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THE FABRIC of the COSMOS, Brian Greene, 2004
```(annotated and with added bold highlights by Epsilon=One)
```(annotated and with added bold highlights by Epsilon=One)
Chapter 14 – Up in the Heavens and Down in the Earth
The Hunt for Extra Dimensions
Before 1996, most theoretical models that incorporated extra dimensions imagined that their spatial extent was roughly Planckian (10^-33 centimeters). As this is seventeen orders of magnitude smaller than anything resolvable using currently available equipment, without the discovery of miraculous new technology Planckian physics will remain out of reach. But if the extra dimensions are "large," meaning larger than a hundredth of a billionth of a billionth (10^-20) of a meter, about a millionth the size of an atomic nucleus, there is hope.
As we discussed in Chapter 13, if any of the extra dimensions are "very large" — within a few orders of magnitude of a millimeter — precision measurements of gravity's strength should reveal their existence. Such experiments have been under way for a few years and the techniques are being rapidly refined. So far, no deviations from the inverse square law characteristic of three space dimensions have been found, so researchers are pressing on to smaller distances. A positive signal would, to say the least, rock the foundations of physics. It would provide compelling evidence of extra dimensions accessible only to gravity, and that would give strong circumstantial support for the braneworld scenario of string/M-theory.
If the extra dimensions are large but not very large, precision gravity experiments will be unlikely to detect them, but other indirect approaches remain available. For example, we mentioned earlier that large extra dimensions would imply that gravity's intrinsic strength is greater than previously thought. The observed weakness of gravity would be attributed to its leaking out into the extra dimensions, not to its being fundamentally feeble; on short distance scales, before such leakage occurs, gravity would be strong. Among other implications, this means that the creation ot tiny black holes would require far less mass and energy than it would in a universe in which gravity is intrinsically far weaker. In Chapter 13, we discussed the possibility that such microscopic black holes might be produced by high-energy proton-proton collisions at the Large Hadron Collider, the particle accelerator now under construction in Geneva, Switzerland, and slated for completion by 2007. That is an exciting prospect. But there is another tantalizing possibility that was raised by Alfred Shapere, of the University of Kentucky, and Jonathan Feng, of the University of California at Irvine. These researchers noted that cosmic rays — elementary particles that stream through space and continually bombard our atmosphere — might also initiate production of microscopic black holes.
Cosmic ray particles were discovered in 1912 by the Austrian scientist Victor Hess; more than nine decades later, they still present many mysteries. Every second, cosmic rays slam into the atmosphere and initiate a cascade of billions of downward-raining particles that pass through your body and mine; some of them are detected by a variety of dedicated instruments worldwide. But no one is completely sure what kinds of particles constitute the impinging cosmic rays (although there is a growing consensus that they are protons), and despite the fact that some of these high-energy particles are believed to come from supernova explosions, no one has any idea of where the highest-energy cosmic ray particles originate. For example, on October 15, 1991, the Fly's Eye cosmic ray detector, in the Utah desert, measured a particle streaking across the sky with an energy equivalent to 30 billion proton masses. That's almost as much energy in a single subatomic particle as in a Mariano Rivera fastball, and is about 100 million times the size of the particle energies that will be produced by the Large Hadron Collider. 6 The puzzling thing is that no known astrophysical process could produce particles with such high energy; experimenters are gathering more data with more sensitive detectors in hopes of solving the mystery.
For Shapere and Feng, the origin of super-energetic cosmic ray particles was of secondary concern. They realized that regardless of where such particles come from, if gravity on microscopic scales is far stronger than formerly thought, the highest-energy Cosmic ray particles might have just enough oomph to create tiny black holes when they violently slam into the upper atmosphere.
As with their production in atom smashers, such tiny black holes would pose absolutely no danger to the experimenters or the world at large. After their creation, they would quickly disintegrate, sending off a characteristic cascade of other, more ordinary particles. In fact, the microscopic black holes would be so short-lived that experimenters could not search for them directly; instead, they would look for evidence of black holes through detailed studies of the resulting particle showers raining down on their detectors, the Pierre Auger Observatory — with an observing area the size of Rhode Island — is now being built on a vast stretch of land in western Argentina. Shapere and Feng estimate that if all of the extra dimensions are as large as 10^-14 meters, then after a year's worth of data collection, the Auger detector will see the characteristic particle debris from about a dozen tiny black holes produced in the upper atmosphere. If such black hole signatures are not found, the experiment will conclude that extra dimensions are smaller. Finding the remains of black holes produced in cosmic ray collisions is certainly a long shot, but success would open the first experimental window on extra dimensions, black holes, string theory, and quantum gravity.
Beyond black hole production, there is another, accelerator-based way that researchers will be looking for extra dimensions during the next decade. The idea is a sophisticated variant on the "space-between-the¬cushions" explanation for the loose coins missing from your pocket.
A central principle of physics is conservation of energy. Energy can manifest itself in many forms — the kinetic energy of a ball's motion as it flies off a baseball bat, gravitational potential energy as the ball flies upward, sound and heat energy when the ball hits the ground and excites all sorts of vibrational motion, the mass energy that's locked inside the ball itself, and so on — but when all carriers of energy have been accounted for, the amount with which you end always equals the amount with which you began. 7 To date, no experiment contradicts this law of perfect energy balance.
But depending on the precise size of the hypothesized extra dimensions, high-energy experiments to be carried out at the newly upgraded facility at Fermilab and at the Large Hadron Collider may reveal processes that appear to violate energy conservation: the energy at the end of a collision may be less than the energy at the beginning. The reason is that, much like your missing coins, energy (carried by gravitons) can seep into the cracks — the tiny additional space — provided by the extra dimensions and hence be inadvertently overlooked in the energy accounting calculation. The possibility of such a "missing energy signal" provides yet another means for establishing that the fabric of the cosmos has complexity well beyond what we can see directly.
No doubt, when it comes to extra dimensions, I'm biased. I've worked on aspects of extra dimensions for more than fifteen years, so they hold a special place in my heart. But, with that confession as a qualifier, it's hard for me to imagine a discovery that would be more exciting than finding evidence for dimensions beyond the three with which we're all familiar. To my mind, there is currently no other serious proposal whose confirmation would so thoroughly shake the foundation of physics and so thoroughly establish that we must be willing to question basic, seemingly self-evident, elements of reality.
As we discussed in Chapter 13, if any of the extra dimensions are "very large" — within a few orders of magnitude of a millimeter — precision measurements of gravity's strength should reveal their existence. Such experiments have been under way for a few years and the techniques are being rapidly refined. So far, no deviations from the inverse square law characteristic of three space dimensions have been found, so researchers are pressing on to smaller distances. A positive signal would, to say the least, rock the foundations of physics. It would provide compelling evidence of extra dimensions accessible only to gravity, and that would give strong circumstantial support for the braneworld scenario of string/M-theory.
If the extra dimensions are large but not very large, precision gravity experiments will be unlikely to detect them, but other indirect approaches remain available. For example, we mentioned earlier that large extra dimensions would imply that gravity's intrinsic strength is greater than previously thought. The observed weakness of gravity would be attributed to its leaking out into the extra dimensions, not to its being fundamentally feeble; on short distance scales, before such leakage occurs, gravity would be strong. Among other implications, this means that the creation ot tiny black holes would require far less mass and energy than it would in a universe in which gravity is intrinsically far weaker. In Chapter 13, we discussed the possibility that such microscopic black holes might be produced by high-energy proton-proton collisions at the Large Hadron Collider, the particle accelerator now under construction in Geneva, Switzerland, and slated for completion by 2007. That is an exciting prospect. But there is another tantalizing possibility that was raised by Alfred Shapere, of the University of Kentucky, and Jonathan Feng, of the University of California at Irvine. These researchers noted that cosmic rays — elementary particles that stream through space and continually bombard our atmosphere — might also initiate production of microscopic black holes.
Cosmic ray particles were discovered in 1912 by the Austrian scientist Victor Hess; more than nine decades later, they still present many mysteries. Every second, cosmic rays slam into the atmosphere and initiate a cascade of billions of downward-raining particles that pass through your body and mine; some of them are detected by a variety of dedicated instruments worldwide. But no one is completely sure what kinds of particles constitute the impinging cosmic rays (although there is a growing consensus that they are protons), and despite the fact that some of these high-energy particles are believed to come from supernova explosions, no one has any idea of where the highest-energy cosmic ray particles originate. For example, on October 15, 1991, the Fly's Eye cosmic ray detector, in the Utah desert, measured a particle streaking across the sky with an energy equivalent to 30 billion proton masses. That's almost as much energy in a single subatomic particle as in a Mariano Rivera fastball, and is about 100 million times the size of the particle energies that will be produced by the Large Hadron Collider. 6 The puzzling thing is that no known astrophysical process could produce particles with such high energy; experimenters are gathering more data with more sensitive detectors in hopes of solving the mystery.
For Shapere and Feng, the origin of super-energetic cosmic ray particles was of secondary concern. They realized that regardless of where such particles come from, if gravity on microscopic scales is far stronger than formerly thought, the highest-energy Cosmic ray particles might have just enough oomph to create tiny black holes when they violently slam into the upper atmosphere.
As with their production in atom smashers, such tiny black holes would pose absolutely no danger to the experimenters or the world at large. After their creation, they would quickly disintegrate, sending off a characteristic cascade of other, more ordinary particles. In fact, the microscopic black holes would be so short-lived that experimenters could not search for them directly; instead, they would look for evidence of black holes through detailed studies of the resulting particle showers raining down on their detectors, the Pierre Auger Observatory — with an observing area the size of Rhode Island — is now being built on a vast stretch of land in western Argentina. Shapere and Feng estimate that if all of the extra dimensions are as large as 10^-14 meters, then after a year's worth of data collection, the Auger detector will see the characteristic particle debris from about a dozen tiny black holes produced in the upper atmosphere. If such black hole signatures are not found, the experiment will conclude that extra dimensions are smaller. Finding the remains of black holes produced in cosmic ray collisions is certainly a long shot, but success would open the first experimental window on extra dimensions, black holes, string theory, and quantum gravity.
Beyond black hole production, there is another, accelerator-based way that researchers will be looking for extra dimensions during the next decade. The idea is a sophisticated variant on the "space-between-the¬cushions" explanation for the loose coins missing from your pocket.
A central principle of physics is conservation of energy. Energy can manifest itself in many forms — the kinetic energy of a ball's motion as it flies off a baseball bat, gravitational potential energy as the ball flies upward, sound and heat energy when the ball hits the ground and excites all sorts of vibrational motion, the mass energy that's locked inside the ball itself, and so on — but when all carriers of energy have been accounted for, the amount with which you end always equals the amount with which you began. 7 To date, no experiment contradicts this law of perfect energy balance.
But depending on the precise size of the hypothesized extra dimensions, high-energy experiments to be carried out at the newly upgraded facility at Fermilab and at the Large Hadron Collider may reveal processes that appear to violate energy conservation: the energy at the end of a collision may be less than the energy at the beginning. The reason is that, much like your missing coins, energy (carried by gravitons) can seep into the cracks — the tiny additional space — provided by the extra dimensions and hence be inadvertently overlooked in the energy accounting calculation. The possibility of such a "missing energy signal" provides yet another means for establishing that the fabric of the cosmos has complexity well beyond what we can see directly.
No doubt, when it comes to extra dimensions, I'm biased. I've worked on aspects of extra dimensions for more than fifteen years, so they hold a special place in my heart. But, with that confession as a qualifier, it's hard for me to imagine a discovery that would be more exciting than finding evidence for dimensions beyond the three with which we're all familiar. To my mind, there is currently no other serious proposal whose confirmation would so thoroughly shake the foundation of physics and so thoroughly establish that we must be willing to question basic, seemingly self-evident, elements of reality.