You may have heard that there’s some sort of big science machine scheduled to turn on in Europe. Very soon, in fact: first (official) beam at the Large Hadron Collider is supposed to occur around 9:30 Central European Summer Time (3:30 a.m. Eastern, if I have done the math correctly) on Wednesday. Call it Tuesday night, for us West Coasters.

The folks at the History Channel recognize the importance of the event, and they’ve recruited Friend of CV David E. Kaplan, a particle theorist at Johns Hopkins, to host a special show entitled the Next Big Bang. And we, of course, have recruited David to tell you a little about the show. (In the picture, David is the one wearing glasses.)

(p.s. This LHC game is surprisingly educational. Via DILigence.)

Update: Hey, I guess this is a preview? Well, not of the History Channel documentary in particular, but closely related (and see David struggling with a bad hair day). Via symmetry breaking.

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Hello All. This post represents shameless advertising for a television program which I am hosting on the History Channel this week. The show is a one-hour program about the Large Hadron Collider (LHC) experiment outside of Geneva and will air the day before the first proton bunch circulates the entire 27 km ring (September 9th, 8pm/midnight EDT/PDT, 7pm/11pm CDT, 6pm/10pm MDT). The show will visually describe the complexity and scale of the experiment and some of the potential discoveries we hope to make (the Higgs particle, supersymmetry, dark matter, extra dimensions).

For many reasons this is an amazing moment in the history of science (many which have probably been repeated on this blog before). [Indeed -- ed.] There are roughly 75 countries with at least one institution (university or lab) which has contributed to the construction of this machine. The list includes strange bedfellows: India and Pakistan, Israel and Iran and the United States, Greece and Turkey, Russia and Georgia, all of western Europe, most of eastern Europe, some of northern Africa and south America, Japan, China, S. Korea, etc. This unlikely team has constructed the biggest single machine in the history of the planet after over 20 years since the first plans were laid. At 10,000 scientists, this project represents the modern day pyramids.

What gets me though is that high-energy physics have not really seen a discovery that has directly shaken the standard model of particle physics for thirty years. The discovery of neutrino masses were a surprise, but fit nicely in the standard model if there is new physics at (unreachably) high energies. Dark matter was certainly a surprise, but could potentially only couple to us gravitationally, and again not uproot the standard model. The same can be said about dark energy to an even greater extreme. However, an unexpected particle has not been discovered since the seventies. The seventies were the time that not only the standard model was discovered experimentally, but its underpinnings, quantum field theory, was confirmed as the correct underlying description of all matter interactions (other than gravitational). The (perhaps, not so) amazing thing is, the surprising discoveries stopped by the end of the seventies, and we have been confirming the standard model even since.

The implication is that almost the entire particle physics community, both theorists and experimentalists, who are actively working on LHC physics have never been involved in a surprising discovery. This large community of scientists have been building up to this moment for their entire careers. The scale of these experiments are such that one can really only expect one discovery per generation, and this one is ours.

The show is not perfect, but there are some stunning analogies. I did not write the show, but I fact-checked most of it. There is no attempt to scare the viewer with ‘disaster scenarios’, but simply an attempt to cover what the physicists are constructing and what they expecting or hoping to discover. There is also a bit of history of particle physics.

Enjoy the show. I’ll stay connected so I can answer any questions that come up.

Insertion of the tracker into the CMS detector. Photo by Maxmillion Price, copyright CERN. Click for full size.

Today’s episode of lazy-bloggers-solicit-guests-to-fill-in features Joel Corbo, a graduate student in physics at Berkeley. Joel and friends were disappointed by some features of the graduate-school experience, and (unusually) decided to actually do something about it — they founded the Compass Project, which supports excellence in science education, especially for women and minorities.

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My name is Joel Corbo, I’m a physics Ph.D. student, and I’m a little frustrated.

My trajectory through the US educational system has been a great one. I have parents who care deeply about me and my future and who believe in the value of a strong education. Because they cared, I went to an elementary school that laid a good foundation and allowed me to attend a high school that was more academically rigorous than many colleges (both of these schools were private, although the latter was also free). I also majored in physics at MIT.

My story may sound typical, at least in certain circles, but there are a few more details to add to the story. My dad is a recent immigrant without a high school education who worked as a maintenance man in the NYC Housing Projects, and my mom is the daughter of Puerto Rican immigrants and a lucky survivor of the NYC public school system. I was the first person in my immediate family to go to college. Statistically speaking, I shouldn’t have succeeded — but I did.

Looking back at my education, it’s obvious to me that a huge factor contributing to my success was the presence of people in my life who believed in me and supported me: my parents, my teachers, and my peers. Even at MIT, which is primarily recognized for the quality of its research (and rightly so), I found a physics department that openly cared about undergraduate education, where teaching was valued and done well and which fostered a community of undergrads who learned from and supported each other.

So, why the frustration? My relatively rosy view of physics education was shaken up not long after starting grad school at UC Berkeley (By the way, I don’t want to single out Berkeley as particularly flawed, as I’m sure its problems are shared by virtually every physics department in the US to one extent or another. However, I can only write about what I know and this is where I am). Back in the cocoon of the MIT undergrad experience, I came to believe that physics was awesome for two main reasons: (1) because it answers deep, fundamental questions about how the world works and (2) because it is a community driven, collaborative exercise that thrives on the effective sharing of knowledge among its practitioners. In my mind, grad school would build upon these dual pillars of awesomeness and help me become (1) a great researcher and (2) a great teacher.

The jury is still out on the great researcher thing, but it turns out that, in principle, grad school has precisely zero to do with becoming a good teacher. Oh, you can TA a class here and there, as long as that doesn’t get in the way of what grad school is “really” all about. The unfortunate thing is that the lack of value assigned to teaching seems very systemic, to the point of being embedded in the culture; perhaps this attitude appears to benefit physics in the short-term by weeding out all but the most “serious” students, but in the long run it does nothing but damage.

The damage done to grad students is fairly obvious. First of all, if they are not provided with encouragement and avenues to become better teachers, then they won’t improve their teaching skills as well as they could have. If you happen to believe that an essential part of being a physicist is the ability to pass physics on to future generations of students, to inspire them to follow in the footsteps of their intellectual ancestors, then it is hard to justify allowing people to graduate with PhDs who have not demonstrated the ability to do just that. Of course, this happens all the time.

Secondly, there are always some grad students, including me, who have a deep interest in teaching (I remember deciding in high school that the only way to know if I really understood something was to try teaching it to someone else — so I can genuinely say that education has been on my mind for a long time). When people with such a passion are met with disinterest or even disdain by the people they want to emulate (successful physicists), the blow to their motivation can be severe. After all, who wants to stick around when their interests and talents aren’t valued or supported? I’ve heard it implied (and sometimes even said outright) that such students aren’t “serious enough” about physics and therefore aren’t worth keeping around, but without a crystal ball, who can really say which student will end up making important contributions to the field?

Let’s put the grad students aside for now (didn’t we just talk about that?), and spend some time looking at how undergrads are damaged by this attitude. Teaching is the single most fundamental service an academic department provides to undergraduates, and if, on average, a department is not interested in teaching well, the implication is that it’s not interested in serving undergrads in any way. But serving undergrads is vital to the survival of an academic discipline, because some of those undergrads are that discipline’s future experts. As I stated above, I was fortunate enough to attend schools that did serve their students well, but I can talk about the opposite through my observations as a TA.

Many students arrive at their undergraduate institution with a substantial number of long-held academic “bad habits”, especially in the sciences. High school has managed to convince many students that physics is a dogmatic, memorization-centered subject. As a result, they don’t have the skills necessary to solve real physics problems, because all that they have learned to do is to pattern-match and to plug-and-chug. Still, popular science books and NOVA specials have kept them interested enough that many intend to pursue the physical sciences as undergrads. Once they get to college, however, their passion for physics is quickly squelched by a number of factors:

1. Because they don’t have the skills necessary to problem-solve, model-build, and generally think like physicists, these students actually don’t know how to effectively learn physics as it is typically presented in a large lecture-based class. This doesn’t mean that these students are stupid, or somehow not worth teaching. It simply means that there are things they need to be taught other than “the material” in order to help them become better learners. Unfortunately, many of them come away feeling like they don’t have what it takes to be physicists (as though there is some intrinsic “physicsness” that they are lacking) and so they leave the field.
2. The typical introductory physics sequence, at least at Berkeley, is very isolating for potential physics majors. The vast majority of people in those classes are engineering students who are there because their departments require that they take physics; they have largely no interest in physics for its own sake. This makes it very difficult for potential physics majors to identify each other — they are like needles in an apathetic haystack. This situation is exacerbated by the fact that even the physics department cannot identify these potential majors. So, these students end up isolated from the department, from upperclassmen physics majors, and from each other – that is to say, from the physics community – for the three semesters it takes them to get through introductory physics. However, an important part of the excitement of physics is the collaboration with peers, the shared goal of building knowledge through interaction and discussion and asking “What if”. Without that, it’s incredibly difficult to paint physics as an interesting field, to really sell the idea of being physicists to these students beyond the level that NOVA can, and so they leave the field.
3. The problems of interaction and perceived lack of “physicsness” are magnified for a certain set of students: women and underrepresented minorities. At this point, so much has been said about the lack of women and minorities in all levels of physics due to the “leaky pipeline” that I don’t have much to add to the subject. For this discussion, the important point to note is that in addition to the issues that their well-represented peers also face, they have to face majoring in a field where they don’t see people like themselves. They arrive at the seemingly logical but erroneous conclusion that success in physics is unattainable unless you are a white male, and so they leave the field.

So, here are three of many reasons why undergrads might leave the field of physics – notice that none of these reasons have anything to do with these students’ ability to be good physicists. If the physics community wants to recruit the best minds into its ranks, it stands to reason that these impediments must be removed, but not enough people seem interested in doing so. Hence, my frustration.

[More below the fold...] Continue reading ‘Guest Post: Joel Corbo on Graduate School and Teaching’

For his final guest post, Tom looks at a topic right up our alley: Einstein’s thoughts about religion. The difference being that he knows what he’s talking about, having written a book on Einstein.

Many thanks to Tom for chipping in this week. His previous posts are here and here, and don’t forget the Inverse Square Blog.

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The Jewishness of Albert Einstein.

I’m a bit late to this particular party, but I hear that there was a bit of a media and blog hullabaloo about a letter by Albert Einstein that was auctioned last month for 170,000 pounds. That doubles the previous record for an Einstein letter, and at least part of the reason for its record price seems to have been its content — what seemed to some a startlingly blunt assessment of religion in general. He wrote:

“The word God is for me nothing more than the expression and product of human weaknesses, the Bible a collection of honourable, but still primitive legends which are nevertheless pretty childish.”

To get down to cases close to home:

“For me the Jewish religion like all other religions is an incarnation of the most childish superstitions.”

To be sure, he acknowledged, he was happy to identify himself as one of “the Jewish people to whom I gladly belong and with whose mentality I have a deep affinity…” But clearly belonging to a community did not make him blind, deaf or dumb.

The reason I ignored this at first is that after fifteen years in the Einstein game I’m pretty tired of WWED appeals to authority, all that pouring through the great man’s quotations to find something to support whatever view one may have had in the first place.

The reason I’m picking it up now is that the letter raises a question that allows us with only a little leap of the imagination to begin to gather the intense pressure of the experience of being Jewish in Europe in the first few decades of the last century – especially if you were smart, prominent, public.

Just to get it out of the way: there is nothing surprising about this letter. Just five years earlier Einstein wrote that, when he was young he had experienced a bout of real piety, until:

“Through the reading of popular scientific books I soon reached the conviction that much in the stories of the Bible could not be true. The consequence was a positively fanatic orgy of freethinking, coupled with the impression that youth is intentionally being deceived by the state through lies.”

That revelation remained with him throughout his life, and he never made a secret of it. He refused to claim a religious affiliation in the papers he filed with the Austro-Hungarian government to take up a professorship in Prague. Told he had to claim something, he declared he was of the “Mosaic” faith – a construction that conveyed his disdain for the whole notion pretty well, IMHO.

And so it went. In 1915, he told one correspondent that, “I see with great dismay that God punishes so many of His children for their ample folly, for which obviously only He himself can be held responsible,” …. “Only His nonexistence can excuse him.”

Those who followed this malign, non-existent deity were fools. When he visited Palestine in 1921, Einstein was much impressed by the sight of Jews constructing cities and a way of life out of raw dirt and effort. But the sight of traditional Jews praying at the Wailing Wall seemed to him the “dull-witted clansmen of our tribe.” They made such spectacles of themselves, “praying aloud, their faces turned to the wall, their bodies swaying to and fro,” that to Einstein, it was “a pathetic sight of men with a past but without a present.”

That’s enough: the point is that Einstein made it clear in public, and even more so in private communications that have been in the public record for decades now, that revealed religion in general and orthodox Judaism in particular had no hold on him at all. When he used the term God, it was mostly just an off-hand short-hand: “God does not play dice” was another way of saying, as he did in the EPR paper, that “no reasonable definition of reality could be expected to permit” the excesses of modern quantum theory.

But all this begs the question why Einstein bothered to claim Jewishness, if Judaism itself as a practice and a body of belief had no hold on him.

Einstein himself gave two answers. The first was he saw in Judaism a framework and a fair amount of thought about how to live ethically with others. His take on the tradition pulled out of Judaism “the democratic ideal of social justice, coupled with the ideal of mutual aid and tolerance among all men” and a passion for “every form of intellectual aspiration and spiritual effort.” This is religion as heuristic – and specifically, Judaism as a sustained body of inquiry into certain problems that interested him.

The second, of course, was that he had no choice. Whatever he may have believed, others defined him: “When I came to Germany,” he wrote some years later as part of an explanation for his conversion to Zionism, “I discovered for the first time that I was a Jew, and I owe this discovery more to Gentiles than to Jews.”

It was more than the casual anti-Semitism that he experienced or perceived, dating back to his failure to get an academic job after finishing his college degree. Rather, Einstein’s strong identification not just as a person of Jewish background, but as a highly public member of both the Berlin Jewish community and the nationalist Zionist movement, is one measure of just how rapidly the nature of German anti-Semitism changed in the immediate aftermath of defeat in World War I.

I go into this at some length in this tome – from which most of the above comes, in one form or another. See chapter ten if you’re interested. In this venue, I want to make just two points abstracted out of that much longer story.

Continue reading ‘Guest Post: Tom Levenson on Einstein, Religion, and Jewishness’

For his second guest post, Tom follows in our proud tradition of fearless eclecticism,
mixing neuroscience and current events with a bit of materialistic philosophizing. His first post was here, and his third is here.

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Burrowing into tragedy: a story behind the story of the Iraq War Suicides.

My thanks to all here who gave me such a warm welcome on Monday (and, again, to Sean for asking me here in the first place).

This post emerges out of this sad story of a week or so ago.

Over Memorial Day weekend this year there was a flurry of media coverage about the devastating psychological toll of the Iraq and Afghanistan wars. The single most awful paragraph in the round-up:

“According to the Army, more than 2,000 active-duty soldiers attempted suicide or suffered serious self-inflicted injuries in 2007, compared to fewer than 500 such cases in 2002, the year before the United States invaded Iraq. A recent study by the nonprofit Rand Corp. found that 300,000 of the nearly 1.7 million soldiers who’ve served in Iraq or Afghanistan suffer from PTSD or a major mental illness, conditions that are worsened by lengthy deployments and, if left untreated, can lead to suicide.”

(For details and a link to a PDF of the Army report – go here.)

This report, obviously, is the simply the quantitative background to a surfeit of individual tragedy – but my point here is not that war produces terrible consequences.

Rather, the accounts of the Iraq War suicides — 115 current or former servicemen and women in 2007 – struck me for what was implied, but as far as I could find, not discussed in the mass media: the subtle and almost surreptitious way in which the brain-mind dichotomy is breaking down, both as science and as popular culture.

How so? It is, thankfully, becoming much more broadly understood within the military and beyond that “shell shock” is not malingering, or evidence of an essential weakness of moral fiber. PTSD is now understood as a disease, and as one that involves physical changes in the brain.

The cause and effect chain between the sight of horror and feelings of despair cannot, given this knowledge, omit the crucial link of the material substrate in which the altered and destructive emotions can emerge. PTSD becomes thus a medical, and not a spiritual pathology.

(This idea still faces some resistance, certainly. I launched my blog with a discussion of the attempt to court martial a soldier for the circumstances surrounding her suicide attempt. But even so, the Army is vastly further along in this area that it was in the Vietnam era and before.)

Similarly, depression is clearly understood as a disease with a physical pathology that underlies the malign sadness of the condition. (H/t the biologist Louis Wolpert for the term and his somewhat oddly detached but fascinating memoir of depression.)

This notion of the material basis of things we experience as our mental selves is not just confined to pathology. So-called smart drugs let us know how chemically malleable our selves can be.

More broadly, the study of neuroplasticity provides a physiological basis for the common sense notion that experience changes who we perceive ourselves to be.

All this seems to me to be a good thing, in the sense that (a) the study of the brain is yielding significant results that now or will soon greatly advance human well being; and (b) that the public seems to be taking on board some of the essential messages. The abuses (overmedication, anyone?) are certainly there. But to me, it is an unalloyed good thing that we have left the age of shell shock mostly behind us.

At the same time, I’m a bit surprised that the implications of this increasingly public expression of an essentially materialist view of mind haven’t flared up as a major battle in the science culture wars.

Just to rehearse the obvious: the problem with cosmology for the other side in the culture war is that it conflicts with the idea of the omnipresent omnipotence of God. The embarrassment of evolutionary biology is that it denies humankind a special place in that God’s creation, destroying the unique status of the human species as distinct from all the rest of the living world.

Now along comes neuroscience to make the powerful case that our most intimate sense of participating in the numinous is an illusion.

Instead, the trend of current neuroscience seems to argue that the enormously powerful sense each of us has of a self as distinct from the matter of which we are made is false. Our minds, our selves may be real—but they are the outcome of a purely material process taking place in the liter or so of grey stuff between our ears.

(There are dissenters to be sure, those that argue against the imperial materialism they see in contemporary neuroscience. See this essay for a forceful expression of that view.)

I do know that this line of thought leads down a very convoluted rabbit hole, and that’s not where I am trying to go just now.

Instead, the reports of the Iraq suicides demonstrated for me that the way the news of the materiality of mind is is slipping into our public culture without actually daring (or needing) to speaking its name.

That the problem of consciousness is still truly unsolved matters less in this arena than the fact of fMRI experiments that demonstrate the alterations in brain structure and metabolism associated with the stresses of war or the easing of the blank, black hole of depression. The very piecemeal state of the field helps mask its potentially inflammatory cultural implications.

To me this suggests two possibilities. One is that it is conceivable that when the penny finally drops, we might see backlash against technological interventions into the self like that which has impeded stem cell research in the U.S.

On the other hand, I don’t think that the public can be motivated or even bamboozled into blocking the basic science in this field. Too much rests on the work; any family that has experienced Alzheimers knows just how urgent the field may be — not to mention anyone with a loved one in harms way.

This actually gives me hope for a shift in the culture war. For all the time and energy wasted over the last several years defending the idea of science against attacks on evolution, with the cosmologists taking their lumps too – the science of mind could force a shift in the terms of engagement decisively in the right direction.

Or I could be guilty of another bout of wishful thinking. Thoughts?

Image: Brain in a Vat, article illustration. Offered in homage to my friend and source of wisdom, Hilary Putnam, who introduced the brain-in-a-vat thought experiment in this book. Source: Wikimedia Commons.

A little treat for loyal CV readers: Tom Levenson is a professor of science writing at MIT, and the proprietor of the Inverse Square Blog, one of the most erudite scientifically-minded outposts in this blogosphere of ours. I’ve been enjoying how Tom writes engagingly about science while mixing in cultural and artistic references, so I asked if he would like to guest-blog a bit here at CV. This is the first of three posts he’ll be contributing; look for the other two later this week. [Here is two, and here is three.]

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Monday Isaac Newton blogging: A little light reading, Principia edition.

Update: See correction below.**

To introduce myself to the Cosmic Variance community (at Sean’s very kind invitation), let me just admit up front that I am a glutton for punishment.

Exhibit A: last year I read the Principia for pleasure.*

That’s not exactly right– it is more accurate to say that in the context of writing a book on Isaac Newton’s role as currency cop and death penalty prosecutor, I found myself reading the Principia as literature rather than the series of proofs it appears to be. Just like John Locke, who had to ask Christiaan Huygens if he could take the mathematical demonstrations on faith (Huygens said he could), I read to see what larger argument Newton was making about the ways human beings could now make sense of material experience. (This is, by the way, the only connection I can imagine that Locke and I share.)

What I got out of the exercise, more than anything else, was a reminder of how something we now mostly take for granted is in fact truly extraordinary: taken all in all, it seems genuinely remarkable that cosmology exists at all as a quantitative, empirical science.

That is: it is not obvious – or at least it wasn’t, all that long ago, that it would ever be possible to treat the universe as a whole as an object of study – especially given our very constrained vantage point from within that which we want to examine.

Most accounts of the story of modern cosmology more or less unconsciously downplay the strangeness of the claim that we can in fact make sense of the universe as a whole. They begin – mine did — with Einstein and the 1917 paper “Cosmological Considerations in the General Theory of Relativity, (to be found in English translation here.) Cosmology in this telling becomes more or less an inevitable extension of a recent advance in theoretical physics; the change in worldview precedes this extension of the apparatus of general relativity into a new calculation.

I recant: though I certainly wrote my version of this basic tale, reading Newton has reminded me of the much more radical change in the understanding of what it is possible to think about that had to precede all that cosmology (among much else) has achieved.

It certainly was not clear that the universe as a whole was subject to natural philosophical scrutiny in 1684, the year of Edmond Halley’s fortunate visit to Trinity College, Cambridge, and his more-or-less innocent question about the curve traced by a planet, assuming “the force of attraction towards the sun to be reciprocal to the square of their distance from it? that would produce an elliptical planetary orbit with the sun at one focus.

An ellipse inverse square relationship, Newton told Halley.

How did he know?

Why – he had calculated it.

By 1686, Newton had extended and revised his off-the-cuff answer into the first two books of Principia, both titled “The Motion of Bodies.” These pursued the implications of his three laws of motion through every circumstance Newton could imagine, culminating in his final demolition of Cartesian vortex physics.

But even though he had worked through a significant amount of mathematical reasoning developing the consequences of his inverse square law of gravitation, he left the ultimate demonstration of the power of these ideas for book three.

Books one and two had been “strictly mathematical,” Newton wrote. If there were any meat and meaning to his ideas, though, he must “exhibit the system of the world from these same principles.”

To make his ambitions absolutely clear Newton used the same phrase for the title of book three. There his readers would discover “The System of the World.”

This is where the literary structure of the work really comes into play, in my view. Through book three, Newton takes his audience through a carefully constructed tour of all the places within the grasp of his new physics. It begins with an analysis of the moons of Jupiter, demonstrating that inverse square relationships govern those motions. He went on, to show how the interaction between Jupiter and Saturn would pull each out of a perfect elliptical orbit; the real world, he says here, is messier than a geometer’s dream.

He worked on problems of the moon’s motion, of the issues raised by the fact that the earth is not a perfect sphere, and then, in what could have been a reasonable resting point for the book as a whole, he brought his laws of motion and gravity literally down to earth, with his famous analysis of the way the moon and the sun influence the tides.

Why not stop there? The story thus far had taken gravity from the limits of the observed solar system to the ground beneath each reader’s feet. More pragmatically – it told a story whose significance Newton’s audience would have grasped immediately: the importance of understanding the rules governing tides was obvious enough to the naval powers of the day.

No matter. Newton kept on going. The last section of his world-system turned to the celestial and seemingly impractical: the motion of comets, in an analysis of the track of the great comet of 1680.

Newton presented his findings through two different approaches: one produced by collecting all the data points he could of traveler’s observations and plotting the comet’s track against those points; and the other in which he selected just three points and calculated the path implied.

The two analyses matched almost exactly, and both showed that this comet did not complete a neat, elliptical orbit. Rather, it traced a parabola.

Newton knew what he had done. He was no accidental writer. A parabola, of course, is a curve that keeps on going – and that meant that at the end of a very long and very dense book, he lifted off again from the hard ground of daily reality and said, in effect, look: All this math and all these physical ideas govern everything we can see, out to and past the point where we can’t see anymore.

Most important, he did so with implacable rigor, a demonstration that, he argued, should leave no room for dissent. He wrote “The theory that corresponds exactly to so nonuniform a motion through the greatest part of the heavens, and that observes the same laws as the theory of the planets and that agrees exactly with exact astronomical observations cannot fail to be true.” (Italics added).

And now, finally, to get back to the point: this, I would argue, was the essential first and in some ways the most difficult step in the foundations of cosmology. With it Newton transformed the scale of the universe we inhabit, making it huge, perhaps infinite. Even more important, he demonstrated that a theory that could not fail to be true made it possible to examine one phenomenon — matter in motion under the influence of gravity — throughout all space.

That thought thrilled Newton’s contemporaries – Halley caught the mood in his dedicatory poem to the Principia, writing that “Error and doubt no longer encumber us with mist;/….We are now admitted to the banquets of the Gods;/We may deal with laws of heaven above; and we now have/The secret keys to unlock the obscure earth….” To catch a distant echo of that euphoria, just imagine what it would have been like to contemplate that ever receding comet, fifteen years into its journey towards who knew where at the time of Newton’s writing, and know that its behavior was knowable through an extraordinary act of human invention.

It’s a whole ‘nother story to ask what it would take to create a similar sense of pride and pleasure in a general audience today. But just to get the discussion going, I’d suggest that one of the oddities of contemporary cosmology as presented to the public is the degree to which the universe at large has become more homey; the very success in making the argument that there is a continuous scientific narrative to be told from the Big Bang to the present makes it harder to see just how grand a claim that is.

So, to end with an open invitation to this community: what would make current physical ideas as powerful and as intelligibly strange as Newton was able to make his story of a comet traveling from and to distances with out limit?

Last housekeeping notes: in one of the more premature bits of self-promotion in publishing history, the Newton material discussed above derives from my book, tentatively titled Newton and the Counterfeiter, coming early next year from Houghton Mifflin Harcourt (and Faber, for those of you across the pond).

Also – my thanks again to Sean Carroll for welcoming me here. If you want to see what I do when I’m at home, check out The Inverse Square Blog.

*If you are minded to pick up a copy of Principia, get this edition. Not only is it a well made book, easy to look at, well printed, with clear diagrams, it comes with the invaluable guide to reading the Principia written by I. Bernard Cohen. Accept no substitutes.

**Thanks to reader and award-winning physics teacher David Derbes for catching my inversion of the problem Halley put to Newton. Let this be a lesson to me: blog in haste; check one’s notes at leisure; repent in public.

Image: Woodcut by Jiri Daschitzsky, “The Great Comet of 1577.” Source: Wikimedia Commons.

You may have heard some of the buzz about a new result concerning the direct detection of dark matter particles in an underground laboratory. The buzz originates from a new paper by the DAMA/LIBRA collaboration; David Harris links to powerpoint slides from Rita Bernabei, leader of the experiment, from her talk at a meeting in Venice.

The new experiment is an upgrade from a previous version of DAMA, which had already been on record as having recorded a statistically significant signal of the form you would expect from the collision of weakly interacting massive particles (WIMP’s) with the detector. The experiment uses a challenging technique, in which their focus is not on eliminating all possible backgrounds so as to isolate the dark-matter signal, but to look at the annual modulation in that signal that would presumably be caused by the Earth’s orbital motion through the cloud of dark matter in the Solar System: you expect more events when we are moving with a high velocity into the dark-matter wind. Other workers in the field have not been shy about expressing skepticism, but the DAMA team has stood their ground; as Jennifer notes in her report from the recent APS Meeting, the DAMA collaboration home page currently features a quote from Kipling: “If you can bear to hear the truth you’ve spoken/ twisted by knaves to make a trap for fools,/ ……………you’ll be a Man my son!”

To help provide some insight and context, we’ve solicited the help of a true expert in the field — Juan Collar of the University of Chicago. I got to know Juan back in my days as a Midwesterner, and a trip to his bustling underground experimental empire was always a highlight of anyone’s visit to the UofC physics department. You can hear him talk about his own work in this colloquium at Fermilab; he’s agreed to post for us about his views on the new DAMA result, and more general thoughts on what it takes to search for 25% of the universe. I promise you won’t be bored.

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My dear friend Sean has me blogging: hey, I’ll try anything once. On the subject of the recent DAMA results no less, as per his request. I am normally a bit of a curmudgeon but… Sean, you really want the worst of me out there permanently on the internets, don’t you?

I’ll try to keep this to the point. A bard I am not, nor the subject invites any poetry. I have therefore chosen brief eruptions of flatulence as the metric and style for this piece. The result of indigestion, you see. I’ll start with the most negative, so as to end up on a brighter note:

• The modulation is undeniable by now. I don’t know of any colleagues who doubted these data were blatantly modulated already back in 2003, when “the lady” (DAMA) decided to keep mum for a while. However, to conclude from something this mundane that the experiment “confirms evidence of Dark Matter particles in the galactic halo with high confidence level” or that there is “an evidence for the presence of dark matter particles in the galactic halo at 8.2 sigma confidence level” is simply delusional. There is evidence for a modulation in the data at 8.2 sigma, stop. Compatible with what would be expected from some dark matter particles in some galactic halo models, full stop. Anything beyond this is wanting to believe, and it smears on the rest of us in the field. Of course, of course… there is no other observed process in nature that peaks in the summer and goes through a low in winter, so this must be dark matter, right? (Occam is turning in his grave, rusty razor still in hand. He is thinking a remake of that opening scene in “Un chien andalou”, with help from this little lady. I am channeling him loud and clear).
• Someone should take the DAMA folks aside for a beer, make them see the following. If one day soon we are all convinced that this effect was DM-induced (see below for what that will really take), they will be recognized for one of the greatest discoveries in the history of science, without them having to look desperate or foolish today. Or making the rest of us in the field do, by association: thanks DAMA, for cheapening the level of our discourse to truly imbecilic levels. (Sean, if you edit this I will scratch the paint off your car. I may not write blogs, but I do read them: I know how to hurt you).
• Deep breath. Having cleared the air some (or just made it toxic, whatever), it is not DAMA’s fault that there is a penury of signatures in this field of ours, laboratory searches for particle dark matter. The one possible exception to this is a detector with good recoil directionality and sufficient target mass to be truly competitive, but we don’t know of a good enough way to do this as of today (“good enough” folds in the price tag). People are still trying. The diurnal modulation in the DM signal that would be sensed by such a device is wickedly rich in features, extremely hard for nature to imitate with anything else. The annual modulation resides on the other side of this spectrum of complexity. It is the poor man’s smoking-gun to DM “evidence”. Inspected carefully, it is disappointingly feeble: different models of the halo can shift the phase of this modulation completely, turning expected maxima into minima and vice-versa, changing the expected amplitude as well. Add to this the fact that essentially every possible systematic effect able to pass for a “signal” can be yearly-modulated, for one reason or another. That’s the ones we can presently think of, and the ones yet to be proposed. To grow convinced that we have observed dark matter in the lab we’ll require a number of entirely different techniques, using a variety of targets, all pointing at the same WIMP (mass, cross sections), with additional back-up information from accelerator experiments and from gamma-ray satellite observations (so-called indirect searches). All of those lines crossing at one point, so to speak. This I (for one) will call “evidence”. I know of no single existing or planned DM experiment, including those I participate in, that would be able to make anything close to a bulletproof claim on its own. My advice to any overambitious individuals looking for a quick kill is to look elsewhere in physics. WIMP hunting is not it, no matter how important the discovery of these particles might be.

Continue reading ‘Guest Post: Juan Collar on Dark Matter Detection’

Michelangelo is a grad student at Berkeley who had the fun opportunity to write a diary for the Economist that will continue through this week about his adventures doing particle physics in Antarctica. I would say more, but he does a pretty good job himself!

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First off, I’d like to thank Sean for giving me the chance to write this guest post. It’s not every day (in fact, this would be the first time) that I get to write something for a blog that I both read and enormously respect. This is an especially great opportunity since the scions of CV are graciously allowing me to do a bit of shameless self-promotion for a five-part journal, being published this week, that I got to write for the website of the Economist magazine.

Maybe I should back up and introduce myself. I’m currently a fifth year physics PhD student at UC Berkeley. My research is on the IceCube project, a neutrino physics experiment located at the South Pole. Basically, we’re building the world’s largest particle detector out of the polar icecap itself. Using hot water, we melt holes 2,500 m down into the ice and install very sensitive light detectors. This allows us to study the particle debris that results from collisions of high-energy neutrinos in the ice. Ultimately, we’re hoping to learn about the basic physics of neutrinos as well about the properties of some of the violent astrophysical objects that might produce them and send them hurtling through the universe towards our detector.

This means that I do what many physicists do. I sit in front of a computer, writing code and analyzing data. I do calculations and simulations. I drink coffee and talk and argue with colleagues. But it also means that I get to do something only a smaller subset of astrophysicists and physicists get to do. I get to travel to a really exotic location to do fieldwork.

I think this is an aspect of being a physicist that sometimes gets overlooked. It’s true that astronomers have always gone to mountaintops to build the best telescopes, and particle physicists have always traveled to underground accelerator facilities. However, fanning out to other locations to take advantage of particular natural features is something that has become increasingly important as we build bigger, deeper detectors to try to understand weak signals and/or rare and exotic phenomena. In recent years, physicists have been traveling to the vast Argentinean plains to understand the origins of the highest energy cosmic rays, particles that are constantly bombarding Earth. Folks who study the CMB and other long-wavelength radiation have been heading up to the high-altitude Atacama Desert (here, here, and here) and to the South Pole to take advantage of their thin, dry atmospheres. Selection and planning has been moving forward for a deep underground facility for doing basic neutrino and dark matter physics.

All this means that graduate students for years to come will have the exciting opportunity to go out into the field to do their work. While the research itself is exciting, traveling to these exotic locations brings us in contact with scientists from other fields doing all sorts of other great science. For those of us who get to go to Antarctica, we meet people on the cutting edge of climate and atmospheric research. For those working underground, they may encounter earth scientists or researchers study life in extreme environments. All of which make for rich and stimulating conversations and experiences.

This brings me back to the shameless self-promotion. When the Economist opportunity came up to share some of my experiences traveling, living, and doing research at the South Pole, I jumped at it. I’ve tried to squeeze in as much basic climate science and physics as I could, so if you’re interested, check it out here…

Science or Sociology?
Joseph Polchinski, 5/20/07

This is a continuation of the on-line discussion between Lee Smolin and myself, which began with my review of his book and has now continued with his response. A copy of this exchange (without the associated comment threads) is here.

Dear Lee,

Thank you for your recent response to my review. It will certainly be helpful in clarifying the issues. Let me start with your wish that I do more to address the broader issues in your book. When I accepted the offer to review these two books, I made two resolutions. The first was to stick to the physics, because this is our ultimate goal, and because it is an area where I can contribute expertise. Also, keeping my first resolution would help me to keep the second, which was to stay positive. I am happy that my review has been well-received. Your response raises some issues of physics, and these are the most interesting things to discuss, but I will also address some of the broader issues you raise, including the process of physics, ethics, and the question in the title. Let me emphasize that I have no desire to criticize you personally, but in order to present my point of view I must take serious issue both with your facts and with the way that they are presented.

Regarding your points:

The fictitious prediction of a non-positive cosmological constant. This is a key point in your book, and the explanation that you now give makes no logical sense. In your book you say (A) “… it [a non-positive cosmological constant] was widely understood to be a consequence of string theory.” You now justify this by the argument that a non-positive cosmological constant is a consequence of unbroken supersymmetry (true), so A would follow from (B) Unbroken supersymmetry was widely understood to be a consequence of string theory. But even if this were true, it would not support your story about the observation of the dark energy leading to a “genuine crisis, … a clear disagreement between observation and a prediction of string theory.” There would already have been a crisis, since supersymmetry must obviously be broken in nature; seeing the dark energy would not add to this. But in fact the true situation, as you can find in my book or in many review articles, was closer to the opposite of B than to B: (B’) Supersymmetry is broken in almost all Calabi-Yau vacua of heterotic string theory. We have no controlled examples because at least one modulus rolls off, usually to a regime where we cannot calculate. The solution to this problem may have to wait until we have a non-perturbative formulation of gravity, or even a solution to the cosmological constant problem.

In your response you largely raise issues surrounding B’, including the Witten quote, but I want to return to what you have actually written in your book. It is a compelling story, which leads into your discussion of “a group of experts doing what they can to save a cherished theory in the face of data that seem to contradict it.” It surely made a big impression on every reader; it was mentioned in several blogs, and in Peter Shor’s Amazon review. And it never happened. It is an example of something that that happens all too often in your book: you have a story that you believe, or want to believe, and you ignore the facts.

You go on to challenge the ethics of string theorists in regard to how they presented the issue of moduli stabilization in their talks and papers. I am quite sure that in every colloquium that I gave I said something that could be summarized as “We do not understand the vacuum in string theory. The cosmological constant problem is telling us that there is something that we do not understand about our own vacuum. And, we do not know the underlying principle of string theory. These various problems may be related.” The cosmological constant and the nature of string theory seemed much more critical than the moduli stabilization problem, and these are certainly what I and most other string theorists emphasized.

This scientific judgment has largely been borne out in time. In 1995-98 these incredible new nonperturbative tools were developed, and over the next few years many string theorists worked on the problem of applying them to less and less supersymmetric situations, culminating in the construction of stabilized vacua. Obviously many questions remain, and these are widely and openly debated. It seems like a successful scientific process: people knew what the important problems were, worked in various directions (a fair number did work on moduli stabilization over the years), and when the right tools became available the problem was solved. As you point out, the stabilization problem is nearly one hundred years old, and now string theorists (primarily the younger generation, I might add) have solved it. You are portraying a crisis where there is actually a major success, and you are creating an ethical issue where there is none.

AdS/CFT duality. You raise the issue of the existence of the gauge theory. There are two points here. First, Wilson’s construction of quantum field theory has been used successfully for 40 years. It is used in a controlled way by condensed matter physicists, lattice gauge theorists, constructive quantum field theorists, and many others. To argue that a technique that is so well understood does not apply to the case at hand, the scientific ethic requires that you do more than just say Not proven! Sociology! as you have done. You need to give an argument, ideally pointing to a calculation that one could do, or at least discuss, in which one would get the wrong answer.

I have given a specific argument why we are well within the domain of applicability of Wilson: there are 1+1 and 2+1 dimensional versions of AdS/CFT, which are also constructions of quantum gravity, and for which the gauge theory is super-renormalizable (and there are no chiral fermions): the counterterms needed to reach the supersymmetric continuum limit can be calculated in closed form - thus there is an algorithmic definition of the gauge theory side of the duality. You could perhaps argue that there will be a breaking of supersymmetry that will survive in the continuum limit, and we could sit down and do the calculation. But I know what this answer is, because I have done this kind of calculation many times (it is basically just dimensional analysis). Similar calculations, for rotational invariance and chiral symmetry, are routine in lattice gauge theory.

As a further ethical point, in your book you state that it is astounding that Gary Horowitz and I ignore the question of the existence of the gauge theory, and you then use this to make a point about groupthink (this is in your chapter on sociology). While you were writing your book, you and I discussed the above points in detail, so you knew that we had not ignored the issue but had thought about it deeply. You do not even acknowledge the existence of a scientific counterargument to your statement, and in saying that Gary and I ignore the issue you are omitting facts that are known to you in order to turn an issue of science into one of sociology. Again you impose your own beliefs on the facts; thus I am reluctant to accept as accurate the various statements that you attribute elsewhere to anonymous string theorists and others.

You raise again the issue of a weak form of Maldacena duality. As you know, it is very difficult to find a sensible weak form that is consistent with all the evidence and yet not the strong form. In my review I have gone through your book and papers and identified three proposals, and explained why each is wrong. Again, you have not acknowledged the existence of scientific counterarguments, but have just reasserted your original point. If your arguments had been made in a serious way, I would expect that you would have given some deep thought to them and be ready to defend them.

There are some interesting points, one of which I will turn to next.

The role of rigor and calculation. Here we disagree. Let me give some arguments in support of my point of view. A nice example is provided by your paper `The Maldacena conjecture and Rehren duality’ with Arnsdorf, hep-th/0106073.

You argue that strong forms of the Maldacena duality are ruled out because Rehren duality implies that the bulk causal structure is always the fixed causal structure of AdS_5, and so there cannot be gravitational bending of light. But this would in turn imply that there cannot be refraction in the CFT, because the causal structure in the bulk projects to the boundary: null geodesics that travel from boundary to boundary, through the AdS_5 bulk, connect points that lie on null boundary geodesics. Now, the gauge theory certainly does have refraction: there are interactions, so in any state of finite density the speed of propagation will be less than 1. (Since Rehren duality does not refer to the value of the coupling, this argument would hold even at weak coupling, where the refraction can be calculated explicitly.)

You have emphasized that Rehren duality is rigorous, so apparently the problem is that you have assumed that it implies more than it does. Generally, rigorous results have very specific assumptions and very precise consequences. In physics, which is a process of discovery, this can make them worse than useless, since one tends to assume that their assumptions, and their implications, are broader than they actually are. Further, as this example shows, a chain of reasoning is only as strong as its weakest step. Rigor generally makes the strongest steps stronger still - to prove something it is necessary to understand the physics very well first - and so it is often not the critical point where the most effort should be applied. Your paper illustrates another problem with rigor: it is hard to get it right. If one makes one error the whole thing breaks, whereas a good physical argument is more robust. Thus, your paper gives the appearance of rigor, yet reaches a conclusion that is physically nonsensical.

This question of calculation deserves further discussion, and your paper with Arnsdorf makes for an interesting case study, in comparison with mine with Susskind and Toumbas, hep-th/9903228. (I apologize for picking so much on this one paper, but it really does address many of the points at issue, and it is central to the discussion of AdS/CFT in your various reviews.) You argue that there are two difficulties with AdS/CFT: that strong forms of it are inconsistent with the bending of light by gravitational fields, and that the evidence supports a weaker relation that you call conformal induction. We also present two apparent paradoxes: that the duality seems to require acausal behavior, and negative energy densities, in the CFT. The papers differ in that yours contains a handful of very short equations, while ours contains several detailed calculations. What we do is to translate our argument from the imprecise language of words to the precise language of equations.

We then find that the amount of negative energy that must be `borrowed’ is exactly consistent with earlier bounds of Ford and Roman, gr-qc/9901074, and that a simple quantum mechanical model shows that an apparent acausality in the classical variables is in fact fully causal when one looks at the full quantum state. Along the way we learn something interesting about how AdS/CFT works.

This process of translation of an idea from words to calculation will be familiar to any theoretical physicist. It is often the hardest part of a problem, and the point where the greatest creativity enters. Many word-ideas die quickly at this point, or are transmuted or sharpened. Had you applied it to your word-ideas, you would probably have quickly recognized their falsehood. Further, over-reliance on the imprecise language of words is surely correlated with the tendency to confuse scientific arguments with sociological ones.

Finally, I have recently attended a number of talks by leading workers in LQG, at a KITP workshop and the April APS meeting. I am quite certain that the standard of rigor was not higher than in string theory or other areas of physics. In fact, there were quite a number of uncontrolled approximations. This is not necessarily bad - I will also use such approximations when this is all that is available - but it is not rigor. So your insistence on rigor does not actually describe how science is done even in your own field.

Continue reading ‘Guest Post: Joe Polchinski on Science or Sociology?’

Today at Fermilab, the MiniBooNE experiment announced to a packed auditorium their long-awaited results looking for neutrino oscillations. Below is a guest post from Dr. Heather Ray, a scientist at Los Alamos National Lab, who has been working on the experiment for several years. I have known Heather since she was a graduate student on the CDF experiment at Fermilab, when she was at the University of Michigan. To the right is a photo of her with her significant other, Ivan Furic.

MiniBooNE Neutrino Experiment Results

by Dr. Heather Ray, Los Alamos National Lab

Neutrinos, a fundamental particle of nature, are believed to oscillate, or change from one type to another. In the long list of experiments which have claimed an observation of neutrino oscillations, one stands apart : LSND. The LSND result doesn’t fit in with our picture of oscillations from other experiments, and as such is highly controversial. The MiniBooNE experiment was designed to explore the LSND result, to conclusively prove or disprove the claimed oscillations. MiniBooNE announced it’s first results today (April 11th, 2007). The illustrious rulers of the Cosmic Variance blog have asked me to write a bit about this result. So, let the amazing neutrino story begin!

In the Standard Model of physics there are three main categories of fundamental particles: quarks, leptons, and gauge bosons, or force carriers. The leptons are the electron, muon, and tau, as well as their partner neutrinos : ν_e, ν_μ, and ν_τ. Neutrinos in the Standard Model have no charge and are massless. Imagine for a minute that neutrinos do have mass. If they have mass then they are able to oscillate, or change type. Neutrinos have definitively been observed changing from one type into another, yet the Standard Model of physics says that neutrinos do not have mass. The simplest solution to this conundrum is to allow the neutrinos to have mass.

In more technical terms we say that the weak eigenstates ( ν_e, ν_μ, and ν_τ) are made up of a combination of mass eigenstates. For example, in a two-neutrino scenario, at the time of creation the muon neutrino is a combination of the two mass eigenstates :

|ν_μ(0)> = -sin θ |ν₁> + cos θ |ν₂>

where the probability for two-neutrino oscillations is given by :

P_osc = sin²(2θ) * sin²[ (1.27 * Δm² * L) / E ]

The probability has two terms which are constrained by the design of the experiment (L, the distance from the neutrino source to the detector, and E, the energy of the neutrino beam), and two terms which are fit for when performing a two-neutrino oscillation analysis (Δm² and sin²(2θ), where θ is the mixing angle between the two neutrino states and Δm²_ab = m²_a - m²_b).

Neutrino physicists illustrate the current status of neutrino oscillations using a two dimensional plot that is the function of the two fit parameters. Oscillation results from the solar and atmospheric sectors (the tiny red and blue dots) have been observed and confirmed by several experiments. The set of several independent measurements allows us to constrain the range of fit parameters for those oscillations. The LSND result, which spans the upper third of this plot, has a large allowed region in parameter space.

In the Standard Model there are only three neutrinos, all of which interact with matter. The Δm² is the mass squared difference between the two neutrino states. These three results represent three differences, or splittings, between the mass states. If the Standard Model of physics is correct and there are 3 and only 3 neutrinos, a summation law should exist : Δm²₁₃ = Δm²₁₂ + Δm²₂₃. You can see that even at LSND’s lowest allowed Δm² point the summation law does not hold.

If LSND’s observation is found to be a true fact of nature, the Standard Model of physics cannot fully accommodate/explain neutrino interactions! This “breaking” of the Standard Model is very exciting to physicists, and indicates there is new physics that we haven’t previously thought possible. Many things could be true - there could be new allowed interactions for neutrinos (Lorentz Violation, CP/CPT violation, the list goes on!), or there could be additional particles - sterile neutrinos, which don’t interact with other matter but only can been seen through mixing with other neutrinos.

To properly explore the LSND signal we need an experiment that has the same experimental constraints (L/E, from the oscillation probability formula), so that the entire allowed region of LSND can be explored. The follow-up experiment also should have more events (smaller statistical errors), and a different signal signature, backgrounds, and sources of systematic errors. This experiment is MiniBooNE.

MiniBooNE is located at Fermi National Accelerator Laboratory, in Batavia, IL. To produce our neutrino beam we start with an 8 GeV beam of protons from the Booster. The proton beam enters a magnetic focusing horn where it strikes a beryllium target. The interactions of the protons+Be produce positively and negatively charged mesons (pions and kaons). The positively charged mesons will decay to produce a neutrino beam, while the negatively charged mesons will decay to produce an anti-neutrino beam.

There are still a lot of mysteries surrounding the interactions of neutrinos. We don’t yet know if neutrinos mix with the same probability as anti-neutrinos. Therefore, the LSND result, which claims observation of anti-ν_μ → anti-ν_e oscillations, needs to be explored using both neutrinos and anti-neutrinos. For the first check of LSND we chose to focus the positively charged mesons, which means we’re looking for ν_μ → ν_e oscillations. This choice was solely dictated by physics : the proton + Be interactions produce far more positively charged mesons. This means the rate of collection for our neutrino sample is much higher than our rate of collection for an anti-neutrino sample. We chose to collect the quick data set first, and then proceed with analyzing that data while collecting the anti-neutrino data set.

The mesons decay in flight into the neutrino beam seen by the detector : K⁺ / π⁺ →μ⁺ + ν_μ, where the ν_μ comprise the neutrino beam seen at MiniBooNE. These mesons decay in flight in our vacuum decay region. Following the decay region is an absorber, put in place to stop any muons and undecayed mesons. The neutrino beam then travels through approximately 450 meters of earth before entering the MiniBooNE detector.

MiniBooNE is a 12.2 meter diameter sphere. The detector is filled with pure mineral oil and lined with photomultiplier tubes (PMTs). PMTs work like a reverse light bulb - instead of putting in electricity to produce light the PMTs collect light from neutrino interactions in our detector and output an electrical pulse. There are two regions of the MiniBooNE detector : an inner light-tight region and an optically isolated outer region known as the veto region, which aids in vetoing cosmic backgrounds.

Neutrinos interact with material in the detector. It’s the outcome of these interactions that we look for. These neutrino interactions in the MiniBooNE detector leave a distinct mark in the form of Cerenkov and scintillation light. Cerenkov light is produced when a charged particle moves through the detection medium with a velocity greater than the speed of light in the medium (v > c/n). This produces an electro-magnetic shock wave, similar to a sonic boom. The shock wave is conical and produces a ring of light which is detected by the PMTs. We can use Cerenkov light to measure the particle’s direction and to reconstruct the interaction vertex. This effect occurs immediately with the particle’s creation and is known as a prompt light signature.

Charged particles moving through the detector also may deposit energy in the medium, exciting the surrounding molecules. The de-excitation of these molecules produces scintillation light. This is an isotropic, delayed light source, and provides no information about the track direction. We can however use the PMT timing information to locate the point, or vertex, where the neutrino interaction occurred.

We can use the patterns of light seen in our PMTs to determine what type of neutrino interacted in our detector. In the charged-current quasi-elastic events, a neutrino interaction in the detector will produce the lepton partner of the neutrino. For example, an electron neutrino interaction will produce an electron, and a muon neutrino interaction will produce a muon. Electrons travel for only a very short time before their velocity falls below the Cerenkov threshold. They multiple scatter along the way, as well. This leaves a fuzzy Cerenkov ring in the detector. Muons tend to travel for a much longer distance. As they travel through the detector they lose energy, and the angle at which the Cerenkov light is being emitted shrinks. Muons also emit scintillation light. The signature of a muon in the detector isn’t one of a ring, as in the case of an electron. It is instead a filled in circle of light. Neutral pions decay into two photons, which then pair produce. The electrons from this pair production each create a ring in the detector.

MiniBooNE is performing a blind analysis. This means that we can either :

• see some of the information in all of the data : we can check the charge per PMT as a function of time, to verify our detector isn’t failing,
• see all of the information in some of the data : we are able to select data sets which will have no oscillation events present, if we assume maximal allowed oscillations from LSND. We can use these data sets to then tune and verify our Monte Carlo simulation.

but we can’t see all of the information in all of the data. Having access to all of the information in all of the data is unblinding. Prior to unblinding we had to have all components of the analysis completely fixed. We aren’t allowed to go back and change event selection cuts or error estimates once we unblind.

Our oscillation analysis can be boiled down to this simple algorithm : determine a set of event selection cuts which will isolate the electron neutrino events but remove the majority of all other events. There are a certain amount of electron neutrino events inherent in the beam, which come from kaon decays. There are also a small amount of other types of events (delta decays, pi0 events) which will pass the electron neutrino cuts, but which are not from true electron neutrino events. The sum of the estimated intrinsic electron neutrino events plus the fake events is the total number of events we expect to see, if no oscillations are present. We compare the number of events observed in data to the number expected, as a function of the reconstructed neutrino energy in these events. If we observe oscillations we should see an excess of data events over the expectation, whose shape will change as a function of the oscillation parameters.

This plot shows the MiniBooNE final sensitivity, compared to the prediction from our 2003 Run Plan. Curves are shown overlaid on the allowed LSND region.

This plot shows the final result from the likelihood analysis. Data points are the black dots. The expected event spectrum is shown in red, and is broken down into the intrinsic electron neutrino and fake event shapes in the green and blue.

We have two separate analyses that are used in the oscillation search : one which depends on likelihood variables, and one which depends on a boosted decision tree. These two analyses have a small overlap in event composition, and provide a good check of each other. We have complete confidence in our analysis if these two analyses find the same result. Before we unblinded our data we had to decide which of the two analyses we would call our primary analysis, for the purpose of quoting numbers in publications. We made this decision based on the expected sensitivity found using Monte Carlo. The sensitivity is the amount of parameter space allowed by the LSND result that we expect to be able to probe. Our sensitivity studies showed that with the likelihood analysis we were able to achieve a sensitivity which agrees quite well with the sensitivity MiniBooNE was designed for. This is something to be quite proud of! We also agreed that the final result quoted for the two neutrino oscillation search would be from 475 MeV to 3 GeV, based on the LSND best fit region.

MiniBooNE unblinded on Monday, March 26th, 2007. When we opened the box we found no evidence for an excess of events over the background prediction. The MiniBooNE neutrino data set agrees with the no neutrino oscillation hypothesis, in the range of reconstructed neutrino energy from 475 MeV to 3 GeV. The probability that MiniBooNE and LSND both are due to two-neutrino oscillations is only 2%. The likelihood and the boosting analyses also agree quite well in measured excess events.

I’m sure by now you’ve noticed that MiniBooNE observes an excess of events in the low energy region. We first saw this excess two weeks ago, when we unblinded. At that point we began working like madmen to determine what those events could possibly be. We’re rechecking our detailed understanding of various low energy background events: interactions in the dirt surrounding the detector, radiative delta decays, low energy neutral current events, you name it, we’re exploring it. Obviously, we don’t want to make any additional comments about this excess until we’re certain that we’ve performed all possible checks on our event predictions in that region. We’re hoping to have this excess mystery resolved in the next few months.

MiniBooNE’s neutrino oscillation analysis was the first of two analyses needed to conclusively explore the LSND result. An anti-neutrino oscillation analysis will also need to be performed. At MiniBooNE it would take many more years to accumulate the data set needed to perform this anti-neutrino analysis. Instead, I’m hoping that we can continue this exploration at the Spallation Neutron Source, at Oak Ridge Laboratory in TN. The SNS is designed to be a world-class neutron facility. One of the side-effects of the process which produces the neutrons is that you get an amazing neutrino beam for free! Funding permitting I’m hoping we can begin taking data at the SNS within 3 to 4 years. At the SNS we could perform neutrino and anti-neutrino measurements simultaneously (no need to switch the horn polarity!), look for oscillations, sterile neutrinos, and look for CP/CPT violation in the neutrino sector.

References

• S. Ahmed et al. [SNO Collaboration], Phys. Rev. Lett. 92, 181301 (2004), arXiv.org/nucl-ex/0309004
• G. Fogli et al., Phys. Rev. D 67, 093006 (2003), arXiv.org/hep-ph/0303064
• A. Aguilar et al. [LSND Collaboration], Phys. Rev. D 64, 112007 (2001), arXiv.org/hep-ex/0104049
• http://www-boone.fnal.gov
• D. Smith, “Calculating the Probability for Neutrino Oscillations”, http://physicsx.pr.erau.edu/Office/oscillations.pdf
• http://en.wikipedia.org/wiki/Cherenkov\_radiation
• http://nevis1.nevis.columbia.edu/heavyion/e910
• The HARP Collaboration, CERN-SPSC/2003-027, SPSC-P-325
• S. Kopp, “The NuMI Neutrino Beam at Fermilab”, arXiv.org/physics/0508001
• http://www.phy.ornl.gov/workshops/nusns/

You may have read here and there about the genteel discussions concerning the status of string theory within contemporary theoretical physics. We’ve discussed it on CV here, here, and even way back here, and Clifford has hosted a multipart discussion at Asymptotia (I, II, III, IV, V, VI).

We are now very happy to host a guest post by the man who wrote the book, as it were, on string theory — Joe Polchinski of the Kavli Institute for Theoretical Physics at UC Santa Barbara. Joe was asked by American Scientist to review Peter Woit’s Not Even Wrong and Lee Smolin’s The Trouble With Physics. Here is a slightly-modified version of the review, enhanced by footnotes that expand on some more technical points.

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This is a review/response, written some time ago, that has just appeared in American Scientist. A few notes: 1) I did not choose the title, but at least insisted on the question mark so as to invoke Hinchliffe’s rule (if the title is a question, the answer is `no’). 2) Am. Sci. edited my review for style, I have reverted figures of speech that I did not care for. 3) I have added footnotes on some key points. I look forward to comments, unfortunately I will be incommunicado on Dec. 8 and 9.

All Strung Out?

Joe Polchinski

The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next. Lee Smolin. xxiv + 392 pp. Houghton Mifflin, 2006. $26.

Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law. xxi + 291 pp. Basic Books, 2006. $26.95.

The 1970s were an exhilarating time in particle physics. After decades of effort, theoretical physicists had come to understand the weak and strong nuclear forces and had combined them with the electromagnetic force in the so-called Standard Model. Fresh from this success, they turned to the problem of finding a unified theory, a single principle that would account for all three of these forces and the properties of the various subatomic particles. Some investigators even sought to unify gravity with the other three forces and to resolve the problems that arise when gravity is combined with quantum theory.

The Standard Model is a quantum field theory, in which particles behave as mathematical points, but a small group of theorists explored the possibility that under enough magnification, particles would prove to be oscillating loops or strands of “string.” Although this seemingly odd idea attracted little attention at first, by 1984 it had become apparent that this approach was able to solve some key problems that otherwise seemed insurmountable. Rather suddenly, the attention of many of those working on unification shifted to string theory, and there it has stayed since.

Today, after more than 20 years of concentrated effort, what has been accomplished? What has string theory predicted? Lee Smolin, in The Trouble With Physics, and Peter Woit, in Not Even Wrong, argue that string theory has largely failed. What is worse, they contend, too many theorists continue to focus their efforts on this idea, monopolizing valuable scientific resources that should be shifted in more promising directions.

Smolin presents the rise and fall of string theory as a morality play. He accurately captures the excitement that theorists felt at the discovery of this unexpected and powerful new idea. But this story, however grippingly told, is more a work of drama than of history. Even the turning point, the first crack in the facade, is based on a myth: Smolin claims that string theorists had predicted that the energy of the vacuum — something often called dark energy — could not be positive and that the surprising 1998 discovery of the accelerating expansion of the universe (which implies the existence of positive dark energy) caused a hasty retreat. There was, in fact, no such prediction [1]. Although his book is for the most part thoroughly referenced, Smolin cites no source on this point. He quotes Edward Witten, but Witten made his comments in a very different context — and three years after the discovery of accelerating expansion. Indeed, the quotation is doubly taken out of context, because at the same meeting at which Witten spoke, his former student Eva Silverstein gave a solution to the problem about which he was so pessimistic. (Contrary to another myth, young string theorists are not so intimidated by their elders.)

As Smolin charts the fall of string theory, he presents further misconceptions. For example, he asserts that a certain key idea of string theory — something called Maldacena duality, the conjectured equivalence between a string theory defined on one space and a quantum field theory defined on the boundary of that space — makes no precise mathematical statements. It certainly does. These statements have been verified by a variety of methods, including computer simulations [2]. He also asserts that the evidence supports only a weak form of this conjecture, without quantum mechanics. In fact, Juan Maldacena’s theory is fully quantum mechanical [3].

A crucial principle, according to Smolin, is background independence — roughly speaking, consistency with Einstein’s insight that the shape of spacetime is dynamical — and Smolin repeatedly criticizes string theory for not having this property. Here he is mistaking an aspect of the mathematical language being used for one of the physics being described. New physical theories are often discovered using a mathematical language that is not the most suitable for them. This mismatch is not surprising, because one is trying to describe something that is different from anything in previous experience. For example, Einstein originally formulated special relativity in language that now seems clumsy, and it was mathematician Hermann Minkowski’s introduction of four-vectors and spacetime that made further progress possible.

Continue reading ‘Guest Blogger: Joe Polchinski on the String Debates’

I first met Chanda (briefly) when she was visiting the University of Chicago as a summer undergraduate research student. Since then we’ve corresponded occasionally about life as a physicist and which general relativity textbook is the best. She emailed me a thoughtful response to a couple of posts about string theory and the state of physics (here and here), and I thought it would be good to have those thoughts presented as a full-blown guest post rather than just a comment; happily, Chanda agreed.

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A few months ago, Sean posted an entry on this blog addressing his concerns about Dr. Lee Smolin’s (then forthcoming) book, The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next. Dramatically titled and well-hyped, Lee’s book was sure to arouse strong emotions and plenty of debate on publication. However, it managed to do that even before it was out, and the commentary on Sean’s entry included correspondence from Lee as well as several other great contemporary thinkers in theoretical physics. The dialogue was inspired, passionate, argumentative, sometimes rude, and always exploratory.

But something was missing. I wondered how there could be a discourse about the marketplace of ideas and about who gets to participate in science without a component that addresses the obvious (at least for those of us with some relationship to the US academic system): the community of scientists in the United States is overwhelmingly homogeneous, white (of European descent) and male. That sounds like a pretty narrow marketplace to me, given that over half of the US population is either female or a member of an underrepresented minority group or both. Surely this must mean that we are under-utilizing our potential talent pool in our drive to better understand the physical world.

As a member of the National Society of Black Physicists’ (NSBP) Executive Committee and Editor of their newsletter, I like to stay on top of the statistics related to these issues, so let me mention a few to satisfy those who like to see data. (All stats are borrowed from the NSF unless otherwise noted.) At the moment, only about 12% of doctoral degrees in physics go to women. The number going to people identified as Black/African-American hovers around an average of 14 per year out of an average 738 total degrees. That’s 1.8% despite making up about 12% of the population. Further investigation uncovers the (to me) monumental tragedy that almost no other field in science and technology is doing worse at diversifying than ours, physics. (See Dr. Shirley Malcolm’s symposium paper from AIP’s 75th Anniversary celebration.)

Knowing all this, I want to share with you how shocking it is to me when I have regular conversations with my peers who express to me that they don’t see a problem. And if they do express concerns to me, a lot of the time it’s guys who want more women in the field because they want to find dates. Sorry guys, we’re here because we’re interested in physics, not you, and on top of that, some of us like women better! And yes, sometimes it’s just a joke, but sometimes it’s hard to tell, and believe me, we’ve heard that one many, many times before. On the topic of seeing more people of color (Blacks, Latina/os, etc.) most often I am met with disinterested silence or an insistence (the knowledge base this derives from is always hazy, in my opinion) that there’s nothing the physics community can do to resolve the issue because the problem is in the high schools and has nothing to do with post-secondary academe.

This attitude is disappointing, to say the least. First of all, the numbers contradict these sentiments. While it is true that there are deeply troubling issues facing the K-12 education system in the US, especially in low-income neighborhoods which are disproportionately populated by people of color, women and other underrepresented groups fall out of the pipeline at all stages, from the post-baccalaureate to the post-doctorate level, and they do so at a much higher rate than white men. Clearly something is happening. What is happening is far too full a topic to tackle here, but perhaps I will be invited to say more about it in the comments section. I invite readers to participate in a knowledge-based discourse about this issue.

On the other hand, if you’re having a hard time figuring out why you should care about diversity, the President of Princeton can offer you a helping hand. In the 2003 Killam Lecture at the University of British Columbia, Princeton University President Shirley M. Tilghman identified four reasons for why we should care about diversity in science. I paraphrase them here:

1. If we aren’t looking at the entire talent pool available, scientific progress will be slower by default.
2. It’s possible that women and other underrepresented minorities will identify unique scientific problems that their majority peers might not.
3. Science will find it increasingly difficult to recruit the brightest minorities as other fields diversify and therefore look attractive to members of underrepresented groups. An attractive work environment is essential to competing on the job market for the best thinkers.
4. The scientific establishment ought to pursue diversification as a matter of fairness and justice.
   In a small (statistically insignificant) survey of various scientists and leaders in scientific organizations, I found that the question of “why is diversity in science important?” is addressed in these four points. While point four is possibly closest to my heart, I think that points one and two are two of the strongest arguments out there. (An aside: As I am tidying up this essay, one professor writes me and says that he finds four to be most compelling! I hope that others will agree.)

I would like to reflect on point one in the context of work in theoretical physics, specifically in quantum gravity and cosmology. If we are to take seriously the concept that what we seek in physics is truth and a better understanding, don’t we want to have the broadest pool of talent available to participate in the process? I think this applies to people and ideas alike. Until we have a theory that pulls out ahead of the others, what are we doing arguing about whose theory is doing better? Right now, neither loops, nor strings, nor triangles, nor anything else has ANY data to back it up, so perhaps the best thing we can all do on that front is get back to work.

An aside to that last remark: It’s hard to get to work when no one will hire you. It remains true that even if I do good work in my field, if my field is not strings, I will have a difficult time finding a