Most symmetries of dynamical systems have the profound
consequence that some quantity in the system is conserved. This was proved
by the German mathematician Emmy
Noether in 1915 when she showed that a symmetry
implies a conservation law, and vice versa. For example, the results
of experiments do not depend on where the experiments are performed, or
on the orientation of the apparatus. The consequence of these particular
symmetries is that linear and angular momenta are both conserved. Similarly,
the laws of physics do not depend on the time at which they are determined,
a symmetry which has the consequence that energy is conserved. Furthermore,
experiments are insensitive to the overall phase of quantum-mechanical
wavefunctions - a property known as gauge invariance - which leads to
the conservation of electric charge.
interesting article. . .
of Space and Time By Lee Smolin
We perceive space and time
to be continuous, but if the amazing theory of loop quantum gravity
is correct, they actually come in discrete pieces
Little more than 100 years ago most
people--and most scientists--thought of matter as continuous. Although
since ancient times some philosophers and scientists had speculated
that if matter were broken up into small enough bits, it might turn
out to be made up of very tiny atoms, few thought the existence
of atoms could ever be proved. Today we have imaged individual atoms
and have studied the particles that compose them. The granularity
of matter is old news.
In recent decades, physicists and
mathematicians have asked if space is also made of discrete pieces.
Is it continuous, as we learn in school, or is it more like a piece
of cloth, woven out of individual fibers? If we could probe to size
scales that were small enough, would we see "atoms" of
space, irreducible pieces of volume that cannot be broken into anything
smaller? And what about time: Does nature change continuously, or
does the world evolve in series of very tiny steps, acting more
like a digital computer?
"Do not take the lecture
too seriously . . . just relax and enjoy it. I am going to tell
you what nature behaves like. If you will simply admit that
maybe she does behave like this, you will find her a delightful,
entrancing thing. Do not keep saying to yourself "But how
can it be like that?" because you will get...into a blind
alley from which nobody has yet escaped. Nobody knows how it
can be like that." Feynman
planet from the early universe Ron Cowen
Astronomers have found the
oldest and most distant planet known in the universe. OLD ORB.
Artist's view of the planet (top) orbiting a pulsar and its
white dwarf companion (two brightest dots at lower left) in
the globular cluster M4. NASA and G. Bacon/STScI Residing 7,200
light-years away, the planet weighs 2.5 times as much as Jupiter
and formed when the universe was an infant.
times for strings Ignatios Antoniadi
The "hidden" dimensions
of string theory may be much larger than was previously thought
and may soon come within experimental reach, together with the
strings themselves. Ignatios Antoniadis gives an introduction
to string physics and describes how it may soon be testable
at particle colliders.
at the Lunar Poles That
the Moon harbors ice at high latitudes is well known. The source
of that water, however, may come as something of a surprise
Richard R. Vondrak ,Dana H. Crider
Since the 1960s planetary scientists
have considered the possibility that the Moon may harbor deposits
of water ice in permanently shaded regions near the lunar poles.
In 1998, the Lunar Prospector mission largely confirmed the
presence of water ice on the Moon. The authors explore how that
ice came to be and suggest what information the study of lunar
ice cores may one day provide to scientists interested in the
ancient history of the Earth-Moon system.
free-electron lasers into the X-ray regime Elke Plönjes, Josef Feldhaus and Thomas
Möller Imagine making movies
of chemical reactions, watching surfaces melt in real time
or taking photographs of individual molecules. These are just
some of the ambitious goals that researchers have set for
powerful new X-ray "free-electron lasers" that are
currently being developed at the Deutsches Elektronen Synchrotron
(DESY) laboratory in Hamburg and the Stanford Linear Accelerator
Center (SLAC) in California.
Microwave Anisotropy Probe
of the Big Bang The
CMB The Big Bang theory predicts that the early universe was
a very hot place and that as it expands, the gas within it cools.
Thus the universe should be filled with radiation that is literally
the remnant heat left over from the Big Bang, called the “cosmic
microwave background radiation”, or CMB.
bone crushers could soon be back Warming
set to return Southern Ocean to its cut-throat past. TOM CLARKE
Global warming could threaten
Antarctic sea life with a re-invasion by powerful, fast-moving
predators that deserted the area 35 million years ago, researchers
announced this week at a meeting in London.
thing that children are taught in science class is that all
"stuff" is made of matter. But that's not quite true.
All of the common stuff around us, from the air we breathe to
the computers we surf the web with, is made of matter - but
that's not everything in the universe.
In the beginning, scientists think, the big bang created both
matter and antimatter. Every particle in atoms - from protons
and electrons to the tinier quarks that make up those particles
- has a corresponding antiparticle. Much of the original antimatter
annihilated itself when it collided with ordinary matter, releasing
energy. But somewhere out there lurk wandering patches of antimatter.
energy tops science class By Paul Rincon
A series of breakthroughs in the quest to
identify the mysterious fabric of the Universe has topped a list
of the 10 key scientific advances of 2003. The
winning discoveries provide further evidence that the Universe is
composed largely of dark matter and dark energy. The eagerly awaited
top 10, which is compiled annually by the journal Science, is always
controversial, and this year's proves to be no exception.
Gamma ray bursts -- those terrific
and mysterious flashes of high-energy light now considered to
be probes to the farthest reaches of the Universe and earliest
moments of time -- may have yet another secret to reveal: quantum
gravity. Not yet observed in nature, quantum gravity is the
long-sought missing link between Einstein's General Relativity
and Quantum Mechanics, the two incongruous pillars of modern
physics. NASA's Gamma-Ray Large Area Space Telescope (GLAST),
planned for a 2005 launch, may be able to detect for the first
time the effects of quantum gravity in the speed of gamma-ray
burst photons, according to two NASA scientists.
Cars Not Best Way To Cut Pollution
As politicians and the public leap aboard the
hydrogen fuel bandwagon, a University of California, Berkeley,
energy expert suggests we all step back and take a critical
look at the technology and consider simpler, cheaper options.
In a paper appearing in the July 18 issue of Science magazine,
Alex Farrell, assistant professor of energy and resources at
UC Berkeley, and David Keith, associate professor of engineering
and public policy at Carnegie Mellon University, present various
short- and long-term strategies that they say would achieve
the same results as switching from gasoline-powered vehicles
to hydrogen cars. "Hydrogen cars are a poor short-term
strategy, and it's not even clear that they are a good idea
in the long term," said Farrell. "Because the prospects
for hydrogen cars are so uncertain, we need to think carefully
before we invest all this money and all this public effort in