Skip to comments.The 11 Greatest Unanswered Questions of Physics
Posted on 02/17/2002 12:35:18 PM PST by The Raven
Here's a tale of modern physics: Two scientists work at the same university in different fields. One studies huge objects far from Earth. The other is fascinated by the tiny stuff right in front of him. To satisfy their curiosities, one builds the world's most powerful telescope, and the other builds the world's best microscope. As they focus their instruments on ever more distant and ever more minuscule objects, they begin to observe structures and behaviors never before seenor imagined. They are excited but frustrated because their observations don't fit existing theories.
One day they leave their instruments for a caffeine break and happen to meet in the faculty lounge, where they begin to commiserate about what to make of their observations. Suddenly it becomes clear to both of them that although they seem to be looking at opposite ends of the universe, they are seeing the same phenomena. Like blind men groping a beast, one scientist has grasped its thrashing tail and the other its chomping snout. Comparing notes, they realize it's the same alligator.
This is precisely the situation particle physicists and astronomers find themselves in today. Physicists, using linear and circular particle accelerators as their high-resolution "microscopes," study pieces of atoms so small they can't be seen. Astronomers, using a dozen or so new supersize telescopes, also study the same tiny particles, but theirs are waiting for them in space. This strange collision of information means that the holy grail of particle physicsunderstanding the unification of all four forces of nature (electromagnetism, weak force, strong force, and gravity)will be achieved in part by astronomers.
The implications are exciting to scientists because bizarre marriages of unrelated phenomena have created leaps of understanding in the past. Pythagoras, for example, set science spinning when he proved that abstract mathematics could be applied to the real world. A similar leap occurred when Newton discovered that the motions of planets and falling apples are both due to gravity. Maxwell created a new era of physics when he unified magnetism and electricity. Einstein, the greatest unifier of them all, wove together matter, energy, space, and time.
But nobody has woven together the tiny world of quantum mechanics and the big world we see when we look through a telescope. As these come together, physicists realize they are getting very close to a single "theory of everything" that accounts for the fundamental workings of nature, the long-sought unified field theory.
About two years ago, after a presentation by the National Research Council's board on physics and astronomy that showed the converging agendas of the two fields, NASA administrator Daniel Goldin suggested a special report that would detail how much astronomers and physicists could benefit from one another's insight. Recently, the council's committee on the physics of the universe released that report. It details 11 profound questions, some of which may be answered within a decade. If so, science is likely to make one of its greatest leaps in history.
But first, what we don't know.
What is dark matter?
All the ordinary matter we can find accounts for only about 4 percent of the universe. We know this by calculating how much mass would be needed to hold galaxies together and cause them to move about the way they do when they gather in large clusters. Another way to weigh the unseen matter is to look at how gravity bends the light from distant objects. Every measure tells astronomers that most of the universe is invisible.
It's tempting to say that the universe must be full of dark clouds of dust or dead stars and be done with it, but there are persuasive arguments that this is not the case. First, although there are ways to spot even the darkest forms of matter, almost every attempt to find missing clouds and stars has failed. Second, and more convincing, cosmologists can make very precise calculations of the nuclear reactions that occurred right after the Big Bang and compare the expected results with the actual composition of the universe. Those calculations show that the total amount of ordinary matter, composed of familiar protons and neutrons, is much less than the total mass of the universe. Whatever the rest is, it isn't like the stuff of which we're made.
The quest to find the missing universe is one of the key efforts that has brought cosmologists and particle physicists together. The leading dark-matter candidates are neutrinos or two other kinds of particles: neutralinos and axions, predicted by some physics theories but never detected. All three of these particles are thought to be electrically neutral, thus unable to absorb or reflect light, yet stable enough to have survived from the earliest moments after the Big Bang.
What is dark energy?
Two recent discoveries from cosmology prove that ordinary matter and dark matter are still not enough to explain the structure of the universe. There's a third component out there, and it's not matter but some form of dark energy.
The first line of evidence for this mystery component comes from measurements of the geometry of the universe. Einstein theorized that all matter alters the shape of space and time around it. Therefore, the overall shape of the universe is governed by the total mass and energy within it. Recent studies of radiation left over from the Big Bang show that the universe has the simplest shapeit's flat. That, in turn, reveals the total mass density of the universe. But after adding up all the potential sources of dark matter and ordinary matter, astronomers still come up two-thirds short.
The second line of evidence suggests that the mystery component must be energy. Observations of distant supernovas show that the rate of expansion of the universe isn't slowing as scientists had once assumed; in fact, the pace of the expansion is increasing. This cosmic acceleration is difficult to explain unless a pervasive repulsive force constantly pushes outward on the fabric of space and time.
Why dark energy produces a repulsive force field is a bit complicated. Quantum theory says virtual particles can pop into existence for the briefest of moments before returning to nothingness. That means the vacuum of space is not a true void. Rather, space is filled with low-grade energy created when virtual particles and their antimatter partners momentarily pop into and out of existence, leaving behind a very small field called vacuum energy.
That energy should produce a kind of negative pressure, or repulsion, thereby explaining why the universe's expansion is accelerating. Consider a simple analogy: If you pull back on a sealed plunger in an empty, airtight vessel, you'll create a near vacuum. At first, the plunger will offer little resistance, but the farther you pull, the greater the vacuum and the more the plunger will pull back against you. Although vacuum energy in outer space was pumped into it by the weird rules of quantum mechanics, not by someone pulling on a plunger, this example illustrates how repulsion can be created by a negative pressure. ....
Even if you tickle me.
when the virtual particles 'pop in' they're the dark matter, and when they 'pop out' they're the dark energy.
Question 12: What is that stuff between al gores ears?
That would be the weak force.
Genesis 1:1 "In the beginning, God created the heaven and the earth."
Crashing branes and cosmic acceleration may power an infinite cycle in which our universe is but a phase.
by JR Minkel, with additional reporting by George Musser
Some questions are disquieting because they can be answered in only one of two equally mind-boggling ways. For instance, are we the sole intelligent beings in the universe, or will we find others? Another discomforting doozy is this: did the universe begin at some remote time in the past, or was it always here?
The big bang clearly marks some kind of first. That fearsome flash of energy and expansion of space set in motion everything our eyes and telescopes can see today. But on its own, the big bang theory would leave us in a curved universe where matter and energy aren't well mixed. In fact, we now know that spacetime is flat and that galaxies and radiation are evenly distributed throughout. To shore up the big bang theory, cosmologists proposed that the universe began with a burst of exponential expansion from a single uniform patch of space, whose stamp remains on the cosmos to this day. Such inflationary cosmologies have worked so well they've crowded out all the competition.
During this past year, however, one group of researchers has started to challenge that idea's preeminence, though the field of cosmology has yet to be completely taken with the new approach. Drawing on some cutting-edge but unproved notions in particle physics, the challengers interpret the big bang as a violent clash between higher-dimensional objects. In the latest installment to the saga, the authors of this interpretation have found a way to turn that single clash into a never-ending struggle that rears its fiery head every trillion years or so, making our universe just one phase in an infinite cycle of birth and rebirth.
Such cyclic ideas are not new. In the 1930s, the late Richard Tolman of the California Institute of Technology wondered what would happen if a closed universein which all matter and energy are ultimately compacted in a big crunchwere to survive its closure and burst forth again. Unfortunately, as Tolman realized, the universe would gather entropy during each new cycle; to compensate, it would have to grow every time like a runaway snowball. And just as a snowball has to begin at some point in time, so, too, would such a universe.
Then in the 1960s, physicists proved that a big crunch, too, must culminate in a singularitya point stuffed with infinite matter and heatwhere general relativity breaks down. The laws of physics are thus up for grabs. "The idea of a cyclic universe has been around for a long time," says Andreas Albrecht of the University of California at Davis, a co-inventor of inflation, "and it has always been plagued by a fundamental problem: what physics causes the collapsing universe to bounce back into the expanding phase?"
One potential way of getting around that problem is by supposing that elementary particles such as electrons, photons and quarks are really just manifestations of tiny strings of energy jiggling in higher dimensions. The thing is, such a string theory requires the universe to have at least 10 dimensions, as opposed to the usual three in space and one in time that we perceive. "In string theory you learn one thingyou are in higher dimensions," says string theorist Burt Ovrut of the University of Pennsylvania. "Then the question is, where does our real world come from? That's a damn good question."
Paving the way for an answer in 1995 were Petr Horava, then at Princeton University, and Ed Witten of Princeton's Institute for Advanced Studies, who showed that strings could also exist in a more fundamental, 11-dimensional theory. They collapsed one of these dimensions mathematically into a minuscule line, yielding an 11-dimensional spacetime, flanked on either side by two 10-dimensional membranes, or branes, colorfully dubbed "end of the world" branes. One brane would have physical laws like our own universe. From there, Ovrut and colleagues reasoned that six of those 10 dimensions could be made extremely small, effectively hiding them from everyday view and leaving the traditional four dimensions of space and time.
Early in 2001, cosmologists Justin Khoury and Paul Steinhardt of Princeton, another inflationary pioneer, Neil Turok of the University of Cambridge, and Ovrut put their branes to work on the big bang. By turning back the clock in string theory, they found that as our universal brane passed through its starting singularity in reverse, it went suddenly from a state of intense but finite heat and density to one that was cold, flat and mostly empty. In the process, it shed another kind of brane into the 11-dimensional gap. Run forward in time, the big bang appeared as nothing more than two branes smacking into each other like cymbals. They christened this process the ekpyrotic model, after the ancient Greek "conflagration" cosmology wherein the universe is born in and evolves from a fiery explosion.
Without a better understanding of the singularity in string theory, however, the group could not study what would happen as our brane expands after the collision; the model only provided for a contracting universe. Then later last year, the group discovered in collaboration with Nathan Seiberg of the Institute for Advanced Study that the singularity could be interpreted as a collision between the two "end of the world" branes, in which only the gap dimension separating them shrinks down to zero for an instant. "So what looks sort of disastrously singular, when you describe it as a brane collision, is not very singular at all," Turok explains. This scenario remains a conjecture, Seiberg notes, but is mathematically identical to the description of the big bang singularity in general relativity.
The ekpyrotic model had seemed a little contrived up to this point, notes Alan Guth of the Massachusetts Institute of Technology, another author of inflation. The pre-bang universe had to be dark, flat and infinite, seemingly by fiat. But why should it have begun in such a state? The answer, according to the latest work from Steinhardt and Turok, has to do with dark energy, the force that is driving the galaxies apart at ever-increasing speeds.
As the universe accelerates, it will become harder for light to travel between distant corners of space. Over time, galaxies will become isolated from their neighbors; stars will wink out; black holes will evaporate quantum mechanically into radiation; even that radiation will be diluted in a sea of space. The universe could end up much as the ekpyrotic model suggests it should appear before the big bang.
Steinhardt and Turok accordingly have proposed that the dark energy, combined with the milder singularity of the ekpyrotic model, provides a tidy way of setting up a cyclic universe. Our brane and its counterpart would bounce off each other as usual, but instead of going their separate ways, they would smack each other again and again as if connected by a spring. This attractive force between branes would in fact be a special case of the kind of force that inflationary cosmologies posit to explain the early universe's blowup.
The branes' oscillating motion would work to pump space into our universe like a bellows, explaining the acceleration that we see today. So "when you ask why is the universe the way it is," Turok explains, "well, it's because it has to be that way in order to repeat the next time around." And because each brane is already infinitely large and flat, there would be no first cycle to worry about.
The model is intriguing in drawing the ultimate link between early inflation and the current acceleration of the universe, Albrecht remarks, but "the case would be a lot more compelling if they were able to really show that a cyclic universe is possible." Guth is also unmoved. He explains that although he awaits the day when cosmology merges with string theory, he expects inflation to be that cosmology. In general, not all physicists are convinced that colliding branes can generate the small fluctuations in matter and energy density that inflation neatly resolves. Such minute variations in these quantities are required to explain the way in which stars and galaxies clump together and the detailed properties of the cosmic microwave background radiation.
In the ekpyrotic model, the necessary fluctuations are supposed to arise as the branes ripple quantum mechanically, so that different areas would strike one another and take off expanding first. The ekpyrotic camp is convinced these ripples can generate the exact variations we see today. "I think it's surprising how well this model works in terms of reproducing everything we see and yet being so different," Steinhardt remarks. "That's quite shocking and, I think, important, because we thought we were converging toward something that was a unique cosmic story."
But the singularity remains as another hurdle. Despite the recent advance, no one is certain whether features such as brane ripples could actually pass unmolested from big crunch to bang. "What happens at the singularity?" Seiberg ponders. "This is a big open question." So although the singularity in string theory may be, as Turok says, the "mildest possible" one, it is still a wild card.
The dealing isn't done, however, making it too soon to say if colliding branes will hold or fold. Perhaps it will attract new players with even more imaginative ideas. "I happen to think the cyclic model is a real intriguing one," Steinhardt says. "It has a lot of new ingredients that people haven't had a chance to play with. When they play they might find other interesting things that we missed." Or not.
But I suspect the new understandings will also reflect a great deal of what we currently call Mysticism. Particularly of the ancient variety - not the wannabe brand honed by massive hits of acid - but those of philosophers from ancient cultures that seem to have some very common themes. Fritjof Capra's "The Tao of Physics" is a classic along this theme - which sought to liberate physics from the vestiges of a classically mechanistic view of the universe.
I suspect you're right, owing to the fact that there are so many flavors of mysticism, and so many of them are so vague, that surely one of them can be stretched until it looks like the new physics, regardless of what that physics happens to be.
I finishing watching Stephen Hawkings DVD "The Universe" a while back, and I was struck with his arrogance about how much modern science knows when a gaping hole exists in Astrophysics namely: "Where is 97% of the mass of the Universe hiding?"
His attitude was one of distain for Religion, while at the same time, blind to how much faith is required to be an atheist.
Otherwise it was the best movie I've ever seen about the modern search for Truth and Astrophysics.
Your post rightly starts with a mention of Dark Matter, so I consider it Right On!
Or maybe it's an SEP - Somebody Else's Problem. Always good to run into another "HitchHiker's Guide to the Galaxy" fan. Or was that from "Life, the Universe, and Everything"?
...wow - I was just looking for other Douglas Adams quotes - and found out he's dead at age 49 from a heart attack! There goes another icon....
Are neutrinos Catholic?
That would be the axis of evil
people who are dumb(dense)--non philosophical-imaginative(belly up)?? Never mind!!
I've always referred to it as the "Primal Fart" but then, I'm an engineer, not a physicist!
Damn, still having co-ord problems after the GO TERPS Drinkfest!