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of the atom at any given moment increased proportionally. The goal of the experiment was to keep the gas atoms packed together closely enough that during this process—as their momentum |
got lower and lower, and their wavelengths got larger and larger—their waves would begin to overlap. This interplay of position and movement in three dimensions with the relative distances between |
particles is known as the phase-space density and is the key factor in forming a condensate. In essence, the momentum of the atoms would become so precisely pinpointed (near zero) |
that their position would become less and less certain and there would be a relatively large amount of space that would define each atom’s position. As the atoms slowed to |
almost a stop, their positions became so fuzzy that each atom came to occupy the same position as every other atom, losing their individual identity. This odd phenomenon is a |
Bose-Einstein As their experimental conditions neared the realm of Bose-Einstein condensation, Cornell and Wieman noticed an abrupt rise in the peak density of their sample, a type of discontinuity that |
strongly indicates a phase transition. The Colorado physicists estimated that after progressive evaporative cooling of the rubidium, they were left with a nugget of about 2,000 atoms of pure condensate. |
and Wieman then released the atoms from the "trap" in which they had been cooling and sent a pulse of laser light at the condensate, basically blowing it apart. They |
recorded an image of the expanding cloud of atoms. Prior to the light pulse, when the density dropped after the atoms were released, the physicists believed the temperature of the |
condensate fell to an amazing frigidity of 20 nanoKelvins (20 billionths of one degree above absolute zero). The image showed a larger, expanding sphere of particles with a smaller, more |
concentrated elliptical-looking center. Cornell and Wieman observed that when a gas is constrained and then released (in an extreme example, as in a bomb), thermodynamics specifies that it will expand |
outward equally in all directions regardless of the shape in which it had been contained. This occurs because the particles in that gas, even if the gas was very cold, |
were moving in all different directions with various energies when the gas was This rule of uniform expansion does not hold for a Bose-Einstein condensate. Because the particles were all |
acting in exactly the same manner at the time of the light pulse, their expansion should give some indication of the shape of the space they had previously inhabited. The |
uneven, elliptical-looking clump of atoms in the center of the image recorded by Cornell and Wieman thus gave further proof that a condensate had formed. Bose-Einstein characteristics have been observed |
in other systems, specifically, in superfluid liquid helium-4 and in superconductors. It is believed that liquid helium-4 at a sufficiently low temperature is composed of two components mixed together, the |
colder of which is a Bose-Einstein condensate. Liquid helium-4, which at very low temperatures is also a superconductor of heat, behaves in dramatic ways, trickling up the sides of containers |
and rising in Electrical superconductors are also boson-related phenomena. In superconductors, which are also formed by supercooling, electrical resistance disappears. In this case it is the electrons within a substance’s |
atoms, rather than the atoms themselves, that condense. The electrons pair up, together forming a particle of zero spin. These paired electrons merge into an overall substance that flows freely |
through the superconductor, offering no resistance to Thus, once initiated, a current can flow indefinitely in a superconductor. Quantum mechanics solved all of the great difficulties that troubled physicists in |
the early years of the 20th century. It gradually enhanced the understanding of the structure of matter, and it provided a theoretical basis for the understanding of atomic structure (see |
Atom and Atomic Theory) and the phenomenon of spectral lines: Each spectral line corresponds to the energy of a photon transmitted or absorbed when an electron makes a transition from |
one energy level to another. The understanding of chemical bonding was fundamentally transformed by quantum mechanics and came to be based on Schrödinger’s wave equations. New fields in physics emerged—condensed |
matter physics, superconductivity, nuclear physics, and elementary particle physics (see Physics)—that all found a consistent basis in quantum mechanics. FURTHER DEVELOPMENTS: In the years since 1925, no fundamental deficiencies have |
been found in quantum mechanics, although the question of whether the theory should be accepted as complete has come under discussion. In the 1930s the application of quantum mechanics and |
special relativity to the theory of the electron (see Quantum Electrodynamics) allowed the British physicist Paul Dirac to formulate an equation that referred to the existence of the spin of |
the electron. It further led to the prediction of the existence of the positron, which was experimentally verified by the American physicist Carl David Anderson. The application of quantum mechanics |
to the subject of electromagnetic radiation led to explanations of many phenomena, such as bremsstrahlung (German, "braking radiation," the radiation emitted by electrons slowed down in matter) and pair production |
(the formation of a positron and an electron when electromagnetic energy interacts with matter). It also led to a grave problem, however, called the divergence difficulty: Certain parameters, such as |
the so-called bare mass and bare charge of electrons, appear to be infinite in Dirac’s equations. (The terms bare mass and bare charge refer to hypothetical electrons that do not |
interact with any matter or radiation; in reality, electrons interact with their own electric This difficulty was partly resolved in 1947-49 in a program called renormalization, developed by the Japanese |
physicist Shin’ichirô Tomonaga, the American physicists Julian S. Schwinger and Richard Feynman, and the British physicist Freeman Dyson. In this program, the bare mass and charge of the electron are |
chosen to be infinite in such a way that other infinite physical quantities are canceled out in the equations. Renormalization greatly increased the accuracy with which the structure of atoms |
could be calculated from first principles. Theoretical physicist C. Llewellyn Smith discusses the discoveries that scientists have made to date about the electron and other elementary particles—subatomic particles that scientists |
believe cannot be split into smaller units of matter. Scientists have discovered what Smith refers to as sibling and cousin particles to the electron, but much about the nature of |
these particles is still One way scientists learn about these particles is to accelerate them to high energies, smash them together, and then study what happens when they collide. By |
observing the behavior of these particles, scientists hope to learn more about the fundamental structures of the universe. Electrons: The First Hundred Years The discovery of the electron was announced |
by J. J. Thomson just over 100 years ago, on April 30, 1897. In the intervening years we have come to understand the mechanics that describe the behavior of electrons—and |
indeed of all matter on a small scale—which is called quantum mechanics. By exploiting this knowledge, we have learned to manipulate electrons and make devices of a tremendous practical and |
economic importance, such as transistors and lasers. Meanwhile, what have we learned of the nature of the electron itself? From the start, electrons were found to behave as elementary particles, |
and this is still the case today. We know that if the electron has any structure, it is on a scale of less than 1018 m, i.e. less than 1 |
billionth of 1 billionth of a meter. However, a major complication has emerged. We have discovered that the electron has a sibling and cousins that are apparently equally fundamental. The |
sibling is an electrically neutral particle, called the neutrino, which is much lighter than the electron. The cousins are two electrically charged particles, called the mu and the which also |
have neutral siblings. The mu and the tau seem to be identical copies of the electron, except that they are respectively 200 and 3,500 times heavier. Their role in the |
scheme of things and the origin of their different masses remain mysteries — just the sort of mysteries that particle physicists, who study the constituents of matter and the forces |
that control their behavior, wish to resolve. We therefore know of six seemingly fundamental particles, the electron, the mu, the tau and their neutral siblings, which—like the electron—do not feel |
the nuclear force, and incidentally are known generically as leptons. What about the constituents of atomic nuclei, which of course do feel the nuclear force? At first sight, nuclei are |
made of protons and neutrons, but these particles turned out not to be elementary. It was found that when protons and neutrons are smashed together, new particles are created. We |
now know that all these particles are made of more elementary entities, called quarks. In a collision, pairs of quarks and their antiparticles, called antiquarks, can be created: part of |
the energy (e) of the incoming particles is turned into mass (m) of these new particles, thanks to the famous equivalence e = mc2. The quarks in the projectiles and |
the created quark-antiquark pairs can then rearrange themselves to make various different sorts of new particles. Today, six types of quarks are known which, like the leptons (the electron and |
its relations) have simple properties, and could be elementary. In the past 30 years a recipe that describes the behavior of these particles has been developed. It is called the |
"Standard Model" of particle physics. However, we lack a real understanding of the nature of these particles, and the logic behind the Standard Model. What is wrong with the Standard |
Model? First, it does not consistently combine Einstein’s theory of the properties of space (called General Relativity) with a quantum mechanical description of the properties of matter. It is therefore |
Second, it contains too many apparently arbitrary futures—it is too baroque, too byzantine—to be complete. It does not explain the role of the mu and the tau, or answer the |
question whether the fact that the numbers of leptons and quarks are the same—six each—is a coincidence, or an indication of a deep connection between these different types of particles. |
On paper, we can construct theories that give better answers and explanations, and in which there are such connections, but we do not know which, if any, of these theories |
is correct. Third, it has a missing, untested, element. This is not some minor detail, but a central element, namely a mechanism to generate the observed masses of the known |
particles, and hence also the different ranges of the known forces (long range for gravity and electromagnetism, as users of magnetic compasses know, but very short range for the nuclear |
and the so-called weak forces, although in every other respect these forces appear very similar). On paper, a possible mechanism is known, called the Higgs mechanism, after the British physicist |
Peter Higgs who invented it. But there are alternative mechanisms, and in any case the Higgs mechanism is a generic idea. We not only need to know if nature uses |
it, but if so, how it is realized in detail. Luckily the prospects of developing a deeper understanding are good. The way forward is to perform experiments that can distinguish |
the different possibilities. We know that the answer to the mystery of the origin of mass, and the different ranges of forces, and certain other very important questions, must lie |
in an energy range that will be explored in experiments at the Large Hadron Collider, a new accelerator now under construction at CERN [also known as the European Laboratory for |
Particle Physics] near Geneva. The fundamental tools on which experimental particle physics depends are large accelerators, like the Large Hadron Collider, which accelerate particles to very high energies and smash |
them together. By studying what happens in the collisions of these particles, which are typically electrons or protons (the nuclei of hydrogen atoms), we can learn about their natures. The |
conditions that are created in these collisions of particles existed just after the birth of the universe, when it was extremely hot and dense. Knowledge derived from experiments in particle |
physics is therefore essential input for those who wish to understand the structure of the universe as a whole, and how it evolved from an initial fireball into its present |
The Large Hadron Collider will therefore not only open up a large new window on the nature of matter, when it comes into operation in 2005, but also advance our |
understanding of the structure of the universe. However, although it will undoubtedly resolve some major questions and greatly improve our knowledge of nature, it would be very surprising if it |
established a "final theory." The only candidate theory currently known which appears to have the potential to resolve all the problems mentioned above—the reason for the existence of the mu |
and tau, reconciliation of general relativity with quantum mechanics, etc.—describes the electron and its relatives and the quarks, not as pointlike objects, but as different vibrating modes of tiny strings. |
However, these strings are so small (10-35 m) that they will never be observed If this is so, the electron and the other known particles will continue forever to appear |
to be fundamental pointlike objects, even if the—currently very speculative—"string theory" scores enough successes to convince us that this is not the case! FUTURE PROSPECTS: Quantum mechanics underlies current attempts |
to account for the strong nuclear force and to develop a unified theory for all the fundamental interactions Nevertheless, doubts exist about the completeness of quantum theory. The divergence difficulty, |
for example, is only partly resolved. Just as Newtonian mechanics was eventually amended by quantum mechanics and relativity, many scientists—and Einstein was among them—are convinced that quantum theory will also |
undergo profound changes in the future. Great theoretical difficulties exist, for example, between quantum mechanics and chaos theory, which began to develop rapidly in the 1980s. Ongoing efforts are being |
made by theorists such as the British physicist Stephen Hawking, to develop a system that encompasses both relativity and quantum mechanics. Breakthroughs occurred in the area of quantum computing in |
the late 1990s. Quantum computers under development use components of a chloroform molecule (a combination of chlorine and hydrogen atoms) and a variation of a medical procedure called magnetic resonance |
imaging (MRI) to compute at a molecular level. Scientists used a branch of physics called quantum mechanics, which describes the activity of subatomic particles (particles that make up atoms), as |
the basis for quantum computing. Quantum computers may one day be thousands to millions of times faster than current computers, because they take advantage of the laws that govern the |
behavior of subatomic particles. These laws allow quantum computers to examine all possible answers to a query at one time. Future uses of quantum computers could include code breaking and |
large database queries. Quantum Time Waits for No Cosmos THE INTRIGUING notion that time might run backwards when the Universe collapses has run into difficulties. Raymond Laflamme, of the Los |
Alamos National Laboratory in New Mexico, has carried out a new calculation which suggests that the Universe cannot start out uniform, go through a cycle of expansion and collapse, and |
end up in a uniform state. It could start out disordered, expand, and then collapse back into disorder. But, since the COBE data show that our Universe was born in |
a smooth and uniform state, this symmetric possibility cannot be applied to the real Universe. Physicists have long puzzled over the fact that two distinct "arrows of time" both point |
in the same direction. In the everyday world, things wear out -- cups fall from tables and break, but broken cups never re- assemble themselves spontaneously. In the expanding Universe |
at large, the future is the direction of time in which galaxies are further apart. Many years ago, Thomas Gold suggested that these two arrows might be linked. That would |
mean that if and when the expansion of the Universe were to reverse, then the everyday arrow of time would also reverse, with broken cups re-assembling themselves. More recently, these |
ideas have been extended into quantum physics. There, the arrow of time is linked to the so-called "collapse of the wave function", which happens, for example, when an electron wave |
moving through a TV tube collapses into a point particle on the screen of the TV. Some researchers have tried to make the quantum description of reality symmetric in time, |
by including both the original state of the system (the TV tube before the electron passes through) and the final state (the TV tube after the electron has passed through) |
in one mathematical description. Murray Gell-Mann and James Hartle recently extended this idea to the whole Universe. They argued that if, as many cosmologists believe likely, the Universe was born |
in a Big Bang, will expand out for a finite time and then recollapse into a Big Crunch, the time-neutral quantum theory could describe time running backwards in the contracting |
half of its life. Unfortunately, Laflamme has now shown that this will not work. He has proved that if there are only small inhomogeneities present in the Big Bang, then |
they must get larger throughout the lifetime of the Universe, in both the expanding and the contracting phases. "A low entropy Universe at the Big Bang cannot come back to |
low entropy at the Big Crunch" (Classical and Quantum Gravity, vol 10 p L79). He has found time-asymmetric solutions to the equations -- but only if both Big Bang and |
Big Crunch are highly disordered, with the Universe more ordered in the middle of its life. Observations of the cosmic microwave background radiation show that the Universe emerged from the |
Big Bang in a very smooth and uniform state. This rules out the time-symmetric solutions. is that even if the present expansion of the Universe does reverse, time will not |
Glucose is a type of sugar. It comes from food, and is also created in the liver. Glucose travels through the body in the blood. It moves from the blood to cells with the help of a hormone called insulin. Once glucose is in those cells, it can be used |
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