Weighty Spin

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Remember E = mc2? It’s the one equation that you are allowed to include in your popular-physics book (unless you’re George Gamow, who couldn’t be stopped). Mark gave a nice explanation of why it is true some time back, and I babbled about it some time before that. For a famous equation, it tends to be a bit misunderstood. A profitable way to think about it is to divide both sides by the speed of light squared, giving us m = E/c2, and take this as the definition of what we mean by mass. The mass of some object is just the energy it has in its rest frame — according to special relativity, the energy (not the mass!) will be larger if the object is moving with respect to us, so the mass of an object is essentially the energy intrinsic to its state, rather than that imparted by its motion. Energy is the primary concept, and mass is derived from it. Interestingly, the dark energy that makes up 70% of the energy of the universe doesn’t really have “mass” at all, since it’s not made up of objects (such as particles) that can have a rest frame — it’s a smooth field filling space.

qna_190.jpg All of which is to say that the mainstream media have let us down again. C. Clairborne Ray, writing in the New York Times, attempts to explain whether a spinning gyroscope weighs more than a stationary one, and answers “The weight stays the same; there is no known physical reason for any change.” Actually, there is! The spinning gyroscope has more energy than the non-spinning one. As a test, we can imagine extracting work from the spinning gyroscope — for example, by hooking it up to a generator — in ways that we couldn’t extract work from the stationary gyroscope. And since it has more energy, it has more mass. And the weight is just the acceleration due to gravity times the mass — so, as long as we weigh our spinning and non-spinning gyroscopes in the same gravitational field, the spinning one will indeed weigh more.

Admittedly, it’s a very tiny difference — the energy will increase by an amount proportional to the speed of the spinning gyroscope, divided by the speed of light, that quantity squared, which is really tiny. Nothing you’re going to measure at home. (I’m guessing it’s never even been measured in any laboratory, but I don’t know for sure.) And the article is correct to emphasize that there is no difference in mass that depends on the direction of spin of the gyroscope — that would violate Lorentz invariance, which is something worth looking for in its own right, but would be a Nobel-worthy discovery for anyone who found it.

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Guest Post: Juan Collar on Dark Matter Detection

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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!”

Juan Collar 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.

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Sixers!!!!

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My beloved Philadelphia Seventy-Sixers, under the coaching tutelage of local hero Maurice Cheeks, have returned to the playoffs after a two-year absence. It wasn’t easy; they started the season with an ugly 5-13 record, but turned it around late to slip into the seventh seed in the tepid Eastern Conference. Their efforts earned them a series with the Detroit Pistons, a perennial power who finished with the league’s second-best record. Pistons center Rasheed Wallace has played in more playoff games than all 15 members of the 76ers roster combined.

But this afternoon, the plucky Sixers came back from a 15-point third-quarter deficit to beat the Pistons in their first game. That’s why they play the games. In their honor, here is my favorite video of Mo Cheeks, back from when he was coaching Portland a few years ago: chokes me up every time I watch it.

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When I have seen by Time’s fell hand defaced

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April is Poetry Month, just like it was last year. We’re celebrating with Shakespeare’s Sonnet 64, about the Second Law of Thermodynamics.

When I have seen by Time’s fell hand defaced
The rich proud cost of outworn buried age;
When sometime lofty towers I see down-razed
And brass eternal slave to mortal rage;
When I have seen the hungry ocean gain
Advantage on the kingdom of the shore,
And the firm soil win of the watery main,
Increasing store with loss and loss with store;
When I have seen such interchange of state,
Or state itself confounded to decay;
Ruin hath taught me thus to ruminate,
That Time will come and take my love away.
This thought is as a death, which cannot choose
But weep to have that which it fears to lose.

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On Choosing a Graduate School: A Dialogue

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A: Hey, what’s up? You’re looking a little anxious these days.

B: I know. We’re getting close to the romance deadline.

A: The romance deadline?

B: Yeah, in a couple of days I have to decide who I’ll be going out with for the next five years or so.

A: Oh, right, I forgot. Have you decided between boyfriend and girlfriend?

B: I’ve thought about it a lot, and I definitely want a girlfriend.

A: That’s cool. But don’t you worry that the standards are higher if you say you want a girlfriend? I’ve heard that boyfriends are much easier.

B: I heard that, too. But girls are what I’m really passionate about.

A: Couldn’t you just get a boyfriend first, and then switch if you don’t like it?

B: Some people try that, but it can be awkward. Better to just be honest about your intentions from the start.

A: Fair enough. So did you get any acceptances?

B: Yeah, two different women have agreed to date me. Cindy and Alyssa. But I have to choose one.

A: Hey, that’s great that you go two offers. Have you made a choice yet?

B: Well, I had coffee with Alyssa, and we really hit it off — she’s beautiful, and charming, and laughed at my jokes. I definitely think we would get along well over the next few years. I met Cindy, too; she’s a knockout, and clearly very talented, but there wasn’t as much of a spark there.

A: That can happen. So are you going to choose Alyssa?

B: I’m tempted, but the thing is — Cindy’s US News ranking is much higher.

A: Her what?

B: Every year, US News puts out rankings of boyfriends and girlfriends. Now, Alyssa is a solid top-20 girlfriend, but Cindy is top five! I’m really worried I’d be making a mistake by passing up the opportunity to go out with Cindy. Everyone has heard of her.

A: That sounds a little weird to me. How do they come up with these rankings?

B: Nobody knows, really. But everyone takes them very seriously. Still, I keep hoping that the NRC will update their boyfriend/girlfriend rankings soon. Those are supposed to be much more scientific.

A: NRC?

B: The National Romance Council.

A: But look, you seem to have really hit it off with Alyssa. Who cares that US News ranks Cindy higher? The concept of a “boyfriend/girlfriend ranking” just doesn’t make sense — what matters is how well you personally get along with them, not some pseudo-objective measure of excellence.

B: It’s easy to say that, but this is a big decision. I’m really worried that, ten years from now when I’m ready to get married, my prospective spouse is not going to be nearly as impressed that I went out with Alyssa than if I had gone out with Cindy.

A: Come on, it’s five years of your life that we’re talking about here. Your chances of eventually being happily married would seem to be a lot better if you choose someone you’re likely to be happy with right now.

B: You’re right, I know. Well, I hope Cindy won’t be disappointed. I don’t think she’s used to being turned down.

A: Don’t worry. I’m pretty sure she’ll get over it.

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The Truth, Respectfully

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So you’re listening to a talk, and the speaker introduces a crucial step which you know — or perhaps only suspect — to be completely incorrect. What do you do? Do you raise your hand and point out the mistake? Or file it away momentarily, planning to ask them about it in private afterwards?

And does your answer change if the speaker is a senior scientist who will some day be writing you a letter of recommendation? What if it’s a fellow graduate student giving their first-ever technical seminar, and you know them to be intimidated by all these smart people in the room?

A Lady Scientist and PhysioProf have been talking about these issues. The former wonders whether there shouldn’t be some solidarity among grad students not to make each other look bad during journal club presentations, while the latter says that good perceptive critical questions are always in order.

My own attitude is pretty straightforward, and close to PP’s: it’s never impolite or out of order to ask appropriately probing questions about the material being presented at a scientific talk, regardless of the status of the speaker or the audience. It’s science, and we’re all on the same side; it doesn’t do anyone any favors to hide the truth in order to save someone’s feelings. Science is bigger than any of us, no matter how young and inexperienced or old and respected (feared) we may be. Not only should listeners feel free to ask any reasonable question of the speaker, but speakers should be honest enough to admit when they have said something that might be incorrect, rather than twisting around to find justifications for a slip-up. We’ve all made them; or at least I have.

To the extent that there is any sort of competition going on, it should not be “speaker vs. audience,” but rather “all of us vs. the natural world.” However, having staked out that absolutist position, it’s extremely important to recognize that we live in the real world. For one thing, many audience members tend to blur the distinction between “asking a good question” and “being an asshole.” There are people out there, one must admit, who tend to view seminar questions as a venue for them to demonstrate how smart they are, rather than learning about the subject matter in an open and collegial environment. There’s no excuse for that, and the guilty parties deserve to be smacked around, if only symbolically. Still, it’s no reason for the rest of us to equate hard questions with egotistical puffery, nor to soft-pedal questions that really are sincere. The biggest benefit of a talk, from the viewpoint of the speaker, would be to actually learn something from the questions and comments offered by the audience.

The other complication is that there is a competition going on, whether we like it or not. I personally don’t like it, and would vastly prefer to live in a utopia of unlimited resources where such competitions were unnecessary. But in the real world, there is a limited collection of goods — jobs especially, but other rewards of the profession — and a large number of people competing for them. And that competition never turns off. Academics are always judging each other, inevitably, and will use those judgments when it comes time to recommend or hire or give prizes to each other. So a real seminar is not simply a value-neutral examination of the facts; it’s a social milieu, in which interactions have real consequences.

Which is not to say that we should ever shy away from asking hard questions. But there are different ways to ask hard questions, and there’s nothing wrong with choosing the tone in which such questions are asked to match the occasion. Graduate students giving their first seminars need to learn that they will get asked tough questions, and that it’s okay — it’s not a devastating critique of their worthiness as scientists, it’s simply part of the process to which we are all ultimately subject.

A common technique to help students ease into the responsibility of giving talks is to have students-only seminars where the faculty are not permitted. The motivation for such things is admirable, but ultimately I don’t think they are a good idea. (As a disgruntled senior colleague once said, “Sometimes I learn something from listening to the students.”) Breaking down the barriers between “faculty” and “students,” and beginning to think of everyone as “researchers” and “colleagues,” should be an important goal of graduate school. It can all be intimidating at first, but it’s ultimately beneficial to learn to treat these artificial hierarchies as administrative annoyances, not natural categories.

The most successful graduate students are the ones that start thinking of themselves as colleagues right away. Go to the seminars, sit in the front, ask good questions, participate in the informal discussions afterward. It’s a big universe out there, and we’re all struggling to understand it, and working together is our only hope.

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Energy Doesn’t Grow on Trees

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Funny thing about energy: it’s conserved! At least when the spacetime background is time-translation invariant, which is a very good approximation here in the Solar System. We bring you this reminder because a knowledge of basic physics can occasionally be helpful when formulating public policy.

ethanol.jpg In particular, biofuels (such as ethanol) and hydrogen are not actually sources of energy — given the vagaries of thermodynamics, it costs more energy to create them than we can get by actually using them, as there will inevitably be some waste heat and entropy produced. Almost all of the useful energy we have here on Earth comes ultimately from nuclear reactions of one form or another — either directly, from nuclear power plants, or indirectly from fusion in the Sun. There is of course direct solar power, but even fossil fuels and biofuels are simply storage systems for energy that can be traced eventually back to sunlight. The question is, what is the best way of capturing and using that sunlight — where “best” is going to be some interesting function of cheapest, cleanest, most easily transportable, and most sustainable.

People seem to be gradually catching on to the fact that biofuels are an especially wasteful and dirty energy storage system. Paul Krugman devoted a column the other day to how ethanol is a boon to Archer Daniels Midland, but terrible for the world’s food supply. (We told you the Farm Bill was a travesty.) And Time has published a cover story on the “Clean-Energy Scam.”

Propelled by mounting anxieties over soaring oil costs and climate change, biofuels have become the vanguard of the green-tech revolution, the trendy way for politicians and corporations to show they’re serious about finding alternative sources of energy and in the process slowing global warming. The U.S. quintupled its production of ethanol–ethyl alcohol, a fuel distilled from plant matter–in the past decade, and Washington has just mandated another fivefold increase in renewable fuels over the next decade…

But several new studies show the biofuel boom is doing exactly the opposite of what its proponents intended: it’s dramatically accelerating global warming, imperiling the planet in the name of saving it. Corn ethanol, always environmentally suspect, turns out to be environmentally disastrous. Even cellulosic ethanol made from switchgrass, which has been promoted by eco-activists and eco-investors as well as by President Bush as the fuel of the future, looks less green than oil-derived gasoline.

As an uneducated guess, I would imagine that in the medium run the world will have to turn to (Earth-based!) nuclear power for its energy needs. In the longer run, solar will be the way to go, although the amount of solar power we can reasonably collect here on Earth is somewhat limited. We’ll likely have to solve the problem of how to efficiently beam power down from orbit, after which we can build big million-square-kilometer solar power collectors in space. Not in my lifetime, I would bet.

Eventually the Sun will run out, of course. But there are other Suns. In the even longer run, once all of the stars have run out and we are all virtual processes running on a computer, perhaps we can tap into the Hawking radiation from the supermassive black hole at the galactic center. Once that is gone and the universe has settled into empty de Sitter space, we’ll be in thermal equilibrium. At that point there’s probably little hope, no matter what optimists like Freeman Dyson might tell you.

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You Would Have to Be Destroyed

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Trevor Paglen has written a fun book, peeking discreetly into the “black world” of secret military projects by reproducing the patches worn by workers on the projects. The patches are surprisingly artful and whimsical, often invoking wizards and dragons to heighten the aura of mystery. Here is a typical example:

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In this case, the motto has proven accurate, as Paglen was unable to figure out what unit was associated with the patch. Probably has nothing to do with pornographic movies, but you never know.

The staging areas for many of the secret operations are in the Southwest U.S., including the Area 51/Groom Lake facility where conspiracy theorists are convinced that the government is harboring alien technology from the crash at Roswell. Which, as you might imagine, makes for great source material for the patches. This one comes from the 509th Bomb Wing, in charge of the B-2 Stealth Bomber — the 509th used to be based in Roswell, although it has now moved to Missouri.

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“Gustatus Similis Pullus” translates from Latin as “Tastes Like Chicken.” Get it? “To Serve Man?” If not, there is a subtle knife and fork on the patch just to drive home the message.

Other times, the emphasis on secrecy is more overt. This patch is from the 22nd Military Airlift Squadron, which would fly C-5’s to deliver classified aircraft from aerospace plants in Southern California to testing facilities around the country.

2006_noyfb_patch.jpg

“NOYFB,” in case you were wondering, stands for “None of your fucking business.”

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