[Clippings from the May 2004 article in Sci American --here,) if it's still there.
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Supersymmetry means an equal matching between bosons (particles that transmit forces) and fermions (particles that make up matter).
. . String theorists are discovering that what they thought were completely different theories were in fact different ways of looking at the same theory! This period in string history has been given the name the second string revolution. And now the biggest rush in string research is to collapse it all into one theory, which some people want to call M theory, for it is the Mother of all theories.
. . M theory is is the unknown eleven-dimensional theory whose low energy limit is the supergravity theory in eleven dimensions. However, many people have taken to also using M theory to label the unknown theory believed to be the fundamental theory from which the known superstring theories emerge as special limits.
Hawking showed, loosely speaking, that a black hole could leak particles with a quantum wavelength similar to the hole's radius.
Because energy and mass are equivalent, the energy of gravitational attraction itself generates gravity!
The particles we observe --quarks, photons, electrons and all the rest-- represent different oscillations of the loops of string.
It's the point-like nature of the electron that makes quantum electrodynamics so vexing. Replace the point with oscillations of a line, and the infinities don't occur in the first place. What's more, superstring theory contains a loopy oscillation that looks like a "graviton", a hypothetical quantum particle that bears the same relationship to the gravitational field as the photon does to the electromagnetic field.
. . According to quantum field theory, the vacuum has some strange properties. Heisenberg's uncertainty principle implies that even in empty space, subatomic particles such as electrons and photons are constantly popping into being from nowhere, then fading away again almost immediately. This means that the quantum vacuum is a seething frolic of evanescent "virtual particles".
. . Although these particles lack the permanence of normal matter, they can still have a physical influence. For example, a pair of mirrors arranged facing one another extremely close together will feel a tiny force of attraction, even in a perfect vacuum, because of the way the set-up affects the behaviour of the virtual photons. This has been confirmed in many experiments.
. . An expanding or contracting Universe would create particles out of a pure vacuum. In effect, the stretching of space jiggles up some of the virtual particles and turns them into real particles.
COSMOLOGY: The Myth of the Beginning of Time
By Gabriele Veneziano: a theoretical physicist at CERN, was the father of string theory in the late 1960s --an accomplishment for which he received this year's Heineman Prize of the American Physical Society and the American Institute of Physics. After string theory made its comeback as a theory of gravity in the 1980s, Veneziano became one of the first physicists to apply it to black holes and cosmology.
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String theory suggests that the big bang was not the origin of the universe but simply the outcome of a preexisting state.
. . Was the big bang really the beginning of time? Or did the universe exist before then? Such a question seemed almost blasphemous only a decade ago. Most cosmologists insisted that it simply made no sense --that to contemplate a time before the big bang was like asking for directions to a place north of the North Pole. But developments in theoretical physics, especially the rise of string theory, have changed their perspective. The pre-bang universe has become the latest frontier of cosmology.
. . After nearly 30 years of arguing that a black hole destroys everything that falls into it, Stephen Hawking is saying he was wrong. It seems that black holes may after all allow information within them to escape. It was Hawking's own work that created the paradox. In 1976, he calculated that once a black hole forms, it starts losing mass by radiating energy. This "Hawking radiation" contains no information about the matter inside the black hole and once the black hole evaporates, all information is lost.
. . But this conflicts with the laws of quantum physics, which say that such information can never be completely wiped out. Hawking's argument was that the intense gravitational fields of black holes somehow unravel the laws of quantum physics.
. . Other physicists have tried to chip away at this paradox. Earlier in 2004, Samir Mathur of Ohio State University in Columbus and his colleagues showed that if a black hole is modelled according to string theory - in which the universe is made of tiny, vibrating strings rather than point-like particles - then the black hole becomes a giant tangle of strings. And the Hawking radiation emitted by this "fuzzball" does contain information about the insides of a black hole
. . Hawking's black holes, unlike classic black holes, do not have a well-defined event horizon that hides everything within them from the outside world. In essence, his new black holes now never quite become the kind that gobble up everything. Instead, they keep emitting radiation for a long time, and eventually open up to reveal the information within.
[In the Big-Bang scenario,] As you play cosmic history backward in time, the galaxies all come together to a single infinitesimal point, known as a singularity--almost as if they were descending into a black hole. Each galaxy or its precursor is squeezed down to zero size. Quantities such as density, temperature and spacetime curvature become infinite. The singularity is the ultimate cataclysm, beyond which our cosmic ancestry cannot extend.
. . The unavoidable singularity poses serious problems for cosmologists. In particular, it sits uneasily with the high degree of homogeneity and isotropy that the universe exhibits on large scales. For the cosmos to look broadly the same everywhere, some kind of communication had to pass among distant regions of space, coordinating their properties. But the idea of such communication contradicts the old cosmological paradigm.
. . A less widely known way to solve the puzzle follows the second alternative by getting rid of the singularity. If time did not begin at the bang, if a long era preceded the onset of the present cosmic expansion, matter could have had plenty of time to arrange itself smoothly. Therefore, researchers have reexamined the reasoning that led them to infer a singularity.
. . One of the assumptions --that relativity theory is always valid-- is questionable. Close to the putative singularity, quantum effects must have been important, even dominant. Standard relativity takes no account of such effects, so accepting the inevitability of the singularity amounts to trusting the theory beyond reason. To know what really happened, physicists need to subsume relativity in a quantum theory of gravity. The task has occupied theorists from Einstein onward, but progress was almost zero until the mid-1980s.
. . The second approach, which I consider more promising, is string theory --a truly revolutionary modification of Einstein's theory. String theory grew out of a model that I wrote down in 1968 to describe the world of nuclear particles (such as protons and neutrons) and their interactions. Despite much initial excitement, the model failed. Only later was it revived as a candidate for combining general relativity and quantum theory.
. . The basic idea is that elementary particles are not pointlike but rather infinitely thin one-dimensional objects, the strings. The large zoo of elementary particles, each with its own characteristic properties, reflects the many possible vibration patterns of a string.
. . Once the rules of quantum mechanics are applied to a vibrating string --just like a miniature violin string, except that the vibrations propagate along it at the speed of light-- new properties appear.
. . First, quantum strings have a finite size. Were it not for quantum effects, a violin string could be cut in half, cut in half again and so on all the way down, finally becoming a massless pointlike particle. But the Heisenberg uncertainty principle eventually intrudes and prevents the lightest strings from being sliced smaller than about 10-34 meter. This irreducible quantum of length, denoted ls, is a new constant of nature introduced by string theory side by side with the speed of light, c, and Planck's constant, h. It plays a crucial role in almost every aspect of string theory, putting a finite limit on quantities that otherwise could become either zero or infinite.
. . Second, quantum strings may have angular momentum even if they lack mass. In classical physics, angular momentum is a property of an object that rotates with respect to an axis. The formula for angular momentum multiplies together velocity, mass and distance from the axis; hence, a massless object can have no angular momentum. But quantum fluctuations change the situation. A tiny string can acquire up to two units of h of angular momentum without gaining any mass. This feature is very welcome because it precisely matches the properties of the carriers of all known fundamental forces, such as the photon (for electromagnetism) and the graviton (for gravity). Historically, angular momentum is what clued in physicists to the quantum-gravitational implications of string theory.
. . Third, quantum strings demand the existence of extra dimensions of space, in addition to the usual three. Whereas a classical violin string will vibrate no matter what the properties of space and time are, a quantum string is more finicky. The equations describing the vibration become inconsistent unless spacetime either is highly curved (in contradiction with observations) or contains six extra spatial dimensions.
. . Fourth, physical constants --such as Newton's and Coulomb's constants, which appear in the equations of physics and determine the properties of nature--no longer have arbitrary, fixed values. They occur in string theory as fields, rather like the electromagnetic field, that can adjust their values dynamically. These fields may have taken different values in different cosmological epochs or in remote regions of space, and even today the physical "constants" may vary by a small amount. Observing any variation would provide an enormous boost to string theory.
. . One such field, called the dilaton, is the master key to string theory; it determines the overall strength of all interactions. The dilaton fascinates string theorists because its value can be reinterpreted as the size of an extra dimension of space, giving a grand total of 11 spacetime dimensions.
Tying Down the Loose Ends
. . Finally, quantum strings have introduced physicists to some striking new symmetries of nature known as dualities, which alter our intuition for what happens when objects get extremely small. I have already alluded to a form of duality: normally, a short string is lighter than a long one, but if we attempt to squeeze down its size below the fundamental length ls, the string gets heavier again.
. . Electrons may be strings whose ends can move around freely in three of the 10 spatial dimensions but are stuck within the other seven.
. . All the magic properties of quantum strings point in one direction: strings abhor infinity. They cannot collapse to an infinitesimal point, so they avoid the paradoxes that collapse entails. Their nonzero size and novel symmetries set upper bounds to physical quantities that increase without limit in conventional theories, and they set lower bounds to quantities that decrease. String theorists expect that when one plays the history of the universe backward in time, the curvature of spacetime starts to increase. But instead of going all the way to infinity (at the traditional big bang singularity), it eventually hits a maximum and shrinks once more. Before string theory, physicists were hard-pressed to imagine any mechanism that could so cleanly eliminate the singularity.
. . Conditions near the zero time of the big bang were so extreme that no one yet knows how to solve the equations. Nevertheless, string theorists have hazarded guesses about the pre-bang universe. Two popular models are floating around.
. . The first, known as the pre-big bang scenario, which my colleagues and I began to develop in 1991, combines T-duality with the better-known symmetry of time reversal, whereby the equations of physics work equally well when applied backward and forward in time. The combination gives rise to new possible cosmologies in which the universe, say, five seconds before the big bang expanded at the same pace as it did five seconds after the bang. But the rate of change of the expansion was opposite at the two instants: if it was decelerating after the bang, it was accelerating before. In short, the big bang may not have been the origin of the universe but simply a violent transition from acceleration to deceleration.
. . In the standard theory, acceleration occurs after the big bang because of an ad hoc inflaton field. In the pre-big bang scenario, it occurs before the bang as a natural outcome of the novel symmetries of string theory.
. . According to the scenario, the pre-bang universe was almost a perfect mirror image of the post-bang one. If the universe is eternal into the future, its contents thinning to a meager gruel, it is also eternal into the past. Infinitely long ago it was nearly empty, filled only with a tenuous, widely dispersed, chaotic gas of radiation and matter. The forces of nature, controlled by the dilaton field, were so feeble that particles in this gas barely interacted.
. . As time went on, the forces gained in strength and pulled matter together. Randomly, some regions accumulated matter at the expense of their surroundings. Eventually the density in these regions became so high that black holes started to form. Matter inside those regions was then cut off from the outside, breaking up the universe into disconnected pieces.
. . Inside a black hole, space and time swap roles. The center of the black hole is not a point in space, but an instant in time. As the infalling matter approached the center, it reached higher and higher densities. But when the density, temperature and curvature reached the maximum values allowed by string theory, these quantities bounced and started decreasing. The moment of that reversal is what we call a big bang. The interior of one of those black holes became our universe.
. . The other leading model for the universe before the bang is the ekpyrotic ("conflagration") scenario. Developed [in 2001] by a team of cosmologists and string theorists. The ekpyrotic scenario relies on the idea that our universe is one of many D-branes floating within a higher-dimensional space. The branes exert attractive forces on one another and occasionally collide. The big bang could be the impact of another brane into ours.
. . In short, all three models match the data.
. . Gravitational waves of certain sizes would leave a distinctive signature in the polarization of the microwave background [see "Echoes from the Big Bang," by Robert R. Caldwell and Marc Kamionkowski; Scientific American, January 2001]. Future observatories, such as European Space Agency's Planck satellite, should be able to see that signature, if it exists--providing a nearly definitive test.
. . So, when did time begin? Science does not have a conclusive answer yet, but at least two potentially testable theories plausibly hold that the universe--and therefore time--existed well before the big bang. If either scenario is right, the cosmos has always been in existence and, even if it recollapses one day, will never end.
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