This week I served on an oral exam committee for a thesis proposal in experimental particle physics (nice job Elisabetta). All went extremely well, and I was able to ask a few (I hope useful) questions, and also witness the way in which the people closer to the subject matter – the experimentalists – questioned their candidate. One thing I took away from this experience was a renewed admiration for the extremely hands-on way in which experimentalists, particularly those working in a subject that continually challenges one’s intuition, understand the concepts and quantities they deal with.

As a specific example, one question concerned how far various particles traveled from a primary interaction vertex in a detector. Obviously, a correct answer to this question requires the knowledge of an awful lot of physics. However, there are rough estimates one can do knowing a few simple facts such as the speed of light. Of course, we all know the speed of light, which we denote as c. Most of us physicists first learned that c is about 3 times 10⁸ meters per second. If you are in my field you are more likely to use different units; namely those in which c=1. However, neither of these choices of unit is particularly suited to calculating something useful for a collider experiment or, indeed, to making an on the fly estimate of a human-sized quantity.

The experimentalists in the room all use, of course, standard sets of units familiar to us all. However, they keep in their heads a bunch of handy human-sized versions, that just aren’t part of my (and I suspect many theorists’) usual way of thinking. In the case above, the relevant example is that light travels one foot per nanosecond (not metric, I know, but one meter per 3.3 nanoseconds somehow doesn’t have the same ring to it). I know the conversion takes hardly any time, and I know this isn’t a particularly scientifically deep piece of knowledge, but I think having a human-scale idea of uncommonly large physical numbers provides a very nice feel for the concept that just isn’t captured by the ways in which we normally, abstractly, think of them.

So I’m interested to know what other common sense statements of uncommonly large or small physical quantities our wise and worldly readers might have at their fingertips. Feel free to chime in in the comments.

We’ve all become familiar with contrails — the cloud tracks that planes leave as they fly through certain altitudes:

But, did you know that ships apparently do the same thing? (I sure didn’t!)

UW professor of Atmospheric Sciences Cliff Mass (author, and weather blogger) has a nice post up discussing the phenomena. Both contrails and “ship trails” are produced when microparticulates released by combustion serve as seeds for condensation. Over the ocean, the typical droplet size is much smaller in ship exhaust than in the natural cloud cover, producing different reflectivity, leading to high contrast white ship trails. As Cliff discusses, you can imagine the interest that national security agencies might have in this effect….

(EDIT: Actually, it turns out that contrails are primarily due to vapor released during combustion, not nucleation, so the processes are somewhat different.)

One of the big questions for people who believe in extra dimensions is: Why don’t we see them? Sure, we have methods for hiding them, usually by making them really tiny, but then we need to ask: Why are they tiny?

Matt Johnson, Lisa Randall and I just came out with a paper that takes a partial stab at this question: Dynamical Compactification from de Sitter Space. (And a similar-sounding paper came out the same day from Jose Blanco-Pillado, Delia Schwartz-Perlov, and Alex Vilenkin.) It’s an intriguing idea, if I do say so myself: starting with nothing more complicated than a higher-dimensional spacetime with a positive vacuum energy and an electromagnetic field (or a higher-dimensional generalization thereof), you will automatically get quantum fluctuations into lower-dimensional spacetimes! If we really believe in extra dimensions, we need to understand how regions with different effective dimensionalities are cosmologically related, and this is a step in that direction.

Normally I’d blog all about it, but on this occasion we’re outsourcing to a guest blogger. My collaborator Matt Johnson is a postdoc at Caltech, and before that was a grad student at UC Santa Cruz, where he worked with Anthony Aguirre — a previous guest-blogger of ours! We like to keep things in the family.

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Extra dimensions. Sounds preposterous at first. Well, perhaps more accurately, it sounds preposterous to most people who don’t do high-energy theory. But, really I assure you, there are many well-motivated reasons why us wacky theorists like to ponder the existence of extra dimensions.

For one, as shown long ago by Kaluza and Klein, it is possible to get Maxwell’s equations of electromagnetism in four dimensions by taking 5 dimensional General Relativity and wrapping one of the spatial dimensions up in a circle too small to see. The smaller the circle is, the harder it is to move in this “other direction,” and so there is no danger in getting lost on the way home. In this way, Maxwell’s equations have an elegant geometrical origin and gravity and electricity & magnatism are combined into one force (5 dimensional gravity).

Another strong motivation comes from string theory, which is only a consistent quantum theory of gravity if there are 10 or 11 dimensions in total. Again, since we don’t see them, it is necessary to hide the existence of the extra dimensions. Inspired by the fact that it was possible to hide one extra dimension by wrapping it up in a circle, generally the extra 6 or 7 dimensions are thought to be “compactified” into a very small compact geometry like a sphere or a torus.

At this point, the five-year-old in the audience is insistently asking, “If you have all these extra dimensions, and you are telling me that they are wrapped up into this tiny ball, how did they get wrapped up in the first place? Why are the four dimensions we see so large, and the others so small?”

After nearly a century of thinking about the existence of extra dimensions, there are surprisingly few plausible answers to this very simple question. One of the few answers was proposed by Brandenberger and Vafa. They studied the thermodynamics of strings in a torus-shaped hot early-universe, and found that miraculously it is favorable for only four of the dimensions to become large. Pretty nice, if the universe is a torus and all the dimensions started out small and compact. But, it would be nice to have some alternatives in case this turns out not to be viable.

Sean Carroll, Lisa Randall, and I recently wrote a paper that revisits the five-year-old’s question. We wanted to start with the very simplest model that has extra dimensions and solutions in which some of them can be compactified. A minimal set of ingredients needed to accomplish this includes 1) D-dimensional gravity, 2) a positive D-dimensional cosmological constant, and 3) a (D-4)-form gauge field (think E&M, but with more indices). This theory has long been known to have solutions where 4 of the dimensions are non-compact and (D-4) of them correspond to directions on a sphere, whose size is stabilized by the energetics of curvature and a background Electric or Magnetic field.

More interestingly, we showed that some of the spacetimes that are solutions to this theory contain a four-dimensional universe that lives behind the event horizon of an extended object, a “p-brane” or “black brane,” that is embedded in a background D-dimensional spacetime. Moreover, there are mechanisms that dynamically give rise to such objects, thanks to the magic of quantum mechanics, and this leads to an explanation for why some number of extra dimensions became compact!

Sounds complicated, but you can actually go a long way towards understanding what we did by considering plain-old four dimensional black holes. (more…)

Because of the growth of entropy, we have a very different epistemic access to the past than to the future. In retrodicting the past, we have recourse to “memories” and “records,” which we can take as mostly-reliable indicators of events that actually happened. But when it comes to the future, the best we can do is extrapolate, without nearly the reliability that we have in reconstructing the past.

However — the human brain, as most readers of this blog probably know, was not intelligently designed. It’s doesn’t have the high-level structure of a computer program, where all the processes are carefully planned to achieve some goal. (The lower-level structures share the mechanical features of any other physical system, but that’s of little help here.) Evolution nudges the genome in useful directions, but it can only work with the raw materials it’s given; it doesn’t have the luxury of starting from scratch. So over and over in biological organisms, we find features that were originally developed for one purpose being re-engineered for something else.

As it turns out, the way that the human brain goes about the task of “remembering the past” is actually very similar to how it goes about “imagining the future.” Deep down, these are activities with very different functions and outcomes — predicting the future is a lot less reliable, for one thing. But in both cases, the brain goes through more or less the same routine.

That’s what Daniel Schacter at Harvard and his friends have discovered, by doing functional MRI studies of brains subjected to different kinds of cues. (Science News report, Nature review article, Charlie Rose interview.) Subjects are inserted gently into the giant magnetic field, then asked to either conjure up a memory or imagine a future scenario about some particular cue-word. What you see is that the same sites in the brain light up in both cases. The brain on the left in this image is remembering the past — on the right, it’s concocting an imaginary scenario about the future.

Further confirmation comes from studies of amnesiacs, who famously can’t remember the past. But if you ask the right questions, you find that they also have significant problems imagining their own future.

We tend to assume that the brain must be like a computer — when we want to access a memory, we simply pull up a “file” stored somewhere on the brain’s hard drive, and take a look at its contents. But that’s not it at all. Schacter believes that pieces of data relevant to any particular memory — times, images, sounds — are stored piecemeal in different parts of the brain. When we want to “remember” something, another part of the brain assembles these pieces into a (hopefully) coherent picture. It’s like running a new simulation every time you need a memory, and it’s the same thing we do when we try to imagine some event in the future.

Everyone has heard that memories can be unreliable, but many of us don’t appreciate the extent to which that is true. It’s not the case that “real” memories are stored once and for all deep in the darkest recesses of the brain, and it’s just a matter of digging them up. False memories — conjured from any number of sources, from gradual embellishment to direct suggestion by others — seem precisely as vivid and real to us as accurate memories do. For a good reason: the brain uses the same tools to construct the memory from the available raw materials. A novel and a history book look the same on the printed page.

My colleague Mark Devlin is the Principal Investigator of the Balloon-borne Large-Aperture Submillimeter Telescope (BLAST). BLAST is a terrific experiment that does a number of different things, including studying the history of galaxy formation through measuring the cosmic infrared background produced by star-forming galaxies. Over at In the Dark, Peter Coles has also discussed BLAST, since the Cardiff group are a big part of the experiment.

This week has been a big one for BLAST, seeing the publication of a host of papers, including a major results paper appearing in Nature.

In addition, the science writer for the Philadelphia Inquirer – Faye Flam – wrote a nice article about the experiment, describing not only the results, but the compelling and dramatic story associated with the recovery of the data. Remarkably, Mark’s brother, Paul, who is a documentary film maker, was covering the experiment when all the excitement happened, and the result is a riveting movie titled, not surprisingly, BLAST!, that is being screened on Wednesday evening on the Penn campus. We saw a clip of this at our recent launch event for the Center for Particle Cosmology, and it’s a great advertisement for science as an exciting endeavor.

I hope you’ll see the movie, so I won’t spoil the surprise by discussing what happened. But here’s the trailer!

An interesting short interview with Ed Witten in this week’s New Scientist. Mostly straightforward stuff, but it’s always good to hear what smart people are thinking. Witten is spending the year on sabbatical at CERN; like many people, he was sort of hoping to be there when the first physics results from the LHC appeared, but reality intervened an that’s looking increasingly unlikely. Happily, CERN has developed electronic means of communication whereby interesting findings may be promulgated to researchers who are not within close physical proximity to the lab.

Longtime CV readers may be interested in Witten’s take on the String Wars:

The 1980s and 90s were dotted with euphoric claims from string theorists. Then in 2006 Peter Woit of Columbia University in New York and Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, published popular books taking string theory to task for its lack of testability and its dominance of the job market for physicists. Witten hasn’t read either book, and compares the “string wars” surrounding their publication – which played out largely in the media and on blogs – to the fuss caused by the 1995 book The End of Science, which argued that the era of revolutionary scientific discoveries was over. “Neither the publicity surrounding that book nor the fact that people lost interest in talking about it after a while reflected any change in the intellectual underlying climate.”

That sounds about right. For the most part, actual string theorists simply went about their business, trying to figure out what this fascinating but difficult theory really is. The irony is that a major point of the anti-string books was that the public hype concerning string theory didn’t paint an accurate picture of its more problematic features — which was true. But the backlash books gave the public a misleading impression in the other direction, leading to the somewhat amusing appearance of my own piece in New Scientist explaining that the theory was for the most part chugging along as before. Hype cuts in every direction, and it feeds on drama, not on accuracy.

There is certainly some feeling that the near-term growth area in high-energy theory is not string theory, but phenomenology (or arguably particle astrophysics). Certainly those are the people who seem to be getting the jobs these days. The explanation there is pretty straightforward: data! Or at least the promise thereof. It’s hard to do physics with little to go on other than thought experiments, but one gets by when relatively few real experiments are available. Increasingly, that’s no longer the case.

But it’s been a long time since we’ve had a good string-wars thread, so here you go. For old time’s sake.

The largest organism on Earth, and probably the oldest multicellular organism, is named Pando. Kind of a cutesy name for such an impressive specimen, don’t you think?

If you were to meet Pando — which you could easily do, if you paid a visit to Fishlake National Forest in Utah — it would look like a forest of Quaking Aspen trees. But if you happened to be equipped to do DNA testing on plant specimens, you would realize that all of the trees were genetically identical. That’s because they’re all part of the same tree, sharing a common root system. One tree springs from a seed, long ago, and spreads out roots; but then more trees erupt from those roots, and the process simply continues. Individual “trees” might die, but that’s like you or me losing a toenail; Pando lives on. It weighs in at over six million kilograms, and is likely more than 80,000 years old (although it might be much older).

I have nothing especially profound to say about Pando, I just think it’s cool. But when you have arrow-of-time on the brain, everything resonates. Unlike most other multicellular organisms, there’s no reason why Pando should ever die, absent dramatic external factors. As long as its environment remains hospitable, Pando could live forever. Monocellular organisms, of course, do this all the time; they split into “children” which are genetically identical (up to mutations), so it’s legitimate to say that any given bacterium has lived for many millions of years. The birth/growth/death cycle is not absolutely necessary to the existence of life — it’s just useful, if life wants to avoid the very real possibility that the environment does dramatically change for the worse. Giving birth to children with slightly different genetic makeups — and then getting out of their way, by dying — gives the species a fighting chance to adapt and survive in the face of dramatic changes around it. (Update: some termites have a different strategy.)

Meanwhile, Pando abides. Good for it.

The kind way to say it is: “Humans are really good at detecting patterns.” The less kind way is: “Humans are really good at detecting patterns, even when they don’t exist.”

I’m going to blatantly swipe these two pictures from Peter Coles, but you should read his post for more information. The question is: which of these images represents a collection of points selected randomly from a distribution with uniform probability, and which has correlations between the points? (The relevance of this exercise to cosmologists studying distributions of galaxies should be obvious.)

The points on the right, as you’ve probably guessed from the set up, are distributed completely randomly. On the left, there are important correlations between them.

Humans are not very good at generating random sequences; when asked to come up with a “random” sequence of coin flips from their heads, they inevitably include too few long strings of the same outcome. In other words, they think that randomness looks a lot more uniform and structureless than it really does. The flip side is that, when things really are random, they see patterns that aren’t really there. It might be in coin flips or distributions of points, or it might involve the Virgin Mary on a grilled cheese sandwich, or the insistence on assigning blame for random unfortunate events.

Bonus link uncovered while doing our characteristic in-depth research for this post: flip ancient coins online!

An arxiv find, via David Hogg (via Facebook, via the internet).

The gravitational force law in the Solar System
Authors: Jo Bovy (NYU), Iain Murray (Toronto), David W. Hogg (NYU, MPIA)

Abstract: If the Solar System is long-lived and non-resonant (that is, if the planets are bound and have evolved independently through many orbital times), and if the system is observed at any non-special time, it is possible to infer the dynamical properties of the Solar System (such as the gravitational force or acceleration law) from a snapshot of the planet positions and velocities at a single moment in time. We consider purely radial acceleration laws of the form a_r= -A [r/r₀]^-α, where r is the distance from the Sun. Using only an instantaneous kinematic snapshot (valid at 2009 April 1.0) for the eight major planets and a Bayesian probabilistic inference technique, we infer 1.989<α<2.052 (95-percent confidence). Our results confirm those of Newton (1687) and contemporaries, who inferred α=2 (with no stated uncertainty) via the comparison of computed and observationally inferred orbit shapes (closed ellipses with the Sun at one focus; Kepler 1609). Generalizations of the methods used here will permit, among other things, inference of Milky-Way dynamics from Gaia-like observations.

So: instead of noting that an inverse-square behavior for the force of gravity fits the data, assume that gravity obeys an inverse power law and fit for the power. (It’s two, to within the errors.) Of course there have been many higher-precision tests of gravity in the Solar System than this one; the new thing here is that the data are simply the positions and velocities of all the planets at one particular moment in time, no direct dynamical measurements. A little bit of Bayesian voodoo magic, and there you go.

What I want to know is, what makes the authors so convinced that their instantaneous kinematic snapshot is valid tomorrow?

Here it is almost the end of March, and none of us has blogged about the International Year of Astronomy 2009.

There are a whole bunch of cool events of various sorts around the world. Ray Jayawardhana at Toronto started the year off with a great ad campaign on Toronto busses and elsewhere called Cool Cosmos. Here are a couple of examples:


Pretty cool to see that while you’re standing on a bus.

Later this week starts 100 hours of Astronomy, running April 2–5. The focus is a worldwide marathon of amateur astronomers watching the sky, culminating in a star party during the final 24 hours, which coincides with the 3rd annual International Sidewalk Astronomy night. If you have a telescope and know how to use it, get out there! And if you don’t, now’s your chance to find one! Astronomical observatories will be participating via Around the World in 80 Telescopes, which will be a live webcast starting on Mauna Kea (with Gemini, Subaru, UKIRT, Keck, CFHT, SMA, CSO all participating) and then heading west until it gets back around to Lick and Palomar 24 hours later. In addition to the webcast, you can also follow 100 Hours on twitter Impressively, in New York City, they managed to get the park lights turned off at 8pm this friday for their star party — great opportunity to see a dark(er) night in NYC!

In case 100 days isn’t enough, there is also a podcast called 365 days of astronomy, which has a daily podcast from a variety of sources and on a wide range of astronomy related topics.

Of course, there is also a blog, Cosmic Diary which includes bloggers from ESA, ESO, JAXA, and NASA, so you can hear about the life of professional astronomers all over. Check em out!

I’m sure I’ve missed some of the most interesting events, so feel free to leave them in the comments.