Posted on 10/16/2001 9:45:09 PM PDT by gcruse
Quantum Stew: How Physicists Are Redefining Reality's Rules
By GEORGE JOHNSON
Struggling to understand the strange
implications of modern physics,
readers in the 1930's and 40's turned to a
popular children's book for adults called
"Mr. Tompkins in Wonderland" by the
physicist George Gamow. In a series of
dreams, Mr. Tompkins finds himself in
surreal surroundings where the constants of
nature have been changed so that matter
behaves in ways that defy common sense.
In one dream, a number known as Planck's constant, which governs the
intensity of quantum theory's perplexing effects, is cranked up so high that
ordinary objects behave like elementary particles, which have the curious
ability to act like both hard little kernels and ethereal waves. Things as large
as billiard balls suddenly behave like electrons, spreading out all over the
table, following many different paths at once. A visit to a quantum pool hall
leaves poor Mr. Tompkins feeling drunk. The reason "quantum elephantism"
doesn't really happen, the book explains, is that Planck's constant is
extremely small, affecting only the tiniest objects — electrons and photons
but not billiard balls.
But nothing involving quantum theory is ever so clear-cut. Recent
experiments are demonstrating that quantum weirdness is not limited to the
atomic realm. In late September, a team of Danish physicists reported that a
phenomenon called quantum entanglement — the "spooky action at a
distance" that troubled Einstein — can affect not just individual particles but
clusters of trillions of atoms. And last week, the Nobel Prize in Physics was
awarded for experiments showing how quantum mechanics can be exploited
to make a couple of thousand atoms crowd together into a single superatom
— what the scientists called "a kind of smeared-out, overlapping stew."
Experiment by experiment, the abstractions of quantum theory are taking on
substance, impinging on phenomena closer to home. Physicists are
developing a new finesse — getting a feel for quantum mechanics by playing
with atoms the way their predecessors mastered Newtonian physics by
fooling around with swinging pendulums or marbles rolling down inclined
planes.
The practice is paying off with a deeper understanding of reality's rules. In
Mr. Tompkins's time, the difference between the mysterious quantum realm
and the hard-edged world of everyday life was assumed to be simply a
matter of size. Much beyond the magnitude of an atom, as quantum effects
faded, objects took on definite positions in space and time. In recent years
the situation has revealed itself as somewhat more subtle. Whether an object
is dominated by quantum fuzziness has less to do with how big it is than with
how well it can be shielded from outside disturbances — tiny vibrations,
bombarding air molecules or even particles of light.
Larger things are indeed harder to isolate from the roiling environment —
hence the predictable behavior of billiard balls. But with their delicate touch,
physicists are steadily bringing the quantum ambiguities further into the
macroscopic domain.
Consider the case of quantum entanglement. A subatomic particle can spin
clockwise or counterclockwise like a top — but with a quantum twist. As
long as it remains isolated from its environment, it lingers in a state of limbo,
rotating both clockwise and counterclockwise at the same time. Only when it
is measured or otherwise disturbed does it randomly snap into focus,
assuming one state or the other. "And" becomes "either/or."
Stranger still, two subatomic particles can be linked so that they must rotate
in opposite directions. Force one to spin clockwise and the other instantly
begins spinning counterclockwise, no matter how far they are separated in
space.
In the past, experimenters had entangled two photons this way, and last year,
in a major leap, they quantum mechanically tethered four atoms together. The
recent excitement came when physicists at the University of Aarhus in
Denmark reported in the Sept. 27 issue of Nature that they had briefly
entangled two clouds consisting of trillions of cesium atoms. In one cloud
most of the atoms were spinning one way; in the other cloud most were
spinning, mirrorlike, in the opposite direction.
Correlating groups of atoms this way may find a use in quantum computers,
devices where calculations are performed using single atoms or particles as
counters. (Think of them as quantum abacus beads.) Theoreticians have
proved that a quantum computer, if one can be built, could solve problems
now considered impossible.
The experiments that won this year's Nobel in physics involved synchronizing
atoms in a different but equally counterintuitive way.
Because of their quantum nature, atoms (like the particles they are made of)
act like waves. The slower they move, the more stretched-out they become,
dropping in pitch like a musical note sliding down the scale. Take a rarefied
gas — atoms darting around in a container — and cool it so that the motion
becomes slower and slower. Each atom's wavelength will widen until finally,
as the temperature nears absolute zero, they all overlap, forming an exotic
substance called a Bose-Einstein condensate. Imagine 2,000 billiard balls
merging into one.
It is impossible for us denizens of the macro world to really picture such a
state. We would have to have grown up in a universe with different
constants, like the ones in Mr. Tompkins's dreams. As Gamow put it in his
preface, "Even a primitive savage in such a world would be acquainted with
the principles of relativity and quantum theory, and would use them for his
hunting purposes and everyday needs."
As for developing quantum instincts, physicists are working their way up to
the level of savages, striking sparks, building their first fires.
I may be getting dopey, but where does the article say that?
No, size isn't the issue!
Get your mind out of the gutter. I'm discussing quantum mechanics here.
A single system, if excited to a sufficiently high quantum number, will exhibit classical behavior. For an example, solve the standard harmonic oscillator for a quantum number of 200. If you've got a sufficiently large number of identical systems, then you'd get classical behaviour as well, if one took the average of some simultaneous measurement on all of them.
Okay. But isn't the goal to go the other way?
That is, have macrosystems exhibit quantum
behavior? Dumb question, prolly.
Hey, talk to me anytime. I loves to learn. :)
If my old prof. explained one more variation of the Ideal gas law, I think I might have strangled him. :)
This is like telling a joke and forgetting the punchline.
Because of their quantum nature, atoms (like the particles they are made of) act like waves. The slower they move, the more stretched-out they become, dropping in pitch like a musical note sliding down the scale.
Why do I always find crap like that in articles written for a popular audience. If it isn't crap, I'm really missing something.
An example of outcome-based journalism, I guess.
"Physicist" has already, and thankfully, spared my from trying to explain what the author really meant. Funny, isn't it, that his "explanations" are invariably more succinct and precise than the original "quote" that he is explaining.
Now, if I only knew what a "Compton wavelength" is, I'd be all set .... I guess looking that up is a reasonable exercise for the student.
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