Black holes, string theory and more: Q&A with physicist Brian Greene

Theoretical physicist and science popularizer Brian Greene presenting at the World Science Festival.
Theoretical physicist and science popularizer Brian Greene presenting at the World Science Festival. (Image credit: World Science Festival/Greg Kessler)

Brian Greene is one of the foremost scientists and science communicators of our time.

Greene, a theoretical physicist at Columbia University, has been working for decades to advance our understanding of the universe and how it works. That work includes significant discoveries in the field of string theory, one of the most promising "theory of everything" candidates put forward to explain all known phenomena in the cosmos.

He also makes it a priority to spread the word of such discoveries to the masses. Greene co-founded and chairs the World Science Festival, for example, and has written a number of best-selling, critically acclaimed popular-science books. These include "The Elegant Universe" (W.W. Norton, 1999), "The Fabric of the Cosmos" (Knopf, 2004), "The Hidden Reality" (Knopf, 2011) and "Icarus at the Edge of Time" (Knopf, 2008), an illustrated children's book that features imagery captured by NASA's famous Hubble Space Telescope.

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And last month, Greene taught a free online course via Varsity Tutors. The interactive lecture, which he geared toward kids from fourth to eighth grade, was called "Adventures in Astrophysics: Black Holes." (If you missed it, you can watch a replay on YouTube.) He plans to do another Varsity Tutors course soon, this time about cosmology and the Big Bang. caught up with Greene to discuss the importance of science education, why black holes are so important and interesting, and whether a theory-of-everything breakthrough could be on the horizon. The following conversation has been edited for length. Why did you think it was important to do a Varsity Tutors course, and why that particular topic? Why did you choose black holes?

Brian Greene: I generally think that, if we can reach the younger generation and get them excited about scientific ideas — it sounds hackneyed, but I think we have a shot of bettering the world, of changing the world. Instilling that sense into the next generation is utterly vital.

And so many kids in the science classroom, their view of science is, it's all about memorizing this fact or coming up with a solution to this or that problem. And yes, that's important, but it's the big ideas that really matter. And black holes are among the most enigmatic and exciting areas of forefront science, and you don't need to know a lot to understand the basic ideas. So I geared the discussion to between fourth and eighth graders, and from the comments that I saw when they summarized what they'd learned in various postings, it felt like a lot of them got it, which is exciting. It feels like we're really coming into a golden age of black-hole astronomy. We've got the Event Horizon Telescope, which recently gave us our first direct image of a black hole, and we're seeing a lot of black-hole mergers, thanks to the LIGO project. Do you feel like we're finally getting a better handle on these objects? And if so, what could that tell us about the universe?

Greene: Yeah, hugely so. There was a time, 10 years ago, when you could still make an argument that maybe black holes aren't real; they're just a figment of the mathematics. But with LIGO, with the collision of two black holes giving that first ripple in the fabric of space, and with EHT — even the Nobel Prize this year was awarded to work on black-hole physics. So black holes have really come into their own. 

For someone like me who works on cutting-edge theoretical ideas — we're struggling now to merge black holes and quantum mechanics, to get a full understanding. And black holes are the prime theoretical laboratory for pushing our ideas to the limit. When we can fully understand black holes and quantum mechanics, I suspect, our understanding of the universe is going to jump to a new level.

Related: Historic first images of a black hole show Einstein was right (again) And are you optimistic that that's going to happen relatively soon? Do you think there are breakthroughs on the horizon?

Greene: Yeah. There are many of us, those people who work on string theory and quantum gravity — focusing upon black holes is really the predominant occupation at the moment. 

And there's so much exciting work that's happening that I would suspect that, even a year or two from now, our understanding today will look relatively primitive to the new ideas that will be developed. Along those lines: There's some news that just came out — the g-2 experiment. Do you have any thoughts about g-2 and about what that might mean for our understanding of physics? [Editor's note: The g-2 research team spotted excessive wobbling by subatomic particles called muons, suggesting that some exotic type of matter or energy may be pushing on them.] 

Greene: I'm happy to answer. I would preface it by saying that results at this level of confidence — the so-called three sigma, four sigma — they do come and go. So one has to take it all with a grain of salt until we reach, say, the five sigma — that confidence level where the chance of being a statistical fluke is like one in a million or one in a few million, as opposed to one in a thousand, which is where we are at the moment.

But, putting that to the side: If the result stands the test of time, it would suggest that the Standard Model of particle physics, which has been the gold standard of understanding, may need to be revisited. Perhaps there's a fifth force in addition to the known four forces of nature. That would be enormously exciting. Perhaps there are other particles that we've yet to discover that are shifting the value of the muon magnetic moment. 

Again, I do want to stress one thing: A [different] paper came out in Nature just two days ago — Yeah, and that one didn't see the shift [that the g-2 experiment did].

Greene: That's right. And that's using theoretical methods, but being very careful, and computer calculations. And they claim that the value that's being seen matches precisely what you'd expect from the vanilla Standard Model of particle physics, without any changes. So it's very much a fluid situation. Let's just wait and see how it all falls out. It strikes me that these sorts of findings show the importance of doing things like the online course that you just taught. People often kind of throw up their hands after they read a study that says "We've made a big breakthrough," and then they see another study that says, "Actually, no." A lot of people don't know how science works, that it's a process that builds upon things step by step and everything is always in flux. So, is that something that you try to get across to these kids when you're teaching them about science?

Greene: It is. Because, if you view science as just a body of facts that are static, then you're missing the drama of the discovery, where people put forward ideas, others react to the ideas, people test and observe and come back. It's a wonderful, dynamic process of human discovery. And when you see science in that light, it brings it to life in a way that a textbook of facts can never. Yeah. And I find, talking to people, and to kids especially — they often don't even view scientists as real people. They're seen as caricatures, the wild-haired guys in the movies. I think it's really important to emphasize to kids that scientists are just people like them or like their mom or dad, and that's something that most people don't internalize really.

Greene: Yeah, it's an important lesson. And there have been attempts — television shows, you know, "The Big Bang Theory" perhaps being the most prominent of them. But again, in "The Big Bang Theory," the scientists were somewhat caricatures, right? So, it was good that it was mainstream, but still there's a tendency, as you say, to see scientists as this weird collection. 

In any large group, there are weird folks. But the vast majority of people are just like everybody else and just focus their attention on a certain class of questions.

Related: The biggest unsolved mysteries in physics Yeah. So, to go back to the very big questions: Scientists are trying to come up with a theory of everything, to find one that stands the test of time. Do you still feel like that's going to be string theory? Has what we've learned over the past five or 10 years changed any of your thinking on the biggest questions?

Greene: Well, just to be clear: Although I'm known for working on string theory and bringing string theory to general populations, I have never, ever said I believe in string theory. I have always said I have confidence that this is an interesting idea worthy of our attention that may ultimately be the final theory, but we just don't know yet.

So my assessment is pretty stable; it's pretty much the same. In the last few years, there have been great theoretical breakthroughs in string theory. There's been less contact with experiment than I would have hoped. I'd hoped that the Large Hadron Collider would reveal some of the hints of string theory. That has not happened. But that may well mean that the theory needs a bigger, better, more powerful machine to probe it, and that is not unexpected. 

So I'd say that developments are happening at a fast and furious pace on the theoretical side in the hope that we'll have some connection to experiment or observation in the not-too-distant future. But that's difficult to predict. Are there any experiments or projects in particular whose results you're most looking forward to seeing in this regard? What could help us make progress? 

Greene: Right now, it's likely that if we do get any insight from observations into string theory, it could come from, say, verifying gravitational wave observatories that might be able to probe the outskirts of a black hole with unprecedented precision. It's conceivable that in these kinds of experiments we might get a hint. 

But if you're asking me in my heart of hearts, I think it's probably going to be the case that in our lifetime we're not going to get that observation or experimental insight. It may be the next generation or the generation beyond that. That's kind of depressing from an individual perspective, because we all want to know; we all want to get the answers. But science is a process, and we've only been at this — trying to marry all the forces of nature and everything into one cohesive whole — for about a century, right? 

Greene: Even less at some level. There was work a century ago, but I'd say [the last] 50 years is when the real work has happened. And we're trying to push our understanding so far beyond the reach of today's experiments that it's not surprising that it may take a few generations to get there.

We're trying to answer some of the deepest questions that have ever been asked. How did the universe begin? How do the fundamental forces integrate with one another? What are the fundamental ingredients? These are questions that, in one way or another, we've been asking for a thousand, or a couple thousand, years. And if it takes another handful of decades before we get real insight, that's just how it is. But do you have confidence that our brains are actually capable of plumbing these depths? We're basically apes that evolved to survive on the savanna on a timescale of 70ish years. Are we capable of actually getting to the bottom of these mysteries that may be much, much deeper than we can possibly comprehend?

Greene: I'm fundamentally an optimistic person, so I've always imagined that the answer to that question is yes. But you look around this planet, and there are intelligent creatures like dogs and cats, who I suspect don't understand Einstein's general theory of relativity. Maybe I'm wrong; maybe the dogs and cats are all laughing at us right now. Maybe they've got the final answer. 

But putting that to the side: Yeah, it could be that our brains are simply too limited to access the final answer, even though it's staring us in the face right now. But you go forward, you push as hard as you can. We haven't hit any insurmountable obstacles yet, and so we maintain our optimism and try to find the final answer.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook. 

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Mike Wall
Senior Space Writer

Michael Wall is a Senior Space Writer with and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.