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Sean Carroll

theoretical physicist,

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LIVE TED Conversation: Join TED Speaker Sean Carroll

LIVE conversation with cosmologist Sean Carroll, TEDxCaltech Speaker and author of From Eternity to Here: The Quest for the Ultimate Theory of Time.

The conversation will open at 4pm (Eastern Standard Time), May 19, 2011. Sean is on vacation in Las Vagas, and will join us from his hotel room there to discuss the cosmos, the nature of time, and the science of poker.

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Closing Statement from Sean Carroll

Thanks for participating, everyone! I hope some light was shed. It's been a pleasure chatting with the TED community.

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    May 19 2011: Hello Sean, and thanks for taking the time to do this. NASA published on their website today (http://www.jpl.nasa.gov/news/news.cfm?release=2011-149) news about the Hubble telescope aiding in confirming that it is dark energy and not gravity, that is causing the accelerated expansion of the Universe. How does this affect the current cosmological models of the future of the Universe? Does it bring us much closer to knowing exactly what the fate of the Universe is going to be? What are your thoughts about this?

    Thanks!
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      May 19 2011: This basically helps confirm the picture cosmologists have been putting together over the last couple of decades. It snapped into place in 1998 with the discovery that the universe is accelerating. So we now think that the universe is 4% ordinary matter, 23% dark matter, and 73% dark energy. It's a robust model that fits a very wide variety of observations.

      Of course we want to know -- why is it like that? There we are still very unsure, but a lot of specific models for dark matter and dark energy (and the preference for matter over antimatter) are being carefully studied.
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        May 19 2011: Thanks! But does this lean the scales towards a specific model? I mean, whether expansion will carry on forever versus a "Big Crunch" scenario? Or are we still far from knowing which is true?
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          May 19 2011: The simplest interpretation of the current data is in terms of a truly fixed vacuum energy, which would imply that the universe will expand forever, not recollapse into a crunch. But we can't be sure that the vacuum energy won't some day go away, given how poorly we understand it.
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    May 19 2011: Do you believe physics will ever be able to account for consciousness?
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      May 19 2011: I believe that consciousness, like every property of matter and energy, is ultimately compatible with the underlying laws of physics. That is, we don't need to violate the laws of physics to explain consciousness. On the other hand, it might not be helpful to think in terms of particles and forces if we want to create a theory of consciousness, just as we don't invoke particles and forces when we talk about hurricanes or the stock market. We'll have to see how the research goes.
      • May 19 2011: If consciousness' "code" includes "logics" of 300 million years of evolution I think it would be impossible to gain artificially same level of deep process unity for the system of the individual consciousness. Emotions are needed for consciousness and there is no emotions without huge amount of cumulative evolutive processes in DNA based "biomatter".
        We can produce some kind of bastard copy versions of DNA-based programmed biomachines but they could not gain the independent consciousness to have self controlled and balanced emotions. 300 million year programming can not be copied. I think.
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    May 19 2011: Hello again, Dr. Carroll. What has your work on the arrow of time revealed about the possibilities of time travel? We know that in theory, time travel to the future is possible, given that we develop the necessary technology. But what about travelling to the past? It has been said to be impossible, with both physical and philosophical difficulties, but has your work and that of others revealed any possible scenario or mechanism?

    Also, I recently learned about a theoretical particle called a Tachyon, which is said to have the ability to travel faster than the speed of light. How is this even possible?

    Thanks!
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      May 19 2011: It's easy to be hypothetical! The laws of physics say that you can't go from moving slower than light to moving faster than light. Ordinary particles always move more slowly, but it's possible to imagine "tachyons" that always move faster -- they literally can't stay still! However, this is completely hypothetical, and there's no evidence at all that they really exist.

      Likewise, we can imagine time travel, but it's hard to take seriously the possibility that it's for real. As I write about in my book, some of the conceptual problems with time travel come down to the arrow of time -- how can you remember the past but not the future if your past is in your future? That doesn't say it's impossible, but gives a hint of how hard it would be to make time travel real.
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        May 19 2011: Thanks for the answer! :)

        That's pretty much what I figured about tachyons, but wanted to hear from someone who actually knows what he's talking about.
  • May 19 2011: Why would it not be the case that to an observer crossing the event horizon of a black hole, the interior remains eternally black? I.e, in the direction of the singularity, all is black?

    A photon that just crosses the EH cannot escape, so why would a photon say 1 meter inside the EV ever be able to travel >= 1 meter from the EV?

    And if that is the case, how can any photons inside the EH and just past the observer ever bounce back from anything inside the BH into the in-falling observers eyeballs?

    Thanks!
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      May 19 2011: The singularity inside a black hole is a funny thing -- it's not in the center of the hole, it's in the *future*. You will run into it as surely as you will run into tomorrow. But you don't actually see it. When we see things, we're looking at light that comes from events in the past. So things don't go dark when you cross an event horizon -- light from events in your past can still reach you.
      • May 19 2011: Ok, so my brain just exploded a little bit, but I'm used to that now when discussing these subjects :)

        If I cross the EH of a BH and I'm looking *opposite* my direction of travel (away from the singularity) I understand that what I would see would not look any different to me than before I crossed the EH. - just normal looking stars as before.

        I still don't know what I would see if I look *toward* the singularity. If the singularity were at a point in space, I expect no photons to ever reach me from anywhere between my current location and the singularity, so I would not be able to "see" anything in that direction.

        If the singularity is not in space but "in the future" as you say, then again, I would expect not to see anything from it since it hasn't happened yet.

        I guess I can simplify by just asking you : If I cross the EH of a BH and look towards where I perceive is the "center", what would I see? As an example, say I fall into the BH with a friend who is 1 meter in front of me as we both approach the BH. I see him disappear as he cross the EH. Then I cross. I would think I should never be able to see him again. How do photons from his body bounce back into my eyeballs once we're both inside the EH?

        Thanks again!
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          May 19 2011: You would see light from whatever fell into the black hole before you, or whatever made the black hole in the first place. You can't see the singularity, which really is in your future!

          The point is: you don't see your friend disappear as he crosses the event horizon. You just see him emit light more and more slowly. But if you chase him in, it doesn't slow down infinitely, like it would if you remained outside.
  • May 19 2011: Hi Sean, Thank you for your work in disseminating physics to non-specialists like me. Are you familiar with Godel's work on time? I believe he presented a paper when he was at the Institute for Advanced Study at Princeton. What's the word on the street among physicists on his work on time?
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      May 19 2011: Godel wrote a paper in which he derived a solution to general relativity (Einstein's equation) which had a startling property -- time looped back on itself. So in this (very hypothetical) universe, you could travel "forward in time" and eventually end up meeting yourself in the past!

      Physicists (including me) have thought long and hard about this phenomenon of "closed timelike curves," which is the sophisticated way to think about traveling backwards in time. The smart money says that they are purely hypothetical, and can't exist in the real world; but we don't know for sure.
      • May 19 2011: Thank you. A follow-up question. So I take it then that Godel's derivation is consistent with Einstein's equations. Have physicists thought of experiments which would would get at the question of "closed timelike curves"?
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          May 19 2011: It is consistent with Einstein's equation, that's right. And these days we know of many other examples; search for "wormhole time machine" or "cosmic string time machine."

          Unfortunately, we're talking about phenomena that are very, very far away from experimental accessibility. Unless you have a couple of black holes or cosmic strings in your back yard and know of some clever way of manipulating their gravitational fields.
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    May 19 2011: Hi everyone. Looking forward to chatting about time, the universe, cosmology, physics, and what have you.
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    May 19 2011: I love your talks, all of them... just fantastic!
    I'm wondering one thing tho... in the whole world of cosmology/physics, does the Fractal geometry play any role? is the whole Mandelbrot Set idea taken into consideration somewhere?
    Cheers
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      May 19 2011: Fractals haven't played a big role in cosmology as yet, but we're open-minded. Years ago people wondered whether the distribution of galaxies was fractal, but it turns out to be a lot smoother than that on large scales.

      These days people take very seriously the possibility that the *multiverse* is a fractal -- but that's obviously speculation, far outside what we know how to test.
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        May 19 2011: Thanks a lot... I have another question If I may...
        When it comes to our understanding of time(4th dimension, right?), in terms of physical equations do we assign a certain "speed" to time? we know it's direction, but what I'm trying to ask is, does our perception of the rate of it's progression affect the empirical/scientific approach to time?
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          May 19 2011: Time doesn't have a speed. "Speed" means "how much of something passes as time passes," which makes no sense when "something" is "time."

          Or, if you like: the speed of time is always one second per second.
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    May 19 2011: Thank you very much for insights and clear language.
  • May 19 2011: Thanks for answering question Sean really appreciate it. What do you think of the prospects of measuring the B mode are? Will Planck find it? or a ground based experiment?
  • May 19 2011: Hello Sean, Thanks so much for your time. Probably too late for an answer, but I'll give it a try: if there are multiverses out there, what would "contain" them? absolute void, not even space?
  • May 19 2011: oh and by the way Baum Frampton model:
    http://arxiv.org/abs/astro-ph/0612243
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    May 19 2011: Is the categorification of physics a worthwhile pursuit in your opinion?
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      May 19 2011: That depends on what the results will be. It's pretty math, but physics is all about the results.
  • May 19 2011: Hi Dr Caroll, what are your thoughts on the resolution of the Turok measurement problem of inflation via a matter bounce from loop quantum cosmology. Does such a bounce if real imply singualrities cant form and what does it mean for the validity of the Guth Borde Vilenkin theorem that says inflation must have a singularity at the beginning?
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      May 19 2011: The Guth-Borde-Vilenkin theorem doesn't really say there must be singularities. They made some assumptions, and those assumptions might not apply to the real world. Most crucially, they are only talking about classical spacetimes, not quantum gravity. Everyone believes (or should believe) that singularities aren't "real" in quantum gravity; but we don't know what takes their place.

      I'm not very optimistic about loop quantum cosmology or loop quantum gravity more generally, but I'm happy to be proven wrong.
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    May 19 2011: Dr. Carroll, can you recommend good books for laymen to learn about the newest developments in cosmology?

    Thanks!
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    May 19 2011: Best introduction to theoretical cosmology for a mathematically inclined undergraduate?
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    May 19 2011: wonderful f you to do this..thanks.

    A second is an earth bound measurement..a solar system bound measurement. Why is it we think it is useful to understand cosmology..the age of the universe..its acceleration even the speed of light itself?
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      May 19 2011: Because we hypothesize that the basic laws of physics are the same all over, and then we test that hypothesis -- and so far it's succeeded! The mass of the electron, the strength of gravity, etc. -- these are all things we can measure far away and long ago, and they seem to have the same values that they do nearby.
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  • May 19 2011: What is the status of the Baum Frampton model, is that still on the table?
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      May 19 2011: I don't know what that one is -- which probably means it's not very popular, if it's a cosmological model.
  • May 19 2011: Hi professor,
    Is it possible to multiverse to be seen as many universes, each one includes another rather than separated ones, I think in such situation the accelerated expending can be explained by the impact of a bigger universe on the smaller one that is included in it?
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      May 19 2011: When cosmologists talk about the "multiverse" these days, they are usually (not always) thinking of just a really big universe, in which different regions have different physical characteristics. It's a confusing terminology, but has become somewhat standard.
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    May 19 2011: A friend asked me to post this question here, because he cannot connect right now...

    Dr. Carroll: I wanted to ask you about the famous "Drake equation" to calculate the number of intelligent civilizations in the Universe. The number that results from the equation is very arbitrary because it depends on many unknown factors (at least at the time Drake presented it and Carl Sagan mentioned it in Cosmos). Do you know if there is any new data that can help us fine-tune the values that we give each of the variables so that they can be more accurate with reality?

    Thanks!
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    May 19 2011: How well considered is the many-worlds interpretation by academics?
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      May 19 2011: I think it's the favorite possibility among most cosmologists and QM experts. But we admit that there are unanswered questions, such as the role of probabilities.
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        May 19 2011: Do more lean towards a discrete set of universes or a continuum of universes?
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        May 19 2011: In your view does the MWI remove the "need" for randomness as was traditionally formulated in quantum mechanics?
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          May 19 2011: I think it changes it, without completely solving the problem. In MWI, the wave function of the universe evolves in a completely deterministic and reversible way. But "where we are" in the wave function seems to necessarily require a random component. Observations are irreducibly random, as far as we can tell.
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        May 19 2011: Thanks. Impressive job answering so many questions in such a short time.
  • May 19 2011: Hello! How you think physics helps you at the poker table? Does it help you to come up with the right strategy?

    Thanks!
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      May 19 2011: See my reply to Ben elsewhere in the thread. I do think it helps -- but it's far from sufficient! This trip I've been doing well, but it's not always the case. Nerves of steel might be more important.
  • May 19 2011: Sean, you stated that: "...empty space essentially lasts forever (but) since empty space gives off radiation there's actually thermal fluctuations and it cycles around all the different possible combinations of the degrees of freedom that exists in empty space."

    Does this make life possible in the empty space era?
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      May 19 2011: Yes, absolutely, at least in principle. Indeed, that's the big question: if a model like this is right, why aren't we random fluctuations in an otherwise empty space, rather than finding ourselves in a warm and inviting universe filled with stars and galaxies?
  • May 19 2011: How deeply do you believe in multiverses? Do you think about the other Sean Carrolls? If so, does it affect you? If not why not?
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      May 19 2011: I'm open-minded, which I think is the right policy when there's so much we don't know. Right now my favorite approaches to explaining the arrow of time involve a multiverse, but maybe we just haven't thought of the right model yet.

      I think about other Sean Carroll's, but they don't bother me. I hope they're having fun. (Statistically, some are and some are not.)
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    May 19 2011: If a property of entropy is to expand over time, if there were a way to reverse linear time, would there also be a corresponding way to reverse entropy or would it continue to expand as it's not a property related to the progression of time?
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      May 19 2011: The only real way to define "reversing time" is by "decreasing entropy." In the real world, you can set up a system of just a few atoms in which that happens (and people have done so), but once you get more than a few dozen particles it becomes infeasible.
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        May 19 2011: Since time is affected by a Black Hole, what effect would result from entropy encountering the event horizon of a black hole or the hole's powerful center?
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          May 19 2011: Nothing special happens at the event horizon, actually; it's just a point of no return, past which you can't get back to the outside world. Black holes absorb new material and more entropy all the time -- the effect is just that you get a bigger black hole!
  • May 19 2011: Conceptually, Dr. Carroll, that the universe is more likely to manifest an apple pie, rather than an orchard, some sugar, an oven, and then from these an apple pie, makes sense to me. This is an example of the random fluctuations of matter and energy in our universe - and furthermore that all possible "states" will one day be achieved. IF time is not circular but infinite, then do all of the finite configurations only happen once? I.e. If we detonate a bomb (one could substitute 'universe' here), which scatters its material far and wide, could a future configuration ever include the exact same bomb?
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      May 19 2011: This is a very good questions that cosmologists are currently tackling, known as "the measure problem." In an infinite universe, it's very possible that everything that can happen actually does happen, infinitely many times. But if two things happen infinitely often, which is more likely? Of course we answer questions like this all the time, but in the case of an infinite universe the rules are just not clear right now. But we're trying. (And of course the universe might not be infinite after all.)
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    May 19 2011: Hello again Dr. Carroll, this is a weird question and barely related, but I wanted to know what your thoughts on free will are. Is it possible to have some limited sort of free will given that we are entirely made of matter and matter has to obey the laws of physics? Some have posited quantum mechanics as a possible mechanism for free will, but do you think that makes any sense? These questions are commonly replied to by philosophers and I would love to hear the opinion of a physical scientist. :)

    Thanks!
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      May 19 2011: I don't think quantum mechanics has anything to do with free will. Free will is a property of the way we think about macroscopic human beings, just like temperature is a property of the way we think about macroscopic collections of atoms. It is just as "real" as temperature is, even though it's completely absent in our microscopic particle-physics description of reality.
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        May 19 2011: Thank you Dr. Carroll, that's what I thought. It always seemed strange to me that quantum mechanics was even proposed since its laws break down at our scale of the Universe. However, what are your general thoughts about free will? Do you think it exists or is it just an illusion? I know it's a weird question to ask, but it's something that philosophers talk about all the time, generally with complete disregard of what the actual sciences have discovered, and would love to hear the thoughts of a physicist.

        Thanks!
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          May 19 2011: I think free will is an emergent property, and it exists just as much as all other emergent properties do. Whether you think that counts as "really" existing is up to you!
  • May 19 2011: Hi prof Carroll, I'm a futurist person, always thinking about the future and trying to imagine it, my question would be, humans will eventually live in other planets, how much time would it take and which planet would be chosen? I know biology is not your area, but will humans be genetically modified for the planet or the other way around?

    Thanks!
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      May 19 2011: Too far outside my domain for me to give a reliable answer, I'm afraid. I do suspect that in the not-too-distant future we will experience great advances in biological engineering and modification, as well as human/mechanical interfaces. Unfortunately, planets outside our Solar System are so far away that getting to them is not a near-future prospect.
      • May 19 2011: So you think that even with biological modification, humans will never live in a planet inside the solar system?

        Thanks!
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          May 19 2011: You mean "outside"? I don't say "never" about almost anything, as there's so much we don't currently know.
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    May 19 2011: is there any advise you would give to someone thinking about becoming a physicist?
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      May 19 2011: Read a lot of books, take a lot of courses, and work really hard. And don't forget to have fun along the way, otherwise what's the point?

      Keep an open mind and always be learning new things.