Science

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.

Greetings from sunny Palo Alto, California, where we’re having the 2005 incarnation of the SLAC Summer Institute, Gravity in the Quantum World and the Cosmos. It’s an annual two-week school, aimed primarily at graduate students in physics, covering topics of interest to SLAC (the Stanford Linear Accelerator Center). Until recently, gravity didn’t qualify as something of interest to anyone working at a particle accelerator, but times have changed — gravity was the subject of SSI in 1998, and again this year. These days, considerations of dark energy, extra dimensions, and string theory are of direct interest to particle physicists.

I got to speak first, giving a three-hour General Relativity Primer. For lecture notes, we handed out my No-Nonsense Introduction to General Relativity that’s been on the web for a while; the online transparencies were scanned in from the actual lectures I gave. The idea was to give a complete intro to GR (metrics, geodesics, tensors, covariant derivatives, curvature, Einstein’s equation, some solutions), aimed at physics graduate students who hadn’t been exposed to GR. (Sadly, there are still plenty of physics grad students who have never been exposed to GR.)

I really wanted to give a blackboard talk, but the organizers talked me out of it, claiming (correctly, as it turns out) that blackboard in SLAC’s lecture hall is practically unreadable. But I didn’t want to use powerpoint, as it’s nearly impossible to move at a pedagogically appropriate pace when you speak from pre-made slides. As a compromise, I wrote the transparencies in real time as I lectured. It worked okay, although I realized that the main test of endurance wasn’t talking for three hours, but rather staring into the light of an overhead projector for three hours as I was writing the transparencies. In the afternoon I fielded questions for another two hours at a discussion section, so the organizers squeezed their money’s worth from me.

Yesterday morning we heard from Alessandra Buonnano on gravitational waves and Gabriella Gonzalez on actual gravitational-wave detectors, such as the LIGO observatory. All seems to be going well at LIGO, and Gabriella mentioned that they’ve even detected something — but it turned out to be an airplane flying overhead. We’re still waiting for the direct detection of an honest gravitational wave.

In the afternoon, we had Ken Nordtvedt talking about testing GR by bouncing lasers and radar signals around the solar system, and Shane Larson talking about the LISA mission. LISA will (assuming all goes according to plan) consist of three satellites flying in formation five million kilometers apart, measuring passing ripples in the geometry of spacetime by bouncing lasers back and forth. What’s that you say? You think the satellites should feature more powerful lasers, and be located twenty million kilometers from each other? Shane has set up a sensitivity curve generator, allowing you to determine how the noise limits of the satellite will change as a function of such parameters. Once you’ve hit on your favorites, it’s up to you to convince NASA to go along.

You know how, when you take snowboarding lessons, they teach you to look in the direction you want to go, not at obstacles you want to avoid? Well they do, and it’s good advice — keep looking at that tree and your subconscious will steer you right into it. Works for driving, too.

I’m not sure if it’s the same psychological phenomenon, but this is what I was reminded of when reading a post by Chris at Mixing Memory about some puzzling psychology experiments. Apparently, just being exposed to words expressing the concept of rudeness is enough to make people behave more rudely.

Bargh, Chen, and Burrows set out to demonstrate the existence of “automatic social behavior.” They conducted three experiments, each targeting different behaviors. In the first experiment, they first gave participants a scrambled sentence test, which involves presenting five scrambled words and asking the participant to form a grammatically correct sentence out of four of them as quickly as possible. They developed three different lists of scrambled words, one of which primed the concept RUDE, another that primed POLITE, and a third that was neutral with respect to rudeness/politeness. The primes in these lists included adjectives, adverbs, or verbs that were associated with the concepts (e.g., brazen, aggressively, or disturb for RUDE, and considerate, patiently, and respect for POLITE). While the participant was completing the scrambled sentence test, the experimenter left and began talking to a confederate (an experimenter posing as another participant). When the participant finished, he or she came out of the room to look for the experimenter to receive instructions for the next task (as the experimenter had instructed). However, the participant always found the experimenter talking to the confederate. Bargh et al. then measured the time it took for the participant to interrupt the conversation between the experimenter and the confederate.

Guess what they found. Of the participants who did the RUDE version of the sentence test, more than 60% interrupted in under ten minutes (they cut it off at ten minutes — can you imagine how frustrated some of those participants were after standing there for ten minutes?), whereas fewer than 20% of the POLITE-primed participants interrupted in that time. The neutral list participants were in between at around 40%. The RUDE participants also interrupted a full 3 minutes sooner than neutral participants, and almost 4 minutes sooner than the POLITE participants.

As Chris says, it gets weirder. My favorite was how people move more slowly after reading words associated with older people. So how do you think our personalities are affected by reading too many blogs?

Last week in Paris, I walked along the north-south line connecting the Observatoire de Paris to the Palais du Luxembourg. A line of longitude: in fact, the line of longitude, if the French had had their way a little over a century ago. A politico-scientific battle was being fought in the late nineteenth century over the location of the Prime Meridian. Parisians, thinking only of considerations of nature and philosophy, argued that the line of zero longitude should go through l’Observatoire; the rest of the world, crass materialists that they were, noted that over seventy percent of the world’s shipping was already using Greenwich (nine minutes and twenty-one seconds to the west of Paris) as its standard of longitude. The French lost out to the British, prefiguring a similarly heated tussle over who would host the Olympic Games over a hundred years later.

These issues figure prominently in the book I was reading during my trip, Peter Galison’s Einstein’s Clocks, Poincare’s Maps: Empires of Time. It is a paradigmatic example of a engaging work of intellectual history, as it has a definite theme that is at once simple, interesting, and true. Einstein and Poincare, the obscure German theoretical physicist and the celebrated French mathematician and philosopher, were pivotal figures in the development of the special theory of relativity, whose centenary we are celebrating this year. Relativity has a reputation as an esoteric theory, and Einstein and Poincare are often thought of as abstract thinkers divorced from mundane matters of technology and experimentation. Galison argues convincingly that these thinkers’ practical concerns with the measurement of time — Einstein judging clock designs at his patent office in Bern, Poincare as President of the Bureau of Longitude — were in fact crucial to their recognition of the need for a new understanding of the fundamental nature of time itself.

In a Newtonian universe, time is universal — the amount of time elapsed between two events is precisely and uniquely defined, even if the events are widely separated in space. It may be difficult to actually measure the time between events, and this task was a constant preoccupation of nineteenth-century astronomers, surveyors, politicians, and businessmen. It’s easy enough to use the sun to determine your local time, but the advent of railroads made it necessary (as several unfortunate accidents proved) to sensibly coordinate time among far-flung locales, a program that eventually led to our current system of time zones. In the course of standardizing time across broad expanses of geography, it became clear that synchronization was an operational concept — you had to bounce some signals back and forth between locations, and taking into account the travel time of the signals themselves was of primary importance. Poincare’s work on longitude was intimately connected to precisely this problem, as was Einstein’s experience with novel clock designs. (At one point subterranean Paris featured tubes that would carry pulses of compressed air from a central station to clocks throughout the city, which would use the pulses as reference standards to guarantee as precise a degree of synchronization as possible. Einstein would have seen numerous proposals for electrical versions of such schemes.)

By itself, the need to synchronize time via exchanged signals does not lead you to relativity; it is equally characteristic of Newtonian absolute time. But when combined with the principle of relativity and the invariance of the speed of light, this insight led Einstein to understand that the notion of simultaneity of distant events is not universal, but depends on one’s frame of reference. (In general relativity, in which spacetime is curved, we need to go even further — the notion of simultaneity is not simply frame-dependent, it is completely ill-defined.) Time goes from being an absolute characteristic of the universe to something individual and personal, a measure of the distance traversed by a particular object through spacetime. Poincare (following Hendrik Lorentz) had worked his way to similar conclusions, but it was Einstein who showed how to completely abandon the absolute Newtonian time that other physicists felt still lurked unobserved in the background.

Did someone say that scientists are individual idiosyncratic human beings? Gleaming mathematical edifices like the special theory of relativity can give the impression of having dropped from the sky; it’s nice to be reminded of the messy contingent ways that real people happen to stumble upon them.