Re: ba manca-v-as

On Dec 15, 6:57 pm, progea <pro...@xxxxxxxxxxx> wrote:
On Dec 15, 6:37 pm, Leru-i ler si iarasi ler <bazilk...@xxxxxxxxx>
wrote:

progea wrote:
Deci cum'~ai, "partea intreaga a inversei constantei de elasticitate"
sau cum gogul meu?
Modulul lui Young sau ce gogul meu, manca'mi'ai?

"constanta de structura fina" ("alpha"), alpha^{-1}~137.0359...http://en..wikipedia.org/wiki/Fine-structure_constant

Aha, multzam'~ai, colindatoare. Pai atuncea'nsamna ca partea'ntreaga'i
zero, un gogul, nu'~ai?

Si daca constanta nu-i constanta, adika alpha ie beta,
aaaaaaaaaaaaa ??

http://www.sciencedaily.com/releases/2007/04/070402153241.htm

si....

In constant search of 'alpha'

* 12 September 2006
* Exclusive from New Scientist Print Edition.
* Mason Inman


On a hefty aluminium table in a Harvard University basement sits the
biggest atom in the world. The culmination of 20 years' work, it looks
like two oil drums stacked on top of one another, sprouting wires and
tubing out the top. The drums contain powerful superconducting
magnets, charged metal plates and special refrigerators - all to do a
job usually accomplished by the nucleus of a hydrogen atom. "God would
be ashamed if he made an apparatus this big," says Harvard physicist
Gerald Gabrielse, the device's creator. "He did it with just the
proton."

The point of this "home-made atom" is to confine an electron and trick
it into behaving as if it were in a real atom. By precisely
controlling the electron's movements, Gabrielse can probe the
innermost workings of electromagnetism, the force that shapes much of
our everyday world and binds us together. It's not just about fridge
magnets, radio waves and television: electromagnetism is responsible
for virtually everything we see and feel, from the redness of roses to
the toughness of diamonds.

One pure number dictates the strength of this force. The number -
known as alpha, the fine-structure constant - is one of nature's most
precisely known quantities and one of the fundamental determinants of
the universe's behaviour.

"Everywhere you have electromagnetic interactions, in principle you
can measure alpha," says Andrzej Czarnecki of the University of
Alberta in Edmonton, Canada. "The number of digits we can get
characterises how well we understand the system. There's a great race
between different areas of physics to get alpha to the most decimal
places." There's a good reason for doing this, too, because measuring
alpha even more precisely could lead to the discovery of new physical
laws and phenomena.
Flying colours

Following years of progress in refining their estimates for alpha,
researchers hit a roadblock, and they have been stuck there for nearly
20 years. Now Gabrielse and his colleagues have made the breakthrough.
By combining measurements from their home-made atom with the latest
theoretical developments, they have calculated the most precise value
yet for alpha. Combined with other measurements of alpha, this new
value could challenge our understanding of how light and matter
interact on the quantum level - one of the great pillars of modern
physics.

Alpha first popped up in 1915 in a theory that explained the colours
of light emitted by excited atoms, as in fireworks and burning stars.
Early versions of quantum physics described why each element shines
with a few characteristic colours. The German physicist Arnold
Sommerfeld explained why this atomic light has "fine structure", in
which each band of light splits into two or more subtle bands. In this
model, alpha was thought to represent the ratio of an electron's
velocity in an atom to the speed of light.

Throughout the 1930s, some researchers thought the numerical value of
alpha might be a simple fraction, 1/137. It didn't turn out that way,
and since then physicists have striven to add more decimal places to
alpha's value. They have no clue why alpha has this particular value;
all they can do is measure it.

By the 1940s, alpha had taken on a starring role in the development of
a quantum theory of electromagnetism. Quantum electrodynamics, or QED,
explained how matter interacts with light and put a quantum face on
familiar phenomena like light-bending lenses and iron-attracting
magnets. In the theory, alpha determines the strength of particle
interactions. Its most important effect is on how tightly atoms hold
on to their electrons. "If alpha changed tomorrow, all the atoms in
the world would suddenly change their size," Gabrielse says. That
would alter chemical reactions and probably prevent the existence of
life as we know it.

If most quantum physics seems weird, QED is weirder. At its heart lie
numerous strange quantum phenomena, one of the most bizarre being
"virtual" particles. Because of the uncertainty principle, a virtual
particle can pop briefly into existence, borrowing energy from empty
space, only to disappear like a ghost an instant later. Virtual
particles are crucial to QED's predictions and provide a quantum-level
mechanism for how electromagnetic fields work. The exact value of
alpha is intimately tied to the behaviour of these virtual particles,
so spooling out more decimal places of alpha can test whether QED
continues to match experimental results.

To calculate the effects that virtual particles have on, say, an
electron, physicists work out how many ways an electron can get from
point A to point B (see Diagram). One route is a straight line,
another a wiggly path. There are also other more convoluted
possibilities: as an electron cruises along, it can spit out a virtual
photon, only to reabsorb it an instant later. Or the electron can spit
out two, three or more virtual photons - and each of these can do a
pirouette, transforming into an electron and a positron. Like a
magician's assistant who gets sawn into pieces and then reassembled,
the virtual particles recombine with the original electron.

Keeping pace with improving experiments, theorists are continually
updating QED to capture more permutations of the virtual photons. This
takes time: there are an infinite number of possibilities, and
extending the calculations to encompass each additional virtual photon
and its transformations requires exponentially more equations to be
solved. Toichiro Kinoshita of Cornell University in Ithaca, New York,
who has led many of the efforts to push quantum electrodynamics
further, and his collaborator Makiko Nio of the Theoretical Physics
Laboratory in Wako, Japan, recently completed a new level of
calculations (Physical Review D, vol 73, 013003). It took years to set
up computers to crunch through the 891 equations involved, which they
solved approximately. Undaunted, the researchers have started on the
next level, which will require solving more than 12,000 equations.

All the experimental results so far have matched QED's predictions,
making it arguably the best-tested theory in all of physics. In fact,
QED paved the way for the standard model of particle physics, a hugely
successful conglomeration of theories that explain the quantum
workings of all the fundamental forces except gravity.

Historically, though, QED's success was a surprise. "It was just
patched together out of bits and pieces, in order to explain some
experiments," says Freeman Dyson, one of the theory's architects, now
at the Institute for Advanced Study in Princeton, New Jersey. "We
didn't expect it to last," he adds. "Every time there was a new
experiment, we all expected that the theory would be proved wrong in
some interesting way. Instead, each experiment still agrees with the
theory. That's sort of a disappointment."

Because QED has been so robust, pushing the theory has not led to any
new physics. Six decades later, researchers are pushing their
experiments further to see if QED will break. If there is a
discrepancy, it could signal the existence of particles never before
seen, or it could force physicists to give up their cherished
assumption that electrons are point particles with no size or
structure.

There could also be bigger repercussions. If QED ever failed to mesh
with experiments, physicists could suddenly find themselves looking
for a way to fix the theory, and the standard model too. "There is no
wiggle room if theory and experiment disagree," Kinoshita says. Some
fundamental aspect of QED - quantum mechanics, say, or special
relativity - might have to be modified. "If quantum electrodynamics is
wrong, that would be a fantastic discovery," says Gabrielse. "The
stakes are really high here."

That's where Gabrielse's home-made replica of the hydrogen atom comes
in. Like the real thing, it features a lone electron that can inhabit
various quantum energy states. Gabrielse's device uses meticulously
tuned electric and magnetic fields to trap the electron in a sugar-
cube-sized space (see Diagram). The greatest challenge is keeping the
electron in a given energy state, as any fluttering between states
would lead to fuzziness in the measurements. Specially designed
refrigerators cool it to within 0.1 degrees of absolute zero so it
will stay in its lowest energy level indefinitely, and a near-perfect
vacuum gets rid of particles the electron could bump into.

Gabrielse's team measured a quantum property of the electron called
its magnetic moment, which makes it act as if it contains a tiny bar
magnet. The magnetic moment dictates the strength and direction of the
electron's response to a magnetic field, and through QED it is
directly related to alpha. They found that if they shot a photon into
the trap they could spur the electron to perform quantum leaps into
higher energy levels. Gabrielse's contraption employs electrodes to
measure the state of the electron, making it wiggle up and down. It
resonates at different rates depending on its quantum state, so
measuring these vibrations is a bit like listening to a guitar's notes
to figure out where the player is pressing the strings. Comparing the
electron's vibration rates across its different states gives a read-
out of the magnetic moment.

Early versions of quantum theory predicted that the electron's
magnetic moment would have a value of exactly 2, but in QED, the
virtual particles - governed by alpha - subtly tweak the value of the
electron's magnetic moment. In Gabrielse's experiment, the electron's
magnetic moment weighed in at 2.00231930436170. This measurement has
improved the value of the electron's magnetic moment to a precision of
one part per trillion - six times better than previous results
(Physical Review Letters, vol 97, 030801). "It's tough to measure
anything this well," says Barry Taylor of the US National Institute of
Standards and Technology in Gaithersburg, Maryland. "I'm very
impressed."

Taking their new measurement, Gabrielse and his colleagues plugged it
into the latest QED theory to get a new value for alpha:
1/137.035999710. They honed down the fine-structure constant's
uncertainty by a factor of 5, the first improvement in precision since
1987 (Physical Review Letters, vol 97, 030802). This brings its
exactitude to better than one part per billion, the equivalent of
measuring the distance from New York to San Francisco with flea-sized
precision. "It's irresistible to see how precisely we can pin down
these quantities," says Gabrielse.
Twitchy atoms

Meanwhile, researchers have been looking out for another, equally
precise way to obtain the value of alpha, only this time doing it
without reference to quantum electrodynamics. Comparing this
measurement with Gabrielse's, which does use QED, will put the theory
to its toughest test yet.

The best hope for this seems to come from atomic recoil experiments.
Here, physicists measure the tiny twitches of atoms when they kick
back after emitting a photon. By measuring the twitches and the colour
of the light that comes out, researchers can derive a value of alpha
that is independent of QED. Two groups - one led by François Biraben
at the Ecole Normale Supérieure in Paris, France, and another by Carol
Tanner at the University of Notre Dame in Indiana - have recently
reported precise measurements of alpha (Physical Review Letters, vol
96, 033001, and Physical Review A, vol 73, 032504).

So, for the first time in 20 years there is a method to measure alpha
that could eventually compete with techniques based on the electron's
magnetic moment. For now, Gabrielse's magnetic-moment method gives a
value of alpha that is about 10 times as precise as Biraben's and
Tanner's. If improved, however, the latter approach will set the stage
for the ultimate test of QED's accuracy. "This atomic recoil method is
coming to the rescue, potentially," says Czarnecki.

Will QED survive intact? "It has worked so remarkably well for a
number of years that it has made it into all of our textbooks as if
it's the gospel truth," Gabrielse says. "Most of us would be surprised
if it breaks down, just because we've failed to make it break down
after trying so hard." He adds, "We're in a rut, for very good reason,
in using these field theories. They have been wildly successful. But
it seems to me sort of part of the human experience to always ask, 'Is
that the whole story? Or is there something more to it?'"

Call it the romance of the next decimal place: physicists never know
what new phenomena may be lurking just around the corner.
Is alpha changing?

Recent studies suggest that the fine-structure constant, alpha, might
not be constant, but may have changed over billions of years (New
Scientist, 3 July 2004, p 6). If confirmed, that would suggest the
ground rules of physics are shifting under our feet. While
measurements of alpha are homing in on its value today, we can get an
idea of what alpha was in the past by looking at ancient processes.

Distant stars, whose light has travelled for billions of years before
reaching Earth, could reveal the value of alpha long ago. Researchers
look at the characteristic wavelengths of light emitted by the
elements in these stars, similar to measurements that introduced alpha
in the first place. A natural nuclear reactor in west Africa could
also reveal whether alpha has changed in the past 2 billion years. At
Oklo, in Gabon, enough uranium naturally clumped together to start a
nuclear reaction. Alpha affects the rate of this reaction and thus
determined which radioactive isotopes are left there today.

Both types of studies have yielded mixed results. For now, it seems,
most physicists think the fine-structure constant is still just that -
constant.
.

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