Category: Science

Sorry for the radio silence around here of late. I don’t know about anyone else, but I’ve been traveling like a mad person. The good news is that I just got back from UC Davis, where I had the chance to meet John Conway for the first time in person.

The bad news is: no time for blogging. But I recently received an email pointing out that some links have died in an old post, which I proceeded to update. And that gave me the idea of stooping to a classic blogospheric move in times of sparse content: reposting old stuff! So here is the post in question, from several years ago. If people don’t complain too loudly, maybe we’ll dig up some more ancient blogging and bring it back to the surface.

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Quantum mechanics, as we all know, is weird. It’s weird enough in its own right, but when some determined experimenters do tricks that really bring out the weirdness in all its glory, and the results are conveyed to us by well-intentioned but occasionally murky vulgarizations in the popular press, it can seem even weirder than usual.

Last week was a classic example: the computer that could figure out the answer without actually doing a calculation! (See Uncertain Principles, Crooked Timber, 3 Quarks Daily.) The articles refer to an experiment performed by Onur Hosten and collaborators in Paul Kwiat‘s group at Urbana-Champaign, involving an ingenious series of quantum-mechanical miracles. On the surface, these results seem nearly impossible to make sense of. (Indeed, Brad DeLong has nearly given up hope.) How can you get an answer without doing a calculation? Half of the problem is that imprecise language makes the experiment seem even more fantastical than it really is — the other half is that it really is quite astonishing.

Let me make a stab at explaining, perhaps not the entire exercise in quantum computation, but at least the most surprising part of the whole story — how you can detect something without actually looking at it. The substance of everything that I will say is simply a translation of the nice explanation of quantum interrogation at Kwiat’s page, with the exception that I will forgo the typically violent metaphors of blowing up bombs and killing cats in favor of a discussion of cute little puppies.

So here is our problem: a large box lies before us, and we would like to know whether there is a sleeping puppy inside. Except that, sensitive souls that we are, it’s really important that we don’t wake up the puppy. Furthermore, due to circumstances too complicated to get into right now, we only have one technique at our disposal: the ability to pass an item of food into a small flap in the box. If the food is something uninteresting to puppies, like a salad, we will get no reaction — the puppy will just keep slumbering peacefully, oblivious to the food. But if the food is something delicious (from the canine point of view), like a nice juicy steak, the aromas will awaken the puppy, which will begin to bark like mad.

It would seem that we are stuck. If we stick a salad into the box, we don’t learn anything, as from the outside we can’t tell the difference between a sleeping puppy and no puppy at all. If we stick a steak into the box, we will definitely learn whether there is a puppy in there, but only because it will wake up and start barking if it’s there, and that would break our over-sensitive hearts. Puppies need their sleep, after all.

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Craig Venter and colleagues have achieved a remarkable milestone: they designed a genome, and brought it to life. More specifically, they’ve synthesized a chromosome consisting of over a million DNA base pairs, and implanted it in a bacterial cell to replace the cell’s original genome. That cell then reproduced, giving birth to offspring that only had the synthetic genome. See the Venter Institute press release, discussion in Nature (pdf), more discussion at Edge, and some background from Carl Zimmer. Update: and here is the paper.

Who knows exactly what this means as yet — but it’s important! You can argue if you like about whether it’s really “artificial life” — that argument has already started, and already seems boring. There are also speculations about designing microorganisms to help us solve problems like global warming or (let’s say) massive oil spills. Not completely crazy speculations, either. But there’s a long way to go before anything like that is coming off a biological assembly line. And eventually we’ll be going much further than that, beyond designer microorganisms into much weirder terrain. This isn’t a culmination, it’s just a start.

People sometimes ask, “Is the universe a black hole?” Or worse, they claim: “The universe is a black hole!” No, it’s not, and it’s worth getting this one straight.

If there’s any quantitative reasoning behind the question (or claim), it comes from comparing the amount of matter within the observable universe to the radius of the observable universe, and noticing that it looks a lot like the relationship between the mass of a black hole and its Schwarzschild radius. That is: if you imagine taking all the stuff in the universe and putting it into one place, it would make a black hole the size of the universe. Slightly more formally, it looks like the the universe satisfies the hoop conjecture, so shouldn’t it form a black hole?

But a black hole is not “a place where a lot of mass has been squeezed inside its own Schwarzschild radius.” It is, as Wikipedia is happy to tell you, “a region of space from which nothing, including light, can escape.” The implication being that there is a region outside the black hole from which things could at least imagine escaping to. For the universe, there is no such outside region. So at a pretty trivial level, the universe is not a black hole.

You might say that this is picking nits, and the existence of an outside region is beside the point if the inside of our universe resembles a black hole. That’s fine, except: it doesn’t. You may have noticed that the universe is actually expanding, rather than contracting as you might expect the interior of a black hole to be. That’s because, if anything, our universe bears a passing resemblance to a white hole. Our universe (according to conventional general relativity) has a singularity in the past, out of which everything emerged, not a singularity in the future into which everything is crashing. We call that singularity the Big Bang, but it’s very similar to what we would expect from a white hole, which is just a time-reversed version of a black hole.

That insight, plus four dollars or so, will get you a grande latte at Starbucks. The spacetime solution to Einstein’s equation that describes a universe expanding from the Big Bang is very similar to the time-reversal of a black hole, but you don’t really learn much from making that statement, especially because there is no outside; everything you wanted to know was already there in the original cosmological language. Our universe is not going to collapse to a future singularity, even though the mass is enough to allow that to happen, simply because it’s expanding; the singularity you’re anticipating already happened.

Still, some folks will stubbornly insist, there has to be something deep and interesting about the fact that the radius of the observable universe is comparable to the Schwarzschild radius of an equally-sized black hole. And there is! It means the universe is spatially flat.

You can figure this out by looking at the Friedmann equation, which relates the Hubble parameter to the energy density and the spatial curvature of the universe. The radius of our observable universe is basically the Hubble length, which is the speed of light divided by the Hubble parameter. It’s a straightforward exercise to calculate the amount of mass inside a sphere whose radius is the Hubble length (M = 4π c³H^-3/3), and then calculate the corresponding Schwarzschild radius (R = 2GM/c²). You will find that the radius equals the Hubble length, if the universe is spatially flat. Voila!

Note that a spatially flat universe remains spatially flat forever, so this isn’t telling us anything about the universe now; it always has been true, and will remain always true. It’s a nice fact, but it doesn’t reveal anything about the universe that we didn’t already know by thinking about cosmology. Who wants to live inside a black hole, anyway?

We had a great time last night at a panel discussion on extrasolar planets, right here at my very own institution of Caltech, sponsored by our very own Discover magazine, and hosted by our very own Bad Astronomer. The panelists included Gibor Basri, John Johnson, Sara Seager, and Tori Hoehler. They did a great job at getting across the most important message: this is a field that has taken a tremendous leap forward in the past ten years, and is poised to make comparable strides in the years to come. A lot of the excitement right now centers on the Kepler satellite, which is on track to find hundreds of extrasolar planets. You can get an idea of recent progress from a graph of extrasolar planets discovered over the years.

From the perspective of the person on the street, planets are pretty cool — but life on other planets is what’s really cool. (Or would be, if we found it.) And frankly, it’s not even the prospect of life that gets people going; it’s the idea of intelligent extraterrestrial life. Tori mentioned that he was slightly surprised, some years ago when there was a report (later discredited) that we had found evidence for life on meteorites from Antarctica, that people didn’t make a big deal out of it — it was exciting, but not Earth-shattering. I suspect that microbes, no matter where they’re from, aren’t going to shatter most people’s Earths; that will take some sort of greeting, friendly or otherwise.

Still, it’s amazing what has been done, and the prospects for doing more are pretty breathtaking. Here’s one idea that I find pretty clever: searching for the Red Edge. You know how plants appear to be really bright in infrared photographs? That’s because they reflect a lot of infrared light, but tend to absorb regular visible red light. In a spectrum, where we decompose the reflected light into different wavelengths, this phenomenon shows up as a sharp “edge” as you go from infrared (on the right here) to red light. The idea would be that something similar should happen even for very different kinds of life — so if you found a planet whose spectrum featured the red edge, that would be a promising place to hope for finding life.

I have no way of judging how feasible this technique really is — in particular, I’m always skeptical of claims that rely on alien forms of life resembling ours in any way. (The authors do emphasize that an extraterrestrial red edge might not be at the same wavelength as ours.) But I like it because it relies on an underlying truth of which I am quite fond — the fact that life relies on the increase of entropy. The specific wavelengths at which different kinds of life might reflect light can undoubtedly be very different from biosphere to biosphere; but what won’t change is the general idea that a planet full of life will re-radiate energy with a much higher entropy than what it absorbs. That’s the deep principle underlying the red edge; plants absorb visible light, and radiate at longer wavelengths with higher entropy. If we eventually find life on other planets, I’d personally be pleased if entropy were at the bottom of it all.

For a long time now, my day job has been “theoretical physicist,” as a quick glance at my papers will confirm. But it was not always thus! Very few people are actually born as theoretical physicists. When I was an undergraduate astronomy major at Villanova, I wasn’t thinking about quantum field theory or differential geometry; I was working on photometric studies of variable stars. My personal favorite star was Epsilon Aurigae, a mysterious eclipsing binary. One of the very few stars out there that has both a Facebook page and a Twitter feed. And now Epsilon is in the news again!

Among this star’s claims to fame is that it has the longest period of any known eclipsing binary: over 27 years. But it’s not just about facile record-holding; this system is truly puzzling, especially the nature of the secondary (the thing that eclipses the primary star). The basic problem is that the eclipse has a fairly flat bottom, as seen in this light curve from the previous eclipse in 1982-84.

A flat-bottomed light curve is usually associated with a total eclipse; the secondary completely blocks the light from the primary for a while. But in this case, the spectrum of the system seemed to remain unchanged, indicating that most of the light was still coming from the primary star, even in the middle of the eclipse. This led Huang in 1965 to propose a clever model, in which the secondary is actually a disk seen edge-on; the eclipse is therefore not total, but the disk blocks out part of the light without emitting much of its own. And indeed, with modern infrared telescopes we can discern the light from the secondary — it does look like a relatively cold disk, about four astronomical units in radius, with a hot central star.

The 1982-84 eclipse raised a problem with Huang’s model, however. If you look closely at that light curve above, you’ll notice that it gets brighter right near the middle. (The gap in data is from when the star was behind the Sun and unobservable.) Your first guess is that this is probably just a fluctuation in the in brightness of the primary star; but it turns out that this can’t be right. The primary is indeed variable, but its color changes in lockstep with its brightness, an effect that can be measured by observing with different filters. And the mid-eclipse brightening shows no variation in color. It’s not due to variability in the primary; somehow the disk is letting more light past, right during mid-eclipse.

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The primary goal of the European Space Agency’s Planck satellite is to provide a map of the cosmic microwave background with unprecedented precision. But along the way, you have to take into account that there is stuff in between us and the farthest edges of the universe — in particular, there’s all sorts of dust here in our home galaxy. You can even become famous just studying dust; one of the most highly cited papers in all of astrophysics is a 1997 map of galactic dust.

Dust isn’t only an annoyance — it’s also pretty. Planck hasn’t released any data about the CMB yet, but they just released a map of the cold dust in our local vicinity, looking for all the world like an abstract expressionist painting. (I want to suggest a particular artist, but my mind is blanking.) Click to embiggen.

It’s a false-color image, of course; the dust is very cold (tens of degrees above absolute zero), and the image is constructed from microwaves, not from visible light. You can see the plane of the galaxy, and the filamentary structures arising from all the churning of the interstellar medium from supernovae, star formation, magnetic fields, and so on.

Okay, pretty time is over. Let’s see the CMB.

When we think about the “meaning of life,” we tend to conjure ideas such as love, or self-actualization, or justice, or human progress. It’s an anthropocentric view; try to convince blue-green algae that self-actualization is some sort of virtue. Let’s ask instead why “life,” as a biological concept, actually exists. That is to say: we know that entropy increases as the universe evolves. But why, on the road from the simple and low-entropy early universe to the simple and high-entropy late universe, do we pass through our present era of marvelous complexity and organization, culminating in the intricate chemical reactions we know as life?

Yesterday’s book club post referred to a somewhat-whimsical vision of Maxwell’s Demon as a paradigm for life. The Demon takes in free energy and uses it to maintain a separation between hot and cold sides of a box of gas — a sustained departure from thermal equilibrium. But what if we reversed the story? Instead of thinking that the Demon takes advantage free energy to help advance its nefarious anti-thermodynamic agenda, what if we imagine that the free energy is simply using the Demon — that is, the out-of-equilibrium configurations labeled “life” — for its own pro-thermodynamic purposes?

Energy is conserved, if we put aside some subtleties associated with general relativity. But there’s useful energy, and useless energy. When you burn gasoline in your car engine, the amount of energy doesn’t really change; some of it gets converted into the motion of your car, while some gets dissipated into useless forms such as noise, heat, and exhaust, increasing entropy along the way. That’s why it’s helpful to invent the concept of “free energy” to keep track of how much energy is actually available for doing useful work, like accelerating a car. Roughly speaking, the free energy is the total energy minus entropy times temperature, so free energy is used up as entropy increases.

Because the Second Law of Thermodynamics tells us that entropy increases, the history of the universe is the story of dissipation of free energy. Energy wants to be converted from useful forms to useless forms. But it might not happen automatically; sometimes a configuration with excess free energy can last a long time before something comes along to nudge it into a higher-entropy form. Gasoline and oxygen are a combustible mixture, but you still need a spark to set the fire.

This is where life comes in, at least according to one view. Apparently (I’m certainly not an expert in this stuff) there are two competing theories that attempt to explain the first steps taken toward life on Earth. One is a “replicator-first” picture, in which the key jump from chemistry to life was taken by a molecule such as RNA that was able to reproduce itself, passing information on to subsequent generations. The competitor is a “metabolism-first” picture, where the important step was a set of interactions that helped release free energy in the atmosphere of the young Earth. You can read some background about these two options in this profile of Mike Russell (pdf), one of the leading advocates of the metabolism-first view.

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I’ve been meaning to link to this post at the arXiv blog, which is a great source of quirky and interesting new papers. In this case they are pointing to a speculative but interesting paper by Martin Perl and Holger Mueller, which suggests an experimental search for gradients in dark energy by way of atom interferometry.

But I’m unable to get past this part of the blog post:

The notion of dark energy is peculiar, even by cosmological standards.

Cosmologists have foisted the idea upon us to explain the apparent accelerating expansion of the Universe. They say that this acceleration is caused by energy that fills space at a density of 10^-10 joules per cubic metre.

What’s strange about this idea is that as space expands, so too does the amount of energy. If you’ve spotted the flaw in this argument, you’re not alone. Forgetting the law of conservation of energy is no small oversight.

I like to think that, if I were not a professional cosmologist, I would still find it hard to believe that hundreds of cosmologists around the world have latched on to an idea that violates a bedrock principle of physics, simply because they “forgot” it. If the idea of dark energy were in conflict with some other much more fundamental principle, I suspect the theory would be a lot less popular.

But many people have just this reaction. It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.)

The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved.

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