Month: December 2009

Having written about the speculation, I suppose I should finish the story.  In a few talks yesterday, the CDMS dark matter detection experiment announced its latest results.  I didn’t hear the talks (at least one was streamed on the web, but I was in bed by then.)

They saw two events in their detector which look like what you’d expect from dark matter particle interactions, but two isn’t enough to conclude anything.  The group has elaborate procedures for calculating how many background events (i.e., events that look like dark matter but aren’t) might be seen.  Two is more than the expected number, but not by all that much: they estimate there’s a 23% chance of getting two background events.  To say they’d seen dark matter, that probability would’ve needed to be a tiny number.

Of course, it’s possible that these were dark matter events.  If so, I guess it means that the experiment is right at the edge of having sufficient sensitivity to detect dark matter particles.  This’d be great, because then future experiments would presumably be able to provide a real detection.

The group said they were planning to post a paper on the arXiv, but it’s not up yet.  Maybe they meant they’d submit it yesterday, and it’d appear today, or maybe they just didn’t get everything finished when they said they would.  I imagine it’ll be up soon.  In the mean time, there’s a brief summary of the result on their web site.  For those who want a bit more detail and can’t wait for the paper, JoAnne Hewett from Cosmic Variance was liveblogging one of the talks.

Having spread an unfounded rumor about the CDMS dark matter search last week, I thought I’d point out what’s actually happening.  This is oldish news, so some of you probably already know it, but for those who don’t, according to the CDMS web site,

The CDMS collaboration has completed the analysis of the final CDMS-II runs, which more than doubled the total data from all previous runs combined. The collaboration is working hard to complete the first scientific publication about these new results and plans to submit the manuscript to arXiv.org (http://arXiv.org) before the two primary CDMS talks scheduled for Thursday, December 17, 2009 at Fermilab and at SLAC. Jodi Cooley, the CDMS analysis coordinator and a professor from Southern Methodist University, will present the talk at SLAC at 2 p.m. PST, and Lauren Hsu, a scientist from Fermilab, will present the talk at Fermilab at 4 p.m. CST. A Web cast of Cooleys talk will be available on the CDMS Web site.

So they will be releasing results on (roughly) the date mentioned in the original rumor, and they do plan on having a paper available.  The part that’s apparently not true is the part about Nature.

I’ve heard some people speculating about the likelihood that the new results will contain something big and exciting, such as a claimed detection of dark matter.  I don’t know the folkways of this particular culture well enough to know whether the way this data release is being handled suggests a big result or not.  (I understand a bit about the astronomy and astrophysics culture, but dark matter detection experiments have mostly inherited the culture of particle physics, which is quite different.)

Anyway, like lots of other people I’ll be going to the arXiv the day the article is supposed to appear to find out.

As I mentioned a while ago, my Dad was awarded the Wallace H. Coulter award for lifetime achievement by the American Society of Hematology.  The award ceremony was this past week at the meeting in New Orleans.  Here’s the Society’s announcement about the award.

I just updated my previous post to make clear that the rumor about CDMS having detected dark matter probably isn’t true.  Thanks to the people who pointed out the update to the place I got the rumor from, which includes an email from an editor at Nature indicating that they haven’t accepted such a paper. Sorry for my credulousness.

Brent Follin, in a comment to my post, makes some observations about the Nature editor’s email:

It seemed a little strong and personal to be an actual email from someone writing a professional email, but it did point out that December 18th is a Friday, and Nature is published on a Thursday (with the articles normally released by Wednesday's evening news). So this rumor could be true, but I doubt the Nature publication (which makes me doubt the rumor). Also, Dr. Sage seemed pretty pissed about the mention of an "embargo", which she says is unfounded–that authors can, for instance, post on Arxiv before the publication date.

A couple of comments:

First, I agree with Brent that the tone seems very odd.  Maybe he just had a bad day.  Second, I went and checked out Nature‘s embargo policy.  It’s true that the policy allows posting on the arXiv, but it forbids talking to the press until a week before publication.

I’m not sure how that policy serves a useful purpose:  journalists can read the arXiv, you know!  Is it really better for them to see the article but not be able to talk to the authors for clarification?   But it’s their journal, so they can do what they want.

UPDATE:  Never mind.  It looks like the whole thing’s not true.  See the “important update.”  Sorry!  (I agree with Brent in the comments to this post that the tone of the email from the Nature editor is quite odd, by the way.)

A rumor is apparently going around that the CDMS experiment may be about to announce that they’ve directly detected dark matter particles.

CDMS is one of several experiments that try to observe dark matter particles directly interacting with their apparatus as they pass through.  These experiments are always placed deep underground to shield them from “ordinary” cosmic rays; this one is in a mine in Minnesota.

Evidence for the rumor: The collaboration had a paper accepted in Nature.  Nature usually only publishes high-profile results.  If CDMS had a non-detection to report (even if it set a new and interesting upper limit), Nature would be less likely to accept it. Nature articles are embargoed until publication, meaning that the collaboration can’t release the results or talk about them until December 18.  Members of the collaboration have canceled seminars before that date and scheduled talks at a number of universities to take place on that date.

So it definitely sounds like they have something exciting to say.  If they really have directly detected dark matter particles in the lab, needless to say this would be a Really Big Deal.

There’s a good post on RealClimate about the nature of certainty and uncertainty in science.  The hook for the post is a Wall Street Journal Op-Ed headlined “The Climate Science Isn’t Settled.”  The point of the post is to explain why this phrase is almost always a misleading rhetorical trick:

The phrase "the science is settled" is associated almost 100% with contrarian comments on climate and is usually a paraphrase of what 'some scientists' are supposed to have said. The reality is that it depends very much on what you are talking about and I have never heard any scientist say this in any general context – at a recent meeting I was at, someone claimed that this had been said by the participants and he was roundly shouted down by the assembled experts.

The reason why no scientist has said this is because they know full well that knowledge about science is not binary – science isn't either settled or not settled. This is a false and misleading dichotomy. Instead, we know things with varying degrees of confidence – for instance, conservation of energy is pretty well accepted, as is the theory of gravity (despite continuing interest in what happens at very small scales or very high energies) , while the exact nature of dark matter is still unclear. The forced binary distinction implicit in the phrase is designed to misleadingly relegate anything about which there is still uncertainty to the category of completely unknown. i.e. that since we don't know everything, we know nothing.

Although the author is (naturally) most interested in talking about climate science, the post is really about the nature of science in general, and a lot of what it says applies more broadly.  In particular, creationists and relativity-denialists (who, astonishingly, still exist) talk in very similar ways to those described here.

I’m giving a talk here in Paris tomorrow on the question of how to interpret the cosmological redshift.  The talk is based on the paper David Hogg and I wrote last year.  I said a bit about the argument of the paper in a previous post.  I’ll quickly recap the big idea, but then I want to comment on some followups to our paper: a blog post by Sean Carroll (from way back when we first posted our paper) and a recent paper by MichaÅ‚ Chodorowski.

First the background.  The most important fact in cosmology is that the light from distant galaxies is redshifted.  This fact is the observational basis for the idea of the expanding Universe.  Most of the time, when you see a redshift, it’s a Doppler shift — that is, it’s caused by the fact that the observed object is moving away from you.  In the cosmological context, though, people often say that the observed redshift has a different explanation: they say that the galaxies aren’t “really” moving, but rather that space itself is expanding, which causes the light to be stretched out in wavelength.  Hogg and I argue for the rehabilitation of the idea that the galaxies are moving and the observed redshift can be regarded as a Doppler shift.

The main thing to realize about all this is that we’re talking purely about a question of interpretation: everybody (at least, everybody sane)  agrees on the physics — the argument is only about what words to wrap around the physics.    Sean Carroll expressed this well in his blog post:

 These are not arguments about the theory €” everyone agrees on what GR predicts for observables in cosmology. These are only arguments about an analogy, i.e. the translation into English words.

…

The point is, arguments about analogies (and, by extension, the proper words in which to translate some well-accepted scientific phenomenon) are not "right" or "wrong." The analogies are simply "useful" or "useless," "helpful" or "misleading." And which of these categories they fall into may depend on the context.

This is 100% right.  We argue in our paper that the expanding-space picture in cosmology (particularly the metaphor of a stretching rubber sheet or balloon)  is misleading, in that it leads people to some incorrect intuitions about the nature of space, and we try to suggest different a way of looking at things.  But our way has its flaws too: all of these verbal descriptions are at best incomplete at capturing the whole picture.  We try to argue forcefully in favor of our way of looking at things, but the truth is that I’d be happies if people are exposed to several different ways, not just ours.

Here’s something else Sean says:

On the other hand, there is another pernicious mistake that people tend to make: the tendency, quite understandable in Newtonian mechanics, to talk about the relative speed between two far-away objects. Subtracting vectors at distinct points, if you like. In general relativity, you just can't do that. And realizing that you just can't do that helps avoid confusions along the lines of "Don't sufficiently distant galaxies travel faster than light?" And reifying a distinction between the Doppler shift and the cosmological redshift is a good first step toward appreciating that you can't compare the velocities of two objects that are far away from each other.

The beginning of this is exactly right: A central idea of general relativity is that you can’t compare vectors at distant points, which means that there’s no well-defined way to talk about the velocity of A relative to B, when A and B are far apart.   But I think he goes completely off the rails in the last sentence.

Sean seems to think that the expanding-rubber-sheet metaphor helps to convey the idea that you can’t talk about velocities of distant objects, but I think it does precisely the opposite.  The rubber-sheet picture virtually demands that you think of things that are just sitting still on the rubber sheet as being “really” at rest.  In its most extreme form (found in a lot of textbooks and pop-science books), the rubber-sheet metaphor says that distant galaxies are not moving with respect to us — that is, not that their velocities with respect to us are undefined, but that they’re zero!

In effect, the rubber sheet acts in people’s minds like a sort of aether, i.e., a preferred frame to use in defining all motions.  This is precisely the opposite of how you want to understand space in relativity.

Anyway, that post of Sean’s is ancient history (more than a year old).  Here’s something much more recent: the paper by Chodorowski called “The kinematic component of the cosmological redshift.” Chodorowski adopts a similar philosophy to us in some ways: he, like us, observes that if you want to talk about the velocity of a distant galaxy, you need to adopt a prescription for “carrying” its velocity vector over to us.  But while we argue that the most natural prescription is to carry the vector along the light path, he says that it’s more natural to carry the vector from one location to the other at a fixed moment in time.  Figure 1 of his paper illustrates this:

I tend to disagree with his assessment of which path is more natural.  The reason is that the comparison we want to make is between the galaxy’s velocity at the time of emission and our velocity now.  That is, the two vectors we want to compare live at the lower right and upper left of his diagram. If you follow Chodorowski’s path from right to left across the bottom, you then have to make a right turn and head up the t axis.  (He also considers the alternative where you go straight up first and then across.)  My preferred path is along the solid curve in the figure.

But the truth is that these “naturalness” questions are always matters of taste.  Chodorowski’s point of view is a very reasonable and sensible one, and this is a very nice paper.

Chodorowski’s two options correspond in effect to figuring out (a) the relative velocity of the galaxy and us back then (at the time of emission), and (b) the relative velocity of the galaxy and us now.  Hogg and I calculate something that can best be described as (c) the velocity of the galaxy then relative to us now.   In my dream world, people who want to understand the nature of the redshift would examine what happens when you do all three possibilities, and why they’re different.