Macro Roundup

great Witten interview,

Natalie Wolchover, "A Physicist’s Physicist Ponders the Nature of Reality,"Quanta Magazine, November 18, 2017, https://www.quantamagazine.org/edward-witten-ponders-the-nature-of-reality-20171128/Physicists are talking more than ever lately about dualities, but you’ve been studying them for decades. Why does the subject interest you?What are some of these newfound facets of dualities?That’s the idea that space-time and everything in it emerges like a hologram out of information stored in the entangled quantum states of particles.Given this web of relationships and the issue of how hard it is to characterize all duality, do you feel that this reflects a lack of understanding of the structure, or is it that we’re seeing the structure, only it’s very complicated?What do you see as the relationship between math and physics?You can’t imagine it at all?I’ve heard about the mysterious (2,0) theory, a quantum field theory describing particles in six dimensions, which is dual to M-theory describing strings and gravity in seven-dimensional AdS space. Does this (2,0) theory play an important role in the web of dualities?Dualities sometimes make it hard to maintain a sense of what’s real in the world, given that there are radically different ways you can describe a single system. How would you describe what’s real or fundamental?The latter.What would you say about the prospect of an even more optimistic idea that there could be one single quantum gravity description that really does help you in every case in the real world?Are you speaking of M-theory?You proposed M-theory 22 years ago. What are its prospects today?What’s an example of something else we might need?

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No.

No. He only talks about things you shouldn’t do and things you should do in trying to arrive at a more fundamental description of physics.

Yes. The way he phrases it is broader: He wants to explain the meaning of existence. That was actually why I thought you were asking if I wanted to explain the meaning of existence.

In my career I’ve only been able to take small jumps. Relatively small jumps. What Wheeler was talking about was an enormous jump. And he does say at the beginning of the essay that he has no idea if this will take 10, 100 or 1,000 years.

My favorite pastime is tennis. I am a very average but enthusiastic tennis player.

I think when I was younger I always thought the next thing I did might be the best thing in my life. But at this point in life I’m less persuaded of that. If I waste a little time reading somebody’s essay, it doesn’t seem that bad.

I wouldn’t call him a hero, necessarily, no. Really I just became curious what he meant by “it from bit,” and what he was saying. He definitely had visionary ideas, but they were too far ahead of their time. I think I was more patient in reading a vague but inspirational essay than I might have been 20 years ago. He’s also got roughly 100 interesting-sounding references in that essay. If you decided to read them all, you’d have to spend weeks doing it. I might decide to look at a few of them.

I tend to assume that space-time and everything in it are in some sense emergent. By the way, you’ll certainly find that that’s what Wheeler expected in his essay. As you’ll read, he thought the continuum was wrong in both physics and math. He did not think one’s microscopic description of space-time should use a continuum of any kind — neither a continuum of space nor a continuum of time, nor even a continuum of real numbers. On the space and time, I’m sympathetic to that. On the real numbers, I’ve got to plead ignorance or agnosticism. It is something I wonder about, but I’ve tried to imagine what it could mean to not use the continuum of real numbers, and the one logician I tried discussing it with didn’t help me.

Don’t expect me to be able to tell you anything useful about it — about whether he was right. When I was a beginning grad student, they had a series of lectures by faculty members to the new students about theoretical research, and one of the people who gave such a lecture was Wheeler. He drewa pictureon the blackboard of the universe visualized as an eye looking at itself. I had no idea what he was talking about. It’s obvious to me in hindsight that he was explaining what it meant to talk about quantum mechanics when the observer is part of the quantum system. I imagine there is something we don’t understand about that.

I tend to assume that space-time and everything in it are in some sense emergent.

I’m trying to learn about what people are trying to say with the phrase “it from qubit.” Wheeler talked about “it from bit,” but you have to remember that this essay was written probably before the term “qubit” was coined and certainly before it was in wide currency. Reading it, I really think he was talking about qubits, not bits, so “it from qubit” is actually just a modern translation.

The other night I was reading an old essay by the 20th-century Princeton physicist John Wheeler. He was a visionary, certainly. If you take what he says literally, it’s hopelessly vague. And therefore, if I had read this essay when it came out 30 years ago, which I may have done, I would have rejected it as being so vague that you couldn’t work on it, even if he was on the right track.

I really doubt I can say anything useful. I guess I suspect that there’s an extra layer of abstractness compared to what we’re used to. I tend to think that there isn’t a precise quantum description of space-time — except in the types of situations where we know that there is, such as in AdS space. I tend to think, otherwise, things are a little bit murkier than an exact quantum description. But I can’t say anything useful.

Maybe a bulk description of the quantum properties of space-time itself, rather than a holographic boundary description. There hasn’t been much progress in a long time in getting a better bulk description. And I think that might be because the answer is of a different kind than anything we’re used to. That would be my guess.Are you willing to speculate about how it would be different?You’re referring toInformation, Physics, Quantum, Wheeler’s 1989 essay propounding the idea that the physical universe arises from information, which he dubbed “it from bit.” Why were you reading it?Observing a quantum system irreversibly changes it, creating a distinction between past and future. So the observer issue seems possibly related to the question of time, which we also don’t understand. With the AdS/CFT duality, we’ve learned that new spatial dimensions can pop up like a hologram from quantum information on the boundary. Do you think time is also emergent — that it arises from a timeless complete description?Do you consider Wheeler a hero?Why do you have more patience for such things now?Do you ever take your mind off physics and math?In contrast to Wheeler, it seems like your working style is to come to the insights through the calculations, rather than chasing a vague vision.And he was talking about explaining how physics arises from information.I see. Does he have any hypotheses?Do you have any ideas about the meaning of existence?

Personally, I thought it was extremely clear it existed 22 years ago, but the level of confidence has got to be much higher today because AdS/CFT has given us precise definitions, at least in AdS space-time geometries. I think our understanding of what it is, though, is still very hazy. AdS/CFT and whatever’s come from it is the main new perspective compared to 22 years ago, but I think it’s perfectly possible that AdS/CFT is only one side of a multifaceted story. There might be other equally important facets.

M-theory is the candidate for the better description.

Well, unfortunately, even if it’s correct I can’t guarantee it would help. Part of what makes it difficult to help is that the description we have now, even though it’s not complete, does explain an awful lot. And so it’s a little hard to say, even if you had a truly better description or a more complete description, whether it would help in practice.

Well, one thing I’ll tell you is that in general, when you have dualities, things that are easy to see in one description can be hard to see in the other description. So you and I, for example, are fairly simple to describe in the usual approach to physics as developed by Newton and his successors. But if there’s a radically different dual description of the real world, maybe some things physicists worry about would be clearer, but the dual description might be one in which everyday life would be hard to describe.

What aspect of what’s real are you interested in? What does it mean that we exist? Or how do we fit into our mathematical descriptions?

Yes, that’s the pinnacle. In terms of conventional quantum field theory without gravity, there is nothing quite like it above six dimensions. From the (2,0) theory’s existence and main properties, you can deduce an incredible amountabout what happens in lower dimensions. An awful lot of important dualities in four and fewer dimensions follow from this six-dimensional theory and its properties. However, whereas what we know about quantum field theory is normally from quantizing a classical field theory, there’s no reasonable classical starting point of the (2,0) theory. The (2,0) theory has properties that sound impossible when you first hear about them. So you can ask why dualities exist, but you can also ask why is there a 6-D theory with such and such properties? This seems to me a more fundamental restatement.

If there’s a radically different dual description of the real world, maybe some things physicists worry about would be clearer, but the dual description might be one in which everyday life would be hard to describe.

No, I can’t. Traditionally it was thought that interacting quantum field theory couldn’t exist above four dimensions, and there was the interesting fact that that’s the dimension we live in. But one of the offshoots of the string dualities of the 1990s was that it was discovered that quantum field theories actually exist in five and six dimensions. And it’s amazing how much is known about their properties.

I find it hard to believe there’s a new formulation that’s universal. I think it’s too much to hope for. I could point to theories where the standard approach really seems inadequate, so at least for those classes of quantum field theories, you could hope for a new formulation. But I really can’t imagine what it would be.

I prefer not to give you a cosmic answer but to comment on where we are now. Physics in quantum field theory and string theory somehow has a lot ofmathematical secretsin it, which we don’t know how to extract in a systematic way. Physicists are able to come up with things that surprise the mathematicians. Because it’s hard to describe mathematically in the known formulation, the things you learn about quantum field theory you have to learn from physics.

There’s another curious fact that you might want to consider, which is that quantum field theory is very central to physics, and it’s actually also clearly very important for math. But it’s extremely difficult for mathematicians to study; the way physicists define it is very hard for mathematicians to follow with a rigorous theory. That’s extremely strange, that the world is based so much on a mathematical structure that’s so difficult.

I’m not certain what we should hope for. Traditionally, quantum field theory was constructed by starting with the classical picture and then quantizing it. Now we’ve learned that there are a lot of things that happen that that description doesn’t do justice to. And the same quantum theory can come from different classical theories. Now, Nati Seibergwould possibly tell you that he has faith that there’s a better formulation of quantum field theory that we don’t know about that would make everything clearer. I’m not sure how much you should expect that to exist. That would be a dream, but it might be too much to hope for; I really don’t know.

Yes. Then there are dualities in math, which can sometimes be interpreted physically as consequences of dualities between two quantum field theories. There are so many ways these things are interconnected that any simple statement I try to make on the fly, as soon as I’ve said it I realize it didn’t capture the whole reality. You have to imagine a web of different relationships, where the same physics has different descriptions, revealing different properties. In the simplest case, there are only two important descriptions, and that might be enough. If you ask me about a more complicated example, there might be many, many different ones.

TheAdS/CFT dualityconnects a theory of gravity in a space-time region called anti-de Sitter space (which curves differently than our universe) to an equivalent quantum field theory describing that region’s gravity-free boundary. Everything there is to know about AdS space — often called the “bulk” since it’s the higher-dimensional region — is encoded, like in a hologram, in quantum interactions between particles on the lower-dimensional boundary. Thus, AdS/CFT gives physicists a “holographic” understanding of the quantum nature of gravity.

It’s open-ended because there are so many different kinds of dualities. There are dualities between a gauge theory and another gauge theory, or between a string theory for weak coupling and a string theory for strong coupling. Then there’s AdS/CFT duality, between a gauge theory and a gravitational description. That duality wasdiscovered 20 years ago, and it’s amazing to what extent it’s still fruitful. And that’s largely because around 10 years ago, new ideas were introduced that rejuvenated it. People had new insights about entropy in quantum field theory — the whole story about “it from qubit.”

People keep finding new facets of dualities. Dualities are interesting because they frequently answer questions that are otherwise out of reach. For example, you might have spent years pondering a quantum theory and you understand what happens when the quantum effects are small, but textbooks don’t tell you what you do if the quantum effects are big; you’re generally in trouble if you want to know that. Frequently dualities answer such questions. They give you another description, and the questions you can answer in one description are different than the questions you can answer in a different description.

When I arrived at his office at the appointed hour on a summery Thursday last month, Witten wasn’t there. His door was ajar. Papers covered his coffee table and desk — not stacks, but floods: text oriented every which way, some pages close to spilling onto the floor. (Research papers get lost in the maelstrom as he finishes with them, he later explained, and every so often he throws the heaps away.) Two girls smiled out from a framed photo on a shelf; children’s artwork decorated the walls, one celebrating Grandparents’ Day. When Witten arrived minutes later, we spoke for an hour and a half about the meaning of dualities in physics and math, the current prospects of M-theory, what he’s reading, what he’s looking for, and the nature of reality. The interview has been condensed and edited for clarity.

Researchers pore over his work and hope he’ll take an interest in theirs. But for all his scholarly influence, Witten, who is 66, does not often broadcast his views on the implications of modern theoretical discoveries. Even his close colleagues eagerly suggested questions they wanted me to ask him.

That’s extremely strange, that the world is based so much on a mathematical structure that’s so difficult.

Physics luminaries since Albert Einstein, who lived out his days in the same intellectual haven, have sought to unify gravity with the other forces of nature by finding a more fundamental quantum theory to replace Einstein’s approximate picture of gravity as curves in the geometry of space-time. M-theory, which Witten proposed in 1995, could conceivably offer this deeper description, but only some aspects of the theory are known. M-theory incorporates within a single mathematical structure all five versions ofstring theory, which renders the elements of nature as minuscule vibrating strings. These five string theories connect to each other through “dualities,” or mathematical equivalences. Over the past 30 years, Witten and others have learned that the string theories are also mathematically dual to quantum field theories — descriptions of particles moving through electromagnetic and other fields that serve as the language of the reigning “Standard Model” of particle physics. While he’s best known as a string theorist, Witten has discovered many new quantum field theories and explored how all these different descriptions are connected. His physical insights have led time and again to deep mathematical discoveries.

During a visit this fall, I spotted Witten on the Institute’s central lawn and requested an interview; in his quick, alto voice, he said he couldn’t promise to be able to answer my questions but would try. Later, when I passed him on the stone paths, he often didn’t seem to see me.

Among the brilliant theorists cloistered in the quiet woodside campus of the Institute for Advanced Study in Princeton, New Jersey, Edward Witten stands out as a kind of high priest. The sole physicist ever to win theFields Medal, mathematics’ premier prize, Witten is also known for discovering M-theory, the only candidate for a unified physical “theory of everything.” A genius’s genius, Witten is tall and rectangular, with hazy eyes and an air of being only one-quarter tuned in to reality until someone draws him back from more abstract thoughts.

A Physicist’s Physicist Ponders the Nature of Reality

Speed of evolution

“…In a paper published last month in Science, researchers reported that a new animal species had evolved by hybridization — and that it had occurred before their eyes in the span of merely two generations. The breakneck pace of that speciation event turned heads both in the scientific community and in the media. The mechanism by which it occurred is just as noteworthy, however, because of what it suggests about the undervalued role of hybrids in evolution….”

Jordana Cepelewicz, “New Bird Species Arises From Hybrids, As Scientists Watch,” Quanda Magazine, December 13, 2017,

https://www.quantamagazine.org/new-bird-species-arises-from-hybrids-as-scientists-watch-20171213/

New Bird Species Arises From Hybrids, as Scientists Watch

The rapid, unorthodox emergence of a new finch in the Galápagos hints that speciation isn’t rare. New hybrid species may quietly appear and disappear without anyone noticing.

It’s not every day that scientists observe a new species emerging in real time. Charles Darwin believed that speciation probably took place over hundreds if not thousands of generations, advancing far too gradually to be detected directly. The biologists who followed him have generally defaulted to a similar understanding and have relied on indirect clues, gleaned from genomes and fossils, to infer complex organisms’ evolutionary histories.

Some of those clues suggest that interbreeding plays a larger role in the formation of new species than previously thought. But the issue remains contentious: Hybridization has been definitively shown to cause widespread speciation only in plants. When it comes to animals, it has remained a hypothesis (albeit one that’s gaining increasing support) about events that typically occurred in the distant, unseen past.

Until now. In a paper published last month in Science, researchers reported that a new animal species had evolved by hybridization — and that it had occurred before their eyes in the span of merely two generations. The breakneck pace of that speciation event turned heads both in the scientific community and in the media. The mechanism by which it occurred is just as noteworthy, however, because of what it suggests about the undervalued role of hybrids in evolution.

In 1981, Peter and Rosemary Grant, the famous husband-and-wife team of evolutionary biologists at Princeton University, had already been studying Darwin’s finches on the small Galápagos island Daphne Major for nearly a decade. So when they spotted a male bird that looked and sounded different from the three species residing on the island, they immediately knew he didn’t belong. Genetic analysis showed he was a large cactus finch (Geospiza conirostris) from another island, either Española or Gardner, more than 60 miles away — too great a distance for the bird to fly home.

Tracking the marooned male bird’s activity, the Grants observed him as he mated with two female medium ground finches (G. fortis) on Daphne and produced hybrid offspring. Such interbreeding by isolated animals in the wild is not uncommon, though biologists have usually dismissed it as irrelevant to evolution because the hybrids tend to be unfit. Often they cannot reproduce, or they fail to compete effectively against established species and quickly go extinct. Even when the hybrids are fertile and fit, they frequently get reabsorbed into the original species by mating with their parent populations.

The new lineage of birds began when a male Geospiza conirostris finch marooned on Daphne Major island mated with a native G. fortis females. (These photos show birds representative of those species, and not the actual parental birds.)

But something different happened with the hybrids on Daphne: When they matured, they became a population distinct from Daphne’s other bird species by inbreeding extensively and exclusively — siblings mating with siblings, and parents mating with their offspring.

In short, an incipient hybrid species, which the researchers dubbed the Big Bird lineage, had emerged within two generations. Today, six generations have passed, and the island is home to around 30 Big Bird finches. “If you were a biologist none the wiser to what had happened,” said Leif Andersson, a geneticist at Uppsala University in Sweden and one of the study’s co-authors, “and you started studying these birds, you’d think there were four different species on the island.”

Where Hybrids Thrive

On Daphne Major, the conditions may have been just right for hybrid speciation. “It shows what is possible, given the right circumstances,” Peter Grant said, and it sends “a valuable message about the importance of rare and unpredictable events in evolution. These have probably been underestimated.”

The Big Bird lineage became reproductively isolated so quickly because those birds could not successfully attract mates among the island’s resident species, which preferred their own kind. Big Bird finches couldn’t pass muster: They had relatively large beaks for their body size, and they boasted a unique song. These differences prevented gene flow between the hybrids and the native medium ground finches from which they had descended, leading to a distinct hybrid population. (In their Science paper, the Grants and their colleagues noted that the species status of Big Bird finches is still unofficial because no one has yet tested whether the birds will breed with their ancestral finches on Española and Gardner. But they cited reasons to suspect that the Big Bird lineage is reproductively isolated from them as well.)

The fact that this niche was available for Big Bird to occupy is likely a result of the particularly young, isolated and often extreme environment of the Galápagos. “Conditions on the islands really help drive the speciation process,” said Scott Edwards, an ornithologist at Harvard University.

The same might be said for small and isolated environments elsewhere, such as mountaintops or ponds. By contrast, speciation isn’t likely to occur this way in less isolated regions, said Trevor Price, an ecologist at the University of Chicago. In those areas, where competition for resources is already fierce, a new hybrid species like Big Bird would find no niche for itself.

But hybrid species may have had more widespread opportunities to establish themselves in the past. Perhaps, Price suggested, this rapid production of species could have occurred in the aftermath of the meteorite impact that caused mass extinction on Earth millions of years ago. At that time, there were resources and potential niches, and not enough species to fill them.

Fractal Speciation

Some experts think that even today hybrid speciation may be far from rare. Under the most commonly accepted speciation model, called allopatric speciation, populations get geographically separated — by a change in a river’s course, say, or the formation of a mountain, or diverging migrations — and then adapt to distinct competitors and environments over long periods. If the groups ever meet again, they may no longer be similar enough to interbreed.

But very fast hybrid speciation events like the one the Grants saw on Daphne may often occur in bursts — only to end with the new species dying out before we have time to observe it. “Speciation is common. It’s happening all around us,” said James Mallet, an evolutionary biologist at Harvard University. “It’s just that usually we don’t recognize the divergent lineages that are appearing as separate species.”

He continued, “I believe that speciation is more of a continuum than people have been thinking.” At one extreme, species can be cleanly divided from one another, with no interbreeding and exchange of genes. But to greater or lesser degrees, hybridization could also allow genes to flow into a species from others, and the resulting hybrids could sometimes develop an identity of their own, even if only temporarily.

Mallet argues that hybrid species could very well be appearing all the time, only to collapse and disappear just as quickly, with the hybrids either going extinct or being absorbed back into a parent population. “To me, that shows the abundance of opportunities for speciation,” he said. Just because many hybrids go extinct — a fate that is still very likely to befall the Big Bird lineage, according to Mallet — doesn’t mean that hybridization is not a real source of new species in nature.

Instead, he sees speciation as an almost fractal process that can be observed in ecosystems over different timescales. “If you look at the macro level over millions of years,” Mallet said, “you’ll see a few species evolving and going extinct.” But at the micro level, on the order of dozens of years or less, species may be forming and dissolving all the time. Most biologists simply don’t have the long-term data to show it, he said.

What made it possible to identify such an event in Darwin’s finches was the Grants’ decades of careful fieldwork, followed by detailed genomics studies. The new paper illustrates “the value of continuous, long-term studies in nature,” Peter Grant said. Without that, “we would not have detected or been able to interpret the immigration of an individual of one species and interbreeding with a member of the resident species.”

Although biologists do not yet know how much animal hybrid speciation occurs outside the Galápagos, they are becoming increasingly aware of hybrids as a powerful agent of evolution. “I think this paper really increases that signal,” Edwards said. “Researchers like me are going to be looking for it much more regularly.”