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EEE / A chip made with carbon nanotubes, not silicon, marks a computing milestone

« on: February 28, 2020, 10:02:46 PM »

Inside a new microprocessor, the transistors — tiny electronic switches that collectively perform computations — are made with carbon nanotubes, rather than silicon. By devising techniques to overcome the nanoscale defects that often undermine individual nanotube transistors (SN: 7/19/17), researchers have created the first computer chip that uses thousands of these switches to run programs.

The prototype, described in the Aug. 29 Nature, is not yet as speedy or as small as commercial silicon devices. But carbon nanotube computer chips may ultimately give rise to a new generation of faster, more energy-efficient electronics.

This is “a very important milestone in the development of this technology,” says Qing Cao, a materials scientist at the University of Illinois at Urbana-Champaign not involved in the work.
The heart of every transistor is a semiconductor component, traditionally made of silicon, which can act either like an electrical conductor or an insulator. A transistor’s “on” and “off” states, where current is flowing through the semiconductor or not, encode the 1s and 0s of computer data (SN: 4/2/13). By building leaner, meaner silicon transistors, “we used to get exponential gains in computing every single year,” says Max Shulaker, an electrical engineer at MIT. But “now performance gains have started to level off,” he says. Silicon transistors can’t get much smaller and more efficient than they already are.

Because carbon nanotubes are almost atomically thin and ferry electricity so well, they make better semiconductors than silicon. In principle, carbon nanotube processors could run three times faster while consuming about one-third of the energy of their silicon predecessors, Shulaker says. But until now, carbon nanotubes have proved too finicky to construct complex computing systems.

One issue is that, when a network of carbon nanotubes is deposited onto a computer chip wafer, the tubes tend to bunch together in lumps that prevent the transistor from working. It’s “like trying to build a brick patio, with a giant boulder in the middle of it,” Shulaker says. His team solved that problem by spreading nanotubes on a chip, then using vibrations to gently shake unwanted bundles off the layer of nanotubes.

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EEE / A predicted superconductor might work at a record-breaking 200° Celsius

« on: February 28, 2020, 10:02:16 PM »

The steamiest summer day would be no sweat for this potential superconductor.

Scientists have calculated that a hydrogen-rich compound could conduct electricity without resistance at temperatures up to about 200° Celsius — well above the 100° C boiling point of water. If that prediction is confirmed experimentally, the material would stand in stark contrast to all other known superconductors, which must be cooled below room temperature to work (SN: 12/15/15).

Superconductors’ need for cool conditions makes them difficult to use. So physicists are on a quest to find a superconductor that can stand the heat, which could revolutionize how electricity is transmitted and save vast amounts of energy.

The newly predicted superconductor — a compound of hydrogen, magnesium and lithium — comes with its own complications, however. It must be squeezed to extremely high pressure, nearly 2.5 million times the pressure of Earth’s atmosphere, physicist Hanyu Liu and colleagues, of Jilin University in Changchun, China, report in the Aug. 30 Physical Review Letters.

Scientists previously have used similar techniques to predict that a pressurized compound of lanthanum and hydrogen would be superconducting at higher temperatures than any yet known. That prediction seems likely to be correct: In 2018, physicist Russell Hemley and colleagues reported signs that the compound is superconducting up to a record-breaking −13° C (SN: 9/10/18).

If the new calculation is confirmed, the purported superconductor would smash Hemley and colleagues’ temperature record. “This is an important prediction using a level of theory that has proven quite accurate,” says Hemley, of the University of Illinois at Chicago, who was not involved in the research.

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EEE / Einstein’s general relativity reveals new features of a pulsar

« on: February 28, 2020, 10:01:56 PM »

Finally, scientists have their finger on the pulse.

Spinning dead stars, known as pulsars, blast powerful beams of radio waves into space. As a pulsar spins, its beams sweep past Earth, producing a pulsating signal similar to a lighthouse’s flashes. Astronomers now have mapped the structure of the beams of one pulsar, using observations made over decades. The technique relies on Albert Einstein’s theory of gravity, general relativity, and simultaneously reconfirms that the theory is correct, the scientists report in the Sept. 6 Science.

The result allowed researchers to “view the beam of a pulsar in a whole new way,” says astrophysicist Victoria Kaspi of McGill University in Montreal, who was not involved with the new study. 

Pulsars are a type of neutron star, a dense remnant left behind when a star explodes. Powerful magnetic fields direct radio waves from a pulsar outward in beams. Typically, those beams pass by Earth at a fixed angle, and scientists can glimpse only a single slice through a beam as it rotates — like viewing a lighthouse beacon through a tiny slit.

But the newly mapped pulsar, known as PSR J1906+0746, was unusual: It was part of a duo, orbiting with another neutron star, about 20,000 light-years away from Earth (SN: 12/18/15). According to general relativity, if a pulsar spins at an angle misaligned with the pair’s orbit — which this one does — the pulsar will precess. That means that the axis on which the pulsar is spinning rotates, much like a wobbling top.

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EEE / A new magnetic swirl, or skyrmion, could upgrade data storage

« on: February 28, 2020, 10:01:34 PM »

Magnetic swirls called skyrmions have gotten a new twist.

Scientists have created a new version of the atomic whirlpools, in which the tiny magnetic fields of individual atoms in a material arrange into a swirl pattern. Known as antiferromagnetic skyrmions, the new structures have some advantages that could make them easier to work with than previously found varieties, researchers report September 2 in Nature Materials. If so, that development could bolster hopes for using skyrmions to store data and to create smaller, speedier hard drives (SN: 2/7/18).

Skyrmions previously have been created in materials known as ferromagnets, in which the tiny magnetic field of each atom aligns with its neighbors’. Those aligned fields are the source of ferromagnets’ ability to affix kids’ doodles to the fridge. But scientists hadn’t yet created skyrmions in antiferromagnets, where each atom’s magnetic field points opposite to its neighbor’s, cancelling out the magnetic field.

Antiferromagnets are difficult to work with. So the researchers created a synthetic version, layering magnetic materials so that the magnetization in one layer cancelled out the other layer, mimicking an antiferromagnet’s lack of a magnetic field. By tweaking the properties of each layer, the team optimized the conditions for producing skyrmions, and then imaged them using magnetic force microscopy.
“I’m really impressed,” says materials scientist Axel Hoffmann of the University of Illinois at Urbana-Champaign. “This was really a tour de force kind of effort.”

Scientists think skyrmions could improve on standard hard drives by packing more data into less space, but for that, skyrmions have to be small.  Larger versions of the magnetic whorls are hundreds of nanometers in size — and they won’t cut it. The researchers predict that, with fine-tuning, they could shrink the antiferromagnetic skyrmions down to 10 nanometers in diameter.

“This is the regime of skyrmion size that becomes really interesting,” says physicist Vincent Cros of Unité Mixte de Physique CNRS/Thales in Palaiseau, France, a coauthor of the study.

Skyrmions can also be moved around within a material via electric currents, allowing scientists to shuttle data from one place to another, for example, when it’s time to read out the data. That could avoid the need for the fragile moving parts found in traditional hard drives. But there’s a problem: Thanks to their swirling patterns, skyrmions tend to speed off at an angle to the input current, making them hard to control. But antiferromagnetic skyrmions, with their alternating orientations, are effectively pulled in two directions at once. That means they should travel straight ahead, relative to the current, and could be easier to manipulate.

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EEE / Physicists may be a step closer to solving the mystery of proton size

« on: February 28, 2020, 09:59:48 PM »

If protons wore clothing, the label might read “XXS.”

For nearly a decade, scientists have been arguing over the size of the puny subatomic particles: extra small, or extra extra small. A new measurement bolsters the case that protons are more petite than once thought, researchers report in the Sept. 6 Science.

Until 2010, the proton’s radius was measured at about 0.88 femtometers, or millionths of a billionth of a meter. But then a new type of measurement — based on exotic atoms made with muons, the heavy cousins of electrons — clashed with that figure, registering a proton size of about 0.84 femtometers (SN: 4/18/17). 

One way to test the proton’s radius is by measuring the separation between the energy levels in which hydrogen atoms can exist — different states in which the atom’s electron carries a certain amount of energy. That energy difference depends on the size of the proton.

By measuring the separation between two such energy levels, physicist Eric Hessels of York University in Toronto and colleagues have pegged the radius at about 0.83 femtometers, in good agreement with the 2010 value.

The result adds to a small heap of recent studies that have claimed a slightly slimmer proton physique, including a 2017 measurement, made by considering a different set of energy levels in hydrogen atoms (SN: 10/5/17), and an estimate reported in October 2018, based on scattering electrons off of protons (SN: 11/2/18). However, a study published in May 2018 went against the slim-proton trend, falling in line with the original, larger value of the radius.

The inability to settle on a size is impeding researchers’ ability to test essential tenets of physics, like quantum electrodynamics, the theory that describes interactions of electrically charged particles. But resolving the debate is likely to be no small feat.

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EEE / Gravitational waves from a ringing black hole support the no-hair theorem

« on: February 28, 2020, 09:59:27 PM »

For black holes, it’s tough to stand out from the crowd: Donning a mohawk is a no-no.

Ripples in spacetime produced as two black holes merged into one suggest that the behemoths have no “hair,” scientists report in the Sept. 13 Physical Review Letters. That’s another way of saying that, as predicted by Einstein’s general theory of relativity, black holes have no distinguishing characteristics aside from mass and the rate at which they spin (SN: 9/24/10).

“Black holes are very simple objects, in some sense,” says physicist Maximiliano Isi of MIT.

Detected by the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, in 2015, the spacetime ripples resulted from a fateful encounter between two black holes, which spiraled around each other before crashing together to form one big black hole (SN: 2/11/16). In the aftermath of that coalescence, the newly formed big black hole went through a period of “ringdown.” It oscillated over several milliseconds as it emitted gravitational waves, similar to the way a struck bell vibrates and makes sound waves before eventually quieting down.
Reverberating black holes emit gravitational waves not at a single frequency, but with additional, short-lived frequencies known as overtones — much like a bell rings with multiple tones in addition to its main pitch.

Measuring the ringing black hole’s main frequency as well as one overtone allowed the researchers to compare those waves with the prediction for a hairless black hole. The results agreed within 20 percent.

That result still leaves some wiggle room for the no-hair theorem to be proved wrong. But, “It’s a clear demonstration that the method works,” says physicist Leo Stein of the University of Mississippi in Oxford, who was not involved with the research. “And hopefully the precision will increase as LIGO improves.”

The researchers also calculated the mass and spin of the black hole, using only waves from the ringdown period. The figures agreed with the values estimated from the entire event — including the spiraling and merging of the original two black holes — and so reinforced the idea that the resulting black hole’s behavior was determined entirely by its mass and spin.

But just as a mostly bald man may sport a few strands, black holes could reveal some hair on closer inspection. If they do, that might lead to a solution to the information paradox, a puzzle about what happens to information that falls into a black hole (SN: 5/16/14). For example, in a 2016 attempt to resolve the paradox, physicist Stephen Hawking and colleagues suggested that black holes might have “soft hair” (SN: 4/3/18).

“It could still be that these objects have more mysteries to them that will only be revealed by future, more sensitive measurements,” Isi says.

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EEE / How an astrophysicist chased a star from the Halo games to real life

« on: February 28, 2020, 09:58:59 PM »

The Soell system was the site of a galactic disaster. The ancient Forerunners fought a long war against intelligent parasites called the Flood. As a last resort, the Forerunners built a ring-shaped superweapon orbiting the moon of Soell’s largest planet. Triggering the weapon, the Halo Array, wiped out the Flood, the Forerunners and all other intelligent life in the galaxy. For millennia, the star Soell was forgotten — until humans found the Halo.

The popular video game Halo and its fictional stars were Julián Alvarado Gómez’s obsession 15 years ago. As a young man in Bogotá, Colombia, he played Halo and its offshoots competitively. Today, at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., he’s studying an actual star, Iota Horologii.

His goal is to map the star’s magnetic field over time along with its gusting stellar wind, the stream of energetic particles that defines a star’s territory and batters its planets. This work will help him understand what our star was like in its youth and how it influenced the start of life on Earth.

“One of the big difficulties we have in our understanding of the sun is that we only have one sun,” says the 35-year-old astrophysicist. Getting to know another star that has a similar mass and temperature as the sun, referred to as a “sunlike” star, would shore up astronomers’ grasp of the sun. And it would offer details on how sunlike stars may affect potential life on their orbiting planets.
Alvarado Gómez’s road to this stellar career was rocky. In 2003, he was about a third of the way through an undergraduate physics degree at the National University of Colombia in Bogotá when financial trouble put his studies on hold. He filled his time with friends and Halo. “We were practicing a lot,” he jokes.

The game offered a solution to his money woes when he came in third place in a Halo tournament. His friend Julián Hernández took first. The two caught the attention of Microsoft representatives looking for skilled players to help advertise the game by playing in public.
From there, he and Hernández earned a salary for playing at events, plus the occasional bonus for winning a tournament. It was fun, Alvarado Gómez says, and it paid the bills.

After two years, he returned to school to study physics again. “That was a very hard semester,” he says. After an early focus on the sun, he shifted toward the stars for his Ph.D. at the European Southern Observatory in Garching, Germany.

That shift has brought Alvarado Gómez to some surprising differences between the sun and other sunlike stars. His research has also revealed how a star might protect its planets from its own energetic outbursts. Yet the work didn’t take him as far from Soell as he’d thought.

Fickle fields
The best way to get to know a star is through its magnetic field, Alvarado Gómez says. The sun is the most familiar example: Our star’s temperamental behavior and periodic mood swings are thought to exist thanks to changes in magnetism.

Magnetic fields help heat the sun’s wispy outer atmosphere, the corona, to millions of degrees Celsius. Those magnetic fields also help drive a stream of charged particles out into space (SN Online: 8/11/17). That solar wind blows a bubble that defines the boundary of the solar system (SN Online: 12/10/18). It can also batter unprotected planets; scientists think the solar wind stripped away much of Mars’ atmosphere.

When tangled magnetic field lines on the solar surface suddenly snap, powerful eruptions of plasma called coronal mass ejections break free (SN: 4/13/19, p. 15). When strong CMEs hit Earth, they can fry satellites, shut down power grids and damage living cells.

The sun’s magnetic activity waxes and wanes in about an 11-year cycle. The peak, or maximum, rages with sunspots, CMEs and bright radiation flashes called flares, while the minimum is relatively quiet.

In another magnetic quirk, the direction of the sun’s dominant magnetic field flips at the peak of each cycle (SN: 3/3/01, p. 139). As the sun’s inner engine reorganizes itself, the south magnetic pole switches to the north, and vice versa. This polarity reversal has ripple effects on the solar wind that extend to the edges of the solar system.

Other stars share much of this magnetic fickleness. About 60 percent of sunlike stars show signs of magnetic cycles of varying lengths depending on the stars’ ages. Young sunlike stars of a few hundred million years have shorter cycles and emit more flares than the 4.6-billion-year-old sun.

“You can use other stars to show snapshots of the sun at earlier and later periods in its evolution,” says stellar physicist Travis Metcalfe of the Space Science Institute in Boulder, Colo. “What was the sun like in the past? What will it be like in the future?”

From what we can tell, stars seem to tame their magnetic frenzies as they age. They calm down by losing mass through their stellar winds and CMEs (SN: 8/31/19, p. 11).

But young stars can be rough on their planets. Stinging winds and violent outbursts could wipe out life as efficiently as the Halo Array, unless something stopped or blocked them (SN Online: 3/5/18).
The perfect star
To find out when a star’s planets face the most danger, Alvarado Gómez needed a magnetic view of a young sun, from the field on its surface to the edges of its stellar wind.

Finding one turned out to be a massive undertaking. The perfect star had to be similar to the sun in mass and temperature, two features that determine a star’s life span. And it needed an observable magnetic cycle in its corona. “If [stars] have corona, they have stellar winds,” Alvarado Gómez says.

That final requirement was the trickiest. Astrophysicists have measured the cycles of fewer than 100 stars, based on variations in a particular wavelength of near-ultraviolet light that can be seen from ground-based telescopes. Tracking these cycles takes time, but is relatively easy.

The coronas of stars other than the sun, however, are best observed through the high-energy X-rays they emit. “I wanted a star in which I could be confident that I know what the activity cycle looks like,” Alvarado Gómez says. “The best proxy for that — the best of all — is the X-rays.” But the only way to see these X-rays is from space, a much more difficult and expensive prospect.

Very few stars have had their magnetic cycles recorded in X-rays. When Alvarado Gómez was searching for his target star in 2014, there were only four solar mass stars that would work, and three of them were in orbits with another star. Alvarado Gómez ruled those out, fearing the companion stars could mess things up. “There was only one star left,” he says. “Iota Horologii.”
Back to the Halo
Iota Horologii, located about 56 light-years (530 trillion kilometers) from Earth, is similar to the sun in temperature, size and mass. At about 625 million years old, it’s the youngest star with a detected magnetic activity cycle.

Its age is more or less the age of the sun when life appeared on Earth, says astrophysicist Jorge Sanz-Forcada of the Center for Astrobiology in Madrid. “This is a way to observe how the sun was at the moment when life appeared.” The star has a planet, too. Unfortunately, it’s an uninhabitable gas giant, but its orbit lasts almost a full Earth year: 307 days.

Even better for observers, Iota Horologii has the shortest magnetic activity cycle observed to date. It peaks and falls over just 1.6 years, Sanz-Forcada, Metcalfe and astrophysicist Beate Stelzer of Eberhard Karls University in Tübingen, Germany, reported in 2013 in Astronomy & Astrophysics. Researchers could observe the star’s full cycle almost seven times in the time it takes the sun to cycle once.

To go after Iota Horologii, Alvarado Gómez got access to every telescope he could, stockpiling more data than astronomers usually get for a single star. “This became a much bigger project than what was envisioned,” he says. “We wanted to map the magnetic cycle. But then we realized that there’s much more that you can do.”

Between October 2015 and September 2018, he and colleagues observed the star using the High Accuracy Radial velocity Planet Searcher, or HARPS, spectrograph at La Silla Observatory in Chile. Then he teamed up with Sanz-Forcada’s group to watch the star in X-rays, ultraviolet and visible light using a trio of space telescopes.

He also gathered another 13 years of data from previous observations of Iota Horologii in a wavelength of light that tracks magnetism on stars’ surfaces. “We were able to trace it back all the way to 2002 — when I was playing Halo,” Alvarado Gómez says. He now has data on more cycles for Iota Horologii than astronomers have for the sun in certain wavelengths.
“These types of observations are rare, especially for new stars that haven’t been observed a lot in the past,” Metcalfe says. “It’s enormously helpful to our understanding of where cycles come from and where they’re going.”

In late 2017, when Alvarado Gómez was writing the first paper on the HARPS observations, he learned Iota Horologii had a potential Halo connection that blew him away.

While searching for information about the star, he stumbled on a fan site laying out the case that the Halo star Soell is supposed to be Iota Horologii. The two have matching planets and similar properties and positions in the sky. It’s possible, says Frank O’Connor, Halo franchise creative director at 343 Industries in Redmond, Wash. “Our normal process includes referencing our sci-fi against current scientific consensus, understanding and data. So it almost certainly got checked against real star systems … and may indeed be the same one,” he says.

“I just find it amazing,” Alvarado Gómez says. He recalled his graduate adviser, astronomer Gaitee Hussain at ESO, telling him that stellar physicists fall a little bit in love with the objects they study. “I was already in that process with Iota Horologii,” he says. He took the potential connection to the game that helped get him back to school as “a sign that I should keep working on it.”

Far beyond the sun
For all that Iota Horologii resembles the sun, its magnetic life looks subtly different in important ways, Alvarado Gómez and colleagues found. Those differences could hold clues to how sunlike stars change over time, and whether those changes influence their planets.

For one thing, Iota Horologii’s magnetic field flips like the sun’s — but faster. The sun flips once every cycle, so it takes two cycles to return to its original configuration. Iota Horologii’s cycle is 1.6 years, but its polarity flips every 1.2 years. That speedy somersault could suggest that the internal engine that drives a star’s magnetic field is different in young stars than in older ones.

Iota Horologii’s magnetic activity cycle is also surprisingly stable, according to a paper the team posted September 3 at arXiv.org that will also appear in Astronomy & Astrophysics. Four cycles in a row lasted the same amount of time and reached the same activity levels.

“We never expected so much regularity,” Sanz-Forcada says. “In the sun, [the cycle] is not so regular.”

The sun’s highest activity level varies from one cycle to the next; the most recent solar cycle had one of the wimpiest peaks ever recorded (SN: 11/2/13, p. 22). No one is sure why. But if Iota Horologii represents the sun in its youth, then the sun’s cycles may have been more consistent a few billion years ago.

Alvarado Gómez is working on figuring out what all the data mean for Iota Horologii’s stellar wind — and by extension, what winds could do to planets. He’s making the first maps of the strength and direction of Iota Horologii’s entire magnetic field at every point of the star’s surface. He’ll then use the maps to build computer simulations of the shape and strength of Iota Horologii’s stellar wind.

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EEE / A new experiment slashes the maximum possible mass of tiny neutrinos

« on: February 28, 2020, 09:57:57 PM »

The maximum possible mass of a barely there particle has just gotten smaller.

Subatomic particles called neutrinos are extremely lightweight. Now, scientists with the KATRIN experiment in Karlsruhe, Germany, have shrunk the potential mass range for these runts of the particle litter. Neutrinos must have a mass of 1.1 electron volts or less, the researchers report in a paper posted September 13 at arXiv.org and in a talk on the same day at the Topics in Astroparticle and Underground Physics conference in Toyama, Japan.

That new number — about half of the previous ceiling on neutrino mass — means that it would take more than 460,000 neutrinos to reach the mass of an electron, “and possibly a lot more,” says physicist Diana Parno of Carnegie Mellon University in Pittsburgh.

Parno and colleagues studied decays of tritium, a radioactive variety of hydrogen (SN: 9/19/18). In each such decay, a type of neutrino known as an electron antineutrino is emitted, along with an electron. By measuring the electrons’ energy, the scientists tested how much of the decays’ energy went into the mass of the neutrinos.

To make matters more complicated, neutrinos typically don’t have a well-defined mass. Due to the intricacies of quantum mechanics, the particles are made up of three different mass states at once. What KATRIN measures is an “effective mass,” a combination of those three masses.

Even tighter constraints on neutrino masses have been made using studies of how matter clumped together in the early universe. But those mass limits apply only if physicists’ understanding of the rules of the cosmos is correct (SN: 3/12/08). KATRIN’s new estimate requires no such assumption.

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EEE / Can time travel survive a theory of everything?

« on: February 28, 2020, 09:57:37 PM »

In many universes, typically those on TV shows or in movies, time travel is not much more difficult than driving downtown in any major city during rush hour. Sure, the traffic can get gnarly, but no law of physics prevents you from reaching your destination eventually.

In real life, time travel isn’t so easy. In fact, it’s probably impossible, a fantasy more farfetched than visiting Alice’s Wonderland, finding gold at the end of a rainbow or cleansing all the hate speech off of Facebook.

Yet time travel does not necessarily violate the laws of physics. In Einstein’s theory of gravity — general relativity — space and time are merged as spacetime, which allows for the possibility of pathways that could bend back to the past and loop back to the future.

Such paths are known as closed timelike curves. They’re a little like great circles around the surface of the Earth — if you start out in one direction and keep going straight, eventually you come back to where you started from. In that case the Earth’s curvature
Nobody thinks that general relativity’s time loops would be practical for time travel even if they are possible. For one thing, they might exist only under certain circumstances — the universe would have to be rotating, and not expanding — as the mathematician Kurt Gödel showed in the 1940s. But the universe is expanding, and probably isn’t rotating, so that dampens the prospects for revisiting the Stone Age or acquiring a pet dinosaur.

Besides, even if such pathways did exist, building a ship to traverse them would cost more than all the DeLoreans (and all other transportation vehicles) ever made. It would need a cruising speed of 140,000 miles per second. And with no place to stop for gas (or whatever), the fuel tank would have to be more than a trillion times the size of an oil tanker.

So for practical purposes, time travel’s time has not yet arrived. But even if it’s possible only in principle, the potential ramifications for the basic physics of the universe might make it worth the time to investigate it. Time loops might not enable you to traverse the cosmos in a TARDIS, but perhaps could still help you understand the cosmos more deeply.

A first step would be to attempt to figure out exactly what the relevant laws of physics really are. Einstein’s general relativity is great, but indubitably not the last word about the physics of the universe. After all, it coexists uneasily with quantum mechanics, which rules the subatomic world and presumably, since everything is made of subatomic stuff, the rest of the universe as well. Whether the quantum–general relativity combo truly permits time travel might depend on what the ultimate correct theory combining the two turns out to be.

Several candidate theories have been developed for merging general relativity and quantum mechanics into a unified theory. It’s an open question whether these candidates would allow time travel in something like the way general relativity does, philosopher Christian Wüthrich of the University of Geneva notes in a new paper.

It’s possible, he says, that a theory that supersedes general relativity might still in some way include the equivalent of general relativity’s timelike loops. And even if the basic theory does not include such loops, they still might emerge in practice.

“Although the fundamental theory would then remain inhospitable to time travel itself, it would tolerate the possibility of time travel at some other, less fundamental, scale,” Wüthrich writes in his paper, posted online in June. “Depending on what the relationship between the fundamental theory and emergent spacetime may be in each case, we may find that the emergent, macroscopic spacetime structure permits time travel.”

Reviewing the major proposals for quantum gravity theories does not provide a lot of hope, though. One approach, known as causal set theory, requires sets of events to be ordered in a proper cause-and-effect relationship. So its central idea seems to rule out closed timelike curves.

Another popular approach, known as loop quantum gravity, envisions space to be constructed of fundamental loops (kind of like “atoms of space”). This view has encountered technical difficulties, one of which is how to work time into the picture with space. “Thus, we seem to be faced with a temporally innocuous structure in which no meaningful sense of time travel is permitted,” Wüthrich writes.

It’s possible that the networks of these “atoms of space” could produce high-level spacetime that did contain closed timelike curves. But analysis of the details at this stage of loop quantum gravity’s development does not offer much reason for optimism, Wüthrich concludes.

Time travel’s future might look a little brighter if the correct approach to quantum gravity turns out to be string theory, currently the most popular contender. In string theory, matter’s basic particles are tiny vibrating snippets of energy, called “strings” because they extend in one dimension. Multiple versions of string theory have been constructed, suggesting that they are different manifestations of a more fundamental master theory known as M-theory.

“As M-theory does not yet exist, it is impossible to determine its verdict on time travel,” writes Wüthrich. But investigations of various string theory scenarios do suggest that the ultimate theory would, in fact, naturally incorporate closed timelike curves.

Even if time loops exist in the fundamental theory, though, there’s still no guarantee that they would be preserved in the emergent large-scale spacetime that would be relevant in real life. For that matter, Wüthrich points out, predicting the existence of time travel loops might be taken as evidence against the theory, considering the serious likelihood that time travel really isn’t possible at all.

So whether general relativity’s time loops will survive in a deeper theory remains an open question. “A more fundamental theory may well admit structures amounting to closed timelike curves and thus permit time travel,” Wüthrich asserts. “This clearly remains a live option at the present stage of knowledge.”

In any case, investigating whether quantum gravity theories retain general relativity’s time travel loophole can illuminate many tough questions that must be answered to develop a successful theory and understand how it relates to general relativity. “For this reason alone,” Wüthrich writes, “the question of time travel beyond general relativity is worth our while.”guides you back to your previous point in space; with closed timelike curves, the geometry of spacetime guides you back to an earlier moment in time.

10

EEE / CO2 from champagne bottles can form shock waves like those seen in rocket exhaus

« on: February 28, 2020, 09:57:07 PM »

You may not be able to blast a bottle of champagne off in the backyard, but it turns out that sparkling wine is its own kind of bottle rocket.

New high-speed videos reveal that the plume of carbon dioxide released from a popped bottle of bubbly can contain a Mach disk — a kind of visible shock wave typically seen in supersonic exhaust streams from jets and rockets. These shock waves appear when the pressure of the exhaust outflow is more than about five times as high as the surrounding air.

In champagne bottles stored at room temperature, carbon dioxide gas in the bottle’s neck is at least seven times as pressurized as ambient air. So when the bottle is uncorked, the gas that gushes out — at more than twice the speed of sound — forms a Mach disk in its plume. Within about a millisecond, the pressure inside the bottle’s throat is closer to that of the surrounding air, and the shock wave vanishes, researchers report September 20 in Science Advances.
“The discovery of these Mach disks was a complete surprise,” says Gérard Liger-Belair, a physicist at the University of Reims Champagne-Ardenne in France. The original intent of the study, he says, was to investigate how bottle temperature affects the appearance of a champagne plume.

In experiments with champagne stored at 20° and 30° Celsius, Liger-Belair’s team confirmed previous findings that bottle temperature influences plume hue: Warmer champagne puffed out white-gray plumes, and cooler bottles exhaled deep blue.
That’s because carbon dioxide is less soluble at higher temperatures, making the gas trapped inside a 30° bottle more pressurized. When the bottles are uncorked, gas in the 30° bottle undergoes a greater pressure drop, and therefore a bigger temperature drop, than CO2 freed from the 20° bottle.

“The lower the [final] temperature, the easier the transformation” of carbon dioxide gas into dry ice, Liger-Belair says. Gas from a 30° bottle forms large ice crystals that scatter all wavelengths of visible light, giving the plume its whitish hue. Meanwhile, gas from a 20° bottle forms smaller crystals that preferentially scatter shorter, bluer wavelengths of light — similar to the way that small atmospheric molecules paint the sky blue.

11

EEE / Rumors hint that Google has accomplished quantum supremacy

« on: February 28, 2020, 09:56:29 PM »

A leaked paper suggests that Google has achieved a milestone known as quantum supremacy, using a quantum computer to perform a calculation that couldn’t be achieved even with the world’s most powerful supercomputers.

It’s a hotly anticipated goal, and one intended to mark the beginning of a new era of quantum computation (SN: 6/29/17). But it’s also largely symbolic: The calculation in question serves no practical purpose and is designed to be difficult for classical computers, standard computers that are not rooted in quantum physics.

On September 20, the Financial Times reported that a scientific paper, briefly published on a NASA website before being removed, claims that Google has built a quantum computer that achieved quantum supremacy. It’s a benchmark that the company’s quantum researchers, led by physicist John Martinis of the University of California, Santa Barbara, have set their sights on for years (SN: 3/5/18). An apparent plain-text version of the paper, posted anonymously on the site Pastebin, has since been circulating among scientists and on Twitter. A spokesperson for Google declined to comment to Science News.

According to the Pastebin version of the paper, Google created a quantum computer named Sycamore with 54 quantum bits called qubits, 53 of which were functional. The researchers used it to perform a series of operations in 200 seconds that would take a supercomputer about 10,000 years to complete.
The calculation consists of performing random operations on the qubits and reading out the result. After doing this many times, the researchers are left with a nearly random assortment of numbers, one that is extremely difficult to reproduce with a classical computer.

Despite its lack of applications, quantum supremacy has been billed as a major breakthrough in the quest for a quantum computer that could eventually perform useful calculations that are not possible with classical computers. “This dramatic speedup relative to all known classical algorithms provides an experimental realization of quantum supremacy on a computational task and heralds the advent of a much-anticipated computing paradigm,” the text of the Pastebin paper reads.

The machines might eventually be capable of defeating encryption techniques used to secure certain transmissions, such as financial transactions made by computers. But that advance will require many more qubits and a method to correct the errors that inevitably creep into quantum calculations. “While this is a milestone, it is *very* far from being a quantum computer that can compute anything useful,” physicist Jonathan Oppenheim of University College London wrote on Twitter.

Not everyone agrees that quantum supremacy is a useful benchmark. “Quantum computers are not ‘supreme’ against classical computers because of a laboratory experiment designed to essentially (and almost certainly exclusively) implement one very specific quantum sampling procedure with no practical applications,” IBM’s director of research Dario Gil wrote in a statement sent to Science News.

IBM is developing their own line of quantum computers (SN: 11/10/17), and researchers there prefer to talk about “quantum advantage,” which they define as “the point at which quantum applications deliver a significant, practical benefit beyond what classical computers alone are capable.” The new result falls short of that standard.

12

EEE / Sean Carroll’s new book argues quantum physics leads to many worlds

« on: February 28, 2020, 09:56:00 PM »

Quantum physics is about multiplicity.

Its equations describe multiple possible outcomes for a measurement in the subatomic realm. Physicists have devised a dozen or two different interpretations of what that really means. And in turn, dozens and dozens of books have been written to explain, defend or deny the validity of those various interpretations.

Caltech physicist Sean Carroll’s Something Deeply Hidden defends one of the most provocative of those interpretations: that multiple possible measurement outcomes imply a multiplicity of universes. Known as the Many-Worlds Interpretation, that view contends that all the possible outcomes of quantum experiments actually come true.
Measuring the spin of an electron, for instance, might yield the result that the spin axis points either up or down. When the measurement is made, the universe splits, branching into two copies, one with the spin up, the other with the spin down. As each measurement is made, this view of quantum theory insists, additional universes are instantly created.

“The theory describes many copies of what we think of as ‘the universe,’ ” Carroll writes, “each slightly different, but each truly real in some sense.” If you want to know where these branches are, he says, “There is no ‘place’ where those branches are hiding; they simply exist simultaneously, along with our own, effectively out of contact with it.”

Many Worlds is a well-known quantum interpretation, originated in the 1950s by American physicist Hugh Everett III. It was mostly ignored for a long time. But in recent decades, many physicists have found it (or variants of it) preferable to the traditional view of quantum mechanics associated with Danish physicist Niels Bohr.

That standard approach is often glibly derided as “shut up and calculate,” since all the quantum math does is provide a recipe for calculating the likelihood of different experimental results. It doesn’t have anything to say about what unseen, or deeply hidden, mechanisms might be responsible for the recipe. And all competing interpretations, it seemed, predicted the same observable results.

But maybe not. Carroll argues that the various interpretations are actually “well-constructed scientific theories, with potentially different experimental ramifications.”

Carroll echoes Everett in contending that the key mathematical expression in quantum physics, known as the wave function, should be taken seriously. If the wave function contains multiple possible realities, then all those possibilities must actually exist. As Carroll argues, the wave function is “ontic” — a direct representation of reality — rather than “epistemic,” a merely useful measure of our knowledge about reality for use in calculating experimental expectations. In epistemic interpretations, “the wave function isn’t a physical thing at all, but simply a way of characterizing what we know about reality.”

In the ontic view, favored by Carroll, reality as a whole is one comprehensive universal wave function. We split up into copies of ourselves as we travel along the branching paths of events that the wave function encompasses. Or, as Carroll suggests, you can think of the process “as dividing the existing universe into almost identical slices.”

As quantum books go, Carroll’s is exceptionally clear, conversational and enjoyable. He has a knack for linguistic lubrication that helps make some highly technical concepts reasonably smooth to swallow. His is by far the most articulate and cogent defense of the Many-Worlds view in book-length depth with a close connection to the latest ongoing research (in the arena known as quantum foundations).

There are some minor shortcomings. Carroll’s historical passages are sketchy and sometimes misleading. The atoms proposed by Greek philosophers were not pointlike, as Carroll writes — they had size and shape and possibly even parts. And the last major salvo of Bohr’s quantum debate with Albert Einstein was not papers on quantum entanglement in 1935, but Bohr’s 1949 essay on the debate in a collection of papers about Einstein, and Einstein’s reply.

Toward the end of the book, the clarity of Carroll’s narrative diminishes somewhat — no doubt, as he acknowledges, because he has passed from the realm of established physics to the current unsettled search for the correct theory combining quantum physics with gravity. From that search, recent work indicates, an understanding of the quantum origins of space and time might emerge.

As for the many quantum worlds, Carroll’s case is strong but not conclusive. As he notes, a process known as quantum decoherence is “absolutely crucial to making sense” of the Many-Worlds view, explaining what happens when measurements select one possibility out of the wave function. In essence, decoherence occurs when microscopic quantum objects get entangled with the macroscopic environment, ensuring that only one result is observed by an experimenter on one branch. The other outcome occurs in another branch.

But other quantum experts use decoherence to explain quantum phenomena without invoking multiple universes. And as Carroll admits, the decoherence process does not require belief in the reality of the other branches. It just seems to him (and many others) to be the most elegant explanation for quantum mysteries.

So it remains the case that the ultimate definitive account of how to properly explain quantum mechanics remains unwritten. That secret remains hidden, if perhaps not quite as deeply as it once was.

13

EEE / Monika Schleier-Smith leads elaborate quantum conversations

« on: February 28, 2020, 09:55:19 PM »

“I like it if I can run uphill and be rewarded with a view of the bay,” says Monika Schleier-Smith. She’s talking about a favorite spot to exercise around Palo Alto, Calif., but the sentiment also applies to her scientific work. A physicist at Stanford, Schleier-Smith, 36, has a reputation for embracing the uphill climb. She’ll push, push, push the smallest details of an experiment until she achieves what others thought near impossible.

Her reward? Seeing large ensembles of atoms do her bidding and interact with one another over distances that are incredibly vast, at least for the quantum realm.

“She tends to persist,” says Harvard physicist Susanne Yelin, who follows Schleier-Smith’s research. She gets results, even though “everything that exists in nature” is working against her experiments.
Quantum physics describes a microworld where many possibilities reign. Unobserved atoms and particles don’t have clearly defined locations, and information can be shared by widely spaced parts of a system. “We have equations that describe quantum mechanics well, but we can’t solve them when we are dealing with more than a few particles,” Schleier-Smith says.

That’s a shame, because understanding how large numbers of these small entities interact is essential to figuring out how our world works at the most fundamental level. Getting atoms to behave in just the right ways also has some practical benefits. It could lead to the most precise clocks yet, a boon for precision measurement, and to quantum computers that can solve problems that are too hard for today’s supercomputers.

Schleier-Smith’s experimental setups use elaborate tabletop arrangements of mirrors, lasers, vacuum chambers and electronic parts to cool atoms, pin them in place and then manipulate them with light. It’s a clutter of essential components, the construction of which requires an exacting understanding of the physics at play plus engineering know-how.

14

EEE / Andrea Young uncovers the strange physics of 2-D materials

« on: February 28, 2020, 09:54:44 PM »

Speaking with Andrea Young feels like watching a racehorse holding itself back at the starting gate. We met on the campus of the University of California, Santa Barbara, where he’s a condensed matter physicist, to chat about his work on 2-D materials. His mind seems to be working faster than the conversation can flow. My sense is, once the reins are loosened — and he’s back in the lab — he’ll take off.

Young’s colleagues confirm that’s the case. “He’s a whirlwind,” says physicist Raymond Ashoori of MIT. When Young was a postdoc in his lab, Ashoori says, it felt like “an idea a minute.”

Young, 35, has a way with substances shaved to the thickness of a single atom, such as the sheets of carbon known as graphene. His research has revealed new states of matter, and advanced scientists’ understanding of the strange physics that arises when materials are sliced thin.
“Things change a lot when you change the number of dimensions,” Young says.

As a graduate student at Columbia University, Young helped create a new type of material that transformed how scientists study graphene. Along with physicists Cory Dean, Philip Kim and colleagues, Young devised a technique for layering graphene with other materials, in particular another compound that forms 2-D sheets called hexagonal boron nitride. The combination makes the sometimes-finicky graphene easier to work with. And the material’s electrons can be coaxed to behave in unusual ways, interacting strongly with one another, for example.  Reported in Nature Nanotechnology in 2010, the technique was quickly adopted by scientists around the world. “Everybody uses it now,” Ashoori says.

15

EEE / This year’s SN 10 enjoy the journey, not just the discovery

« on: February 28, 2020, 09:54:14 PM »

After nearly four years of painstaking work, in 1902 Marie Curie produced one-tenth of a gram of radium chloride from several tons of uranium ore. It took her another eight years to isolate pure radium. The effort won her a second Nobel Prize and cemented her legacy as one of science’s most tenacious minds. “One never notices what has been done; one can only see what remains to be done,” Curie once wrote to her brother, Józef Skłodowski.

Doing science, and doing it well, can be frustrating, tedious and messy. There are long days at the computer, finicky experimental setups, do-overs and dead ends. And for one researcher featured on the pages that follow — digging into goat poop. Yet this year’s SN 10: Scientists to Watch appear to take it in stride. Why? They enjoy the work.

For the fifth consecutive year, Science News is spotlighting 10 early- and mid-career scientists who are persistent enough to make headway on science’s big questions. Some are tackling problems of societal importance: studying how climate change will affect food supplies, for example, or trying to make education more equitable. Others are seeking knowledge to answer fundamental questions, such as how the chemistry of space gives rise to the chemistry of life. Members of this year’s group are developing new tools to see deep into cells or into the mind, and are finding new routes to green fuels (thank you, goats)