Particles Our Universe

MICRO BLACK HOLES could be created by particle accelerators such as the Large Hadron Collider smashing together protons (yellow) at high energies. The holes would evaporate rapidly by emitting Hawking radiation of Standard Model particles (blue) and gravitons (red).

berger of the University of Washington. They expect preliminary results this year.

The idea of extra dimensions in effect continues the Coper-nican tradition in understanding our place in the world: The earth is not the center of the solar system, the sun is not the center of our galaxy, our galaxy is just one of billions in a universe that has no center, and now our entire three-dimensional universe would be just a thin membrane in the full space of dimensions. If we consider slices across the extra dimensions, our universe would occupy a single infinitesimal point in each slice, surrounded by a void.

Perhaps this is not the full story. Just as the Milky Way is not the only galaxy in the universe, might our universe not be alone in the extra dimensions? The membranes of other three-dimensional universes could lie parallel to our own, only a millimeter removed from us in the extra dimensions [see illustration on preceding page]. Similarly, although all the particles of the Standard Model must stick to our own membrane universe, other particles beyond the Standard Model in addition to the graviton might propagate through the extra dimensions. Far from being empty, the extra dimensions could have a multitude of interesting structures.

The effects of new particles and universes in the extra dimensions may provide answers to many outstanding myster ies of particle physics and cosmology. For example, they may account for the masses of the ghostly elementary particles called neutrinos. Impressive new evidence from the Super Kamiokande experiment in Japan indicates that neutrinos, long assumed to be massless, have a minuscule but nonzero mass [see "Detecting Neutrino Mass," by Edward Kearns, Takaaki Kajita and Yoji Totsuka; Scientific American, August 1999]. The neutrino can gain its mass by interacting with a partner field living in the extra dimensions. As with gravity, the interaction is greatly diluted by the partner being spread throughout the extra dimensions, and so the neutrino acquires only a tiny mass.

Parallel Universes

A nother example is the mystery in cosmology of what con-x \ stitutes "dark matter," the invisible gravitating substance that seems to make up more than 90 percent of the mass of the universe. Dark matter may reside in parallel universes. Such matter would affect our universe through gravity and is necessarily "dark" because our species of photon is stuck to our membrane, so photons cannot travel across the void from the parallel matter to our eyes.

Such parallel universes might be utterly unlike our own, having different particles and forces and perhaps even being confined to membranes with fewer or more dimensions. In one intriguing scenario, however, they have identical properties to our own world. Imagine that the wall where we live is folded a number of times in the extra dimensions [see illustration on preceding page]. Objects on the other side of a fold will appear to be very distant even if they are less than a millimeter from us in the extra dimensions: the light they emit must travel to the crease and back to reach us. If the crease is tens of billions of light-years away, no light from the other side could have reached us since the universe began.

Dark matter could be composed of ordinary matter, perhaps even ordinary stars and galaxies, shining brightly on their own folds. Such stars would produce interesting observable effects, such as gravitational waves from supernovae and other violent astrophysical processes. Gravity-wave detectors scheduled for completion in a few years could find evidence for folds by observing large sources of gravitational radiation that cannot be accounted for by matter visible in our own universe.

The theory we have presented here was not the first proposal involving extra dimensions larger than 10-35 meter. In 1990 Ignatios Antoniadis of École Polytechnique in France suggested that some of string theory's dimensions might be as large as 10-19 meter, but he kept the scale of quantum gravity near 10-35 meter. In 1996 Petr Horava of the California Institute of Technology and Edward Witten of the Institute for Advanced Study in Princeton, N.J., pointed out that a single extra dimension of 10-30 meter would neatly unify gravity along with the supersymmetric unification of the other forces, all at 10-32 meter. Following this idea, Joseph Lyk-ken of Fermi National Accelerator Laboratory in Batavia, Ill., attempted to lower the unification scale to near 10-19 meter (without invoking large extra dimensions). Keith Di-enes of the University of Arizona and Emilian Dudas and Tony Gherghetta of CERN observed in 1998 that extra dimensions smaller than 10-19 meter could allow the forces to unify at much larger distances than 10-32 meter.

Since our proposal in 1998 a number of interesting varia-

DRIVE

TORSION OSCILLATOR at the University of Colorado looks for changes in gravity from 0.05 to 1.0 millimeter. Piezoelectrics vibrate the tungsten source mass (blue) like a diving board. Any forces acting between the source mass and the tungsten detector (red) produce twisting oscillations of the detector (inset; oscillations are exaggerated), which are sensed by electronics. A gold-

tions have appeared, using the same basic ingredients of extra dimensions and our universe-on-a-wall. In an intriguing model, Lisa Randall of Princeton University and Raman Sundrum of Stanford proposed that gravity itself may be concentrated on a membrane in a five-dimensional space-time that is infinite in all directions. Gravity appears very weak in our universe in a natural way if we are on a different membrane.

For 20 years, the conventional approach to tackling the hierarchy problem, and therefore understanding why gravity is so weak, has been to assume that the Planck scale near 10-35 meter is fundamental and that particle physics must change near 10-19 meter. Quantum gravity would remain in the realm of plated shield (yellow) suppresses electrostatic forces, and suspension from brass isolation stacks stops vibrations from traveling from the source to the detector. Electrostatic shields enclosing the apparatus are not shown. Results at room temperature (300 kelvins) are expected this year. For maximum sensitivity, liquid helium will cool the apparatus to four kelvins.

theoretical speculation, hopelessly out of the reach of experiment. In the past two years we have realized that this does not have to be the case. If there are large new dimensions, in the next several years we could discover deviations from Newton's law near 6 x 10-5 meter, say, and we would detect stringy vibrations or black holes at the LHC. Quantum gravity and string theory would become testable science. Whatever happens, experiment will point the way to answering a 300-year-old question, and by 2010 we will have made decisive progress toward understanding why gravity is so weak. And we may find that we live in a strange Flatland, a membrane universe where quantum gravity is just around the corner. E3

The Authors

Further Information

NIMA ARKANI-HAMED, SAVAS DIMOPOULOS and GEORGI DVALI thought up the extra-dimension theory while they were together at Stanford University in February 1998. Arkani-Hamed was born in Houston in 1972 and received a Ph.D. in physics at the University of California, Berkeley, in 1997, where he returned as an assistant professor in 1999. When he's not exploring theoretical possibilities beyond the Standard Model of particle physics, he enjoys hiking in the High Sierra and the California desert. Dimopoulos grew up in Athens, Greece, received a Ph.D. from the University of Chicago and has been a professor of physics at Stanford since 1979. His research has mostly been driven by the quest for what lies beyond the Standard Model. In 1981, together with Howard Georgi of Harvard University, he proposed the supersymmetric Standard Model. "Gia" Dvali grew up in what is now Georgia and in 1992 received his Ph.D. in high-energy physics and cosmology from Tbilisi State University. In 1998 he became an associate professor of physics at New York University. He enjoys overcoming gravity by high mountaineering and rock and ice climbing.

The Theory Formerly Known as Strings. Michael Duff in Scientific American, Vol. 278, No. 2, pages 64-69; February 1998.

The Elegant Universe : Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. Brian Greene. W. W. Norton, 1999.

Flatland: A Romance of Many Dimensions. Edwin A. Abbott. Text available from the Gutenberg project at http://promo.net/cgi-promo/pg/t9. cgi?entry=97 on the World Wide Web.

An introduction to tabletop gravity experiments is available at http://mist.npl.washington.edu/ eotwash/

An introduction to string theory is available at http://superstringtheory.com/

Male Sexual Circuitry

by Irwin Goldstein and the Working Group for the Study of Central Mechanisms in Erectile Dysfunction ive hundred years ago Leonardo da Vinci made an observation about the penis that rings true even today for many men and their partners. The Renaissance scientist, inventor and artist—one in a long line of investigators who have attempted to solve the riddle of penile rigidity—observed that this seemingly wayward organ has a will of its own. "The penis does not obey the order of its master, who tries to erect or shrink it at will, whereas instead the penis erects freely while its master is asleep. The penis must be said to have its own mind, by any stretch of the imagination," he wrote.

Da Vinci, who dissected cadaverous penises from men who had been executed by hanging, was the first scientist to recognize that during an erection, the penis fills with blood. In his perception that the penis acts of its own free will, however, this multitalented scholar was wrong.

Far from having a mind of its own, the penis is now known to be under the complete control of the central nervous system—the brain and spinal cord. As William D. Steers, chair of the department of urology at the University of Virginia, has noted, any disturbance in the network of nerve pathways that connects the penis and the central nervous system can lead to erection problems.

In the past few decades the study of erections has been redefined. Thanks to advances in molecular biology, we now have a better understanding of the processes within the penis that lead to erection and detumescence, the return of the penis to a flaccid state. Armed with this knowledge, we have begun to explore how the brain and spinal cord control erections and other sexual functions. The field is still young, but we are optimistic that these efforts will lead to new therapies for the millions of men who suffer from sexual dysfunction— and we expect that some of these findings will also inform treatments for women. Although research on women has lagged far behind that on men, we are beginning to elucidate the striking similarities—as well as differences—be-tween the sexes in regard to sexual function.

An erection is a carefully orchestrated series of events, with the central nervous system in the role of conductor. Even when the penis is at rest, the nervous system is at work. When a man is not sexually aroused, parts of the sympathetic nervous system actively limit blood flow to the penis, keeping it limp. The sympathetic nervous system is one of two branches of the autonomic nervous system—the part of the central nervous system that controls largely "automatic" internal responses, such as blood pressure and heart rate.

Dynamic Balance

Within the penis, and throughout the nervous system, a man's sexual response reflects a dynamic balance between excitatory and inhibitory forces. Whereas the sympathetic nervous system tends to inhibit erections, the para-sympathetic system—the other branch of the autonomic nervous system—is one of several important excitatory pathways. During arousal, excitatory signals can originate in the brain, triggered perhaps by a smell or by the sight or thought of an alluring partner, or by physical stimulation of the genitals.

Regardless of where the signals come from, the excitatory nerves in the penis respond by releasing so-called proerec-tile neurotransmitters, including nitric oxide and acetylcholine. These chemical messengers signal the muscles of the penile arteries to relax, causing more blood to flow into the organ. Spongy chambers inside the penis fill up with blood. As these expand, they compress the veins that normally drain blood from the penis. This pressure squeezes the veins until they are nearly closed, trapping blood within the chambers and producing an erection. (Viagra—also known as silde-nafil—works by slowing the breakdown of one of the chemicals that keeps the muscles relaxed, thereby holding blood in the penis.)

During an erection, the penis not only receives nerve signals but also sends them to the spinal cord and brain. The penis has an unusually high density of specialized tactile receptors; when these receptors are stimulated, their signals course to the spinal cord and brain, where they influence nerve pathways from these higher centers. So although the penis does not "think" for itself, it keeps the brain and spinal cord well apprised of its feelings. After a man climaxes or the arousal has diminished, the erection quickly subsides. The sympathetic nervous system again limits blood flow into the penis, which returns to its soft state.

Circumstances that increase the activity of the sympathetic nervous systemsuch as stress or exposure to cold—can temporarily shrink the penis by making it more flaccid. Conversely, switching off the activity of the sympathetic nervous system enhances erections. Nocturnal erections are a good example of this phenomenon. These occur primarily during rapid eye movement (REM) sleep, the stage in which dreaming occurs. During REM sleep, sympathetic neurons are turned off in the locus coe-ruleus, a specific area of the brain stem, the part of the brain that connects to the spinal cord. According to one theory, when this sympathetic brain center is quiet, proerectile pathways predominate, allowing nocturnal erections to occur. We often refer to such erections as "battery-recharging mechanisms" for the penis, because they increase blood flow, bringing in fresh oxygen to reenergize the organ. (Episodes of nocturnal arousal also occur in women. Four or five times a night—that is, during each episode of REM—women experience la-

ANCIENT GREEK HERM, or square pillar, from 510 B.C. illustrates the connection between the brain and the penis that has long mystified many and that is currently the subject of so much research.

bial, vaginal and clitoral engorgement.)

Some erections, called reflexive erections, are generated entirely in the spinal cord. Much like touching a finger to a hot burner triggers a rapid withdrawal of the hand, physical stimulation of the penis can set off a spinal erection reflex in some situations. So crucial is reproduction to our perpetuation as a species that it appears that the capacity to create an erection has been wired into nerve circuits near the base of a man's spine.

In humans, most of the evidence for this finding has come from observations of soldiers with spinal cord injuries, particularly veterans wounded in World War II. Before then, the general belief was that men with spinal cord injuries were permanently and completely impotent and sterile. Although we now know that this view is mistaken, it is understandable. The spinal cord is the information superhighway for the nerv-

The brain is the most important sex organ. One of its roles in male sexuality is to keep the penis under control.

ous system, shuttling nerve stimuli to and from the brain and the peripheral nerves of the rest of the body. If the spinal cord is damaged, this flow of nerve impulses can be interrupted in myriad ways, depending on where the injury occurs and how extensive it is.

Yet, as physician Herbert Talbot reported in a classic study in 1949, men with severe or complete spinal cord injuries often continue to have erections. In his examination of 200 men with paraplegia, two thirds were able to achieve erections, and some were able to engage in vaginal intercourse and have an orgasm. Even though devastating war injuries left these men paralyzed and unable to control many basic bodily functions, the ability to have erections was often preserved.

These observations—and information from studies in laboratory animals as far back as the 1890s—led to the dis covery that an "erection-generating center" is located in the sacral segments of the spinal cord (that is, just above the tail end of the spine, between the S3 and T12 vertebrae). Physical stimulation of the penis sends sensory signals via the pudendal nerve to this erection center. The incoming signals activate connector nerve cells called interneurons, which then stimulate nearby parasympathetic neurons. These neurons send erection-inducing signals from the sacral spine to the penile blood vessels. As long as this reflex arc remains intact, an erection is possible.

The Brain's Brakes

Observations of men and laboratory animals with spinal cord damage have led to another intriguing finding: when the brain is disconnected from the erection-generating center in the spinal cord, erections typically occur more frequently and with less tactile stimulation than they did before the injury. For instance, Benjamin D. Sachs, an experimental psychologist at the University of Connecticut, found in 1979 that spinal transection in rats caused an increase of more than 1,000 percent in the number of erections and a 94 percent reduction in the time it took for the animals to become erect.

It seemed as if, in the disconnection of the brain from the body, some inhibitory control over erections was removed. This proved to be the case. In 1990 physiologists Kevin E. McKenna and Lesley Marson, then at Northwestern University, identified the brain center that keeps the brakes on spinal-mediated erections. They found that a specific cluster of neurons in the hindbrain (an evolutionarily ancient part of the brain that controls such basic functions as blood pressure and heart rate) is in charge of this central inhibition. When McKenna and Marson destroyed this group of neurons—called the para-gigantocellular nucleus, or PGN—in a male rat's brain, the inhibition disappeared, causing more frequent and intense erections.

These researchers then made another significant discovery about the brain's role in suppressing erections. They found that the PGN neurons send most

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