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NOVA’s Nobel Poll: Scientists Predict the 2013 Laureates

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<p>
  The selection process for the Nobel prizes—this year’s to be announced starting October 7—is highly secretive, but that doesn’t stop the speculation. In that spirit, we surveyed dozens of scientists in the fields of physics, chemistry, and physiology, asking who they think could win this year’s Nobel Prize.
 </p>
<h2>
  Physics
 </h2>
<p>
  For the physics prize, almost all of our respondents predicted that it will honor the discovery of the Higgs boson,
  <a href="http://www.pbs.org/wgbh/nova/physics/higgs-boson-revealed.html">
   announced tentatively in July 2012
  </a>
  and with
  <a href="http://www.pbs.org/wgbh/nova/next/physics/higgs-boson-discovered/">
   greater confidence in March 2013
  </a>
  . But with thousands of scientists involved in the discovery,
  <a href="http://www.pbs.org/wgbh/nova/physics/blog/2012/03/a-higgs-by-any-other-name/">
   eight or more
  </a>
  contributing to the origination of the theory, and the prize allowed to go to no more than three winners, how will the Nobel committee divvy up the laurels?
 </p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/nobel-medal.width-990_CHpDg2c.jpg"/>
<figcaption>The Nobel Prize medal
        
    </figcaption>
</figure>
<p>
  Most of the physicists responding to our poll felt that the prize would go to Peter Higgs and Francois Englert, who independently invoked a new field—known today as the Higgs field—to explain why subatomic particles have mass. Their ideas were published within just weeks of each other. As Peter G. O. Freund, a theoretical physicist at the University of Chicago, wrote, “For the better part of the past half century the phenomenon discovered by Englert, Brout [Englert’s coauthor] and Higgs has been a crucial ingredient of the standard model of physics, and repeatedly verified indirectly along with this model. Now, after the phenomenon’s beautiful direct verification at the LHC, it is high time to honor its discoverers with the Nobel Prize.”
 </p>
<h2>
  Chemistry
 </h2>
<p>
  The chemists selected David A. Evans, a synthetic organic chemist at Harvard. Evans’s work focuses on both total synthesis and stereoselectivity. Total synthesis involves creating synthetic versions of compounds found in nature. The pharmaceutical industry relies on the field’s techniques heavily, as they allow researchers to mine the natural world for compounds that may prove beneficial as treatments, even if those compounds are incredibly rare. Evans is something of a total synthesis wizard—he and his lab have synthesized over 50 natural products throughout the years.
 </p>
<p>
  Stereoselectivity, Evans’s other speciality, is where one chemical reaction spits out different three-dimensional orientations of the same molecule, or stereoisomers. Specifically, Evans is interested in enantiomers, a specific class of stereoisomer that involves mirror images of the same molecule. Evans has been developing techniques that vary the levels of stereoselectivity in the hopes that someday we’ll be able to completely control the configuration of the final product. Like total synthesis, it’s important in the development and production of pharmaceuticals. The painkiller naproxen is a perfect example. One enantiomer relieves pain, while the other simply causes liver damage. Producing and selecting the right enantiomer is crucial.
 </p>
<h2>
  Medicine
 </h2>
<p>
  For medicine and physiology, unfortunately our poll didn’t garner enough results to say anything conclusive, but one widely-watched Nobel prediction site, ScienceWatch, published by Thomson Reuters, has identified as possible winners A. Paul Alivisatos, Chad A. Mirkin, and Nadrian C. Seeman for their work on DNA nanotechnology, a promising and relatively new field that uses the unique properties of both DNA and nanoparticles to study biological processes and treat the human body. Mirkin’s research, for example,
  <a href="http://www.pbs.org/wgbh/nova/next/tech/nanoparticles-and-the-future-of-medicine/">
   uses gold nanoparticles studded with nucleic acids
  </a>
  to deliver drugs targeted at specific cells.
 </p>
<p>
  It’s important to keep in mind that these predictions are just that, predictions. We’ll have to wait until next week to see if any pan out. But in the meantime, you can
  <a href="http://svy.mk/19mOIwz">
   voice your opinion in our public Nobel poll
  </a>
  and see which scientists the crowd feels are this year’s frontrunners.
 </p>
</section>
<p>
 Photo credit: Public domain
</p>

A Window into New Physics

<p>Bursts of radio waves lasting just milliseconds could reveal the physics behind dark matter or quantum gravity theories.</p>

How LIGO Detected Gravitational Waves

<p>
 For 13 years, the scientists of LIGO—the most ambitious, and expensive, project in the history of the National Science Foundation—had been waiting.
</p>
<p>
 LIGO, the Laser Interferometer Gravitational-Wave Observatory, has been twenty-five years and more than half a billion dollars in the making. It involves 900 scientists and engineers, including many whose entire careers have been spent designing, building, and preparing to analyze data streaming in to LIGO. Their goal: To confirm, once and for all, Einstein’s century-old idea that gravity travels across space-time in the form of gravitational waves.
</p>
<div class="related"></div>
<p>
 Well,
 <a href="http://www.pbs.org/wgbh/nova/next/physics/gravitational-waves-discovered/">
  they did
 </a>
 .
</p>
<p>
 This morning, the LIGO team announced that they had picked up gravitational waves from the collision of two orbiting black holes. “We have detected gravitational waves from a binary black hole system,” says Matthew Evans, an assistant professor of physics at MIT and a LIGO collaborator. “They’re the largest stellar mass black holes we’ve ever observed, and the first time we’ve observed such things merging together.”
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/gravitational-waves.width-990_2wDuZta.jpg"/>
<figcaption>A visualization of two black holes merging based on general relativity's predictions.
        
    </figcaption>
</figure>
<p>
 The collision landed on the detectors like a half-second thump—somewhat literally. “I would say heard more than saw in the sense that gravitational waves signal is much more like audio signal than a video signal,” Evans says. “We can literally put the signal on a speaker and listen to it.”
</p>
<p>
 It sounds like a happy ending, but in reality, this first detection is just the beginning of a brand new way of looking at the universe, one that will pull back the covers on some of the most extreme events in the universe, like black hole collisions and supernova explosions. We’ve been blind—but we’re about to see a lot more.
</p>
<h2>
 A New Way of Seeing
</h2>
<p>
 For the entire history of civilization, we’ve explored the universe in just one way, using light—that is, electromagnetic waves. “Everything that you and I know about the universe—every time we see a picture on the web, or in the news—it has been something that’s been learned with light,” says Shane Larson, a gravitational wave researcher at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics.
</p>
<p>
 For millennia, humans have used visible light, the kind that you can see with your naked eyes, to learn about the cosmos. In the last century or so, we’ve expanded that view, building telescopes that can see infrared, ultraviolet, radio, X-rays and gamma rays. Each kind of light reveals something different about the cosmos. Objects that are invisible to the eye may be glaringly bright in radio waves, or vice versa; infrared light can pierce right through dust clouds that obscure visible light.
</p>
<div class="pullquote">
<p>
 “If an object’s motion is changing, its gravitational field changes, too.”
</p>
</div>
<p>
 But gravitational waves are something else entirely: they are gravity’s messengers, ripples in the fabric of space that reverberate out from the source of a gravitational disturbance. You can’t see them, but you can feel them, like you feel the wake of a passing speedboat on the water. The bigger the boat, the bigger the waves, and so it goes with gravitational waves. Really hefty objects—supermassive black holes containing as much matter as hundreds of millions of suns—make big billows in space-time. Littler things—a moon, a mouse—leave barely a quiver.
</p>
<p>
 Big or small, if an object is just sitting still, its gravitational field is static and unchanging. But if the object’s motion is changing, or if its mass is being dramatically rearranged, like in an explosion or implosion, the gravitational field changes, too, and that change produces gravitational waves. A black hole sitting peacefully in isolation is silent and invisible to gravitational wave detectors, but a black hole on a collision-course spiral around another black hole should be positively shrieking. Which is exactly what the LIGO team heard.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/ligo-beam-tube.width-990_ZawH4Mk.jpg"/>
<figcaption>Tubes connect the LIGO optics so the laser beam can pass between them.
        
    </figcaption>
</figure>
<p>
 LIGO started listening for gravitational waves back in 2002, but, after eight years, it was shut down without recording even one unambiguous gravitational wave detection. But that was okay: Initial LIGO, as the first phase of operations was called, was, in a sense, just for practice. Last September, the real games began. Freshly upgraded with technology that didn’t even exist when the LIGO first came online, LIGO, now rebranded Advanced LIGO, is able to detect gravitational waves coming from sources some ten times farther from Earth than before.
</p>
<div class="ad"></div>
<p>
 LIGO is called an observatory, but it isn’t like any astronomical observatory you might be imagining. Each site—there are two currently—is actually a big, L-shaped tube called an interferometer. Each arm is a little more than a meter wide and extends for two and a half miles. Where the two arms meet, a 180 Watt laser beam is split in two; the twin beams travel down the arms, where they hit ultra-precise mirrors and bounce back and forth some 400 times before meeting again at a light sensor.
</p>
<p>
 Because light is a wave, the two laser beams combine to make an interference pattern of bright and dark areas, or “fringes,” at that light sensor. Analyze the fringes, and you can tell how far each light wave traveled to cover the distance between the mirror and the sensor. More importantly, you can tell if that distance is changing over time. If a gravitational wave passes by, one arm of the observatory’s “L” should be stretched while the other gets squeezed, producing a telltale change in the fringe pattern.
</p>
<h2>
 What Kind of Noise Annoys an Interferometer?
</h2>
<p>
 But it gets more complicated. Everything from passing trucks to distant ocean waves can shake the mirrors, causing “noise” that muddies the measurements. That’s why LIGO isn’t just one machine, but a pair of facilities with
 <a href="https://ligo.caltech.edu/page/ligo-detectors">
  identical-twin designs
 </a>
 , one located in Livingston, Louisiana and the other Hanford, Washington. By putting almost 2,000 miles between the two instruments, physicists can discriminate between local jiggling, which will only be felt by one detector, and authentic cosmic gravitational waves, which should be felt by both.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/ligo-aerial5.width-990_ewpthoE.jpg"/>
<figcaption>LIGO's Hanford, Washington, facility
        
    </figcaption>
</figure>
<p>
 Still, picking out the tiny signal expected from a gravitational wave from mundane background vibrations is like trying to hear the crickets chirping at an AC/DC concert. The trick is to isolate LIGO’s mirrors from external shaking as perfectly as possible. To do that, Advanced LIGO has a completely revamped isolation system that exploits seven different layers of technology to effectively “float” the optics. For LIGO’s first run, the mirrors were hung from simple pendulums. This time around, the mirrors are heavier, and each one is suspended from welded glass fibers that hang from a quadruple pendulum—that is, a pendulum that hangs from a pendulum that hangs from a pendulum that hangs from a pendulum. This quadruple pendulum system is great at cushioning the mirrors from very high frequency vibrations.
</p>
<p>
 “Though LIGO’s suspension system is much more complex than a car’s, it operates on a similar principle,” says MIT research scientist Fabrice Matichard, who leads design and testing of Advanced LIGO’s seismic isolation system. “When you go very fast, the wheels follow the bumps, but the wheels are decoupled from the frame,” so you feel a smooth ride. With LIGO, it’s like that, but times four.
</p>
<p>
 To handle lower-frequency tremors, Advanced LIGO is using a new, “active” isolation system that senses and corrects for vibrations in real-time. Outside the LIGO vacuum chamber, a hydraulic system neutralizes the slow swaying coming from distant effects like tides. Inside the vacuum chamber, another set of seismometers triggers two stages of magnetic actuators that counteract mid-frequency vibrations. This active isolation system also keeps in check the Achilles’ heel of the pendulum system: its inconvenient tendency to resonate at certain frequencies, making the internal vibrations stronger instead of weaker.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/sus_llo_fibers.width-990_XGfXjI4.jpg"/>
<figcaption>A member of the pendulum suspension system
        
    </figcaption>
</figure>
<p>
 Even with all this noise-canceling technology, today, much of the day-to-day business of operating LIGO is trying to understand all the background noise that it picks up. And now, after eight years spent acclimating to the standard noise of the first LIGO run, physicists find themselves back at the beginning.
</p>
<p>
 “The instrument is so different” after the upgrade, says LIGO spokesperson Gabriela González, a professor at Louisiana State University, that the “noise looks very different—in a good way.” In fact, the very same active seismic isolation system designed to keep the noise out can introduce new noise as it pushes against the mirrors, noise that in the past would have been imperceptible. Like hunters skilled at finding their prey against one landscape, LIGO scientists must now relearn how to pick out their quarry in a totally new environment.
</p>
<h2>
 The Thrill of the Chase
</h2>
<p>
 Which is where physicists like Rob Owen come in. “You have to know what you’re looking for before you can check whether it’s in the data,” says Owen, an assistant professor at Oberlin College who is also part of the Simulating Extreme Spacetimes (SXS) collaboration.
</p>
<p>
 Owen and his colleagues use computer simulations to solve the equations of Einstein’s general relativity and predict what different sources of gravitational waves will look like to LIGO and other gravitational wave detectors. Computer codes like theirs and those being developed in other labs show how gravitational waves should ring out from
 <a href="http://www.black-holes.org/explore/movies">
  a panoply
 </a>
 of exotic cosmic collisions: black holes merging with black holes, neutron stars and white dwarfs corkscrewing into each other, and every combination thereof. Their work will help LIGO scientists find what they’re looking for in a forest of data.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/rs232_modecleaners-group003.width-990_nkPVPB2.jpg"/>
<figcaption>These output mode cleaners help improve the quality of laser light.
        
    </figcaption>
</figure>
<p>
 Before today’s announcement the smart money was on neutron stars as the source of the first signals. Neutron stars are the “bread and butter” of LIGO, González says. Less camera-shy than black holes, neutron stars—ultra-dense, fiercely magnetized stellar corpses left behind by supernova explosions—have already been observed by radio telescopes in death-spiral orbits that will ultimately lead them to crash and coalesce, perhaps birthing a black hole in the process. In fact, some of the strongest indirect evidence for gravitational waves had come from observations of a pair of neutron stars known as the
 <a href="http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm">
  Hulse-Taylor pulsar
 </a>
 , named for its discoverers, who also won a Nobel prize for their work.
</p>
<div class="pullquote">
<p>
 “We know next to nothing about these black holes.”
</p>
</div>
<p>
 Astrophysicists guesstimate that a single neutron star pair merger happens in our galaxy every 10,000 years or so. “That’s too long to wait,” González says, but by expanding LIGO’s reach to include hundreds of thousands or even millions of galaxies, we can tip the odds in our favor. Right now, LIGO should be able to pick up the signal of a garden-variety neutron star merger if it happens within about 200 million light years of Earth. (The Milky Way, for comparison, is about 100,000 light years across.) As LIGO scientists better understand the sources of noise in the LIGO system, they expect to be able to extend LIGO’s reach to some 650 million light years. At that point, González estimates, LIGO should be picking up tens of events each year.
</p>
<p>
 Fortunately, the expanded search technique also applies to the short wavelength gravitational waves from binary black holes, which LIGO is most attuned to detecting. The observatory is most sensitive to small black holes, but there was a chance—which was proven correct—that it could sense in the range of tens or hundreds of solar-masses. Those have proved almost totally resistant to observation with other kinds of observatories. “We know next to nothing about these black holes,” González says.
</p>
<h2>
 The End of the Beginning
</h2>
<p>
 Ultimately, LIGO won’t be searching alone. Already, the LIGO group is working in partnership with the European GEO and VIRGO gravitational wave detectors, with plans for collaborations with new instruments, like LIGO-India and the Japanese KAGRA project, in development.
</p>
<p>
 Furthermore, just as traditional telescopes are only sensitive to a certain range of light wavelengths—ultraviolet, say, or radio—LIGO and similarly sized detectors are only built to pick up high-frequency gravitational waves. That means that while LIGO can detect gravitational waves from merging stellar-mass black holes and possibly neutron stars, they are probably deaf to a thick slice of the gravitational wave spectrum, including the low-frequency waves physicists think should be thrumming out from colliding supermassive black holes.
</p>
<p>
 Two decades from now, an orbiting gravitational wave observatory called eLISA may be able to pick up these low-frequency waves. But in the meantime, far from the big-ticket fanfare, radio astronomers have been developing a clever,
 <a href="http://nanograv.org/science/pta.html">
  bargain-basement technique
 </a>
 to try to catch these waves using the “instruments” that nature has provided: millisecond pulsars, rapidly-rotating neutron stars that issue precisely-timed beams of radio waves. By clocking the arrival times of pulsar pulses from across the galaxy, radio astronomers can analyze how space-time is “waving” under the influence of gravitational waves.
</p>
<p>
 The LIGO collaboration also has agreements with 30 different astronomy groups, each poised to swiftly follow up on any future gravitational wave detection with X-ray, ultraviolet, radio, and visible-light telescopes. They are playing a sort of celestial matching game, trying to spot a transitory light source that coincides with the gravitational wave signal. They want to see what LIGO hears.
</p>
<p>
 But it isn’t easy. The problem: LIGO gives terrible directions. Together, the twin LIGO detectors can only localize a gravitational wave source to a patch of sky that’s many hundreds of square degrees. In a field that large, other telescopes are practically guaranteed to find some now-you-see-it-now-you-don’t object. Telling the difference between a true match and a coincidence may be impossible. A decade from now, González and her colleagues hope, LIGO will be just one part of a network of detectors that can provide the kind of localization electromagnetic astronomers need to make confident matches.
</p>
<p>
 For now, though, LIGO—one of the most audacious astrophysical experiments—has proven that gravitational waves exist and that we can detect them. “We couldn’t make a baby step,” Larson says. “We had to do the big experiment to get the data at all.” Which is exactly what they did.
</p>
<div class="newsletter"></div>
<p>
 Photo credits: LIGO, NASA,
 <a href="https://www.flickr.com/photos/tobin/625793393/">
  Tobin/Flickr (CC BY-SA)
 </a>
</p>

<p>Upgrades to the observatory sharpened its hearing, allowing scientists to detect the waves after just 16 days.</p>

Are Black Holes Real?

<p>
 Not so long ago, black holes were like unicorns: fantastical creatures that flourished on paper, not in life. Today, there is wide scientific consensus that black holes are real. Even though they can’t be observed directly—by definition, they give off no light—astronomers can infer their hidden presence by watching how stars, gas, and dust swirl and glow around them.
</p>
<p>
 But what if they’re wrong? Could something else—massive, dense, all-but-invisible—be concealed in the darkness?
</p>
<div class="pullquote">
<p>
 A telescope as big as the Earth could tell a black hole from an exotic imposter.
</p>
</div>
<p>
 While black holes have gone mainstream, a handful of researchers are investigating exotic ultra-compact stars that, they argue, would look exactly like black holes from afar. Well, almost exactly. Though their ideas have been around for many years, researchers are now putting them to the most stringent tests ever, looking to show once and for all that what looks and quacks like a black hole really is a black hole. And if not? Well, it could just spark the next revolution in physics.
</p>
<p>
 The game-changer is a new experiment called the
 <a href="https://www.reddit.com/r/IAmA/comments/3ihgbr/were_scientists_on_the_event_horizon_telescope">
  Event Horizon Telescope
 </a>
 (EHT). The EHT is a network of telescopes that are sensitive to radio waves about a millimeter long and linked together using a technique called very long baseline interferometry. Baseline refers to the distance between the networked telescopes: the longer the distance, the finer the details the telescope can pick out. It’s impossible—or at least impractical—to build a single telescope as big as planet Earth, but astronomers can achieve the same “zoom” factor by linking telescopes on opposite continents. Just like that, the universe goes from standard-definition to HD: a switch powerful enough to tell a black hole from an exotic imposter.
</p>
<p>
 Meanwhile, scientists have directly detected gravitational waves for the
 <a href="http://www.pbs.org/wgbh/nova/next/physics/gravitational-waves-discovered/">
  first time
 </a>
 using the Laser Interferometer Gravitational-Wave Observatory, also known as LIGO. Gravitational waves—ripples in the fabric of space-time that Einstein predicted should radiate out from the site of any gravitational disturbance—represent an
 <a href="http://www.pbs.org/wgbh/nova/next/physics/advanced-ligo/">
  entirely new way to see the cosmos
 </a>
 , and with enough data, they could finally confirm—or contradict—the existence of black holes.
</p>
<div class="related"></div>
<h2>
 Black Hole Anatomy
</h2>
<p>
 On its own, a black hole looks like nothing: black-on-black, indistinguishable from the empty space that surrounds it. But supermassive black holes, which are believed to sit at the core of almost every galaxy in the universe, surrounded by stars and other galactic detritus that accumulates around the edge like soap suds circling the bathtub drain. By studying those “suds,” astronomers can answer questions about the central black hole.
</p>
<p>
 The best-studied black hole candidate in the universe is the one called Sagittarius A*, which lives at the center of our very own Milky Way galaxy. By tracking the orbits of stars circling around Sagittarius A*, they have deduced that Sagittarius A* packs some 4 million times the mass of the Sun into a region of space much smaller than the solar system. Their conclusion: it could only be a supermassive black hole.
</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/m106.width-990_cXnFdL6.jpg"/>
<figcaption>Supermassive black holes are thought to sit at the center of every galaxy, including this one, Messier 106. But what if something else lurked in the starry froth?
        
    </figcaption>
</figure>
<p>
 To confirm that suspicion, they would like to see up to the edge of the black hole—the event horizon, a sort of line in the sand that separates the “inside” of the black hole from the “outside” and beyond which nothing can escape. From the perspective of a telescope on Earth, the event horizon should look like a
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2015/08/the-shadow-of-a-black-hole/">
  dark shadow
 </a>
 surrounded by a bright ring of light. The exact shape of this ring and shadow are predicted by the equations of general relativity, plus the properties of the black hole and its surroundings.
</p>
<div class="ad"></div>
<h2>
 An Earth-Sized Telescope
</h2>
<p>
 That’s where the EHT comes in. Since the EHT first started taking data, it has been building its telescope roster, and with each new member, it gets closer to making the first true image of a black hole shadow.
</p>
<p>
 The EHT is like an all-star team of telescopes: Most days, its millimeter-wave dishes run their own experiments independently, but for one or two weeks a year, they team up to become the EHT, taking new data and running tests during the brief window when astronomers can expect clear weather at sites from Hawaii to Europe to the South Pole.
</p>
<p>
 “It sounds too good to be true that you just drop telescopes around the world and ‘poof!’ you have an Earth-sized telescope,” says Avery Broderick, a theoretical astrophysicist at University of Waterloo and the Perimeter Institute. And in a way, it is. The EHT doesn’t make pictures. Instead, it turns out a kind of mathematical cipher called a Fourier transform, which is like the graphic equalizer on your stereo: it divvies up the incoming signal, whether its an image of space or a piece of music, into the different frequencies that make it up and tells you how much power is stored in each frequency. So far, the EHT has only given astronomers a look at a few scattered pixels of the Fourier transform. When they compare those pixels to what they expect to see in the case of a true black hole, they find a good match. But the job is like trying to figure out whether you’re listening to Beethoven or the Beastie Boys based only on a few slivers of the graphic equalizer curve.
</p>
<div class="pullquote">
<p>
 Black holes are mothers of cosmic paradox.
</p>
</div>
<p>
 Now, the EHT is about to add a superstar player: the Atacama Large Millimeter Array, a telescope made up of 66 high-precision dishes sited 16,000 feet above sea level in Chile’s clear, dry Atacama desert. With ALMA on board, the EHT will finally be able to make the leap from fitting models to seeing a complete picture of the black hole’s shadow. EHT astronomers are now rounding up time at all of the telescopes so that they can take new data and assemble that first coveted image in 2017.
</p>
<p>
 And if they don’t see what they expect? It could mean that the black hole isn’t really a black hole at all.
</p>
<p>
 That would come as a relief to many theorists. Black holes are mothers of cosmic paradox, keeping physicists up at night with the puzzles they present: Do black holes really
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2013/12/do-black-holes-destroy-information/">
  destroy information
 </a>
 ? Do they really contain
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2015/12/are-singularities-real/">
  infinitely dense points
 </a>
 called singularities? Black holes are also the battlefield on which general relativity and quantum mechanics clash most dramatically. If it turns out that they don’t actually exist, some physicists might sleep a little better.
</p>
<p>
 But if they’re not black holes, then what could they be? One possibility is that they are dark stars made up of bosons, subatomic particles that, unlike more familiar electrons and protons, obey strange rules that allow more than one of them to be in the same place at the same time. Boson stars are highly speculative—astronomers have never seen one, as far as they know—but theorists like Vitor Cardoso, a professor of physics at Técnico in Lisbon and a distinguished visiting researcher at Sapienza University of Rome, hypothesize that some or all of the objects we think are supermassive black holes could actually be boson stars in disguise.
</p>
<p>
 Physicists classify particles into two different categories: fermions, which include protons, electrons, neutrons, and their components; and bosons, like photons (light particles), gluons, and Higgs particles. Every star that we’ve ever seen shining is dominated by fermions. But, Cardoso says, given a starting environment rich in bosons, bosons could “clump” together gravitationally to form stars, just as fermions do. The early universe might have had a high enough density of bosons for boson stars to form.
</p>
<p>
 But not every boson is a suitable building block for a boson star. Gravity won’t hold together a clump of massless photons, for instance. Higgs particles are massive enough to be bound together by gravity, but they aren’t stable—they only exist for tiny fraction of a second before decaying away. Theorists have speculated about ways to stabilize Higgs particles, but Cardoso is more intrigued by the prospect that other, yet-undiscovered heavy bosons, like
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2015/07/my-enchanted-evening/">
  axions
 </a>
 , could make up boson stars. In fact, some physicists hypothesize that massive bosons like these could be responsible for dark matter—meaning that boson stars wouldn’t just be a solution to the riddle of black holes, they could also tell us what, exactly, dark matter is.
</p>
<h2>
 Gravastars
</h2>
<p>
 Boson stars aren’t the only black hole doppelgänger that theorists have dreamed up. In 2001, researchers proposed an even more speculative oddity called a
 <a href="http://arxiv.org/abs/gr-qc/0109035">
  gravastar
 </a>
 . In the gravastar model, as a would-be black hole collapses under its own weight, extreme gravity combines with quantum fluctuations that are constantly jiggling through space to create a bubble of exotic spacetime that halts the cave-in.
</p>
<p>
 Theorists don’t really know what’s inside that bubble, which is both good and bad news for gravastars: Good news because it gives theorists the flexibility to revise the model as new observations come in, bad news because scientists are rightly skeptical of any model that can be patched up to match the data.
</p>
<p>
 When the data does come in, physicists have a checklist of sorts that should help them know which of the three—black hole, boson star, or gravastar—they’re looking at. A gravastar should have a bright surface that’s distinguishable from the glowing ring predicted to loop around a black hole. Meanwhile, if the object at the center of the Milky Way is actually a boson star, Cardoso predicts, it will look more like a “normal” star. “Black holes are black all the way through,” Cardoso says. “If really the object is a boson star, then the luminous material can in principle pile up at its center. A bright spot should be detected right at the center of the object.”
</p>
<h2>
 A New View
</h2>
<p>
 Most physicists have placed their bets on Saggitarius A* and other candidates being black holes, though. Boson stars and gravastars already have a few strikes against them. First, when it comes to scientific credibility, black holes have a major head start. Astronomers have a solid understanding of the process by which black holes form and have direct evidence that other ultra-dense objects, like white dwarfs and neutron stars, which could merge to form black holes, really do exist. The alternatives are more speculative on every count.
</p>
<div class="newsletter"></div>
<p>
 Furthermore, Broderick says, astronomers have looked for the telltale signature of boson stars and gravastars at the center of the Milky Way—and haven’t found it. “The stuff raining down on the object will give up all its kinetic energy—all the gravitational binding energy tied up in the kinetic energy of its fall—resulting in a thermal bump in the spectrum,” Broderick says —that is, a signature spike in infrared emission. In 2009, astrophysicists
 <a href="http://arxiv.org/abs/0903.1105">
  reported
 </a>
 that they had found no such bump coming from Sagittarius A*, and in 2015, they
 <a href="http://arxiv.org/abs/1503.03873">
  announced
 </a>
 that it was missing from the nearby massive galaxy M87, too.
</p>
<div class="pullquote">
<p>
 “To confirm or rule out boson stars entirely, we need ‘louder’ observations.”
</p>
</div>
<p>
 Cardoso doesn’t see this as a death-knell for the boson star model, though. “The field that makes up the boson star hardly interacts with matter,” he says. To ordinary matter, the surface of a boson star would feel like frothed milk. “We do not yet have a complete model of how these objects accrete luminous matter,” Cardoso says, “so I think that it’s fair to say that this is still an open question.” He is less optimistic about gravastars, which he describes as “artificial constructs” that are likely ruled out by the latest observations.
</p>
<p>
 As the LIGO experiment gathers more data, theorists will get more opportunities to test their exotic hypotheses with gravitational waves. As two massive objects—say, a supermassive black hole and a star—spiral toward each other on the way toward a collision, gravitational waves carry away the energy of their motion. If one member of the spiraling pair is a black hole, the gravitational wave signal will cut off abruptly as the star passes through the black hole’s event horizon. “It gives rise to a very characteristic ringdown in the final stages of the inspiral,” Cardoso says. Because the alternative models have no such horizon, the gravitational wave signal would keep on reverberating.
</p>
<p>
 Most astronomers believe that the waves LIGO detected were given off by the collision of two black holes, but Cardoso thinks that boson stars shouldn’t be ruled out just yet. “The data is, in principle, compatible with the two colliding objects being each a boson star,” he says. The end result, though, is probably a black hole “because it rings down very fast.”
</p>
<p>
 LIGO is not designed to pick up signals at the frequency at which supermassive objects like Sagittarius A* are expected to “ring.” (LIGO is tuned to recognize gravitational waves from smaller black holes and dense stars like neutron stars.) But supermassive black holes and boson stars are in the sweet spot for the planned space-based gravitational wave telescope eLISA (the Evolved Laser Interferometer Space Antenna), slated for launch in 2034. “To confirm or rule out boson stars entirely, we need ‘louder’ observations,” Cardoso says. “EHT or eLISA are probably our best bet.”
</p>
<h2>
 Taking the Pulse
</h2>
<p>
 In the meantime, astronomers could measure waves from these extremely massive objects by precisely clocking the arrival times of radio pulses from a special class of dead stars called pulsars. If astronomers spot pulses arriving systematically off-beat, that could be a sign that the space they’ve been traveling across is being stretched and squeezed by gravitational waves. Three collaborations—NANOGrav in North America, the European Pulsar Timing Array, and the Parkes Pulsar Timing Array in Australia—are already scanning for these signals using radio telescopes scattered around the globe.
</p>
<p>
 To Broderick, though, the big question isn’t which model will win out, it’s whether these new experiments can find a flaw in general relativity. “For 100 years, general relativity has been enormously successful, and there’s no hint of where it breaks,” he says. Yet general relativity and quantum mechanics, which appears equally shatterproof, are fundamentally incompatible. Somewhere, one or both must break down. But where? Boson stars and gravastars might not be the answer. Still, exploring these exotic possibilities forces physicists to ask the questions that might lead them to something even more profound.
</p>
<p>
 “We expect that general relativity will pass the EHT’s tests with flying colors,” Broderick says. “But the great hope is that it won’t, that we’ll finally find the loose thread to pull on that will unravel the next great revolution in physics.”
</p>
<p>
 Image Credit: X-ray: NASA/CXC/Caltech/P.Ogle et al; Optical: NASA/STScI; IR: NASA/JPL-Caltech; Radio: NSF/NRAO/VLA
</p>

<p>Two even more exotic possibilities could explain the curious data physicists have gathered over the years.</p>

Gravity from Huchra's Lens causes light from the quasar Einstein Cross to bend around it..

Does Dark Matter Ever Die?

<div class="related"></div>
<p>Dark matter is the unseen hand that fashions the universe. It decides where galaxies will form and where they won’t. Its gravity binds stars into galaxies and galaxies into galaxy clusters. And when two galaxies merge, dark matter is there, sculpting the product of the merger. But as for what dark matter actually is? No one knows.</p><p>Here’s the short list of what we do know about dark matter. Number one: There’s a lot of it, about five times more than “ordinary” matter. Two: It doesn’t give off, reflect, or absorb light, but it does exert gravity, which is what gives it a driver’s-seat role in the evolution of galaxies. Three: It’s stable, meaning that for almost 13.8 billion years—the current age of the universe—dark matter hasn’t decayed into anything else, at least not enough to matter much. In fact, the thinking goes, dark matter will still be around even when the universe is quintillions (that’s billions of billions) years old—maybe even forever.</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/solar-system-gravity.width-990_h0CAMaB.jpg"/>
<figcaption>Though invisible, dark matter exerts gravity just like other matter.
        
    </figcaption>
</figure>
<p>Theoretical physicists dreaming up new ideas about dark matter typically start with these three basic principles. But what if the third—the requirement that dark matter be stable over the cosmic long haul—is wrong? That’s the renegade idea behind a new dark matter proposal called “Dynamical Dark Matter.” Though it’s still on the fringe of dark matter physics (“It’s as far as you can get from the traditional approaches,” says physicist Keith Dienes of the University of Arizona, who first developed the idea with Lafayette College theorist Brooks Thomas), it’s been gaining traction and attracting collaborators from particle physics, astrophysics, and beyond.</p><p>And dark matter is a field that could use some new ideas. While astronomers have been picking up dark matter’s fingerprints all over the universe for at least a century, physicists can’t seem to get a fix on a single dark matter particle. It’s not for lack of trying. Particle hunters have looked for signs of them in flurries of particles set loose by colliders like the Large Hadron Collider (LHC). They have buried germanium crystals and tanks of liquid xenon and argon deep underground—beneath mountains and in old gold mines—and looked for dark matter particles pinging off the atomic nuclei inside. The result: Nothing, at least not anything that physicists can agree on.</p><p>Meanwhile, the astrophysical evidence for dark matter keeps building up. Take one universal mystery: Astronomers, after clocking how fast stars are circling around in galaxies, have found that stars skimming a galaxy’s perimeter are going just about as fast as closer-in stars. But based on everything we know about how gravity works, they should actually be going a lot slower—unless there is some invisible mass pulling on them. Then, there are galaxy clusters: Galaxies within them are jouncing around so quickly that they should fly apart, absent some invisible mass is holding them all together. Noticing a theme here? Even the cosmic microwave background radiation, the closest thing we have to a baby picture of the newborn universe, has patterns in it that can only really be explained by dark matter. So, if dark matter is so ubiquitous, why can’t we find it?</p><p>Some researchers are beginning to wonder if they’ve been searching for the wrong thing all along. Most (though not all) dark matter detectors are designed to find hypothetical particles called WIMPs—short for “weakly interacting massive particles.” WIMPs are an appealing dark matter candidate because they emerge naturally from a beyond-the-standard-model theory called supersymmetry, which posits that the all the fundamental subatomic particles have as-yet-undiscovered partners.</p><p>As physicists worked out the properties of those still unseen particles, they noticed that one was a startlingly good match for dark matter. It would interact with other particles via gravity and something called the weak force, which only works when particles get within a proton’s-width of each other. Plus, it would be stable, and there could be just enough of it to account for the missing mass without upsetting with the evolution of the universe.</p><p>The appeal of WIMPs is “almost aesthetic,” says Jason Kumar, a physicist at the University of Hawaii: it speaks to physicists’ love of all that is simple, symmetrical, and elegant. But, Kumar says, “It’s now becoming very hard to get these models to fit with the data we’re seeing.” That doesn’t mean that the WIMP model is wrong, but it does put researchers in the mood to consider ideas that, ten years ago, might have been brushed off as theoretical footnotes. Like, for instance, the idea that dark matter that isn’t stable after all.</p>
<div class="ad"></div>
<h2>A Destabilizing Influence</h2><p>Dienes and Thomas were newcomers to dark matter when they first hatched the idea of Dynamical Dark Matter. They were so new to the field that, at first, they didn’t even worry about stability. Together, they began sketching a new kind of dark matter. First, they thought, what if dark matter weren’t just one kind of particle, but a whole bunch of different kinds? Second, what if those particles could decay? Some might disappear within seconds, but others could stick around for trillions of years. The trick would be getting the balance right, so that the bulk of the dark matter would linger until at least the present day.</p><p>Dienes and Thomas called their new framework “Dynamical Dark Matter,” and started sharing it at talks and academic conferences. The reaction, according to Dienes: “A boatload of skepticism.”</p><p>“People kept asking about stability,” Dienes remembers. “But we were not thinking about stability in the traditional way.”</p><p>Why are physicists so sure that dark matter is stable, anyway? Galaxies from long ago—the ones astronomers see when they look billions of light years out into the universe—aren’t more weighed-down by dark matter than our nearby, present-day specimens, at least not at the level of precision that astronomers can measure. Plus, if dark matter decayed into lighter, detectable particles, the little shards would fly out into space with a lot of energy, which we would be able to measure on Earth. And if the decay started in the universe’s baby days, it would disrupt the formation of the elements, shifting the chemistry of the cosmos.</p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/galaxy-gravity.width-990_Hb3uncj.jpg"/>
<figcaption>Galaxies far away from Earth aren't any more massive than those nearby.
        
    </figcaption>
</figure>
<p>Dynamical Dark Matter resolves the stability problem through a balancing act. If most of dark matter is tied up in particles that live a long time—longer than the age of the universe—that leaves room for a small share of dark matter to be made up of particles that vanish quickly. “It’s a balancing between lifetimes and abundances,” Dienes says. “This balancing is the new underlying principle that replaces mere stability.”</p><p>At first glance, this might sound contrived. Why should everything work out just so? But Dienes, Thomas, and their collaborators have discovered several scenarios that naturally produce just the right combination of particles. “It turns out there are a lot of interesting ways in which these things can come about,” Thomas says. Dynamical Dark Matter remains agnostic about what the dark matter particles are or how they came to be. “It’s not just a single model for dark matter, like a particle that’s a candidate,” he says. “It’s a whole new framework for thinking about what dark matter could be.”</p><p>Dynamical Dark Matter is one of a growing number of “multi-component” dark matter models that welcome in multiple particles. “The key differentiator for Dynamical Dark Matter is that it’s not just a random collection of particles,” Kumar says. “There are just a couple of parameters that describe everything about it.”</p><h2>A Shrinking Slice of Pie</h2><p>Today, dark matter makes up about 85% of the “stuff” in the universe, out-massing regular matter by a factor of five to one. But if the Dynamical Dark Matter framework is right, one day, dark matter will fizzle out entirely. The process will start slowly. Then, as a larger share of dark matter hits its expiration date, the die-out will speed up until, ultimately, dark matter goes extinct.</p><p>That won’t happen for a long, long time—long after dark energy, that other cosmic mystery force, stretches the universe to the brink of nothingness. (But that’s <a href="http://www.pbs.org/wgbh/nova/next/physics/physics-in-1-trillion-years/">another story</a> .) So one might ask: Who cares if a teeny weeny bit of dark matter goes “poof” if no one misses it?</p><p>Scientists searching for dark matter particles do.</p><p>That’s because, at dark matter detectors, Dynamical Dark Matter particles should leave a more complicated set of fingerprints than WIMPs. While WIMPs should make a relatively simple “clink” against the ordinary particles inside a detector, Dynamical Dark Matter (or any other brand of multiplex dark matter) would make a jumbled-up jangle. “If there is only one dark-matter particle, there is a well-known ‘shape’ for this recoil spectrum,” says Dienes, describing the detector read-out. “So seeing such a complex recoil spectrum would be a smoking gun of a multi-component dark-matter scenario such as Dynamical Dark Matter.”</p><p>Particle collider experiments could also distinguish Dynamical Dark Matter from WIMPs. “Dynamical dark matter basically provides a very rich spectrum of very different types of collider signatures, some very different from conventional dark matter,” says Shufang Su, a physicist at the University of Arizona. With Dienes and Thomas, Su is trying to predict the traces Dynamical Dark Matter would leave in data from particle colliders like the LHC.</p><p>Su was attracted to the dynamical dark matter model by the idea that dark matter could be a whole panoply of particles instead of just one, which would leave a distinctive signature on the visible particles produced in the LHC’s smash-ups. “These changes could be very dramatic and very different from what would occur if there is only a single dark matter species,” Su says. “If one dark matter particle leads to a single peak, Dynamical Dark Matter could lead to multiple peaks and perhaps even peculiar kinks.”</p>
<div class="newsletter"></div>
<p>Then there’s the decay factor. Depending on how long Dynamical Dark Matter particles live, some might fall apart almost as soon as they are created. Others might last long enough to travel some length of the detector, or escape entirely. “Even though it’s still dark matter, it could have a totally different signature,” Su says.</p><p>Peer into the universe’s deep unknowns to explore the mysteries of dark matter and energy.</p><p>While Su is thinking about how to detect Dynamical Dark Matter at colliders here on Earth, Kumar is thinking about whether it could explain something that has been puzzling astronomers: a mysterious excess of high-energy positrons in space. Dark matter researchers have suggested the positrons could be coming from WIMPs, which spit them out as they collide with and annihilate other WIMPs. The trouble, Kumar says, is that this process should only produce positrons up to a certain maximum energy before shutting down; so far, astronomers haven’t found such a cut-off. Dynamical dark matter just might be able to make positrons at the energy levels astronomers observe.</p><p>Of course, Dynamical Dark Matter is just one of many alternatives to WIMPs. There are also SIMPS, RAMBOs, axions, sexaquarks—the list goes on. Until physicists make a clear-cut detection, theorists will have plenty of headroom to dream up new ideas.</p><p>“The main message is that this is an interesting alternative. We are not claiming that it is necessarily better,” Dienes says. “The field is wide open, and data will eventually tell us.”</p>

<p>A new theory suggests that dark matter may be more dynamic than we currently assume.</p>

Welcome to The Nature of Reality

<section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>
<p>
  When I was seven, I understood relativity perfectly. I
  <em>
   got
  </em>
  it. At least I thought I did. And it was glorious.
 </p>
<p>
  It all started with
  <em>
<a href="http://en.wikipedia.org/wiki/Flight_of_the_Navigator">
    The Flight of the Navigator
   </a>
</em>
  , that movie about the kid who accidentally stumbles upon a space ship and has a grand old afternoon traversing the galaxy in the company of its cargo of friendly creatures. The problem? When he returns to Earth he finds that eight years have elapsed. He has been declared missing; when he rings the bell at his old house, a stranger answers the door.
 </p>
<p>
<em>
   The Flight of the Navigator
  </em>
  is a story about
  <a href="http://www.pbs.org/wgbh/nova/physics/spacetime-trip.html" target="_blank" title="A Trip Through Spacetime">
   time dilation
  </a>
  . The ship was traveling close to the speed of light, so time passed significantly more slowly for its passengers than it did for those back on Earth. That’s special relativity for you. It seemed easy, elegant, amazing.
 </p>
<p>
  For the next few years, that was my relationship with relativity. Then came the equations, and suddenly relatively didn’t seem so easy any more. Elegant, maybe. But that first spark of intuition got lost in symbols, equations, calculations and miscalculations. So I wondered: Why can’t we have it both ways? Is it possible to be both wide-eyed and rigorous, methodical and amazed?
 </p>
<p>
  That’s a balance even physicists struggle with. As MIT theorist Max Tegmark told NOVA, “When I was in high school, physics was my most boring subject….It was only later that I realized that physics isn’t at all about just solving some equations. It’s about figuring out what reality is all about.”
 </p>
<p>
  That’s what we’re doing on this blog: taking on the biggest questions of all, the ones that get at the very nature of reality. What happens to information inside a black hole? What are we really talking about when we talk about teleportation? What if the laws of physics can’t be unified? Every week, we’ll have a new post from a scientist or science writer–look for James Stein, Sean Carroll, and Frank Wilczek in the coming weeks and months–with plenty of room for you to join the discussion with your own questions and ideas.
 </p>
<p>
  We hope you’ll let us know what subjects in fundamental physics you want to hear about—the stranger, the better. Can neutrinos really go
  <a href="http://www.nature.com/news/2011/110922/full/news.2011.554.html">
   faster than the speed of light
  </a>
  ? What is this
  <a href="http://www.pbs.org/wgbh/nova/physics/spacetime-lemonade.html">
   spacetime
  </a>
  thing anyway? Is our universe just one of countless others in a
  <a href="http://www.pbs.org/wgbh/nova/physics/johnson-multiverse.html">
   multiverse
  </a>
  ? Join us as we dive deep and discover how physics is changing what we thought we knew about the nature of reality.
 </p>
</section>

Debating the Meaning of Quantum Mechanics

<section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>
<p>
  Why is quantum mechanics like cricket?
 </p>
<p>
  Because for me, no matter how many times the rules are explained, I can’t seem to get my head around what the game is actually about.
 </p>
<p>
  Is quantum theory a system of equations? A description of the behavior of invisible particles? A philosophy for the post-post-modern age?
 </p>
<p>
  And how strange is it that we even have to ask? Unlike other scientific theories, quantum physics is so slippery that its formalism—the equations that add up to a mathematical representation of what we humans call reality—is divorced from its physical interpretation. Sure, we can solve the Schrödinger equation for the case of a particle stuck in a box, but what is that telling us about how the natural world really works?
 </p>
<p>
  This isn’t a question you’d even think to ask about classical mechanics. Remember Newton’s Second Law, the one relating force to mass and acceleration? Its formalism is
  <strong>
   F=ma
  </strong>
  , and its interpretation is pretty simple: If you want to know the force an object is exerting, just multiply its mass by its acceleration.
 </p>
<p>
  That’s
  <strong>
   F=ma
  </strong>
  . But what about:
 </p>
<p>
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://staging.pbs.org/wgbh/nova/media/images/schrodinger-equation.width-990_7R0gBt6.png"/>
<figcaption>
</figcaption>
</figure>
<br/>
</p>
<p>
  “Quantum mechanics needs an explanation worse than other theories do because others always had a physical picture that guided the formulation of the mathematics,” explains
  <a href="http://faculty.washington.edu/jcramer/">
   John Cramer
  </a>
  , a physicist at the University of Washington who also happens to be the author of his own interpretation of quantum mechanics—more on that later. Newton had his (possibly apocryphal) apples, his inclined planes, his cannonballs. Werner Heisenberg, one of the “fathers” of quantum mechanics, by contrast, had some elegant mathematics, a vision more akin to numerology than to a picture of the physical world, in Cramer’s view.
 </p>
<p>
  “The Copenhagen interpretation is like a religious text,” says MIT physicist Max Tegmark. “It leaves a lot open to interpretation.”
 </p>
<p>
  Yet Heisenberg, like his colleague
  <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-bio.html">
   Niels Bohr
  </a>
  , felt that quantum mechanics needed no further interpretation. This view, which is now known as the Copenhagen interpretation, holds that there is no “objective reality” lurking beneath the formalism. If the equations say that I have a 50% chance of measuring a particle in a certain state—say,
  <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html">
   spin up
  </a>
  —and then I go ahead and measure it in that state, what more is there to say? To guess at what the particle was doing before I made the measurement would be worse than speculation; nothing can be said about the particle except in the context of a measurement. “Reality” is no more and no less than what our instruments and senses reveal it to be. The Copenhagen interpretation may give you a headache, but according to Anton Zeilinger, the University of Vienna physicist most famous for his
  <a href="http://www.scientificamerican.com/article.cfm?id=the-gedanken-experimenter">
   teleportation experiments
  </a>
  , “It works, is useful to understand our experiments, and makes no unnecessary assumptions.”
 </p>
<p>
  Still, many physicists find this notion unsatisfying. “Quantum mechanics is full of strange things that cry out for an interpretation,” says Cramer. There’s the problem of “
  <a href="http://www.pbs.org/wgbh/nova/physics/spooky-action-distance.html">
   spooky action at a distance
  </a>
  ,” the apparent connection between “entangled” particles that seems to violate the finite speed of light; and there’s Einstein’s famous discomfort with the idea that no reality exists outside of our own perceptions. As Einstein put it: “Do you really think the moon isn’t there if you aren’t looking at it?”
 </p>
<p>
  There’s also a niggling problem with exactly what defines “looking at it”—or, in quantum-speak, what defines a “measurement.” If we truly cannot say anything definite about a particle until after we’ve measured its state, then the act of measuring it must be pretty special. But why? What happens in that moment? Physicists often talk about it as the “collapse of the wavefunction”—that is, the moment when all of the possible particle states represented in the probability equation called the wavefunction collapse into a single, measured state. The instantaneous collapse of an entity that wasn’t physically real to start with is weird in itself. But physicist Steven Weinberg pointed to another weak link in this interpretation in a 2005 article in
  <em>
   Physics Today
  </em>
  : “The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe.”
 </p>
<p>
  If not Copenhagen, then what? Let’s take a quick tour of a handful of the (many!) competing interpretations of quantum mechanics.
 </p>
<ul>
<li>
<strong>
    Copenhagen interpretation
   </strong>
   : This is the interpretation we’ve just met, and the one you’ll see in most physics books—though even Heisenberg and Bohr didn’t always agree on the particulars. To put it in terms of our cricket analogy, let’s say that you’re following a cricket match on your cell phone. Actually let’s make it a baseball game because, as I’ve already confessed, I don’t understand cricket. So you’re using one of those apps that updates the box score every time you press “refresh,” but you can’t actually see the game in progress. According to the Copenhagen interpretation, there is no game—just the results you get when you ping the server. So it’s no use talking about whether the batter is getting into the pitcher’s head, or the appearance of the rally squirrel, or even the trajectory the ball takes on its way into the first baseman’s glove. The box score is real; the game isn’t.
  </li>
</ul>
<ul>
<li>
<strong>
    Consistent histories
   </strong>
   : The Copenhagen interpretation applies to a situation in which an observer (the baseball fan) makes a measurement (checks the score) on some external system. But what happens when the observer is himself part of the system—say, the shortstop? That’s the problem that a special breed of physicists called quantum cosmologists encounter when they attempt to study the entire universe as a single quantum system. The Copenhagen interpretation falls short in this case, but the consistent histories interpretation, developed in the 1980s and early 1990s, does away with external “observers” and “measurements”—they are treated as part of one big system.
  </li>
</ul>
<ul>
<li>
<strong>
    Many worlds
   </strong>
   : We talked earlier about the problem of the collapsing wavefunction. But what if the wavefunction never actually collapses? What if every possibility it represents really does happen in its own universe? With every measurement, each universe branches off into countless others, each of which in turn branches into ever more universes. The many worlds interpretation was first proposed in the 1950s by the young physicist Hugh Everett, and though it never gained much traction at the time, its star is now ascending: In the film
   <a href="http://www.pbs.org/wgbh/nova/manyworlds/">
    Parallel Worlds, Parallel Lives
   </a>
   , Tegmark called the many worlds interpretation “one of the most important discoveries of all time in science,” and he and his colleagues
   <a href="http://arxiv.org/abs/1008.1066">
    recently posited
   </a>
   that Everett’s parallel universes might be congruent with the parallel universes
   <a href="http://arxiv.org/abs/astro-ph/0101507">
    proposed by cosmologists
   </a>
   .  Of course, plenty of physicists can’t stomach the idea of a multiplicity of fundamentally unobservable universes. Yet—back to baseball for a moment—there is something appealing about an interpretation that insists upon the existence of a universe in which the baseball rolls squarely into Buckner’s glove; an interpretation that guarantees that every heartbreaker in our universe is shadowed by a heroic comeback in another; an interpretation in which the Red Sox
   <em>
    and
   </em>
   the Yankees win, year after year after year.
  </li>
</ul>
<ul>
<li>
<strong>
    Transactional interpretation
   </strong>
   : The transactional interpretation might solve some of quantum theory’s biggest quandaries, if you can get your head around the idea of a wave with negative energy that travels back in time. The transactional interpretation was first proposed in the 1980s by John Cramer, and suggests that the wavefunction includes not just one but two probability waves—the familiar one that travels forward in time, plus an exotic twin that travels backward. When they meet, they exchange a “handshake” across space-time, says Cramer; at other points, they cancel each other out completely, removing any telltale traces of the journey backward in time.
  </li>
</ul>
<p>
  So, is there any way to know which interpretation is right or wrong? “Unless you can catch an interpretation deviating from the mathematics, you can’t rule it out,” says Cramer. And though
  <a href="http://en.wikipedia.org/wiki/Afshar_experiment">
   some experiments
  </a>
  could maybe, possibly tip the scales in favor of one interpretation or another, there is no consensus that any of the contenders above have been favored or nixed by experiment. Perhaps, some physicists argue, the pursuit of an interpretation is a flawed endeavor. “There is no logical necessity of a realistic worldview to always be obtainable,” wrote Christopher Fuchs and Asher Peres in a
  <em>
   Physics Today
  </em>
  opinion piece titled, transparently, “
  <a href="http://www.physicstoday.org/resource/1/phtoad/v53/i3/p70_s1?bypassSSO=1">
   Quantum Theory Needs No ‘Interpretation’
  </a>
  .” “If the world is such that we can never identify a reality independent of our experimental activity, then we must be prepared for that, too.” Perhaps the interpretation problem isn’t a problem of quantum physics at all, but a problem of human beings.
 </p>
</section>

Kate Becker

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