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.
Not necessarily. See "The Matter of the Bullet Cluster". According to these observers, gravitational lensing may make it possible for us to observe the effects of the mass of concentrations of dark matter on the light from distant galaxies.
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Right Whale: " Less than a century ago there were no galaxies. Everything was inside the Milky Way. Sometime between 1922 and 1960 the universe acquired galaxies..."
Not quite: from the northern hemisphere, there have always been two galaxies visible to the unaided eye: M31, the "Great Andromeda Galaxy" and our own galaxy -- visible from the inside-out as the "Milky Way". However, I agree that it wasn't until suitable telescopes were developed that we realized what they were -- and that there are lots of other galaxies.
BTW, & FWIW, in the photo at the above link, there are only a handful of nearby stars visible; all the rest of those white "spots" are galaxies...
In another comment, you bemoaned the lack of "C/E debaters" on this thread. Along those lines, I'll add that I can't comprehend how the "YEC" folks can "stick to their guns" after having seen photos like the one above -- or the Hubble Ultra Deep Field images... To me, those images make it impossible for me to accept Earth as the center of everything -- or the proposition that everything is only a few millenia old...
Right. We had a thread on that last week. NASA Finds Direct Proof of Dark Matter.
That's a great thread! I'm bummed out that I missed it "in real time"...
I came back and the thread was still extant so I have to assume your time-jump was either unsuccessful or incomplete....;]
[or maybe you're just still in the bathroom?]
"I like Bananarama."
I remember Wonderama.
[has anbody here seen an aardvark?]
Heh heh heh. They were hoping you'd be so dazzled by their extreme brilliance, you'd forget to ask.
In fact, the answer is "Turtles all the way down..."
Cheers!
P.S. Did they misspell the name of the magazine, "Seed"? (Of course, both spellings relate to plants.)
There is nothing, not even in the infamous thermodynamics that gives a preference to the direction of time.
Do you mean "intrinsic" or "overwhelmingly probable" preferenc...?
Details, please...?
Cheers!
Leave DU out of this! :-)
Cheers!
What did Bush know, and when did he know it?
Or to quote Feynman...
I wonder why
I wonder why
I wonder why I wonder
I wonder why I wonder why I wonder why I wonder...
Cheers!
This sentence just puzzles me.
I saw that quote in a Dorothy Sayers novel, but don't know the original source--what is it, please?
Cheers!
I thought relativity said that there is no preferred inertial reference frame.
Cheers!
Need the isotropy be "absolute" or just "as well as we can measure"...?
Thinking here of string theories and collapsed dimensions which seem to argue against isotropy.
(Reaching vainly for straws)--has anyone done an analogy to Minkowski space-time, using collapsed dimensions in addition to the convention Cartesian (or spherical polar, I don't mind) coordinates?
...or am I just re-inventing the square wheel?
Cheers!
Thank you so much for the ping and for sharing your insights and the links!
Perhaps they are complementarities in the sense intended by Niels Bohr, who said that two mutually exclusive principles are both necessary to the "complete description" of the system of which they are the "mutually exclusive" parts.
In short, seeming incompatibilities or even outright mutually exclusive principles may find logical resolution at a higher level of understanding -- of Truth that harmoniously integrates both into a more comprehensive description of reality.
But then, people today tend to expect that Einstein's remark is the correct one: If you have two mutually exclusive principles, then at least one of them must be wrong....
I love Einstein. But I don't agree with this statement. Aristotle's law of the excluded middle is crushing human intelligence these days, leading to a digital style of thinking that does not accord with direct human experience of the real world.
Well, FWIW. That and a buck-twenty-five (plus tax) might get you a cup of coffee....
Good night, dear Right Whale!
Supposedly, Roger Bacon built a human head out of clay (or wood or something); he was going to ask the head a question but the head just said, "Time is. Time was. Time is past." Then a lightening bolt came through a window and destroyed the head.
Zeus (or Thor) must have not been amused.
I agree, thus my contention that time is more a measurement of location.
Something I've always wondered: If a universe spontaneously forms & inflates inside a parent universe, what would happen to anything or anyone who's already inside the parent universe, say within 100 million light years away? Seems to me that would be quite destructive, as in pretty much completely destroying the whole parent universe.
The following paragraph from page 3 of Anthony Aguirre & Steve Gratton, "Inflation Without a Beginning: a null boundary proposal" (2003, PDF format) is somewhat relevant, I think (my red fonting):
"An observer within a bubble can never leave, but will eventually be encountered by an encroaching bubble wall after a typical time τcoll, where τcoll-1 is related to the r-integral of Eq. (5) by some transformation between the bubble observer's proper time τ and cosmic time t. Since this rate depends on t−t0, a patient and very sturdy oberver could in principle discover the global time at which it formed by counting the frequency of incoming bubbles."
"A patient and very sturdy observer"...nice turn of phrase...
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