Sean Carroll

Author: Sean Carroll

  • The Universe is Not a Black Hole

    People sometimes ask, “Is the universe a black hole?” Or worse, they claim: “The universe is a black hole!” No, it’s not, and it’s worth getting this one straight.

    If there’s any quantitative reasoning behind the question (or claim), it comes from comparing the amount of matter within the observable universe to the radius of the observable universe, and noticing that it looks a lot like the relationship between the mass of a black hole and its Schwarzschild radius. That is: if you imagine taking all the stuff in the universe and putting it into one place, it would make a black hole the size of the universe. Slightly more formally, it looks like the the universe satisfies the hoop conjecture, so shouldn’t it form a black hole?

    But a black hole is not “a place where a lot of mass has been squeezed inside its own Schwarzschild radius.” It is, as Wikipedia is happy to tell you, “a region of space from which nothing, including light, can escape.” The implication being that there is a region outside the black hole from which things could at least imagine escaping to. For the universe, there is no such outside region. So at a pretty trivial level, the universe is not a black hole.

    You might say that this is picking nits, and the existence of an outside region is beside the point if the inside of our universe resembles a black hole. That’s fine, except: it doesn’t. You may have noticed that the universe is actually expanding, rather than contracting as you might expect the interior of a black hole to be. That’s because, if anything, our universe bears a passing resemblance to a white hole. Our universe (according to conventional general relativity) has a singularity in the past, out of which everything emerged, not a singularity in the future into which everything is crashing. We call that singularity the Big Bang, but it’s very similar to what we would expect from a white hole, which is just a time-reversed version of a black hole.

    That insight, plus four dollars or so, will get you a grande latte at Starbucks. The spacetime solution to Einstein’s equation that describes a universe expanding from the Big Bang is very similar to the time-reversal of a black hole, but you don’t really learn much from making that statement, especially because there is no outside; everything you wanted to know was already there in the original cosmological language. Our universe is not going to collapse to a future singularity, even though the mass is enough to allow that to happen, simply because it’s expanding; the singularity you’re anticipating already happened.

    Still, some folks will stubbornly insist, there has to be something deep and interesting about the fact that the radius of the observable universe is comparable to the Schwarzschild radius of an equally-sized black hole. And there is! It means the universe is spatially flat.

    You can figure this out by looking at the Friedmann equation, which relates the Hubble parameter to the energy density and the spatial curvature of the universe. The radius of our observable universe is basically the Hubble length, which is the speed of light divided by the Hubble parameter. It’s a straightforward exercise to calculate the amount of mass inside a sphere whose radius is the Hubble length (M = 4π c3H-3/3), and then calculate the corresponding Schwarzschild radius (R = 2GM/c2). You will find that the radius equals the Hubble length, if the universe is spatially flat. Voila!

    Note that a spatially flat universe remains spatially flat forever, so this isn’t telling us anything about the universe now; it always has been true, and will remain always true. It’s a nice fact, but it doesn’t reveal anything about the universe that we didn’t already know by thinking about cosmology. Who wants to live inside a black hole, anyway?

  • Blogginess

    A handful of fun things that shouldn’t pass unremarked:

    • Natalie Wolchover, an aspiring science writer, has started a fun blog called Facto Diem. For those of you who didn’t attend Catholic school, that’s Latin for “Fact of the Day.” (Or a close enough facsimile.) I didn’t even know there were that many facts in the world!
    • In the more venerable sections of the blogosphere, Chad Orzel is running a poll concerning the most amazing application of lasers. Considering that “death ray” is not among the options, it’s a pretty good list.
    • We should also link to Scientia Pro Publica #27, over at Melliferax. (Clearly Latin is the lingua franca of the science blogosphere.) Most of the posts involve living things in some way or another, but they should nevertheless be of interest to those of us with more inorganic inclinations.

  • Guest Post: Malcolm MacIver on War with the Cylons

    Malcolm MacIver We’re very happy to have a guest post from Malcolm MacIver. See if you can keep this straight: Malcolm is a professor in the departments of Mechanical Engineering and Biomedical Engineering at Northwestern, with undergraduate degrees in philosophy and computer science, and a Ph.D. in neuroscience. He’s also one of the only people I know who has a doctorate but no high school diploma.

    With this varied background, Malcolm studies connections between biomechanics and neuroscience — how do brains and bodies interact? This unique expertise helped land him a gig as the science advisor on Caprica, the SyFy Channel’s prequel show to Battlestar Galactica. He also blogs at Northwestern’s Science and Society blog. It’s a pleasure to welcome him to Cosmic Variance, where he’ll tell us about robots, artificial intelligence, and war.

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    It’s a pleasure to guest blog for CV and Sean Carroll, a friend of some years now. In my last posting back at Northwestern University’s Science and Society Blog, I introduced some issues at the intersection of robotics, artificial intelligence (AI), and morality. While I’ve long been interested in this nexus, the most immediate impetus for the posting was meeting Peter Singer, author of the excellent book ‘Wired for War’ about the rise of unmanned warfare, while simultaneously working for the TV show Caprica and a U.S. military research agency that funds some of the work in my laboratory on bio-inspired robotics. Caprica, for those who don’t know it, is a show about a time when humans invent sentient robotic warriors. Caprica is a prequel to Battlestar Galactica, and as we know from that show, these warriors rise up against humans and nearly drive them to extinction.

    a-centurian-cylon-in-battlestar-galactica--2Here, I’d like to push the idea that as interesting as the technical challenges in making sentient robots like those on Caprica are, an equally interesting area is the moral challenges of making such machines. But “interesting” is too dispassionate—I believe that we need to begin the conversation on these moral challenges. Roboticist Ron Arkin has been making this point for some time, and has written a book on how we may integrate ethical decision making into autonomous robots.

    Given that we are hardly at the threshold of building sentient robots, it may seem overly dramatic to characterize this as an urgent concern, but new developments in the way we wage war should make you think otherwise. I heard a telling sign of how things are changing when I recently tuned in to the live feed of the most popular radio station in Washington DC, WTOP. The station had commercial after commercial from iRobot (of Roomba fame), a leading builder of unmanned military robots, clearly targeting military listeners. These commercials reflect how the use of unmanned robots in the military has gone from close to zero in 2001 to over ten thousand now, with the pace of acquisition still accelerating. For more details on this, see Peter Singer’s ‘Wired for War’, or the March 23 2010 congressional hearing on The Rise of the Drones here.

    (more…)

  • Hawking: Beware the Alien Menace!

    Okay, that’s a bit alarmist. But Stephen Hawking has generated a bit of buzz by pointing out that contact with an advanced alien civilization might not turn out well for us backward humans. In fact, we should just try to keep quiet and avoid being noticed.

    “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,” he said.

    Prof Hawking thinks that, rather than actively trying to communicate with extra-terrestrials, humans should do everything possible to avoid contact.

    He explained: “We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet.”

    To which I can only say: yeah. Sounds about right. If aliens were sufficiently enlightened to be utterly peace-loving and generous, it would be great to have back-and-forth contact with them. But it’s also possible that they would simply wipe us out — not necessarily in a Mars Attacks! kind of invasion, but almost without noticing (as we have done to countless species here on Earth already). So how do you judge the risk? (Dan Drezner gives the interplanetary-security perspective.)

    It’s like the LHC doomsday scenarios, but for real — the sensible prior on “murderous aliens” is much higher than on “microscopic black hole eats the Earth.” Happily, a face-to-face chat seems unlikely anyway. Nothing wrong with listening in, on the unlikely chance that the aliens are broadcasting their communications randomly throughout the galaxy. Besides, a little advance warning wouldn’t hurt.

    Update: I had forgotten that we had already discussed this a couple of years ago. Old bloggers tend to repeat themselves.

  • The Red Edge

    We had a great time last night at a panel discussion on extrasolar planets, right here at my very own institution of Caltech, sponsored by our very own Discover magazine, and hosted by our very own Bad Astronomer. The panelists included Gibor Basri, John Johnson, Sara Seager, and Tori Hoehler. They did a great job at getting across the most important message: this is a field that has taken a tremendous leap forward in the past ten years, and is poised to make comparable strides in the years to come. A lot of the excitement right now centers on the Kepler satellite, which is on track to find hundreds of extrasolar planets. You can get an idea of recent progress from a graph of extrasolar planets discovered over the years.

    Extrasolar Planets by the Bushel

    From the perspective of the person on the street, planets are pretty cool — but life on other planets is what’s really cool. (Or would be, if we found it.) And frankly, it’s not even the prospect of life that gets people going; it’s the idea of intelligent extraterrestrial life. Tori mentioned that he was slightly surprised, some years ago when there was a report (later discredited) that we had found evidence for life on meteorites from Antarctica, that people didn’t make a big deal out of it — it was exciting, but not Earth-shattering. I suspect that microbes, no matter where they’re from, aren’t going to shatter most people’s Earths; that will take some sort of greeting, friendly or otherwise.

    Still, it’s amazing what has been done, and the prospects for doing more are pretty breathtaking. Here’s one idea that I find pretty clever: searching for the Red Edge. You know how plants appear to be really bright in infrared photographs? That’s because they reflect a lot of infrared light, but tend to absorb regular visible red light. In a spectrum, where we decompose the reflected light into different wavelengths, this phenomenon shows up as a sharp “edge” as you go from infrared (on the right here) to red light. The idea would be that something similar should happen even for very different kinds of life — so if you found a planet whose spectrum featured the red edge, that would be a promising place to hope for finding life.

    The Red Edge

    I have no way of judging how feasible this technique really is — in particular, I’m always skeptical of claims that rely on alien forms of life resembling ours in any way. (The authors do emphasize that an extraterrestrial red edge might not be at the same wavelength as ours.) But I like it because it relies on an underlying truth of which I am quite fond — the fact that life relies on the increase of entropy. The specific wavelengths at which different kinds of life might reflect light can undoubtedly be very different from biosphere to biosphere; but what won’t change is the general idea that a planet full of life will re-radiate energy with a much higher entropy than what it absorbs. That’s the deep principle underlying the red edge; plants absorb visible light, and radiate at longer wavelengths with higher entropy. If we eventually find life on other planets, I’d personally be pleased if entropy were at the bottom of it all.

  • From Eternity to Book Club: Chapters Fifteen and Sixteen

    And we’ve reached the final installment of the From Eternity to Here book club. Chapter Fifteen is entitled “The Past Through Tomorrow,” in an oblique allusion to Robert Heinlein, my favorite author when I was younger. We’re going to throw in the Epilogue for good measure.

    Excerpt:

    What we’ve done is given the universe a way that it can increase its entropy without limit. In a de Sitter universe, space grows without bound, but the part of space that is visible to any one observer remains finite, and has a finite entropy—the area of the cosmological horizon. Within that space, the fields fluctuate at a fixed temperature that never changes. It’s an equilibrium configuration, with every process occurring equally as often as its time-reverse. Once baby universes are added to the game, the system is no longer in equilibrium, for the simple reason that there is no such thing as equilibrium. In the presence of a positive vacuum energy (according to this story), the entropy of the universe never reaches a maximum value and stays there, because there is no maximum value for the entropy of the universe—it can always increase, by creating new universes.

    This is the chapter where we attempt to put it all together. The idea was that we had been so careful and thorough in the previous chapters that in this one we could be fairly terse, setting up ideas and knocking them down with our meticulously-prepared bludgeon of Science. I’m not sure if it actually worked that way; one could argue that it would have been more effective to linger lovingly over the implications of some of these scenarios. But there was already a lot of repetition throughout the book (intentionally, so that ideas remained clear), and I didn’t want to add to it.

    Of course my own current favorite idea involves baby universes pinching off from a multiverse, and I’m certainly happy to explain my reasons in favor of it. But there are also good reasons to be skeptical, especially when it comes to our lack of knowledge concerning whether baby universes actually are formed in de Sitter space. What I hope comes across is the more generic scenario: a multiverse where entropy is increasing locally because it can always increase, and does so both toward the far past and the far future. While there’s obviously a lot of work to be done in filling in the details, I haven’t heard any other broad-stroke idea that sounds like a sensible dynamical origin for the arrow of time. (Which isn’t to say that one won’t come along tomorrow.)

    Chapter 16 is the Epilogue, where I reflect on where we’ve been and what it all means. I talk a little about why thinking about the multiverse is a very respectable part of the scientific endeavor, and how we should think about the fact that we are a very tiny part of a very big cosmos. Finally, I wanted to quote the very last paragraph of text in the book, at the end of the Acknowledgments:

    I’m the kind of person who grows restless working at home or in the office for too long, so I frequently gather up my physics books and papers and bring them to a restaurant or coffee shop for a change of venue. Almost inevitably, a stranger will ask me what it is I’m reading, and—rather than being repulsed by all the forbidding math and science—follow up with more questions about cosmology, quantum mechanics, the universe. At a pub in London, a bartender scribbled down the ISBN number of Scott Dodelson’s Modern Cosmology; at the Green Mill jazz club in Chicago, I got a free drink for explaining dark energy. I would like to thank every person who is not a scientist but maintains a sincere fascination with the inner workings of nature, and is willing to ask questions and mull over the answers. Thinking about the nature of time might not help us build better TV sets or lose weight without exercising, but we all share the same universe, and the urge to understand it is part of what makes us human.

    Among those people who share a fascination with the inner workings of nature, I of course include people who regularly read this blog. So — thanks!

  • Media Frenzy

    The final book club installment is still percolating, don’t worry. I’ve been traveling like a crazy person, which has pushed blogging into the background. In the meantime, here are a couple of interviews elsewhere in the infosphere.

    First is a New York Times interview with me. It’s very short, but we cover a lot of ground — science education, time travel, entropy, the movies, and my love life. Such plenitude of topics in a tiny piece will necessarily lead to compression, and Jerry Coyne is already complaining that I give short shrift to the complicated reality of aging — and he’s right!

    71020603Second and more fun, in Wired I am on the other side of the interviewer’s table, talking to Lost creators Damon Lindelof and Carlton Cuse. How cool is that? That was a great time, as we chatted excitedly about time, narrative, wormholes, fate and determinism, the role of science in television, and so on. These guys have given an incredible amount of thought into their show at every level — the characters, the mythology, and what it all means. And they wanted to ask me questions about cosmology and how scientists think, which I’m always happy to talk about. I got hooked on the show only after participating in Lost University, but now Tuesdays at 9:00 p.m. is the high point of my week. Only a few more episodes to go — which means that people who haven’t seen it can finally order the complete DVD selection, which is really the way to see it. (Just note that Season Three drags a bit, especially near the beginning.)

  • Census Day Looms

    Groups of people with whom I disagree (so many, many groups…) should not hand in their census forms. That way they will be under-represented in official figures and basically count less. And do you really want to be in the government’s database when the black helicopters come?

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    Just kidding. Only two days left, hand in your census forms! Even people I don’t like.

  • What to do about the Pope?

    When it comes to religion, I’m more interested in scientific and philosophical questions — Does God exist? Can science say anything about the supernatural? — than in sociological or political ones — Is religion good and or evil?, etc. So there was not much temptation to wade in on Pope Benedict’s recent troubles, or the wider issue of sex scandals in the Catholic Church.

    Now, happily, that temptation has dipped to zero, since Phil Plait has done such a good job. Read the whole thing, as they say. Roughly, Phil notes that the Pope seems to be responsible for some very bad things; that he should be brought to justice for any wrong-doings; that there is some relevance to concerns of the skeptical community, insofar as the Church invokes supernatural explanations; but finally, that the strategy should not be simply one of proclaiming superiority and tarring religion as evil and demanding heads on plates. Catholics and other believers, whether we disagree with them or not, are human beings who will understandably be upset and troubled at the recent news. We don’t help to convert them to atheism or naturalism or skepticism by shoving the shortcomings of their leaders in their faces in the midst of a crisis; reason and rational discourse should be more our style. It’s a nuanced argument, which means it’s guaranteed to be misunderstood and caricatured, since even God can’t control the natural impulses of the internet.

    Let’s be clear: I want religion to vanish. I think that religious beliefs are wrong, and that the world would be a better place if everyone accepted the real world for what it is. And I believe that many of the actions of the Church when it comes to pedophilia certainly deserve the label “evil,” whatever one might think of the people who perpetrated them.

    So the question is, how to bring about the rationalist utopia in which people’s actions are based on reason and reflection rather than faith and hierarchy? I agree with Phil’s answers, as I’ve argued in other contexts. One of the primary tenets of a rationalist philosophy should be that we should be especially skeptical about claims that we want to be true. Our personal preferences don’t have any effect on the truth, so we need to guard against confirmation bias and lazy acceptance of ideas that make us happy. One great example is the idea that we’re going to make the world a better and more rational place by telling everyone how much smarter we are than everyone else, and how evil and stupid our enemies are. The Pope’s recent actions, it seems clear, are some combination of evil and stupid. But now is just not the time for patting ourselves on the back. A lot of people have been deeply hurt, directly or indirectly, and we should be able to show just a modicum of restraint. Not giving up or keeping quiet, but picking our spots. After all, we don’t have to win by being obnoxious — we can win by being right.

  • From Eternity to Book Club: Chapter Fourteen

    Welcome to this week’s installment of the From Eternity to Here book club. We’re on to Chapter Fourteen, “Inflation and the Multiverse.” Only one more episode to go! It’s like the upcoming finale of Lost, with a slightly lower level of message-board frenzy.

    Excerpt:

    There is a lot to say about eternal inflation, but let’s just focus on one consequence: While the universe we see looks very smooth on large scales, on even larger (unobservable) scales the universe would be very far from smooth. The large-scale uniformity of our observed universe sometimes tempts cosmologists into assuming that it must keep going like that infinitely far in every direction. But that was always an assumption that made our lives easier, not a conclusion from any rigorous chain of reasoning. The scenario of eternal inflation predicts that the universe does not continue on smoothly as far as it goes; far beyond our observable horizon, things eventually begin to look very different. Indeed, somewhere out there, inflation is still going on. This scenario is obviously very speculative at this point, but it’s important to keep in mind that the universe on ultra-large scales is, if anything, likely to be very different than the tiny patch of universe to which we have immediate access.

    This is a fairly straightforward chapter, trying to explain how inflation works. Given that by this point the reader already is familiar with dark energy making the universe accelerate, and with the fine-tuning problem represented by the low entropy of the early universe, the basic case isn’t that hard to put together. Of course we have an additional non-traditional goal as well: to illuminate the tension between the usual story we tell about inflation and the “information-conserving evolution of our comoving patch” story we told in the last chapter. Here’s where I argue that inflation is not the panacea it’s sometimes presented as, primarily because it’s not that easy to take all the degrees of freedom within the universe we observe and pack them delicately into a tiny patch dominated by false vacuum energy. Put that way, it doesn’t seem all that surprising, but too many people don’t want to get the message.

    This is also the chapter where we first introduce the idea of the multiverse. (The multiverse occupies less than 15 pages or so in the entire book, but to read some reactions you would think it was the dominant theme. The publicists and I must share some of the blame for that perspective, as it is an irresistible thing to mention when talking about the book.) Mostly I wanted to demystify the idea of the multiverse, presenting it as a perfectly natural outgrowth of the idea of inflation. What we’re supposed to make of it is of course a different story.

    Looking back, I think the chapter is a mixed success. I like the gripping narrative of the opening pages. But the actual explanation of inflation is kind of workmanlike and uninspiring. I really put a lot of effort into coming up with novel explanations of entropy and quantum mechanics, which didn’t simply rehash the expositions found in other books; but for inflation I didn’t try as hard. Partly simply because of looming deadlines, partly because I was eager to get to the rest of the book. Hopefully the basic points are more or less clear.