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.
...gniP
!snorom ,yaw gnorw eht gniog er'uoY
(in a quantum sense)
We always seek but do not always find...
Sean M. Carroll & Jennifer Chen, Spontaneous Inflation and the Origin of the Arrow of Time (PDF)Abstract:We suggest that spontaneous eternal inflation can provide a natural explanation for the thermodynamic arrow of time, and discuss the underlying assumptions and consequences of this view. In the absence of inflation, we argue that systems coupled to gravity usually evolve asymptotically to the vacuum, which is the only natural state in a thermodynamic sense. In the presence of a small positive vacuum energy and an appropriate inflaton field, the de Sitter vacuum is unstable to the spontaneous onset of inflation at a higher energy scale. Starting from de Sitter, inflation can increase the total entropy of the universe without bound, creating universes similar to ours in the process. An important consequence of this picture is that inflation occurs asymptotically both forwards and backwards in time, implying a universe that is (statistically) time-symmetric on ultra-large scales.
Sean M. Carroll & Jennifer Chen, Does Inflation Provide Natural Initial Conditions for the Universe? (PDF)Abstract:If our universe underwent inflation, its entropy during the inflationary phase was substantially lower than it is today. Because a low-entropy state is less likely to be chosen randomly than a high-entropy one, inflation is unlikely to arise through randomly-chosen initial conditions. To resolve this puzzle, we examine the notion of a natural state for the universe, and argue that it is a nearly-empty spacetime. If empty space has a small vacuum energy, however, inflation can begin spontaneously in this background. This scenario explains why a universe like ours is likely to have begun via a period of inflation, and also provides an origin for the cosmological arrow of time.
bump
I can hardly understand this stuff, but I have this theory about time: All of our theories about it are completely wrong.
I'm reminded of St. Augustine's famous remark: "What is time? If no one asks me, I know; if I wish to explain it to one that asketh, I know not."
"time is just natures way of keeping everything from happening all at once"
Naw, it's just me. I've read about time, and it always seems to be about to fall into place, and then I step back and it just seems completely theoretical and based on a certain mindset. I think time is all about assumptions. But then, i can't understand even the assumptions.
Time: It's what keeps everything from happening at once!
Mark
Another classic...
Time is unidirectional..
I would also posit that time is actually a constant..
It is only in a relativistic sense that time is "perceived" as moving faster or slower..
I think we will find that the speed of light itself is not the constant we believe it to be..
It can be surpassed..
What the consequences may be in doing so, I do not know..
I will bet, however, that time travel is not one of them..
We may eventually be able to traverse vast distances very quickly, but it won't affect time itself..
Just my two cents worth..
Time is just a means to measure something's existence. In other words, it's arbitrary. Gravity is the force that needs to be understood to move to a higher plane.
'Time flies like an arrow; fruit flies like a banana' ping
Somehow reminds me of
" . . . time, times and half a time . . . "
Let's hear it for the Chambers Brothers!
Oh, yeah, baby!
Time is unidirectional..I would also posit that time is actually a constant..
It is only in a relativistic sense that time is "perceived" as moving faster or slower..
Should we understand you to mean that you deny that photons experience no lapse of time?
I think we will find that the speed of light itself is not the constant we believe it to be..
It can be surpassed..
The constancy of the speed of light in a vacuum is a cornerstone of special relativity, an experimentally well-tested theory that we've found no cause to emend just yet. You may recall, though, that light has been slowed to a stop in recent laboratory experiments. Only tachyonic stuff could exceed the speed of light in a vacuum, and we've seen no evidence of such stuff so far.
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