In March-1999 the American Physical Society (APS) held its 100th Anniversary Meeting in Atlanta. It was the largest such physics meeting ever held, with an attendance of about 11,500 physicists, most of whom gave contributed or invited papers, with as many as 60 sessions of papers being presented at the same time. It's of course impossible to try to capture the impact of such a torrent of scientific information in the few thousand words of a column such as this. Instead, I'll focus only on a few of the new physics ideas, developments, and discoveries presented at the Atlanta APS Meeting that caught my fancy.
Super-Lasers - In the past few years laser physicists have made very rapid progress in increasing the power and shrinking the time-width of laser pulses. A prime example of increased power is the Petawatt Laser Project at the Lawrence Livermore National Laboratory in California, where physicists have been able to produce focused laser beams with intensities of up to 1020 watts per square centimeter. At that intensity a coherent light wave has an electric field of about 2 × 1013 volts/meter. Such huge electric fields propel atomic electrons to energies measured to be in excess of 100 million electron volts (MeV).
Livermore physicists directed this laser beam at a foil of gold backed by uranium and reported that nuclear gamma rays, neutrons and positrons, as well as fission fragments were detected after each laser shot. Thus, lasers of sufficient intensity can produce nuclear reactions. These are the first observations of (1) antimatter positron creation with lasers, (2) laser induced fission, and (3) laser-produced electrons with energies above 100 MeV.
Another Livermore group directed a more modest tabletop laser with an intensity of "only" 1017 watts/cm2 but with a time width of 3.5 × 10-14 seconds onto a jet of deuterium gas. The gas jet exploded, producing the fusion of some of the deuterium nuclei into heluim-3, accompanied by an energetic neutron with a characteristic energy of 2.45 MeV. About 10,000 neutrons per laser shot were produced. This doesn't solve the problem of finding a practical way of extracting useful nuclear energy from deuterium fusion, but it could provide a fairly cheap tabletop generator of energetic neutrons.
This process is highly efficient, in that almost all of the laser energy is converted into ion kinetic energy. Therefore, let me speculate about its potential as a space drive. One can envision a laser-driven liquid deuterium engine in which deuterium is ejected as a propellant, with very efficient conversion of electrical energy to laser energy to ion kinetic energy, and with the added bonus of an energy boost from the nuclear fusion of a fraction of the fuel.
Complex Molecules from Space - Astronomers are coming to realize that some very interesting chemistry occurs is space, and that the chemical products may provide the precursors of life on planets such as ours. Diffuse interstellar bands were discovered in 1921 by Mary Lea Heger of California's Lick Observatory. She found broad absorption spectra in starlight that came not from the stars themselves but from some unknown medium through which the starlight was passing. Closer examination later showed that the diffuse interstellar bands had a complex structure that did not correspond to any of the known interstellar molecules (H2, CO, CNO, NH3, etc.) The origin of diffuse interstellar bands has remained a mystery for seven decades.
Now analytical work at NASA's Ames Laboratory has revealed the probable source of the diffuse interstellar bands to be polycyclic aromatic hydrocarbons or PAHs. These are flat "chicken-wire" molecules made of linked hexagoal benzene rings of carbon, with hydrogen atoms on their edges. The family of PAH molecules, with names like napthalene, pyrene, coronene, and chrysene, tend to be stable, colorful, fluorescent, and biochemically active. They are common in our environment, for example they are a principal component of soot from a smoky fire. Recent studies suggest that they are among the most common molecules in the interstellar medium, with only H2 and CO more abundant. Infrared and optical spectroscopy in the laboratory have now demonstrated a close match between the structure of diffuse interstellar bands and the absorption lines of the family of PAH molecules.
The scenario that emerges is that the young massive stars of the early universe produced carbon in stellar burning and then dispersed it into the interstellar medium in supernova explosions. This atomic carbon found its way to dust grains, where it accumulated. On this surface carbon and hydrogen could organize themselves into PAH molecules, then evaporate when heated. The result is that most stars, including our sun, are surrounded by a gas cloud including high concentrations of many PAH molecules.
With planet formation, these PAH molecules are captured. For example, fluorescence measurements indicate that asteroids in orbits beyond Neptune probably have surface concentrations of PAH compounds. On earth-like planets with liquid water, the PAH molecules will accumulate in the oceans. The Ames scientists showed that shining ultraviolet light on PAH compounds can convert them into organic compounds that are present henna, aloe, and St. John's wort. Thus, PAH compounds under the influence of sunlight can seed the oceans with complex organic molecules that are the precursors of the earliest biomolecules. It appears that a link in the chain that leads to planetary life has been discovered.
Atom Lasers and 4-Wave Atom Mixers- In previous columns I've described the atom laser (see Analog July-97.) While optical lasers produce light of a single wavelength with the waves all in phase, atom lasers produce a beam of atoms, all with exactly the same velocity and all the matter waves in phase (i.e., coherent). The first atom laser produced its atom beam as a spreading pulse, as the atoms of a Bose-Einstein condensate (see Analog March-96.) were simultaneously ejected from the trap that held them.
At the Atlanta APS Meeting Theodor Haensch of the Max Planck Institute for Quantum Optics in Munich described a new atom laser that produces a narrow and continuous beam of atoms, potentially with a beam radius as small as a nanometer (10-9 meters). Possible applications range from "writing" nano-scale structures with beams of atoms to ultra-precise measurements that test our present understanding of gravity and quantum mechanics.
Also in a previous column I discussed the 4-wave mixing technique in a non-linear optical crystal for producing a time-reversed light (see Analog June-85.) At the Atlanta APS Meeting Nobel Laureate William Phillips of the NIST Laboratory in Gaithersburg, MD described his investigation of non-linear effects with an atom laser, which produced a 4-wave mixer for atoms. The NIST researchers overlapped three Bose-Einstein condensates of sodium atoms moving in different directions as three beams which produced a fourth wave, while conserving energy and momentum.
Let me inject an idea here. In the NIST work the beams were NOT moving in opposite directions. However, if two of the beams are coherent and in opposite directions, the created fourth beam is the time reverse of the third beam, which is not required to be coherent. Thus, a modification of the NIST technique might be used to precisely reverse the velocities of a group of atoms forming a beam. Since the fourth beam is the time-reverse of the third, it will go back along precisely the same path taken by an incident beam of atoms. If these are gas atoms escaping from a confining vessel, the new beam will converge back to that vessel, retracing the path of the third beam. If the atoms of the third beam scattered on the way out, the atoms of the fourth beam will reverse this scattering on the way back in. This may offer a new way of testing the second law of thermodynamics (entropy always increases) in a new regime, exact time reversal, where it has not previously been investigated.
Direct CP Violation and the Matter/Antimatter Asymmetry - As discussed in my most recent previous column (see Analog, July/August-99), it is now clear from astrophysical evidence that the universe is completely dominated by matter, even in very distant regions. There are far more electrons than positrons and far more protons than antiprotons in our universe. There is not the slightest hint of antimatter stars or galaxies.
This is a problem for particle physics because, at the particle level, energetic processes normally produce matter particles and antimatter particles (e.g., electrons and positrons, quarks and anti-quarks, etc.) in pairs, so that the balance between matter and antimatter is preserved.
The only known exception to this is the so called "CP Violation" observed in the decay of "strange" K0 mesons. Here the term CP violation means that if a given system were transformed by interchanging matter and antimatter (charge-exchange or "C") and by reversing the three space directions (parity or "P"), the resulting system is qualitatively changed. The universe as a whole is an example of CP violation because it contains more matter than antimatter.
The K0 meson CP violation was discovered in 1962 by observing that Klong mesons, which normally decay into three pi mesons, were about two times in 1000 spontaneously transforming themselves into Kshort mesons and decaying instead into two pi mesons.
A number of experiments have attempted to determine whether the Klong meson could also directly decay into two pi mesons without first being transformed into a Kshort. This is of great interest because it distinguishes between possible explanations for the origins of the CP violation. The "superweak theory" which would invoke a new force to produce the CP violation, predicts that the Klong must always be transformed to a Kshort before it could decay into two pi mesons. Quantum chromodynamics, the "standard model" of particle physics, predicts that about 4 times in 10,000,000 the Klong can decay directly to two pi mesons without prior conversion to a Kshort.
At the Atlanta APS Meeting, the KTeV collaboration reported on their Fermilab experiment that observed the direct Klong decay to two pi mesons. This was a very difficult experiment because the quantity measured is so small. The group used two beams of Klong mesons, one of which was made to interact with a block of matter a short distance from their detector, so that Kshort mesons were generated in the second beam.
By comparing decays of the two beams into two neutral pi mesons (p°+p°) and into two charged pi mesons (p++p-), they were able to extract the probability of direct decay. Surprisingly, they observed that the direct decay occurs about 6 times in 10,000,000, considerably in excess of the normal predictions of the standard model. Therefore, the result falsifies the superweak theory, but it also pushes the standard model in a new direction, suggesting that the strange quark, present in K mesons, is less massive than has been previously estimated.
How is this related to the matter-antimatter asymmetry of the universe? We know that only about one extra matter particle in 100 million matter-antimatter particle pairs in the early universe would be needed to explain the present dominance of matter. The CP violation observed in these K meson experiments could produce this matter excess, but it is not strong enough to produce a matter asymmetry of the required size.
We must be missing something important. The only way of finding out what is missing is to explore the nature of the CP violation more thoroughly. The new direct CP violation measurement did this to some extent, and indeed produced a surprisingly large effect. The new "B-factories" that are about to come into operation in the next year can be depended upon to provide further insights into the nature of the matter-antimatter asymmetry and to add to our understanding of how the universe works.
There were many other new items of physics presented at the Atlanta APS Meeting that I don't have the space to discuss (including my own paper presented at the meeting.) I hope, however, that the above may communicate some of the excitement, interest, and enthusiasm of this greatest gathering of the physics community.
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Columns Online: Electronic
reprints of about 194 "The Alternate View" columns by John G.
Cramer, previously published in
Additional information on the physics topics discussed above can also be found
at the APS Physics News Update site at: http://www.aip.org/physnews/update