Sean Carroll

theoretical physicist,

This conversation is closed.

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.

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.
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    May 19 2011: Kind of new this entropy idea, has me wondering. In a closed system is entropy a factor? I mean if something is in a state of rest then there is no entropy correct? Being that there are no forces either disruptive or orderly.
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      May 19 2011: There can certainly be entropy in closed systems, or when things aren't changing. The gas in a box in a state of equilibrium has a very well-defined entropy.
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        May 19 2011: So entropy is just the possibility of change with the same outcome? No matter which atom is near which in the box it is still full of the gas. So with extremely high entropy is there instability in the form of atoms, being that the electron and other components can switch around?
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          May 19 2011: Not necessarily; it depends on the physical conditions within the substance. Entropy likes to increase, but energy is conserved at the same time. If it costs energy for electrons to hop around, they will tend to stay put.
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        May 19 2011: Is it that entropy increases with energy then? If that box of gas were frozen then there would be less entropy? Or would the entropy be the same because there is the same amount of atoms? So does volume play a factor in entropy?
  • May 19 2011: ok....so are there any growth patterns....that the universe seems to favor...because of the fundamental laws......?
    Is it possible to trace the fundamental asymmetries/symmetries all the way back....to the initial high entropy state.....and say ok....this particle(higgs boson for example) arose from this form...and then from then on....evolve all the emergent types of growth...into higher complexity forms.....
    i guess my question...if it makes sense....is did form or content come first.....or both together.....(I studied music...so I really dont have much technical knowledge)
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      May 19 2011: I think you have to distinguish between entropy, which characterizes the particular configuration a system is in, and the underlying laws of physics, which give you the building blocks of different systems. In the current way we understand physics, the Higgs boson and so forth are really built into the fundamental structure of reality. It's just a matter of actually bringing it to life, which we're trying to do (for example) at the Large Hadron Collider.
      • May 19 2011: so does....symmetry/asymmetry and the compression of information....in the fundamental laws....play a role.....in those configurations.......so that say an anti proton / proton mixture.....such as those in a neutron star....might have less or more entropy....based on the fundamental laws...
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          May 19 2011: It plays some role, yes. Entropy counts the number of configurations that look the same, and the number of configurations certainly depends on the fundamental laws of physics.
  • May 19 2011: hello Sean, It is very interesting this conversation and in the philosofical arena I hear about entropy as part of a process in time that goes like around a circle, like a cycle... Everything in the material world get involved in this entropic process but at certain point there is a phenomena that is called the neguentropic point. It can be like to start again another cycle of time... In some way the arrow of time you think its possible to go in circles???
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      May 19 2011: It's possible, since there's not that much we're sure of yet in this game. But the challenge is to build a respectable model based on plausible physics, then to connect it to the observable universe so it can be tested. This isn't easy, but we're trying.
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    May 19 2011: Is the level of entropy is in direct relation to the state of sequence patterns occurring near the point of measure?
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      May 19 2011: I'm not sure what that means, but I don't think so. The entropy of a state is (the logarithm of) the number of states that look macroscopically indistinguishable from it.
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        May 19 2011: This is hard for me to relate. It seems that there are sequences of occurrences in the universe. Some occurrences are that there are these three occurrences in a row, for example, some are that there are patterns of occurrences. Then there are different type of occurrences. So some spots of the universe would be riddled with occurrences and some spots with be empty of them. Is there a relation between the entropy and the occurrence factor. Something like a high entropy indicates the high rate of occurrences. I say near the point of measure because if the relationship exist then at another point in the universe where the occurrence rate is lower then so would the entropy be.

        Thank you for the responses
  • May 19 2011: sean, thanks for your time! just wondering... when two people watch a funny movie they laugh... when they watch a sad movie, they cry... would that count as a repeatable finding for a scientific experiment? would it prove that there is a dimension of emotions?
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      May 19 2011: I think you would have to design a more careful scientific protocol than that. It might be possible to think of "emotions" as a dimension, but it's clearly not the same kind of dimension as time and space -- so one could argue that you're just abusing the terminology.

      But emotions are real, and hard to study. Human sciences in general are much more difficult than physics and astronomy.
      • May 19 2011: ah. thx for your response! when i was a little kid, one of my teachers had me read a book called flatland or something... and it said that to find the next dimension, you just had to go "inside yourself" or something like that lol. i guess that's what i've been imagining all these years!
  • May 19 2011: Hello Sean,

    Does that all mean that there is actually nothing like time? Is it just a our "feeling" of the changes around, states changes, fluctuations? Without any change around us would we be in present time time? Ok, just said there is no time in fact :-).
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      May 19 2011: I'm someone who believe that time really exists. Otherwise how would we know when to participate in the TED chat?
  • May 19 2011: What dictates a symmetry?
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      May 19 2011: Symmetries happen when you can make some sort of transformation and leave the underlying dynamics invariant. For example, you can measure the mass of the electron here, and then measure it a million miles away; you'll get the same answer.
      • May 19 2011: Thanks, yes certainly, but why should it be so? I mean what dictates this situation?
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          May 19 2011: Who knows? We're just lucky the universe is like that, because symmetries make things much easier to understand.
  • May 19 2011: Hello Sean. I know you're leaning towards the multiverse being true, but I wanted to ask a question that assumes that the Universe is all there is (multiverse doesn't exist). It's been explained that the expansion of space is "self-contained" but our intuition of course fails us miserably when trying to visualize this because our brains always assume that an expansion is expanding into some pre-existing volume. Do you know of any way on how to best visualize a "self-contained" expansion?
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      May 19 2011: Sadly, no. The mathematics are perfectly clear and unambiguous, but the translation into words is hard, because as you say this kind of thing is outside our experience. Space is expanding, but it's not expanding "into" anything at all.

      Some people find it helpful to think of space as fixed, and everything in it to be *shrinking*. That's mathematically equivalent and perfectly okay, if it helps you out.
  • May 19 2011: I believe at one point you suggested that periodicity gave us a way of intuiting the existence of time. (Please correct me if I'm wrong.) This idea leads me to wonder how anything can behave in time: the capacities or affordances of time, if you will. It seems things can be periodic or aperiodic, measured or unmeasured, successive or simultaneous, continuous or discontinuous, static or dynamic, and goal-oriented or directionless. At first glance, might it seem that I'm missing anything? I'm very hesitant to think this might be an exhaustive list.. Perhaps you know of any resources on this topic? Thank you!
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      May 19 2011: I don't want to say the list is exhaustive, but things can certainly behave in very different ways with respect to time. Time exists by itself, but my remarks about "periodicity" have to do with how we *measure* time. Our universe is full of things that repeat themselves in synchronicity with other things -- we call such objects "clocks." Without them we wouldn't be able to measure time very accurately, although (given the laws of physics as we understand them) time would still exist.
  • May 19 2011: OK with hypotheticals! OK then;
    Just spitbalin here, but, lets say an atom was travelling at the speed of light exactly, lets say that somehow it was aligned so that it was traveling such that the subatomic components were aligned horizontally along the direction of travel of the whole atom (which "could" happen for an instant maybe?).
    When the atom is traveling one direction at the speed of light would the particles that are travelling around the atom horizontally be traveling slower than the speed of light when traveling away from the direction of travel of the whole atom and faster than the speed of light when travelling toward the direction of travel of the whole atom?
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      May 19 2011: Sorry to be a wet blanket, but the right answer is: atoms can't move at the speed of light. No object with mass can do that; only massless particles like photons.
      • May 19 2011: Wet blankets are fine where appropriate.
        What about all the things caught in the well of the black hole?
        Do they just burn off as they travel around and into the center and what speeds could those things reach?
        Also, is there any possibility of charged particles orbiting each other in the same manner?
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          May 19 2011: In a black hole, everything gets squeezed together until they create a singularity of infinite density. Speeds go up and up along the way.
  • May 19 2011: Hi Prof Carroll
    Many eastern philosophies regard time, at least in relation to our earthbound experiences, as being circular, whatever has happened will happen again, rather like the print of a movie, the swastika being the ancient symbol of the never ending circle of time. What are your thoughts on such ideas, seems quite a sophisticated view of time from a seemingly technologically unsophisticated past.
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      May 19 2011: I personally don't think that time is circular, although other people take that possibility very seriously. One big problem is the arrow of time: in our observable universe entropy increases, but you can't increase forever along a circle; at some point the entropy would have to start decreasing again. It's interesting to contemplate, but I don't know of a really compelling model along those lines.
  • May 19 2011: i know this sounds cliche, but i keep hearing that we've discovered signs of life out there. if we have, are we actually able to prove that there is life out there?
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      May 19 2011: We haven't! Yet. Unless the government is hiding something from me.

      It might be hard to know that we've discovered life, if it takes the form of an unusual and complicated but subtle chemical reaction on some object here in the Solar System. If we find aliens and start chatting with them, hopefully we will be able to recognize signs of intelligence.
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    May 19 2011: Is it possible for time to go backwards? I keep hearing the second law of thermodynamics as proof that it can't. But are there sensible cosmological models which might allow it to move in reverse?
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      May 19 2011: It's hard to say what it would mean for "time to go backwards." If entropy were decreasing rather than increasing, we would simply change our definitions of "past" and "future" to compensate. What matters is that entropy is changing in a consistent way. Certainly there are cosmological models in which entropy evolves very differently in different parts of the universe, but those parts are generally widely-separated and non-interacting.
  • May 19 2011: Hi. My question is a little off track.....but I think it has to do with the idea of entropy just not on the cosmological scale. I recently watched a youtube video by a physicist at the University of Boulder. (Albert Bartlett) His talk was basically about how we often fail to understand exponential growth, in regards to population, and economies. I read your book "The Arrow of Time", and I can not claim to have absorbed or understood all of the implications of the thought experiments; however, i found the part about entropies relation to complexity....and maxwells demon's relation to life to be some of the most mind blowing parts that I read. I wonder if you could elaborate a connection between those concepts and exponential growth(if there is any).......i figured because of the relationship of thermodynamics....to the consumption of resources that there is one........
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      May 19 2011: Exponential growth is important -- but so are other kinds of evolution that start off exponential and turn into something complete different, such as yeast growth curves. The universe is a complex place, and we should be open to all sorts of possibilities, rather than finding one favorite idea and applying it too indiscriminately.

      But you're right, the connection between entropy, complexity, and life is endlessly fascinating.
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      May 19 2011: We don't know much about the universe before the Big Bang, so that's hard to say! Black holes typically have an event horizon and an "outside," which the universe does not, so I don't think that's a great analogy.

      http://blogs.discovermagazine.com/cosmicvariance/2010/04/28/the-universe-is-not-a-black-hole/

      The (apparent) event horizon of an exploding black hole actually *shrinks*, as the hole loses mass. The constants all stay the same -- as far as we know, we obviously haven't done that experiment.

      For #4, I do believe that all options are open, although I'm not very fond of "bounces" in general. Hard to arrange things correctly before the bounce.
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          May 19 2011: You should take a look at the blog post I linked to, which goes into more detail. Of course there could be an outside to the universe, but it doesn't seem to be a crucial element of the part we can observe. If anything, the universe is like a *white* hole, not a black hole: we have a singularity in our past, not our future.
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          May 19 2011: I have oft wondered if gravity was caused by the outer universe. Somewhat of a pressure from the otherside. I was thinking of many factors such as when a super nova explodes creating a black whole is it because the center of the star was crushed to a critical mass or is it because the outside layers are slow to return to a smooth state afterwards. If the star was "sandwiched' between to layers of the otherside then when it went super nova the small area of the star grew very forcefully, the otherside reacting somewhat as a non-Newtonian liquid is settling back into the void created by the super nova. The reason then that light does not escape is the black hole is essentially a vacuum, trying to reach an equilibrium in the amount of energy in the void. With time the otherside will settle to a smooth sate but while that void is it will pull in energy to replace what was expelled during the super nova.
  • May 19 2011: Hi Sean, big fan of your work as well! Something that bothers me - is it possible that we as humans are way too fine-tunned into our limited environment and so are our senses and thinking and our views on cosmology are clouded by that? I am afraid we are asking the wrong questions - time before big bang is just not a legitimate question as are many others. Do you believe we are really asking the right questions? : )
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      May 19 2011: Well, we never know until we find the final answers! What's important is not that we're asking exactly the right questions, because it's hard to know what those are. What matters is that we're making progress, and I certainly think that's true.

      Our brains did not evolve to study physics or cosmology; yet, we're doing a very good job. We know how old the universe is, and what it's made of! Those are impressive accomplishments by any measure. But we'll have to see what the future holds.
  • May 19 2011: Hi Sean,
    Let me add my hearty gratitude to all the TED contributors, I've been thoroughly entertained, gratefully educated and quizzically intrigued (wishing I hadn't just downed that beer not 5mins ago) but nevertheless I'm thankful that TED has been the best site on the net that I've found, so far :)

    To my question, one I couldn't answer for my nearly 7 year old friend, Louis - The Plasma streaming from the sun that's directed by the Earth's magnetosphere which has a north and a south - Does that mean the the Aurora Borealis and Australis are made of either positive or negative particles and if so which is which?

    My apologies if the is a bit Earthbound for you, being a 'timelord' and all, but I just thought you might know the answer. Thanks :)
    Bill
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      May 19 2011: This actually is too Earthbound for me to give an expert-level answer! But the North and South poles are *magnetic*, not *electric*, which makes a difference. Both light up because of interactions with both positive and negative particles.
  • May 19 2011: Another question I have. Given that the universe is expanding with increasing acceleration,we can say that time will never end ( I guess?). But did it have a starting point? As far as I know the classical theory of Big Bang says it started at the moment of Big Bang , but the new concepts in physics predict there might be "something" before the Big Bang. (Multiverse). With the multiverse idea, how can we deduce something about the beginning of time?
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      May 19 2011: Short answer is, we don't know. The Big Bang could very well have been the beginning, or it could just be a phase the universe (or our small part of it) went through. There are models of both kinds on the market. I personally suspect there was something before the Big Bang, but it's good to keep an open mind at this point.
      • May 19 2011: Thank you for your answer Prof. Carroll! A small question I have: What was/would be our best argument against "fine-tuning" (anthropic principle), before the multiverse idea/if the multiverse idea starts getting rejected on a large scale?
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          May 19 2011: Fine-tuning is hard to quantify, and it depends a lot on which particular feature we are talking about. You have the "hierarchy problem" of particle physics, the cosmological constant problem, and the entropy problem. It might turn out that all three of these have separate, non-anthropic explanations. If I knew what they were, I'd write a paper!
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      May 19 2011: I think it affects my everyday life quite a bit. When you fly to a physics conference, and you're in the airport waiting to board, it's usually not hard to pick out the other physicists. But I try to blend in to some extent.

      Physics helps you think quantitatively about the world, understand hypothesis-testing, and get some feeling for the uncertain values of real-world measurements. All very useful skills!
    • May 19 2011: It depresses us, thats how. And it seems that all those minds working on explaining that distant phenomena will themselves never live to experience them.
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        • May 19 2011: Dont you compare yourself, your existence with the proportions of the universe? what is your answer to extracting the knowledge about the planets around the nearest star? is there life there(probably)?how many varieties? how will you prove that hypothesis? and when? The big questions with our current computational capacity and state of development take huge amount of time. Maybe sending AI and wait hundreds or thousands of years for information to come back?
          I agree that it is fun to answer questions, but the point of answering is to know more and more and at the end to know it all. Then you can play. That went too far :)
          My point is that first we have to concentrate our efforts to extend our live long enough, that is how we beat time! Then, everything will be different.
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    May 19 2011: On a separate note: How have the cards been treating you? And does your physics training help around the poker table?
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      May 19 2011: Cards have been treating me very well. Here's the tiny "trophy" you get for winning a tournament at Mandalay Bay:

      http://yfrog.com/h2gsh9tj

      I think the physics training does help; Michael Binger, a PhD in particle theory from Stanford, is a very successful professional poker player who finished 3rd in the World Series of Poker a few years ago. My wife Jennifer Ouellette looked into this phenomenon in an article for Discover magazine:

      http://blogs.discovermagazine.com/cosmicvariance/2010/10/06/physicists-playing-poker/
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        May 19 2011: !!! Michael Binger was my office-mate in grad school!

        We've gone different paths...
      • May 19 2011: Hello Professor Carroll,

        I am also a big fan of your work, my favorite living physicist actually. I can tell by following you on twitter that you are a fan of poker, as am I. It says you are on vacation in Vegas right now (I live in Vegas). Which casino will you be playing at tonight? Mandalay Bay? Maybe I can just so happen to run in to you tonight, it would be great to meet you!

        I have no physics question ready for you at the moment, but I want to thank you for doing this. I enjoy watching every one of your talks: you are a brilliant man and an excellent communicator.

        Thank you,
        Russ
  • May 19 2011: Hi Prof. Carroll, I would like to learn your views about the nature of reality. As you know, there are pretty radical views about the nature of reality in science communities : Like holographic universe, mathematical universe hypothesis, or even crazy ideas like "Are we living in a simulation?". Do you think solving the mystery of time is crucial for understanding the nature of reality? Best Regards from a big fan of your work.
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      May 19 2011: The big questions are very hard, and I think it's useful to pursue both "top-down" -- "Let's imagine reality is really like *this*, and see where that takes us" -- and "bottom-up" -- "Let's try to explain the mass of the Higgs boson and the patterns in the cosmic microwave background" -- approaches.

      With that in mind: my personal feeling is that we can learn a lot by trying to tackle the problem of time, which is why that's what I'm doing! I'm sure holography will be part of the final answer; the other ideas, not so much.
  • May 19 2011: Hi Sean, big fan of your work. My question is what are your thoughts on the Higgs Boson particle? Do you believe in this proposed particle? If not what are your theories regarding where/how the mass is distributed?
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      May 19 2011: Thanks! I definitely believe in either the Higgs, or something that is practically the same. The role of the Higgs boson is to explain why the symmetry of the electroweak theory of particle physics is broken -- which it certainly is. Something is breaking that symmetry, whether it's what we think of as the Higgs boson or something else. People have tried, but the Higgs idea is still leading by a long way. See:

      http://blogs.discovermagazine.com/cosmicvariance/2007/11/06/higgs-101/
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      May 19 2011: That's a tricky question. We say that "entropy increases with time," but that's really because we *define* the past to be the direction of lower entropy, and the future to be the direction of higher entropy. What's really going on is "entropy is changing by a large amount, consistently over the history of the universe." That's a bit of a puzzle, because it's easy to imagine entropy just being stuck near some very large value and occasionally fluctuating downward and coming back up. That's what would happen in a box of gas that lasted forever, for example.

      Given that we don't know why the increase of entropy is so dramatic within the universe we observe, it's certainly possible that it changes in other ways in other universes. At least, if we (hypothetically) compare them to our own situation. People in those universes wouldn't notice anything unusual -- they would still remember "the past," the direction of lower entropy from their local perspective.
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          May 19 2011: It's hard, because when gravity becomes important entropy becomes hard to calculate. But I know what would happen: the entropy would go up.
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          May 19 2011: I have to say "maybe," since there's so little about this that we know for sure. But it's not a necessary consequence, according to our current ideas.
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    May 19 2011: Hi Sean, thanks for doing this!

    How did you get interested in the problem of the arrow of time? What about that question appeals to you?
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      May 19 2011: The arrow-of-time problem is just intrinsically interesting -- who doesn't want to know how time works? Fortunately for me, it turns out that one of the biggest unanswered questions in the subject comes from cosmology, my home area -- why did the Big Bang have such a low entropy? (I.e., why was it so orderly, rather than random?)

      Also, this is an area where philosophy can really contribute to the progress of science, which I find interesting. Figuring out the arrow of time isn't simply a matter of doing a few calculations, it's really about making sure we're asking the right questions.
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        May 19 2011: Ooh! Do you have a favorite 'wrong' question? Or a place where philosophy pointed out a much more productive line of questioning? (I'm a sucker for interdisciplinary success stories....)
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          May 19 2011: I should have something specific in mind, but I don't. Many physicists will claim that Boltzmann's H-theorem completely explains why entropy increases in one direction of time but not the other, which is manifestly untrue. The need for a "past hypothesis" -- an assumption that the early universe takes a particular low-entropy form -- has been recognized by some physicists, but really clarified and emphasized by philosophers.
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      May 19 2011: Hi Nafissa--

      That's exactly true. Light travels at the speed of light, which takes time. We're seeing the Sun as it really was about 8 minutes ago, and distant galaxies as they were millions or billions of years ago. (There's a slight clarification here that relativity doesn't let you define "right now" in a rigorous way for distant objects, but you get the basic point.)
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          May 19 2011: Well, it means that spacetime doesn't divide up into "time" and "space" in a clean and unique way. That was one of Einstein's biggest insights when he invented the theory of relativity. What is "now" to us here can be past, present, or future to someone very far away, depending on how things are defined.

          It sounds like a mess, but the math is actually quite unambiguous and pretty!