Earth remained habitable for billions of years because of good luck

“…People have the idea that the universe started in a special, ordered state, and it's been getting disordered ever since then. We're suggesting it's completely the other way around. The universe, in our view, starts in the most disordered way possible and, at least up to now, it's been getting ever more interesting. This, on the face of it, looks like a more positive view of the world. It's not quite like that awful image of the heat death. Now it may be, nevertheless, that the universe in some senses will die, but in a very beautiful form. This is fairly speculative, but certainly we are suggesting that the universe, up to now at least, is not getting more disordered; it's getting more interesting and more structured, and this corresponds to what we see. Certainly, near the Big Bang you and I couldn't be talking to each other the way we are now. Here we are with modern technology talking to each other in London. It's a pretty amazing universe we live in, and my colloaborators and I think that we possibly have the underlying explanation of why that can be. It lies in certain mathematical theorems, which in the past, when you had to have steam in a box, led to the steam, if it was originally in a small corner of the box, spreading out and becoming very uniformly distributed in the box. You would have what is called equilibration taking place. However, if there's no box, the system can spread out, and as it does that, it can take a much more interesting shape. It's that removal of the box that seems to suggest why the universe is so extraordinarily interesting. This is potentially a totally different way of thinking about things. It's all to do with saying that there isn't a box there at all. There's a nice analogy with this back when Kepler was trying to work out what the planets did. He thought about certain observations that Tycho Brahe had made of a comet in 1577. Brahe had shown that the comet was very far away. It couldn't be up in the sky the way people had thought comets were meteorological things in the atmosphere of the Earth because Brahe had showed it was fairly far away. Kepler went further in saying it was very far away, and it must have gone clean through the crystal spheres, which people thought carried the planets around in the sky. Kepler said, "Henceforth the planets must find their way through the void, like the birds through the air. We must philosophize about these things differently."And what I'm suggesting is that, quite amazingly, all that we have thought about entropy and how disorder grows has all been because people have been thinking in terms of everything being in a box. If the universe is not in a box, just like Kepler said, we must philosophize about things differently. That changes things totally….”

Julian Barbour, "The Universe Is Not In A Box,"Edge, September 11, 2019, https://www.edge.org/conversation/julian_barbour-the-universe-is-not-in-a-box

The Universe Is Not in a Box

One of the great books in science was published in 1824 by a young Frenchman called Sadi Carnot. It is one of the most wonderful books, the title of which is Reflections on the Motive Power of Fire. In about six pages, Carnot explains how you would make a steam engine that would work with the absolute maximum efficiency possible. It was almost entirely ignored, and he died before anything much could come out of it. It was rediscovered in 1849 when William Thomson, who later became Lord Kelvin, wrote a paper that publicized this work. Within a couple of years, thermodynamics had been created as a science.

It caused a tremendous lot of excitement from the 1850s onwards. The key thing about this work of Carnot's is that if you have a steam engine, the steam has to remain in a cylinder in a box. You want the steam engine to work continuously, so you keep on having to bring the steam and the cylinder back to the condition it was before. It's remarkable that the development of what's called statistical mechanics—to understand how steam behaves—led to the discovery of entropy, one of the great discoveries in the history of science, and with it the mystery of the arrow of time. And it all followed out of this work of Carnot on how steam engines work. And moreover, it was very anthropocentric thinking about how human beings could exploit coal to drive steam engines and do work for them. At that stage, nobody was thinking about the universe as a whole; they were just thinking about how they could make steam engines work better.

This way of thinking, I believe, has survived more or less unchanged to this day. You still find that people who work on this problem of the arrow of time are still assuming conditions that are appropriate for a steam engine. But in the 1920s and early 1930s, Hubble showed that the universe was expanding, that we live in an expanding universe. Is that going to be well modeled by steam in a box? My belief is that people haven't realized that we have to think out of the box. We have to think in different ways. My collaborators and I keep on finding ways in which the mathematics that was developed before to understand systems confined in a box have to be modified with quite surprising consequences and, above all, possibly to explain why we have an incredibly powerful sense of the passage of time, why the past is so different from the future.

THE UNIVERSE IS NOT IN A BOX

I'm asking questions that I've been asking myself now for a very long time. Basically, what is time? What is motion? Even more so, why do we have such a strong sense of moving forward in time, and that things in many ways are getting more and more interesting in the universe?

This has been a puzzle for a long time in science. All the laws of physics suggest there's no distinguished direction of time. The laws of physics work in exactly the same way. For example, if you film two billiard balls colliding and run it backwards, it looks exactly the same. However, you take a film of someone diving into a swimming pool and run that back, it looks completely different. It's been an issue now for about 170 years—why that can be if the laws suggest it should be exactly the same.

About seven years ago, I got an idea that might solve that problem, which came out of more fundamental questions I'd been asking. Just how do you define distance? How do you define motion? We always see things relative to other objects in the universe. Newton had introduced this concept called absolute space. It's a little bit like a room with the walls taken away. In this room where I'm sitting with you now, my position relative to the chair and the walls is perfectly clear. You can see that. Newton somehow imagined that we move in a space, which is as if the room was there but the walls have somehow disappeared. This has been a mystery in physics for a very long time, and it was the stimulus that led Einstein to create his general theory of relativity, which, in the end, finished up in rather a muddled state because Einstein didn't try and do it directly; he worked indirectly.

I'd never forgotten this issue of why things seem to unfold in one direction when the laws that Newton first found, and then Einstein found, and more recent laws of particle physics all suggest that you can run things in either direction of time and it should look the same. As I said, when you run a film backwards of two billiard balls colliding, it looks exactly the same, but when you run a film of somebody diving into a swimming pool backwards, it looks totally different. This fundamental fact was noticed in a paper in 1852 by William Thomson, who later became Lord Kelvin. He published a short paper, the title of which was "On a Universal Tendency in Nature to the Dissipation of Mechanical Energy."

Everything seems to be running down, and we see this everywhere we look. We all get older in the same direction. We never meet anyone getting younger. Moreover, all the billions upon billions of stars that the astronomers see in the sky are all getting older in the same direction. Their ageing process is very well understood. Where does this fantastic sense of direction come from if you can't see it in the underlying laws?

Quite by chance, an idea came to me about seven years ago. There was a famous bit of work done in 1772 by the great Italian-French mathematician Joseph Lagrange, who was studying the Earth-Moon-Sun system. That was a problem that gave Newton headaches. Lagrange made the interesting discovery that if the system has either zero energy or positive energy—so, it's not a bound system like the solar system where the planets can never escape from the sun—as time goes on, the size of the system in both directions of time, both to the past and the future, will grow without limit. It will go up to infinity in both directions, and there's just one unique point where the size is at its minimum. I suddenly thought, well, maybe that's something to do with the problem of where the arrow of time comes from. Why? Because if you start from the central point, the system gets bigger and, under gravity, more structured. It's very uniform at the point of minimal size, and as you go away from it in one direction, it gets more structured. And as you go in the other direction, it also gets more structured. This applies not only to the three particles that Lagrange was studying, but any number of particles you have.

It occurred to me that if you're thinking about a system of objects in the real universe, the whole of the background universe defines the direction of time for you. And you just say, well, this system went through minimal size and then it grew again. That was the past, and that's the future. But if you're saying this is a toy model of the whole universe, that background arrow, with which we're all so familiar, just isn't there. This situation in which I have a uniform distribution of particles and in both directions away from it a more clustered, more structured distribution, if I was to show that to my grandson who's nearly five and say, "If you were to point to interesting things that are happening, which direction would they go?" He would say, "One arrow goes from here in that direction, and one goes in the other direction."

This then gave me the idea of a Janus point, from the Roman god Janus who looks in two opposite directions at once. If this was just my model universe and I was asked to say in which direction time flows, I would have to say there are two directions from that central Janus point. There's one arrow of time going this way and one arrow going that way. If we had observers inside this universe, they would think, well, maybe this is the beginning of time back here, this Janus point, and we're going forward to the future away from it. If they could look backwards, they would see confusion here where everything is rather uniform—a bit like a swarm of bees that are uniformly distributed and moving in all sorts of different directions at once. You can do the calculations on the computer and show this happening. Then there would be another universe on the other side where time is going in the other direction, but the people on this side couldn't see what's happening on that side and vice versa.

This has the potential to completely resolve the mystery of how the laws can be symmetric, but you can be in a situation where you only see asymmetric things happening, where there's a very pronounced sense of direction. The overall solution in this case is completely symmetric. The two halves are qualitatively similar; they differ in details, but they're basically the same. So, the solution is symmetric, but because observers can only be on one side or the other, they see things very asymmetrically. There's a profound sense of the direction of time on either side of the Janus point, although the symmetry of the law and the symmetry of the solution are respected. That was the idea that occurred to me back in 2012, and I've been developing that with collaborators since then.

Back in 1999, The End of Time came out, which was very much concerned with the same issue of where our sense of coming from a past and going to a future comes from. At that stage, I was still thinking that size had some real objective meaning. It's quite clear as I sit in this room that I'm smaller than the room, and you can see that because you can see the background of the walls of the room. Size is always relative to something else. My hands are basically the same size, but my fingers are shorter than my complete hand. Science should always be about ratios, particularly if you're thinking about the whole universe.

Twenty years ago, I was just beginning to develop these ideas that it's only ratios that count, and that overall size of the complete universe may be a dangerous concept. I was just beginning to develop these ideas in The End of Time. There are just three pages where I talk about shape space, and I can illustrate that idea quite easily. Think of a triangle, the simplest nontrivial geometrical object. It's amazing how much you can get out of a triangle. A triangle has a shape and a size, and in many ways the shape is clearly much more fundamental. Suppose I hold an equilateral triangle up in front of my face and move it backwards and forwards, the shape doesn't change, but the size does as the image is projected onto the retina of my eyes. I started then to think that it would be better just to think about the shape of the universe and how that changes, and not worry so much about the size.

In the course of developing these ideas, I had this new insight about how the universe might have a minimum size and grow either side of it and become more structured. One could characterize this change of structure in a way that tells you how the shape is changing, not how the size is changing. This has led to quite an interesting way of thinking about Einstein's great theory of general relativity, which is talking not about space-time and how the curvature of space and time changes, but how the shape of the universe at any instant changes. My collaborators and I call this shape dynamics. In many ways, it's a more fundamental way of looking at general relativity. It's removing all the parts of this wonderful theory that are not absolutely essential and leaving all the bits that you must have, otherwise it would fall apart. You've always got to have three sides of a triangle; if you take one of them away, you've lost your triangle. It's gone. The shape is absolutely minimum. You've got to have the three sides or two internal angles, and this is boiling things down to the absolute bare minimum.

Surprisingly, this hasn't been done in science before, in thinking about the universe. It's not been a way that people have thought about it. Though there are some important parts of general relativity that do rely upon that. I was lucky to work for several years with someone who had done important work on that. In fact, all the work that's now done with predicting what gravitational waves would look like when two black holes circle around each other and then collide and give off gravitational waves—all those predictions couldn't be done without this work, which is really talking about how the shape of the universe changes in accordance with Einstein's theory. Hidden inside that wonderful theory of Einstein's, there is a theory of shape dynamics, which I hope we have brought out.

An increasing number of people are taking that seriously, so this has been an important input in my story. Quite a lot of the ideas that I had in my book from twenty years ago have survived, with the redundant elements taken out. The most important ideas have survived. And this first step to thinking about the shape of the universes was already in the book then.

The End of Time made certain predictions that required very difficult mathematics to be done. In a way, that book of mine was based much more on intuition than the present ideas I've got, which do have solid mathematical results behind them in a way that the other book didn't. I'm very pleased about that. The starting point of the new ideas is a very important result in Newton's theory, which is now 250 years old, and it's a very solid result. It's not so much something being disproved by an observation as mathematical results being brought forward, which are much more secure and on a sure basis.

If we are on a promising new direction, we may be able to make predictions about the universe, which nobody has thought that the universe would look like. Maybe if things work out well in twenty years from now, a space probe might be sent up to make observations and confirm these things. Meanwhile, it looks as if the Janus-point idea has the potential to solve a very longstanding problem.

I've discussed this with some leading physicists. None of them have found anything wrong with it. The first paper was published in Physical Review Letters back in 2014, which is the top physics journal in the world. The editors must've been quite worried about our paper because we were making fairly big claims. They sent it to five referees, three of which said it was very interesting and definitely worth publishing. One referee just said it was wrong and can't be published, but didn't give any reason. And the fourth one was skeptical, but we won that referee round. Then the editors chose the paper as "Editors' Choice." They had a comment piece written about it by a well-known expert in quantum gravity. It attracted a lot of attention online, and there were a number of features about it, so it went down well. We've given talks at many seminars, and nobody yet has come up with a flaw.

I've got three main collaborators now. One is an Englishman, one is a German, and one is an Italian. The Englishman is David Sloan, who's a cosmologist and has also worked in quantum gravity with Abhay Ashtekar, who's fairly well-known in the quantum gravity field. Then there's Tim Koslowski, who also has a background in quantum gravity and was at the Perimeter Institute for several years. He's now in Germany. Then there's Flavio Mercati, the Italian.

One of the great books in science was published in 1824 by a young Frenchman called Sadi Carnot. It is one of the most wonderful books, the title of which is Reflections on the Motive Power of Fire. In about six pages, Carnot explains how you would make a steam engine that would work with the absolute maximum efficiency possible. It was almost entirely ignored, and he died before anything much could come out of it. It was rediscovered in 1849 when William Thomson, who later became Lord Kelvin, wrote a paper that publicized this work. Within a couple of years, thermodynamics had been created as a science.

It caused a tremendous lot of excitement from the 1850s onwards. The key thing about this work of Carnot's is that if you have a steam engine, the steam has to remain in a cylinder in a box. You want the steam engine to work continuously, so you keep on having to bring the steam and the cylinder back to the condition it was before. It's remarkable that the development of what's called statistical mechanics—to understand how steam behaves—led to the discovery of entropy, one of the great discoveries in the history of science, and with it the mystery of the arrow of time. And it all followed out of this work of Carnot on how steam engines work. And moreover, it was very anthropocentric thinking about how human beings could exploit coal to drive steam engines and do work for them. At that stage, nobody was thinking about the universe as a whole; they were just thinking about how they could make steam engines work better.

This way of thinking, I believe, has survived more or less unchanged to this day. You still find that people who work on this problem of the arrow of time are still assuming conditions that are appropriate for a steam engine. But in the 1920s and early 1930s, Hubble showed that the universe was expanding, that we live in an expanding universe. Is that going to be well modeled by steam in a box? My belief is that people haven't realized that we have to think out of the box. We have to think in different ways. My collaborators and I keep on finding ways in which the mathematics that was developed before to understand systems confined in a box have to be modified with quite surprising consequences and, above all, possibly to explain why we have an incredibly powerful sense of the passage of time, why the past is so different from the future.

In the modern age, certainly in physics and cosmology, there's tremendous competition for ideas. It's quite difficult for people to notice new ideas often because each scientist is working on his/her own idea, so they generally don't look at other things. There are certainly a lot of problems with confirming the speculative ideas that theoreticians in string theory and loop quantum gravity have been trying to develop.

Where things may become very interesting but will take time is from astronomical observations. It is amazing the new instruments that are being developed. Only a few days ago we had the results of the Event Horizon Telescope, which has shown us what a black hole looks like. That was a major project to get going. These things that rely on observations of the whole universe can take up to a couple of decades or longer from the first plan through to either building a huge telescope in Northern Chile in the Atacama Desert or putting a telescope in space. It takes a tremendous amount of time.

It's getting incredibly expensive and difficult now to go beyond this great discovery of the Higgs boson at the Large Hadron collider in Geneva. These take a lot of time. People are a bit discouraged by that. I'm hoping we might be able to find something that could be tested. Maybe the material is already there, it just needs to be looked at in a different way. Or maybe the Janus-point idea can suggest a new experiment that could be made.

The idea we have that the history of the universe consists of two parts where what we normally think of as the Big Bang, the start of time, is just a middle point in the timeline of the universe with two arrows of time pointing away from it in opposite directions, suggests that we should be able to say something rather precise about what the conditions are like at that critical central point. This would, say, give an idea of how the universe started in a way that's more precise than now, which then might lead to predictions as to what the universe should look like now. That's a hope that we're working on. We did have some ideas about that which are not totally discouraging, but it's a long way to develop them.

There is a theory called inflation, which explains the structure of the universe now. It's very widely accepted by cosmologists, but it's not without its difficulties. It's a little bit ad hoc. It makes one wonderful prediction, but a lot of other ones you have to know what the answer is and then adjust the theory to make sure it gives that. It's possible that the ideas we're developing could solve some of those problems for inflation, or show that inflation is not necessary at all. That would be definitely an interesting development. It probably wouldn't be welcomed by the people who work on inflation, but there are quite a lot of people who distrust inflation. One of the problems that people who do like inflation admit is that it doesn't have a rival theory at the moment. If we were able to develop some serious alternative to it, people would welcome that because it's never a healthy situation where you have just one theory and that theory isn't 100% predictive. It makes one prediction, which is confirmed very well indeed, but there are a lot of other things it can't predict and you have to put in by hand. People are not totally happy with inflation.

One of the things that is most interesting about this is how it completely changes the way people think about the universe ever since the law of entropy increase was discovered in the 19th century as a result of the work that Sadi Carnot had originally done on the steam engine. Then there was a great German scientist called Rudolf Clausius. He was the person who developed the notion of entropy and then said that the entropy of the universe tends to a maximum, and this also matched the idea of the heat death of the universe, that the universe is going to be a completely bleak future with nothing interesting there. We could completely change that by saying that there's a lot of evidence the universe has been getting a much more interesting shape.

This is maybe because of the anthropocentric way people thought about the steam engine. The steam engine was doing work for human beings. What may not have been noticed is that the steam engine was making the universe into a more interesting shape, a more interesting story. There's a very famous image of the evolution of the universe, which NASA has put out, showing how it starts from a mysterious quantum Big Bang, then there's an inflationary period, then stars and galaxies form, and finally you get life forming on the Earth and ever more detailed things happening. This goes completely counter to the idea that the entropy, which means the disorder of the universe, is increasing from the Big Bang to now.

People have the idea that the universe started in a special, ordered state, and it's been getting disordered ever since then. We're suggesting it's completely the other way around. The universe, in our view, starts in the most disordered way possible and, at least up to now, it's been getting ever more interesting. This, on the face of it, looks like a more positive view of the world. It's not quite like that awful image of the heat death. Now it may be, nevertheless, that the universe in some senses will die, but in a very beautiful form. This is fairly speculative, but certainly we are suggesting that the universe, up to now at least, is not getting more disordered; it's getting more interesting and more structured, and this corresponds to what we see.

Certainly, near the Big Bang you and I couldn't be talking to each other the way we are now. Here we are with modern technology talking to each other in London. It's a pretty amazing universe we live in, and my colloaborators and I think that we possibly have the underlying explanation of why that can be. It lies in certain mathematical theorems, which in the past, when you had to have steam in a box, led to the steam, if it was originally in a small corner of the box, spreading out and becoming very uniformly distributed in the box. You would have what is called equilibration taking place. However, if there's no box, the system can spread out, and as it does that, it can take a much more interesting shape.

It's that removal of the box that seems to suggest why the universe is so extraordinarily interesting. This is potentially a totally different way of thinking about things. It's all to do with saying that there isn't a box there at all.

There's a nice analogy with this back when Kepler was trying to work out what the planets did. He thought about certain observations that Tycho Brahe had made of a comet in 1577. Brahe had shown that the comet was very far away. It couldn't be up in the sky the way people had thought comets were meteorological things in the atmosphere of the Earth because Brahe had showed it was fairly far away. Kepler went further in saying it was very far away, and it must have gone clean through the crystal spheres, which people thought carried the planets around in the sky. Kepler said, "Henceforth the planets must find their way through the void, like the birds through the air. We must philosophize about these things differently." And what I'm suggesting is that, quite amazingly, all that we have thought about entropy and how disorder grows has all been because people have been thinking in terms of everything being in a box. If the universe is not in a box, just like Kepler said, we must philosophize about things differently. That changes things totally.

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

might have to retire my "entropy" quip

"...In the new story of the arrow of time, it is the loss of information through quantum entanglement, rather than a subjective lack of human knowledge, that drives a cup of coffee into equilibrium with the surrounding room. The room eventually equilibrates with the outside environment, and the environment drifts even more slowly toward equilibrium with the rest of the universe. The giants of 19th century thermodynamics viewed this process as a gradual dispersal of energy that increases the overall entropy, or disorder, of the universe. Today, Lloyd, Popescu and others in their field see the arrow of time differently. In their view, information becomes increasingly diffuse, but it never disappears completely. So, they assert, although entropy increases locally,the overall entropy of the universe stays constant at zero.“The universe as a whole is in a pure state,”Lloyd said. “But individual pieces of it, because they are entangled with the rest of the universe, are in mixtures.”..."

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“We can discuss the fact that an hour ago, our brains were in a state that was correlated with fewer things,” he said. “But our perception that time is flowing — that is a different matter altogether. Most probably, we will need a further revolution in physics that will tell us about that.”

The backdrop for the steady growth of entanglement throughout the universe is, of course, time itself. The physicists stress that despite great advances in understanding how changes in time occur, they have made no progress in uncovering the nature of time itself or why it seems different (both perceptually and in the equations of quantum mechanics) than the three dimensions of space. Popescu calls this “one of the greatest unknowns in physics.”

According to the scientists, our ability to remember the past but not the future, another historically confounding manifestation of time’s arrow, can also be understood as a buildup of correlations between interacting particles. When you read a message on a piece of paper, your brain becomes correlated with it through the photons that reach your eyes. Only from that moment on will you be capable of remembering what the message says. As Lloyd put it: “The present can be defined by the process of becoming correlated with our surroundings.”

Not to mention a bit of philosophy.

Twenty-six years after Lloyd’s big idea about time’s arrow fell flat, he is pleased to be witnessing its rise and has been applying the ideas in recent work on theblack hole information paradox. “I think now the consensus would be that there is physics in this,” he said.

Sean Carroll, a theoretical cosmologist at the California Institute of Technology, is employing the new formalism in his latest work on time’s arrow in cosmology. “I’m interested in the ultra-long-term fate of cosmological space-times,” said Carroll, author of “From Eternity to Here: The Quest for the Ultimate Theory of Time.” “That’s a situation where we don’t really know all of the relevant laws of physics, so it makes sense to think on a very abstract level, which is why I found this basic quantum-mechanical treatment useful.”

“We’ve been thinking more and more about what we can do with quantum machines,” saidPaul Skrzypczykof the Institute of Photonic Sciences in Barcelona. “Given that a system is not yet at equilibrium, we want to get work out of it. How muchuseful work can we extract? How can I intervene to do something interesting?”

Some researchers expressed doubt that this abstract approach to thermodynamics will ever be up to the task of addressing the “hard nitty-gritty of how specific observables behave,” as Lloyd put it. But the conceptual advance and new mathematical formalism is already helping researchers address theoretical questions about thermodynamics, such as the fundamental limits of quantum computers and even the ultimate fate of the universe.

Despite the recent progress in calculating equilibration time scales, the new approach has yet to make headway as a tool for parsing the thermodynamic properties of specific things, like coffee, glass or exotic states of matter. (Several traditional thermodynamicists reported being only vaguely aware of the new approach.) “The thing is to find the criteria for which things behave like window glass and which things behave like a cup of tea,” Renner said. “I would see the new papers as a step in this direction, but much more needs to be done.”

One aspect of time’s arrow remains unsolved. “There is nothing in these works to say why you started at the gate,” Popescu said, referring to the park analogy. “In other words, they don’t explain why the initial state of the universe was far from equilibrium.” He said this is a question about the nature of the Big Bang.

“The universe as a whole is in a pure state,” Lloyd said. “But individual pieces of it, because they are entangled with the rest of the universe, are in mixtures.”

In the new story of the arrow of time, it is the loss of information through quantum entanglement, rather than a subjective lack of human knowledge, that drives a cup of coffee into equilibrium with the surrounding room. The room eventually equilibrates with the outside environment, and the environment drifts even more slowly toward equilibrium with the rest of the universe. The giants of 19th century thermodynamics viewed this process as a gradual dispersal of energy that increases the overall entropy, or disorder, of the universe. Today, Lloyd, Popescu and others in their field see the arrow of time differently. In their view, information becomes increasingly diffuse, but it never disappears completely. So, they assert, although entropy increases locally, the overall entropy of the universe stays constant at zero.

Consequently, a tepid cup of coffee does not spontaneously warm up. In principle, as the pure state of the room evolves, the coffee could suddenly become unmixed from the air and enter a pure state of its own. But there are so many more mixed states than pure states available to the coffee that this practically never happens — one would have to outlive the universe to witness it. This statistical unlikelihood gives time’s arrow the appearance of irreversibility. “Essentially entanglement opens a very large space for you,” Popescu said. “It’s like you are at the park and you start next to the gate, far from equilibrium. Then you enter and you have this enormous place and you get lost in it. And you never come back to the gate.”

“When Lloyd proposed the idea in his thesis, the world was not ready,” saidRenato Renner, head of the Institute for Theoretical Physics at ETH Zurich. “No one understood it. Sometimes you have to have the idea at the right time.”

In 2009, the Bristol group’s proof resonated with quantum information theorists, opening up new uses for their techniques. It showed that as objects interact with their surroundings — as the particles in a cup of coffee collide with the air, for example — information about their properties “leaks out and becomes smeared over the entire environment,” Popescu explained. This local information loss causes the state of the coffee to stagnate even as the pure state of the entire room continues to evolve. Except for rare, random fluctuations, he said, “its state stops changing in time.”

Advances in quantum computing have since turned quantum information theory into one of the most active branches of physics. Lloyd is now a professor at the Massachusetts Institute of Technology, recognized as one of the founders of the discipline, and his overlooked idea has resurfaced in a stronger form in the hands of the Bristol physicists. The newer proofs are more general, researchers say, and hold for virtually any quantum system.

“I was darn close to driving a taxicab,” Lloyd said.

The idea, presented in his 1988 doctoral thesis, fell on deaf ears. When he submitted it to a journal, he was told that there was “no physics in this paper.” Quantum information theory “was profoundly unpopular” at the time, Lloyd said, and questions about time’s arrow “were for crackpots and Nobel laureates who have gone soft in the head.” he remembers one physicist telling him.

“What’s really going on is things are becoming more correlated with each other,” Lloyd recalls realizing. “The arrow of time is an arrow of increasing correlations.”

Using an obscure approach to quantum mechanics that treated units of information as its basic building blocks, Lloyd spent several years studying the evolution of particles in terms of shuffling 1s and 0s. He found that as the particles became increasingly entangled with one another, the information that originally described them (a “1” for clockwise spin and a “0” for counterclockwise, for example) would shift to describe the system of entangled particles as a whole. It was as though the particles gradually lost their individual autonomy and became pawns of the collective state. Eventually, the correlations contained all the information, and the individual particles contained none. At that point, Lloyd discovered, particles arrived at a state of equilibrium, and their states stopped changing, like coffee that has cooled to room temperature.

The idea that entanglement might explain the arrow of time first occurred toSeth Lloydabout 30 years ago, when he was a 23-year-old philosophy graduate student at Cambridge University with a Harvard physics degree. Lloyd realized that quantum uncertainty, and the way it spreads as particles become increasingly entangled, could replace human uncertainty in the old classical proofs as the true source of the arrow of time.

<img class="block fit-x fill-h fill-v is-loaded mxa" alt="Seth Lloyd, now an MIT professor, came up with the idea that entanglement might explain the arrow of time while he was in graduate school at Cambridge University in the 1980s." src="https://d2r55xnwy6nx47.cloudfront.net/uploads/2014/04/Seth-Lloyd2-340x520.jpg

“Entanglement is in some sense the essence of quantum mechanics,” or the laws governing interactions on the subatomic scale, Brunner said. The phenomenon underlies quantum computing, quantum cryptography and quantum teleportation.

When two particles interact, they can no longer even be described by their own, independently evolving probabilities, called “pure states.” Instead, they become entangled components of a more complicated probability distribution that describes both particles together. It might dictate, for example, that the particles spin in opposite directions. The system as a whole is in a pure state, but the state of each individual particle is “mixed” with that of its acquaintance. The two could travel light-years apart, and the spin of each would remain correlated with that of the other, a feature Albert Einstein famously described as “spooky action at a distance.”

Quantum uncertainty then gives rise to entanglement, the putative source of the arrow of time.

If the new line of research is correct, then the story of time’s arrow begins with the quantum mechanical idea that, deep down, nature is inherently uncertain. An elementary particle lacks definite physical properties and is defined only by probabilities of being in various states. For example, at a particular moment, a particle might have a 50 percent chance of spinning clockwise and a 50 percent chance of spinning counterclockwise.An experimentally tested theoremby the Northern Irish physicist John Bell says there is no “true” state of the particle; the probabilities are the only reality that can be ascribed to it.

The tendency of coffee — and everything else — to reach equilibrium is “very intuitive,” saidNicolas Brunner, a quantum physicist at the University of Geneva. “But when it comes to explaining why it happens, this is the first time it has been derived on firm grounds by considering a microscopic theory.”

“Finally, we can understand why a cup of coffee equilibrates in a room,” saidTony Short, a quantum physicist at Bristol. “Entanglement builds up between the state of the coffee cup and the state of the room.”

Popescu, Short and their colleaguesNoah LindenandAndreas Winterreported the discoveryin the journal Physical Review E in 2009, arguing that objects reach equilibrium, or a state of uniform energy distribution, within an infinite amount of time by becoming quantum mechanically entangled with their surroundings.Similar resultsbyPeter Reimannof the University of Bielefeld in Germany appeared several months earlier in Physical Review Letters. Short and a collaboratorstrengthened the argumentin 2012 by showing that entanglement causes equilibration within a finite time. And, in work that was posted on the scientific preprint site arXiv.org in February, two separate groups have taken thenextstep, calculating that most physical systems equilibrate rapidly, on time scales proportional to their size. “To show that it’s relevant to our actual physical world, the processes have to be happening on reasonable time scales,” Short said.

Now, physicists are unmasking a more fundamental source for the arrow of time: Energy disperses and objects equilibrate, they say, because of the way elementary particles become intertwined when they interact — a strange effect called “quantum entanglement.”

Surely, he said, time’s arrow is not steered by human ignorance. And yet, since the birth of thermodynamics in the 1850s, the only known approach for calculating the spread of energy was to formulate statistical distributions of the unknown trajectories of particles, and show that, over time, the ignorance smeared things out.

“In classical physics, we were struggling,” saidSandu Popescu, a professor of physics at the University of Bristol in the United Kingdom. “If I knew more, could I reverse the event, put together all the molecules of the egg that broke? Why am I relevant?”

But to the bafflement of generations of physicists, the arrow of time does not seem to follow from the underlying laws of physics, which work the same going forward in time as in reverse. By those laws, it seemed that if someone knew the paths of all the particles in the universe and flipped them around, energy would accumulate rather than disperse: Tepid coffee would spontaneously heat up, buildings would rise from their rubble and sunlight would slink back into the sun.

Coffee cools, buildings crumble, eggs break and stars fizzle out in a universe that seems destined to degrade into a state of uniform drabness known as thermal equilibrium. The astronomer-philosopher Sir Arthur Eddington in 1927 cited the gradual dispersal of energy as evidence of an irreversible “arrow of time.”

Natalie Wolchover, "Time’s Arrow Traced to Quantum Source,"Quanta Magazine, April 16, 2014, https://www.quantamagazine.org/quantum-entanglement-drives-the-arrow-of-time-scientists-say-20140416/ Time’s Arrow Traced to Quantum Source

"...where is everybody?" Tim Urban, “The Fermi Paradox,” Wait But Why, May 21, 2014, https://waitbutwhy.com/2014/05/fermi-paradox.html The Fermi Paradox

There’s also a debate over what percentage of those sun-like stars might be orbited by an Earth-like planet (one with similar temperature conditions that could have liquid water and potentially support life similar to that on Earth). Some say it’s as high as 50%, but let’s go with the more conservative 22% that came out ofa recent PNAS study. That suggests that there’s a potentially-habitable Earth-like planet orbiting at least 1% of the total stars in the universe—a total of100 billion billion Earth-like planets.

So there are100 Earth-like planetsfor every grain of sand in the world. Think about that next time you’re on the beach.

Moving forward, we have no choice but to get completely speculative. Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life (if that’s true, every grain of sand would represent one planet with life on it). And imagine that on 1% ofthoseplanets, the life advances to an intelligent level like it did here on Earth. That would mean there were 10 quadrillion, or10 million billion intelligent civilizations in the observable universe.

Moving back to just our galaxy, and doing the same math on the lowest estimate for stars in the Milky Way (100 billion), we’d estimate that there are1 billion Earth-like planets and 100,000 intelligent civilizations in our galaxy.The Drake Equationprovides a formal method for this narrowing-down process we’re doing.

SETI (Search for Extraterrestrial Intelligence) is an organization dedicated to listening for signals from other intelligent life. If we’re right that there are 100,000 or more intelligent civilizations in our galaxy, and even a fraction of them are sending out radio waves or laser beams or other modes of attempting to contact others, shouldn’t SETI’ssatellite arraypick up all kinds of signals?

But it hasn’t. Not one. Ever.

Where is everybody?

It gets stranger. Our sun is relatively young in the lifespan of the universe. There are far older stars with far older Earth-like planets, which should in theory mean civilizations far more advanced than our own. As an example, let’s compare our 4.54 billion-year-old Earth to a hypothetical 8 billion-year-old Planet X.

If Planet X has a similar story to Earth, let’s look at where their civilization would be today (using the orange timespan as a reference to show how huge the green timespan is):

The technology and knowledge of a civilization only 1,000 years ahead of us could be as shocking to us as our world would be to a medieval person. A civilization 1 million years ahead of us might be as incomprehensible to us as human culture is to chimpanzees. And Planet X is 3.4billionyears ahead of us…

There’s something calledThe Kardashev Scale, which helps us group intelligent civilizations into three broad categories by the amount of energy they use:

AType I Civilizationhas the ability to use all of the energy on theirplanet. We’re not quite a Type I Civilization, but we’re close (Carl Sagan created a formula for this scale which puts us at a Type 0.7 Civilization).

AType II Civilizationcan harness all of the energy of theirhost star. Our feeble Type I brains can hardly imagine how someone would do this, but we’ve tried our best, imagining things like aDyson Sphere.

AType III Civilizationblows the other two away, accessing power comparable to that ofthe entire Milky Way galaxy.

If this level of advancement sounds hard to believe, remember Planet X above and their 3.4billionyears of further development. If a civilization on Planet X were similar to ours and were able to survive all the way to Type III level, the natural thought is that they’d probably have mastered inter-stellar travel by now, possibly even colonizing the entire galaxy.

One hypothesis as to how galactic colonization could happen is by creatingmachinerythat can travel to other planets, spend 500 years or so self-replicating using the raw materials on their new planet, and then send two replicas off to do the same thing. Even without traveling anywhere near the speed of light, this process would colonize the whole galaxy in 3.75 million years, a relative blink of an eye when talking in the scale of billions of years:

Source:Scientific American: “Where Are They”

Continuing to speculate, if 1% of intelligent life survives long enough to become a potentially galaxy-colonizing Type III Civilization, our calculations above suggest that there should be at least 1,000 Type III Civilizations in our galaxy alone—and given the power of such a civilization, their presence would likely be pretty noticeable. And yet, we see nothing, hear nothing, and we’re visited by no one.

So where is everybody?

_____________________

Welcome to the Fermi Paradox.

We have no answer to the Fermi Paradox—the best we can do is “possible explanations.” And if you ask ten different scientists what their hunch is about the correct one, you’ll get ten different answers. You know when you hear about humans of the past debating whether the Earth was round or if the sun revolved around the Earth or thinking that lightning happened because of Zeus, and they seem so primitive and in the dark? That’s about where we are with this topic.

In taking a look at some of the most-discussed possible explanations for the Fermi Paradox, let’s divide them into two broad categories—those explanations which assume that there’s no sign of Type II and Type III Civilizations because therearenone of them out there, and those which assume they’re out there and we’re not seeing or hearing anything for other reasons:

Explanation Group 1: There are no signs of higher (Type II and III) civilizations because thereareno higher civilizations in existence.

Those who subscribe to Group 1 explanations point to something called the non-exclusivity problem, which rebuffs any theory that says, “There are higher civilizations, but none of them have made any kind of contact with us because they all _____.” Group 1 people look at the math, which says there should besomany thousands (or millions) of higher civilizations, that at leastoneof them would be an exception to the rule. Even if a theory held for 99.99% of higher civilizations, the other.01% would behave differently and we’d become aware of their existence.

Therefore, say Group 1 explanations, it must be that there are no super-advanced civilizations. And since the math suggests that there arethousandsof them just in our own galaxy, something else must be going on.

This something else is calledThe Great Filter.

The Great Filter theory says that at some point from pre-life to Type III intelligence, there’s a wall that all or nearly all attempts at life hit. There’s some stage in that long evolutionary process that is extremely unlikely or impossible for life to get beyond. That stage is The Great Filter.

If this theory is true, the big question is, Where in the timeline does the Great Filter occur?

It turns out that when it comes to the fate of humankind, this question is very important. Depending on where The Great Filter occurs, we’re left with three possible realities:We’re rare, we’re first, or we’re fucked.

1. We’re Rare (The Great Filter isBehindUs)

One hope we have is that The Great Filter is behind us—we managed tosurpassit, which would mean it’sextremely rarefor life to make it to our level of intelligence. The diagram below shows only two species making it past, and we’re one of them.

This scenario would explain why there are no Type III Civilizations…but it would also mean thatwecould be one of the few exceptions now that we’ve made it this far. It would mean we have hope. On the surface, this sounds a bit like people 500 years ago suggesting that the Earth is the center of the universe—it implies that we’respecial. However, something scientists call “observation selection effect” suggests that anyone who is pondering their own rarity is inherently part of an intelligent life “success story”—and whether they’re actually rare or quite common, the thoughts they ponder and conclusions they draw will be identical. This forces us to admit that being special is at least a possibility.

And if we are special, when exactly did we become special—i.e. which step did we surpass that almost everyone else gets stuck on?

One possibility: The Great Filter could be at the very beginning—it might be incredibly unusual for life to begin at all.This is a candidate because it took about a billion years of Earth’s existence to finally happen, and because we have tried extensively to replicate that event in labs and have never been able to do it. If this is indeed The Great Filter, it would mean that not only is there no intelligent life out there, there may beno other life at all.

Another possibility: The Great Filter could be the jump from the simple prokaryote cell to the complex eukaryote cell.After prokaryotes came into being, they remained that way for almosttwo billionyears before making the evolutionary jump to being complex and having a nucleus. If this is The Great Filter, it would mean the universe is teeming with simple prokaryote cells and almost nothing beyond that.

There are a number of other possibilities—some even think the most recent leap we’ve made to our current intelligence is a Great Filter candidate. While the leap from semi-intelligent life (chimps) to intelligent life (humans) doesn’t at first seem like a miraculous step, Steven Pinkerrejects the ideaof an inevitable “climb upward” of evolution: “Since evolution does not strive for a goal but just happens, it uses the adaptation most useful for a given ecological niche, and the fact that, on Earth, this led to technological intelligence only once so far may suggest that this outcome of natural selection is rare and hence by no means a certain development of the evolution of a tree of life.”

Most leaps donotqualify as Great Filter candidates. Any possible Great Filter must be one-in-a-billion type thing where one or more total freak occurrences need to happen to provide a crazy exception—for that reason, something like the jump from single-cell to multi-cellular life is ruled out, because it has occurred as many as46 times, in isolated incidents, just on this planet alone. For the same reason, if we were to find a fossilized eukaryote cell on Mars, it would rule the above “simple-to-complex cell” leap out as a possible Great Filter (as well as anything before that point on the evolutionary chain)—because if it happened onbothEarth and Mars, it’s almost definitely not a one-in-a-billion freak occurrence.

If we are indeed rare, it could be because of a fluky biological event, but it also could be attributed to what is called theRare Earth Hypothesis, which suggests that though there may be many Earth-likeplanets, theparticularconditions on Earth—whether related to the specifics of this solar system, its relationship with the moon (a moon that large is unusual for such a small planet and contributes to our particular weather and ocean conditions), or something about the planet itself—are exceptionally friendly to life.

2. We’re the First

For Group 1 Thinkers, if the Great Filter isnotbehind us, theonehope we have is that conditions in the universe are just recently, for the first time since the Big Bang, reaching a place that would allow intelligent life to develop. In that case, we and many other species may beon our wayto super-intelligence, and it simply hasn’t happened yet. We happen to be here at the right time to become one of the first super-intelligent civilizations.

One example of a phenomenon that could make this realistic is the prevalence of gamma-ray bursts, insanely huge explosions that we’ve observed in distant galaxies. In the same way that it took the early Earth a few hundred million years before the asteroids and volcanoes died down and life became possible, it could be that the first chunk of the universe’s existence was full of cataclysmic events like gamma-ray bursts that would incinerate everything nearby from time to time and prevent any life from developing past a certain stage. Now, perhaps, we’re in the midst of anastrobiological phase transitionand this is the first time any life has been able to evolve for this long, uninterrupted.

3. We’re Fucked (The Great Filter isAheadof Us)

If we’re neither rare nor early, Group 1 thinkers conclude that The Great Filtermustbe in our future. This would suggest that life regularly evolves to where we are, but thatsomethingprevents life from going much further and reaching high intelligence in almost all cases—and we’re unlikely to be an exception.

One possible future Great Filter is a regularly-occurring cataclysmic natural event, like the above-mentioned gamma-ray bursts, except they’re unfortunately not done yet and it’s just a matter of time before all life on Earth is suddenly wiped out by one. Another candidate is the possible inevitability that nearly all intelligent civilizations end up destroying themselves once a certain level of technology is reached.

This is why Oxford University philosopher Nick Bostromsays that“no news is good news.” The discovery of even simple life on Mars would be devastating, because it would cut out a number of potential Great Filtersbehindus. And if we were to find fossilizedcomplexlife on Mars, Bostrom says “it would be by far the worst news ever printed on a newspaper cover,” because it would mean The Great Filter is almost definitelyaheadof us—ultimately dooming the species. Bostrom believes that when it comes to The Fermi Paradox, “the silence of the night sky is golden.”

Explanation Group 2: Type II and III intelligent civilizationsareout there—and there are logical reasons why we might not have heard from them.

Group 2 explanations get rid of any notion that we’re rare or special or the first at anything—on the contrary, they believe in theMediocrity Principle, whose starting point is that there is nothing unusual or rare about our galaxy, solar system, planet, or level of intelligence, until evidence proves otherwise. They’re also much less quick to assume that thelackof evidence of higher intelligence beings is evidence of their nonexistence—emphasizing the fact that our search for signals stretches only about 100 light years away from us (0.1% across the galaxy) and suggesting a number of possible explanations. Here are 10:

Possibility 1) Super-intelligent life could very well have already visited Earth, but before we were here.In the scheme of things, sentient humans have only been around for about 50,000, a little blip of time—if contact happened before then, it might have made some ducks flip out and run into the water and that’s it. Further, recorded history only goes back 5,500 years—a group of ancient hunter-gatherer tribes may have experienced somecrazyalien shit, but they had no good way to tell anyone in the future about it.

Possibility 2) The galaxy has been colonized, but we just live in some desolate rural area of the galaxy.The Americas may have been colonized by Europeans long before anyone in a small Inuit tribe in far northern Canada realized it had happened. There could be an urbanization component to the interstellar dwellings of higher species, in which all the neighboring solar systems in a certain area are colonized and in communication, and it would be impractical and purposeless for anyone to deal with coming all the way out to the random part of the spiral where we live.

Possibility 3) The entire concept of physical colonization is a hilariously backward concept to a more advanced species.Remember the picture of the Type II Civilization above with the sphere around their star? With all that energy, they might have created a perfect environment for themselves that satisfies their every need. They might have crazy-advanced ways of reducing their need for resources and zero interest in leaving their happy utopia to explore the cold, empty, undeveloped universe.

An even more advanced civilization might view theentire physical worldas a horribly primitive place, having long ago conquered their own biology and uploaded their brains to a virtual reality, eternal-life paradise. Living in the physical world of biology, mortality, wants, and needs might seem to them the way we view primitive ocean species living in the frigid, dark sea. FYI, thinking about another life form having bested mortality makes me incredibly jealous and upset.

Possibility 4) There are scary predator civilizations out there, and most intelligent life knows better than to broadcast any outgoing signals and advertise their location.This is an unpleasant concept and would help explain the lack of any signals being received by the SETI satellites. It also means that we might be the super naive newbies who are beingunbelievablystupid and risky by ever broadcasting outward signals. There’s a debate going on currently about whether we should engage in METI (Messaging to Extraterrestrial Intelligence—the reverse of SETI) or not, and most people say we should not. Stephen Hawking warns, “If aliens visit us, the outcome would be much as when Columbus landed in America, which didn’t turn out well for the Native Americans.” Even Carl Sagan (a general believer that any civilization advanced enough for interstellar travel would be altruistic, not hostile)calledthe practice of METI “deeply unwise and immature,” and recommended that “the newest children in a strange and uncertain cosmos should listen quietly for a long time, patiently learning about the universe and comparing notes, before shouting into an unknown jungle that we do not understand.” Scary.Thinking about this logically, we should disregard all the warnings get the outgoing signals rolling. If we catch the attention of super-advanced beings, yes, they might decide to wipe out our whole existence, but that’s not that different than our current fate. And maybe, instead, they’d invite us to upload our brains into their eternal virtual utopia, which would solve the death problem and also probably allow me to achieve my childhood dream of bouncing around on the clouds. Sounds like a good gamble to me.

Possibility 5) There’s only one instance of higher-intelligent life—a “superpredator” civilization (like humans are here on Earth)—who isfarmore advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level.This would suck. The way it might work is that it’s an inefficient use of resources to exterminate all emerging intelligences, maybe because most die out on their own. But past a certain point, the super beings make their move—because to them, an emerging intelligent species becomes like a virus as it starts to grow and spread. This theory suggests that whoever was thefirstin the galaxy to reach intelligence won, and now no one else has a chance. This would explain the lack of activity out there because it would keep the number of super-intelligent civilizations to just one.

Possibility 6) There’s plenty of activity and noise out there, but our technology is too primitive and we’re listening for the wrong things.Like walking into a modern-day office building, turning on a walkie-talkie, and when you hear no activity (which of course you wouldn’t hear because everyone’s texting, not using walkie-talkies), determining that the building must be empty. Or maybe, as Carl Sagan has pointed out, it could be that our minds work exponentially faster or slower than another form of intelligence out there—e.g. it takes them 12 years to say “Hello,” and when we hear that communication, it just sounds like white noise to us.

Possibility7) Wearereceiving contact from other intelligent life, but the government is hiding it.This is an idiotic theory, but I had to mention it because it’s talked about so much.

Possibility8) Higher civilizations are aware of us and observing us (AKA the “Zoo Hypothesis”).As far as we know, super-intelligent civilizations exist in a tightly-regulated galaxy, and our Earth is treated like part of a vast and protected national park, with a strict “Look but don’t touch” rule for planets like ours. We wouldn’t notice them, because if a far smarter species wanted to observe us, it would know how to easily do so without us noticing. Maybe there’s a rule similar to theStar Trek’s“Prime Directive” which prohibits super-intelligent beings from making any open contact with lesser species like us or revealing themselves in any way, until the lesser species has reached a certain level of intelligence.

Possibility 9) Higher civilizations are here, all around us. But we’re too primitive to perceive them.Michio Kakusums it uplike this:

Lets say we have an ant hill in the middle of the forest. And right next to the ant hill, they’re building a ten-lane super-highway. And the question is “Would the ants be able to understand what a ten-lane super-highway is? Would the ants be able to understand the technology and the intentions of the beings building the highway next to them?

So it’s not that we can’t pick up the signals from Planet X using our technology, it’s that we can’t even comprehend what the beings from Planet Xareor what they’re trying to do. It’ssobeyond us that even if they really wanted to enlighten us, it would be like trying to teach ants about the internet.

Along those lines, this may also be an answer to “Well if there are so many fancy Type III Civilizations, why haven’t they contacted us yet?” To answer that, let’s ask ourselves—when Pizarro made his way into Peru, did he stop for a while at an anthill to try to communicate? Was he magnanimous, trying to help the ants in the anthill? Did he become hostile and slow his original mission down in order to smash the anthill apart? Or was the anthill of complete and utter and eternal irrelevance to Pizarro? That might be our situation here.

Possibility 10) We’re completely wrong about our reality.There are a lot of ways we could just betotallyoff with everything we think. The universe might appear one way and be something else entirely, like ahologram. Or maybewe’rethe aliens and we were planted here as an experiment or as a form of fertilizer. There’s even a chance that we’re all part of a computer simulation by some researcher from another world, and other forms of life simply weren’t programmed into the simulation.

________________

As we continue along with our possibly-futile search for extraterrestrial intelligence, I’m not really sure what I’m rooting for. Frankly, learning either that we’re officially alone in the universe or that we’re officially joined by others would be creepy, which is a theme with all of the surreal storylines listed above—whateverthe truth actually is, it’s mindblowing.

Beyond its shocking science fiction component, The Fermi Paradox also leaves me with a deep humbling. Not just the normal “Oh yeah, I’m microscopic and my existence lasts for three seconds” humbling that the universe always triggers. The Fermi Paradox brings out a sharper, more personal humbling, one that can only happen after spending hours of research hearing your species’ most renowned scientists presentinsanetheories, change their minds again and again, and wildly contradict each other—reminding us that future generations will look at us the same way we see the ancient people who weresurethat the stars were the underside of the dome of heaven, and they’ll think “Wow they really hadnoidea what was going on.”

Compounding all of this is the blow to our species’ self-esteem that comes with all of this talk about Type II and III Civilizations. Here on Earth, we’re the king of our little castle, proud ruler of the huge group of imbeciles who share the planet with us. And in this bubble with no competition and no one to judge us, it’s rare that we’re ever confronted with the concept of being a dramatically inferior species to anyone. But after spending a lot of time with Type II and III Civilizations over the past week, our power and pride are seeming a bit David Brent-esque.

That said, given that my normal outlook is that humanity is a lonely orphan on a tiny rock in the middle of a desolate universe, the humbling fact that we’re probably not as smart as we think we are, and the possibility that a lot of what we’re sure about might be wrong, sounds wonderful. It opens the door just a crack that maybe, just maybe, there might be more to the story than we realize.

The science world isn’t in total agreement about what percentage of those stars are “sun-like” (similar in size, temperature, and luminosity)—opinions typically range from 5% to 20%. Going with the most conservative side of that (5%), and the lower end for the number of total stars (1022), gives us 500 quintillion, or500 billion billion sun-like stars.

As many stars as there are in our galaxy (100 - 400 billion), there are roughly an equal number of galaxies in the observable universe—so for every star in the colossal Milky Way, there’s a wholegalaxyout there. All together, that comes out to the typically quoted range ofbetween 1022and 1024total stars, which means that forevery grain of sand on Earth, there are10,000 starsout there.

When confronted with the topic of stars and galaxies, a question that tantalizes most humans is, “Is there other intelligent life out there?” Let’s put some numbers to it (if you don’t like numbers, just read the bold)—

A really starry sky seemsvast—but all we’re looking at is our very local neighborhood. On the very best nights, we can see up to about 2,500 stars (roughly one hundred-millionth of the stars in our galaxy), and almost all of them are less than 1,000 light years away from us (or 1% of the diameter of the Milky Way). So what we’re really looking at is this:

________________

Physicist Enrico Fermi felt something too—”Where is everybody?”

Some people stick with the traditional, feeling struck by the epic beauty or blown away by the insane scale of the universe. Personally, I go for the old “existential meltdown followed by acting weird for the next half hour.” But everyone feelssomething.

Everyone feels something when they’re in a really good starry place on a really good starry night and they look up and see this:

Discovery News, “Fireball Meteorities Spark Space Age ‘Gold Rush’,” Discovery News, May 14, 2012, http://www.nbcnews.com/id/47411677/ns/technology_and_science-science/t/fireball-meteorites-spark-space-age-gold-rush/

Fireball Meteorites Spark Space Age 'Gold Rush'

Scientists are on an epic treasure hunt for meteorite fragments from a spectacular fireball that lit up the daytime sky over California last month.

The space rocks came from a minivan-size asteroid that plunged through Earth's atmosphere and exploded into a dazzling daytime fireball over California and parts of Nevada on April 22. Meteorite fragments were scattered around Sutter's Mill, an old sawmill in Coloma, Calif. — the same region where the first gold nugget was found, triggering the Gold Rush of 1848.

Now, NASA has a meteorite rush on its hands, one just as exciting as the California's Gold Rush, the agency said.

Scientists and meteorite hunters have descended on the area in hopes of finding precious space rocks that may contain clues about the solar system's history, as well as the origins of molecules that support life.

Fragments from the so-called Sutter's Mill Meteorite fell to Earth on April 22 at 7:51 a.m. PDT (10:51 a.m. EDT). At least one space rock landed in a horse pasture outside of Lotus, Calif., in the Sierra Nevada mountains, according to NASA officials. Merv de Hass, who owns the farm, found the meteorite, but has since donated it to NASA.

"If I could contribute to science in some small way, then that would be great," de Hass said in a statement. "I'm looking forward to the results."

The de Hass family has let NASA researchers comb the land for more fragments.

"I feel like I have done a service to my country," said Eugena de Haas, who lives on the land where the meteorite was found.

The meteorite found by de Hass is very rare, and scientists are interested in studying it because it could contain molecules that explain how the building blocks of life on Earth may have been delivered from space, agency officials said.

Piecing together clues about the meteor could also help astronomers understand the early solar system and how the planets formed.

"This is among the most chemically primitive meteorites," Greg Schmidt, deputy director of the NASA Lunar Science Institute (NLSI), said in a statement. "It's like asking 'how did life on Earth begin?' and then having a fossil fall right in your back yard. This is exciting stuff — who knows what's inside? The Sutter's Mill Meteorite could be the most profound sample collected in over 40 years."

Peter Jenniskens, a meteor astronomer with the SETI Institute, is working with the NLSI and is leading the search. As he finds the meteorites, Jenniskens is making note of their exact location, so that the science team will be able to better track how the meteorites fell to Earth.

But the space rock is a rare carbonaceous chrondrite, which decomposes quickly in damp conditions, so the scientists are hoping to locate any other specimens before they are ruined.

"I am grateful this meteorite was found quickly," Jenniskens said. "We need to recover as much material as possible from the damp environment before weather affects the rocks too badly."

To help with the search, a helium-filled zeppelin flew slowly over the area with a trained group of observers to relay potential coordinates for ground teams to investigate. The zeppelin, which is owned and operated by Airship Ventures, carried a high definition camera, and observers used binoculars and cameras to spot possible impact sites.

"I suspect this is the first time in history that anyone has searched for meteorites with an airship," Schmidt said.

So far, the meteorite found by the de Haas family is one of the largest fragment found, but the meteorite search is expected to continue for the next few months, NASA officials said.

"The de Haas family has welcomed NASA's involvement with open arms," NLSI director Yvonne Pendleton said in a statement. "I want to express my personal gratitude to them. They should be commended for their contribution to scientific discovery."

David Jones, “An extragalactic mystery: where do high-energy cosmic rays come from?,”The Conversation, April 23, 2012, https://theconversation.com/an-extragalactic-mystery-where-do-high-energy-cosmic-rays-come-from-6623

An extragalactic mystery: where do high-energy cosmic rays come from?

It’s been the defining question of high-energy astrophysics for the past century: where do cosmic rays come from?

New findings from the IceCube Neutrino Observatory at the South Pole have brought us closer to understanding the origin of these strange “rays” – charged particles that originate somewhere beyond our planet and that reach Earth with varying energy levels and in different forms.

A recent announcement by IceCube scientists suggests the newly constructed observatory has found evidence allowing them to rule out gamma-ray bursts – the most energetic explosions in the known universe – as the most likely source of the highest-energy cosmic rays.

This is big news, make no mistake. But to understand the significance of this finding, we first need to take a look at the history of cosmic-ray research.

Cosmic(-ray) history

Some 100 years ago, the pioneering Austrian physicist Victor Hess jumped in a hot-air balloon and climbed to the dizzying height of 5.3km above the earth’s surface.

Why? Because he, as with many scientists of his day, was wondering where exactly ionising radiation was coming from.

Ionising radiation is comprised of particles that can react with matter; radiation such as that seen in the fallout from the Fukushima nuclear disaster.

(It is important to understand, however, that the ionising radiation that was a mystery in the early 20th century is stopped by the Earth’s atmosphere. It was the by-products of the radiation’s interaction with the atmosphere that was being observed.)

Hess’s major achievement was to show that as altitude increases, so do the levels of ionising radiation – and so the radiation must have been coming from space. This radiation was thus given the title “cosmic rays” and Hess was awarded the 1935 Nobel Prize in Physics for his troubles.

Ever since, scientists such as myself have been wondering where these cosmic rays actually came from.

What we know

While we are still in the dark about some major aspects of the origins of cosmic rays, we’ve made considerable progress.

We know cosmic rays are made up of charged particles: primarily protons (roughly 89%), helium (roughly 9%) and electrons (roughly 1%) with heavier elements (up to iron) making up the other 1%.

We also know cosmic rays arrive at Earth with an impressive energy range: from millions of electron volts – an electron volt (eV) is the amount of energy gained by the charge of a single electron when it is moved across an electric potential difference of one volt – up to more than 1021 eV (the number 1 with 21 zeroes after it!).

These 1021 eV particles are the most energetic particles ever observed – subatomic particles with the energy of a vigorous tennis serve!

Given the huge energy range with which cosmic rays arrive at Earth, it seems clear we need several explanations for their origin.

Some from the sun

The sun supplies many of the low-energy cosmic rays seen on Earth, while sources within our galaxy probably make up most of the rest. The cosmic rays at intermediate energies are probably created through the explosion of dead stars (known as supernovae), their remnants and other detritus from their lives.

But there are still large gaps in our knowledge. There is precious little evidence about the exact site(s) of cosmic-ray acceleration – cosmic rays being sped up to extremely high energies – above what the sun or the Milky Way can produce.

At modest energies, there is very good evidence – from X-ray and radio telescopes – that the remnants of supernovae can accelerate cosmic-ray electrons. But we still have no strong evidence where cosmic-ray protons, which, you’ll recall, make up roughly 90% of cosmic rays, are accelerated, even in our own cosmic backyard.

And even after a century of intense study, we still don’t know the exact source of the highest-energy cosmic rays; subatomic particles with energies above about 1015 eV. These particles are found above what is known as the “knee” of the cosmic ray spectrum (see image above) and are thought to come from extragalactic sources.

So, how do we go about determining the source of these high-energy cosmic rays, observationally? There are two methods: direct and indirect detection.

Direct detection of cosmic rays

Due to the fact cosmic rays are made up of charged particles, they are – unlike photons (light particles) – deflected by ambient magnetic fields. As a result, the exact location of their acceleration is lost.

But the relative weakness of galactic and intergalactic magnetic fields implies this does not occur for sufficiently energetic cosmic rays – rays with energy levels above 1019 eV.

Thus it was a great breakthrough in 2007 when the Pierre Auger Observatory – a vast array of tanks and fluorescence (light) detectors dedicated to directly detecting cosmic rays at the highest energies – reported there was a statistically significant correlation between cosmic rays and active galactic nuclei – the centres of galaxies which contain supermassive black holes.

In other words, it appeared as if high-energy cosmic rays were coming from active galactic nuclei.

Unfortunately, this evidence is getting worse. While scientists at the Pierre Auger Observatory were sure their data were correct and their result was statistically significant, in statistics, nothing is certain.

A small number of results will actually be a meaningless blip and, unfortunately for the researchers at the Pierre Auger Observatory, this result was one of them. Their signal is, sadly, disappearing into the statistical muck of random noise.

Therefore, for now, we need a non-interacting proxy for detecting cosmic rays: the neutrino.

Indirect detection of cosmic rays

Neutrinos are almost-massless, charge-less particles (their name means “little neutral one”) which are created in many particle interactions. You might remember that neutrinos were at the centre of the faster-than-light kerfuffle earlier this year.

The neutrino’s lack of charge and mass, however, makes them incredibly difficult to detect and as a result, neutrino detectors have to be enormous.

As it turns out, one of the best detection mediums is the ice of Antarctica and so, to detect neutrinos from outer space, the one-cubic-kilometre IceCube observatory was built at the South Pole.

The gamma-ray burst link

For the vast majority of the past century, without enormous detectors such as IceCube, the only source of information about the acceleration of the highest-energy cosmic rays was purely theoretical.

Many of these theories have posited that mysterious bursts of gamma rays (photons with very high energies), called gamma-ray bursts, could provide a meaningful flux of particles at the highest energies.

Gamma-ray bursts, the most energetic events in the known universe, are observed at a rate of a few per galaxy per century. They last from milliseconds to minutes and are followed by afterglows emitted at X-ray to radio wavelengths.

Despite their rarity, the vast expanse of our universe makes (or according to IceCube’s current research, made) these events a great potential source for the highest-energy cosmic rays.

We know that neutrinos are produced in the same interactions that should produce the gamma rays that give gamma-ray bursts their name. Hence, the theory goes, neutrinos should be observed from gamma-ray bursts.

Given this, it leaves particle physics in a precarious position, given that neutrinos from these gamma-ray bursts are not seen by IceCube.

The importance of no neutrinos

There are two schools of thought about the sources of cosmic rays at the highest energies. Either they’re created bottom-up – accelerated from lower energies, such as in gamma-ray bursts – or top-down – where more-energetic particles interact in some way and lose energy, creating cosmic rays.

Top-down models usually posit the existence of exotic dark matter particles (although recent results suggest that even dark matter is not on as certain scientific footing as it once was).

The crux of the problem, really, is that other than active galactic nuclei and gamma-ray bursts, there are no known, credible sources that are abundant enough to create these high-energy cosmic rays.

This fact leads us to a startling conclusion: either the IceCube data is wrong (as with the Pierre Auger Observatory results, we might have seen a random fluctuation of the data masquerading as a statistically significant result), our understanding of gamma-ray bursts is wrong, or we have to appeal to a completely new paradigm of physics.

In this new paradigm, cosmic rays at the highest energies might be created by unknown particles, (possibly) under the influence of novel particle physics.

This could lead to this century’s Albert Einstein or Victor Hess showing the rest of us what we’ve been missing this whole time.

One thing’s for sure: this is a very exciting time to be a particle physicist.