian and Nieuwenhuizen.

Posted in Quantum Mechanics | 46 Comments

Fall QM Course

This year I’ll be teaching a new version of the same course on quantum mechanics aimed at mathematicians that I taught during the 2012-3 academic year (there’s a web-page here). During the last course I started writing up notes, and have spent a lot of the last academic year working on these, the current version will always be here. At this point I have a few of the last chapters to finish writing, as well as a long list of improvements to be made in the earlier ones. I’ll be teaching the course based on these notes, and hope to improve them as I go along, partly based on seeing what topics students have trouble with, and what they would like to hear more about.

I’ve learned a lot while doing this, and it now seems like a good idea to write something on the blog discussing topics that don’t seem to me to be dealt with well in the standard textbook treatments. Johan de Jong’s Stacks Project has recently added a sloganerator (written by Pieter Belmans and Johan Commelin), which has inspired me to try to organize things as slogans. Slogan 0 to appear soon….

Posted in Quantum Mechanics | 8 Comments

Quantum Connection Could Revitalize Superstrings

Finally back from vacation, postings may appear somewhat more regularly…

Science journalist Tom Siegfried has been one of the most vociferous proponents of string theory for many, many years (see here), but even his faith seems like it might be failing as the decades roll on. His latest on the topic starts out:

Sometimes it’s nice to reflect nostalgically on the last couple of decades of the 20th century. You know, the era of Madonna and Duran Duran, Cheers and The X-Files, McGwire and Sosa, the Macarena, and superstring theory.

The article does try and mount an argument that string theory may not be moribund, with the hope for the future coming from a new paper by Bars and Rychkov entitled Is String Interaction the Origin of Quantum Mechanics?. The idea here seems to be that if you assume you somehow have a fully consistent string field theory, not based on quantum mechanics, then the occurrence in this theory of non-commutative phenomena would “explain” quantum mechanics. To me, this seems to deserve some sort of award for the most desperate attempt yet to justify string theory, but Siegfried is a fan, explaining:

For decades, explaining why nature observes the mysterious rules of quantum physics has perplexed physicists everywhere. Nobody could explain why those rules worked. The connection between string physics and the quantum math may now lead the way to an answer.

I’ll write more soon about those “mysterious rules of quantum physics”, but I just don’t see at all how string field theory (which supposedly is based on quantum mechanics…) makes anything about quantum mechanics less mysterious.

Siegfried of course is not just a fan of string theory, but also of the multiverse, so he ends with:

On top of all that, the string-quantum connection suggests an intriguing insight into the nature of reality. Quantum physics is notorious for implying the existence of multiple realities, as articulated in the “many worlds” interpretation of quantum mechanics. Superstring theory has also annoyed many physicists by forecasting the existence of a huge “landscape” of different vacuum states, essentially a multiverse comprising multiple universes with a wide range of physical properties (many not suitable for life, but at least one that is). If string interactions really are responsible for the rules of quantum physics, maybe there’s some connection between the multiple quantum realities and the superstring landscape. For fans of the late 20th century, it seems like an idea worth exploring.

One thing remarkable about this is that he has another piece that recently appeared, an interview with Katherine Freese, where he tries to convince her about the multiverse, but doesn’t get anywhere:

Theory predicts vastly more vacuum energy than the amount actually observed. Wouldn’t this huge disparity be explained if there are multiple universes, a multiverse, and each has a different density of vacuum energy? Then the reason we have a low amount in ours is because that’s the only way we could exist in it.

I don’t like that idea. A lot of people like it because of string theory. Originally people thought that string theory would give a unique solution to the vacuum-energy equations. But it turns out that in string theory there are maybe 10-to-the-500th different vacuum states. So the idea is that they’re all out there, but we have to live in one with a value of the cosmological constant close to the one we have. But I don’t like anthropic arguments. They rely on the fact that human life can only come to exist under certain conditions, so that of the many universes out there it’s not surprising we live in the one that supports our type of life. That’s not a good enough explanation for me. I feel there are physics problems that we have to answer, and we can answer them in this universe, in this piece of the universe we live in. I think it’s our job to try to do that, and it’s not good enough for me to give up on it and say, well, it has to have this value because otherwise we couldn’t exist. I think we can do better than that. I know, I’m old-fashioned.

Isn’t part of the question whether there is a multiverse or not? If you had really strong evidence that there is a multiverse, then the anthropic explanation becomes better motivated. Inflation, the rapid burst of expansion right after the Big Bang, supposedly can produce a multiverse by way of “eternal inflation.”

I do believe in inflation, so can inflation give you a multiverse or not? Because if it can, then I’m forced to consider this possibility. I recently wrote a paper with Will Kinney on this. We concluded that what we observe in the cosmic microwave background radiation is not giving rise to eternal inflation. So how do you know that ever happened?

Are the recent results on the cosmic microwave background from the BICEP2 experiment relevant to this issue?

If you take the BICEP data literally, which I’m not saying you should, you never have eternal inflation. So you don’t have to have eternal inflation, if you ask me. I was very happy about that.

Posted in This Week's Hype | 22 Comments