Information theory makes sense of the Second Law of thermodynamics and much of the formalism of quantum mechanics. Black holes are bad ass not because they destroy matter per se, but because they destroy information. The holographic principle holds that the universe has an unexpectedly limited capacity to store and process information, perhaps indicating that space and time are not fundamental ingredients of nature, but derived from some deeper level.

Might information be more basic than the material things that carry it? As physicist John Wheeler famously put it, can we get “it from bit”? Does that idea even make sense? Information is always information about something, isn’t it? There is quite a knot of questions to disentangle. I expect that a Who’s Who of physicists and philosophers will enter the contest, not to mention new voices with provocative ideas.

Scientific American is a co-sponsor of the contest, which, in practical terms, means that one of the editors will serve on the official judging panel and the magazine will consider the top-placed winners for publication. The article by David Tong in December, “The Unquantum Quantum”, came out of the third essay contest, “Is Reality Digital or Analog?” The editors are now considering essays from the fourth, “Which of Our Basic Physical Assumptions Are Wrong?”

The contest will accept entries through the end of June. The fun part is that you don’t need to submit an essay to participate. All the essays are available online for reading, remarking, and rating. FQXi uses the community ranking to short-list entries for the official judging panel, and the institute plans to announce the winners in October.

This video shows the result. It’s part two of a video project I’ve been working on with John Matson, Sci Am’s associate editor for physics, and Eric Olson, the magazine’s video guru. In part one, we and our colleague Mary Karmelek dramatized what quantum entanglement means, metaphorically. Now you get to see the non-metaphorical version.

I’d gotten to know Colgate professor Enrique Galvez a decade ago for his studies of the orbital angular momentum of light. I went back to him because of his reputation as a pioneer of quantum experiments that college students could do in a lab course, and he kindly set aside a day to demonstrate them for us. The video focuses on the famous EPR experiment that Einstein devised and published in a famous paper with Boris Podolsky and Nathan Rosen in 1935. At the end, it mentions the elaboration developed by physicist John Bell in the mid-1960s, which proved that entanglement represents a type of nonlocality—or, as Einstein put it, “spooky action at a distance.”

The experiment entails creating pairs of photons that must then run a gauntlet of polarizing filters (shown in photo above). The polarizers are oriented so that an individual photon has a 50% chance of getting through. When both photons get through their respective polarizers, the equipment registers a “coincidence.” For a pair of unentangled photons, that has a 25% chance of happening—it’s equivalent to flipping two coins and seeing two heads. For entangled photons, however, the probability ranges from 0% to 50% depending on the relative polarizer orientation. The photons are correlated in a way the ordinary laws of chance do not allow. It is as if you flipped two coins and both always landed on the same side.

Like many physics experiments, when you first see the setup, you focus on the taking in all the complexity. Much of the equipment on the lab bench is technically essential but conceptually irrelevant; it ensures the alignment of light, for example. The data readouts require some interpretation, too: to translate coincidence rates to a probability, you need to account for the efficiency of the particle detectors. “The actual doing of an EPR measurement is not very glamorous,” Galvez admits. But then it dawns on you what you’re seeing. The photons are acting in unison even though no known force or influence links them. And they do so despite being separated by the width of a hand, which, for an infrared photon, might as well be a million miles.

In fact, I was so inspired by Galvez’s and others’ efforts to streamline these experiments that I recently developed my own el-cheapo version, which you can do at home for a few hundred dollars.

Photo credit: Eric R. Olson/Scientific American

For physicists trying to make sense of quantum mechanics, Albert Einstein’s thinking remains highly relevant. “This guy saw more deeply and more quickly into the problems that plague us today,” one quantum physicist told me. The latest volume of the Collected Papers of Albert Einstein, which contains Einstein’s publications, draft papers, letters, and scribblings from January 1922 through March 1923, shows that his deep concerns with the quantum predated his well-known duels with Niels Bohr and played a major role in shaping the emerging theory. Many of these writings have not been made public before. I’ve invited Tilman Sauer of Caltech, a senior editor at the Einstein Papers Project, to describe some of the new things that have turned up. —George Musser

Albert Einstein and Niels Bohr famously clashed in the late 1920s and early 1930s over the interpretation of quantum mechanics and the phenomenon we now call quantum entanglement. However, Einstein had already been probing quantum mechanics for many years, both theoretically and experimentally. He proposed not only thought experiments but also real experiments that, when conducted, were crucial in shaping physicists’ understanding of the emerging theory. Details of Einstein’s ideas and new insights into his involvement with quantum theory come to light in the course of our work on his papers.

Bohr had postulated that electrons move around atomic nuclei only in certain stationary orbits, and that they send off radiation only when they fell from one such orbit to another one of lower energy. Equating the energy of the emitted light to the energy difference of the two orbits, he was able to explain the spectrum of hydrogen. It was a spectacular breakthrough. But despite their empirical success, Bohr’s postulates challenged basic tenets of classical electrodynamics. Everybody was puzzled. Bohr had opened the door to a world of mystery and surprise.

Einstein, too, was challenged and provoked by Bohr’s bold theory. Himself a founding father of the early quantum theory and author of the light quantum hypothesis, he sensed that here was a problem that could not be reconciled with classical concepts and imperatively required new ideas and approaches.

In late 1921, Einstein devised what he thought would be a crucial experiment to determine the nature of the light emission process. When electrons move from one orbit to the next, is light emitted by atoms sent out instantaneously, as a quantum, or gradually, as a continuous wave? He proposed studying the process using so-called canal rays, which are particles accelerated by a voltage in a glass tube; they stream through pores (“canals”) in the cathode of the tube. After passing through the cathode, they may reneutralize or not, but in any case can send out light (by Bohr’s mechanism of electrons jumping to a lower energy orbit). The light is shifted in frequency because of the Doppler effect, indicating that the light-emitting particles are moving fast.

If you send light produced in this way through a dispersive medium, Einstein predicted, the wave fronts should be deflected if the emission process were classical. When Hans Geiger and Walther Bothe conducted the experiment, using carbon disulfide gas as the medium, they saw no such deflection. Einstein took this as evidence that light is emitted as a particle rather than as a wave. But as the new volume of Einstein’s Collected Papers makes clear, Einstein had made a mistake. As was pointed out to him by his friend Paul Ehrenfest (shown in the photo above with his son and Einstein), his analysis was flawed. He had not distinguished correctly between group velocity and phase velocity. When you correct for this, neither of the two theoretical alternatives results in a deflection. Retracting his original manuscript, Einstein graciously published an analysis of classical wave propagation through dispersive media, so others would not run into the same trap.

But he did not let up. Again and again, he devised experiments designed to shed light on the choice between quantum and classical concepts. Some ideas were short-lived, others were being put to the test. Ehrenfest was fascinated. He dreamed of locking Einstein and Bohr together in a room to fight it out, thus anticipating the famous debates between the two about the Copenhagen interpretation a few years later at the 1927 Solvay conference.

Indeed, in the early 1920, Einstein examined most matters in light of the quantum question. Few others probed as deeply as he did. Take superconductivity. The mysterious complete and sudden loss of electric resistivity at liquid helium temperatures had first been observed only some 10 years earlier in Leyden for the case of mercury, and until 1923 it was only in Leyden that Kamerlingh Onnes had the facilities to produce the phenomenon. Einstein hypothesized that the charge current in superconductors is produced by electrons moving on chains of Bohr orbits without emitting radiation. In superconducting metals, atoms are aligned in such a way that their orbits tangentially osculate each other, thus allowing electrons to pass smoothly from one atom’s orbit to the next. If that were so, he reasoned, interfaces between different metals should not be superconducting. It was a clever idea. But when Onnes performed the experiment and found superconductivity in a ring consisting of alternating pieces of lead and tin, Einstein sighed: “Yet another glimmer of hope for understanding is dashed.”

In Frankfurt at the same time, Otto Stern and Walther Gerlach were sending silver atoms through a strong, inhomogeneous magnetic field in order to see whether they carried a magnetic momentum and, if so, whether that momentum was quantized in space, as quantum theory postulated. They found that the beam of silver atoms split into two rays, corresponding to two different possibilities of aligning their magnetic momentum in the magnetic field. When Einstein and Ehrenfest discussed the experiment during one of Einstein’s visits to Leyden, they immediately realized that the Stern-Gerlach experiment cannot be explained classically. They did a little calculation (the photo at left shows an excerpt from Ehrenfest’s notes) estimating how long it would take a silver atom to align in a magnetic field by classical emission of radiation through Larmor rotation. They found that this would take 100 years, as compared to the few microseconds that the atoms had available during their time of flight. Confronted with the first example of a genuine quantum measurement process, Einstein and Ehrenfest immediately realized the fundamental puzzle of what later became a textbook example of the wavefunction collapse.

The Einstein Papers Project aims at making accessible Einstein’s writings and correspondence in a carefully annotated, scholarly edition. The editorial project builds on the Albert Einstein Archives, an archival collection of his published and unpublished writings and extensive correspondence deposited at the Hebrew University of Jerusalem. A catalogue of the collection is accessible in a database with more than 80,000 records and many facsimiles. The editorial project is organized chronologically. With the latest volume, the published series now covers Einstein’s life and work up to his 44th birthday. Who knows what other nuggets of insight will turn up in the volumes to come?

Photos © Museum Boerhaave, NL.

As a warm-up exercise, I sandwich my source of entangled photons—a disk of radioactive sodium-22—between my two Geiger counters (see diagram and photo below) and leave the system to run overnight, measuring how often the Geigers click at the same time. If gamma-ray photons are indeed emerging two by two in opposite directions, the coincidence rate should vary strongly when I change the alignment of the two Geigers. And that is what I see.

When the Geigers are pointing straight at each other, each clicks about 900 times per minute and both do so in unison about 4 times per minute. This is about 40% greater than the expected rate of accidental coincidences. There are various subtleties in separating accidental and genuine coincidence rates and in estimating statistical errors, but the signal I observe is something like 10 standard deviations above the noise. When I rotate one of the Geigers out of alignment, the coincidence rate drops precipitously. For a 25° angle, it only about 15% greater than the accidental rate, which is still statistically significant, if barely. For 45° and 90°, it is equal to the expected accidental rate. So I can tentatively conclude I’m seeing pairs of gammas—one or two of them per minute! This is no mean accomplishment given how crude the equipment is.

Just because the gammas emerge in pairs doesn’t mean they are entangled, though. To check for entanglement, I measure the photons’ polarization with a technique called Compton polarimetry. A pair of aluminum cubes bought at OnlineMetals.com serve as gamma-ray prisms, scattering photons in directions that depend on their polarization. The two gammas produced by the annihilation of an antielectron and electron are linearly polarized at right angles to each other, so they should scatter off the aluminum in perpendicular directions.

Here’s where the physics gets spooky. Each individual photon scatters in a random direction, yet the random direction one photon takes is related to the random direction its partner does. The gammas act in synchrony. How can they do that, if they’re truly random? Einstein concluded that the photons either are not truly random or are acting on each other at a distance.

In a first attempt to observe this effect, I sandwich the sodium-22 disk in between the two cubes and put a Geiger on one face of each cube (see photo below). I start by pointing the Geigers in the same direction and letting them sit overnight to count the coincidences. In the morning, I move one Geiger to a different face of its cube, so that the two detectors are now perpendicular to the other, and leave the system to run all day. I continue cycling through different ways to align the detectors either parallel or perpendicular to each other. Entanglement should betray itself as an asymmetry in the coincidence rate.

And indeed that’s what I see. About one coincidence occurs per minute on average, and the rate is consistently greater when the Geigers are perpendicular. It looks like entanglement in action!

A wise graduate student would hesitate to show this result to his or her faculty advisor, though. The perpendicular rate stands a couple of standard deviations above the expected accidental-coincidence rate, but the parallel rate swims in the noise. So the asymmetry might well be a fluke of statistics or a subtle bias in the setup.

To improve on the experiment, I need to beat down the accidental rate—in particular, the rate caused by gammas traveling straight from the sodium to the Geiger counter rather than scattering off the aluminum. I enclose the radioactive sodium in a so-called collimator: a lead storage canister in which I drilled a 1/2-inch hole at either end. A couple of hundred gammas per minute leak out through each hole, forming a pair of gamma-ray beams. The lead squelches off-axis radiation by a factor of about four.

With the collimator, the coincidence rate drops by a factor of 10, but now exceeds the predicted accidental rate for both orientations. The perpendicular rate is the higher of the two, again as the Compton-polarimetry theory predicts for entangled photons.

This still isn’t anything to call the Nobel committee about. At best, it implies the detection of one entangled pair of photons every 20 minutes, and with such a meager trickle, who knows what subtle bias might be operating. What was iffy for the pioneering Bleuler and Bradt experiment can only be more so for my apparatus. Then again, all I’m seeking is a suggestive demonstration, not a research-grade system.

A possible next step would be to special-order a stronger sodium-22 source, which would bring the particle rates in my experiment up to the level of Bleuler and Bradt’s, at the price of posing a greater radiation hazard. Another idea would be to try scatterers besides aluminum cubes. Beyond that, however, I think you exhaust the el-cheapo options and have to dig deeper into your wallet, starting with replacing the Geigers counters with scintillation counters, as Wu and Shaknov used. These are more efficient at picking up radiation; create shorter electrical pulses for each particle they detect, which reduces the probability of accidental coincidences; and measure particle energy, which would help to sift out annihilation-produced photons. But such instruments are pricier and fussier.

A useful guide to further refinements is Leonard Kaskay’s Ph.D. dissertation from 1972. A student of Wu, Kasday systematically went through the possible sources of error: multiple scattering, geometric misalignment, unwanted photons, and more. He was able to achieve enough precision to show that the gammas violated a mathematical inequality derived by theorist John S. Bell, confirming that he was seeing spooky action at a distance rather than some mundane effect.

These kinds of experiments are notoriously tricky, so please share your thoughts and advice—not to mention your attempts to reproduce! Wait till your friends hear that you’re an amateur quantum physicist in your spare time.

I got the idea last fall while building a supersimple cloud chamber. I was having trouble seeing particle tracks in my initial designs and wanted to check whether the radioactive materials I had scrounged really were radioactive. So I dug out a pair of Geiger counters that Aware Electronics in Wilmington, Delaware, gave me years ago for a post-9/11 review of consumer radiation detectors. Each time you hear a Geiger counter click, it has detected a single particle–which, in the case of gamma radiation, means a single high-energy photon. It struck me that the Geigers might substitute for the single-photon detectors that account for much of the cost of that €20,000 system.

Doing a literature survey, I found that the very first entanglement experiment, performed by Ernst Bleuler and H.L. Bradt and independently by R.C. Hanna in 1948, used Geigers. It was the precursor of better-known work by Chien-Shiung Wu and Irving Shaknov, who reproduced the results using more sensitive detectors. In a strange twist of physics history, these researchers didn’t associate their work with entanglement per se, let alone with Einstein’s puzzlement over the phenomenon. Several years after the fact, theorists David Bohm and Yakir Aharonov realized that the experiments had breathed life into the famous EPR thought experiment of Einstein, Boris Podolsky, and Nathan Rosen.

Today, physics students at Caltech, the University of Edinburgh, and elsewhere routinely perform the Wu-Shaknov experiment. But what about the rest of us who lack the resources of such august institutions? With the encouragement of David Prutchi, who practically invented the category of amateur quantum physicist, I decided to try to recreate the Bleuler-Bradt experiment in my basement workshop.

The parts I use are:

• two Aware RM-60 Geiger counters
• disk of radioactive sodium-22
• 1-inch-diameter plastic tube and wood plugs
• Aware coincidence box
• Geiger-to-iPad interface box (built from Radio Shack parts)
• iPad running Geiger Bot app
• cables for the above
• lead can
• two aluminum bars

Besides being cheap, Geigers have two major advantages as single-photon detectors. First, by detecting gamma rays rather than visible or infrared photons, they don’t require darker-than-night conditions, which are the bane of optical entanglement experiments. Second, it’s easy to buy sources of entangled gamma-ray photons. A small disk of radioactive sodium-22 runs $80, compared to $1,400 for a source of entangled optical photons. When the sodium atoms decay, they give off antielectrons, or positrons, which in turn annihilate with nearby electrons to create entangled pairs of gamma-ray photons. (As someone raised on Star Trek, I got quite a thrill ordering antimatter over the Internet.)

The disk is rated as safe to handle, although it’s not something you’d want to carry around in your pocket. At a distance of about a foot, its emission is comparable to the background level in my basement. The gammas spray out equally in all directions. The disk is also designed to let positrons escape from one side, in case you want some. I use a short wooden dowel to block these errant particles, as well as hold the disk in place inside a plastic tube.

Theory says the entangled gammas in a pair fly off in exactly opposite directions, so I look for them by placing a Geiger on either side of the sample. Paired photons should cause both Geigers to click in unison. To count these simultaneous hits, I wire the Geigers to Aware’s coincidence box (nothing more than a NAND gate, a standard electronic component), plug the c-box output into the mic jack of an iPad using a simple interface, and run an app called Geiger Bot. In pre-iOS times, Sci Am’s Amateur Scientist columnist, Shawn Carlson, hacked a pedometer to accomplish much the same.

Before hunting for entangled photons, I test the Geigers and c-box by measuring the background radiation—specifically, cosmic rays. When a high-energy particle from deep space hits Earth’s atmosphere, it shatters into a fusillade of particles, notably muons, that rain down upon the ground. A muon will rip through both Geiger counters and register as a coincidence. You can tell you’re detecting cosmic rays, rather than some other form of radiation, because the coincidence rate depends on the Geigers’ positioning. Stacked one of top of the other, the devices register particles traveling vertically downward, and Geiger Bot counts about 3 coincidences per minute. This value matches estimates of the cosmic-ray flux for such a setup. Lined up horizontally side by side, the Geigers become insensitive to muons from on high and the coincidence rate drops by a factor of 100. (If you’re a cosmic-ray geek, or bored, you can watch my readings on the Cosm data-logging website. Please let me know if you put your own cosmic-ray detector online.)

Occasionally, the system also picks up spurious coincidences: two unrelated particles that just so happen to hit both Geiger counters at nearly the same time. For cosmic-ray detection, this is a non-issue; in the absence of any radioactive materials, each Geiger clicks only 20 times per minute on average, so a day might pass before both go off simultaneously by chance. When you’re working with radioactive samples, however, the effect becomes significant, since the Geigers are clicking hundreds or thousands of times a minute. The rate of accidental coincidences scales up with the square of the Geiger readings and, in Bleuler and Bradt’s experiment, was comparable to the rate of genuine coincidences. It’s unfortunate that the two rates are similar—it means any detection of entanglement will be marginal at best. But this source of error is unavoidable when using Geiger counters. Pricier kinds of detectors have sharper time resolution and pick up fewer chance overlaps.

Having tested the electronics, I’m ready to start the experiment. In the next post, I’ll describe what I found.

Yet this is precisely what physicists have been doing to make sense of high-temperature superconductors and plasmas of nuclear particles. Both of these states of matter are about as un-black-hole-like as you can imagine. They don’t suck you to your death—indeed, the force of gravity plays no role in them at all—and they don’t split open the very foundations of physics. They are hard to understand in much the same way Earth’s climate is: the laws governing their constituents are perfectly well-known, but there are just so damned many constituents.

In the course of studying black holes, however, string theorists have discovered unexpected parallels, or “dualities,” between gravitational systems and non-gravitational ones. These correspondences may be purely mathematical or may reflect deeper physical linkages, but either way, you can leverage your knowledge of one domain to solve problems in another. In the January issue of Sci Am, Harvard physicist Subir Sachdev describes how to take analyses of gravitational phenomena and apply them to otherwise intractable problems regarding superconductors. Sabine Hossenfelder at Backreaction blogged on this topic recently, too, although she presumed a comfort level with vector fields and critical points.

But what about running the dualities in the other direction, using laboratory measurements of extreme materials to probe exotic gravitational physics? At an afternoon coffee-and-cookie break this spring at the Kavli Institute for Theoretical Physics, string theorist Ramy Brustein of Ben-Gurion University in Israel told me a way to do just that. He and Joey Medved of Rhodes University in South Africa have since written up their proposal. An expert on nuclear plasmas, Raju Venugopalan at the Brookhaven National Lab, likes the idea of returning the favor that string theorists have paid his subject area. “Can these experiments be used to learn about aspects of gravity?” Venugopalan wonders. “That would just be a phenomenon.”

The experiments in question entail smashing gold or lead nuclei together to create plasmas of quarks and gluons. When Brookhaven’s RHIC accelerator, following up earlier discoveries at CERN, first created these plasmas in 2005, physicists were flummoxed. They’d predicted the plasmas would behave like a gas, since quarks and gluons interact only weakly under the conditions that RHIC achieved. But the particulate debris betrayed pressure gradients that a gas cannot sustain. The plasmas must actually be liquid. Evidently the sheer number of particles compensated for the inherent weakness of their interactions.

Theorists were at a loss to calculate basic parameters of the fluid, such as viscosity—loosely speaking, the friction of fluid flow. The best they could manage was a rough argument based on Heisenberg’s uncertainty principle. Viscosity depends on the energy of the fluid’s constituent particles and the average time between successive particle collisions, and the uncertainty principle relates these two quantities, thereby implying a minimum possible value to the viscosity (as explained here). Even a so-called superfluid can’t evade Dr. Heisenberg’s strictures. A gas actually has a fairly large viscosity, since its particles are spaced farther apart and collide less frequently than those in a liquid. (A technical note: by “viscosity,” I really mean the ratio of viscosity to density.)

But what exactly the minimum value should be, theorists couldn’t tell, until Dam Son of the University of Washington and his colleagues applied duality. They equated the viscosity of a fluid to gravitational waves caroming off a black hole in higher-dimensional space—which, even for a physicist, is not an analogy that springs to mind. “That was a big surprise,” Brustein says. “The fact you can calculate hydrodynamical parameters from gravity was not understood.” The answer: 1/4π, in the appropriate units. The viscosity measured by RHIC comes close. Water, some 400 times more viscous, is molasses in comparison.

Surprisingly, the minimum value is the same for all fluids, whatever they are made of. Through the logic of duality, this universality has a simple explanation: Viscosity is equivalent to a gravitational phenomenon, and according to Einstein’s general theory of relativity, gravitation is blind to compositional details.

This is the line of reasoning Brustein hopes to flip around. The way he tells the story, it all started on an extended visit to CERN during the snowy winter in Europe two years ago. Brustein was out shoveling his driveway in the French village of Thoiry and got talking to his neighbor. Turned out the neighbor was the technical director of the ALICE experiment, which is CERN’s answer to RHIC. Not long after, Brustein bumped into the ALICE team leader at a formal dinner. Clearly it was meant to be. Some months later, Brustein sat with ALICE scientists in the CERN cafeteria and sketched out his ideas on a napkin (see photo above). Even if they don’t work out, Brustein has at least checked off two items on physicists’ list of 1000 things to do before you die: (1) napkin sketch, and (2) CERN cafeteria, a storied hangout where scientists have come up with such ideas as the World Wide Web.

Brustein’s insight was that viscosity is not the only fluid property you can measure. Shape is another. If the duality is valid, viscosity and shape will be related in a way that pins down the corresponding theory of gravity. “He’s looking for new observables that are a bit more discriminating than viscosity,” Venugopalan says.

For instance, if Einstein’s general relativity governs the gravitational dual, the minimum viscosity will equal 1/4π and the plasma should be spherically symmetrical. Nuclear physicists would not expect an ephemeral roiling fireball to have such symmetry, so this counts as a strong and significant prediction. “It’s an actual way of proving the quark-gluon plasma has a gravitational dual,” Brustein says. Things get even more interesting if Einstein’s theory is only an approximation to a deeper theory, as string theory holds. Then the viscosity value will differ from 1/4π and may no longer be universal among substances; the plasma shape will gain some angular structure (a quadrupolar correlation function, to be technical). So the experiment is able to probe post-Einsteinian physics.

To be sure, these measurements would not probe the law of gravity that governs our universe, but only the law of gravity that is implicit in the plasma dynamics. That is to say, the plasma’s fluid behavior can be thought of as related to some hypothetical universe where gravity acts a certain way. That universe may or may not be a model for ours. What the measurements would do, however, is test the general concept of duality, which currently has the status of a conjecture, and validate it as a tool in the search for a unified theory.

Brustein’s biggest challenge is not the physics per se; it is to persuade RHIC and ALICE experimentalists to take the data he needs. Typically experimentalists measure just the numbers of particles coming out in different directions, rather than the details of the particles’ energy and momentum. Venugopolan cautions: “Though I appreciate where Brustein is coming from and it would be indeed great if one can make an empirical determination of these questions, there are a large number of nontrivial issues to resolve before one gets there.” Particle experimentalists are busy people these days and have no shortage of ideas for what to look for. So Brustein might have to eat a lot more cookies and shovel more driveways to convince them.

Napkin sketch courtesy of Ramy Brustein

I first heard about their brainstorm while visiting the Kavli Institute for Theoretical Physics in Santa Barbara this spring, and the team—Polchinski and fellow Santa Barbarans Don Marolf, Ahmed Almheiri, and James Sully—wrote it up over the summer. Polchinski blogged about it a few months ago, and another theorist who helped to usher in the idea, John Preskill, did so last week. Polchinski’s talk to the New York University physics department drew a standing-room-only crowd, not a single person snuck out early, and he was still fending questions an hour after it ended.

Almost as much has been written about Hawking’s original paradox (including by me) as about the fiscal cliff, so I’ll jump straight to the new version. Step #1 of the argument is what Polchinski and his co-authors call the “no-drama” principle. According to current theories of physics, a black hole is mostly just empty space. Its perimeter or “event horizon” is not a material surface, but just a hypothetical location that marks the point of no return. Once inside, you are gripped too tightly by gravity ever to get back out. By then, falling at nearly the speed of light, you have a few seconds to look around before you reach the very center and get crushed into oblivion. But nothing noticeable should happen at the moment of crossing. One of Einstein’s great insights was that observers who are freely falling—whether into a black hole or toward the ground—don’t feel the force of gravity, since everything around them is falling, too. As they say, it’s not the fall that kills you; it’s the sudden stop at the end.

An outside observer knows you’re doomed, but likewise doesn’t think anything untoward happens upon passing through the event horizon. Indeed, this observer never sees anything actually cross over. Because of a kind of gravitational mirage, things seem to slow down and freeze in time. All the stuff piling up at the horizon forms a ghostly membrane, which obeys the usual laws of physics and has conventional properties such as viscosity and electrical conductivity.

Step #2 is to relate these two viewpoints. To the infalling observer, space looks like a vacuum, and in quantum theory, a vacuum is a very special state of affairs. It is a region of space that is empty of particles. It is not a region that is empty of everything. There’s no getting rid of the electromagnetic field and other fields. (If you could, the region would not merely be empty, but nonexistent.) A particle is nothing more or less than a vibration one of these fields, and what makes a vacuum a vacuum is that all the possible vibrations cancel one another precisely, leaving the fields becalmed. To maintain this finely balanced condition, the vibrations must be thoroughly quantum-entangled with one another.

To the outgoing observer, the horizon (or membrane) cleaves space in two, and the vibrations no longer appear to cancel out. It looks like there are particles flying off in every direction. This is perfectly compatible with the infalling observer’s viewpoint, since the fields are what is fundamental and the presence of particles is a matter of perspective. To put it differently, emptiness is a holistic property in quantum physics—true for a region of space in its entirety, but not for individual subregions.

For consistency between the two viewpoints, the outside observer infers that each particle he or she sees has a doppelgänger inside the horizon. The two are quantum-entangled, like those particles in laboratory experiments you read about. (Watch this lighthearted video that my colleagues made earlier this year to explain entanglement.) Individually, both particles behave completely randomly, but together they form a matched pair. See the diagram at left: the infalling observer sees vacuum state a, the outside observer sees entangled particles b and b′. Particle b is part of what physicists call the Hawking radiation.

Step #3 is to consider the long-term fate of the hole. Like everything else in this world, black holes must decay—quantum mechanics mandates it. In the process, a hole must gradually release everything that fell in. If Joe Polchinski jumps into a black hole, he will get scrambled with all the other theorists who have done the same, and the morbid gruel will emerge particle by particle in the Hawking radiation. Though mangled beyond recognition, each martyr to the cause of knowledge can still be separated out and pieced back together. To enable this reconstruction, the particles of the Hawking radiation must be thoroughly entangled with one another.

So, by step #2, each particle flying away from the hole must be thoroughly entangled with its doppelgänger inside the hole. By step #3, the particle must also be thoroughly entangled with other particles that are flying away from the hole. These two conclusions clash, because quantum mechanics says that particles are monogamous. They can’t be thoroughly entangled with more than one other partner at a time. They can be partially entangled, but that is not enough to ensure consistency between the observers’ view or to reconstruct the infalling physicists.

This formulation of the black-hole paradox vindicates Hawking’s original argument. For years physicists hoped that the devil lay in the details—that more precise calculations would reveal an escape route—only to be serially disappointed. Now they have officially given up hope. One of the basic premises must be wrong—which is to say, something deep about modern physics must be wrong. “You need huge changes, not just quantum-gravitational corrections, to invalidate Hawking’s argument,” Polchinski told the assembled multitudes at NYU.

More surprisingly, Polchinski and his co-authors have shown that a popular approach known as black-hole complementarity, championed by Leonard Susskind of Stanford University, isn’t up to the task, either. Susskind reasoned that, although infalling and outside observers might see different and mutually incompatible events, no single observer can be both infalling and outside, so no single observer is ever faced with a direct contradiction. In that case, the paradox is only ever conceptual—suggesting it is somehow illusory, the product of thinking about the situation in the wrong way. But Polchinski and colleagues showed that a single observer can catch a particle in the act of polygamy by first lingering outside the hole and then jumping in.

The least radical conclusion is that the no-drama principle is false. Someone falling into a black hole doesn’t pass uneventfully through the horizon, but hits a wall of fire and is instantly incinerated. “I think it’s crazy,” Polchinski admitted. But in order for a black hole to decay and its contents to spill out, as quantum mechanics demands, the infalling observer can’t see just a vacuum. The firewall idea strikes me as similar to past speculation that black holes are somehow material objects—so-called black stars or dark matter stars—rather than merely blank space.

“I spent 20 years confused by this,” Polchinski said, “and now I’m as confused as ever.” It would be nice to answer the question, if only so that no one ever has to undertake the journey to answer the question.

Diagrams courtesy of Joseph Polchinski

In the latest issue of Make magazine, I learned about an extraordinary book that could do for quantum homebrewers what Popular Electronics magazine did for Jobs and Wozniak in the ’70s. Written by the father-daughter team of David and Shanni Prutchi, Exploring Quantum Physics Through Hands-On Projects takes you from basic particle-wave demonstrations all the way up to quantum random-number generators. The associated website follows up with detailed advice and other projects. Had you asked me a month ago, I would have said that an atomic clock is surely beyond the capacity of even the most dedicated hobbyist. Now I know better.

I happened to be down in Philadelphia, near where the Prutchis live, for a conference two weeks ago, and David (shown above with his other two daughters, Abigail and Hannah—Shanni was still at school) kindly gave me a tour of their basement lab. The first clue that it wasn’t just any suburban house was the radio telescope in the backyard, which Shanni used to map the Milky Way’s hydrogen gas in fifth-grade. The living room tchotchkes included an old Civil Defense radiation meter and blown-glass Crookes tubes. The lab itself, about the size of a one-car garage, is a work of art in its own right, lined with snazzy electronics, such as spectrum analyzers, as well as contraptions out of a Victorian experimental philosophy lab, such as van de Graaf generators. It’s better outfitted than some academic labs I’ve seen, but David modestly insisted they cobbled it together from Home Depot, eBay, and dumpster dives at local high-tech firms. The book offers tips for how to do the same.

More than equipment, though, experimentation requires persistence. Shanni said she shudders at the effort it took to prove quantum spooky action at a distance. The principle is simple (see this video my colleagues and I made); the practice is a different story. For instance, the experiment requires light detectors that are sensitive enough to detect single photons, which means they are also prone to the tiniest sliver of stray light. David and Shanni kept them dark by enclosing the apparatus in a set of nested black boxes, which they had to open and close to make any adjustment—and an experiment of this sort requires a lot of adjustments. “I hated that black box,” Shanni told me. “There were days I didn’t want to do the experiment. It was very frustrating, but it all paid off. We were actually able to disprove local hidden variables—and I’m a high-school senior.”

Photos by George Musser

You’ll need the following:

1. sponge
2. rubbing alcohol (92%)
3. clear plastic cup
4. tape
5. black construction paper
6. foil cupcake liner
7. blu-tack
8. foil tray
9. air duster (one of those spray cans you use to blow crumbs off computer keyboards)
10. bright LED flashlight

The real innovation here is the air duster. The difluoroethane it releases is cold—cold enough to supercool alcohol vapor, which is what you need for a cloud chamber. The supercooled vapor will condense along the paths of ionizing particles like a tiny contrail.

The one thing you probably don’t readily have is a source of ionizing particles, but you’d be surprised how many household items are mildly radioactive, and scientific suppliers sell test sources. I bought a chunk of uranium ore from United Nuclear. (If nothing else, receiving a box from a company called United Nuclear will impress your friends.) Even if you don’t have a source, you can use the cloud chamber to see cosmic rays—energetic particles from outer space.

The plastic cup is the chamber proper. Here’s how you prepare it:

1. Cut a piece of sponge a few inches square, soak it in rubbing alcohol, and wedge it into the bottom of a clear plastic cup, holding it in place with tape.
2. Mount your radioactive source, if you have one, to the inside of the cup just below the rim. I found it important to mount the source to the cup rather than just lay it on the bottom of the chamber; that way, you slow down how fast the source gets coated with alcohol.
3. Cut a circle of black construction paper to match the rim of the cup. This will provide a dark backdrop to view particle tracks.
4. Flatten the cupcake foil, center the paper circle on it, and tape it down.
5. Press blu-tack along the cup rim. This will be the air seal between the cup and foil.
6. Turn the cup upside-down.
7. Press the cupcake foil to the rim. The paper circle should be inside the cup. Smush the blu-tack into any gaps so that you have a decent air seal.
8. Place your chamber above the foil tray to catch difluoroethane released from the spray can. I mounted the cup on a chemistry lab stand, but you can just hold it with your hand (which is better in some ways, because your hand warms the sponge, increasing alcohol evaporation).
9. Shine the flashlight into the chamber. I got the best results when the flashlight was nearly horizontal and touching the cup to minimize reflection off the plastic.

YMMV, so play around.

To operate the chamber, turn off the room lights, hold the air sprayer upside-down, and spray the foil for a couple of seconds. Repeat every 10 seconds or so to keep the foil cold. A sign that it’s cold enough will be that ice crystals form on the outside of the foil.

Inside the cup, a mist of alcohol droplets forms almost immediately along the bottom, within a centimeter of the construction paper. If you have a radioactive source, you should start to see tracks radiating from it. If not, you’ll see a cosmic ray streak across the bottom of the chamber every 20 seconds or so. If you can’t see anything, change the illumination angle. I got significantly better results by replacing the foil and paper with a square of aluminum sheet metal covered in black electrical tape.

The spray cans are exhausted quickly. I bought a four-pack of them. To prolong the particle fireworks show, buy some dry ice—my local ice cream store sells it—and lay the chamber on a block of it.

Happy particle hunting!

Photos by George Musser

Other cosmologists think they just need to learn how to count better. In April I went to a talk by Leonard Susskind (silhouetted in the photo above), who has been arguing for a decade that you don’t need to count all the parallel universes, just those that are capable of affecting you. Forget the causally disconnected ones and you might have a shot at recovering your empiricist credentials. “Causal structure is, I think, all important,” Susskind said. He presented a study he did last year with three other Stanford physicists, Daniel Harlow, Steve Shenker, and Douglas Stanford. I didn’t follow everything he said, but I was enamored of a piece of mathematics he invoked, known as p-adic numbers. As I began to root around, I discovered that these numbers have inspired an entire subfield within fundamental physics, involving not just parallel universes but also the arrow of time, dark matter, and the possible atomic nature of space and time.

Lest you think that the whole notion of parallel universes was ill-starred to begin with, cosmologists have good cause to think our universe is just one member of a big dysfunctional family. The universe we see is smooth and uniform on its largest scales, yet it hasn’t been around long enough for any ordinary process to have homogenized it. It must have inherited its smoothness and uniformity from an even larger, older system, a system permeated with dark energy that drives space to expand rapidly and evens it out—the process known as cosmic inflation. Dark energy also destabilizes the system and causes universes to nucleate out like raindrops in a cloud. Voilà, our universe.

Other bubbles are nucleating all the time. Each gains its own endowment of dark energy and can give rise to new bubbles—bubbles within bubbles within bubbles, an endless cosmic effervescence. Even our universe has a dab of dark energy and can birth new bubbles. The space between the baby bubbles expands, keeping them isolated from one another. A bubble has contact only with its parent.

The process produces a family tree of universes. The tree is a fractal: no matter how closely you zoom in, it looks the same. In fact, the tree is a dead ringer for one of the most famous fractals of all, the Cantor set.

In a simplified case, if you start with a single universe, by the Nth generation, you have 2^N of them. You label each universe by a binary number giving its position in the structure. After the first bubble nucleation, you have two universes, the inside and outside of the bubble: 0 and 1. In the first generation, universe 0 spawns 00 and 10, and universe 1 spawns 01 and 11. Then, universe 00 gives birth to 000 and 100, and so it goes.

The process goes on forever, approaching a continuum of universes (the red line at the top of the diagram) indexed by numbers with an infinity of bits. The fun thing is that these numbers are not standard-issue infinite-digit numbers like 1.414… (√2) or 3.1415… (π), which mathematicians call “real” numbers—the ones you find on a grade-school number line. Instead they are so-called 2-adic numbers with very different mathematical properties. In a more general setup, each universe could fork into p universes rather than just two, hence the general term p-adic.

Mathematicians came up with p-adic numbers in the late 19th century as an alternative way, besides real numbers, to fill in the spaces between integers and integer fractions to make an uninterrupted block of numbers. In fact, Russian mathematician Alexander Ostrowski showed that p-adics are the only alternative to the reals.

Unfortunately, mathematicians have done a good job of smothering the beauty beneath formal definitions, theorems, lemmas, and corollaries that dot every ‘i’ but never tell you what they’re spelling out. (My mathematician friends, too, complain that math texts are as compelling to read as software license agreements.) It wasn’t until I heard Susskind’s description in terms of counting parallel universes that I had a clue what p-adics were or appreciated their sheer awesomeness.

What differentiates p-adics from reals is how distance is defined. For them, distance is the degree of consanguinity: two p-adics are close by virtue of having a recent common ancestor in their family tree. Numerically, if two points have a common ancestor in the Nth generation, those points are separated by a distance of 1/2^N. For instance, to find a common ancestor of the numbers 000 and 111, you have to go all the way back to the root of the tree (N=0). Thus these numbers are separated by a distance of 1—the full width of the multiverse. For the numbers 000 and 110, the most recent common ancestor is the first generation (N=1), so the distance is 1/2. For 000 and 100, the distance is 1/4.

To put it another way, if someone gives you two p-adic numbers, you determine the distance between them using the following procedure. Line them up, one on top of the other. Compare the rightmost bits. If they’re different, stop! You’re done. The distance is 1. If they’re the same, shift to the left and compare the next bits over. If they’re different, stop! The distance is 1/2. Keep going until you find the first bit that is different. This bit—and none other—determines the distance.

This distance rule messes with your mind. Two parallel universes that look nearby can be far apart because they lie on different branches of the tree. Likewise, two points that look far apart might be nearby. In the figure at left, universe ‘B’ is closer to universe ‘C’ than to ‘A’. What is more, the number 100 is smaller than the number 10, since it is closer to the far left side of the multiverse. With p-adics, you gain precision by adding digits to the left side of the number rather than to the right. Accordingly, mathematician Andrew Rich and undergraduate Matthew Bauman have dubbed them “leftist numbers.”

p-adics can be added, subtracted, multiplied, and divided like any other self-respecting number, but their leftist proclivities change the rules and make arithmetic unexpectedly easier. To add two p-adics, you start with the most significant digit (on the right) and add them one by one toward the least significant digits (on the left). With reals, on the other hand, you start with the least significant digit, and you’re out of luck if you have a number such as π with an infinite number of digits.

The weirdness doesn’t stop there. Consider three p-adic numbers. You can think of them as the three corners of a triangle. Oddly, at least two sides of the triangle must have the same length; p-adics, unlike reals, don’t give you the liberty to make the sides all different. The reason is evident from the tree diagram: there is only one path from one number to the other two numbers, hence at most two common ancestors, hence at most two different lengths. In the jargon, p-adics are “ultrametric.” On top of that, distance is always finite. There are no p-adic infinitesimals, or infinitely small distances, such as the dx and dy you see in high-school calculus. In the argot, p-adics are “non-Archimedean.” Mathematicians had to cook up a whole new type of calculus for them.

Prior to the multiverse study, non-Archimedeanness was the main reason physicists had taken the trouble to decipher those mathematics textbooks. Theorists think that the natural world, too, has no infinitely small distances; there is some minimal possible distance, the Planck scale, below which gravity is so intense that it renders the entire notion of space meaningless. Grappling with this granularity has always vexed theorists. Real numbers can be subdivided all the way down to geometric points of zero size, so they are ill-suited to describing a granular space; attempting to use them for this purpose tends to spoil the symmetries on which modern physics is based.

By rewriting their equations using p-adics instead, theorists think they can capture the granularity in a consistent way, as Igor Volovich of the Steklov Mathematical Institute in Moscow argued in 1987. The resulting dynamics might even explain dark matter and the mechanics of cosmic inflation.

Naturally, having found a new toy to play with, physicists immediately wonder how to break it. Susskind and his colleagues took the tree of parallel universes, lopped off some of its branches, and figured out how it would deform the p-adics. Those pruned branches represented infertile baby universes: those born with zero dark energy or a negative density of the stuff. Just as pruning a real tree might seem destructive but actually helps it to grow, pruning the tree of universes mucks up its symmetry but does so in a good cause: it explains, the team argued, why time is unidirectional—why the past is different from the future.

p-adics are a case study of how a concept mathematicians invented for its own beauty might turn out to have something to do with the real world. What a bonus that they may be more real than the reals.

Photograph courtesy of Gary Smaby. Bubble figure courtesy of George Musser. Tree figures courtesy of Daniel Harlow, Stanford University.

It shows a galaxy riddled with great big clumps of gas, at least three of which (labeled A, B, and D) are so strongly ionized that they must contain giant black holes, with a mass of 3 to 20 million solar masses. Such a triplet is rare, and what makes it even more remarkable is that there are no debris trails or other signs that the galaxy is the product of the collision of several galaxies. Evidently all three holes are homegrown—the first known example of multiple giant black holes forming in the same galaxy. Kevin Schawinski and his colleagues published this Hubble Space Telescope image last fall, and he told me about it at the American Astronomical Society meeting in January.

Usually when astronomers marvel at supermassive black holes, they focus on holes that date to the first billion years of cosmic history. Those ancient holes managed to reach their enormous proportions in not a lot of time. The rapidity of their growth, astronomer Jenny Greene argued in our January issue, means they probably formed by the direct collapse of immense gas clouds. To have assembled the holes in the usual multistep process—stars are born, stars die, dead stars collapse to black holes, black holes agglomerate—would likely have taken too long.

But the discovery by Schawinski’s team poses the opposite problem. The galaxy lies at a cosmic redshift of 1.35, corresponding to a cosmic age of 4.5 billion years—more than enough time for a black hole to grow big. If anything, it was too much time. The era of supermassive black hole formation should have long since been over. Moreover, the holes, being heavy, should all have sank to the middle of the galaxy and fused into one. Thus the triplet indicates black holes were continuing to form. That multistep process might operate after all.

The gaseous clumps that host the holes, inelegant though they may appear, are themselves fascinating. Our own galaxy has giant interstellar gas clouds, but these clumps are a different order of giant. Spanning thousands of light-years and containing up to a billion solar masses, they are to our galaxy’s clouds what Jupiter is to Earth. Astronomers began noticing them in the mid-’90s as the Hubble, Keck, and other telescopes started to make photo albums of galaxies at redshifts beyond 1. People’s initial instinct was to see clumpy galaxies as the aftermath of galactic collisions, but they turned out to be galaxies whose interstellar gas had curdled.

Galaxies back then contained proportionately much more gas—10 to 50 percent by mass, versus a few percent for today’s Milky Way—and the gas was roiled by vast rivers flowing in from intergalactic space (like the flow in the artist’s conception at right). Instead of passively riding on the gravity of other matter, gas actively reshaped the galaxy. It fragmented under its own sheer weight and strong turbulent motions, taking stars along for the ride, and most clumps went on to create mega black holes.

Clumpy galaxies have about the same mass, size, and numbers as the Milky Way. They have generally flattened disk shapes and, as BX442 shows, all the prerequisites for spiral patterns. So they appear to represent the turbulent adolescent phase of systems like our galaxy. “All this suggests that they are the typical progenitors of Milky Way-like spiral galaxies,” says astronomer Frédéric Bournaud.

To hear astronomers play down the importance of major galactic collisions is to hear the creak of a pendulum swinging back. When I started my career in science-writing nearly 20 years ago, astronomers had taken to collisions with the zeal of new converts. And indeed they have confirmed the importance of massive pileups to the creation of the very largest galaxies. But they now think that most galaxy evolution, star formation, and supermassive black hole growth are driven by internal processes and the occasional tickling from a minor close galactic encounter, like the putative mini-galaxy that may have set in motion the spiral pattern in BX442. For instance, the galaxies in the image at left all have actively feeding supermassive holes, but only the one at the top left is undergoing a major collision (and it is the sole example in a sample of 28). The beauty of galaxies needs no kick from the outside. It comes from within.

Credits: NASA, ESA, Kevin Schawinski (first and third images); ESA–AOES Medialab (cold flow artist’s conception)

Unruh bit off a piece of the central question in the search for a unified theory: What happens to stuff that falls into a black hole? No place else in the universe brings modern theories into such direct conflict. Einstein’s general theory of relativity says black holes are one-way streets: their gravity is so intense that nothing going down the drain can ever get back out again. Quantum theory says black holes are two-way streets: all processes are reversible in time, so whatever falls in has to be able to get back out in some form or other.

People who specialize in relativity, such as Unruh, not surprisingly tend to blame quantum theory for the trouble. They suggest that the time-reversibility of quantum theory, or, more strictly, its unitarity, fails. For their part, people who specialize in quantum theory tend to find fault with relativity. They think Einstein’s brainchild must break down, loosening the hole’s gravitational clutches.

In 1984 three people in the latter camp came up with a knockout blow. Tom Banks, Michael Peskin, and Leonard Susskind claimed that if unitarity failed, so would the laws of conservation of energy and momentum. Their reasoning was a straightforward application of the second law of thermodynamics. Any irreversible process creates entropy and heat. Under ordinary circumstances, the heat comes from burning fuel or some other source of energy; here, it has no source. It comes out of nothingness.

A failure of basic conservation laws is bad enough. What makes it really bad is that its effects would not be confined to black holes out in deep space, but should afflict our planet, too. Black holes pop into and wink out of existence all around us all the time. Quantum physicists are ordinarily untroubled by this ethereal effervescence—it is a measure of the weirdness of their theories that space could sizzle with short-lived black holes, like so many Pop Rocks, and theorists scarcely bat an eye. It gets their attention only if energy conservation fails.

Susskind has compared the failure of energy conservation to being pregnant: there’s no such thing as just a little energy non-conservation. Once you have any, you’re cooked. Literally. All those black holes would turn space into a giant heating coil and roast us to a temperature of 10³² degrees. The fact we are not being roasted alive means unitarity holds, ergo relativity must break down.

Not so fast, says Unruh. He is an avuncular, frizzy-haired Canadian who has a way of making you question things you thought you were sure about. One morning at the workshop, which was being held at the Kavli Institute for Theoretical Physics in Santa Barbara, I went to fill my water bottle and bumped into Unruh and another physicist, Ted Jacobson, in the hallway. We got to talking about our bike rides to campus, and I commented on some bluffs to the east. Unruh asked how tall they were. Jacobson and I both estimated 100 feet. Bill had never even seen the bluffs, but, based on some general remarks about local topography, doubted our estimate so vehemently that Jacobson and I lost all our initial conviction.

His doubts about toasty black holes go back to the mid-’90s, when he and fellow relativity expert Bob Wald cited new work in the physics of materials which suggested that an irreversible process does not necessarily generate heat. Nikolai Prokof’ev and Philip Stamp had recently described how a process can instead cause spinning particles to begin precessing like quantum versions of a wobbling top. Causing a particle to precess does not require any expenditure of energy, so it is not subject to the same thermodynamic restrictions that apply to other processes.

Experiments have since confirmed Prokof’ev and Stamp’s idea, as I learned last year when Stamp spoke at a meeting organized by the Foundational Questions Institute. Stamp breathes the same contrarian fire as Unruh. Quantum theory may be the best-tested theory in the history of science, but that didn’t stop him from cautioning that it may not be the final word: “I don’t think it’s a good idea to be too sure of it.”

In Santa Barbara, Unruh suggested that if the inner mechanism of a black hole behaves like a bunch of wobbly particles, it could swallow material irreversibly without roasting the universe in return. After an hour and a half of vigorous exchange, few if anyone seemed convinced, and Unruh became steadily more self-deprecating. “I accept my and Bob Wald’s position is a minority view,” he said at last. “This is the last gasp. As Popper [sic] said, you just have to wait for us to die out. But sometimes the troglodytes are right.”

Minority view though it is, Unruh’s position still gives his colleagues pause. Three weeks ago, I attended a conference on quantum gravity at the Nordic Institute of Theoretical Physics in Stockholm, organized by physicist and blogger extraordinaire Sabine Hossenfelder. Hossenfelder had the good fortune to bring together German, Italian, and Spanish physicists while their national soccer teams were clashing at the Euro 2012 championship. There’s nothing like epic sporting rivalry and Swedish microbrews to liven up physics discussions.

Chatting over lunch, two of the attendees discovered they had both been thinking about Unruh’s arguments and whether irreversibility necessarily toasted energy and momentum conservation. Jonathan Oppenheim, who has been toying with alternatives to quantum mechanics as a way to unpick the mysteries of black holes, said that holes popping up here and there wouldn’t alter the overall symmetries of space and time, which underpin the conservation laws. Luis Garay discussed how no clock is perfect. Unavoidable timing errors can cause quantum waves to fall out of sync irreversibly without flouting any conservation law. Like Unruh, Oppenheim and Garay were circumspect. It wasn’t that they thought the Banks, Peskin, and Susskind paper had to be wrong. They just thought no one could securely pronounce on it.

A few weeks after our encounter in the hallway at Santa Barbara, Jacobson told me he had checked Google Earth and found that the bluffs were indeed about 100 feet tall. Unruh was wrong about their height after all. Still, he was right to force us to think twice. Unusually among contrarians, he applies his same skeptical instincts to himself. “If you think you know the answer to something, then you stop looking for the answer,” he said during his talk. “For the young people: don’t assume the answers are known, no matter how confident the speaker may be—and that includes me.”

Photo credit: iStockPhoto

It wasn’t the quantum weirdness that wowed me—we should all be used to that by now—but the fact we’re talking about individual atoms here. Atoms. In olden times (before 1990), physicists worried that quantum uncertainty might foil attempts to manipulate matter on this scale. Even Richard Feynman’s famous nanotech lecture in 1959 envisioned atomic artisanship only “ultimately—in the great future.” Today experimentalists see atoms, poke atoms, prod atoms. Atoms’ quantumness has not been a hindrance but a benefit. Bloch’s horse race has the serious purpose of demonstrating that single atoms could store and process data in a quantum computer.

The tough part isn’t so much that atoms are small, but that they don’t stay put and that they bunch up. Bloch’s team and others bring them to heel by cooling them to a temperature of nanokelvins and pouring them into an optical lattice, which, depending on your poetic frame of mind, you might call an optical egg crate or a crystal of light. It consists of laser beams that overlap and interfere, forming a stationary grid of bright and dark spots. The light exerts electromagnetic forces that wrest atoms into these spots (typically the bright ones) and pin them there. The atoms are spaced perhaps 400 nanometers apart, so they reach a density of about 100 trillion atoms per cubic centimeter—which is a lot of atoms per cubic centimeter, but still only about a hundred-thousandth the density of hydrogen gas at room temperature and pressure. So these systems let physicists explore a domain they seldom otherwise enter, a frigid, sparse realm where quantum is king.

In particular, experimentalists are exploring the central problem in the physics of materials (what physicists call condensed matter): how matter makes the transition between different phases. The lattice-imprisoned aggregate of atoms can melt, like an ice cube, except that the transition arises from a rebalancing of energy rather than a change in temperature or pressure.

The atoms’ energy is both kinetic (they jump from one site to another through the process of quantum tunneling) and interactive (they repel one another). When the laser intensity is low, the electromagnetic forces trapping atoms are weak, so kinetic energy dominates and atoms are footloose. They no longer call one site home, but ooze all over the place, acting in unison as one ginormous quantum wave—a phase of matter called a superfluid. When the laser intensity is strong, the interaction energy dominates. Atoms hold one another at arm’s length and stay confined to specific spots on the grid—a phase of matter called a Mott insulator. A decade ago, Bloch and his colleagues gradually dialed up the intensity of their laser to watch the transition from superfluid to insulator.

This ability to fine-tune the system also lets physicists take pictures of individual atoms. First, crank up the interaction energy so that atoms space themselves out. Then, illuminate them with light that they absorb and re-emit. Through an optical microscope, you can see the atoms fluoresce. If their overall density is low enough, each spot is a single atom.

A similar procedure can control atoms one by one. First, shine a laser beam on the atom you’d like to muck with; this shifts its energy levels and makes it vulnerable to microwave radiation. Then, beam in a microwave pulse to flip the atom’s spin. Bloch’s team has applied this technique to create the world’s smallest pixel display (see picture). The atoms stay in place, so this approach differs from the nanosculpture that IBM scientists famously used two decades ago to create the world’s smallest corporate logo. Like the horse race, nanoartistry is practice for building a quantum computer.

There are all sorts of other fun experiments you can do. Last year, Bloch’s team tracked the insulator-superfluid transition and showed that the system goes through a “hidden” phase of matter—a subtly patterned arrangement that conventional theory doesn’t capture. Theorists have drawn on ideas from string theory, of all places, to help explain these transitions. Lately, the team has played with the Higgs mechanism. To most physics aficionados, the name “Higgs” conjures up the boson that particle physicists may finally be on the verge of discovering. But the concept actually originates in condensed matter: it explains why electromagnetic forces do not propagate freely in a superconductor. (In fact, the particle-physics Higgs is a sort of superconductor in which weak nuclear forces do not propagate freely.) Bloch’s team switched on the Higgs mechanism in its simplified system. The “bosons” were collective vibrations of the atoms.

Yet another experiment touches on the fundamental question of what determines the speed of events in the world. The theory of relativity sets the ultimate speed limit, that of light, but in practice the limit is usually much lower. What sets it? It turns out that quantum mechanics has a self-policing tendency, the Lieb-Robinson bound. Bloch and colleagues have observed it for the first time. They began with an insulator, dialed up the interaction energy, and watched the atoms start to self-organize. A wave of activity spread though the system at twice the speed of sound. What governed the velocity was that atoms did not passively roll on the wave, but actively contributed to it. Some quantum gravity theorists have speculated that the speed of light represents the Lieb-Robinson bound of some underlying quantum system out of which space and time emerge.

To me, the atomic hubbub suggests another meaning for the horse-race metaphor. For would-be nanoengineers, atoms are not tiny nuts and bolts. They are like animals, with a mind of their own. You don’t screw them together, but cajole them. If you set the right conditions, they’ll do all the work for you. They are not passive, but a creative force capable of things you might never expect.

Figures courtesy of Immanuel Bloch, Christof Weitenberg, and Peter Schauß, Max Planck Institute for Quantum Optics

I naturally wanted to invite an article for Sci Am about these charismatic megaparticles, but for years I struggled with what the article would say. Although they may be the most powerful electromagnetic radiation known to science—photons with an energy of around a teraelectron-volts (TeV), the kinetic energy of a mosquito concentrated into a single quantum—once you use up all the superlatives in your thesaurus, what was there to say, really? At the time I saw Weekes speak, astronomers had found a grand total of about a dozen celestial sources of TeV gamma rays, and they were the usual suspects: giant black holes and suchlike. Teragammas had revealed nothing about the ecology of the universe which astronomers didn’t already know. They were like animals in a zoo rather than out in the wild: fun to look at before you move onto the baby penguins.

This has all changed in the past couple of years. Observatories have catalogued 136 TeV sources, which is enough to start doing systematic astronomy rather than freak-show physics. They have turned up some striking results, questioning conventional wisdom about pulsars and shedding some light on dark matter.

Blazars, giant black holes that just so happen to be oriented that we are looking down the barrel of the jets they spray out (see picture above), are the largest single category of TeV gamma source outside our galaxy. They are pretty extreme to begin with, but some go all out. They blaze with the intensity of a thousand Milky Way galaxies and can vary in brightness by a factor of five within an hour—a puzzlingly rapid time, too fast even for light to cross from one side of the black hole to the other. “They’re some of the wildest animals in the whole astronomical zoo,” says astrophysicist Chuck Dermer. “The luminosities are just incredible.”

Superlatives aside, last year Christoph Pfrommer, Philip Chang, and Avery Broderick proposed that TeV gammas from blazars play an unappreciated role in heating up intergalactic gas. The injection of thermal energy would prevent the gas from settling into galaxies—especially into small galaxies, whose gravitational fields are too weak to overcome the tendency to dissipate. This may solve one of the most perplexing puzzles in modern cosmology: the fact that dark matter should nucleate lots of miniature galaxies, yet doesn’t seem to do so.

The blazars listed in the TeV catalog are only a small fraction of the ones out there. To our instruments, all the others blur together, forming a diffuse glow spread over the entire sky. In the 1990s, the Compton satellite measured this gamma-ray background up to an energy of 0.1 TeV. Yet when Compton’s successor, the Fermi satellite, went to take a look, the background glow looked so different that it was as if astronomers were seeing it for the first time. The earlier observatory appears to have been miscalibrated at the highest energies.

The upshot is that blazars are not the only things bathing our sky in a diffuse glow of high-energy gammas. Dermer says they account for only about a sixth of the background. The rest must come from pulsars, collisions of cosmic rays produced by supernovae, and maybe the decay or annihilation of dark-matter particles. “We still cannot explain the intensity of the isotropic flux,” says physicist Steve Ritz, one of the leaders of the Fermi project. Astrophysicists gathered to discuss this mystery during a special session of the American Astronomical Society meeting in Anchorage last week.

Pulsars are another example of how recent measurements have forced theorists back to the drawing board. By rights, these hyperdense neutron stars should be denuded of very-high-energy gammas. Although the stars might well produce such gammas near their surface, the surrounding magnetosphere should snuff them out, while gammas produced at higher altitudes should be comparatively wimpy. “A lot of people discouraged us from looking at pulsed emissions from pulsars,” recalls gamma-ray astronomer Nepomuk Otte.

So when the MAGIC observatory saw hints of high-energy pulses from the pulsar at the heart of the Crab Nebula, Otte says few paid any attention. But he and his colleagues kept at it and, last year, Fermi and the VERITAS observatory confirmed photons with up to 0.4 TeV. “This has changed the picture that we have of how gamma rays are produced in the Crab pulsar,” Otte says. A new idea is that streams of electrons and positrons are carrying energy into the outer magnetosphere and converting into gammas there. Astrophysicists had known that neutron stars were complicated, but not this complicated.

The biggest wildcards in teragamma astrophysics are so-called dark accelerators. These are TeV gamma sources that astronomers have yet to see any other way; they do not seem to correspond to any star, nebula, or other discernible object. They are tantalizingly marked “UNID” in the database. They might turn out to be known systems such as pulsar nebulae, but there’s always the hope they are dark matter or some other never-before-seen species. “There’s a lot of speculation about them,” Otte says.

To know for sure what’s going on, astronomers need even more than 136 TeV sources. A thousand would be more like it. So they are now planning the next generation of observatory with telescopes scattered over a square kilometer of land. Like the animals of Madagascar, gammas have broken out of their zoo and returned to the wild—with emphasis on the word “wild.”

Blazar image credit: copyright: ESA/NASA, the AVO project and Paolo Padovani; Telescope image credit: G. Perez, SMM, IAC

For those who know Verlinde, that label hardly fits. He is a brilliant theorist, and the amount of discussion his paper provoked suggested that most of his colleagues saw something in it. The whole story caught the eye of New Scientist and the New York Times, but ultimately we at Sci Am opted for watchful waiting. I caught up with Verlinde this spring during a workshop at the Kavli Institute for Theoretical Physics. He has doubled-down on his original paper, and his colleagues’ reaction hasn’t changed. One told me: “There are a lot of ideas he’s bringing together in an interesting way, but it’s a little hard for us to decipher, so I’m withholding judgment.” All he has really done, though, is take a general sentiment among string theorists and follow it to its logical conclusion.

String theorists and other would-be unifiers of physics face a basic problem. The theories they seek to unify, quantum field theory and Einstein’s general theory of relativity, are well-grounded and well-tested, yet mutually incompatible. Reconciling them will demand that some deeply held intuition must give way. One such intuition is that the world exists within space and time. Participants at the Kavli workshop were inclined to think that space and time are not fundamental, but emergent. The universe we seeing playing out in space and time may be just the surface level, where we float like little boats while leviathans stir in the deep.

Black holes provide the strongest argument for this point of view. The laws of gravity predict that these cosmic vacuum cleaners obey versions of the laws of thermodynamics, which is strange, because thermodynamics is the branch of physics that describes composite systems, such as gases made up of molecules. A black hole sure doesn’t look like a composite system. It just looks like a warped region of space that you would do well to stay away from. For it to be composite, space itself must be.

In that case, black holes represent a new phase of matter. Outside the hole, the universe’s “degrees of freedom”—all that its most fundamental building blocks are capable of—are in a low-energy state, forming what you might think of as a crystal, with a fixed, regular arrangement we perceive as the spacetime continuum. But inside the hole, conditions become so extreme that the continuum breaks apart. “You can make spacetime melt,” Verlinde told me. “This is really where spacetime ends. To understand what goes on, you need to use these underlying degrees of freedom.” Those degrees of freedom cannot be thought of as existing in one place or another. They transcend space. Their true venue is a ginormous abstract realm of possibilities—in the jargon, a “phase space” commensurate with their almost unimaginably rich repertoire of behaviors.

Verlinde’s 2010 paper applied this reasoning to the laws of gravity themselves. Instead of being a fundamental force of nature, as almost all physicists since Newton have thought, gravity may be an “entropic force”—a product of some finer-scale dynamics, much as the pressure force in a gas arises from collective molecular motions. At Kavli he went further and argued that the notion of emergent spacetime transforms our entire conception of the universe. “If you realize there’s much more phase space than we usually assume—much more—you will think about cosmology differently,” he argued.

For starters, dark matter may be a glimpse into the depths. To account for anomalous motions within galaxies and larger systems, astronomers think our universe must be filled with some invisible material that outweighs ordinary matter by a factor of five to one. They have never detected the material directly, though, and for something that is supposed to be so overwhelmingly dominant, dark matter has a puzzlingly subtle effect. The anomalous motions occur only in the unfashionable outskirts of galaxies. Stars and gas clouds out there move faster than they should, but don’t do anything truly wacky—it is as if the gravitational field of the visible galaxy were simply being amplified.

Consequently, some astronomers and physicists suspect there may be no dark matter after all. If you notice the floorboards in your house are sagging, as if there is too much weight on them, you might conclude there is an 800-pound gorilla in the room with you. You see no gorilla, so it must be invisible. You hear no gorilla, so it must be silent. You smell no gorilla, so it must be odorless. After a while, the gorilla seems so improbably stealthy that you begin to think there must be some other explanation for the sagging floorboards—the house has settled, say. Likewise, perhaps the laws of gravity and motion which led astronomers to deduce dark matter are wrong. “I think dark matter will be a sign of another type of physics,” Verlinde said.

The leading alternative to dark matter is known as MOND, for Modified Newtonian Dynamics. Verlinde has reinterpreted MOND not just as a tweak to the laws of physics, but as evidence for a vast substratum. He derived the MOND formula by assuming dark matter is not a novel type of particle but the vibrations of some underlying degrees of freedom—specifically, vibrations produced by random thermal fluctuations. Such fluctuations are muted and become conspicuous only where the average thermal energy is low, such as in the outskirts of galaxies. Astoundingly, Verlinde even derived the five-to-one ratio. “I started seeing this as a manifestation of this larger phase space,” he said.

MOND is super-iffy, as cosmologist Sean Carroll has detailed in a series of blog posts over the years, most recently this one. I’m inclined to agree, but one thing gives me pause. MOND manages to account for a wide range of anomalous galactic motions with one simple formula. Even if MOND doesn’t overturn the laws of physics, it has shown that dark matter behaves in a simple way. All the complicated dynamics of dark matter must somehow settle down into a very regular pattern. Dark-matter modelers tell me they have yet to explain this.

Verlinde bucks conventional wisdom not only on dark matter, but also on much of the rest of cosmology. For instance, he has reintroduced elements of the steady-state theory that most cosmologists thought they had ruled out in the 1960s. In his model, all matter—ordinary as well as dark—consists of vibrations of the underlying degrees of freedom and so is being created and destroyed all the time. In fact, the same degrees of freedom also explain dark energy, thereby unifying all the components of the universe. What differentiates these components is how fast they respond: ordinary matter is the surface chop, dark matter the languid but powerful deep currents, and dark energy the quiet bulk of the sea. As for another leading cosmological theory, cosmic inflation, he doesn’t think much of that, either.

The grander his claims become, the less plausible they seem. Still, Verlinde has captured theorists’ sense that cosmological mysteries signal a new era of physics. The impulse to explain dark matter and dark energy as signatures of a deeper reality, rather than a bolt-on to current theories, arises not only in string theory but also in alternatives such as loop quantum gravity and causal set theory. And if Verlinde is wrong and spacetime really is a root-level feature of our world, what other intuition will have to give way? What other thing that we thought we knew for sure is wrong?

Diagram courtesy of Erik Verlinde