Time Flies Like an Arrow

… And fruit flies like a banana. HA HA HA.

Sean Carroll of Caltech explains how time is just another dimension and the unidirectionality of time’s flow is, in actuality, a big puzzle for scientists to figure out. And he does it in under two minutes.

I’ll grant the “Minute Physics” series’ misnomer, given that it’s about time. It also makes an excellent point I’ve made myself in the past (heh) when debating with theists regarding how “order can’t come from disorder” in a universe with the laws of thermodynamics as they are. I generally point to the initial singularity as a moment of perfect order and everything that has occurred since then as a net increase of entropy. The order we as life have built on our planet, owes entirely to the energy output by the sun making our planet not a closed system. The whole universe works in much the same way, with “order” seeming to increase, but only in places where there’s energy being put into the system from outside sources. And ultimately, having clumps of matter in balls in an expansive universe, instead of evenly distributed throughout the very tiny singularity universe, is disorder rather than order.

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Time Flies Like an Arrow
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10 thoughts on “Time Flies Like an Arrow

  1. 1

    Actually, I’m afraid that having mass organized in stars and planets is less “disorderly” (that is, entropic) than a thin soup of matter evenly spread across the universe.

    We use the word “disorder” for entropy in casual or popular physics writing, because it’s the closest approximation in everyday language. But entropy is actually a statistical concept that doesn’t have an exact match in everyday language, because most people don’t have good statistics intuition. It is technically “entropy”, not common-sense “disorder”, that the 2nd law of thermodynamics says will always increase.

    Entropy is actually the number of different combinations (of particle positions and motions, in the case of physics) that can produce the same macroscopic result. When you roll 2d6 (or a pair of craps or Monopoly dice, if you’re less nerdy than I am) the result with the highest entropy is 7, while 2 and 12 have the lowest entropy. You’re more likely to end up on 7 exactly because it’s high entropy – there’s more ways for it to happen. That’s how the 2nd law of thermodynamics works: you’re more likely to end up with high-entropy states because there’s more ways for them to happen. Except thermodynamics rolls Avogadro’s Number d6, so “more likely” is more like “overwhelmingly probable.”

    A universe full of stars and planets is relatively low entropy, because the particles have fewer places they can be: most of them (not counting the dark matter) are restricted to the spots where the stars and planets are. In a universe that’s a vast uniform soup, there are many more possible positions for a particle – it could be anywhere! – so the entropy is higher. If it weren’t for the restriction of gravity, stars and planets would fly apart into a higher-entropy thin-soup state.

  2. 2

    My understanding of the singularity prior to the Big Bang was that it was a uniform soup of matter that was compressed into the least space necessary to contain all that matter. The expansion thereafter was actually pulling everything apart and giving everything more place to “be”. Therefore, as the expansion proceeds apace, entropy’s increasing just by virtue of there being more possible places for everything to be. So, does what you say invalidate the construct I used to rebut theists, Robert?

  3. 3

    Nope! It just invalidates the last line, which was more of an extra point than an integral part of the case. Sorry I didn’t make that clear.

    I don’t know about anything before the Big Bang, but just afterward, the universe was indeed a uniform soup, though a very hot and dense one. You’re quite right about what the expansion of the universe did to entropy – that’s exactly why we were able to go from uniform soup to stars and planets, because a small uniform soup winds up being less entropic than a big bunch of dense objects. If the universe hadn’t expanded, we’d have had to stay as a soup – everything would be too hot for gravity to hold a star together.

  4. 4

    Very good — I amended the last sentence to make it clearer, then. And “prior to” the Big Bang is only extrapolation about the seed state that must have resulted in the physics we have information about, shortly after the universe got to banging. I suppose “prior to” is the wrong way to phrase it. We know enough about cosmology and the hairy beginnings to get a sense of what that seed condition must have been, but “before time” doesn’t work. Time STARTED then. There’s no before.

    You know, I love this stuff. If I hadn’t gone autodidact computer nerd, I almost certainly would have done theoretical physics.

  5. 5

    Technically, the concept of time starting at the Big Bang is only a plausible explanation. Certainly if there was anything before it, the physics say that whatever it was couldn’t matter to us. But the Big Bang is a singularity, which means a point where our understanding of physics goes all wonky. “Singularity” is one of those words like “dark matter” – it’s basically physics-ese for “buuuuuuh, I dunno?”

    Here’s an alternate model – I don’t think it’s probable, but I don’t know of anything that rules it out. The universe we can see is one side of a two-branch universe. In our branch, the universe is expanding from a Big Bang at the beginning. But in the other branch, the universe was contracting to a Big Crunch at the end. In other words, a whole bunch of widely separated matter fell into a point singularity, and then burst out into what we see now.

    And actually… suppose entropy flows away from the singularity in both directions, so time “works backward” in the other branch… and then populate it with antimatter instead of matter so that the physics all come out the same… then you get a coherent symmetric double-universe that maybe solves two fundamental asymmetries we observe (matter v. antimatter and the arrow of time)… I just blew my own mind.

    Uhh… The point is, we don’t really understand singularities. Time beginning at the Big Bang is just the simplest hypothesis that fits the facts, not anything we’ve proven.

  6. 6

    When I hear the word singularity, I think “matter compressed into the smallest possible space for the amount available”. Though, I realize it has another meaning, being “a singular event”, something that can only happen once. I’m a bit annoyed that the Geek Rapture crowd called their self-feeding tech event “the Singularity” because of that.

    I like the symmetry of the double-universe idea. If some particles are the same as other particles travelling back in time (e.g. a proton is an electron in reverse), and if antimatter is just matter with a different spin, what if that “mirror universe” expands into the “prior” area of the time dimension and it’s the same as our universe only everything’s got an opposite spin — a looking-glass version of our universe with the singularity at the center.

    Of course, we can theorize all sorts of unfalsifiable claims. We haven’t even really figured out this universe though, much less shown any way to prove what’s going on elsewhere.

    One idea I really like is that this universe’s matter emerged from (or “still is”) another universe’s black hole. Since black holes likely have no movement of time within them, all the matter there is in the same sort of timeless state that the singularity at “the beginning” of this universe would have been. It would be very symmetrical if all the matter seeding the initial state of this universe was the result of a black hole eating matter in another.

  7. 7

    It’s actually more like “matter compressed into such a small space that it shouldn’t be possible.” Singularities just make no sense according to all our models of physics, they throw out infinities that break every physics equation we’ve got, but those same models say that singularities have to happen. Which is awesomely puzzling.

    What you said, “matter compressed into the smallest possible space for the amount available,” is more like neutronium, the stuff neutron stars are made of. It’s pretty friggin dense – half a petaton per cubic meter, says wikipedia – but not as dense as a singularity.

  8. 8

    I think the “shouldn’t” is the important part of your clarification. Unless I’m completely off base here, considering that the majority of an atom is empty space, and we’re talking a quantum soup, and not so much a super-dense soup of atoms, wouldn’t that mean a better “compression ratio” without all that extra space holding the bits of the atoms apart?

    The quantum particles that make up atoms are definitely way smaller than atoms themselves — consider that an electron is so small that none of our instruments can actually measure it. And all this quantum “stuff” is so small we have to treat it like a wibbly wobbly “quantum field”.

    I really ought to get some proper training on this. A lay understanding of these concepts is definitely holding me back in this conversation. You have no idea how good it is for me and my curiosity that you’re around and willing to let me pick your brains, Robert.

  9. 9

    Haha, I really do like talking about it. All my friends who were curious about this sort of thing have physics degrees too, and so don’t need me to tell them.

    Your “better compression ratio” idea is exactly how neutronium works. It’s composed entirely of neutrons, packed together just like in the nucleus of an atom, stuck together by the strong nuclear force. The protons are missing because, having like charges, they repel each other electrically, as well as attracting each other with the strong force. The electric repulsion makes a nucleus unstable once you get up near 100 protons. Neutrons, being electrically neutral, avoid that problem, so under the right conditions, the entire mass of a star can compress itself into a single gigantonormous nucleus composed solely of neutrons, and it stays together.

    “Size” is kind of a funky concept on that scale, because neutrons and such behave as much like waves as like particles, but you can take wavelength as a rough equivalent. In neutronium, the neutrons are on the order of one wavelength apart – the quantum mechanical equivalent of being jammed in right next to each other. A chunk of neutronium fifty times the “size” of a single neutron will have about fifty neutrons in it. (I’m really approximating here, since a neutron’s wavelength changes depending on how fast it’s moving, but it gives you the rough idea.) As matter goes, neutronium is fantastically compact – you could send our example chunk through the empty space in a hydrogen atom 1/50 its mass with plenty of room to spare, like throwing a ping pong ball through the St. Louis arch.

    But a singularity is denser even than that. In a singularity, a space the size of a neutron will contain many, many neutrons, which shouldn’t even be possible. Neutrons (and protons and electrons and quarks) just don’t stack on top of each other like that – that’s a rule called the Pauli exclusion principle and we’ve confirmed its predictions many times. Also, in a singularity, our math predicts that spacetime has infinite curvature. That’s a weird concept, but imagine taking two spools of thread and letting them unwind into opposite sides of a black hole. With infinite curvature, the two strings will never meet, even if they’re infinitely long, even though they’re both being forever drawn toward the same point.

    In a singularity, we have no fragging idea what is going on.

  10. 10

    hey, Jason, if you wanna read more about big bang cosmology, check out Ethan’s blog “Starts with a Bang” over at scienceblogs dot com. you can search for his posts on the big bang. good stuff!

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