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There’s More Than One Way To Hunt For Gravitational Waves

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 This is Part 1 of a two-part feature on the search for gravitational waves. Read Part 2
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2014/11/theres-more-than-one-way-to-hunt-for-gravitational-waves-part-2/">
 here
 </a>
 .
 


 Physicists are poised to make the first-ever direct detection of gravitational waves. Will the detection come from a big-budget experiment already a decade deep into the search? Or will one of a handful of dark-horse experiments win an upset?
 

 When Albert Einstein published his general theory of relativity in 1916, he revolutionized physics and reenvisioned the nature of spacetime and gravity: he showed that spacetime was dynamic, not static, and reimagined gravity as the bending and warping of spacetime by massive objects. He also made the startling prediction that gravity travels in waves. Just as objects moving through water cause waves to ripple outward, objects moving through space should produce ripples in spacetime. The more massive the object, the more it will churn the surrounding spacetime, and the stronger the gravitational waves it should produce.
 
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 Most of these ripples would be small, and would dissipate too quickly to be detected, Einstein predicted. But certain bodies, such as merging black holes, supernovae, or orbiting pairs of neutron stars, are massive enough that they might produce detectable
 <a href="http://imagine.gsfc.nasa.gov/docs/features/topics/gwaves/gwaves.html">
 gravitational waves
 </a>
 . Physicists have devised a number of ingenious methods to detect gravitational waves directly, from extremely precise laser
 <a href="http://en.wikipedia.org/wiki/Interferometry">
 interferometry
 </a>
 to clever schemes for using stars, pulsars, even the Earth and moon as gravitational wave detectors.
 

 The big-ticket project is the half-a-billion-dollar
 <a href="http://www.ligo.caltech.edu">
 Laser Interferometer Gravitational Wave Observatory
 </a>
 (LIGO), which boasts two highly sensitive laser interferometers in Louisiana and Washington state. Here’s how it works: Split a laser beam in two and send each beam down one of two long, perpendicular tunnels, each with a mirror at the end. When the laser beams strike the mirrors they will be reflected back to the same spot, where they will recombine and cancel each other out. But if a gravitational wave happens to be passing through, it will warp the space between those mirrors ever so slightly. One beam will travel a longer path than the other, and when they meet up again, they won’t cancel each other out, producing light that will be picked up by a
 <a href="http://space.mit.edu/LIGO/more.html">
 detector
 </a>
 .
 

 LIGO ran from 2002 to 2010—almost a decade—yet failed to detect any gravitational waves. Furthermore, LIGO is sensitive only to a small fraction of the gravitational wave spectrum. Just as optical, radio, x-ray, infrared, and gamma-ray telescopes each reveal different, and complementary, electromagnetic views of the cosmos, says Montana State University physicist Neil Cornish, it will take more than one kind of gravitational wave telescope to “see” the full gravitational wave spectrum. “You can only see [the waves] in their particular [frequency] bands because the frequency they emit is set by the mass of the system,” Cornish explained in an interview. “We need to open up the entire gravitational wave spectrum just like we’ve opened up the entire electromagnetic spectrum [in astronomy].”
 

 LIGO’s range centers on stellar remnant black holes and other celestial objects of similar mass. Tackling the lowest end of the gravitational wave frequency spectrum are the
 <a href="http://www.quantamagazine.org/20140921-big-bang-signal-could-all-be-dust-planck-says/">
 headline-grabbing
 </a>
 BICEP2 and Planck experiments, which are looking for imprints left by gravitational waves from the earliest moments of our universe in the polarization of the cosmic microwave background radiation.
 

 To detect higher-frequency gravitational waves, like those produced by supermassive black holes, astronomers are using pulsars—rapidly rotating neutron stars that beam out regular radio pulses—like beacons on the sloshing sea of spacetime. One such effort is the
 <a href="http://nanograv.org">
 North American Nanohertz Observatory for Gravitational Waves
 </a>
 (NanoGRAV), part of an international consortium that also includes the
 <a href="http://www.epta.eu.org">
 European Pulsar Timing Array
 </a>
 , and the Parkes Pulsar Timing Array in Australia.
 

 The first pulsar was discovered in 1967, when Jocelyn Bell Burnell and Antony Hewish noticed strange, highly regular radio pulses coming from a fixed point in the night sky. They cheekily dubbed the mysterious object LGM-1 (for “little green men”). The signals weren’t coming from alien transmissions, however, but from a rapidly rotating neutron star. Pulsars form when stars more massive than our Sun explode and collapse into neutron stars. As they shrink, they spin faster and faster, because angular momentum is conserved. (Think of what happens when you swing an object around your head on a string: the more you shorten the tether, the faster it goes.) Pulsars also blast out radiation that can be picked up on Earth whenever that beam sweeps into our direction, like the rotating beam of a lighthouse.
 

 The fastest pulsars, spinning hundreds of times per second, make excellent clocks—on par with the best atomic clocks. “That regular rotation of a pulsar is like the swing of a pendulum,” said Cornish, and it enables astrophysicists to precisely time all kinds of astronomical systems. Pulsars have helped astronomers identify distant exoplanets, and provided the
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/press.html">
 first indirect evidence for gravitational waves
 </a>
 back in 1982, when astronomers observed energy leaking out of a binary pulsar system—probably in the form of gravitational radiation.
 

 The NanoGRAV network uses data from telescopes at the Arecibo Osbervatory in Puerto Rico and the Green Bank Telescope in West Virgina to monitor 19 pulsars in the Milky Way that serve as a galactic-scale gravitational wave detector. The method is described on NanoGRAV’s Website as a “cosmic Global Positioning System… looking for tiny changes in the position of the Earth that are due to the shrinking and stretching effect of passing gravitational waves,” although Cornish said the analogy is imperfect. The GPS employs multiple satellites to triangulate the three dimensions of space, thereby pinpointing the location of the source of a signal. NanoGRAV is looking for a common effect in the form of a telltale signature: a “shimmering” effect produced because pulses affected by gravitational waves should arrive slightly earlier or later in response to those ripples in spacetime. While no detection has yet been made, the collaborators are currently combining data from all three arrays to further improve accuracy and precision, according to Cornish. Those results should be released in the next several months.
 

 “Compared to the cost of LIGO, this is the bargain basement way of detecting gravitational waves,” says Cornish. “The NSF has made this major investment using laser interferometers. For a tiny fraction of that, they have a chance to enable detection using pulsar timing. As far as bang for buck, it’s the cheapest way to go about it.”
 


 Go Deeper
 
 

 Author’s picks for further reading
 


 Galileo’s Pendulum:
 <a href="http://galileospendulum.org/2013/03/26/will-we-ever-detect-gravitational-waves-directly">
 Will We Ever Detect Gravitational Waves Directly?
 </a>
 
 Matthew Francis explains how LIGO and similar detectors are advancing the search for gravitational waves.
 

 Nature:
 <a href="http://www.nature.com/news/physics-wave-of-the-future-1.15561">
 Wave of the Future
 </a>
 
 Alexandra Witze previews the launch of the new advanced LIGO.
 

 TED:
 <a href="http://www.ted.com/talks/janna_levin_the_sound_the_universe_makes?language=en">
 The Sound the Universe Makes
 </a>
 
 In this video, astrophysicist Janna Levin explains how gravitational waves are made and LIGO’s role in searching for them.
 
</section>

Will one of a handful of dark-horse experiments be the first to detect gravitational waves?

A Brief History of the Speed of Light

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 One night over drinks at a conference in San Jose, Miles Padgett, a physicist at Glasgow University in Scotland, was chatting with a colleague about whether or not they could make light go slower than its “lawful” speed in a vacuum. “It’s just one of those big, fundamental questions you may want to ask yourself at some point in the pub one night,” he told BBC News. Though light slows down when it passes through a medium, like water or air, the speed of light in a vacuum is usually regarded as an absolute.
 
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 This time, the pub talk proved to be a particularly fruitful exchange. Last month, Padgett and his collaborators
 <a href="http://www.bbc.com/news/uk-scotland-glasgow-west-30944584">
 made headlines
 </a>
 when they revealed their
 <a href="http://arxiv.org/ftp/arxiv/papers/1411.3987.pdf">
 surprising success
 </a>
 : They raced two photons down a one-meter “track” and managed to slow one down just enough that it finished a few millionths of a meter behind its partner. The experiment showed that it is possible for light to travel at a slower speed even in free space—and Padgett and his colleagues did it at the scale of individual photons.
 

 The notion that light has a particular speed, and that that speed is measurable, is relatively new. Prior to the 17th century, most natural philosophers assumed light traveled instantaneously. Galileo was one of the first to test this notion, which he did with the help of an assistant and two shuttered lanterns. First, Galileo would lift the shutter on his lantern. When his assistant, standing some distance away, saw that light, he would lift the shutter on his lantern in response. Galileo then timed how long it took for him to see the return signal from his assistant’s lantern, most likely using a
 <a href="http://en.wikipedia.org/wiki/Water_clock">
 water clock
 </a>
 , or possibly his pulse. “If not instantaneous, it is extraordinarily rapid,” Galileo concluded, estimating that light travels at about ten times the speed of sound.
 

 Over the ensuing centuries, many other scientists improved upon Galileo’s work by devising ingenious new methods for measuring the speed of light. Their results fell between 200,000 kilometers per second, recorded in 1675 by Ole Roemer, who made his measurement by studying eclipse patterns in Jupiter’s moons, and 313,000 kilometers per second, recorded in 1849 by Hippolyte Louis Fizeau, who sent light through a rotating tooth wheel and then reflected it back with a mirror. The current accepted value is 299,792.458 kilometers per second, or 669,600,000 miles per hour. Physicists represent this value with the constant c, and it is broadly understood to be the cosmic speed limit: all observers, no matter how fast they are going, will agree on it, and nothing can go faster.
 

 This limit refers to the speed of light in a vacuum—empty space, with no “stuff” in it with which light can interact. Light traveling through air, water, or glass, for example, will move more slowly as it interacts with the atoms in that substance. In some cases, light will move so slowly that other particles shoot past it. This can create Cherenkov radiation, a “photonic boom” shockwave that can be seen as a flash of blue light. That telltale blue glow is common in certain types of nuclear reactors. (Doctor Manhattan, the ill-fated atomic scientist in Alan Moore’s classic “Watchmen” graphic novel, sports a Cherenkov-blue hue.) It is useful for radiation therapy and the detection of high-energy particles such as neutrinos and cosmic rays—and perhaps one day, dark matter particles—none of which would be possible without the ability of certain materials to slow down light.
 

 But just how slow can light go? In his 1933 novel “Master of Light,” French science fiction writer Maurice Renard imagined a special kind of “slow glass” through which light would take 100 years to pass. Slow glass is very much the stuff of fiction, but it has an intriguing real-world parallel in an exotic form of matter known as a Bose-Einstein Condensate (BEC), which exploits the wave nature of matter to stop light completely. At normal temperatures atoms behave a lot like billiard balls, bouncing off one another and any containing walls. The lower the temperature, the slower they go. At billionths of a degree above absolute zero, if the atoms are densely packed enough, the matter waves associated with each atom will be able to “sense” one another and coordinate themselves as if they were one big “superatom.”
 

 First predicted in the 1920s by Albert Einstein and the Indian physicist Satyendra Bose, BEC wasn’t achieved in the lab until 1995. The Nobel Prize winning research quickly launched an entirely new branch of physics, and in 1999, a group of Harvard physicists realized they could slow light all the way down to 17 miles per hour by passing it through a BEC made of ultracold sodium atoms. Within two years, the same group succeeded in stopping light completely in a BEC of rubidium atoms.
 

 What was so special about the recent Glasgow experiments, then? Usually, once light exits a medium and enters a vacuum, it speeds right back up again, because the reduced velocity is due to changes in what’s known as phase velocity. Phase velocity tracks the motion of a particular point, like a peak or trough, in a light wave, and it is related to a material’s refractive index, which determines just how much that material will slow down light.
 

 Padgett and his team found a way to keep the brakes on in their experiment by focusing on a property of light known as group velocity. Padgett
 <a href="http://www.bbc.com/news/uk-scotland-glasgow-west-30944584">
 likens the effect to a subatomic bicycle race
 </a>
 , in which the photons are like riders grouped together in a peloton (light beam). As a group, they appear to be moving together at a constant speed. In reality, some individual riders slow down, while others speed up. The difference, he explained to BBC News, is that instead of using a light pulse made up of many photons, “We measure the speed of a single photon as it propagates, and we find it’s actually being slowed below the speed of light.”
 

 The Glasgow researchers used a special liquid crystal mask to impose a pattern on one of two photons in a pair. Because light can act like both a particle and a wave—the famous wave-particle duality—the researchers could use the mask to reshape the wavefront of that photon, so instead of spreading out like an ocean wave traveling to the shore, it was focused onto a point. That change in shape corresponded to a slight decrease in speed. To the researchers’ surprise, the light continued to travel at the slightly slower speed even after leaving the confines of the mask. Because the two photons were produced simultaneously from the same light source, they should have crossed the finish line simultaneously; instead, the reshaped photon lagged just a few millionths of a meter behind its partner, evidence that it continued to travel at the slower speed even after passing through the medium of the mask.
 

 Padgett and his colleagues are still pondering the next step in this intriguing line of research. One possibility is looking for a similar slow-down in light randomly scattered off a rough surface.
 

 If so, it would be one more bit of evidence that the speed of light, so often touted as an unvarying fundamental constant, is more malleable than physicists previously thought. University of Rochester physicist Robert Boyd, while impressed with the group’s ingenuity and technical achievement, calmly took the news in stride. “I’m not surprised the effect exists,” he told
 <a href="http://www.sciencenews.org/article/speed-light-not-so-constant-after-all">
 Science News
 </a>
 . “But it’s surprising that the effect is so large and robust.”
 

 His nonchalance might strike non-physicists as strange: Shouldn’t this be momentous news poised to revolutionize physics? As always, there are caveats. When it comes to matters of light speed, it’s important to read the fine print. In this case, one must be careful not to confuse the speed at which light travels, which is just a feature of light, with its central role in special relativity, which holds that the speed of light is constant in all frames of reference. If Galileo measures the speed of light, he gets the same answer whether he is lounging at home in Pisa or cruising in a horse-drawn carriage. The same goes for his trusty assistant. This still holds true, centuries later, despite the exciting news out of Glasgow last month.
 


 Go Deeper
 
 

 Our picks for further reading
 


 arXiv:
 <a href="http://arxiv.org/abs/1411.3987">
 Photons that travel in free space slower than the speed of light
 </a>
 
 Access a full preprint of Padgett and his colleagues’ paper, later published in Science.
 

 Empire of Light:
 <a href="http://books.google.com/books/about/Empire_of_Light.html?id=ROHJnk2JOfIC">
 A History of Discovery in Science and Art
 </a>
 
 Sidney Perkowitz, a condensed matter physicist, looks at the history of our understanding of light in both art and science.
 

<a href="http://www.light2015.org/Home.html">
 International Year of Light
 </a>
 
 The UN General Assembly has proclaimed 2015 as the International Year of Light (IYL 2015). Discover more about the science, technology, and social impact of light at the web site of the IYL.
 

 NOVA:
 <a href="http://www.pbs.org/wgbh/nova/physics/ultracold-atoms.html">
 Ultracold Atoms
 </a>
 
 Take a closer look at Bose Einstein condensates in this Q&amp;A with physicist Luis Orozco.
 
</section>

From Galileo to today, how physicists have measured and modified the speed at which light travels.

There’s More Than One Way To Hunt For Gravitational Waves, Part 2

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 This is Part 2 of a two-part feature on the search for gravitational waves. Read Part 1
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2014/11/theres-more-th…tational-waves/">
 here
 </a>
 .
 


 All of these techniques are exquisitely sensitive, seeking out minute changes. But gravitational waves might have a much stronger impact on matter than previously assumed, thanks to resonant frequencies. It’s something Alexander Graham Bell noticed as a young man: strike a chord on one piano and it will be echoed by a piano in another room. The effect is known as “sympathetic resonance.” Objects like a piano’s strings vibrate at very specific frequencies. If there is another object nearby that is sensitive to the same frequency, it will absorb the vibrations (sound waves) emanated from the other object and start to vibrate in response.
 
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 That’s the essence of a new paper in the Monthly Notices of the Royal Astronomical Society: Letters, proposing that certain stars could absorb energy from gravitational waves that ripple by. Should that happen, the stars would show a temporary marked increase in brightness from that excess energy that
 <a href="http://www.space.com/27510-gravitational-wave-detection-method.html">
 could be measurable
 </a>
 .
 

 Co-author Saavik Ford of CUNY’s Graduate Center compared stars to the bars on a xylophone, each of which has a natural resonant frequency, just like piano strings. Striking those bars in sequence, moving from lower to higher frequencies, is akin to how two merging black holes produce gravitational waves of gradually increasing frequency. “If you have two black holes merging with each other and emitting gravitational waves at a certain frequency, you’re only going to hit one of the bars on the xylophone at a time,”
 <a href="http://www.amnh.org/explore/news-blogs/research-posts/hints-of-gravitational-waves-in-the-stars">
 Ford explained
 </a>
 . “But because the black holes decay as they come closer together, the frequency of the gravitational waves changes and you’ll hit a sequence of notes. So you’ll likely see the big stars lighting up first followed by smaller and smaller ones.”
 

 Perhaps the Earth itself could be used as a gravitational wave detector: it, too, could vibrate like a bell in response to gravitational waves rippling through. Set up a global array of highly sensitive seismometers, and one could conceivably find evidence of such waves in the data That was the gist of a
 <a href="http://adsabs.harvard.edu/full/1969ApJ...156..529D">
 1969 paper
 </a>
 by
 <a href="http://www.webofstories.com/play/freeman.dyson/135">
 physicist Freeman Dyson
 </a>
 .
 

 Dyson’s work was the inspiration for Harvard graduate student Michael Coughlin and a colleague, Jan Harms of the National Institute of Nuclear Physics in Florence, Italy, who were working with seismic data relating to LIGO with an eye toward reducing the noisy background so that a signal would be more easily detected. Coughlin recalled Dyson’s paper and thought such an approach could be useful for setting some vital constraints on background noise, and he and Harms did an initial analysis of terrestrial seismic data.
 

 Then another professor recalled his earlier geophysical work with instruments placed on the moon during the Apollo missions to track so-called “moonquakes.” Those instruments collected lunar seismic data from July 1975 to March 1977. Intrigued,
 <a href="//medium.com/the-physics-arxiv-blog/the-apollo-missions-and-the-hunt-for-gravitational-waves-64696256d3">
 Coughlin and Harms analyzed that older dataset
 </a>
 as well, correlating it with their earlier terrestrial analysis. They published their findings in Physical Review Letters earlier this year.
 

 Coughlin and Harms
 <a href="http://arxiv.org/abs/1409.4680">
 didn’t find any evidence of gravitational waves
 </a>
 in their analysis, nor did they expect to. One reason is that there is a lot of seismic noise from other sources cluttering up the data. The moon might not have Earth’s plate tectonics, atmospheric fluctuations, or volcanic activity, but asteroids routinely hit the moon, causing it to “ring” for weeks from the impact. There is also background noise generated by thermal heating from the Sun and tidal forces.
 

 Cornish pronounced their work a good analysis but said it is unlikely to lead to direct detection of gravitational waves, even if NASA placed upgraded seismometers on the moon with far greater sensitivity to get a better dataset. He suggested that the best way to search for gravitational waves in that frequency range is the space-based
 <a href="https://www.elisascience.org">
 LISA
 </a>
 , now known as the Next Gravitational-wave Observatory (NGO), another very large and pricey collaboration similar to LIGO (in that it uses a similar laser interferometer array) that is still several years’ away from completion. Meanwhile,
 <a href="http://www.symmetrymagazine.org/article/October-2014/to-catch-a-gravitational-wave">
 LIGO is currently undergoing upgrades
 </a>
 , including an additional mirror to increase its sensitivity to other frequencies of gravitational waves, like those produced by binary pulsars.
 

 Still, there remains much uncertainty in the various proposed models for gravitational waves. Coughlin’s and Harms’ null result has helped further constrain the range in which we should expect to see gravitational waves in Earth’s vicinity. “If we thought we knew what the source distribution of gravitational waves looked like in the universe, then it wouldn’t be quite such a useful exercise,” Coughlin said. “Since we don’t, and the cost is relatively low, I don’t see why we shouldn’t try it.”
 


 Go Deeper
 
 

 Author’s picks for further reading
 


 Galileo’s Pendulum:
 <a href="http://galileospendulum.org/2013/03/26/will-we-ever-detect-gravitational-waves-directly">
 Will We Ever Detect Gravitational Waves Directly?
 </a>
 
 Matthew Francis explains how LIGO and similar detectors are advancing the search for gravitational waves.
 

 Nature:
 <a href="http://www.nature.com/news/physics-wave-of-the-future-1.15561">
 Wave of the Future
 </a>
 
 Alexandra Witze previews the launch of the new advanced LIGO.
 

 TED:
 <a href="http://www.ted.com/talks/janna_levin_the_sound_the_universe_makes?language=en">
 The Sound the Universe Makes
 </a>
 
 In this video, astrophysicist Janna Levin explains how gravitational waves are made and LIGO’s role in searching for them.
 
</section>

Quantum Physicists Catch a Pilot Wave

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 In October 1927, some of the greatest minds in physics gathered for the Fifth Solvay International Conference to debate the troubling implications of the then-nascent theory of quantum mechanics. A particularly contentious topic was the perplexing “wave-particle duality,” in which objects we typically think of as particles—like photons and electrons—exhibit wave-like properties as well, and things we think of as waves, like light, sometimes behave like particles.
 

 The French physicist Louis de Broglie proposed a means by which a photon or electron could behave like both a particle and a wave, complementary aspects of the same phenomenon. He reasoned that the particles could be carried along by what he dubbed “pilot waves”—fluid-like ripples in space and time—much like a buoys bobbing along with the tide.
 
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 De Broglie won the Nobel Prize in Physics just two years later, but it wasn’t for pilot waves. His contemporaries largely dismissed his explanation for the dual nature of subatomic particles. Now, more than 80 years later,
 <a href="http://newsoffice.mit.edu/2013/when-fluid-dynamics-mimic-quantum-mechanics-0729">
 a series of experiments
 </a>
 on the behavior of oil droplets bouncing along a vibrating liquid surface have provided a macroscale analog of de Broglie’s pilot waves, replicating some of the stranger properties of quantum mechanics.
 

 Quantum mechanics seeks to describe nature at the level of individual atoms and the particles that comprise them. But when physicists began delving into this strange new realm at the dawn of the 20th century, they discovered that the old, deterministic laws of classical physics no longer apply at that scale. Instead, uncertainty reigns supreme. It is a world governed by probabilities, and many physicists found this disquieting, to say the least. Hence Albert Einstein’s famous declaration at the Solvay conference that God does not play dice with the universe, prompting Niels Bohr to counter, “Einstein, stop telling God what to do.”
 

 At the heart of the discomfort is the question of uncertainty. Flip a coin, and it will land either heads or tails; in principle, with complete information about the coin, the hand doing the flipping, and the movement of air molecules around the flip, it is possible to predict the outcome. In the quantum world, things hover in a fuzzy, nebulous cloud of probability called a wave function that encompasses all potential states, with no prospect of gaining further information. Flip a quantum coin, and it is both heads and tails until we look. Things become definite only when an observation forces them to settle on a specific outcome.
 

 To Einstein, the notion of observation dictating the outcome of an experiment was ridiculous, since it denied the existence of a solid underlying reality. Even Schrödinger, inventor of the wave function, was deeply disturbed by the implications of what he’d helped create, memorably declaring, “I don’t like it, and I’m sorry I ever had anything to do with it.”
 

 De Broglie’s alternative pilot wave theory was an attempt to restore that underlying solid reality. Instead of the wave function, de Broglie’s pilot wave theory employs two equations, one describing an actual wave and the other describing the path of an actual particle and how it interacts with, and is guided by, the wave equation. It is deterministic, like a classical coin flip. In principle, at least, we can glean sufficient information to plot a particle’s path, something that is not allowed in Bohr’s interpretation of quantum mechanics.
 

 While the idea of pilot wave theory never really caught on, it stubbornly refused to die. A physicist named David Bohm proposed a modified version in the 1950s that also failed to gain much traction. But perhaps the pilot wave’s time has come at last.
 

 The latest resurgence of interest began in Paris about ten years ago, when Yves Couder and Emmanuel Fort of Diderot University started experimenting with oil droplets bounced off a vat of vibrating liquid. The droplet’s impact causes waves to ripple outward, like tossing a pebble in a pond. If the liquid in the vat vibrates at just the right frequency, usually quite close to the droplet’s natural resonant frequency, the droplet interacts with the ripples it creates as it bounces along, which in turn can affect its path. That’s eerily similar to de Broglie’s notion of a pilot wave. Such a system turns out to be a fantastic means for simulating weird quantum effects like the dual nature of light and matter.
 

 The 19th century physicist Thomas Young demonstrated this with his famous “double-slit” experiment. In the double slit experiment, a series of photons or electrons strike a screen with two slits in it before landing on a detector behind the screen. If you consider photons and electrons to be particles, you would expect the detector light up along the path through the slits and nowhere else. But that’s not what Young found. Instead, Young discovered an interference pattern of alternating light and dark bands, suggesting that the would-be particles were acting like water waves passing through a barrier wall with two openings. But if one places detectors near the slits to “see” which slit each particle went through, then the interference pattern disappears—the waves start acting like particles. It is the essence of quantum weirdness.
 

 Couder and Fort replicated Young’s experiment by steering their bouncing droplets toward such a screen with a slit, helped along by the pilot waves created by the vibrating liquid. While they appear to scatter randomly as they pass through the slit, over time, wavy interference patterns emerge. “Guided” by the pilot waves, the droplets appear to be drawn to those regions where the wavefronts add together, and steer clear of those regions where the wavefronts cancel each other out. Disturbing the pilot wave destroys the interference pattern, much like measuring the path of particles as they hit the screen does.
 

 Last year, pilot wave theory received another boost when MIT physicists Daniel Harris and John Bush used a similar fluid system to mimic a “quantum corral,” in which electrons are trapped within a ring of ions. Harris and Bush made a shallow tray with a circle-shaped trough in the center to serve as the walls of the “corral.” They filled the tray with silicon oil and mounted it on a vibrating stand tuned to a frequency just shy of that required to produce pilot waves spontaneously, without the droplet, according to Bush. Above that threshold, the roiling sea of waves will interfere with the droplet’s walk. Below it, the surface remains smooth except for the waves produced by the bouncing droplet. The closer one tunes the vibrations to that threshold, the more robust and long-lived the generated pilot waves will be.
 

 When the bouncing droplet produced waves, those waves bounced off the walls and interfered with each other, producing pretty interference patterns. They also affected the trajectory of the droplet. At first it looked like it was bouncing along randomly, but over time (around 20 minutes), the droplet was far more likely to drift towards the center of the circle, and increasingly less likely to be found in the rippling rings spreading out from that center. That probability distribution for the single droplet proved very similar to that of an electron trapped in a quantum corral.
 
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 The droplet experiments provide an intriguing analogue or “toy model” for de Broglie’s pilot waves, but there is still no direct evidence of pilot waves at the quantum scale. “Time will tell whether the quantum-like behavior of the walking dropets is mere coincidence,” Bush told me via email. Also, the theory is currently limited to describing the simplest interactions between particles and electromagnetic fields. “It is not by itself capable of representing very much physics,” Oxford University physics philosopher David Wallace told Quanta earlier this year. “In my own view, this is the most severe problem for the theory, though, to be fair, it remains an active research area.”
 

 Nobody is claiming that quantum mechanics is wrong; there is too much experimental evidence that the equations do make accurate predictions about how things work at the subatomic scale. But the implications of the standard interpretations remain troubling. The pioneers of quantum mechanics came up with the most plausible theory they could, given the resources they had, and they transformed modern physics in the process. Contemplating the possibility of pilot wave theory might lead to a fresh interpretation of quantum weirdness, one that prompts physicists to rethink their longstanding assumptions about the true nature of the quantum world. Another transformation could be lurking in the wings.
 


 Go Deeper
 
 

 Author’s picks for further reading
 


 Cocktail Party Physics, Scientific American:
 <a href="http://blogs.scientificamerican.com/cocktail-party-physics/2012/06/27/the-photon-has-two-faces/">
 The Photon Has Two Faces
 </a>
 
 Jennifer Ouellette on quantum interference, the wave-particle duality, and that “cheeky over-achieving upstart” Thomas Young.
 

 Quanta:
 <a href="http://www.simonsfoundation.org/quanta/20140624-fluid-tests-hint-at-concrete-quantum-reality">
 Fluid Tests Hint at Concrete Quantum Reality
 </a>
 
 Natalie Wolchover on macroscale analogs for pilot wave physics.
 
</section>

Experiments with bouncing oil droplets could change our understanding of quantum mechanics and the nature of reality.

Jennifer Ouellette

Articles

There’s More Than One Way To Hunt For Gravitational Waves

A Brief History of the Speed of Light

There’s More Than One Way To Hunt For Gravitational Waves, Part 2

Quantum Physicists Catch a Pilot Wave

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