How did the universe come to be? We don’t know yet, of course, but we know enough about cosmology, gravitation, and quantum mechanics to put together models that standing a fighting chance of capturing some of the truth.

Stephen Hawking‘s favorite idea is that the universe came out of “nothing” — it arose (although that’s not really the right word) as a quantum fluctuation with literally no pre-existing state. No space, no time, no anything. But there’s another idea that’s at least as plausible: that the universe arose out of something, but that “something” was simply “chaos,” whatever that means in the context of quantum gravity. Space, time, and energy, yes; but no order, no particular arrangement.

It’s an old idea, going back at least to Lucretius, and contemplated by David Hume as well as by Ludwig Boltzmann. None of those guys, of course, knew very much of our modern understanding of cosmology, gravitation, and quantum mechanics. So what would the modern version look like?

That’s the question that Anthony Aguirre, Matt Johnson and I tackled in a paper that just appeared on arxiv. (Both of my collaborators have also been guest-bloggers here at CV.)

Out of equilibrium: understanding cosmological evolution to lower-entropy states
Anthony Aguirre, Sean M. Carroll, Matthew C. Johnson

Despite the importance of the Second Law of Thermodynamics, it is not absolute. Statistical mechanics implies that, given sufficient time, systems near equilibrium will spontaneously fluctuate into lower-entropy states, locally reversing the thermodynamic arrow of time. We study the time development of such fluctuations, especially the very large fluctuations relevant to cosmology. Under fairly general assumptions, the most likely history of a fluctuation out of equilibrium is simply the CPT conjugate of the most likely way a system relaxes back to equilibrium. We use this idea to elucidate the spacetime structure of various fluctuations in (stable and metastable) de Sitter space and thermal anti-de Sitter space.

It was Boltzmann who long ago realized that the Second Law, which says that the entropy of a closed system never decreases, isn’t quite an absolute “law.” It’s just a statement of overwhelming probability: there are so many more ways to be high-entropy (chaotic, disorderly) than to be low-entropy (arranged, orderly) that almost anything a system might do will move it toward higher entropy. But not absolutely anything; we can imagine very, very unlikely events in which entropy actually goes down.

In fact we can do better than just imagine: this has been observed in the lab. (more…)

Reading about emergence and reductionism and free will and determinism has led me to finally confront a concept I had vaguely heard about but never really looked into before: downward causation, a term that came to prominence in the 1970′s. (Some other views: here, here, here.) I think it’s a misguided/unhelpful notion, but this is way outside my area and I’m happy to admit that I might be missing something.

Physicists are well aware that there are different vocabularies/models/theories that we can use to describe the same underlying reality. Sometimes you might want to talk about a box of gas as a fluid with pressure and velocity, other times you might want to talk about it in terms of atoms and molecules. Philosophers and psychologists might want to talk about human beings as autonomous agents who do things for reasons, while admitting that they can also be thought of as collections of cells and tissues, or even once again as atoms and molecules. The question is: what is the relationship between these different levels? In fluid mechanics/kinetic theory things are pretty clear, but in the mind/body problem things begin to get murky. (Or at least, there are people who take great pleasure in insisting that they are murky.)

Reductionism notes that some of these descriptions are more complete, and therefore arguably more fundamental, than others. In particular, some descriptions are in terms of entities that are literally smaller than the others; atoms are smaller than neurons, which are smaller than people. The smaller-level descriptions tend to have a wider range of validity; we can imagine answering certain questions in the atomic language that we can’t answer (correctly) in the fluid language, like “what happens if we divide the box in half, and then divide that in half, and so forth a million times?” It therefore seems natural to arrange the descriptions vertically: “lower” levels refer to small-scale descriptions, while “higher” levels refer to macroscopic objects. The claim of reductionism is, depending on who you talk to, that the lower-level description is either “always more complete,” or “capable of deriving the higher-level descriptions,” or “the right way to think about things.”

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Over the last year or so I’ve been devoting quite a bit of my time to exploring the origins and implications of a relatively new class of models known as Galileons. These may turn out to be nothing but mathematical curiosities, but while they’re still interesting I thought I’d try to explain what these theories are. This will be a little more technical than typical posts, but I’m hoping to get across the main reasons people are interested in these ideas even if the technicalities become a little much for some readers.

The resurgence of interest in extra dimensional models of particle physics and gravity during the last thirteen years has led to a number of novel approaches to cosmology, one of which is the fascinating idea of Dvali, Gabadadze and Porrati (DGP). In this picture, one begins by thinking of our four-dimensional world as residing on a brane floating in one extra dimension. The difference between this and other extra-dimensional models is that one imagines gravity as being described by a sum of the action for general relativity in 5d, and a 4d version just defined on the brane. This is rather technical, I know, but the main point is that gravity is described by an unusual but deceptively simple action. We, of course measure our world to be four-dimensional, and so the relevant question to ask of theories like this one is how the extra-dimensional physics manifests itself in the four-dimensional world.

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A few weeks back I wrote about the remarkable milestones passed by the Tevatron and LHC, and prognosticated that if there was ever a time when new discoveries could come out rapidly, this was it, especially for the LHC experiments analyzing a data sample 30 times larger than the previous one.

The result? Nature is being coy – in basically every new particle search for new particles and phenomena conducted by the CMS and ATLAS experiments at the LHC, we see naught but eerie agreement with the predictions for ordinary standard model background.

A huge raft of results has been presented at two large international conferences: the annual European Physical Society meeting on high energy physics in Grenoble, France, and the Particles and Nuclei International Conference (PANIC11) at MIT in Cambridge, MA. I presented the CMS results on the searches for the Higgs boson at the latter on Tuesday…more on that below.

There is a trove of material available online for these two conferences and at the ATLAS and CMS public physics result web sites. Let’s just look at a couple examples, though.

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A website called The Browser has been doing a fun collection of interviews, where they ask experts in different fields to recommend five books, either starting points for non-experts or books that they were influenced by themselves. Read Randall Grahm on wine, Jim Shepard on short stories, Deborah Blum on science and society, or Qiu Xiaolong on classical Chinese poetry.

They asked me about relativity and cosmology, and I decided it would be more helpful to pick recent books that would bring people up to date rather than go for the classics I was reading back in the 70′s. Some of these books aren’t light reading, but it’s a matter of dedication rather than preparation; I think an interested and intelligent person who didn’t know anything about relativity or cosmology could read these and come away with some deep insights.

For more thoughts, check out the full interview.

Update: for obvious reasons, it wouldn’t be considered quite kosher to recommend one’s own books in an interview like this. This has led to the misimpression that I think my books are less than the very best. Not so!

For some reason Nature News was inspired to tweet about my old blog post on Seriously, The Laws of Physics Underlying Everyday Life Are Completely Understood. Which I mentioned on Facebook, which led to an interesting comment, which I then mentioned on Google+… but now it’s substantive enough that I feel like I am neglecting our loyal blog readers! So here is a copy of my G+ comment, and a lament that I suck at proper use of the internet.

Not sure what brought this back to life. Like the Lord of the Rings, this is part of a trilogy; don’t miss the first installment, or the exciting conclusion.

As Michael Salem points out on an alternative social-media site (rhymes with “lacebook”), some of the resistance to this really quite unobjectionable claim comes from a lack of familiarity with the idea of a “range of validity” for a theory. We tend to think of scientific theories as “right” or “wrong,” which is hardly surprising. But not correct! Theories can be “right” within a certain regime, and useless outside that regime. Newtonian gravity is perfectly good if you want to fly a rocket to the Moon. But you need to toss it out and use general relativity (which has a wider range of validity) if you want to talk about black holes. And you have to toss out GR and use quantum gravity if you want to talk about the birth of the universe.

Just because there is something we don’t understand about some phenomenon (superconductivity, cancer, consciousness) does not imply that everything we think we know might be wrong. Sometimes we can say with confidence that certain things are known, even when other things are not.

Not only do theories have ranges of validity, but in some cases (as with the Standard Model of particle physics) we know what the range is. Or at least, we know where we have tested the theory and where we can be confident it is valid. The Standard Model is valid for all the particles and interactions that constitute our everyday existence.

Today we think of ourselves and the stuff we see around us as made of electrons, protons, and neutrons, interacting through gravity, electromagnetism, and the nuclear forces. A thousand years from now, we will still think precisely that. Unless we destroy the planet, or are uploaded into computers and decide that the laws of physics outside the Matrix aren’t that interesting any more.

Sean mentioned yesterday that the next generation space telescope JWST is at risk. In a bit more detail, JWST has been cut in the House appropriations bill:

$4.5 billion for NASA Science programs, which is $431 million below last year’s level. The bill also terminates funding for the James Webb Space Telescope, which is billions of dollars over budget and plagued by poor management.

In all, the House appropriations bill cuts 1.6 billion dollars from the NASA budget. The game is not over yet — the House Appropriations Subcommittee in charge of NASA will consider this bill today, and the full Appropriations committee will meet again to consider the final bill on Wednesday — and of course the Senate will have its own bill. But this is obviously a very ominous sign for NASA astrophysics in general.

JWST is a 6.5 meter IR-optimized telescope, which has been scheduled to launch in 2018. It is certainly true that it has suffered from numerous cost overruns, and has essentially eaten the rest of the NASA astrophysics program. However, nearly all the technical hurdles have now been overcome. And the science reach of JWST is spectacular. It is now the only observatory-class mission planned to operate once the current Great Observatories (Hubble, Spitzer, Chandra) reach their end of life. JWST has been the highest priority for NASA of the Decadal Surveys and essentially every other study commissioned by the field.

Hubble Space Telescope has given us amazing views of the Universe, back to about a billion years after the big bang. However, it has reached its limits there — JWST would allow us to see well into this first billion years, to view the formation of the first stars, galaxies, and black holes, and to study in detail how radiation from these objects reionized the Universe. There are no other planned missions that will allow us to observe this earliest stage of galaxy formation with this level of detail. JWST would also allow us to observe the chemical composition of planets outside the solar system, and to image the disks around stars as they begin planet formation.

It is hard to overstate the impact of HST on astronomy over the last two decades, and in particular on the public’s engagement with astronomy and science in general. There is just something incredibly inspiring and awesome about space-based observatories and the images they produce, that are unmatched by ground-based telescopes. JWST is a natural successor to Hubble in this mission: it has tremendous potential to be a vehicle of wonder. In addition to the science that would be lost, the funding losses to US astronomy, and the set back of our research progress, this loss to the public inspiration and engagement in scientific discovery could be one of the most substantial hits if JWST does not go forward.

I encourage all who are concerned about the next decade of astronomy to contact your representatives and senators as soon as possible. Termination of JWST would reduce the strength and visibility of the US science program as a whole, its impacts would be felt far beyond astrophysics. Killing JWST now also substantially threatens US credibility as an international partner, and sends the message that the US is just not interested in scientific leadership in major projects.

More at the New York Times, the Nature News Blog, Sky and Telescope, and Bad Astronomy. House press release here. The AAS will be releasing a statement later today. Thanks to Garth Illingworth for some useful background.

The case for JWST from a fan at the Vlog brothers: “I do not want to live in a world where we only focus on suck, and never think about awesome.”

Woke up this morning to the happy news that my post “The Fine Structure Constant is Probably Constant” walked away with the Charm Quark (i.e., tied for third place) in this year’s 3QuarksDaily science blogging prizes. Many thanks to Lisa Randall for judging and Abbas Raza and the 3QD crew for hosting. And of course congrats to the other winners:

1. Top Quark: SciCurious, Serotonin and Sexual Preference: Is It Really That Simple?
2. Strange Quark: Anne Jefferson, Levees and the Illusion of Flood Control
3. Charm Quark: Ethan Siegel, Where Is Everybody?

I already have a great nominee for next year’s contest. One of the most confusing things in particle physics is the notion of “chirality.” The related notion of a particle’s “helicity” is relatively easy to explain — is the particle spinning in a left-handed or right-handed sense when compared to its direction of motion? But a massive particle need not have a direction of motion, it can just be sitting there, so the helicity is not defined. Chirality is the same as helicity — left-handed or right-handed — for massless particles moving at the speed of light, but it’s always defined no matter how the particle is moving. It had better be, since the weak interactions couple to particles with left-handed chirality but not ones with right-handed chirality! (And the opposite for antiparticles.)

It all gets a bit heady, and you can’t give a real explanation without going beyond simple pictures and actually talking about the quantum wave function. But Flip Tanedo at Quantum Diaries has given it an heroic effort, which I insist you go read right now. I don’t want to reproduce the whole thing — Flip was more careful and thorough than I ever would have been, anyway — but I will tease you with this one picture.

Isn’t that the cutest pair of elementary particles you’ve ever seen? I smell a Quark in this lepton’s future.

This past week saw two big milestones for the two big operating high energy particle colliders in the world. At these machines, we measure the number of collisions with the rather arcane unit of “inverse barns”, which is essentially a measure of inverse cross sectional area. It’s just like if you are throwing darts at a dart board across the room with your eyes closed: the bigger the dart board, the more likely you are to hit it, and the more darts you throw, the more hits you get.

The term “barn” came from the early days of nuclear physics when Fermi quipped that a nucleus is “as big as a barn.” And so a new physics unit was born: one barn is 10^-28 m², about the size of a big nucleus. At the Tevatron at Fermilab, we’ve just crossed over 10.0 inverse femtobarns of integrated luminosity, after over ten years of operation in what we call Run 2 of the Tevatron. At the LHC at CERN, we just saw the integrated luminosity counter roll over to 1.000 inverse femtobarns. It’s kind of like the difference between your 10-year old car rolling over to 100,000 miles, and your new year-old car rolling over to 10,000 miles.

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In the comments to the previous post, Monty asks a perfectly good question, which can be shortened to: “Is the Higgs boson really necessary?” The answer is a qualified “yes” — we need the Higgs boson, or something like it. That is, we can’t simply take the Standard Model as we know it and extend it to arbitrarily high energies without new physics kicking in.

The role of the Higgs field is to break the symmetry of the electroweak interactions, as discussed here. We think that there is a lot of symmetry underlying particle interactions, but that much of it is hidden from our low-energy view. In technical terms, the electroweak theory of Glashow, Weinberg and Salam posits an “SU(2)xU(1)” symmetry, that somehow gets broken down to “U(1).” That unbroken symmetry gives us electromagnetism, a force carried by a massless particle, the photon. The broken symmetries are still there, but their force-carrying particles become massive when the symmetry breaks — those are the W⁺, W^-, and Z⁰ bosons.

There’s no question that something breaks the symmetry. The question that is worth asking is: “Can we imagine breaking the symmetry without introducing any new particles?”

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