Sean Carroll

Category: Time

  • A Cornucopia of Time Talks

    I don’t suppose “cornucopia” is the right collective noun, but what does one call a collection of talks centered on the subject of time? I previously linked to these talks from our time conference, but it’s clear from the viewing numbers that not nearly enough of you have taken advantage of them. There’s a lot of great stuff here! So let me pick out some of my very favorites, although I promise they are all good.

    Here’s neuroscientist David Eagleman, talking about how we perceive time.

    David Eagleman on CHOICE

    Here’s physicist-turned-complexity-theorist Raissa D’Souza, talking about complexity.

    Raissa D'Souza on COMPLEXITY

    Here’s another physicist-turned-complexity-theorist, Geoffrey West, taking the complexity story even further.

    Geoffrey West on COMPLEXITY

    Here’s former guest-blogger, now Discover blogger, and engineer/roboticist/neuroscientist/philosopher Malcolm Maciver, talking about making choices and the evolution of consciousness.

    Malcolm MacIver on CHOICE

    And to top things off, here’s one of those mock debates (where participants attempt to defend the side they don’t believe in). This time it’s David Albert vs. David Wallace, on the many-worlds interpretation of quantum mechanics.

    A Mock Debate on Quantum Mechanics with DAVID ALBERT and DAVID WALLACE

    Seriously good stuff. There are still more talks not yet up, I’ll let you know.

    Update: I didn’t realize my own talk was up. Here it is.

    Sean Carroll, OPENING PANEL at FQXi Conference on Time

  • Does Time Exist?

    Videos from our Setting Time Aright conference are gradually filtering online, courtesy of the Foundational Questions Institute. Perhaps the very first question that should be asked, of course, is whether the subject of the conference actually exists. So we recruited two well-known partisans on this issue to hash things out. Tim Maudlin is a philosopher of science who has argued forcefully that time is real — and furthermore that the arrow of time is an intrinsic part of reality, not just a byproduct of the low-entropy Big Bang. (Crazy talk.) Julian Barbour is a physicist who is well known for arguing that time doesn’t really exist, we can happily eliminate it from all of our equations of physics. (Even crazier.)

    So we asked them to go at it, with a twist: here Tim defends the proposition that time doesn’t exist, while Julian argues that it is real. I was not the only one to conclude that these guys were just as good at arguing this side as the one they actually believed.

    A Mock Debate on Time with JULIAN BARBOUR AND TIM MAUDLIN

    Well worth watching — both talks are quite brilliant, in very different ways.

  • Ten Things Everyone Should Know About Time

    “Time” is the most used noun in the English language, yet it remains a mystery. We’ve just completed an amazingly intense and rewarding multidisciplinary conference on the nature of time, and my brain is swimming with ideas and new questions. Rather than trying a summary (the talks will be online soon), here’s my stab at a top ten list partly inspired by our discussions: the things everyone should know about time. [Update: all of these are things I think are true, after quite a bit of deliberation. Not everyone agrees, although of course they should.]

    1. Time exists. Might as well get this common question out of the way. Of course time exists — otherwise how would we set our alarm clocks? Time organizes the universe into an ordered series of moments, and thank goodness; what a mess it would be if reality were complete different from moment to moment. The real question is whether or not time is fundamental, or perhaps emergent. We used to think that “temperature” was a basic category of nature, but now we know it emerges from the motion of atoms. When it comes to whether time is fundamental, the answer is: nobody knows. My bet is “yes,” but we’ll need to understand quantum gravity much better before we can say for sure.

    2. The past and future are equally real. This isn’t completely accepted, but it should be. Intuitively we think that the “now” is real, while the past is fixed and in the books, and the future hasn’t yet occurred. But physics teaches us something remarkable: every event in the past and future is implicit in the current moment. This is hard to see in our everyday lives, since we’re nowhere close to knowing everything about the universe at any moment, nor will we ever be — but the equations don’t lie. As Einstein put it, “It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.”

    3. Everyone experiences time differently. This is true at the level of both physics and biology. Within physics, we used to have Sir Isaac Newton’s view of time, which was universal and shared by everyone. But then Einstein came along and explained that how much time elapses for a person depends on how they travel through space (especially near the speed of light) as well as the gravitational field (especially if its near a black hole). From a biological or psychological perspective, the time measured by atomic clocks isn’t as important as the time measured by our internal rhythms and the accumulation of memories. That happens differently depending on who we are and what we are experiencing; there’s a real sense in which time moves more quickly when we’re older.

    4. You live in the past. About 80 milliseconds in the past, to be precise. Use one hand to touch your nose, and the other to touch one of your feet, at exactly the same time. You will experience them as simultaneous acts. But that’s mysterious — clearly it takes more time for the signal to travel up your nerves from your feet to your brain than from your nose. The reconciliation is simple: our conscious experience takes time to assemble, and your brain waits for all the relevant input before it experiences the “now.” Experiments have shown that the lag between things happening and us experiencing them is about 80 milliseconds. (Via conference participant David Eagleman.)

    5. Your memory isn’t as good as you think. (more…)

  • Time Is Out of Joint

    Greetings from Norway, where we’re about to embark on what is surely the most logistically elaborate conference I’ve ever attended. Setting Time Aright starts here in Norway, where we hop on a boat and cross the North Sea to Copenhagen. The get-together is sponsored by the Foundational Questions Institute, although it came together in an unusual way; I was part of a group that was organizing a conference, and we applied to FQXi for funding, at which point they mentioned they were planning almost exactly the same conference at the same time. So we joined forces, and here we are. Unity ’11!

    The topic, if you haven’t guessed, is time. That’s a big subject, one that can hardly be done justice by sprawling books with hundreds of (admittedly quite charming) footnotes. You can see why the conference has to spread over two countries. We’re trying an experiment in interdisciplinarity: while the conference is a serious event meant for researchers, we have a wide variety of specialties represented, including biologists, computer scientists, philosophers, and neuroscientists, as well as the inevitable physicists and cosmologists. (There is also a public event, for those of you who find yourselves in Copenhagen next week.) I can’t wait to hear some of these talks, it should be a blast.

    My job is to open the conference with an introductory talk that hits on some of the big questions. Here are the slides, at least as they are right now; last-minute editing is always a possibility. I think I put enough in there to provoke almost everyone at the conference one way or another.

  • Oedipus and the Riddle

    Thanks to Richard O’Connell for suggesting this Jorge Luis Borges poem as appropriate to the time-travel theme.

    Oedipus and the Riddle

    Quadruped in the dawn, erect at noon,
    and wandering on three legs across the blind
    spaces of afternoon; so the eternal
    Sphinx saw her inconstant brother, Man.
    And to her rocky silence came a man
    who unlocked the riddle in the mirror;
    terrified, he saw the shattering image
    of his destruction and his error.
    We are Oedipus, doomed as he, to be
    the triple beast: child, saviour, suppliant-
    all that we will be, all that we have been.
    It would annihilate us in an instant
    to glimpse our monstrous being; mercifully
    God grants us issue and oblivion.

    Sadly, God grants us nothing of the sort. But happily, we are not annihilated by glimpsing our monstrous being. We may be disappointed, disillusioned, or discombobulated; but those are temporary conditions that we can strive to overcome. Embrace your monstrous being! It’s the only true strategy in the face of Time’s relentless march.

  • The Flow of Time

    I Tweeted the following inscrutable remark. Probably best left unexplained, but upon reflection I can’t resist.

    My consciousness freely travels up and down my world line, but sadly it only carries the memories appropriate to the moment it inhabits.

    The point is that (some) people don’t think about the flow of time in the right way, and this leads to a couple of unfortunate consequences: a difficulty in understanding the psychology of time, and a scattering of entertaining but illogical science-fiction scenarios.

    Modern physics suggests that we can look at the entire history of the universe as a single four-dimensional thing. That includes our own personal path through it, which defines our world line. This seemingly conflicts with our intuitive idea that we exist at a moment, and move through time. Of course there is no real conflict — just two different ways of looking at the same thing. There is a four-dimensional universe that includes all of our world line, from birth to death, once and for all; and each moment along that world line defines an instantaneous person with the perception that they are growing older, advancing through time.

    But if you don’t play too much attention to the way these two views fit together, you are tempted to imagine that “you” might actually, in some set of laws of physics if not actually in our own, go visit different moments in your own life, carrying along the consciousness of your “present” self. Something like that happens in SF stories from Slaughterhouse-Five to Back to the Future. But it’s not consistent — it requires the implicit introduction of a kind of “meta-time” that keeps track of when we visit the ordinary time with which we are familiar. That’s not how nature works; my tweet was trying to point out the inconsistency of taking this idea seriously, subject to the strictures of 140 characters or less. (To be earnestly explicit: if you did manage to travel up and down your world line at will, you would indeed have whatever memories were appropriate to the moment you were inhabiting — which means it would be exactly like not traveling at all.)

    Sometimes, unfortunately, people go further than science fiction. I’ve run into folks who believe that our conscious perception of time passing is actually evidence against modern physics — arguing that we need to change the known laws of physics to account for the flow of time. It’s always conceivable, in principle, that what we think we understand at a basic level is completely wrong. But the evidence had better be pretty overwhelming. The brain is a complicated thing, and I don’t think that our present inability to provide a complete and comprehensive theory of conscious perceptions should be held as compelling evidence that the laws of physics are in need of overthrowing.

  • A Universe Out of Chaos

    How did the universe come to be? We don’t know yet, of course, but we know enough about cosmology, gravitation, and quantum mechanics to put together models that standing a fighting chance of capturing some of the truth.

    Stephen Hawking‘s favorite idea is that the universe came out of “nothing” — it arose (although that’s not really the right word) as a quantum fluctuation with literally no pre-existing state. No space, no time, no anything. But there’s another idea that’s at least as plausible: that the universe arose out of something, but that “something” was simply “chaos,” whatever that means in the context of quantum gravity. Space, time, and energy, yes; but no order, no particular arrangement.

    It’s an old idea, going back at least to Lucretius, and contemplated by David Hume as well as by Ludwig Boltzmann. None of those guys, of course, knew very much of our modern understanding of cosmology, gravitation, and quantum mechanics. So what would the modern version look like?

    That’s the question that Anthony Aguirre, Matt Johnson and I tackled in a paper that just appeared on arxiv. (Both of my collaborators have also been guest-bloggers here at CV.)

    Out of equilibrium: understanding cosmological evolution to lower-entropy states
    Anthony Aguirre, Sean M. Carroll, Matthew C. Johnson

    Despite the importance of the Second Law of Thermodynamics, it is not absolute. Statistical mechanics implies that, given sufficient time, systems near equilibrium will spontaneously fluctuate into lower-entropy states, locally reversing the thermodynamic arrow of time. We study the time development of such fluctuations, especially the very large fluctuations relevant to cosmology. Under fairly general assumptions, the most likely history of a fluctuation out of equilibrium is simply the CPT conjugate of the most likely way a system relaxes back to equilibrium. We use this idea to elucidate the spacetime structure of various fluctuations in (stable and metastable) de Sitter space and thermal anti-de Sitter space.

    It was Boltzmann who long ago realized that the Second Law, which says that the entropy of a closed system never decreases, isn’t quite an absolute “law.” It’s just a statement of overwhelming probability: there are so many more ways to be high-entropy (chaotic, disorderly) than to be low-entropy (arranged, orderly) that almost anything a system might do will move it toward higher entropy. But not absolutely anything; we can imagine very, very unlikely events in which entropy actually goes down.

    In fact we can do better than just imagine: this has been observed in the lab. (more…)

  • Can’t Really Blame Them

    Very excited to learn that my talk from TEDxCaltech is featured on the TED home page today. They have their own comment thread, and in a couple of weeks we’ll have a live call-in “conversation with the speaker” deal. If the Twitters are to be believed, these TED talks are pretty darn popular.

    The talk is a punchy, 15-minute version of my usual cosmology-and-the-arrow-of-time schtick. Glad to see the arrow of time get some more publicity; sophisticated Cosmic Variance readers know all about it, but not everyone is so lucky. When Brian Cox did an episode of Wonders of the Universe that discussed the arrow of time, the comments were all “Wow, what an amazing concept, never heard of that!” Obviously reading the wrong blogs.

    But I can’t help but notice something about the presentation on the TED home page…

    Each talk is advertised by an image from the video; in most cases it’s a picture of the speaker actually giving the talk. But for mine, they (wisely) went with the Hubble Deep Field.

    Lesson: you can’t compete with the universe! It’s bigger, smarter, and prettier, too.

  • Does Time Run Faster When You’re Terrified?

    Neuroscientists have all the fun. When we physicists think about the fundamental nature of time, it largely involves standing hopefully in front of a blackboard and writing the occasional equation, or at best sending clocks on strange journeys. All in the service of very good ideas, of course. But when I give talks about these wonderful ideas, I learn that what people care more about are down-to-earth questions about aging and memory. So not only do neuroscientists get to tackle those questions directly, but they do so by dropping people from tall buildings. How cool is that?

    Dr. David Eagleman on the Discovery Channel

    David Eagleman is an interesting guy, as a recent New Yorker profile reveals. Mild-mannered neuroscientist by day, in his spare time he manages to write fiction as well as iPad-based superbooks. But his research focuses on how the mind works, in particular how we perceive time.

    I’ve written previously about how, as far as the brain is concerned, remembering the past is like imagining the future. Eagleman studies a different neurological feature of time: how we perceive it passing under a variety of different conditions. You might be familiar with the feeling that “time slows down” when you are frightened or in some extreme environment. The problem is, how to test this hypothesis? It’s hard to come up with experimental protocols that frighten the crap out of human subjects while remaining consistent with all sorts of bothersome regulations.

    So Eagleman and collaborators did the obvious thing: they tied subjects very carefully into harnesses, and threw them from a very tall platform. The non-obvious thing is that they invented a gizmo that flashed numbers as they fell, so that they could determine whether the brain really did speed up (perceiving a larger number of subjective moments per objective second) during this period of fear.

    Answer: no, not really. There is a perceptual effect that kicks in after the event, giving the subject the impression that time moved more slowly; but in fact they didn’t perceive any more moments than a non-terrified person would have. Still, incredibly interesting results; for example, when you’re afraid, the brain lays down memories differently than when you’re in a normal state.

    Obviously, of course, these findings need to be replicated. If you’ll excuse me, I’m off to find some grad students and a tall building.

  • Interview on Static Limit

    David Reffkin is a radio host at KUSF in San Francisco. His usual gig is classical music, but once a month he hosts a special called Static Limit where he delves into physics and cosmology. Here’s an interview he did with me a short while back. Right at the beginning we’re talking about this very blog, which I am now using to plug the interview, which is mostly about my book. This is what’s known as “synergy.”

    (Those viewing in an RSS reader, you have to visit the page to click the audio link.)

    David assumes the listeners have been following along previous shows, so we don’t spend too much time defining general relativity and the Big Bang; we go right for the cutting edge. But we also covered a lot of meta ground, about the process of doing physics. He also gave me the most comprehensive list of errata (mostly minor typos) for my book, so I know he read the whole thing!