A Little Learning is a Dangerous Thing

October 28, 2009

Just in time for Halloween, we bring you a terrifying pseudoscientific YouTube video, courtesy of a 2007 Wellness Seminar in Bozeman, Montana. (h/t: PZ Myers of Pharyngula -- perhaps you know him by his hip-hop moniker, P-Zeddy.) The woman is attempting to "explain" the "scientific" basis for homeopathy by invoking the name of Albert Einstein, among other luminaries. Her (highly disjointed and rambling) argument appears to boil down to this:

[pseudospeak on] Einstein said that light times mass is energy in his famous equation, E=mc<2>. But how much mass are we, really? Compress all the mass in the universe so that there is no space at all, and you'd get something the size of a bowling ball. So, really, our body's mass is an infinitesemal amount! Which means we can just cancel out that pesky "m" expression in Einstein's equation to conclude that light is energy. How amazing is that? And since our bodies have almost no mass, really, that means we are made of energy. Energy can't be created or destroyed, only transformed from one state to another state. That is the definition of disease: we have transformed our healthy energy state into a diseased energy state. Homeopathy just transforms one form of energy to another to "heal" disease.

And hey, speaking of vibrations, there's a physicist name Stephen Hawkings who invented the string theory that says the particles in the universe are tiny strings that work by vibration -- those very same vibrations picked up by our eyes and ears. If none of us having any real mass, and everything is energy, that means everything has a vibration to it. We just need to encase some form of energy for later use! Homeopathy is teh awesome! [/pseudospeak off]

Sigh. Really, there's so much wrong or misguided here, it's like shooting fish in a barrel. Seriously, would it have killed her to look up "Stephen Hawkings" on Wikipedia to learn that there is no "s" in his surname, and that he actually works on general relativity? And that string theory was invented in the 1970s by numerous theorists, including Gabriele Veneziano and Leonard Susskind, among others? (Veneziano is the one who first unearthed a long-forgotten equation by Leonhard Euler 200 years earlier, and Susskind found the equation could describe not just the strong nuclear force, but also vibrating elastic particles.)

This is a prime example of how well-meaning but misguided people learn a few cool-sounding physics terms -- thermodynamics! string theory! relativity! -- and try to twist otherwise perfectly valid science around to justify their personal beliefs. But that isn't science, people; it's the classic definition of pseudoscience. Watch the whole scary thing... if you dare! Don't be surprised if your head goes all 'splodey.

Lost Lessons

August 10, 2009

We have rather belatedly found ourselves addicted to Lost in our household. I know, I know, friends have been raving about it for years, but I resisted because, frankly, I had a feeling that once we started watching, we wouldn't be able to stop. I was right: the first four episodes of Season 1 comprise some of the best television I've ever seen. Who knew 40+ people stranded on a desert island could be so dramatically compelling? Now we can't stop watching, and are slogging our way through the DVDs to try and catch up before the final season begins this fall.

From a physics perspective, the most interesting season has to be Season 5, wherein the main story arc rested on time travel/temporal anomalies. We did see a few episodes, in part because Sean was interviewed for the DVD extras, to be released this fall, along with two University of Southern California physicists, Nick Warner (who studied under Stephen Hawking) and Clifford Johnson (who writes the Asymptotia blog, among his many other research and outreach activities).

The writers clearly did their homework when it comes to researching the most famous tropes on time in science fiction. In-jokes abound, along with some pretty geeky debates between characters about what is, and is not, allowed by the laws of physics.

But the folks who put together the DVD package didn't stop with a simple short documentary as a bonus feature. They've launched an associated Website, LOST University, with an actual "curriculum" spanning not just time travel, but philosophy (many of the characters' names are nods to famous philosophers), foreign languages, and Egyptology. (Yes! You can learn the ABCs of hieroglyphics!). 

Jeremy Davies, the actor who plays Daniel Faraday (is he alive? Dead? A flesh-eating zombie? We'll find out in Season 6!), is also featured at LOST University, "teaching" a course on new physics -- scientific studies that have captured his interest over the summer. Apparently he's a longtime Friend of Science, long before landing the role of Faraday. And the cast and crew pool their efforts for a "course" on the basics of jungle survival skills.

Sean's take on this intriguing new venture is here. Those who follow my work regularly know I'm a big fan of using popular culture (film, TV, books, including comics, etc) as "teaching moments" to talk about the underlying science (or lack thereof). LOST University is an uber-cool example of what is now possible when online multimedia meets TV meets science. Yummy science and entertainment goodness, with a twist!

No Fate But What We Make?

June 07, 2009

Terminator: Salvation, the fourth film in the hugely successful franchise, opened pretty strong a few weeks ago at the box office, but faced some criticism for abandoning the time travel underpinnings of its earlier installments, and focusing instead on a futuristic war with the machines. If you look at the progression of themes in the franchise, however, it's clear this was a natural choice for the filmmakers to follow.

In the original, of course, Ah-nold is the bad guy, a robot killing machine machine who travels back in time to take out the mother of the future leader of the resistance -- before she has a chance to meet the father of her child. The robot fails; the future is secure. Terminator 2, easily one of the best sequels in cinematic history, took the point one step further, with a toughened-up Sarah Connor trying to stop Armageddon and making this her mantra: "No fate but what we make." That is, the future isn't written in stone; if we can change the course of events. And she succeeds -- temporarily. 

In Terminator 3, we get another interesting twist. Despite all these attempts to rewrite history, the apocalypse still happens in the end -- just not in quite the same way it was supposed to happen. The implication? The future is inevitable, no matter how much one tries to change it. So the focus in T4 shifts to that inevitable future, choosing instead to ruminate on a new question: what makes us human.

Certain physicists like to nerd-gas about violations of physics in time travel movies. My own spouse, Sean, recently posted some helpful "Rules for Time Travelers" over at Cosmic Variance, outlining his pet peeves about how the topic is often portrayed in film. Three in particular are worth singling out because they relate to characters wanting to travel back in time in order to change the past.

The problem with this, as Sean points out, is that "If something happened, it happened." Based on everything we know thus far about relativity, quantum mechanics, and the like, even if one could travel backwards in time -- and this is a very difficult feat to accomplish -- it would not be possible to change the past, because the past already happened. For much the same reason, you can't travel back in time to a point where your spiffy time machine hasn't been invented yet. This is known in hip physics circles as the "chronology protection conjecture."

But then Sean comes through for sci-fi fans and includes a loophole: you can't change the past unless you go to a parallel universe. I've written about the "Many Worlds" theory before, and these days the notion of a multiverse is no longer grounds for automatic dismissal as a crackpot by the physics community.

Here's the gist: If you travel to a parallel universe, you can change your past, or at least experience a different outcome, because what you're actually doing is traveling to a different branch of the wavefunction. It's not really "your" past; you can only experience one branch of reality at a time. But things may have turned out differently for you in a parallel universe. And thus launched a thousand new sci-fi subplots.

That's certainly the approach taken on Fringe during its season finale: [SPOILER ALERT!]  Olivia starts dimension-hopping as a result of all that messing with her brain and taking some sort of weird (and totally fictional) mind-control drug called cortexifan as a kid. So she's shunting back and forth between her reality and an alternate Earth where Boston is a bombed-out ruin and the inhabitants are preparing to go to war with the "other" Earth. Or something. Also? Main character Olivia's phone is black in one dimension, red in the other, and her co-worker's desk has been moved. ZOMG! Alert the media! As io9 snarkily concludes, "So things are more dangerous and stylish in Parallel Earth."

Who knows what those crazy kids behind the wackiness that is Fringe will come up with next? Now that they've found a semi-plausible means of side-stepping the chronology protection conjecture, Olivia is free to make her own fate -- or rather, to choose the branch of the wavefunction with the best outcome, from her perspective. I hope it's one where Walter Bishop remains his zany, brilliantly wacky self, since an alternative in which he's a buttoned-down, respectable member of the scientific establishment would be too, too dreary....

Image: I Can Haz Cheezburger.

Warp Factor

April 23, 2009

Excitement is mounting over the imminent release of J.J. Abrams' Star Trek "prequel" wherein a promising young actor named Chris Pine attempts to walk in William Shatner's legendary footsteps as the young James T. Kirk. Even if you're not a hard-core Trekkie, it's tough to deny the enormous impact the series (both film and TV) has had on popular culture. "Beam me up Scotty." "He's dead, Jim." "Set phasers to stun." Not to mention the almost certain expendability of any unfortunate crew member wearing a bright red shirt (brilliantly satirized by the character of Guy in the spoof film Galaxy Quest).

And don't forget all that cutting-edge futuristic technology: phasers, the Holodeck, the transporter room, and those nifty handheld devices that inspired a thousand cell phone designs. But perhaps the most famous is the Enterprise's "warp drive", which enables it to travel faster than the speed of light -- something normally in violation of the laws of relativity, which say that nothing with mass can travel faster than light, even the tiniest subatomic particle. 

But is a warp drive possible for real? Alas, Wikipedia tells me that

At present, there is no known way to naturally or artificially establish a separate, finite space-time region or "bubble" - such a region is necessary to locally suspend or encapsulate the spacecraft within its view of a "normal" space-time. Concurrently, external from that region, there would exist a "warped" space-time, through which the separate region travels at velocities exceeding c, the speed of light.

Unless, of course, one happens to have a plentiful supply of antimatter and a "gravimetric field displacement manifold" handy, a.k.a., a warp core. The warp core is the literal heart of the Enterprise, a special kind of reactor in which matter and antimatter annihilate and release energy with 100% efficiency, thereby beating the laws of thermodynamics as well as relativity. When Stephen Hawking guest-starred on an episode of Star Trek: The Next Generation, he was given a tour of the set. Stopping in front of the model of the warp core, he commented, "I'm working on that." (Hawking has been gravely ill this past week, but it's looking like he'll pull through, and we wish him a speedy recovery.)

He's not the only one. Science and science fiction have always inspired one another in turn, and Star Trek has inspired as much physics research as it has drawn upon over the decades. The most promising theory to date was advanced in 1994 by Mexican physicist Michael Alcubierre, who insisted that while relativity forbids faster than light travel when it comes to the fabric of space-time, regions of space also move relative to each other, and some of those regions could, theoretically, move faster than the speed of light.

Alcubierre's notion is that the Enterprise would be enclosed within a highly distorted bubble of space-time, which would shrink in whatever direction the ship was traveling from the front of the ship, and expand behind it. The bubble could then move faster than light. Here's Lawrence Krauss, physics professor and author of The Physics of Star Trek, explaining it all for you in plain English, with just a balloon and a magic marker as props:

Ah, but that's relying on classical relativity. Bring it down to the quantum level and things get quite a bit trickier. Stefano Finazzi of the International School for Advanced Studies in Trieste, Italy, has been working the problem with a few colleagues, and earlier this month posted a paper claiming that "Warp drives would become rapidly unstable once superluminal speeds are reached." Bummer. It's got something to do with "the renormalized stress-energy tensor" which the math shows grows exponentially at faster-than-light speeds, making that cozy little bubble housing the Enterprise dangerously unstable.

Oh, and the bubble would also be filled with Hawking radiation, most likely killing the entire crew. Kirk and Crew don't know how good they have it in their fictional film and TV world, where the laws of physics can be bent at the whims of the writers.

Photo: Visualization of a spaceship in a warp field. Source: Wikipedia (Public Domain).

Fun with Gravity

March 23, 2009

Most of us are accustomed to seeing gravity-defying feats in film and television -- and I'd wager that a high percentage of those reading this blog can relate to Sheldon on The Big Bang Theory when he ranted about the scientific inaccuracies in the first Superman movie. You know, the one where Lois Lane falls from a great height and Superman catches her right before she hits ground? Sheldon's take is that, under actual Earth-like gravitational conditions, poor Miss Lane would have been sliced into three segments by Superman's of arms of steel. His conclusion: "If he really loved her, he'd let her hit the ground. It would be a more merciful death."

Still, who wouldn't love to tweak gravity, just to see what happens? It's not something we can do for real, but video games have become sufficiently sophisticated that new rules of gravity can be applied. Via the Science Punk Blog, I learned (belatedly -- it's a 2007 game) about Gravity Pods, an online game that requires the player to get his/her green rocket to "the purple swirly thing" (there's really no better way to describe it).

It's easier said than done. There are all these "pods", you see, creating gravitational fields that must be carefully navigated, lest they pull your plucky little rocket off-course. Develop a bit of skill, and you can get all Newtonian with the game, using those same gravitational fields "to slingshot your craft to its destination." Best of all -- from a visual aesthetics standpoint -- is that the game creates a colorful "map" out of the trajectories of all your failed rockets. Groovy!

I have no idea what kind of algorithms were used to create those gravitational fields, or how gravity in Gravity Pods works in comparison to how it works in our real world. The study hasn't been done yet. But it has been done for Super Mario Brothers! A nifty Website called Hypertextbook has the analysis of acceleration due to gravity in Super Mario Brothers. (Warning: equations are involved; but don't let that hold you back!) Per the authors:

Gravity is the force which is responsible for keeping us on the ground. It is also the force that prohibits us from jumping 50 feet in the air. However, in Mario's world, gravity does not quite work that way. Mario is able to jump 5 times his height and fall with accelerations that would be deadly to humans.

How do they know this? They did the math, of course, gleaning their data by recording video clips of the little Mario figure falling from ledges as presented in successive iterations (versions) of the game.

Their conclusion? "[G]enerally speaking, the gravity in each Mario game, as game hardware has increased, is getting closer to the true value of gravity on earth." So that's all right.

It's probably bad news for Mario, though: those falls will just get harder and harder to take. He'll still fare better than Lois Lane in Sheldon's alternate Superman scenario.

Photo: Screen shot of Mario falling off a ledge. Source: Hypertextbook.com.

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.

In Rotation

October 24, 2008

Back in August, I wrote about gravitational waves -- those ripples in the fabric of spacetime produced by violent events in the distant universe -- in the context of some recent findings by the Laser Interferometer Gravitational-Wave Observatory (LIGO). Now composer, conductor, percussionist and video artist Andrea Centazzo has put together a solo multimedia musical performance piece, Einstein's Cosmic Messengers, celebrating the quest for gravitational waves. And the world premiere takes place next Thursday, October 30, at 8 PM, at Caltech's Beckman Auditorium. Woo-hoo!

Those in the Los Angeles area might want to head on over for the performance, which will also feature two brief public lectures: on by Caltech's own black hole/gravity expert, Kip Thorne, on "The Warped Side of the Universe," -- I loved Black Holes and Time Warps, along with so many others -- and the second by LIGO's executive director, Jay Marx, "Listening for Ripples in Spacetime" (because that's essentially what LIGO does).

The musical portion of the evening is a five-part "suite," if you will, combining acoustic and digital images accompanied by video imagery -- some of it original, filmed in such locations as the recently restored theater of San Giovanni near Bologna, Italy, and the 13th century Castel del Montea in Apulia, Italy. Other imagery is draw from astronomical data and computer animations. The grand finale -- "Inspiral, Merger and Ringdown" -- features the life of a black-hole binary system; the instruments are based on the actual gravitational waveforms of such a system, transposed into audible sounds. Check it out:

Andrea Centazzo: Inspiral, Merger, and Ringdown (excerpts) from Michele Vallisneri on Vimeo.

I especially like the big swirling iris turning into a rotating black hole binary system, whirling faster and faster as it contracts, much like how the water in a flushing toilet spins around the center of the bowl, rotating faster and faster the closer it gets to the center. That said, the final scene calls to mind the fiery all-seeing eye of Sauron.

I should add that collaborating with Centazzo was theoretical physicist Michele Vallisneri of nearvy Jet Propulsion Lab. Vallisneri is a member of the LIGO collaboration and studies gravitational waves, but he is also intrigued by the "creative interface of science and art, as explored through music, visualization, and computer programs," according to his bio. Kudos to him for straddling the infamous "two cultures." I'm looking forward to the Big Premiere. Maybe we'll see you there!

"We All Believe in Relativity...."

October 13, 2008

It's long been known that song can be a pretty powerful mnemonic device: that is, setting certain facts to music helps anchor them more firmly in long-term memory. The Schoolhouse Rock vignettes from the 1970s are a prime example. An entire generation learned the basics of grammar, American history, and arithmetic by singing along on Saturday mornings ("I'm just a bill/Yes, I'm only a bill/And I'm sittin' here on Capitol Hill...").

Thanks to the glories of YouTube, MIT physics professor Max Tegmark has made a bit of a splash in the physics blogosphere this year with his own mnemonic offering: "The Relativity Song." Sung to the tune of the Beatles' "Yellow Submarine," he adapted the lyrics to help students in his relativity class prepare for the final exam. It's quite possibly the only song to date containing seven different equations -- helpfully spelled out in the subtitled lyrics.

From a musical standpoint, it's an amateur effort, and it certainly can't match the Beatles, but there's something quite charming about seeing a scientist of Tegmark's stature gamely warbling through the occasionally tortured lyrics, and urging his students to join in for the chorus: "We all believe in relativity, relativity, relativity...."

I was reminded of the Tegmark singalong video after stumbling on a fascinating paper by a Boston University physicist named Kaca Bradonjic on the topic of musical relativity. Any musician could list a few ways to change one chord into another, for instance, in order to alter the mood of the resulting music. Bradonjic contends that there is yet another way to change the pitch of a musical note: via the Doppler shift. We've all experienced this when, say, a fire truck rushes past with sirens blaring. As the truck moves toward us, we perceive the tone to be a higher frequency, and as it passes and moves away from us, the pitch appears to shift to a lower frequency. For the person riding on the truck, however, the frequency wouldn't change.

How would this work in a musical setting, such as a live concert? Well, theoretically, says Bradonjic, it should be possible for the mood of any given piece of music to depend on the relative velocities of the musicians on stage and the audience members. This would influence how certain notes were perceived: i.e., a "sad" note would be heard as a "happy" note, or vice versa, depending on how fast and in what direction the listener was traveling.

Practically, however, there are some stumbling blocks to relying on the Doppler shift to change the pitch of a musical note, namely, all those concerned would need to be mounted on independently moving platforms, and those platforms must be able to move at some non-negligible speeds. For instance, in order to hear a C major chord as a more melancholy C minor chord, the listener would need to be traveling at 43 MPH, directly away from the source. Add in all the various shifts in speed and direction needed to play even a short tune, and you've got a logistical (and mechanical) nightmare.

It's a fun little exercise, nonetheless, and perhaps Tegmark can figure out how to employ the scheme in a new version of his catchy little ditty. Now that's relativity we can believe in.

Oh, Pioneer

October 02, 2008

On March 2, 1972, a spacecraft called Pioneer 10 was launched from Cape Canaveral, followed a year later by Pioneer 11, headed for the furthest reaches of our solar system... and beyond. And just like the Energizer Bunny, they're still going, and going, and going, with NASA tracking their position and speed all along. The collected data yielded a surprising result: both Pioneer 10 and 11 appear to be decelerating faster than expected due to the sun's gravity, and scientists haven't been able to figure why. It's slight: each year, the probes veer off course by 8000 thousand miles, a small fraction of the 219 million miles they travel in that time. But that sort of thing adds up as the years pass. And the two Pioneers are in it for the long haul.

Science writer Michael Brooks devotes an entire chapter to the so-called Pioneer anomaly in his new book, 13 Things That Don't Make Sense (which I reviewed for New Scientist). Among other interesting facts, I learned that from the outset the Pioneer probes were unique from other spacecraft, in that they relied on literally "spinning" their way through space: "The spin provides a force that fixes the top's orientation; on Pioneer, the spin meant the mission scientists wouldn't have to worry about firing any thrusters to keep the craft on track," he writes.

Brooks also reports that NASA explicitly designed the probes as a test of Newton's laws  -- specifically, whether the universal theory of gravitation holds true at the edges of our solar system and beyond -- and while most scientists just don't buy the argument that the observed anomalies constitute any true violation of the laws of physics, Brooks opts to play devil's advocate: "The law failed the test; shouldn't we be taking that failure seriously?"

The answer, of course, is that scientists do take this apparent "failure" seriously. They're just not willing to rewrite the laws of physics until they've ruled out every conceivable ordinary explanation. Observational errors are unlikely, given the years of painstaking analysis already completed. Perhaps the slight shift is the result of gravitational forces from unidentified sources (dark matter, for example. Others have suggested we need to account for drag from interplanetary media (dust, solar wind, cosmic rays), or perhaps there are helium gas leaks from Pioneer's thermoelectric generators. Scientists at Jet Propulsion Lab believe the periodic variations result from, say, a tiny variation in Earth's orbit.

On the side of "new physics," there is MOND: MOdified Newtonian Dynamics, which is just what it sounds like: a slight modification of universal gravitation so that it is more consistent with the observed anomalies. Alas, no one has come up with a way to test MOND's predictions yet.

None of these can be said to have earned a consensus among scientists. So the anomaly remains a mystery.

We haven't heard a peep from Pioneer 10 since January 2003; there's no longer any power left with which to send back even a very weak signal. Brooks estimates the plucky little spacecraft is now well past the orbits of Neptune and that erstwhile planet, Pluto, some 8 billion miles away. It's supposed to hit the star Aldebaran (in the Taurus constellation) in about 2 million years or so, assuming Newton's laws hold true, and/or scientists can make adjustments in their calculations of the expected trajectory. How far off course will Pioneer be by then, assuming it hasn't been destroyed? And will our instrumentation be sufficiently advanced that we'd even know if it made it to Aldebaran?

No wonder hopes seem to be pinned to the New Horizons spacecraft mission to Pluto, which is another spin-stabilized design that could yield further insights into the Pioneer anomaly. The European Space Agency, among others, has flirted with the notion of a mission dedicated solely to investigating the anomaly.

But perhaps the mystery has already been solved. Bruno Christophe and colleagues from a French aerospace lab called ONERA (near Paris) have apparently performed the most detailed analysis of the Pioneer data thus far, and last week posted a new paper on arXiv claiming the periodic variations in the Pioneer data are consistent with the effects on radio signals that some of MOND's modifications might cause. It's a daring claim, to say the least, but if their analysis stands up under the rigorous scrutiny that has no doubt already begun, this could be some exciting evidence of new physics beyond the bounds of Einstein's revered general relativity. And that would make the Pioneer spacecraft... true pioneers.

Photo: Artist's conception of Pioneer 10. Source: NASA, via Wikimedia Commons. (Public domain)

Gravity Wave Blues

August 15, 2008

Back in 1974, a pair of scientists located a pair of neutron stars in the Milky Way galaxy, one of which was a pulsar: that is, it emits regular pulses of radio waves easily detectable on Earth. Joseph Taylor and Russell Hulse used those very precise, regular pulses as a kind of clock, and observed the orbiting neutron stars over two decades. And they found a pronounced shift in the timing of those pulses. That meant that the stars had lost energy, carried away by gravitational waves -- just as Albert Einstein had predicted in his theory of general relativity.

Chalk up another victory for Einstein, and one for Taylor and Hulse: they shared a Nobel Prize in 1993 for their discovery. But direct observation of gravitational waves continues to elude scientists. According to general relativity, mass warps the fabric of space-time, and this curvature accounts for what we observe as gravity. When a large celestial mass moves suddenly -- for instance, if a star explodes as a supernova -- some of that curvature will ripple outwards, just like the ripples in a pool of water if you suddenly dropped a rock into its center.

The same thing can happen with a neutron star, the incredibly dense burned-out core that remains after a star explodes. Two of these incredibly dense objects circling each other stir space-time as they move -- much like a very large kitchen mixer -- and this causes ripples of gravitational energy through the fabric of space-time. These ripples are very, very faint, since the waves weaken as they ripple outward, so by the time they reach Earth, they are very weak indeed. But with sensitive enough instruments, we should, in theory, be able to detect them.

How hard can it be to catch a gravitational wave? Pretty darn hard, it turns out. Back in 1969, a physicist  named Joseph Weber at the University of Maryland set up giant cylindrical bars, thinking that should a gravitational wave pass by, it would cause them to vibrate, or ring like a bell. He claimed he 'd succeeded, but no one else could reproduce his results, so it remains a highly disputed experiment, even though several groups today still listen for the telltale ripples using similar detectors.

The most promising type of detector developed so far is called a laser interferometer, an instrument that precisely measures how long it takes light to travel between suspended mirrors, using laser light. Any ripples in space-time should cause the distance measured to change as the gravitational wave passes by, and this change can be picked up by a photodetector. In essence, it acts like a microphone, converting gravitational waves into electrical signals.

The Laser Interferometer Gravitational Wave Observatory (LIGO) has three laser interferometers: two near Richland, Washington, and a third near Baton Rouge, Louisana. You need at least two instruments separated by a great distance to rule out false signals. To date, LIGO hasn't detected any gravitational waves.

In a bid to further increase sensitivity, LIGO scientists combined their search with a fourth detector, the GEO600 in Germany, all scanning the heavens simultaneously for those telltale ripples. They announced their results in an arXiv paper last week, and the news is not encouraging. They collected data for a full month and concluded that "No candidate gravitational wave signals have been identified." Considering the hundreds of millions of dollars spent to date on LIGO, this doesn't bode well for future projects, such as LISA, which -- if built -- would take the search for gravitational waves into space, thereby increasing the chances of making a direct observation.

The good news is that this need not mean that general relativity is "wrong," since many of its other predictions have been verified repeatedly. But it may be incomplete. Until scientists can definitively make such a conclusion, the search for those elusive ripples in space-time will (funding permitting) continue.

Photo: Illustration of ripples in the fabric of space-time, Kip Thorne (Caltech) and T. Carnahan (NASA/GSFC). Source: NASA/Jet Propulsion Laboratory.