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Quantum phenomena wash out as particles interact with the environment, but classical properties survive. Are they selected in a process analogous to evolution by natural selection?

By Philip Ball

Panther Media GmbH/Alamy

IT IS often said that the very small is governed by quantum physics, and the large by classical physics. There seems to be one set of rules for fundamental particles and another for us. But everything, including us, is made of particles. So why cant we too be in superpositions or show wave-like interference when we pass through a doorway, as a photon or electron does when it passes through narrow slits? Ditto any large, inanimate object?

To cut to the chase: we dont know the answer. One of the most intriguing ideas now being tested, however, is that classical reality might emerge through a process analogous to evolution by natural selection.

That notion has its origins in the 1970s, when physicists first came to realise that a particles quantum behaviours of superposition, entanglement and suchlike leak out into its environment, disappearing as a result of interactions with other particles a process called decoherence. The coupling to the macroscopic environment spoils the quantum coherences so fast that they are unobservable, says Jean-Michel Raimond at the Sorbonne University in Paris, France. Experiments have demonstrated that decoherence is a real, physical process, albeit one that happens in the blink of an eye.

What it cant tell us, however, is why various definite properties, such as position or velocity, emerge for us to observe. Why do these properties survive the transition from quantum to classical, while some other quantum features dont?

To Wojciech Zurek at the Los Alamos National Laboratory in New Mexico, it looked a lot like there was some sort of selective filtering going on. That filtering, he realised, is

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Quantum Darwinism: Can evolutionary theory explain objective reality? - New Scientist

FLASHES OF CREATIONGeorge Gamow, Fred Hoyle, and the Great Big Bang DebateBy Paul Halpern

The universe is changing. But scientists didnt realize that a century ago, when astronomers like Edwin Hubble and Henrietta Leavitt discerned that other galaxies exist and that theyre hurtling away from the Milky Way at incredible speeds. That monumental discovery sparked decades of epic debates about the vastness and origins of the universe, and they involved a clash of titans, the Russian-American nuclear physicist George Gamow and the British astrophysicist Fred Hoyle.

In his new book, Flashes of Creation, Paul Halpern chronicles the rise of Gamow and Hoyle into leaders of mostly opposing views of cosmology, as they disputed whether everything began with a Big Bang billions of years ago.

Halpern, a physicist himself at the University of the Sciences in Philadelphia, skillfully brings their fascinating stories to light, out of the shadow of the overlapping quantum physics debates between Albert Einstein and Niels Bohr, which Halpern has written about in an earlier book. Halpern also poses fundamental questions about how science should be done. When do you decide, for example, to abandon a theory? Ultimately, his book seeks to vindicate Hoyle, who in his later years failed to admit his idea had lost.

Until these two bold theoreticians arrived, astrophysics had been stuck at an impasse. Scientists werent sure how to interpret Hubbles observations, and no one understood how the universe created and built up chemical elements. It is clear that the intuitive, seat-of-the-pants styles shared by Gamow and Hoyle were absolutely needed in their time, Halpern writes.

Gamow and Hoyle make for a challenging joint biography, Halpern acknowledges, in part because their parallel stories so rarely intersected. They had only one significant in-person meeting, in the summer of 1956 in La Jolla, Calif., where Gamow had briefly served as a consultant for General Dynamics, the aerospace and defense company. They discussed many ideas in that coastal town, hanging out in Gamows white Cadillac, but for the most part, their debates took place in the pages of physics journals, newspapers and magazines, including Scientific American.

They also frequently appeared on early television and radio programs, becoming among the first well-known science communicators, paving the way for Carl Sagan, Neil deGrasse Tyson, Bill Nye, Carolyn Porco, Pamela Gay and others today. Hoyle wrote the science fiction novel The Black Cloud and the television screenplay A for Andromeda, while Gamow produced One, Two, Three Infinity and the Mr. Tompkins series, whose main characters predicaments illustrated aspects of modern science.

For years, their dueling theories a Big Bang origin of matter and energy (championed by Gamow) versus a steady-state universe that created matter and energy through quantum fluctuations (championed by Hoyle) remained highly speculative. Initially, the Big Bang theory predicted a universe only a couple billion years old, which conflicted with observations of the sun and other stars, known to be much older. Physicists were evenly divided between the two.

But that changed as more evidence emerged, and a key discovery eventually seemed to settle the debate. In 1964, the astronomers Arno Penzias and Robert Wilson noticed a constant signal of radio static with the Holmdel Horn Antenna in New Jersey. After ruling out possible experimental sources of noise (including pigeons and their droppings on the antenna), they deduced that the radio hiss had a cosmic origin. They and their colleagues eventually realized the signal came from relic radiation from the hot fireball of the early universe.

After that, the Big Bang theory quickly became consensus in the field. While Hoyles steady-state idea eventually failed, he made many other significant contributions, especially involving stellar processes and supernova explosions, which he showed could fuse chemical elements into heavier atoms and produce nitrogen, oxygen, carbon and more. In explaining this, and throughout the book, Halpern provides many helpful metaphors and analogies. He also reminds readers that Hoyle, Gamow and their fellow theoretical physicists made these accomplishments well before the heyday of supercomputers.

Halpern doesnt shy away from the characters flaws. In particular, he shows how Hoyles work later in life lay on the fringes of physics, including his controversial panspermia hypothesis, that organic material and even life on Earth came from colliding comets, and his unsuccessful attempts to revive steady-state theory. But this shouldnt cast a pall over his legacy.

Hoyles investment in the theory raises important philosophical and sociological questions about when we should consider an idea proven. Its also the sort of quandary that threads its away through contemporary debates among physicists: about dark matter versus modified gravity theories; about what dark energy is and how the universes inflation happened moments after the Big Bang; and about a persistent discrepancy in measurements of the universes expansion rate, known as the Hubble tension. Halpern unfortunately gives only brief mention to these active areas of research, which owe a lot to Gamow and Hoyle.

At one point in the book, Halpern relates a conversation he had with Geoff Burbidge, a colleague of Hoyles who also continued to support a steady-state model. Cosmology needed alternatives, he argued, not lemmings following their leader over a cliff.

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When the Big Bang Was Just a Theory - The New York Times

Imagine an intact diamond whose innards move with no friction, or a formed ice cube whose tightly-packed contents effortlessly flow. These might sound strange, or even impossible. But to physicists, theyre not too far removed from something theyve recently created: a strange state of matter called a supersolid.

For the past several years, scientists have been creating supersolids at very tiny scales in the lab. Now, a group of physicists have made the most sophisticated supersolid yet: one that exists in two-dimensions, like a sheet of paper. They published their results in Nature last Wednesday.

Its always been a sort of outstanding goal to bring [supersolids] into two dimensions, says Matthew Norcia, a physicist at Innsbruck University in Austria, and lead author of the Nature paper.

So what exactly is a supersolid? At its base, it contains properties of two different states of matter, one mundane and another quite esoteric.

The first of those states is a solid, which is among the most mundane forms of matter. Chances are that youre touching one at this very moment. Importantly, To physicists, a solid is interesting because the atoms inside are held in a rigid structure. Its why you dont, normally, see solid objects flowing like water.

But the second is a state of matter youve probably seen somewhat less: a superfluid. A quirk of quantum mechanics, a superfluid is a substance that acts like a fluid with zero viscosity. Scientists have caught glimpses of superfluids by cooling helium to temperatures barely above absolute zero. They can, and will, effortlessly crawl up walls or slide across surfaces.

A supersolid combines both a solid and a superfluid into one package: a solid that flows like a fluid with no friction, no resistance. If that sounds strange, its all perfectly natural. Its simply a product of quantum mechanics, the peculiar sort of physics that governs the cosmos at the very smallest scales.

To picture a supersolid, consider an ice cube immersed in liquid water, with frictionless flow of the water through the cube, wrote Bruno Labruthe-Tolra, a physicist at Sorbonne Paris North University in France who was not involved with the latest paper, in Nature News & Views that accompanied the new study.

It isnt an entirely new idea; physicists have been proposing it since the 1960s. But for many decades, it wasnt clear if we could make a supersolid on Earth. Only in the 2010s did scientists start making concrete progress towards creating a supersolid in the laboratory.

[Related: What the heck is a time crystal, and why are physicists obsessed with them?]

At first, scientists tried looking for supersolids in supercooled helium. Superfluids occur in helium, whose atomic properties make it ideal, so it seemed logical that you might find supersolids in them, too. But that effort has yet to bear fruit.

Later in the decade, physicists began turning to other elements such as rubidium and lanthanum. When you trap a small number of gaseous atoms and chill them down to fractions of a degree above absolute zero (the very coldest possible temperature, at around -460 degrees Fahrenheit), they condense into a whole suite of quantum weirdness. Thats called a Bose-Einstein condensate.

So, to create a supersolid, you first trap some atoms, then cool them, then play with their interactions. If you tune those correctly, and you tune the shape of the trap correctly, you can get a supersolid, says Norcia, the lead author.

Using this method, in 2019, researchers began to create a basic, one-dimensional supersolid: essentially, a thin supersolid tube in a straight line.

Thats what Norcia and his colleagues at Innsbruck University and the Austrian Academy of Sciences have now done. By tinkering with the device they used to trap atoms and the process they used to condense the atoms, they were able to extend their supersolid from one dimension into two: from a tiny tube into a small sheet.

This demonstration is a key advance because one direct way to prove that a system exhibits superfluidity is to study its properties under rotation, writes Labruthe-Tolra, and this analysis cannot be achieved if the system has only one dimension.

Now that researchers have created a supersolid in two dimensions, can they make one in three dimensions? Can they make a proper supersolid that you can touch?

Probably not soon, according to Norcia, though he says its a question that has crossed physicists minds. Currently, hes uncertain how they would do that with the technology they have.

Instead, for now, the researchers want to study the supersolid theyve created. Even though theyve successfully created a supersolid, physicists still know so little about it.

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We finally have a working supersolid. Here's why that matters. - Popular Science