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What Is Time? One Researcher Shares His Exploration 578

Physicist Sean Carroll has built up a bit of a name for himself by tackling one of the age old questions that no one has been able to fully explain: What is time? Earlier this month he gave an interview with Wired where he tried to explain his theories in layman's terms. "I’m trying to understand how time works. And that’s a huge question that has lots of different aspects to it. A lot of them go back to Einstein and spacetime and how we measure time using clocks. But the particular aspect of time that I’m interested in is the arrow of time: the fact that the past is different from the future. We remember the past but we don’t remember the future. There are irreversible processes. There are things that happen, like you turn an egg into an omelet, but you can’t turn an omelet into an egg."
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What Is Time? One Researcher Shares His Exploration

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  • Time (Score:5, Funny)

    by Anonymous Coward on Friday February 26, 2010 @06:33PM (#31291560)
    About half past ten, give or take a couple of minutes.
  • What Is Time? (Score:4, Insightful)

    by negRo_slim ( 636783 ) <mils_orgen@hotmail.com> on Friday February 26, 2010 @06:33PM (#31291566) Homepage
    • Re: (Score:3, Informative)

      by Jorl17 ( 1716772 )
    • Re:What Is Time? (Score:4, Interesting)

      by N7DR ( 536428 ) on Friday February 26, 2010 @06:55PM (#31291804) Homepage

      As Einstein famously said: "Time is what a clock reads". I always thought that was rather a clever evasion: true but not particularly helpful.

      For many years that quotation was on a poster that greeted one in the lobby of what was then the National Bureau of Standards in Boulder, CO.

      • Re: (Score:3, Interesting)

        by joe_frisch ( 1366229 )

        Another version (Sorry, don't know if it was from Einstein or from the Meisner Thorne Wheeler book) is that time is chosen to make physics look simple. If I plot the position of an object without any forces acting on it, I can choose time to be such that its position is a linear function of time. Since clocks are based on mechanics, this pretty directly turns into time is what a clock measures.

        Once you go beyond that sort of description you need to tread carefully to avoid turning your physics into philosop

    • Or antimatter (Score:5, Insightful)

      by mangu ( 126918 ) on Friday February 26, 2010 @07:03PM (#31291880)

      Thermodynamics is one of two sets of phenomena that are irreversible. The other [wikipedia.org] is rather obscure, but is related to the fact that "ordinary" matter seems to be so much more abundant in our universe than anti-matter.

      All other phenomena in our universe are reversible in time, which raises an interesting question: are we unable to see the future because our brains work on thermodynamic operations?

      Not only biologic brains, but digital computers also depend on non-reversible operations. A two-input AND gate has a "0" output in three different input conditions: "00", "01", and "10". Now imagine a computer that uses a reversible logic system that is reversible, would that computer have a time-symmetric operation?

      • CP != T (Score:3, Interesting)

        The other [wikipedia.org] is rather obscure, but is related to the fact that "ordinary" matter seems to be so much more abundant in our universe than anti-matter.

        Sorry but you are confusing CP (matter/antimatter) symmetry and T (time reversal) symmetry. These are not the same. In addition time reversal violation does NOT mean that a process is irreversible it just means that it prefers to go in one direction over the other.

        Both have been independently shown to be broken: CP in K and B meson decays and T in K and B meson oscillations which might be the source of your confusion. It is also worth pointing out that the combination of all three, called CPT, is expect

      • Re: (Score:3, Insightful)

        by Hurricane78 ( 562437 )

        I think that that question is just as pointless, as asking what was before time, or what is outside of everything. Because “operation” is only defined in terms of a progressing time.

        But you could ask how the world would look if something like that existed, and then compare it to reality with experiments, to find out if it is at all possible. (Just like the final argument of (I think) Bohr against Einstein in the great debate about quantum physics.)

    • by snooo53 ( 663796 ) *
      You know, usually I get really irritated tongue-in-cheek answers, but yours fits the article perfectly (of which reading was a big waste of time IMHO).
  • Time is the goo... (Score:4, Informative)

    by gestalt_n_pepper ( 991155 ) on Friday February 26, 2010 @06:33PM (#31291568)

    that connects state one to state two.

    • by debrain ( 29228 )

      Time is the goo ... that connects state one to state two.

      Goo made up of units of the shortest appreciable differences in states.

    • by eepok ( 545733 ) on Friday February 26, 2010 @07:22PM (#31292082) Homepage

      ding ding ding!

      Time is just another dimension, but one we can only experience shallowly.

      Two points make describe line.
      Two lines make describe plane.
      Two planes make describe space.
      Two states of space describe time.

      Time as we experience time in the same way a single-cell organism on a slide experiences 3-dimensional space.

      • by Digital Vomit ( 891734 ) on Saturday February 27, 2010 @03:09AM (#31295268) Homepage Journal
        A point doesn't have to move across a line.
        A line doesn't have to move across a plane.
        A plane doesn't have to move across space.
        So why, then, is space constantly moving across time, always in the same direction? Is "God" pushing our "space" through "time"?

        Why do we "experience" anything at all? Why are we not just static sequences of space?

        I think time is a little something other than "just another dimension". But who can really say?
  • Easy (Score:5, Funny)

    by Yvan256 ( 722131 ) on Friday February 26, 2010 @06:35PM (#31291582) Homepage Journal

    FORD: No, No listen. Just imagine that you’ve got this ebony bath, right? And it’s conical.

    ARTHUR: Conical? What kind of bath is -

    FORD: No, no, shh, shhh, it’s, it’s, it’s conical okay? So what you do, you fill it with fine white sand right? Or sugar, or anything like that. And when it’s full, you pull the plug out and it all just twirls down out of the plug hole but the thing is

    ARTHUR: Why?

    FORD: No, the clever thing is that you film it happening. You get a movie camera from somewhere and actually film it. But then you thread the film in the projector backwards.

    ARTHUR: Backwards?

    FORD: Yeah, neat you see. So what happens is you sit and you watch it and then everything appears to swirl upwards, out of the plug hole and fill the bath amazing.

    ARTHUR: And that’s how the universe began?

    FORD: No. But it’s a marvellous way to relax.

    TRILLIAN: Funny man.

    FORD: Well it broke the ice didn’t it?

  • by snowraver1 ( 1052510 ) on Friday February 26, 2010 @06:39PM (#31291620)
    Maybe not directly, but you can feed that omelet to a chicken, and then take the resulting egg.
    • by painandgreed ( 692585 ) on Friday February 26, 2010 @07:35PM (#31292262)

      I read it on a plane trip earlier this month and was fairly disappointed with it. In fact, the entire issue made me decide to never bother with Discover magazine again. I have a physics degree and used to not getting any actual math with my physics in mainstream culture. However, everything it in was pretty much uninformative if you've ever even heard of the subject before. Seriously, wikipedia does a better job and is probably more up to date than Discover magazine.

      This aritcle in question, there was no actual discussion of physics. No talk of the lack of time direction in Feynman diagrams [wikipedia.org]. None of the solutions for time travel that can be come up with using Einstein's equations. Nothing really, just a bit like "you can't go back in time and kill your father because then you wouldn't exist to go back and kill your father" logic. Never mind that this isn't actually supported by physics and Tippler showed [wikipedia.org] that acasual time like paths can occur, it completely ignores the many-world interpretation [wikipedia.org] and it's possible relevance to time travel. never mind that you don't have to actually go kill your dad but just showing up is going to cause the same effect simply from your changes in weather do to chaos theory/butterfly effect. I was hoping for a simple article talking about things I already know with the possiblity of a mention of some new development that I could research later, but ended up with no actual physics (and not even a good philosophical discussion) of the subject. [wikipedia.org]

      Real rules for time travellers? Einstein's theories currently say that a time machine is possible but you can't go back in time to a point before the time machine was .turned on'. Entropy is in there so if you are going back in time it's going to take energy to reverse it. What happens when you go back in time to kill your father is an interesting question, but not one that the article actually addresses in any way that actual addresses physics of the subject. My personal hypothesis is that either you can't change history, only fulfill it because it has already happened, or you end up in a different time line. Yay! now we have a testable hypothesis and science. We just need a way to test it.

      • Re: (Score:3, Informative)

        by evanbd ( 210358 )

        My personal hypothesis is that either you can't change history, only fulfill it because it has already happened, or you end up in a different time line. Yay! now we have a testable hypothesis and science. We just need a way to test it.

        You mean the Novikov principle [wikipedia.org]?

    • Fredrick Brown
      "The first time machine, gentlemen," Professor Johnson proudly informed his two colleagues. "True, it is a small-scale experimental model. It will operate only on objects weighing less than three pounds, five ounces and for distances into the past and future of twelve minutes or less. But it works."

      The small-scale model looked like a small scale—a postage scale—except for two dials in the part under the platform.

      Professor Johnson held up a small metal cube. "Our experimental object

  • But... (Score:5, Insightful)

    by Black Parrot ( 19622 ) on Friday February 26, 2010 @06:42PM (#31291648)

    We remember the past but we don't remember the future. There are irreversible processes. There are things that happen, like you turn an egg into an omelet, but you can't turn an omelet into an egg.

    But if time is non-monotonic, wouldn't we un-remember, un-break things, during the backturns?

    How would anyone know if time isn't always forward?

    • Re:But... (Score:5, Interesting)

      by timeOday ( 582209 ) on Friday February 26, 2010 @07:25PM (#31292138)
      In Slaughterhouse Five, Vonnegut advances the theory that the perception of time is simply a limitation built into us - that everything from all times simply exists, but we can only sample it monotonically (like a flat-bed scanner head moving along).

      If the universe were deterministic, then time is essentially meaningless even if it exists, since the start state and dynamics are all you need to know. And if the dynamics are information-preserving, any state (not just the start state) suffices. Apparently there are even deterministic interpretations of quantum mechanics, although I really don't know what that means.

    • Re: (Score:3, Interesting)

      He actually discusses this fairly well in the interview. Here is where it's put most succinctly:

      Wired.com: In this multiverse theory, you have a static universe in the middle. From that, smaller universes pop off and travel in different directions, or arrows of time. So does that mean that the universe at the center has no time?

      Carroll: So that’s a distinction that is worth drawing. There’s different moments in the history of the universe and time tells you which moment you’re talking about. And then there’s the arrow of time, which give us the feeling of progress, the feeling of flowing or moving through time. So that static universe in the middle has time as a coordinate but there’s no arrow of time. There’s no future versus past, everything is equal to each other.

      The essential point is that the 2nd law of thermodynamics is really a backward way of looking at the question. It isn't that entropy increases with time. It's that we define "forward in time" to mean, "the direction of increasing entropy". Our local region of the multiverse happens to have an entropy gradient in one direction, so that's the direction we perceive time to increase in. But other regions of the

  • What Is Time? (Score:5, Interesting)

    by Monkey_Genius ( 669908 ) on Friday February 26, 2010 @06:42PM (#31291652)
    Time is an artificial construct of the Human mind that allows us to mark our pitiful existence in an uncaring universe.
    • Re:What Is Time? (Score:5, Interesting)

      by Toonol ( 1057698 ) on Friday February 26, 2010 @07:02PM (#31291872)
      Time is an artificial construct of the Human mind that allows us to mark our pitiful existence in an uncaring universe.

      Not far from the truth, but I'd say it is an "amazing creation of evolution" that allows us to "experience the unfurling glory of our life in a rich universe."
    • Time is an artificial construct of the Human mind that allows us to mark our pitiful existence in an uncaring universe.

      I tend to agree with you, but it brings up the question of why the effects of time are different on an observer in motion compared to one at rest.

  • Time? (Score:2, Insightful)

    by daffey ( 1726862 )
    A physicist I'm not, nor mathematician, but 'TIME is CHANGE' in my book. No change- no time. What else can you measure it against?
    • Re:Time? (Score:4, Informative)

      by Monkeedude1212 ( 1560403 ) on Friday February 26, 2010 @06:58PM (#31291830) Journal

      What else can you measure time against, or what else can you measure change against? Because you can measure a change in distance, a change in volume, a change in temperature, the list goes on.

      As for measuring time - you can have instances where nothing changes BUT the time - so thus begs the question, what is time if nothing changes?

      Imagine a single Molecule, Well if you can imagine it moving you know it has speed and then you just take the change in distance to find the amount of time that had passed.

      Well, imagine if it didn't have a speed - it wasn't moving. How would you calculate the change in time?

      • Re: (Score:3, Interesting)

        As for measuring time - you can have instances where nothing changes BUT the time - so thus begs the question, what is time if nothing changes?

        Time has effectively ceased if nothing has changed, therefore time is nothing. You've answered your own question, time does not exist, only matter and energy. Rates of change are simply questions of quantities of energy applied.

    • by Toonol ( 1057698 )
      I think that is part of the issue, but there's more problems:

      Change is measured over time. An accelerating change means that more change is happening per time; that means that time is somehow independent of change.

      Also... change happens go forward through time, going backward through time. If you see two atoms collide, the process works forward and backward... but time only seems to go forward. Why does time seem to only exist in one direction?

      It seems to tie heavily into thermodynamics (and, he
  • "We remember the past but we don’t remember the future. There are irreversible processes"

    I am in NO WAY qualified to argue on the subject, but the quoted statement seems like a problem with words and definitions. You can't 'remember' the future because the word 'remember' doesn't apply very well to the word (or usage of the word) 'future.'

    I'll probably be blasted out to hell by an expert in 3...2...1...
    • by sjames ( 1099 ) on Friday February 26, 2010 @08:10PM (#31292604) Homepage Journal

      The semantics is more an artifact of trying to express something that we have no proper words for because it never happens and we can't exactly imagine what it would be like if it did happen.

      At the subatomic level, everything is reversible with equal probability. If a particle can decay into two others, the two others can join to form the particle just as easily. However, at our scale, making all the bits of egg on the floor come back together and the egg then fly up into your hand only happens if you run a movie backwards. Beyond being nearly infinitely funny to first graders, physicists are lead to wonder why that is. What is different between the scales such that equally likely at the small scale becomes "never happens" at ours.

  • v = s/t therefore vt = s therefore t - s/v: Time is simply distance over velocity!

    Honestly it's all very well to swindl^H^H^H^H convince people to give you grant money by investigating "time". I mean, the prospects of having one's very own time machine are incredible.

    Yet one has to ask, (and this is where tenses get complicated, I will resort to the Douglas Adams trans-temporal convention) if anything practical wioll have come from such a study, we would have been receiving visitors from "the future" for a

    • by Eudial ( 590661 )

      v = s/t therefore vt = s therefore t - s/v: Time is simply distance over velocity!

      Distance between what, and velocity of what? What about stationary objects?

  • Humpty Dumpty sat on a wall,
    Humpty Dumpty had a great fall.
    All the king's horses and all the king's men
    Couldn't put Humpty together again.

    A more apt question is this: What is Entropy [wikipedia.org]?

  • Time Travel (Score:5, Funny)

    by Citizen of Earth ( 569446 ) on Friday February 26, 2010 @06:52PM (#31291766)
    Hello. I am a time traveler. Be not afraid. I come from the past and I travel into the future at a rate of one second per second.
  • by joebok ( 457904 ) on Friday February 26, 2010 @06:55PM (#31291794) Homepage Journal

    From St. Augustine's Confessions, Book XI:


    17. At no time, therefore, hadst Thou not made anything, because Thou hadst made time itself. And no times are co-eternal with Thee, because Thou remainest for ever; but should these continue, they would not be times. For what is time? Who can easily and briefly explain it? Who even in thought can comprehend it, even to the pronouncing of a word concerning it? But what in speaking do we refer to more familiarly and knowingly than time? And certainly we understand when we speak of it; we understand also when we hear it spoken of by another. What, then, is time? If no one ask of me, I know; if I wish to explain to him who asks, I know not. Yet I say with confidence, that I know that if nothing passed away, there would not be past time; and if nothing were coming, there would not be future time; and if nothing were, there would not be present time. Those two times, therefore, past and future, how are they, when even the past now is not; and the future is not as yet? But should the present be always present, and should it not pass into time past, time truly it could not be, but eternity. If, then, time present -- if it be time -- only comes into existence because it passes into time past, how do we say that even this is, whose cause of being is that it shall not be -- namely, so that we cannot truly say that time is, unless because it tends not to be?

  • Larry Niven wrote that "A man who can make fire burn backwards is mighty wizard indeed", or words to that effect. One of his short stories, I forget which.
  • I just took a linear algebra course, and to perform a translation on a matrix (each column is a coordinate set, each row is x, y or z coordinates), you first add a dimension, and multiply your matrix by an identity matrix with the wanted translation in the extra dimension. In other words, to move stuff using matrix multiplication, you have to add a dimension. It makes no physical sense, but it is interesting to think of time as this added dimension simply facilitating movement.

    As for the math I am talkin
  • This sounds like the steady state theory. Back then Hoyle was pushing it the idea was that mass comes from nowhere continuously. In this idea entropy just appears in a quiet universe for no reason.

  • by afabbro ( 33948 ) on Friday February 26, 2010 @07:09PM (#31291948) Homepage

    Prove me wrong.

    The future obviously does not exist. The past? Doesn't exist either. Hence, only this present moment exists.

    You can't even prove that the past existed. The only thing we have is present-moment memories, etc. I remember typing "Prove me wrong" but my memory is hardly reliable. If thirty seconds ago you spilled milk on your pants, all you have now is wet, soggy pants, not any "chain of events". Even if you filmed it, all you have is the present-moment series of images, not some actual piece of the past.

    Only this present moment exists. All else is wild speculation and fantasy. Time does not exist.

  • Hello:

    Time will never have an arrow. Spacetime will, from the space part. If you take Minkowski's advice, that one should only think about spacetime, not time or space, then Carroll's question is poorly formed. It is good English, bad mathematical physics. Since Minkowski's observation was based on work with special relativity, people presume is observation applies only for relativistic systems. Sorry, Nature is more consistent than that: one needs to think about spacetime always, even if it contributes

  • TFA is bullshit (Score:3, Interesting)

    by mestar ( 121800 ) on Friday February 26, 2010 @10:39PM (#31293964)

    I got zero new information about time in the article.

    From wikipedia:
    http://en.wikipedia.org/wiki/Entropy [wikipedia.org]

    "Entropy is the only quantity in the physical sciences that seems to imply a particular direction for time, sometimes called an arrow of time. As we go "forward" in time, the second law of thermodynamics states that the entropy of an isolated system tends to increase or remain the same; it will not decrease. Hence, from one perspective, entropy measurement is thought of as a kind of clock"

    Bad car analogy:
    This is silly in a same way if you had an indicator light that would turn on only if you are going forward, and then call that light "a speedometer".

  • by jopet ( 538074 ) on Saturday February 27, 2010 @03:43AM (#31295374) Journal

    Theories that involve the multiverse are, in my opinion, nearly as unscientific and embarrassing as religion or theories that involve "god": you can "explain" nearly everything and you can prove nothing. Give me a break with multiverses.

    How is the question why there is a multiverse that spawns off universes randomly so much nicer that the question why there is a universe? It is equally unanswerable but introduces complexity: let occam's razor cut away the multiverse part until there is anything that is falsifyable about the story.

To write good code is a worthy challenge, and a source of civilized delight. -- stolen and paraphrased from William Safire