Posted on 08/30/2006 1:01:48 AM PDT by snarks_when_bored
An event like the Big Bang is about as likely as billions of coin tosses all coming up heads. Explaining why that is might take us from empty space to other universes--and through the mirror of time.
by Sean Carroll • Posted August 28, 2006 11:53 AM
From the SEPTEMBER issue of Seed:
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The nature of time is such that the influence of the very beginning of the universe stretches all the way into your kitchen—you can make an omelet out of an egg, but you can't make an egg out of an omelet. Time, unlike space, has an obvious directionality—the view in a mirror makes sense in a way that a movie in reverse never would.
The arrow of time in our universe is puzzling because the fundamental laws of physics themselves are symmetric and don't seem to discriminate between the past and future. Unlike an egg breaking on the side of a frying pan, the journey of the planets around the sun would look basically the same if we filmed them and ran the movie backwards. Rather, it must be due to the initial conditions of the universe—a fact that makes the nature of time a question for cosmology. Remarkably, the answers we're beginning to discover are telling us there may be other universes out there in which the arrow of time actually points in reverse.
For some reason, our early universe was an orderly place; as physicists like to say, it had low entropy. Entropy measures the number of ways that you can rearrange the components of a system such that the overall state wouldn't change considerably. A set of neatly racked billiard balls has a low entropy, since moving one of the balls to another location on the table would change the configuration significantly. Randomly scattered balls are high entropy; we could move a ball or two and nobody would really notice.
Low-entropy configurations naturally evolve into high-entropy ones—as any billiards-break shows—for the simple reason that there are more ways to be high entropy than low entropy. The very beginning of time found our universe in an extremely unnatural and highly organized low-entropy state. It is the process by which it is inevitably relaxing into a more naturally disordered and messy configuration that imprints the unmistakable difference between past and future that we perceive.
Naturally, this leads one to wonder why the Big Bang began in such an unusual state. Attempts to answer this question are wrapped up with the question of time and have led me and my colleague Jennifer Chen to imagine another era before the Big Bang, in which the extremely far past looks essentially the same as the extremely far future. The distinction between past and future doesn't matter on the scale of the entire cosmos, it's just a feature we observe locally.
If time is to be symmetric—if the direction of its flow is not to matter throughout the universe—conditions at early times should be similar to those at late times. This idea has previously inspired cosmologists like Thomas Gold to suggest that the universe will someday recollapse and that the arrow of time would reverse. However, we now know that the universe is actually accelerating and seems unlikely to ever recollapse. Even if it did, there is no reason to think that entropy will spontaneously begin to decrease and re-rack the billiard balls. Stephen Hawking once suggested that it would—and he later called that the biggest blunder of his scientific career.
If we don't want the laws of physics to distinguish arbitrarily between past and future, we can imagine that the universe is really high-entropy in both the far past and the far future. How can a high-entropy past be reconciled with what we know about our observable universe—that it began with unnaturally low entropy? Only by imagining that there is an ultra-large-scale universe beyond our reach, where entropy can always be increasing without limit, and that if we went far enough back into the past, time would actually be running backwards.
Such a scenario isn't as crazy as it sounds. Our universe is expanding and becoming increasingly dilute, and the high-entropy future will be one in which space is essentially empty. But quantum mechanics assures us that empty space is not a quiet, boring place; it's alive and bubbling with quantum fluctuations—ephemeral, virtual particles flitting in and out of existence. According to a theory known as the "inflationary universe scenario," all we need is for a tiny patch of space to be filled with a very high density of dark energy—energy that is inherent in the fabric of space itself. That dark energy will fuel a spontaneous, super-accelerated expansion, stretching the infinitesimal patch to universal proportions.
Empty space, in which omnipresent quantum fields are jiggling back and forth, is a natural, high-entropy state for the universe. Eventually (and we're talking about a really, really big eventually) the fluctuations will conspire in just the right way to fill a tiny patch of space with dark energy, setting off the ultra-fast expansion. To any forms of life arising afterward, such as us, the inflation would look like a giant explosion from which the universe originated, and the quiescent background—the other universes—would be completely unobservable. Such an occurrence would look exactly like the Big Bang and the universe we experience.
The most appealing aspect of this idea, Chen and I have argued, is that over the vast scale of the entire universe, time is actually symmetric and the laws truly don't care about which direction it is moving. In our patch of the cosmos, time just so happens to be moving forward because of its initial low entropy, but there are others where this is not the case. The far past and the far future are filled with these other baby universes, and they would each think that the other had its arrow of time backwards. Time's arrow isn't a basic aspect of the universe as a whole, just a hallmark of the little bit we see. Over a long enough period of time, a baby universe such as ours would have been birthed into existence naturally. Our observable universe and its hundred billion galaxies is just one of those things that happens every once in a while, and its arrow of time is just a quirk of chance due to its beginnings amid a sea of universes.
Such a scenario is obviously speculative, but it fits in well with modern ideas of a multiverse with different regions of possibly distinct physical conditions. Admittedly, it would be hard to gather experimental evidence for or against this idea. But science doesn't only need evidence, it also needs to make sense, to tell a consistent story. We can't turn eggs into omelets, even though the laws of physics seem to be perfectly reversible, and this brute fact demands an explanation. It's intriguing to imagine that the search for an answer would lead us to the literal ends of the universe.
—Sean Carroll is a cosmologist at the University of Chicago and the author of a popular textbook on general relativity. He is also a regular contributor to the physics blog Cosmic Variance.
Good one.
The very beginning of time found our universe in an extremely unnatural and highly organized low-entropy state.This set off my illogic alarm. What does he mean, "unnatural"? Isn't it nature itself that we're discussing? How many counterexamples can he offer? If he wants to say that the state of space in the distant past is different from the state of space now, that's fine, but calling it "unnatural" is a non-sequitur.
I think by 'unnatural' Carroll means highly improbable. If there are 10 quadrillion ways for something not to happen and only 1 way for something to happen, it's reasonable English to describe the 1 way as 'unnatural'. He certainly doesn't mean 'non-physical'.
I wish this guy would be more precise.
Check out the references in Post #6 above.
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Exactly. Our Universe is but one of perhaps a nearly infinite number, birthed and informed by the Metaverse.
Yea, but when He sinks the 8-ball; Games Over Dude!
I only like the article because they use the egg example.
This is a little bit off topic, but I read something like this and I think, "Here it comes again." So let me anticipate something.
Some years ago, I went around and around with some people about how low-entropy the early universe was and how could it have started so wound up. Their point, as you can imagine, is that it took an intelligence to design a low-entropy starting universe since low-entropy means "highly ordered."
The only way an unimaginably hot quark-gluon plasma is low-entropy is if it is super-small. It turns out that that's indeed the trick. Within the space available (almost zero), the universe is as high entropy as it can be, a gas so super-hot it doesn't even have baryons, just quarks and gluons.
So, yes, where "order" is set as the inverse of "entropy," then like those pool balls racked in the triangle, the low entropy of the early universe comes from confinement in a small space. (It's a singularity of some sort.) If for some reason that triangular rack is allowed to grow bigger every game, pretty soon the initial game conditions are looking pretty ragged.
But all that is using "order," "disorder," and "entropy" in narrow senses. There's a fallacy of equivocation in thinking this equates to "order" as in keeping your room neat. It most certainly does not.
Even as the total entropy of our universe rose with expansion, also with that expansion the plasma cooled. Baryons formed. Atoms formed. Gas and dust condensed under gravity to form stars and planets. Life arose. The universe is "more disordery" in a limited technical sense of the word. However, in the sense most of us think of it there is more order everywhere and anywhere in the arrangement of matter than existed in the quark-gluon plasma. We're just racked up in a really big, loose rack now.
Mmmmmmmmmmmmm...omelets!
I still maintain that the only people who really understood the 2nd law lived in the 19th century.
Food is another thing altogether, though. ;)
Hmmm. I'm not sure your logic holds up; ironically, it might be the very direction of time that monkey-wrenches the works.
What I mean is, probability becomes meaningless when you look backward in time; it only has meaning when you look forward. Let's say I buy a raffle ticket, and I win. Before my winning ticket was drawn, my odds of winning were very-large to one, against...yet the odds of someone winning the lottery were one-to-one, or unity. Somebody will win.
So when I go to collect my raffle prize, the judge looks at me and says "The odds of you winning instead of all those other entrants are so small, it's highly improbable that you in particular would pull the winning ticket. You must have rigged the game somehow!"
No I didn't. Now that I've won the raffle, my odds of having drawn the winning ticket are one-to-one. It's already been done.
Time has as much directionality as the temperature readings on the thermometer on my porch. Today, BTW, is the fourth frost of the month, and this is a good one--all the leaves have visible frost this time. Temperatures are headed down, and the leaves will be headed for the lawn soon, but this may not be permanent.
The arrow of time is a myth. There is nothing, not even in the infamous thermodynamics that gives a preference to the direction of time.
Also, I am wondering lately why those who disapprove of the scientific tool of evolution even care. They have no need of the tool any more than they need an oil refinery in their yard.
I was just speaking of Carroll's use of the word 'unnatural'. The low-entropy beginning of our cosmos does appear to be very highly improbable...which is not to say it can't happen. Indeed, given a long enough time, just about anything will happen (there are no observers waiting around for most of the time so nobody's counting the hours!).
There is a school of thought that denies such things as the flow of time altogether. There is only now.
Considering that it's a veritable law of the universe that entropy increases over time, it appears to me that a low-entropy beginning of the cosmos is not only not improbable, but is in fact inescapable.
Thanks for that. I have always wanted to write a really good time travel story, but I haven't thought of a truly original idea. I should do a little more reasearch...
I wrote one and submitted it to a monthly newsletter, but it scared them so much they thought they better not publish it. Time is fascinating.
There are definitely some terminology problems here. Similar, really, to those who say: "Laws of nature? Aha! That means there's a law-giver!"
The increase of entropy since the BB is mostly a decrease of heat and compactness. (No doubt, our resident physicists will point out the many ways that's incorrect.) If being hot and condensed is so "ordered" that it implies a supreme intellect, well, the implication escapes me.
The arrow of time is a myth. There is nothing, not even in the infamous thermodynamics that gives a preference to the direction of time.
My understanding is that the directionality of time emerges from the statistics of enormous numbers of interacting particles. At the microscopic level, the laws of physics are reversible; at the macroscopic level, their reversibility is swamped by the correlations of interacting particles.
(BTW RW, I could use an Alaskan cold front about now!)
Sounds like the beginnings of intelligent design to me.
Order is a funny term. It can mean everything quiet and organized, or it can mean uniformity. Uniformity could be the condition in a totally random system as when entropy is maxed out or in a political system where everyone thinks the same.
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