Lightspeed: Edited by John Joseph Adams




Interview: Brian Greene

Brian Greene is a professor of physics and mathematics at Columbia University, and is recognized for a number of groundbreaking discoveries in his field of superstring theory. His books are widely read: The Elegant Universe was a finalist for the Pulitzer Prize and has sold more than a million copies worldwide; The Fabric of the Cosmos spent half a year on The New York Times bestseller list, and inspired The Washington Post to call him the “single best explainer of abstruse ideas in the world today.” His latest book, The Hidden Reality debuted at #4 on The New York Times bestseller list. With producer Tracy Day, he is the co-founder of the World Science Festival.

This interview first appeared on’s The Geek’s Guide to the Galaxy podcast, which is hosted by John Joseph Adams and David Barr Kirtley. Visit to listen to the entire interview and the rest of the show, in which the hosts discuss various geeky topics.


You recently hosted a PBS special called The Fabric of the Cosmos. How did that program come about, and why should people go check it out?

Well, it’s based on a book that I wrote with the same title, The Fabric of the Cosmos. It’s a show that explores some of the strangest features of modern science, but ideas that are well-grounded in mathematical research and observational data. So there’s one program that asks the question: What is space? The stuff that’s all around us. Another asks: What is time? This strange feature of our lives that’s so familiar yet so hard for science to pin down. And then there’s a program on quantum mechanics that explores the micro world, and focuses on a feature known as “entanglement,” where distant objects can somehow communicate with each other even though nothing travels between them. And finally there’s a program on the most far-out of all the subjects, the possibility that our universe is not the only universe, that we might be part of a multiverse.

The challenge in creating a program like this, about space and time and quantum mechanics and the multiverse, is that there’s not that much you can point a camera at to really show what we’re talking about. So the programs rely on a good deal of high-quality computer animation, on which I worked with the animators, as did the whole team at Nova, to try to get the animations to be as close as possible to what it would be like to go to these exotic realms that the programs are about—be it the micro world of quantum physics, or the other universes in the multiverse, or to try to get a sense of what the fabric of space and time might actually look like.

Since our podcast is a show for science fiction fans, we were just curious if you’re a science fiction fan yourself, and if so, who are some of your favorite authors?

Isaac Asimov, I think, is probably my favorite. I think Ray Bradbury would be right up there too. I love it when real science finds a home in a fictional setting, where you take some real core idea of science and weave it through a fictional narrative in order to bring it to life, the way stories can. That’s my favorite thing.

I’ve had various experiences where I’ve been called by Hollywood studios to look at a script or comment on various scientific ideas that they’re trying to inject into a story. You know, I had a great meeting with Jerry Bruckheimer on a film that came out a few years ago called Deja Vu, with Denzel Washington. There was a time travel element to this film, and I went to the studios in Hollywood, and they earnestly wanted to understand special relativity, and the possibility of time travel that comes from Einstein’s insight. And it was a great thing. I had a white board, and I was writing out the equations, and explaining to them all these ideas, and they were really getting it.

But at the end of the conversation—perhaps predictably—they said to me, “But couldn’t we modify things just a little bit so that this could happen, or that could happen?” And they wanted to deviate from the science. And at the end of the day, of course, Hollywood is really dedicated to making the films that will attract the most people into the theater, get the most people into the seats, and I fully understand that.

You know, not to be self-promoting, that’s not what I mean at all, but I did a small piece with Philip Glass called Icarus at the Edge of Time, where I rewrote the myth of Icarus so that the boy doesn’t travel to the sun, but travels to a black hole. And there, the real physics of general relativity dictates how the story unfolds. And to make a long story short, the boy spends an hour near the edge of the black hole, but when he comes back and wants to show his dad what he’s done—because his dad told him not to go—he realizes that it’s 10,000 years later.

Because that’s what would happen—time slows down at the edge of a black hole. So an hour for you could be thousands of years for somebody else who’s further from the black hole. And with Philip Glass, there’s an orchestral score, and a narrator tells the story. And the hope is that people who see this piece—and we’ve now performed it many times around the world—leave with this kind of visceral sense of what general relativity is about. They don’t know the equations, they don’t know the details, but they’ve gone on a fictional ride to the edge of a black hole, and they’ve come back with an intuitive understanding of some real science.

Is that a realistic treatment, in that the ship would be able to survive getting close enough to the black hole for the relativistic effects to take effect?

It’s a great question, and one that I worried about when we were doing this, and it turns out if the black hole is sufficiently big, then yes, it is a realistic rendition of what would happen. But the bottom line is, even if it were not the case, even if the full scientific story could not be realized in this fictional setting, I don’t think it would matter, because my point is, it’s the core science—the science that really drives the narrative. In this particular case, it’s the science of how time behaves at the edge of a black hole. That’s what really matters. So I would say, give yourself license—if you’re a science fiction writer—to bend the rules at the edges in order to make the story work, but if the integrity of the core science that really matters for the story can be kept intact, I think that’s a worthwhile goal to shoot for.

One of my favorite tropes in fantasy and science fiction is the idea of parallel worlds, but in science fiction and fantasy settings, typically what happens is somebody from the real world travels to a parallel world. So assuming that the multiverse is actually real, would it ever be possible to travel to a parallel world?

It’s pretty tough to imagine how that would happen. So, you may know I have a recent book called The Hidden Reality, where I go through nine different variations on the theme of parallel universes. Because there isn’t just one flavor of parallel universe—there’s a version that comes out of quantum mechanics, there’s a version that comes out of cosmology, a version that comes out of string theory, and so forth. But one thing that they do share is it’s pretty tough, if not impossible, to go from one universe to another in any of these versions—in any conventional notion of what it would mean to travel from one universe to another.

So what I mean by that is, let me just give one example. The parallel universe theory that comes out of quantum mechanics is called “the many worlds interpretation of quantum mechanics.” And it emerges because the core idea of quantum physics that we learned about in the 1920s and ’30s is that you can’t predict with certainty the outcome of any experiment. Instead the best you can ever do is predict the probability that you’ll get one outcome or another—say a 30 percent chance of this, 50 percent chance of that, and 20 percent chance of that. Now, the question arose, and is still with us, of when you do a measurement, when you find one and only one outcome, what happened to the other potential outcomes?

And it turns out that the most straightforward reading of the math of quantum mechanics—as realized by a guy named Hugh Everett all the way back in 1957—the most straightforward reading is that the other potential outcomes actually do happen, they just happen in their own separate universe, which would mean that the experimenter, say me, would measure the particle and find it in one location in this universe and think that’s the only outcome, but there’d be another copy of me in a parallel world finding the particle at a different location, and another version of me still in yet another parallel universe that would find the third possible outcome.

So there’d be three of me, if there are three possible outcomes in these three parallel universes, so you could say that I “traveled,” in some sense, to all of them, because there would be a version of me in each of those universes. But the traditional notion of being able to jump from one universe to another, in the way that we see in movies or sometimes read about in books, it’s hard to see how that would have any meaning in this version of parallel universes, and a similar kind of discussion would apply to most of the others as well.

I listened to a lecture where you talked about how if you were to fly deep enough into outer space, you might in effect end up in a parallel universe?

Yeah, you’re absolutely right. So another version of parallel universes comes from far more simple considerations than quantum physics. If space goes on infinitely far, then there’s another flavor of parallel universe theory that emerges. Now, we don’t know that space goes on infinitely far, but it’s certainly a viable possibility that scientists today still seriously consider. And the version of parallel universes that comes out of that is pretty straightforward to grasp. You see, when we look out into space today, even with the most powerful telescope, there’s just so far we can see, because it takes light a certain amount of time to travel through space and reach us. So we only really have access to a chunk of space, if it goes on infinitely far, the chunk that could have sent out a light signal that would reach us by the time we look up today. So it’s roughly 30 to 40 billion light years, the size of that chunk of space. It seems big, but if the universe is infinitely big, that’s just a small little patch—a little city, if you will, in a grand cosmic landscape that would go on much, much further than we have access to.

Now, the reason why that’s interesting is because in any finite region of space, matter can only arrange itself in finite many different configurations. It’s a fairly basic consequence of the laws of physics. And that means that if space goes on infinitely far out there, there have to be duplicates of us, and the argument is quite straightforward. Let me just give an analogy. Imagine I have a deck of cards, and started to shuffle the deck. Well, the cards will come out in different orders. You shuffle again, the cards will come out in a different order still, but since there are only finitely many cards in the deck, there are only finitely many distinct orders of those cards. It’s a big number, but it does mean that if you shuffle the cards enough times, sooner or later the order of the cards has to repeat.

Now, by the same reasoning, since matter could only arrange itself into finitely many different configurations in a given region of space . . . well, if you look region by region by region in an infinite cosmos, sooner or later the arrangements of the particles has to repeat. There aren’t enough different arrangements to go around, just like the shuffle of the deck of cards. Now, I’m just an arrangement of particles, as are you, as is anybody else, as is the earth, the sun, and so on. So if the particle arrangements here repeat someplace way out there, it means that you and I, the sun, the earth, they would be out there too. So that’s a sense in which there would be parallel realities way out there in the cosmos, if space goes on sufficiently far.

And now to your question. You’re right—if in principle you could travel sufficiently far, you might be able to reach those other domains, those other “parallel worlds.” But again physics comes in to pretty much thwart that possibility. First of all, we’re talking about gargantuan distances, distances that are so spectacularly large that we’ll never be able to traverse them—or at least, any conceivable technology that we know of would never be able to travel those distances. But even beyond that, we’ve learned that our universe isn’t static, it’s expanding, and in fact it’s expanding ever more quickly, and because of that there’s actually a barrier, a physical barrier, to how far we could ever traverse space, and that barrier would be too small for us to ever reach these other worlds. So again, the idea of being able to travel to a parallel world is likely one that can’t actually be realized.

Would the matter for all of those infinite worlds have come from our Big Bang, or are we talking about multiple Big Bangs—an infinite number of Big Bangs creating those worlds?

Well, it’s again a great question, and it does call into question the notion of what we mean by “the Big Bang.” You see, if our universe is truly finite in size, then ever further back in time, the size of that universe would be ever smaller, so that way back at time zero, or right near time zero, our entire universe would be a tiny little speck, and then that speck would swell rapidly, and that’s usually the picture we all have in mind when talking about the Big Bang.

But if space goes on infinitely far—this alternative possibility—then ever further back in time, objects in space were ever closer together, but space itself would still extend infinitely far. I mean, if you want to say that way back in time, the universe was half its current size, well, half of infinity, that’s still infinity. A third of infinity, that’s still infinity. So if the universe goes on infinitely far, then even way back at time zero itself, space would go on infinitely far. So the Big Bang would better be thought of, in some sense, as an infinite number of Big Bangs, all happening throughout this infinite spatial expense. And those “Big Bangs,” if you will, will be responsible for all the happenings in these different domains, these different chunks of space populating this infinite expanse—if indeed space does go on infinitely far. It’s a different image of the Big Bang than the one that we traditionally have in mind.

So going back to the Everett multiverse idea, how different could the laws of physics be in those parallel worlds? Are we talking about a different Periodic Table of Elements? Different fundamental constants? Different subatomic particles? What’s the degree of variation there?

Well, in the Everett many-worlds interpretation of quantum mechanics, we aren’t actually imagining that the laws of physics or the properties of particles are varying. There are other versions of parallel universe theory, multiverse theory, that do however have this feature that you’re referring to, of different laws of physics and different particle properties. And the easiest one to grasp there is the one that comes out of a field called inflationary cosmology. So inflationary cosmology is, in some sense, an enhanced version of the Big Bang theory, which seeks to fill in a missing piece in the standard Big Bang proposal.

See, the standard Big Bang tells us how the universe evolved after the bang, but doesn’t tell us what powered the bang itself, and people have tried to fill in that gap, to try to figure out what drove space to rush outward in the first place. A guy named Alan Guth, a great physicist now at MIT, in the 1980s was the first to propose that there might be a naturally occurring kind of cosmic “fuel” that would naturally force space to rush outwards, and he proposed that this would be what drove the bang in the first place. The interesting thing is, as people began to study that proposal in more detail, they found that this fuel that he had proposed—and others like Steinhart and Linde developed further—would be so efficient that it would be virtually impossible to use it all up, which would mean that in inflationary cosmology, the Big Bang giving rise to our universe would not be a unique event. There would be Big Bangs that happened before, there would be Big Bangs that would happen after, in various and far-flung locations, each giving rise to its own expanding domain, each giving rise to its own universe.

And when you study those universes in detail, you find that, indeed, particle properties can vary from one expanding realm to another. Those particle properties and various environmental influences can indeed make the laws of physics appear different from one expanding realm to another, so the variations in that version of the multiverse proposal can be quite, quite significant.

One of my favorite book series is the Chronicles of Amber by Roger Zelazny, in which there are characters who travel between parallel worlds, and they decide to carry swords with them rather than guns, because guns stop working very rapidly when the laws of physics start changing around you. What do you think about that idea?

Indeed, I would suspect that in those other worlds, things could be so different that not only would guns stop working, everything else might stop working too. So they prepared themselves well, but I think what they may not have taken into account is, if the laws of physics vary enough that guns and gunpowder don’t work, it’s probably the case that the laws are such, and they vary to such a degree, that the biological processes that keep us ticking would probably not be happening either.

If there were a material in a parallel world that couldn’t exist in our world—that different laws of physics produced—and you could take that material and bring it to our world, would it fall apart? Would it have special properties?

You know, you can imagine the simplest example of that, where perhaps the basic fundamental particles like electrons and quarks, maybe they exist in those other universes, but maybe their masses are a little bit different, or their electric charges are a little bit different, and that idea is quite compatible with the mathematical formulations that we have of these various multiverse proposals. Now, if you study the properties of matter, and how they depend upon the masses of the basic particles, and the charges of the basic particles, you find something spectacularly interesting. If you change the basic properties of the particles by even a little bit—change masses by 20 or 30 percent, or you change electric charges by 20, 30, 40 percent, you really disrupt the atomic structure that’s responsible for all those elements on the Periodic Table, and the way those elements would exist and combine and behave.

So even modest adjustments to the fundamental physical parameters would rapidly disrupt matter as we know it. So if you tried to take things from one place to another, they would themselves suffer radical disruption. You can imagine that maybe there are other universes where the changes are so slight that matter would suffer only the most modest of changes as it went from universe to universe—if indeed you could transport it from place to place—but in most of these multiverse proposals, the vast majority of the other universes would not be very close in these features to our universe, and therefore matter really could not survive that kind of journey.

Science fiction stories are full of characters traveling through hyperspace, but in The Fabric of the Cosmos you make it sound as if that wouldn’t work, because higher dimensions only exist at very small scales. Is that right?

The most well-studied explanation for how our universe could have more than three dimensions of space, how there could be so-called “hyperspace” and yet we don’t see those dimensions, is indeed the one that you’re referring to: The extra dimensions are all around us, they’re just crumpled to such a fantastically small size that we can’t see them.

Nevertheless, there are other proposals that have come on the scene of physics in the last ten years which imagine that the extra dimensions might be big, and the reason we don’t see them is not because they’re so fantastically small, but because of the way that we see, using light and using the other forces of nature, those forces—except for gravity, it turns out—would be unable to penetrate those other dimensions. Those forces would be locked into our slice of space, our slice of bread, if you will—which is one way of thinking about it—even if there are other slices of bread in the universe, even though there are other dimensions that are off of our slice of bread that fill out the entire loaf. We wouldn’t have access to those dimensions because of the way the forces we have access to behave.

But remarkably, gravity, as I mentioned, is different, and in these theories gravity can penetrate these other, larger dimensions. So again—in a completely fanciful manner—you could imagine communicating across these other dimensions by sending gravitational waves, gravitational signals through these large extra dimensions. You and I, who are held together by the familiar forces—the electromagnetic force, the nuclear forces—we couldn’t literally travel into those extra dimensions, even though they would be big, but we might be able to send signals into them, at least in principle.

How close are we to developing a teleportation device like the transporter in Star Trek?

Well, we’re pretty far. There are experiments going on today where individual particles are being teleported from one location to another. Now, this notion of “quantum teleportation,” which is what I’m referring to, is somewhat different from at least my rudimentary understanding of what the creators of Star Trek had in mind with the transporter. There, I think, the basic idea is the material that makes you up is somehow scrambled or broken up into little pieces, and it’s kind of sent through space and then reassembled at a distant location, on the surface of some distant planet. That’s not the kind of teleportation that physics seems to allow.

Instead, what happens in quantum teleportation is the object that you want to teleport is closely examined in one location, and all the information that defines that object is sent to the remote location, and that information is then used at the remote location to build what can be thought of as an exact duplicate of the object that you started with, so you might want to call that, I don’t know, “quantum Xeroxing” or “quantum faxing” or something of that nature. What makes this a little bit closer to teleportation is that you can establish that the act of measuring the original object destroys it. There’s no way that you can get at all the necessary information to rebuild it without disrupting its basic makeup to such a degree that it really wouldn’t exist any longer at the original location, so if I asked you where the object is, I think the best answer you’d give is, well, it’s at the remote location, because that’s the only object that looks like the original that I started with, since the act of measurement destroyed the original.

So that’s a version of teleportation. Again, it’s only being done with individual particles. Perhaps that will be bumped up to some collection of particles at some point, but it is utterly, utterly beyond the pale to imagine doing this kind of process with the number of particles that make up any macroscopic body like a person, or an object like a car. So I am tempted to say that we’re infinitely far away from teleportation of big objects, but that would perhaps be a little too pessimistic, but we’re nearly infinitely far away.

One of the knocks against the Star Trek transporter from a scientific perspective has always been that it violates Heisenberg’s Uncertainty Principle. Does this quantum teleportation get around that somehow?

Yeah, exactly. So the question is: How do you actually know the information about how an object is built? Because according to Heisenberg, in some sense the act of trying to measure the object affects it or changes it. You don’t learn about the object’s makeup prior to your measurement—your measurement itself impacts that answer. So the whole trick in quantum teleportation is to try to do an end run around that problem.

And the way it’s done is, you don’t actually measure the object itself directly. Instead you bring the object into contact with some other material that’s already in the teleporter, and you measure some joint features of the combined system of the object of interest and the raw material that was already there. And it turns out that with some very clever mathematical manipulations, you can get all the information you need about the object through this more indirect measurement, and that information indeed tells you about what the object was like before you did the measurement. You’re able to avoid the contamination of the measurement itself, and in that way get a pristine result regarding the informational makeup of the object, send that to the remote location, and make use of that pristine information to rebuild the object.

There have been a lot of stories in the news recently about faster-than-light neutrinos. What’s your take on that?

There aren’t any faster-than-light neutrinos, is the quick answer. You know, even when this data was first brought to the public’s attention, six months ago or so, most physicists, me included, looked at it and said, “Yeah, that would be great if it was true,” but our suspicion was that closer examination of the experiment would reveal that there was an error, or something isn’t doing what somebody thinks it is, and at the end of the day they will not stand up to close scrutiny.

And the reason for that simply was that there’s a mountain of experimental support behind Einstein’s special theory of relativity. Anything that challenges that is going to require a similar mountain of experimental support, and one single experiment suggesting that there’s a violation of the speed of light barrier is far from convincing. The interesting thing is that in the last couple of months, the experimenters have indeed found a flaw in the experiment—a faulty fiber optic cable, which they suspect is the culprit. They’re redoing the measurements, and they’ll have the data soon. But there’s already been an independent measurement done at the same location by a different group, and they have found that the neutrinos do not go faster than the speed of light. So I think that’s an idea, however exciting it might have been, that one can pretty much throw away.

There’s a novel by Gregory Benford called Timescape in which scientists use tachyons to send a message backward in time. What do you think about that idea?

Well, the theoretical science—that if you did have a tachyon, you might be able to use it to send a signal back in time—that’s pretty solid, so the basic mathematics of Einstein’s special relativity can be used to confirm that theoretical idea. The obstacle, of course, the thing that makes it so hypothetical, is do tachyons exist? Do objects that travel faster than the speed of light exist? Now, that’s why this report about neutrinos got a lot of interest from the press and from scientists, because it would be exciting. It would be something that would shake our understanding, if indeed tachyons did exist—and neutrinos going faster than the speed of light might well be candidates for that. But the important thing to stress is there’s zero evidence that tachyons exist, and zero evidence that neutrinos go faster than the speed of light and might be candidates for tachyons.

You participated in the 2011 Isaac Asimov Memorial Debate, where your colleague Dr. Jim Gates explained that his recent research leads him to wonder if we’re living in the matrix [at 1:01:30 in the video]. What did you think about that?

I have no idea. You know, Jim is a great scientist, a good friend of mine. I’ve not really followed the ideas that he’s been pursuing of late, and just don’t feel qualified to comment on it.

There was something in The Fabric of the Cosmos where you said that there’s some evidence that our universe is in some sense a 3D projection of information contained in a 2D shell surrounding the universe? What was that all about?

Well, that’s a wonderfully weird collection of ideas that go under the heading of the “holographic principle.” It’s a collection of ideas developed over the last 30 or so years, initially starting with attempts to deeply understand the physics of black holes. Black holes, we all know, are these regions where if an object falls in, it can’t get out, but the puzzle that many struggled with over the decades is, what happens to the information that an object contains when it falls into a black hole. Is it simply lost? You know, if I throw an iPad chock full of all sorts of wonderful apps and books, is all that information lost when it goes into the black hole or not? Now, Stephen Hawking believes that the information is simply lost—it falls into a black hole, gets trapped inside, you’ll never see it again, and that’s that.

The problem is, there’s a pretty basic law of physics that convinces us that information can’t be destroyed. It can be scrambled, it can be transmuted, but ultimately it can’t be destroyed. And black holes seem to be flying in the face of that, and because of that tension a number of physicists—people like Leonard Susskind, Gerard ’t Hooft, others—they tried to see whether the information might not really be lost.

And over the course of many years, they developed an idea that when an object falls into a black hole, yes indeed, it falls in, but a copy of all of its information content gets in some sense “smeared out” on the surface of the black hole, on the horizon of the black hole. Smeared out in some sense like a series of 0s and 1s, the way information is stored in a typical computer. And that idea would suggest that a three-dimensional object inside the black hole could be described by information on a two-dimensional surface that surrounds the black hole.

And it was a few years ago that string theory—the field that I work on—gave really strong evidence to many of us that this idea really might be correct. Now, the reason why that’s particularly interesting is because the space inside a black hole is not really fundamentally different—it isn’t governed by different laws than space outside a black hole, or space anywhere else, for that matter. So if we learn, as we seem to have, that a 3D object inside a black hole can be described by 2D information on a surface that surrounds it, that lesson should be quite general. Which means that 3D objects, even the ones that we’re familiar with—you and me and everything around us—may indeed be describable by information on a 2D surface that surrounds us, a surface that in some sense is at the edge of the universe. Now, this starts to sound like a hologram: a thin 2D piece of plastic which, when illuminated correctly, yields a realistic three-dimensional image. The idea is, we may be that three-dimensional image of this more fundamental information on the 2D surface that surrounds us.

Now, let me just point out, this is a hard idea even for physicists who work on it every day to fully grasp. We’re still trying to really dot the i’s and cross the t’s and understand in detail what this would mean. But there are many who now take this idea very seriously, that we may be a kind of holographic projection.

The World Science Festival is coming up at the end of the month. Do you want to tell us a bit about that?

Sure. The World Science Festival is an event that we hold each year in New York at the end of May. This year it’s May 30th to June 3rd. And the idea is to have a whole collection of programs for kids to adults, from those who know a lot about science to those who don’t, on subjects from cosmology to quantum physics to neuroscience, to sustainability, to issues in psychology, to issues having to do with pandemics and vaccines. I mean, a whole range of science where people can just come and get totally immersed and absorbed and excited by what’s happening at the cutting edge of research.

And our point in this event is to take science out of the classroom, where for many people it’s kind of a boring, dull, drab subject, and to bring the public face to face with the scientists that are pushing the envelope, where they can really experience the drama and wonder of discovery. So if anybody reading this will be in the New York area in that time period, go to, see the wonderful spectrum of programs that are available, and come down and just immerse yourself for a few days in what I consider the greatest drama of the human species—scientific discovery.

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The Geek’s Guide to the Galaxy

The Geek's Guide to the Galaxy

The Geek’s Guide to the Galaxy is a science fiction/fantasy talk show podcast. It is produced by John Joseph Adams and hosted by: David Barr Kirtley, who is the author of thirty short stories, which have appeared in magazines such as Realms of Fantasy, Weird Tales, and Lightspeed, in books such as Armored, The Living Dead, Other Worlds Than These, and Fantasy: The Best of the Year, and on podcasts such as Escape Pod and Pseudopod. He lives in New York.