Physics in a New Era An Overview (2001) / Chapter Skim
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4. Fundamental Laws and Symmetries
4. Fundamental Laws and Symmetries
Pages 70-88
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From page 70... ...
Early on, a new particle was aciclecl to the list: the neutrino, observed as an encl-product in nuclear clecay. This nearly weightless particle plays an essential role in nuclear clecay, the lives of stars, ancl even, possibly, the ultimate fate of our universe.
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From page 71... ...
One celebrated example, the relationship between radioactive decays and electromagnetic phenomena, was originally introduced as an analogy: Enrico Fermi suggested that the force responsible for certain radioactive decays, now known as the weak interaction, might be described in terms of weak charges and weak currents, just as electromagnetism involves ordinary charges and currents. The analogy was not precise—the analogue of the electromagnetic radiation was absent from Fermi's theory.
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From page 72... ...
Study of these phenomena led Steven Weinberg to suggest that the symmetry of the electroweak interaction might be similarly hidden, with similar consequences. In particular, this hidden symmetry turned the long-range interaction into a short-range one.
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From page 73... ...
Just as astronomy has seen amazing advances brought about by the latest generation of large-scale modern instruments, so too have the small distance frontiers of physics been advanced by sophisticated, large-scale facilities. In particle and nuclear physics, where the smallest structures in nature are probed, the increasing capability of particle accelerators, the modern "microscopes" designed for this purpose, along with their instrumentation, have driven the experimental frontier.
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From page 74... ...
Beams from accelerators are successfully used in the treatment of cancer and other diseases. Modern medical imaging techniques such as CAT scans, PET scans, and MRI have their roots directly in technologies developed for particle detectors.
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From page 75... ...
The Strong Force Hidden symmetry also plays a crucial role in the theory of the strong force. The underlying constituents of strongly interacting matter, the quarks, do not appear individually under normal conditions but only as composite bound states called hadrons.
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From page 76... ...
As in the electroweak interactions, the hidden quark symmetry may be restored at very high temperatures or very high energy densities. The newly commissioned Relativistic Heavy Ion Collider (RHIC)
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From page 77... ...
At Brookhaven National Laboratory near New York City, a large accelerator called the Relativistic Heavy Ion Collider (RHIC) has been constructed to accomplish this goal.
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From page 78... ...
which allows a nonzero EDM—does not generate one. One attractive solution of this strong CP puzzle involves a modification of the standard model's Higgs structure.
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From page 79... ...
Some of the questions involve distance scales so small that they will probably never be probed directly in accelerator experiments. But subtle hints of this new physics may be encoded in phenomena like neutrino masses, free proton clecay, ancl family mixing.
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From page 80... ...
Despite 20 years of careful searches with massive detectors located far underground to avoid spurious signals, no proton decay has been seen, ruling out the simplest grand unified theories. However, some of the most appealing versions of these theories incorporating supersymmetry predict that current detectors are very close to having the required sensitivity.
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From page 81... ...
If neutrinos have a mass, then the electron neutrinos produced in the solar core can "oscillate" into muon or tau neutrinos before reaching Earth, thereby escaping detection. The Super-Kamiokande experiment, an enormous 50,000-ton water Cherenkov detector located in a mine deep within the Japanese alps, has recently measured both solar neutrinos and neutrinos produced in our atmosphere by cosmicray interactions.
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From page 82... ...
A new solar neutrino detector that uses heavy water, the Sudbury Neutrino Observatory in Canada, will soon provide new information, determining whether the solar flux contains muon or tau neutrinos. A supernova, the spectacular collapse of a massive star in which the star's outer mantle is ejected, produces prodigious numbers of neutrinos of al I three types, wh ich must then propagate through matter bi 11 ions of times denser than the core of our Sun.
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From page 83... ...
Generally the strength of gravity will depend on a particle's total energy, growing stronger with increasing energy. When a particle's energy reaches the Planck energy, the energy necessary to probe physical phenomena at the Planck length scale gravity becomes as strong as the other forces of nature.
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From page 84... ...
However, the characteristic length scale at which quantum gravity becomes important is the Planck length of 10-33 cm or the corresponding Planck energy scale of 1079 billion eV. This is a length scale a hundred billion billion times smaller than the size of an atomic nucleus and an energy scale 10 million billion times larger than that of the largest accelerator on Earth.
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From page 85... ...
FU N DAMENTAL LAWS AN D SYMMETRI ES string theory suggests that there is something more fundamental than the space-time that is the focus of Einstein's theory. On this distant frontier, gravitational physics at last becomes important for science at the smallest scales considered by contemporary physics.
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From page 86... ...
Although string theory is not directly tied to experiment as quantum theory was during its formation, it does call for radically new ideas, and its future development may prove equally revolutionary. THE LENGTH SCALES OF NATURE Our universe has a size, age, and complexity that dwarfs the realm of human experience.
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From page 87... ...
Nothing illustrates the unity of physics better than these similarities across vastly different length scales. The common ideas of physics have been applied over distances ranging from the realm of string theory to the furthest reaches of our universe.
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