Congratulations to Vikram!

Another young scientist joins the ranks of credentialed scholars. Congratulations to Vikram Duvvuri, who just defended his Ph.D. thesis on “Modified Gravity as an Alternative to Dark Energy”!

This was not an easy one, through no fault of Vikram’s — a certain member of his thesis defense committee got stuck on the East Coast, and had to phone in, after numerous delays. But Vikram kept his cool throughout all of the tense drama, and made it through the defense itself unscathed.

The thesis was based on two papers on which I was a collaborator: Is Cosmic Speed-Up Due to New Gravitational Physics? with Mark Trodden and Michael Turner, and The Cosmology of Generalized Modified Gravity Models with the same authors plus Antonio De Felice and Damien Easson. The idea is to explain the observed acceleration of the universe by modifying gravity rather than introducing dark energy. That is to say, we look out into the universe and see that distant galaxies are accelerating away from us. In the context of Einstein’s general relativity, that can’t happen in a universe consisting of ordinary matter and radiation — we need some form of energy density that persists, rather than dissipating away, as the universe expands. So we fit the data by imagining that about 70% of the universe is some exotic dark energy, perhaps a cosmological constant, that is smoothly distributed through space and nearly-constant in time.

But the other possibility is that Einstein was wrong, and we need to modify general relativity on cosmological scales. There are various ways to do this; one that seems potentially viable is a brane-world construction by Dvali, Gabadadze, and Porrati. Our approach was to ask for the simplest possible modified-gravity model that would make the universe accelerate. So we stuck with four dimensions, no new fields, and just played with the dynamics of the spacetime metric.

Einstein’s equation for general relativity can be derived by minimizing an action, where the action is simply the integral over spacetime of the curvature scalar R. We wanted a new action that looked like Einstein’s when R was large, but looked different when R was small, as in the late universe. So we did the obvious thing: replaced R with R+1/R in the action. Vikram’s thesis was an examination of this model and some more complicated variations on the same theme.

Sadly the original model doesn’t quite work; as noted by Chiba, it is ruled out by tests of gravity in the Solar System. That’s basically because our theory introduces a new degree of freedom. In the weak-field limit, general relativity is a theory of a massless spin-2 particle, the graviton. Our modified action turns on a new degree of freedom, which is a massive (tachyonic) spin-0 particle. This turns out to be fairly generic; if you mess with Einstein’s theory by adding new terms to the action, it almost always happens. Thus, a lesson is learned: general relativity is hard to mess with without running into conflict with experiment. But such messing can nevertheless lead to useful outcomes, like a new doctorate!

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John Bahcall

Sad to hear that John Bahcall passed away on Tuesday. Here is an email from the Institute for Advanced Study.

From: Peter Goddard
Subject: Sad news

To the Institute Community,

It is with regret that I share the sad news of the passing yesterday evening of Professor John Bahcall.

John was the Richard Black Professor in the School of Natural Sciences, and had been with the Institute since 1968, when he arrived here as a Member. He was appointed to the Faculty in 1971.

Many of you are familiar with John and his distinguished career, which is marked by work on models of the Galaxy, dark matter, atomic and nuclear physics applied to astronomical systems, stellar evolution, and quasar emission and absorption lines. John was an expert on the elusive form of radiation known as neutrinos, and was involved for many years with NASA’s Hubble Telescope Working Group.

John was truly a pioneer, who made lasting contributions to the field of astrophysics. He will be greatly missed, and we extend our deep sympathy to his wife Dr. Neta Bahcall and their children Safi, Dan and Orli.

Peter Goddard

Bahcall was known primarily, of course, for his work on the solar neutrino problem. He was the theorist who calculated the expected number of neutrinos that the Sun should be emitting; he worked very closely with Ray Davis, the experimenter who first demonstrated a deficit of solar neutrinos reaching the earth. Davis was awarded the Nobel Prize in 2002, and many people (myself included) were very surprised that Bahcall didn’t share the award.

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The curve of binding energy

Okay, more than one person has sent me this, and it’s even appeared on Daily Kos, so I suppose we must give it a hearing. The BBC is reporting that physics is the key to love. Well, who didn’t know that?

Successful couples are said to have chemistry, but a study by an Oxford graduate suggests that dating may actually have more to do with physics.

Richard Ecob adapted a system for modelling atoms in radioactive decay to investigate how we look for partners.

He found that “super daters”, people who have many short relationships, have a good effect on others’ lives.

This is because they break up weak couples, forcing their victims to find better relationships.

That’s right, folks: when some suave Cassanova or Cassanovette steals your honey from you, it’s all for the greater good, as you will be forced to find a better relationship. It must be true, physics says so.

To model the phenomenon, he wrote a computer program which placed “software singles”, people seeking partners, in an imaginary social network.

Each single had a set of interests, which they also looked for in potential partners.

The research suggested that multiple daters, those who form many relationships, were less effective at finding the right partner than those who remained in one place and let others come to them.

Something tells me that a lot of this research falls in the category of a “thought experiment.”

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Wagging fingers

Eugene Volokh has inspired a useful and uplifting blogosphere meme: condemnation of groups of people whom, although nobody is claiming that they are numerous or influential, we can nevertheless agree are worthy of our scorn. (Indeed, the search for actual examples hasn’t been very fruitful.) His own entry in this game was “Westerners who side with the Iraqi resistance.” As he explains (after some prodding),

Fortunately, the group being criticized is not a vast group. So? They’re still worth condemning.

Capital idea. Next to join in was Belle Waring:

I points the fingerbone of scorn at those inhumanly cruel Republicans who drink puppy blood for breakfast. When I consider the sharp, tiny milk-teeth of those puppies, protruding from gums now white with blood loss, I am filled with a righteous and long-abiding anger.

Again, she is quick to note that there are very few Republicans who fit such a description, but they should surely be denounced. (Lindsay Beyerstein, playing the contrarian, denounces Belle’s denunciation, but is clearly just trying to score some intellectual-virtue points.)

Since one can never denounce such heinous activities too fervently, Brad DeLong chimes in:

I for one, would like to also denounce adherents of the Republican Party who pretend to “adopt” kittens from animal shelters, and then kill them and dissect their little kittenish bodies with knives. I acknowledge that rather few Republicans are in this category, but I insist that these people are very bad.

How true that is.

Not to pile on, but I can’t help but offer my own humble contribution to the rare-but-worthy-of-scorn category. To wit, we should condemn Republicans who attempt to justify the capture and long-term detention of prisoners who are denied counsel and not charged with any crime, and then tortured, sometimes to death, in a misguided attempt to extract useful intelligence from them, even though they may be perfectly innocent. Likewise, Republicans who make fun of such practices by selling witty T-shirts. Oh, and those who advocate public torture of criminals in order to satsify the public’s bloodlust — wouldn’t want to forget them.

Of course, nobody would suggest that such people comprise a vast group. So? They are still worth condemning.

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Rumors of new forces

Eric Adelberger, leader of the experimental gravity group at the University of Washington, left a comment in the discussion about new forces, which is worth elevating to the front page here:

Please don’t get too excited yet about rumors concerning the Eot-Wash test of the 1/r^2 law. We can exclude gravitational strength (|alpha|=1) Yukawa violations of the 1/r^2 law for lambda>80 microns at 95% confidence. It is true that we are seeing an anomaly at shorter length scales but we have to show first that the anomaly is not some experimental artifact. Then, if it holds up, we have to check if the anomaly is due to new fundamental physics or to some subtle electromagnetic effect that penetrates our conducting shield. We are now checking for experimental artifacts by making a small change to our apparatus that causes a big change in the Newtonian signal but should have essentially no effect on a short-range anomaly. Then we will replace our molybdenum detector ring with an aluminum one. This will reduce any signal from interactions coupled to mass, but will have little effect on subtle electromagnetic backgrounds. These experiments are tricky and measure very small forces. It takes time to get them right. We will not be able to say anything definite about the anomaly for several months at least.

I suppose we have to go along, although it’s hard to enforce levels of excitement. More importantly, it’s precisely because these experimenters are so careful that we’ll have every right to be very excited if they ultimately announce that they’ve really discovered a deviation from Newton’s force law!

While we’re waiting, here’s a great review article about tests of gravity at short scales. If you read it now, you’ll be all ready to understand new results as they come in.

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Stories about Nature

As predicted, we had a great time (as it were) talking about the nature of time last night at the Museum of Contemporary Art. Antonia and David gave great presentations, Gretchen moderated with aplomb, and Angel Ysaguirre and the rest of the Illinois Humanities Council crew organized the whole thing with practiced professionalism.

I was happy to have a chance to catch up with David, who has become semi-famous for his appearance in the movie What the #$*! Do We (K)now!? (known informally as “What the Bleep?”). This movie was a travesty of a docu-drama, the basic gist of which was to push on an unsuspecting public certain New Age ideas about how quantum mechanics allows human consciousness to affect reality. David was interviewed by the filmmakers for hours, in which he patiently explained that everything they were saying was wrong. They then sliced his words to make it look like he was agreeing with the spirit of the movie, creating a willful misrepresentation of his views in the final product.

But since I last saw David in December, he had attended an event in Santa Monica in February featuring all of the speakers from the movie (such as Ramtha, a 35,000-year-old warrior spirit who is available for consultation for an appropriate fee). Although billed as a “conference,” it was really an excuse to sell expensive tickets to hundreds of gullible New Agers. The conference organizers were a different group from the filmmakers, who belatedly informed them that this was one person they should have left off the guest list — but too late.

After some hesitation, David decided to go, and thought very carefully about the talk he would give. I can’t do justice to the precision with which he worded his presentation, but the basic message was essentially this: “When you are trying to figure out how the world works, there are two ways to proceed. One is to invent a story about Nature which serves to say something flattering about yourself. The other is to listen to the story that Nature itself tells, no matter what it may turn out to be. What you are doing is the former; science is the latter.”

He was aiming specifically at pseudo-scientific mysticism, but I can’t think of a better characterization of the really fundamental difference between science and religion. There are differences in methods, and of course there are differences in results. But the most important distinction is in the initial attitude one takes toward the world. Real scientists will take what Nature tells them, and make sense of it as honestly and courageously as they can, regardless of what it says about their own place in the cosmos. If there was one lesson that we could spread through science education, that would be my choice.

The punchline was the response from the California audience. The other personalities on the speaker list were of course outraged, and attacked David in increasingly strident tones. But the audience, after the initial shock wore off, quickly took his side. Not really because they had become convinced of the superiority of reason and evidence to mysticism and quackery, but because they had transferred their reverence from the modern-day shamans to the philosophy professor from Columbia. They had found a new guru, who spoke more convincingly than the old ones. The more important lesson, that finding the right guru isn’t really the path to enlightenment, remained elusive.

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Relative importance

I want to say more about those rumored forces mentioned in JoAnne’s post, but tomorrow (Thurs) I’m giving a presentation on Time’s Arrow, and, ironically, I’m running out of time. The event itself should be great fun. It’s sponsored by the Illinois Humanities Council, and will be held at the Museum of Contemporary Art. I’ll talk about time in Einstein’s universe for half an hour, and then we’ll have responses from philosopher David Albert and artist Antonia Contro. Preposterous Universe readers will remember David from my report on a meeting last December. Antonia, in addition to being a talented artist, is the executive director of Marwen, a non-profit organization devoted to teaching disadvanaged youths about art. The moderator will be former Preposterous guest-blogger (and occasional radio host) Gretchen Helfrich, and I’m sure it will be a blast.

The event is sold out, but at some point it will be televised on the Illinois Channel (“like CSPAN for Illinois”). You can also check out two previous events in the Humanities Council’s celebration of the Einstein year: Peter Galison on Einstein and Poincare, and Janna Levin and Rocky Kolb on cosmology.

Here’s a teaser for my talk. How important is the notion of “time,” anyway? I did the obvious thing — I asked Google. So here is the number of search results returned when you search Google for various important concepts.

  • space:                   422,000,000 pages found
  • money:                 262,000,000 pages found
  • fun:                       173,000,000 pages found
  • love:                     170,000,000 pages found
  • sex:                       76,400,000 pages found
  • peace:                   89,900,000 pages found
  • war:                     179,000,000 pages found
  • harry potter:         20,900,000 pages found
  • time:                   972,000,000 pages found

Good news there about love vs. sex. Not so much about peace vs. war. But the important thing is, “time” kicks the rest of the concepts’ collective butts, with nearly a billion pages found. Yet another reason I should get a raise.

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Enlightenment

Things have been busy, but at some point I hope to stop just linking and start actually writing something. In the meantime, why not link to something profound?

Enlightenment is man’s leaving his self-caused immaturity. Immaturity is the incapacity to use one’s own understanding without the guidance of another. Such immaturity is self-caused if its cause is not lack of intelligence, but by lack of determination and courage to use one’s intelligence without being guided by another. The motto of enlightenment is therefore: Sapere aude! Have courage to use your own intelligence!

— Immanuel Kant, in “What Is Enlightenment?”, 1784

Is it too cynical to think that the anti-science attitude on the part of our government is part of a bigger picture, a roll-back of rationality itself? Shakespeare’s Sister examines the evidence, and concludes that it’s not too cynical at all.

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