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Science Talk

From Eternity to Here: Sean M. Carroll's Quest to Understand Time

Sean M. Carroll, theoretical physicist at the California Institute of Technology talks with podcast host Steve Mirsky about his new book From Eternity to Here: The Quest for the Ultimate Theory of Time. Plus, we test your knowledge of some recent science in the news. Web sites related to this episode include preposterousuniverse.com

Sean M. Carroll, theoretical physicist at the California Institute of Technology talks with podcast host Steve Mirsky (pictured) about his new book From Eternity to Here: The Quest for the Ultimate Theory of Time. Plus, we test your knowledge of some recent science in the news. Web sites related to this episode include preposterousuniverse.com 

Podcast Transcription

Welcome to Science Talk, the more or less weekly podcast of Scientific American posted on March 30th, 2010. I'm Steve Mirsky. This week on the podcast:

Carroll:          We all know time, we use it everyday all the time, but it's not that many steps to go from the time that we use to unanswered questions about the universe.

Steve:          That's Sean Carroll, author of the new book, From Eternity to Here: The Quest for the Ultimate Theory of Time. We'll talk to him about the book, and we'll test your knowledge of some recent science in the news. First, Sean Carroll, he is a theoretical physicist at the California Institute of Technology where he studies cosmic inflation, the arrow of time and what happened at or before the big bang. We talked on February 21st when we were both at the annual meeting of the American Association for the Advancement of Science in San Diego.

Steve:          From Eternity to Here—What exactly do you mean by that title?

Carroll:          Well, the point of my book is to talk about the arrow of time—the fact that the past appears to us different from the future even though the underlying laws of physics don't treat them any differently. And the nice thing is that it starts in the kitchen; it starts in our everyday lives, the arrow of time—we remember yesterday, we don't remember tomorrow—but the explanation for it is to be found in the big bang or even what happened before the big bang. So thinking about why the past appears different than the future to us is something that connects our everyday lives to our notions of eternity.

Steve:          I have found the connection by the way between you, the physicist Sean Carroll, and the molecular biologist, evolutionary biologist, Sean Carroll, and that is the egg.

Carroll:          The egg, yes, was very important.

Steve:          He studies how the egg develops and how that relates to evolution overall and for you it's how the egg cannot be unscrambled.

Carroll:          That's right. It means some metaphorical power as the egg is the origin of things and Sean "B. for biologist" Carroll talks about the origin of life and I talk about the origin of the universe.

Steve:          So your book has a very specific architecture. There are four major sections of the book.

Carroll:          Right.

Steve:          And why don't we go through those if you’d like to …

Carroll:          Sure.

Steve:          Just talk about each section, why you built the book this way and where we ultimately wind up.

Carroll:          Well I think it's something where it wouldn't have been good to go absolutely in logical order or a chronological order for that matter in the book. So part 1 is the rapid overview of the terrain, the map of what the issues are. So we talk about, what do you mean by time, what is [the] definition of time. We talk about what is the definition entropy in the second law of thermodynamics that really gives force to the arrow of time.

Steve:          You actually were standing in line at a DMV office.

Carroll:          Yes.

Steve:          And you did an experiment with lay people.

Carroll:          That's right.

Steve:          You want to talk about that just briefly.

Carroll:          Well I think that you know as physicists we have definitions in our brains and we use words like time or energy and dimension, things like that, and it's very, very useful if you're trying to explain these concepts to people on the street, see what they have in mind. And I think it's very enlightening because physicists, if you ask them what time means, [will] talk about coordinates or what clocks measure or things like that, whereas to people on the street who're not trained as physicists, they think of time as some sort of medium through which we move and distinguishing the different aspects of time they think is [an] important thing to do.

Steve:          So you actually asked lay people, while you were line at the DMV, "How do you perceive time? What do you think time is?" and they didn't move away?

Carroll:          They didn't run away, but they didn't give me their spot in line. You know, I ask people in line, I ask people, you know, at restaurants or in airplanes or so forth. That is where sort of, [I could hear] people talk about time in their own words so that I wasn't putting ideas into their heads.

Steve:          I interrupted you. Go back to where we were …

Carroll:          So yes, part 1 of the book is just an overview. What is time? What is entropy? What is universe? What is the arrow of time? How does cosmology fit into things? So you know, so we're going to, sort of, in the rest of the book do all this better, but [I] want to get in people's minds—what are the stakes, what are the questions we're trying that we're trying to answer?

Steve:          Why don't you explain very briefly what entropy is for people who might not be familiar with that answer.

Carroll:          Yeah entropy is the absolutely key concept. It has a long history from the 19th century with people who [were] trying to build better steam engines. But basically entropy is just a measure of how disorderly things are. Things in the universe naturally go from a state of organization, low entropy orderliness, to disorganization, and we call that high entropy. On a more technical level, the entropy measures the uselessness of some amount of energy, you know, useful energy like your gasoline, you can use that gasoline, burn it in your car to get you somewhere. One you've burnt it, the energy of the universe is the same, energy is conserved, but it's now useless; you create a lot of entropy in the course of burning the gas. And the point is the entropy increases with time, that's the second law of thermodynamics. The first law just says the total energy is conserved. The second law says that entropy goes up and up as the universe ages. So that's what gives time an arrow, that's what tells the difference between the past and future, in fact, the entropy was low in the past, it will be higher in the future, in that simple idea underlies all the different ways, in which there's an arrow of time.

Steve:          The way I was taught thermodynamics was the first law is you can't win, the best you can do is break even, and the second law was you can't even break even.

Carroll:          Can't break even, and the third law is you can't even get out of the game. So, you're stuck. And then so in part 2 of the book, we start getting down [to] the nitty-gritty and we think about time in terms of how Einstein explains space and time to us, special relativity, general relativity, gravity as a manifestation of the curvature of space-time, and also the idea of time machines, time travel and you actually travel into the past; and again we do a little foreshadowing, because I say that it's probably not possible to travel to the past, but if you could the thing that seems paradoxical to us or troubling to us about the prospects of time travel can be traced to the arrow of time, can be traced to the fact that if you mix up the past with the future by having a time machine then there is no consistent arrow of time—one persons past is another person's future—and that's what gives us the willies about traveling to the past. So then part 3 is really the heart of the book, part 3 of the four parts [is] where we talk about entropy at a detailed level. We talk about Ludwig Boltzmann, the Austrian physicist, who is the great hero of statistical mechanics in thermodynamics.

Steve:          I think his equation is on his gravestone.

Carroll:          He has an equation on his gravestone and it's not even the equation call the Boltzmann equation. You know, he has more than one really worldly important equation. I think, that Boltzmann deserves to be up there in the pantheon of physicists who people on the street would recognize the names of, you know, the Newton, Galileo, Einstein level of physicists; Boltzmann should be up there. So I hope that my book does something [to] raise his profile. And so we talk about reversibility, for example. I have a whole chapter that I think is talking about a concept that a lot of physics book[s] assume people understand, if you don't go into great detail about namely, the paradigm of how physics works: that if you understand the universe now, and you understand the laws of physics and you can predict with perfect fidelity what the universe will be like at some moment in the future, then you can retrodict what the universe was like at some moment in the past. The universe is defined by its state at any moment in time. That's a very deep concept and [I], sort of, show what that means how it could've been different and how it plays out. And then we go into entropy and Boltzmann and how it works and then the good news about Boltzmann's understanding as he showed why entropy would tend to go up as we observe it to do, but then the hole in his argument, the unsolved puzzle, is why it was ever low in the first place.

Steve:          And one of the key points is entropy was very low at the big bang and it has been increasing ever since, and so then the obvious question was what was it doing being so low at the big bang?

Carroll:          That's absolutely right, because what Boltzmann taught was [that] what entropy is, deep down in its guts, is a way [of] counting the number of ways you can rearrange atoms or elementary particles or what have you without noticing macroscopically, right? So, if we have an egg, you notice that there is an egg shell and a[n] egg white and a yolk, but if you change some of the molecules around within the yolk you wouldn't notice, but if you started changing yolk molecules around with egg white molecules, you would notice. So the more messy and, sort of, mixed up something is the higher the entropy, because there [are] more ways to rearrange it. So if you say that the early universe has a low entropy, which is a true statement, what you're saying is the universe is in a very, very special state, one of only a small number of possible arrangements. So that becomes puzzling, that sounds like a clue to something that we don't [even] understand—why was the early universe so special?

Steve:          You say in the book that the universe, isn't really the way it "should be"?

Carroll:          That's right.

Steve:          So how should it be, how is it not that way and what does that tell us?

Carroll:          Well, that's part 4 of the book. We talk about, you know, now that we've gone through what entropy means and what its puzzle is in part 3, in part 4 we try to relate it cosmology, relate to what we know about the observable universe. And again I try to be clear that the question, what should the universe look like? is not one we have one clear, obviously correct answer to. We can sort of make some guesses, but when we think about entropy and we think that high entropy corresponds to a natural state, there are many, many, many ways to be high entropy. Low entropy corresponds to something delicately tuned, so that's why it's easy for entropy to go up. There's more ways to be high entropy than low entropy. So if the universe is low entropy then it's delicately tuned, and that doesn't seem the way that it should be. [You know,] what did the tuning? It's okay to imagine that the universe is in a low entropy state, but it seems to imply there's an explanation, there's a mechanism, there's some laws of physics that makes the universe that way at early time[s]. So in part 4, I go through a bunch of possible explanations that you might consider, and tell you why none of them are really very satisfying. And in the end I suggest that, well maybe our universe is part of a much bigger multiverse, that the universe we see is not all there is; there are other regions and our big bang is actually part of a much bigger system that has [a high] entropy [but] never[theless] the entropy can get higher and higher by the much bigger universe creating new universes, and we're a baby universe that branched off of a much bigger system. It's very, very speculative, it's not something that I'm proclaiming to be necessarily the right answer, but what I try to emphasize is that something like this is what we need to be thinking about. We need to be thinking about models for the large-scale evolution of the universe, where the big bang is not the beginning, the big bang is explained by something preexisting to that and that's really the only hope I think we have of dynamically coming up with a reason why our observed universe had a low entropy at early times.

Steve:          Our big bang is a budding off of this other, more massive system that we can't get a look at.

Carroll:          That's right and there's various ways that might happen. I talk about one or two ways in the book, but even if we don't know the details of that mechanism yet, it could still be you know, logically a good, sensible answer to the problem. I think that right now, the problem with [the] arrow of time and its connection to cosmology is not just that we don't know which theory is right, but the theor[ies] that we're thinking about these days don't even address the problem, they don't even try. So, I think that, you know, once you have that problem in mind [it affects] what you call a sensible theory of cosmology it's really a very strong constraint on your models.

Steve:          Have you as a reader, maybe as a listener, am I entitled to possibly be a little bit frustrated that you've just added another tortoise to the line of tortoises that are holding up the universe?

Carroll:          Well, I think it's a very good question, but I think that there's an answer to it. I mean, I think that what counts as an explanation is [if you] can explain something complicated in terms of something simple. So, it is certainly true that if you start thinking about the multiverse, you're adding a lot of universes okay; the number of universes goes from one to infinity. So, it does not seem like progress in terms of explanatory concision. However, the multiverse is not the starting point, the multiverse is a prediction [of] a way of thinking about gravity and quantum mechanics, and so if you can blend together what we know about space-time [according] to Einstein with what we know about quantum mechanics [according to] the giants of early 20th-century physics, it seems very natural to think that the universe does not sit there; there's no place the universe can just settle in and stay that way forever. And the good news is that that's what you want to help explain the arrow of time. If there were a way the universe could just sit there unperturbed forever, it probably would. Why wouldn't it? Why wouldn't we have reached that state a long time ago? You have a puzzle that goes back to Boltzmann himself, where he says that if the universe is just an equilibrium—so the universe is like the gas in a room or that is sort of uniformly distributed, not changing over time, just sort of sitting there forever and ever—well, you might think that that's not changing, but really at a subatomic level or the atomic level, there're fluctuations, there's random things occurring. So if you have a box of gas that lasts literally forever, the gas molecules in that box will find themselves in every possible arrangement over the course of time and those arrangements include a human being or a [goldfish] or a solar system if the box of gas is big enough. And that's not a benefit, that's a bit of a problem because that's [says that if] the universe lasts forever, just through its sort of random rearrangements of its constituents, we could be there. You know, something like us could exist; that's not disallowed by the laws of physics. And then why aren't we there? That doesn't seem to be the universe in which we live. We live in this incredibly rich universe that has not just you and me, but a planet and we have a star in a galaxy and there are a hundred billion other galaxies. So it doesn't look to us like we're just random fluctuations in some eternally existing thing, but why not? And that becomes a big puzzle.

Steve:          You know, I felt like my cat was being threatened numerous times in the course of reading your book; you kept saying that my cat just might disappear at any moment.

Carroll:          Well, we tried in the book not to harm any animal[s] either conceptually or literally. I think that it's very funny when you look at all these thought experiments that scientists do, they're always killing cats and dogs and things like that. So, I'm just sort of feeding them and sending them on journeys; I'm not actually killing anybody.

Steve:          I want to go back to something we were talking about. The concept of over 100 years ago that if you understood everything about every particle of the universe at this moment, you could both predict the future and what's the word you use?

Carroll:          Retrodict the past.

Steve:          Retrodict the past? Right, but you discuss in the book how there was this contest for the king of Sweden.

Carroll:          That's right. He had a birthday coming up, and which mathematician was it again?

Steve:          Poincaré. And why don't you tell the story?

Carroll:          So yeah, King Gustav was having a birthday coming up—and it was the 19th century; things were a little bit more highbrow back then—so, he decided to celebrate this birthday by having a mathematics competition. The Swedish mathematicians proposed various puzzles and they said, you know, the best solution to [any of] these puzzles will win the competition. Poincaré, who was a famous French mathematician at the time tried to answer one of these puzzles, which was to basically prove that the solar system, the planets moving around the sun, is stable, that the planets do not go wandering off into the cosmos, if you wait long enough. And he wrote an essay, he claimed to prove it. He didn't prove the whole thing; he just looked at three bodies. So, the sun and two planets, so it was a much simpler system. But he says, yes here’s the proof that its stable and it was a big success, they granted him the prize, they [were] about to print up this essay in the winning journal and he realized he made a mistake. One of the people who read his article found a loophole, send it back to him and in trying to close the loophole, Poincaré actually ended up proving the opposite of what he had claimed, the solar system is not stable. And this is an example of what we now know as chaos theory. And, in fact, given the three planets orbiting each other, you cannot predict with perfect accuracy—starting from incomplete data, starting from imprecise initial conditions—you have very little control over what the far future will look like. So that's as a practical matter, a huge problem for this project of predicting the future, given the present because we never have perfect information about what the present is. On the other hand, that was never a practical matter anyway; we were never claiming to know even approximately the position and velocity of every atom in your body or anything like that. So I think people have made a big deal out of chaos being a roadblock for this dream of determinism: Starting from the present you can predict the future. But I think it's a little bit overblown actually because I think it's a practical problem, not a problem of principle. It still remains true that if you did have perfect information about the state today, you could predict the future and no one ever expected that there was anything other than a matter of principle, no one ever expected that we would have an approximate knowledge of the current state of the universe and therefore approximately predict the future. I think it's a very different question. So at the level of the fundamental laws of physics, the questions about whether or not the future is determined by the present is still a very good question, and chaos theory doesn't quite answer it.

Steve:          Why did you decide that you wanted to write this book. I mean, it's not for everybody, but it's certainly aimed at a nontechnical audience.

Carroll:          Oh, absolutely. I mean it might not be for everybody because no book is, but it's certainly for people who want to think about the universe without, you know, having any preparation for doing so, any training in doing so and I try to connect it to other things. There's you know, little quotes from literature and movies and things like that, but the ideas are big and they're abstract and they require having an open mind and being willing to think about them. It's not like you have to go through lot of equations, but you do have to open your conceptualization of space and time and what that means to ideas that most people aren't familiar enough with, but I think that in fact it's a perfect venue for doing that because time is something we all experience. You don't need to first be convinced that time is interesting, you know, it's not like a black hole or a quark, where you have to first be told what it is. We all know time, we use it everyday, all the time, but it's not that many steps to go from the time that we use to unanswered questions about the universe. And so it's a good way to make this transition from questions we're all familiar with to questions of the origin of everything we see.

Steve:          And it's just fun to think about this stuff.

Carroll:          It's absolutely fun to think about this stuff, especially the slightly counterfactual questions: If the universe, were a little bit different, how would we perceive it? And it can be very, very different, and it's hard for us to quite think in those way because the arrow of time, the increase of entropy is so engrained in how we live. The reason why we remember yesterday and not tomorrow is dependent on the fact that entropy is increasing, so if you don't have a consistent arrow of time, like the time machines, or if we don't have any arrow of time, at all, if you're really in thermal equilibrium, then all of our notions of cause and effect go out the window, and it's a very different way of thinking about the universe.

Steve:          What if any kind of practical things you get out of working on this material?

Carroll:          Well, I think that the things that most excite me are not the practical ones. I'm curiosity driven in my own research. I don't know where the universe came from but when you read about it, it's certainly true that even if we think we understand the mechanics of entropy and how that plays out in eggs or melting ice cubes or something like that, there's a tremendous number of connections that we haven't yet made. I can say that the reason why I remember yesterday but not tomorrow is because entropy is increasing, but okay tell me exactly how that gets reflected in the operation of the brain. And it becomes complicated and we don't know the answers even at the level of computers that are mechanistic and understandable, things like Maxwell’s demon come into play. And really once again [it's] very easy to start asking questions at the cutting edge of things we don't [know]. We understand entropy pretty well, we don't understand complexity, we don't understand simplicity versus complexity, which is sort of parasitic upon the growth of entropy. So, I think there's just a lot of room to move in many different directions in improving connections between what we do know about entropy and how that relates to other areas of science.

Steve:          You have a very nice discussion of Maxwell's demon in the book, for anybody who's looking for it; you could also find it in a Scientific American article from well maybe the 1940s or 1950s, I think.

Carroll:          Right, yeah, I learned only while writing this book that sort of the final nail in the coffin of Maxwell's demon was not nailed in until 1980s. You know, it took us a century to really figure out how to reconcile the thought experiment of Maxwell with everyone's belief that entropy is going to increase in a closed system. For those people who don't know, Maxwell’s demon was a suggestion by Maxwell of a way entropy could spontaneously decrease. He had a little box that was in two halves and he had a little demon that sort of washed the box and let all of the fast moving molecules go into one half and all the slow moving molecules go into the other half. So the box went from being uniform temperature to being hot on one side and cold on the other side; that is a decrease in entropy. But the second law of thermodynamics says that can't happen and of course everyone knows that the answer must be that somehow the demon has the entropy and the demon is increasing its entropy, but putting our fingers on exactly how that happened took a century of work. And it turns out that the way that it happens is the demon needs to have a memory of which molecules are moving fast, which molecules are moving slow and if the demon has a notepad that it can record that information on and that notepad is finite, then at some point you gotta erase that information and that increases the entropy of the universe. And in my book what I do is I use this as a paradigm to think about living organisms. You know, living organisms, as any good creationist will tell you, seemed to fly in the face of the second law of thermodynamics. The earth started fairly simple and now we have all this complicated structure in the biosphere, so where did that come from? And in some sense the living organism is trying to avoid the repercussions of the second law thermodynamics by not winding down. And of course its easy to show that it doesn't violate the second law of thermodynamics because the sun is giving us a lot of low-entropy energy, and we're putting it back in a high-entropy form back into the universe. But the mechanism once again, the actual way in which it plays out—so why do you need these complicated structures like human beings to make that happen? That's something we don't completely understand right now, so I think it's a great area for research.

Steve:          Our physiology is sort of a Maxwell's demon. It keeps us at 98.6 instead of allowing us to go into equilibrium with whatever the ambient temperature is.

Carroll:          That's exactly right, that’'s the point I make in the book and I think that you know, Maxwell’s demon is trying its best to stay out of equilibrium, right, that's what its trying to do and…

Steve:          [Because then] you're dead.

Carroll:          Because then you die, that's right. So any good living organism worth [its] salt doesn't want to die, so it needs to leach energy out of the environment, increase the entropy of that energy and then give it back; so it's increasing the entropy of the universe [while] maintaining its own low-entropy state.

Steve:          You've put in a good word for thermodynamics.

Carroll:          Very happy to do that.

Steve:          Because we don't have equations in the book but if anybody is willing to get into a thermodynamic course and actually see the equations, I just remember thinking it was the coolest thing, no pun intended.

Carroll:          Yep. Well, I think in fact, in my own education I didn't appreciate thermodynamics as much as I should have the first time around. But it's because, you know, the[re's something] physicists love, which is to immediately go to the simplest possible system. [This] goes back to Galileo, right, saying ignore air resistance when we drop things from the Leaning Tower of Pisa; we'll put it in later. You know, it's sort of an annoyance and thermodynamics is all about that annoyance, it's all about the fact that there's a messy world out there with dissipation and friction and noise and things like that but the understanding of that is really a connection between the beautiful fundamental principles of physics and the messy reality of the world. And in fact the connection turns out actually pretty elegant and compelling in its own right.

Steve:          Now, these guys are trying to design better steam engines.

Carroll:          That's right and there was lot of sort of national competition, the French versus the British and so forth to get the best steam engines and along the way, they invented the laws of thermodynamics.

Steve:          Which help us to understand the way the entire universe operates.

Carroll:          Absolutely and vice versa. Hopefully, the universe is going to help us understand the origin of those laws.

Steve:          I thought you [were] going to say help us build better steam engines.

Carroll:          That [would] be good, too, and in fact, that's true. You know, one of these things that a lot other people, even back to Poincaré and Einstein who are fundamental in inventing relativity, we think of, you know, relativity as being one of the most abstract mathematical topics in physics. But we think of Einstein as being the sort of the exemplar of the head in the clouds physicists, but you know, he was a patent clerk and that was not simply an accident. The thing that was being patented when Einstein was working in the patent office were clocks. Telling time was a crucial thing [at] the moment [and] people realized that clocks in different places might behave differently. Poincaré worked in the bureau of longitude making maps and he realized that once again making a map is not necessarily an objective representation of [a] sort of territory. And these very practical concerns helped Einstein and Poincaré and their predecessors who invented thermodynamics to come to out these great abstracts [leaps of] logic.

Steve:          Sean Carroll's book is From Eternity to Here. His Web site is preposterousuniverse.com

And now it's time to play TOTALL……. Y BOGUS. Here are four science stories; only three are true. See if you know which story is TOTALL……. Y BOGUS.

Story number 1: Kidney donors live just as long as people with both kidneys.

Story number 2: India and Bangladesh can stop fighting over who owns the island called New Moore Island by India and South Talpatti by Bangladesh because it's gone, swallowed by the sea.

Story number 3: Authorities in Tampa, Florida have finally captured a rhesus monkey that's been on the loose for at least a year.

And story number 4: A survey of over 50 depiction[s] of the Last Supper painted in the last thousand years finds that the portion sizes keep growing.

(clock ticking)

And time's up.

Story number 1 is true. People who donate one of their kidneys live just as long on an average as nondonors. That's according to a study in the Journal of the American Medical Association. Seems that if you have both kidneys, they're each little slackers working less than full strength. If one kidney is removed, the other gets up to 25 percent bigger, up to 30 percent more efficient. The biggest risk is in the three months immediately after surgery, when donors do have eight times the already very low risk of death that they normally have. That jump in the risk translates to 3.1 deaths out of every 10,000 donors.

Story number 2 is true. The island in dispute by India and Bangladesh has disappeared. It's not clear how much of the disappearance is due to erosion or rising seas or other factors. What is known is that a meter rise in sea level would wipe out about 17 percent of Bangladesh's land, currently home to some 20 million people.

And Story number 4 is true. The portions shown in various paintings of the Last Supper have grown by 70 percent over the last millennium, indicating that the trend toward supersizing is an older one than we may have thought. For more check out the March 25th of the daily SciAm podcast, 60-Second Science.

All of which means that Story number 3, about the capture of that rhesus monkey in Tampa, is TOTALL……. Y BOGUS. Because the critter still roams free. In fact, the latest development is the discovery that he has apparently built up immunity to iocaine powder; I mean to the tranquilizer in the dart they've hit him with. Or he's just got really good at plucking the darts out before the drug can take effect. An official with the Florida Fish and Wildlife Conservation Commission, apparently upset at being asked how the monkey was outsmarting its hunter said, "This monkey is not outsmarting us; this monkey is getting away based on its athletic ability developed over years and years of evolution." And whatever helps us sleep at night. The monkey's Facebook page—that's right, I said it, its Facebook page—is Mystery Monkey of Tampa Bay.

(clock ticking)

Well that's it for this episode. Get your science news at www.ScientificAmerican.com, where you can read Kate Wong's March 24th article on the first human ancestor to be discovered based on DNA sequence evidence rather than anatomy. It's called "No Bones About It: Ancient DNA from Siberia Hints at Previously Unknown Human Relative". And follow us on Twitter, where you will get a tweet every time a new article hits the Web site. Our Twitter name is @SciAm. If you want to follow me, I tweet as @SteveMirsky, because that's my name. For Science Talk, the podcast of Scientific American, I'm that guy. Thanks for clicking on us.

Also check out John Matson's Q&A with Sean M. Carroll, "What Keeps Time Moving Forward? Blame It On The Big Bang"

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