Schroedinger's Dog in Copenhagen

May 27, 2009

My bloggy buddy Chad Orzel, over at Uncertain Principles, has a new book coming out in December, called How To Teach Physics To Your Dog. The dog in question is Emmy, known to Chad's readers as "The Queen of Niskayuna," who has a tendency to browse Chad's physics books when she's bored. And oh, yes, fame will most certainly go to her head.

Chapter 3 deals with the "Copenhagen Interpretation" of quantum mechanics, specifically, the infamous thought experiment known as "Schroedinger's Cat." Back in 1935, physicist Erwin Schroedinger illustrated how ludicrously counter-intuitive the implications of quantum mechanics could be by suggesting one could place a cat in a closed box with a single uranium atom (a highly unstable element), right next to a Geiger counter. The uranium atom has a 50% chance of decaying and emitting an electron, and that tiny bit of radiation would set off the Geiger counter. To up the stakes, Schroedinger pictured a hammer rigged to smash a small vial containing cyanide, should the Geiger counter detect any radiation, instantly killing the poor kitty. (Emmy, not surprisingly, has no moral qualms about this.)

That's not the mind-bending part of the thought experiment. According to quantum mechanics, we have no way of knowing before we open the box whether the cat is alive or dead. And until we do so, the cat inhabits a bizarre superposition of states -- both alive and dead at the same time -- until we open the box and look for ourselves. This constitutes a "measurement" or observation, and the cat's wavefunction collapses into either an alive or dead state. So in that sense, observation determines reality.

"Preposterous!" you say. So did lots of people, including physicists, when the implications of quantum mechanics were first being debated. Albert Einstein was famously skeptical, once asking Niels Bohr if he truly believed that, say, the moon is not there when we don't happen to be looking at it.

Einstein had a point: quantum mechanics only applies to the subatomic world; on the macroscopic level, we don't see those bizarre effects. Cats are alive or dead, not both at the same time. That's because cats are macroscopic objects, made of billions of subatomic particles. And an "observation" doesn't necessarily imply a human (or canine) observer. Any interaction whatsoever within the system -- a particle in the air interacting with a single particle of the cat -- is sufficient to cause the wave function to collapse and destroy any superposition of states. The moon doesn't actually need us to look at it -- although why wouldn't you, on a nice clear night when it's hanging low in the sky?

You can say all of what I just said above, and watch your listeners' eyes glaze over in bafflement. Or you can take the humorous approach, like Chad, and try explaining it in the simplest possible terms to your dog. In the book, Emmy clearly stands in for the Everyman (or Everydog), asking the kind of basic "why is the sky blue" questions that most of us are reluctant to ask for fear of looking, well, stupid. Dogs don't have this hang-up. Here's Chad reading aloud from Chapter 3, with accompanying photos:

Learning Curve

January 19, 2009

Last summer I wrote about Quantum Quest, an animated 45-minute film set entirely in the subatomic world, where the forces of the Core (protons, photons, neutrinos) battle it out with the antimatter forces of the Void to determine the fate of the universe. Now in post-production, the film is starting to generate a bit of buzz, although it's garnered mixed reactions from those who've either heard about it, or had the chance to view a few clips at last year's Comic-Con. (You can see a preview here.)

On the one hand, NASA is heavily involved, and while we're all NASA fans here (at least of all that cool science NASA funds), it's not the first name that pops to mind when you think of quality children's entertainment. On the other, some truly major names have lent their voices to the project, including William Shatner (who voices the Core) and breakout star Chris Pine ("Dave," a plucky little photon), who will play the young Captain Kirk in J.J. Abrams' forthcoming Star Trek "prequel." Add in Samuel Jackson, Jason Alexander, Mark Hamill, Amanda Peet, Brent Spiner, Sandra Oh, and James Earl Jones, and you've got some serious celebrity street cred going -- enough for Variety to sit up and take notice.

I'm of a more optimistic bent than the skeptics, especially since the film's writer/producer, Harry Kloor (Jupiter 9 Productions) stopped by my office last week to give me a sneak preview of where things stand thus far. Kloor is a scientist -- with not one, but two shiny PhDs -- and deeply engaged with education and outreach, but he's also an unabashed sci-fi fan with bona fide writing chops, having written a few episodes of Star Trek: Voyager, among other TV credits.

That makes Kloor kind of the ideal person to tackle a project like Quantum Quest. I saw rough cuts, with incomplete animation (no texturing, for instance) and storyboards as placeholders in a couple of spots, but even so, it's clear that Kloor has taken pains to delight as well as to teach. The science "lessons" are woven into the storyline, rather than dryly lecturing to the audience. Plus, the animation is excellent, incorporating actual footage taken from various NASA missions, including Cassini-Huygens, SOHO, Mars Odyssey, and Mercury Messenger.

But the story reigns supreme. The script has a decent plot, lively pacing, humorous asides, clever physics in-jokes, fun characters, and if the message gets a bit ham-fisted towards the end -- well, it's no more so than any other family flick, and certainly less nauseating than, say, Barney.

Kloor expects to complete the animation by June, and finish the final touches by this fall, shooting for a November release. And he's looking to partner with various organizations to produce educational materials that teachers can use in the classroom, based on the characters in the film. His philosophy is that you don't need to lecture to kids -- you just need to pique their interest and make them want to know more. Do that, and they'll probably do what all of us grown-up geeks did as kids: we heard about something cool and looked it up.

That's a philosophy I happen to agree with: I can totally see kids Googling "neutrino," or wanting to learn more about the Cassini-Huygens mission after seeing the film. There's just one problem: the science in Quantum Quest occasionally goes far beyond anything that's readily accessible to a non-scientist.

Take my personal favorite characters, the Gell-Mann Ghosts sent after Dave and his neutrino pal to keep them from completing their mission. Google "ghost" and "Gell-Mann" (as in physicist Murray Gell-Mann), and you won't find much information. Even Googling "ghosts" and "physics" turns up mostly paranormal debunking sites. That might be because, while physicists do indeed talk about "ghosts" in their research, it's more of a side dish than the main course -- and it can mean more than one thing, depending on the context.

Fortunately, I have my own personal household physicist to enlighten me. In one context, "ghosts" are hypothetical particles that don't exist. (Neutrinos are sometimes called "ghost particles," but this is something else altogether.) If they did exist, they would have negative energy -- and this, says Sean, would be very bad, in part because it would mean theory did not agree with experimental observations: "Empty space would be catastrophically unstable if there were ghosts." (The existence of "ghosts" would be very good, however, if you wanted to build a stable trasversable wormhole. Many theoretical models call for a form of negative energy to hold such a structure open long enough for an object to pass through.)

See, normally, in particle physics, a muon decays into an electron by spitting out neutrinos, and then the electron is stable. Heavy particles decay into lighter particles, per the Standard Model. If there were negative energy, then we should also see this process in reverse: an electron would "decay" into a muon by producing "ghosts." We don't -- ergo, these sorts of "ghosts" are largely confined to some of the more exotic physical theories.

The above seems like the most likely candidate for Kloor's Gell-Mann Ghosts, except these particles are not the same thing as antimatter (remember, the Void's forces are antimaterial): antimatter has positive energy, and actually exists. We can even make small amounts of the stuff in particle accelerators. Also? "Ghosts" in this context don't have anything to do with Murray Gell-Mann.

Gell-Mann was tangentially involved in a different usage of the term "ghosts," related to Feynman diagrams and quantum field theory. Sean tried to explain it to me, and this appears to be the gist: Feynman diagrams are a handy method for calculating how probable a given "event" will be in quantum field theory, but the process requires one to also include -- solely as a "bookkeeping device" to get the math to come out right -- diagrams with particles that can only exist within the diagram and cannot escape into the outside world to be observed. Much like we don't see "free quarks", these "ghosts" can never be observed all by themselves. Physicists are actually still arguing about this.

Apparently the term "ghost" also crops up in the context of cosmology. Sheesh. No wonder there's no good lay language discussions to be found online. I suppose I could have asked Kloor to explain the science behind his Gell-Mann Ghosts, but where's the fun in that? I invite any theoretical physicists out there to weigh in with their own ideas of what might be behind the "ghosts" in Quantum Quest. Maybe we can all get a spot of advanced science education.

Photos: (top) Dave the Photon and The Core, from Quantum Quest. (middle) Coach Mackey and Milton, war hero turned "free quark" miner. (bottom) Dave the Photon's "home," the core of the Sun. Source: Jupiter 9 Productions.

Gravity Gets Loopy

December 15, 2008

A few years ago, at a meeting of the American Physical Society in Tampa, Florida, I attended a press conference on loop quantum gravity (LQG), the leading contender to string theory for a unified Theory of Everything (i.e., a theory that merges general relativity with quantum mechanics). The press conference speakers included Abhay Ashtekar, currently director of the Center for Gravitational Physics at Penn State University. He was one of LQG's founders in the early 1980s (along with Lee Smolin and Carlo Rovelli, among others), and is still among its strongest proponents.

Max Planck spawned the quantum revolution when he introduced the notion of quanta: atoms could only emit or absorb energy in specific amounts -- much like how currency comes in specific denominations. (You can have a $1 bill or a $5 bill, but not a bill for exactly $2.43, for instance.) In LHQ, essentially space itself becomes quantized. Per Smolin, "If you take a volume of space and measure it to very fine precision, you discover that the volume can't be just anything. It has to fall into some discrete series of numbers, just like the energy of an electron in an atom. And just as in the case of the energy levels of atoms, we can calculate the discrete areas and volumes from the theory."

It just so happened that Brian Greene -- author of the bestselling book (and NOVA series) The Elegant Universe, and string theory's most prominent ambassador -- was also at the meeting. After a bit of arm-twisting, he agreed to sit in on the press conference, offering his take on the pros and cons of the rival schools. But if any of the assembled journalists were expecting the fur to fly, they were disappointed: the two men were perfectly cordial and reasonable -- in short, they behaved like the professional scientists they are.

The two approaches certainly have their differences. String theory starts at the quantum level and builds upward to incorporate general relativity, while loop quantum gravity starts out at the top with general relativity and seeks to incorporate quantum mechanics.

But both involve some kind of loop -- loops of string in string theory, and the mathematical equivalent to loops of space in loop quantum gravity. Who knows? As Greene pointed out at that press conference, maybe the two camps could turn out to be a weird sort of duality: opposite ends of the same Theory of Everything. Theorists of the future might end up with something that combines elements of both.

Loop quantum gravity does have at least one advantage over string theory: it is a "background-independent formulation." See, string theory makes one very big assumption: the pre-existence of the fabric of spacetime. Think of the master painter who produces great works, but never stop to wonder where the canvas came from. We talk about the "fabric" of spacetime routinely, but it isn't any kind of tangible material, despite the fact that physicists speak of curving and twisting space-time as if it were. It's more of a mathematical construct on which to drape the master equations of the universe. So explaining where it came from is one of the criteria for a bona fide Theory of Everything. String theorists are in hot pursuit of their own background-independent formulation, but loop quantum gravity already has it: the fabric of space-time emerges directly from the equations.

LQG might be string theory's lesser known cousin, but it's had its share of success (mathematically, at least), such as correctly computing the entropy of a black hole -- a must for any successful theory of quantum gravity. It's yielded some intriguing theoretical possibilities: for instance, last year another Penn State physicist, Martin Bojowald, published a paper in wich he claimed that, unlike Einstein's theory of general relativity, LQC doesn't break down at the point of the Big Bang.

In fact, per Bojowald and Ashtekar, there might not have been so much of a Big Bang, as more of a "big bounce." In their model, the universe eventually stops expanding and contracts, except instead of contracting down to the dreaded singularity, it bounces back, reborn -- literally. Just like the mythical phoenix rising from its own ashes, an old, dying universe gives birth to a new one. The sticking point: all the experimental data so far indicates that the universe is accelerating, expanding outward at an increasing rate because of dark energy, which is the exact opposite of a contraction.

We mere mortals can only grasp the outlines of these deep mathematical theories, of course, so I won't be so presumptuous as to make a call as to whether LQG or string theory will emerge victorious. Maybe we'll never know for sure. Physicists of a more experimental bent tend to dismiss theories that are mostly pretty math with few predictions and/or any means of experimentally testing those predictions. It's a valid criticism. But I like Greene's more laissez-faire attitude. Asked how he'd feel if string theory was proven wrong after devoting most of his career to its advancement, he responded, "Don't you want to know? I do!" Right or wrong, knowing is better. That's what science is all about.

Photo: 3D model of the "loopy" structure of space predicted by loop quantum gravity at very short scale. Source: Carlo Rovelli, Centre de Physique Theorique de Luminy.

LEGO Hawking

October 07, 2008

Is there any limit to the creative potential of LEGOS? Via Talk like a Physicist, I stumbled upon Brick Shelf's whimsical series of photographs featuring a LEGO model of astrophysicist Stephen Hawking. Author of the bestselling A Brief History of Time, Hawking has appeared on Star Trek and The Simpsons, gone weightless in NASA's "vomit comet," and indulged his penchant for making small wagers with his fellow physicists on the outcome of various theories. Most recently, he bet $100 that the Large Hadron Collider wouldn't find the elusive Higgs boson -- mostly because he thinks it'd be more interesting that way.

Hawking isn't betting on what amounts to an LHC long shot: that the collider will produce mini-black holes, which will evaporate within fractions of a second, thereby proving something he predicted at least a decade ago, called "Hawking radiation." There's a less than 1% chance this will happen, which is why Hawking told the New York Times, I'm not holding my breath." But if it did, he'd stand to win more than a small wager; he'd likely snag a Nobel Prize.

The premise of Hawking radiation is that black holes aren't as black as they seem. They can and do emit radiation, and this in turn causes black holes to lose mass over time, eventually winking out of existence. How fast this happens depends on the black hole's mass: the greater the mass, the more slowly it evaporates, and the smaller the mass, the more quickly it evaporates. Mini-black holes would be the size of subatomic particles, and thus would evaporate in fractions of a second.

But the reason we get Hawking radiation in the first place is because of a quirk of quantum mechanics. (Is there anything but quirks in quantum mechanics?) See, empty space isn't really empty. It is teeming with virtual pairs of particles (matter and antimatter, i.e., having the same mass but opposite charges) that spontaneously pop into existence, and just as quickly annihilate into radiation. It happens on such a short time scale that they can't be observed or measured directly, so in this way, the laws of energy conservation aren't violated.

Ah, but if a virtual particle pair pops into existence near a black hole, there's a pretty good chance that one half of the pair will fall in, while the other half is emitted as radiation. Energy appears to come from nothing. That's why, Hawking explained, the mass of the black hole must decrease ever so slightly in response to account for that sudden emission of energy. This happens over and over until, voila! The black hole evaporates completely.

Only time will tell if Hawking wins or loses his bet, or snags that Nobel Prize. He lost a prior bet on a technicality -- ironically stemming from his own pioneering work on Hawking radiation In 1991, he wagered 100 pounds sterling -- plus an article of clothing "embroidered with a suitable concessionary message" -- with two Caltech physicists that naked singularities could not exist. Most physicists believe singularities lie at the heart of black holes, but they are "clothed," that is, hidden from direct observation beyond the event horizon. A naked singularity wouldn't have that safeguard, known as "cosmic censorship."

But if black holes radiate energy and decay over time -- and all signs thus far indicate that they do -- then the protective event horizon must also evaporate over time. So the singularity at the center could be exposed oh-so-briefly at the very moment the black hole winks out of existence. Hawking graciously conceded the bet, although the message on the T-shirt he presented to his colleagues wasn't exactly concessionary. It read, "Nature abhors a naked singularity."

That Nature can be such a prude.

Photos: LEGO Hawking images from Brick Shelf, via Talk Like a Physicist.

Splitting Image

July 23, 2008

Back in 1957, a young Princeton graduate student named Hugh Everett III dared to challenge the prevailing "Copenhagen interpretation" of quantum physics with a controversial theory called Many Worlds: the notion that there could be an infinite number of parallel universes lying just beyond our ken. The notion has spawned a plethora of science fiction stories, but was largely ignored by Everett's peers when he proposed it. Crushed by the rejection, Everett left physics entirely, struggling with depression and drink until he died of a heart attack many decades later. He didn't live to see his theory begin to gain some measure of acceptance by the community of physicists who once scorned him.

Everett's tragic story is the focus of Parallel Worlds, Parallel Lives, a documentary that follows the journey of Everett's son, Mark Oliver Everett, as he discovers the brilliant father he barely knew. Mark Everett is better known as E, lead singer of a cult band called the Eels (their tune "Souljacker" is on permanent rotation on my iPod workout playlist). He knows nothing about physics, which makes him the perfect guide for those who have never heard of Hugh Everett III, or of Many Worlds. The film debuted last fall in England, and made its American premiere in May at the World Science Festival in new York. And NOVA will air the film on PBS this October.

The young Hugh Everett wasn't happy about some of the troubling implications of quantum mechanics: specifically, the notion that observation determines reality -- or at least the outcome of an experiment. In any quantum system (such as a subatomic particle like a photon or electron), every possible outcome is present simultaneously in a sort of superimposed limbo state. The technical term is a superposition of states.

The sum of all those outcomes is described by an equation known as the wave function. It's only when we check to see what happened -- when we observe the quantum system by making a measurement -- that the wave function collapses and all those possibilities reduce to a single "real" event: the outcome we have observed.

But what happens to the other possibilities once the wave function collapses? The strictest interpretation of quantum theory -- espoused by physics giant Neils Bohr, among others -- simply ignored the question, assuming that all other potential outcomes vanish once a measurement is made.

Everett wasn't satisfied with that. He proposed that perhaps the wave function only appears to collapse from our limited vantage point as human observers. Perhaps it continues to evolve, forever splitting into other wave functions in a never-ending tree, with every branch becoming a parallel universe with a different outcome. Every potential outcome gets its own separate universe. There is no such thing as the road not taken.

Many Worlds has its own intriguing implications -- like doppelgangers. Think about it: if there is a separate universe for all possible outcomes of an experiment, for example, then each universe must contain "copies" of those who perform the experiment. So, you might opt to order the kung pao chicken for dinner one night at your local Chinese takeout in this universe, but somewhere else, in a parallel universe, another version of you orders the mapo tofu, and in still another, you order the sweet and sour pork -- a separate universe and a carbon copy of you for every item on the menu.

That's a lot of parallel universes, just for that one decision about what to have for dinner on a given night. There are branchings upon branchings of possible choices, an infinite number of paths our lives can take, and per Many Worlds, each must give rise to its own parallel universe. So there must be an infinite number of other worlds, linked through a vast network of forks in the proverbial road.

Back in 1957, physicists were none too happy with Everett's theory because of this. Taken down to the subatomic level, it would mean the universe must split into 10<100> copies every second, each equally "real" -- although we can only perceive one at a time. (Here's an amusing explanation as to why that even a dog can understand.) This also makes it pretty much impossible to test the Many Worlds hypothesis: how can we prove there are an infinite number of parallel worlds if our perception is limited just to this one?

The finite human brain tends to balk at the notion of infinity; mine certainly does. My husband, a cosmologist, is far more comfortable contemplating such a possibility, as is MIT physicist Max Tegmark, a featured personage in the film. In fact, Tegmark tells Mark Everett in the film that his father's theory is "one of the most important discoveries of all time in science," comparing it to Einstein's theory of relativity and Newton's theory of gravity. There's plenty of physicists who might take issue with that assessment, but only time will tell. In the meantime, Mark Everett has made his peace with his father, and his past. And he's also made a very thought-provoking, heartfelt film that deserves a wider audience.

Photo: Mark Oliver Everett and his doppelganger, in a still from Parallel Worlds, Parallel Lives.