Speaking of writing popular books, I’m at it again. I’m currently hard at work writing The Particle At the End of the Universe, a popular-level book on the Large Hadron Collider and the search for the Higgs boson. If all goes well, it should appear in bookstores at the end of this year or beginning of next. (Ideally, it will go on sale the same day they announce the discovery of the Higgs. I’m trying to bribe the right people to make that happen.) The title is somewhat tentative, so it might change at some point.

This will be a somewhat different book than From Eternity to Here. While both are aimed at a general audience, FETH was a rather lengthy tome that made a careful argument in a hopefully novel way. Anyone could read it, but to get the most out of it you have to really sit and think about certain ideas. Particle, on the other hand, aims to be a fun and narratively gripping page-turner — a book that makes you eager to move quickly to the next chapter, rather than taking a few minutes to let the last one sink into your head. A bodice-ripper, if you will. It will be full of stories and fun anecdotes about the human beings who made the LHC happen and have devoted their lives to searching for the Higgs and particles beyond the Standard Model. A book you would be happy to give to your Grandmom in order to convey some of the excitement of modern physics. (Unless your Grandmom is a particle physicist, in which case she might think it’s at too low a level.)

At the same time, of course, I’m going to try to illuminate the central ideas of the Standard Model in as clear a fashion as I can manage. It won’t just be a list of particles; I’ll cover field theory, gauge bosons, and spontaneous symmetry breaking. All in fine bodice-ripping style. (Maybe get Fabio for the cover?)

If you are a particle physicist yourself, I’m happy to take input. This could take the form of a favorite analogy you like to use to explain some subtle concept, or some physics idea or piece of history you think really doesn’t get the attention it deserves in the popular media. Even better if you have some personal involvement in a fun story — you lost your virginity in the LHC tunnel, or you discovered asymptotic freedom but didn’t get around to publishing it. I’m talking to as many physicists as I can, but I can’t talk to everyone. I’m looking for tales that will make the human side of physics come alive.

Also happy to take input if you’re not a particle physicist! What are the concepts that we don’t do a good job explaining? What are the buzzwords you’ve heard about the don’t make sense? The questions you really want answered?

I sincerely believe the search for the Higgs and whatever might lie beyond is a Big Deal in the history of science, and I hope to convey some of the importance and excitement of this question to as large an audience as possible. I’ll be flitting around the country giving talks when the book comes out, so let me know if you have a big lecture hall full of eager minds that want to hear the latest dispatches from the particle trenches. Should be a fun ride.

When I wrote From Eternity to Here, I was faced with a perennial problem for pop-physics authors: how to write a book that will appeal to the aficionados (although not scientists themselves) who have already devoured everything Brian Greene and Lisa Randall have ever written, but also be understandable and interesting to folks who don’t know much more about Einstein than the fact that he rarely combed his hair? I came across a short blog review that claims I wasn’t entirely successful in balancing the competing requirements, and it might be a fair criticism.

But one small complaint is I’m not sure if he’s quite exactly worked out his audience. Early in the book, I was starting to fear it would be a rehash of stuff I already knew. It’s not. But there were some elementary rehashes that, frankly, I think if someone went into the book not having that knowledge already, they aren’t going to be able to grok the rest. This is not a mathematically demanding book, but it is a conceptually demanding book, and I am not sure if someone who doesn’t have some limited grounding in the mathematics side will be able to make it through the conceptual side without missing a lot.

The diagnosis is completely accurate. On the one hand, I do spend time going over the basics of relativity, quantum mechanics, and logarithms, in ways that hopefully make these ideas accessible to people who haven’t ever tried to understand them before. On the other hand, the meaty middle section of the book is conceptually very challenging for almost everybody, even if you are a regular consumer of popular physics books. We are simply not used to thinking in ways that don’t presume the arrow of time from the start, and some of the issues that arise are highly non-intuitive. I tried to keep things fun and engaging, but there’s no question that certain pages of the book require careful thinking and brain work.

What to do? I’m not sure that, given the material I wanted to cover in this book, there is much else one could do. Probably I could have had less about the basics of relativity, and moved what there is until later in the book. But although the central ideas are conceptually very challenging, I don’t think they’re actually inaccessible to anyone who is willing to put some thought into them, regardless of mathematical background. So I don’t regret that I didn’t write a leaner and more challenging book aimed only at the aficionados, although I appreciate that something along those lines might have been more focused. I think that the aficionados just have to get used to reading introductions to GR and QM from a wide variety of different books — at least until those topics become part of the standard high-school curriculum.

Science keeps advancing, in fits and starts. It was a good week for intriguing results from experiments.

The first bit of news, which has been the subject of the most internet buzz, is a new paper by Chilean astronomers C. Moni Bidin, G. Carraro, R. A. Mendez, and R. Smith, which claims that there’s no evidence for dark matter in the dynamics of stars near the Sun. If this were true, it would imply something funny going on with the distribution of nearby dark matter, which could have significant implications for direct searches here on Earth (see below). It wouldn’t really be much of a threat to the idea of dark matter itself, since there’s plenty of evidence for dark matter elsewhere. But it might mean that the distribution in the Milky Way was very different from the kinds of models we like to use, for example by being much lumpier.

We just heard a great physics colloquium here at Caltech by Katie Freese, who talked about this result very briefly. Her opinion matched those of the skeptics in Ron Cowen’s article linked above: this paper makes a lot of assumptions, some of the a bit dubious, and we would need to see something much more solid before we become convinced. The biggest issue is that they don’t actually measure the DM distribution near the Sun; they try to measure it in a region between 1500 and 4000 parsecs below the galactic plane (which is actually pretty far away), and then fit to a model and extrapolate to what we should have nearby. Read the rest of this entry »

I love Jon Stewart’s work on The Daily Show, which manages to be consistently fresh and intelligent. Their segment on the Large Hadron Collider was sheer brilliance, and I’ve often said that between Stewart and Stephen Colbert, Comedy Central is the best place to go to hear insights from real working scientists on TV these days.

Which is why it was so crushing to listen to this interview he did with Marilynne Robinson, a leader among the movement to reconcile science and religion. I didn’t agree with much of what Robinson said, but then again I didn’t really expect to. Nor did I expect Stewart to challenge her in any way; a “why just can’t we all get along” perspective is very consistent with his way of thinking. But I admit I was hoping he would not misrepresent modern science as thoroughly and lazily as he managed to do here. (It’s a 2010 interview, brought to my attention by Scott Derrickson’s Twitter feed; apologies if these complaints were hashed out elsewhere two years ago.)

If you skip ahead to 2:50, here’s what Stewart has to say: Read the rest of this entry »

Sorry for a post title that will attract the crazies. Carl Zimmer has a story in the New York Times that discusses a growing unease with the practice of science among scientists themselves.

In tomorrow’s New York Times, I’ve got a long story about a growing sense among scientists that science itself is getting dysfunctional. For them, the clearest sign of this dysfunction is the growing rate of retractions of scientific papers, either due to errors or due to misconduct. But retractions represent just the most obvious symptom of deep institutional problems with how science is done these days–how projects get funded, how scientists find jobs, and how they keep labs up and running.

However… essentially all the examples are from biologically-oriented fields. I’ll confess that Carl asked me if there is a similar feeling among physicists, and after some thought I decide that there really isn’t. There are certainly fumbles (faster-than-light neutrinos, anyone?) and scandals (Jan Hendrik Schön being the most obvious), but I don’t have any feeling that the problem is growing in a noticeable way. Biology and physics are fundamentally different, especially because of the tremendous pressure within medical sciences when it comes to any results that might turn out to be medically useful. Cosmologists certainly don’t have to worry about that.

But maybe this is a distorted view from within my personal bubble? Happy to hear informed opinion to the contrary. The relevant kind of informed opinion would actually involve a comparison of the situation today with the situation at some previous time, not just a litany of things you think are dysfunctional about the present day.

Several different things (all pleasant and work-related, no disasters) have been keeping me from being a good blogger as of late. Last week, for example, we hosted a visit by Andy Albrecht from UC Davis. Andy is one of the pioneers of inflation, and these days has been thinking about the foundations of cosmology, which brings you smack up against other foundational issues in fields like statistical mechanics and quantum mechanics. We spent a lot of time talking about the nature of probability in QM, sparked in part by a somewhat-recent paper by our erstwhile guest blogger Don Page.

But that’s not what I want to talk about right now. Rather, our conversations nudged me into investigating some work that I have long known about but never really looked into: David Deutsch’s argument that probability in quantum mechanics doesn’t arise as part of a separate ad hoc assumption, but can be justified using decision theory. (Which led me to this weekend’s provocative quote.) Deutsch’s work (and subsequent refinements by another former guest blogger, David Wallace) is known to everyone who thinks about the foundations of quantum mechanics, but for some reason I had never sat down and read his paper. Now I have, and I think the basic idea is simple enough to put in a blog post — at least, a blog post aimed at people who are already familiar with the basics of quantum mechanics. (I don’t have the energy in me for a true popularization at the moment.) I’m going to try to get to the essence of the argument rather than being completely careful, so please see the original paper for the details.

Read the rest of this entry »

A standard topic in an introductory General Relativity (GR) course is the study of maximally symmetric solutions. These are flat (Minkowski) spacetime, de Sitter spacetime (obtained when the cosmological constant is positive) and Anti-de Sitter spacetime (when the cosmological constant is negative). While this last space has been of great interest in physics during the last fifteen years due to its central role in the correspondence between gauge theories and gravity, it is de Sitter space with which I’ll be concerned here.

The idea of cosmological inflation is our best developed idea of how the physics of the early universe might lead to the observed universe today. This idea has been widely discussed in popular books and beyond, and in this context, many students have heard the loose description that inflation occurs when the universe is in an almost de Sitter state, and undergoes exponentially rapid expansion. There is nothing wrong with this explanation, but one consequence of accepting it before having a thorough grounding in GR is that it seems to imply that de Sitter space is a solution to GR that undergoes a rapid change over time. This leads to a few confused looks when I get to maximally symmetric spaces in my course.

You see, maximal symmetry means that you should be able to look at the space at different places and at different times and the metric should be just the same. So how are we to square that with the idea of an exponentially growing universe? Well, it all comes down to coordinate choices and the crucial existence of other matter in the universe.

Pure de Sitter space – the solution to the Einstein equations with a positive cosmological constant and no other matter sources – is, indeed, a maximally symmetric space. There exist a number of particularly useful coordinate choices for this space. In some cases, these consist of picking a useful time choice, and thus defining a family of spacelike surfaces (the spatial part of the spacetime at a constant value of this time choice). This is referred to as a slicing of the space, and it is, actually, possible to slice the space in three different ways that correspond to cosmologically expanding spaces with flat, positively-curved and negatively curved spatial parts, respectively. These are the ways of describing de Sitter space that are useful when considering inflation. However, there also exists a choice of coordinates in which the metric does not depend on time at all, and the mere existence of such a choice is enough to tell us that there is no fundamental sense in which this is an expanding cosmological spacetime. In fact, from what I just wrote, you might have a related question: even in the cosmological coordinates, what decides if the universe is flat, positively, or negatively curved?

In the case of pure de Sitter space there is no answer to these questions. All the coordinate choices are equally allowed of course, and so we might as well look at the static coordinates, and there is no cosmology here. However, importantly, in cosmology we are never interested in pure de Sitter space. We know that there is other matter in the universe. This may be either in the form of particles like us, or, in the case of inflation, the background field that causes inflation in the first place – the inflaton. These types of matter mean that the behavior of the metric is at best almost de Sitter – the difference from pure de Sitter being that, crucially, there are only certain coordinate systems in which the regular matter is homogeneous and isotropic, whereas for a cosmological constant this is true in all coordinate systems. Thus an almost de Sitter space has less symmetry than pure de Sitter. One is free to transform coordinates as much as one likes, but there will no longer be any choices in which the metric is static!

Of course, we find it most convenient to discuss cosmology in the (Friedmann, Robertson-Walker) coordinates that exploit the natural homogeneity and isotropy of the relevant matter sources. This picks out a slicing of the spacetime, and in this slicing, when the universe is almost de Sitter, the universe does expand almost exponentially rapidly – inflation! This also decides among the flat, positively and negatively curved options for the spatial part of the metric.

So it matters that inflation is “quasi-de Sitter”. It is this that gives sense to statements about inflation beginning, ending, and even operating in the way we usually describe. de Sitter space is beautiful symmetric and rich, but out real universe is somewhat messier, even at its earliest times.

David Deutsch:

“Despite the unrivaled empirical success of quantum theory, the very suggestion that it may be literally true as a description of nature is still greeted with cynicism, incomprehension, and even anger.”

We’re a little bit late here, but I wanted to note that Chinese physicist Fang Lizhi died on Friday in Arizona at the age of 76.

Fang’s research area was quantum cosmology, but he was most well-known for his political activism, fighting against repression in China. Originally a member of the Communist Party, he was expelled for protesting some of the government’s policies. The NYT obituary relates an amusing/horrifying story, according to which Fang attracted the government’s censure by co-authoring a paper entitled “A Solution of the Cosmological Equations in Scalar-Tensor Theory, with Mass and Blackbody Radiation.” Seems pretty innocuous from where we are sitting, but in Communist China the Big Bang model was considered to be a challenge to Engels’s idea that that the universe was infinite, and therefore was deemed heresy. Googling around brought me to this 1988 article in Contemporary Chinese Thought, which shows what Fang was up against. The abstract quotes Lenin, and says in all seriousness “with every new advance in science the idealists distort and take advantage of the latest results of physics to “prove” with varying sleights of hand that the universe is finite, serving the reactionary rule of the moribund exploiting classes.”

In the late 1980′s Fang helped organize resistance to China’s authoritarian regime, in the lead-up to the Tiananmen Square protests. He was fired from his job as a professor, and sought refuge in the American embassy. He was finally permitted to leave the country and emigrate to America in 1990. He finally settled down at the University of Arizona, but continued his work campaigning for human rights.

I’ve reached the cosmology part of my General Relativity (GR) course, and one of the early points that comes up is my traditional rant against confusing three very distinct concepts when thinking about the universe. Roughly stated, these are; What is the shape of the universe? Is the universe finite or infinite? and Will the universe expand forever or recollapse.

When we apply GR to cosmology, we make use of the simplifying assumptions, backed up by observations, that there exists a definition of time such that at a fixed value of time, the universe is spatially homogeneous (looks the same wherever the observer is) and isotropic (looks the same in all directions around a point). We then specialize to the most general metric compatible with these assumptions, and write down the resulting Einstein equations with appropriate sources (regular matter, dark matter, radiation, a cosmological constant, etc.). The solutions to these equations are the famous Friedmann, Robertson-Walker spacetimes, describing the expansion (or contraction) of the universe.

It is important to take a moment to emphasize what we have done here. GR is indeed a beautiful geometric theory describing curved spacetime. But practically, we are solving differential equations, subject to (in this case) the condition that the universe look the way it does today. Differential equations describe the local behavior of a system and so, in GR, they describe the local geometry in the neighborhood of a spacetime point.

Because homogeneity and isotropy are quite restrictive assumptions, there are only three possible answers for the local geometry of space at any fixed point in time – it can be spatially positively curved (locally like a 3-dimensional sphere), flat (locally like a 3-dimensional version of a flat plane) or negatively spatially curved (locally like a 3-dimensional hyperboloid). A given cosmological solution to GR tells you one of these answers around a spacetime point, and homogeneity then tells you that this is the same answer around every spacetime point. This is what we mean when we say that GR tells us about geometry – the shape of the universe – as depicted in the NASA graphic below.

This raises a very different question that is often confused with the one above. If our solution tells us that the universe is locally a 3-sphere (or flat space, or a hyperboloid) around every point, then does that mean it is a 3-sphere, or an infinite flat 3-dimensional space, or an infinite hyperboloid. This is really a question of topology – how is it connected up – which also answers the question of whether the universe is finite or infinite. To illustrate the point, suppose we have solved the cosmological equations of GR, and discovered that at every spacetime point, the universe is locally a flat 3-dimensional space. This is, by the way, what observations actually indicate our universe is like. Then, just off the top of your head, you can think of many different spaces with precisely this same property. One example is, of course, that the universe is indeed a flat, infinite 3-dimensional space. Another is that the universe is a 3-torus, in which if you were to fix time and trace out a line away from any point along the x, y or z-axis, you traverse a circle and come right back to where you started. This is a finite volume space, that is connected up in a very specific way, but which is everywhere flat, just like the infinite example. In two dimensions, one might visualize it as

Of course, I could have only made one or two directions into circles (leaving it still infinite in some directions), or made the space into a finite one with more than one hole, or any number of other possibilities.

This is the beauty of topology, but it is not something that solving the equations of GR tells us. Rather it is an extra input into our solutions. It is, however, something we can test, most precisely through measurements of the Cosmic Microwave Background radiation, as I may discuss in a later post.

Completely independent of questions of topology, the geometry of a given cosmological solution raises another issue that is often mixed up with those of geometry and topology. Suppose that the universe contains only conventional matter sources (regular matter, dark matter and radiation, say), and suppose you know (you might question whether this is truly possible) that this is all it will ever contain. Then the equations easily predict that, in the case of positive spatial curvature, an expanding universe will ultimately reach a maximum size and recollapse in a big crunch, whereas flat or negatively curved universes will expand forever. These are predictions of the destiny of the universe, and often lead to the following connection

However, as I made clear, there are some assumptions that go into the connection between geometry and destiny, and although these may have seemed reasonable ones at one time, we know today that the accelerated expansion of the universe seems to point to the existence of some kind of dark energy (a cosmological constant, for example), that behaves in a way quite different from conventional mass-energy sources. In fact, we know that for sources like this, once acceleration begins, it is easily possible for a positively curved universe, for example, to expand forever. Indeed, in the case of a cosmological constant, this is precisely what happens.

So the universe may be positively or negatively curved, or flat, and our solutions to GR tell us this. They may be finite or infinite, and connected up in interesting ways, but GR does not tell us why this is the case. And the universe may expand forever or recollapse, but this depends on detailed properties of the cosmic energy budget, and not just on geometry. Cosmological spacetimes are some of the simplest solutions to GR that we know, and even they admit all kinds of potential complexities, beyond the most obvious possibilities. Wonderful, isn’t it?