Tag Archives: DoingScience

How Evidence for Cosmic Inflation Was Reduced to Dust

Many of you will have read in the last week that unfortunately (though to no one’s surprise after seeing the data from the Planck satellite in the last few months) the BICEP2 experiment’s claim of a discovery of gravitational waves from cosmic inflation has blown away in the interstellar wind. [For my previous posts on BICEP2, including a great deal of background information, click here.] The BICEP2 scientists and the Planck satellite scientists have worked together to come to this conclusion, and written a joint paper on the subject.  Their conclusion is that the potentially exciting effect that BICEP2 observed (“B-mode polarization of the cosmic microwave background on large scales”; these terms are explained here) was due, completely or in large part, to polarized dust in our galaxy (the Milky Way). The story of how they came to this conclusion is interesting, and my goal today is to explain it to non-experts.  Click here to read more.

The BICEP2 Dust-Up Continues

The controversy continues to develop over the interpretation of the results from BICEP2, the experiment that detected “B-mode” polarization in the sky, and was hailed as potential evidence of gravitational waves from the early universe, presumably generated during cosmic inflation. [Here’s some background info about the measurement].

Two papers this week (here and here) gave more detailed voice to the opinion that the BICEP2 team may have systematically underestimated the possible impact of polarized dust on their measurement.  These papers raise (but cannot settle) the question as to whether the B-mode polarization seen by BICEP2 might be entirely due to this dust — dust which is found throughout our galaxy, but is rather tenuous in the direction of the sky in which BICEP2 was looking.

I’m not going to drag my readers into the mud of the current discussion, both because it’s very technical and because it’s rather vague and highly speculative. Even the authors of the two papers admit they leave the situation completely unsettled.  But to summarize, the main purpose and effect of these papers seems to be this:

Continue reading

Will BICEP2 Lose Some of Its Muscle?

A scientific controversy has been brewing concerning the results of BICEP2, the experiment that measured polarized microwaves coming from a patch of the sky, and whose measurement has been widely interpreted as a discovery of gravitational waves, probably from cosmic inflation. (Here’s my post about the discovery, here’s some background so you can understand it more easily. Here are some of my articles about the early universe.)  On the day of the announcement, some elements of the media hailed it as a great discovery without reminding readers of something very important: it’s provisional!

From the very beginning of the BICEP2 story, I’ve been reminding you (here and here) that it is very common for claims of great scientific discoveries to disappear after further scrutiny, and that a declaration of victory by the scientific community comes much more slowly and deliberately than it often does in the press. Every scientist knows that while science, as a collective process viewed over time, very rarely makes mistakes, individual experiments and experimenters are often wrong.  (To its credit, the New York Times article contained some cautionary statements in its prose, and also quoted scientists making cautionary statements.  Other media outlets forgot.)

Doing forefront science is extremely difficult, because it requires near-perfection. A single unfortunate mistake in a very complex experiment can create an effect that appears similar to what the experimenters were looking for, but is a fake. Scientists are all well-aware of this; we’ve all seen examples, some of which took years to diagnose. And so, as with any claim of a big discovery, you should view the BICEP2 result as provisional, until checked thoroughly by outside experts, and until confirmed by other experiments.

What could go wrong with BICEP2?  On purely logical grounds, the BICEP2 result, interpreted as evidence for cosmic inflation, could be problematic if any one of the following four things is true:

1) The experiment itself has a technical problem, and the polarized microwaves they observe actually don’t exist.

2) The polarized microwaves are real, but they aren’t coming from ancient gravitational waves; they are instead coming from dust (very small grains of material) that is distributed around the galaxy between the stars, and that can radiate polarized microwaves.

3) The polarization really is coming from the cosmic microwave background (the leftover glow from the Big Bang), but it is not coming from gravitational waves; instead it comes from some other unknown source.

4) The polarization is really coming from gravitational waves, but these waves are not due to cosmic inflation but to some other source in the early universe.

The current controversy concerns point 2. Continue reading

In Memoriam: Gerry Guralnik

For those who haven’t heard: Professor Gerry Guralnik died. Here’s the New York Times obituary, which contains a few physics imperfections (though the most serious mistake in an earlier version was corrected, thankfully), but hopefully avoids any errors about Guralnik’s life.  Here’s another press release, from Brown University.

Guralnik, with Tom Kibble and Carl Hagen, wrote one of the four 1964 papers which represent the birth of the idea of the “Higgs” field, now understood as the source of mass for the known elementary particles — an idea that was confirmed by the discovery of a type of “Higgs” particle in 2012 at the Large Hadron Collider.  (I find it sad that the obituary is sullied with a headline that contains the words “God Particle” — a term that no physicist involved in the relevant research ever used, and which was invented in the 1990s, not as science or even as religion, but for $$$… by someone who was trying to sell his book.) The other three papers — the first by Robert Brout and Francois Englert, and the second and third by Peter Higgs, were rewarded with a Nobel Prize in 2013; it was given just to Englert and Higgs, Brout having died too early, in 2011.  Though Guralnik, Hagen and Kibble won many other prizes, they were not awarded a Nobel for their work, a decision that will remain forever controversial.

But at least Guralnik lived long enough to learn, as Brout sadly did not, that his ideas were realized in nature, and to see the consequences of these ideas in real data. In the end, that’s the real prize, and one that no human can award.

Brane Waves

The first day of the conference celebrating theoretical physicist Joe Polchinski (see also yesterday’s post) emphasized the broad impact of his research career.  Thursday’s talks, some on quantum gravity and others on quantum field theory, were given by

  • Juan Maldacena, on his latest thinking on the relation between gravity, geometry and the entropy of quantum entanglement;
  • Igor Klebanov, on some fascinating work in which new relations have been found between some simple quantum field theories and a very poorly understood and exotic theory, known as Vassiliev theory (a theory that has more fields than a field theory but fewer than a string theory);
  • Raphael Bousso, on his recent attempts to prove the so-called “covariant entropy bound”, another relation between entropy and geometry, that Bousso conjectured over a decade ago;
  • Henrietta Elvang, on the resolution of a puzzle involving the relation between a supersymmetric field theory and a gravitational description of that same theory;
  • Nima Arkani-Hamed, about his work on the amplituhedron, a set of geometric objects that allow for the computation of particle scattering in various quantum field theories (and who related how one of Polchinski’s papers on quantum field theory was crucial in convincing him to stay in the field of high-energy physics);
  • Yours truly, in which I quickly reviewed my papers with Polchinski relating string theory and quantum field theory, emphasizing what an amazing experience it is to work with him; then I spoke briefly about my most recent Large Hadron Collider [LHC] research (#1,#2), and concluded with some provocative remarks about what it would mean if the LHC, having found the last missing particle of the Standard Model (i.e. the Higgs particle), finds nothing more.

The lectures have been recorded, so you will soon be able to find them at the KITP site and listen to any that interest you.

There were also two panel discussions. One was about the tremendous impact of Polchinski’s 1995 work on D-branes on quantum field theory (including particle physics, nuclear physics and condensed matter physics), on quantum gravity (especially through black hole physics), on several branches of mathematics, and on string theory. It’s worth noting that every talk listed above was directly or indirectly affected by D-branes, a trend which will continue in most of Friday’s talks.  There was also a rather hilarious panel involving his former graduate students, who spoke about what it was like to have Polchinski as an advisor. (Sorry, but the very funny stories told at the evening banquet were not recorded. [And don’t ask me about them, because I’m not telling.])

Let me relate one thing that Eric Gimon, one of Polchinski’s former students, had to say during the student panel. Gimon, a former collaborator of mine, left academia some time ago and now works in the private sector. When it was his turn to speak, he asked, rhetorically, “So, how does calculating partition functions in K3 orientifolds” (which is part of what Gimon did as a graduate student) “prepare you for the real world?” How indeed, you may wonder. His answer: “A sense of pertinence.” In other words, an ability to recognize which aspects of a puzzle or problem are nothing but distracting details, and which ones really matter and deserve your attention. It struck me as an elegant expression of what it means to be a physicist.

Wednesday: Sean Carroll & I Interviewed Again by Alan Boyle

Today, Wednesday December 4th, at 8 pm Eastern/5 pm Pacific time, Sean Carroll and I will be interviewed again by Alan Boyle on “Virtually Speaking Science”.   The link where you can listen in (in real time or at your leisure) is


What is “Virtually Speaking Science“?  It is an online radio program that presents, according to its website:

  • Informal conversations hosted by science writers Alan Boyle, Tom Levenson and Jennifer Ouellette, who explore the explore the often-volatile landscape of science, politics and policy, the history and economics of science, science deniers and its relationship to democracy, and the role of women in the sciences.

Sean Carroll is a Caltech physicist, astrophysicist, writer and speaker, blogger at Preposterous Universe, who recently completed an excellent and now prize-winning popular book (which I highly recommend) on the Higgs particle, entitled “The Particle at the End of the Universe“.  Our interviewer Alan Boyle is a noted science writer, author of the book “The Case for Pluto“, winner of many awards, and currently NBC News Digital’s science editor [at the blog  “Cosmic Log“].

Sean and I were interviewed in February by Alan on this program; here’s the link.  I was interviewed on Virtually Speaking Science once before, by Tom Levenson, about the Large Hadron Collider (here’s the link).  Also, my public talk “The Quest for the Higgs Particle” is posted in their website (here’s the link to the audio and to the slides).

The Fast and Glamorous Life of a Theoretical Physicist

Ah, the fast-paced life of a theoretical physicist!  I just got done giving a one-hour talk in Rome, given at a workshop for experts on the ATLAS experiment, one of the two general purpose experiments at the Large Hadron Collider [LHC]. Tomorrow morning I’ll be talking with a colleague at the Rutherford Appleton Lab in the U.K., an expert from CMS (the other general purpose experiment at the LHC). Then it’s off to San Francisco, where tomorrow (Wednesday, 5 p.m. Pacific Time, 8 p.m. Eastern), at the Exploratorium, I’ll be joined by Caltech’s Sean Carroll, who is an expert on cosmology and particle physics and whose book on the Higgs boson discovery just won a nice prize, and we’ll be discussing science with science writer Alan Boyle, as we did back in February. [You can click here to listen in to Wednesday’s event.]  Next, on Thursday I’ll be at a meeting hosted in Stony Brook, on Long Island in New York State, discussing a Higgs-particle-related scientific project with theoretical physics colleagues as far flung as Hong Kong.  On Friday I shall rest.

“How does he do it?”, you ask. Hey, a private jet is a wonderful thing! Simple, convenient, no waiting at the gate; I highly recommend it! However — I don’t own one. All I have is Skype, and other Skype-like software.  My words will cross the globe, but my body won’t be going anywhere this week.

We should not take this kind of communication for granted! If the speed of light were 186,000 miles (300,000 kilometers) per hour, instead of 186,000 miles (300,000 kilometers) per second, ordinary life wouldn’t obviously change that much, but we simply couldn’t communicate internationally the way we do. It’s 4100 miles (6500 kilometers) across the earth’s surface to Rome; light takes about 0.02 seconds to travel that distance, so that’s the fastest anything can travel to make the trip. But if light traveled 186,000 miles per hour, then it would take over a minute for my words to reach Rome, making conversation completely impossible. A back-and-forth conversation would be difficult even between New York and Boston — for any signal to travel the 200 miles (300 kilometers) would require four seconds, so you’d be waiting for 8 seconds to hear the other person answer your questions. We’d have similar problems — slightly less severe — if the earth were as large as the sun.  And someday, as we populate the solar system, we’ll actually have this problem.

So think about that next time you call or Skype or otherwise contact a distant friend or colleague, and you have a conversation just as though you were next door, despite your being separated half-way round the planet. It’s a small world (and a fast one) after all.

Why Scientists Can Be Happy Even When They Find Nothing

Appropriate for General Readership

Last week, the LUX experiment reported its results in its search for the dark matter that (speaking roughly) makes up 25% of the stuff in the universe (see here for the first report and here for some Q&A).  [See this article, specifically the “Dark Matter Underfoot” section, for some nontechnical discussion about how experiments like LUX work.]  Shortly thereafter, a number of articles in the media made a big deal out of the fact that, simultaneously,

  1. the LUX experiment did not find evidence of dark matter
  2. yet scientists at the LUX experiment appeared to be quite happy

as though this was contradictory and mystifying. Actually, if you think about it carefully, this is perfectly normal and typical, and not the slightest bit surprising. But to make sense of it, you do also have to understand the levels of “happiness” that the LUX scientists are expressing.

The point is that whenever scientists do an experiment whose goal is to look for something whose precise details aren’t known, there are two stories running simultaneously:

  1. The scientists are trying to do the best experiment that they can, in order that their search be as thorough and as expansive as it could possibly be with the equipment that they have available.
  2. The scientists are hoping that the thing that they are looking for (or perhaps something else equally or more interesting) will be within reach of their search.

Notice that humans have control over the first story. The wiser they are at designing their experiment, and the more skillful they are in carrying it out, the more effective their search will be. But they have no control over the second story. Whether their prey lies within their reach, or whether it lies far beyond, requiring the technology of the distant future, is up to nature, not humans. In short, story #1 is about skill and talent, but story #2 is about luck. Even a great experiment can’t do the impossible, and even one that doesn’t work quite as well as it was supposed to can be fortunate.

Of course, there is some interplay between the stories. A disaster in story #1 precludes a happy ending in story #2; if the experiment doesn’t work, there won’t be any discoveries! And the better is the outcome in story #1, the more probable is a success in story #2; a more thorough search is more likely to get lucky.

The LUX researchers, in order to make a discovery, have to be lucky in several ways, as I described on Thursday.

  • Dark matter (at least some of it) has to be made from particles which are heavier than protons and have uniform properties;
  • These particles have to be rather smoothly distributed through the Milky Way galaxy, rather than bound up in clumps the way ordinary matter is, so that some of them are likely, just by chance, to be passing through the earth;
  • And they have to interact with ordinary matter at a rate that is not insanely small — no less than a millionth of the interaction rate of high-energy neutrinos with ordinary matter.

None of these things is necessarily true, given what we know about dark matter from our measurements of the heavens. And if any one of them is false, no detector similar to LUX will ever find dark matter; we’ll need other methods, some of which are already under way.

Now, in this context, what’s the worst thing that could happen to a group of scientists who’ve built an experiment? The worst thing that could happen is that after spending several years preparing the experiment, they find it simply doesn’t work. This can happen! These are very difficult experiments requiring very special and remarkable techniques, and every now and then, in the history of such experiments, an unexpected problem arises that can’t be solved without a complete redesign, which is usually too expensive and in any case means years of delay. Or something just explodes and ruins the experiment. Something like this is extremely depressing and often deeply embarrassing.

So if instead the experiment works, the scientists who designed, built and ran it are of course very relieved and reasonably happy. And if, because of a combination of hard work and cleverness, it works better than they expected and as well as they could have hoped, they’re of course enormously pleased, and proud of their work!

Now what could make them happier still — even ecstatic, to the point of staying up late drinking entire bottles of champagne? A discovery, of course. Discovering what they’re looking for, or perhaps something they weren’t even looking for, if it is truly novel and of fundamental importance.  If that happens, then they won’t care as much if their experiment worked better than expected… because, if you’re an experimental scientist, there’s nothing, nothing at all, better than discovering something new about nature.

So with this perspective, I think the LUX scientists’ emotions (as conveyed during his talk by Richard Gaitskell of Brown University, the project’s leader) are actually very easy to understand. They are very happy because their experiment works better than they expected and as well as they hoped… maybe even better than that. For this, they get the high respect and admiration of their colleagues. But make no mistake: they’d certainly be a lot happier — overjoyed and humbled — if they’d discovered dark matter. For that, they’d get a place in the history books, major prizes (perhaps a Nobel, if the Nobel Committee could figure out who to give it to), lasting fame, and the almost unimaginable feeling of having uncovered something about nature that no human previously knew, and that (barring a complete collapse of civilization) will never be forgotten. So yes, they’re happy. But not nearly as happy as can be. They’re frustrated, too, just like the rest of us, that nothing’s shown up yet.

However, they’re also hopeful. Since they’ve built such a good experiment, and since they’ve only run it for such a short time so far, they’ll have another very reasonable shot at finding dark matter when they run it for about a full year, in 2014. Not only will they run it longer, they’ll surely also learn, from their experience so far, to be smarter about how they run it. So expect, at the very least, powerful new limits on dark matter from them in eighteen months or so. And maybe, just maybe, something more.