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RE: [escepticos] Desigualdad de Bell
> Cuando uno muere, aquello que nos sustentaba sigue ahí, lo que desaparece
es
> precisamente esa sensorialidad. Para un organismo como nosotros la
ausencia
> total de sensorialidad equivale a la nada más absoluta, sin embargo aún
> sigue quedando ese algo indefinible que constituía nuestro cuerpo, quizás
> desde este punto de vista apreciemos mejor que realidad y existencia no
> definen en absoluto a ese algo insensorial y que quizás sería más
> conveniente redefinir el concepto de realidad como parte del especialísimo
> "display" sensorial que la evolución ha desarrollado en los seres vivos
> conscientes.
[Rafa]
Esto merece aclaración. Nunca me podría imginar que de la mecánica
cuántica pueda salir lo que me parece que dices.
El concepto de realidad no parece tener origen evolutivo *debido* a lo
que dice la cuántica.
[Vicente]
Soy consciente del peligro de relacionar este tipo de razonamientos con
aspectos de la f. cuántica. Lo cierto es que debería ser más cauteloso
máxime cuando se me escapan muchos detalles, sin embargo considerando el
asunto a nivel global creo que este tipo de comentarios son pertinentes, de
hecho los físicos que trabajan en este campo siguiendo una línea
exclusivamente experimental acaban desembocando forzosamente en conclusiones
que desafian frontalmente nuestra concepción digamos..."natural" del mundo,
ya no son capaces de considerar y atrapar una realidad independiente del
experimentador, la realidad *se concreta* como tal cuando un "algo"
intrínsecamente indefinido se mide o sensorializa de alguna manera, no
antes. Es, en todo caso, dependiente del sistema sensor y está
indisolublemente ligada a
el. En este sentido parece que filósofos y científicos han alcanzado, sin
pretenderlo, cierta convergencia a través de distintos caminos.
Aclaro desde ya que no busco una analogía fácil, podría matizar muchas cosas
al respecto pero se me haría muy pesado. Una cosa es segura, la forma en que
se manifiesta la realidad para nosotros está determinada directamente por
nuestra configuración biológica, por el particular diseño de nuestros
órganos. La respuesta que genera nuestro ojo ante muchos millones de fotones
que viajan a velocidades de vértigo puede ser una suave y apacible luz, una
cosa es el fenómeno exterior (aún misterioso para nosotros) y otra esa
sensación lumínica perfectamente definida, que no es sino una herramienta de
supervivencia producto de la evolución, como lo es el calor, la mano, la
amistad, el sueño, el sonido o la inteligencia. De ninguna manera podemos
considerar nuestras sensaciones como inherentes a los fenómenos externos, y
por favor... que esto no es Berkeley esto lo dice cualquier libro serio que
hable de los sentidos.
[Rafa]
Las medidas y "resultados" de un experimento son reales
"a secas" y las partículas materiales y de campo también.
[Vicente]
Tu lo has dicho, las "medidas" concretan una realidad, pero no sólo esa
realidad es coproducida por el aparato de medida y por lo tanto variable
según éste sino que no se puede hablar de realidad en tanto no haya
medición.
Respecto a lo de que existen las partículas per se, sin mediar un acto de
medición, se halla en contradicción con lo que dice la física actualmente.
De entrada el concepto de partícula puede ser útil pero resulta muy ambíguo
e indefinido.
En la edición española de Discover, no estoy seguro si la del mes de enero,
publicaron un artículo sobre un experimento llevado a cabo por un tal David
Pritchard que me dejó lo que se dice pasmao. Este señor ha construido un
sofisticadísimo aparato que, a diferencia de otros que consiguen obtener
interferencias vibratorias a partir de fotones, las obtenía a partir de
!moleculas! de sodio. El artículo venía con multitud de dibujitos y esquemas
de forma que uno se podía hacer una idea de su funcionamiento. En
experimentos con fotones suelen utilizar un "espejo" especial que permite
dividir en dos al fotón incidente, dejando pasar una mitad y reflejando la
otra, en el caso de las moleculas de sodio se trata de una especie de
rendijas que sorprendentemente tambien dividen esa estructura molecular en
haces vibratorios que luego se pueden hacer coincidir para efectuar
interferencias, esas moléculas que imaginamos formando una estructura
tridimensional pueden pasar sin problemas a traves de una especie de colador
haciendo gala de su condición vibratoria, pero no sólo se puede hacer con
una molécula, también es posible con una bacteria y con un piano de cola!
Actualmente las dificultades son de orden práctico no teórico
He hecho una búsqueda en la web de Discover para encontrar artículos en los
que apareciera Pritchard en el texto y me han aparecido dos, los envío
adjuntos por si fueran de interés, aunque no me resisto a hacer aquí un
pequeño quoteo francamente suculento:
Talk of creating interference patterns with bacteria or large molecules--or
even single atoms--leads to the question, What is really going on in the
interferometer? When pressed on how he visualizes the activity in his
machine, Pritchard professes to be an agnostic. ?I think of the
interferometer as a black box. We put things in one end and get a pattern
out the other end.? But what does the atom really do? It?s a mistake even to
ask, he says, for this assumes that there is a reality in that black box,
and that assumption will inevitably get you tangled in the contradictions of
quantum mechanics. The most that one can say is that when observed, the atom
appears as a particle, and when no one is looking, it seems to take on the
form of a wave, which can be described by a wave function--a purely
mathematical construct. Although Pritchard and other physicists fall back on
metaphors, such as water waves splitting up and recombining, Pritchard
cautions against attaching physical significance to the language they use
when they try to put the mathematics into words. ?Quantum mechanics says
there is no reality when you don?t make a measurement,? he says. From this
point of view, the atom waves passing through the interferometer aren?t
real; they are merely convenient fictions invented by physicists to predict
where the atoms will appear when they are observed.
Un saludo.
Vicente.
Interfering Atoms
By
David H. Freedman
Stopping atoms in their tracks is not the only way to get them to
show their wavelike nature. Another way is to throw them at a
grating with slits so small and tightly spaced that each atom wave
passes through two slits at once and is thus split in two. The split
waves can then be recombined to produce an interference
pattern--alternating bands of intensity in which the matter waves
either cancel each other or reinforce each other, just as
interfering light waves do. MIT physicist David Pritchard first
measured such atomic interference in 1988.
Last February Pritchard and his colleagues reported another first:
using the silicon nitride grating shown here, whose slits are just a
few hundred-millionths of an inch apart, they managed to separate
the split atom waves enough to do separate experiments on them. (The
closer the spacing of the slits, the more the waves diverge after
they pass through the grating.) The researchers passed one of the
waves through a gas or an electric field while leaving the other
alone. By observing the effect on the interference pattern--which is
extremely sensitive to any tampering with one of the component
waves--Pritchard and his team made fundamental measurements that
were not possible before. They measured the susceptibility of sodium
atoms to electric fields and the degree to which sodium atom waves
are refracted--bent and attenuated--as they pass through another gas
and the atoms in that gas attract them.
Physicists armed with optical interferometers have been able to make
similar measurements on light waves for the last century or so--but
light waves are 10,000 times longer than atom waves, which means
they can be diffracted with much coarser gratings than the one in
Pritchard?s atom interferometer. Pritchard has managed to send
entire sodium molecules through his device, and in principle, even
something as large as a living bacterium could hurtle through it in
wave form. But quantum mechanical trade-offs mean that such a large
chunk of matter would take thousands of years to pass through the
grating. For the moment at least, physicists must be content with
finally being able to exploit the wave nature of atoms.
Beams of Stuff
By
Robert Pool
To get closer to the true, quantum nature of matter, physicist David
Pritchard has been splitting atoms down the middle, fiddling with
the halves, and then putting them back together. In principle, he
says, he could do the same to a bacterium. Or even a baby grand.
In room 240 of building 26 on the mit campus, physicist David
Pritchard has built an attitude-adjustment machine. That?s not what
he calls it, of course. Officially it is an atom interferometer,
built to measure various properties of atoms with astonishing
accuracy, and this it does quite well. The machine has allowed
Pritchard to pin down some atomic quantities that were once
unmeasurable and to improve the accuracy of other measurements ten
times over--the particle-physics equivalent of adding a couple of
inches to the world high-jump record or shaving several minutes from
the previous best marathon time. But as good as it is at making
measurements, the interferometer is even better at opening eyes and
shaking beliefs. Once you understand what it does, you can never
think of ordinary matter in the same way again.
From the outside, the machine appears to be nothing more than a
cylindrical stainless-steel shell, 11 or 12 feet long and a foot in
diameter, with a dozen airtight ports that provide access for vacuum
pumps, control cables, and various instruments. It is the sort of
container physicists typically use when they want to perform
experiments in a near vacuum. Inside, however, is where its secrets
lie. The first section of the interferometer creates a very narrow
atomic beam--a collection of sodium atoms, all moving in the same
direction and at the same speed--and shoots it down the axis of the
machine. After a few inches, the beam gets split into two
components, one angling left, the other right. Two feet farther on,
the two beams encounter a second device that angles them back toward
the middle, so that a few feet later they meet.
To understand what happens next, you need a passing acquaintance
with wave-particle duality--the notion that matter at the quantum
mechanical scale, the scale of individual subatomic particles, acts
sometimes like a wave, sometimes like a particle, depending on the
circumstances. In this case, when the beams meet they act like two
sets of ripples colliding on a pond--they create an interference
pattern of peaks and valleys. Wherever the crests of two ripples
meet, or two troughs, they add together to create a large peak or
valley; when crest meets trough, they cancel each other out. At the
far end of the interferometer, a detector records the interference
pattern created.
One?s first impression on hearing this description is that the atoms
have done nothing unusual. After all, you could set up a similar
system with water waves and get the same sort of interference
pattern. But then Pritchard adds one detail that changes everything.
He has made the beam so weak that only one atom passes through the
interferometer at a time. Thus, when the atom beam splits in two,
it?s not that some atoms are going right and others left. The entire
beam is a single atom, and when the beam is split, the atom is
split. Suddenly you find yourself scrambling to visualize what has
happened--and failing miserably.
?It is difficult to picture what the atom is doing in there,?
Pritchard admits. Nearly all similar beam-splitting experiments use
fundamental particles, like photons or electrons, that have no
internal structure. Because a photon is a featureless, dimensionless
speck of light, it?s easy to squint mentally and fancy that you see
it as a fuzzy, spread- out wave. Stop squinting, and it?s a point
particle again. You fool yourself into believing that you really
understand wave-particle duality.
But atoms are different. They have a definite, complex structure.
Each sodium atom passing through the interferometer is composed of
34 individual particles--11 protons, 12 neutrons, and 11
electrons--arrayed in the familiar atomic pattern, with the protons
and neutrons in a central nucleus orbited by the electrons. It is
not easy to squint and pretend that these well-defined compound
objects are fuzzy waves. To make imagining even more difficult,
Pritchard has sent sodium molecules--pairs of sodium atoms, 68
particles in all--through the interferometer and shown that these
doubly complex entities also traverse the machine as waves.
The wave nature of an atom is necessarily hidden because, according
to theory, the moment a quantum mechanical object is observed, it no
longer behaves like a wave. It assumes the guise of an ordinary
classical particle--its behavior is not significantly different from
a billiard ball?s. For this reason, ever since Niels Bohr and other
physicists constructed quantum theory back in the 1920s, the details
of the transformation from wave to particle have been shrouded in
mystery. ?In the days of Bohr, we just accepted it as a fact of
life,? Pritchard says. ?Now we want to understand it and hopefully
be able to work with it.? And Pritchard believes he may finally have
built an instrument that can do that.
The beam begins with a bit of sodium metal heated to 1300 degrees
Fahrenheit. Sodium atoms come bubbling off into an atmosphere of
neon, argon, or other noble gas and form a thick sodium vapor. Then,
whoosh, the gas mixture jets through a tiny nozzle at several times
the speed of sound and enters a vacuum chamber, in which it expands
and cools, leaving the sodium beam monochromatic--that is, with all
its atoms moving at about the same speed. This torrent strikes an
inverted funnel with an opening the size of a pinhole, which
?collimates? the beam, allowing only those atoms nearest the
centerline to advance to the next chamber. Farther on, the beam is
collimated again, this time by passing through a narrow slit, and
then again, so that only those few atoms pointed straight down the
axis of the interferometer remain in the beam.
Of the sodium atoms that began the journey, just a minute fraction
are now left to carry on, but the wastefulness is necessary,
Pritchard says. ?You need an atom beam that is sufficiently
monochromatic and well-collimated that it doesn?t wash out the
interference pattern.? If the atoms are moving at different speeds
or in different directions, the interference pattern will be as
difficult to make out as the picture on a scrambled cable tv
channel.
In normal atmosphere, the atoms in the beam would collide with
surrounding atoms before they had traveled a millionth of an inch,
and the beam would dissipate almost immediately. Pritchard?s
chambers, however, are nearly a perfect vacuum. ?An atom in this
vacuum could travel a hundred meters without hitting anything,? he
says. And since the atoms in the beam, all moving in the same
direction and at the same speed, are also unlikely to run into each
other, each sodium atom sails as though through empty space.
Until, that is, it encounters the first beam splitter. To divide the
atom beam into components, Pritchard uses a diffraction grating--a
series of closely spaced microscopic slits cut into a silicon
nitride membrane. The atom approaches the grating like a bb pellet
fired at a picket fence. Each slit in the grating is 100 nanometers
wide--about four- millionths of an inch--as is each of the silicon
nitride slats. An individual sodium atom, on the other hand, appears
only a few tenths of a nanometer across when observed--hundreds of
times smaller than the slits in the grating. If the atom were indeed
a bb-like particle, it would either slam into one of the slats of
the fence or else sail through one of the gaps between the slats.
That doesn?t happen. About half the atoms in the beam do hit and
stick to the slats of the grating, but the half that get to the
other side do not pass through just one slit. Each atom passes
through approximately a hundred slits simultaneously, like an ocean
wave washing through a line of pilings protecting a harbor, and
exits the grating as a hundred individual waves. Once past the
grating, each of these component waves begins to spread out and
collide with the others, canceling out in some places and combining
in others. The resulting diffraction pattern consists of a series of
waves that fan out behind the grating like the tines on a leaf rake.
The strongest wave is the one headed straight down the axis of the
interferometer, the next strongest are the two on either side of it,
and so on. For the interferometer, Pritchard needs just two of
these, and he sets up the apparatus in such a way that only the
straight-ahead wave and the one to its right play a role.
This ratherawkward beam splitter is necessary, Pritchard says,
because there is no better, easier way to separate an atom wave into
two components. In a light interferometer, the beam splitter is a
half-silvered mirror set at an angle to the light beam. Half the
light goes through, and the other half is reflected off at an angle.
But there is no material that is transparent to atoms, nor is there
an easy way to reflect atoms off a surface.
As the two beams move away from the diffraction grating, they
diverge ever so slightly. By the time they reach a second grating
two feet farther on, the distance between the beams is the width of
a very fine hair--about 50,000 nanometers, or two-thousandths of an
inch. Like the first beam splitter, this grating divides each of the
two beams into a fan of spreading waves, and this time Pritchard
uses only one finger from each. From the beam on the left, he
chooses the first component on the right, and from the beam to its
right, he takes the first component on the left. Two feet farther
on, these two sub-beams intersect to create an interference pattern.
To detect the interference pattern, Pritchard uses a third
diffraction grating paired with a ?hot wire? detector. Whenever an
atom comes through this third grating, it strikes a thin rhenium
wire heated to 1500 degrees. At this temperature the rhenium can
suck an electron off the atom, giving the atom a positive electric
charge and making it perceptible to a charge detector behind the
wire. Pritchard counts the atoms hitting the wire for a few
thousandths of a second, moves the grating a little and counts
again, gradually building up a picture of the interference pattern.
When this third diffraction grating is lined up with the
high-intensity parts of the pattern, many atoms will come through.
When the grating is lined up with the low-intensity sections, only a
few will.
The interference pattern, Pritchard notes, is a statistical
phenomenon. It cannot be seen in the behavior of an individual atom,
because the act of striking the detector causes the atom to shed its
quantum mechanical nature. The pattern can be inferred, however, by
watching thousands of atoms and calculating the probability that a
given atom will be found in this place or that. The effect is much
like viewing a pointillist painting--a single atom contributes a
single dot, and together they create a clear image.
So far Pritchard has put only sodium atoms and sodium molecules
through the interferometer, but there is no reason he couldn?t
create interference patterns with larger objects. ?There?s no real
theoretical limit,? he says. ?The practical limit is time.? To
produce a detectable interference pattern, the wavelength of an
object moving through the interferometer needs to stay above a
certain minimum, which for Pritchard?s machine is about a hundredth
of a nanometer. A so-called matter wave, of course, is not to be
confused with an electromagnetic or any other kind of wave. To talk
about the wavelength of a bacterium, say, or a baseball, is to defy
common sense, and yet, using the equations of quantum mechanics, you
can calculate a wavelength for each of these objects. According to
these equations, wavelength decreases as both mass and velocity
increase-- the bigger an object is, the slower it has to move for
the wavelength to stay the same. Therefore, if you wanted to send a
molecule with a hundred sodium atoms through Pritchard?s machine,
the molecule would need to travel through the interferometer at
one-hundredth the sodium atoms? speed, if it were to have the same
wavelength. Larger objects would have to go even more slowly. ?And
at some point,? Pritchard points out, ?you?d have to get a larger
diffraction grating so that things would fit through the slits.?
That would demand slowing things down even more because the
visibility of the interference pattern depends partly on how much
the two beams diverge, which in turn depends on the closeness of the
splits in the grating.
?We calculated that somewhere between a bacterium and an amoeba,
we?d run out of time,? Pritchard jokes. At this size, he says, an
object would need two years to pass through the interferometer
instead of the two milliseconds typical for an atom, and if it took
longer than that he couldn?t attract any graduate students to help
him run the experiments.
Of course, Pritchard notes, a number of practical obstacles would
prevent him from going this far, even if he could figure out how to
make a bacterium beam. The interferometer is exceptionally sensitive
to vibrations, changes in temperature, and other disturbances, and
to obtain an interference pattern with atoms demanded such measures
as hanging the machine from the ceiling to reduce vibrations.
Working with something as large as bacteria would make the
interferometer so sensitive that ?the gravitational pull from a
truck pulling up at the loading dock would screw things up.?
Talk of creating interference patterns with bacteria or large
molecules--or even single atoms--leads to the question, What is
really going on in the interferometer? When pressed on how he
visualizes the activity in his machine, Pritchard professes to be an
agnostic. ?I think of the interferometer as a black box. We put
things in one end and get a pattern out the other end.? But what
does the atom really do? It?s a mistake even to ask, he says, for
this assumes that there is a reality in that black box, and that
assumption will inevitably get you tangled in the contradictions of
quantum mechanics. The most that one can say is that when observed,
the atom appears as a particle, and when no one is looking, it seems
to take on the form of a wave, which can be described by a wave
function--a purely mathematical construct. Although Pritchard and
other physicists fall back on metaphors, such as water waves
splitting up and recombining, Pritchard cautions against attaching
physical significance to the language they use when they try to put
the mathematics into words. ?Quantum mechanics says there is no
reality when you don?t make a measurement,? he says. >From this point
of view, the atom waves passing through the interferometer aren?t
real; they are merely convenient fictions invented by physicists to
predict where the atoms will appear when they are observed.
These convenient fictions can have real payoffs, however, when put
to work in an interferometer. A hundred years ago, some of the most
important measurements in physics were being performed with light
interferometers--such as the Michelson-Morley experiment, which
found the speed of light to be the same in all directions and paved
the way for Einstein?s special theory of relativity. The atom
interferometer is a useful measuring device for precisely the same
reason that the light interferometer is: its interference pattern is
extremely sensitive to conditions along the paths that the two beams
take. If something causes one beam to, say, slow down slightly, the
interference pattern will shift noticeably and the shift will
indicate how big the slowdown was.
This sensitivity has allowed Pritchard to measure the electric
polarizability of the sodium atom--the extent to which a sodium
atom?s electrons are pulled off center by an electric field--with an
accuracy ten times greater than ever before. He did it by placing an
electric field across the path of only one of the beams in his
interferometer and observing the resulting shift in the interference
pattern. In another experiment, Pritchard measured the force of
attraction between sodium and atoms of different gases. This he
accomplished by placing a gas in the path of one arm of the sodium
beam. Each sodium atom sped up as it approached an atom of gas and
slowed down as it left the atom behind, and in the process the
interference pattern shifted almost imperceptibly. By measuring this
subtle shift, Pritchard obtained details about these interactions
between atoms that no one else had.
Pritchard even tried rotating the interferometer at about the same
speed as the hour hand on a clock. Since the atoms in the beam
continued to fly straight, the resulting interference pattern
shifted slightly. By measuring this shift, he was able to reverse
calculate the speed at which the interferometer was rotating.
Eventually, Pritchard suggests, this capability might make atom
interferometers useful in ultrasensitive inertial navigation systems
or other applications that require the detection of tiny movements.
Although using the interferometer as an exquisitely sensitive
yardstick is interesting and potentially very useful, Pritchard?s
favorite experiment was far more academic. It told him nothing that
quantum physicists haven?t known since 1925. Rather it had value as
a demonstration of theory--a particularly vivid and exactingly
controlled demonstration, and one that allowed him to linger at the
boundary at which a particle begins to act like a wave.
The experiment started merely by observing the sodium atom as it
passed through the interferometer. To do this, Pritchard placed a
laser slightly before the second diffraction grating and shone it
across the paths of both arms of the sodium beam. When a photon from
the laser struck a passing atom, it bounced off, providing a way to
?see? the atom. Actually, Pritchard didn?t even bother to place a
photo-detector in the path of the deflected photons. What mattered
was that he could have if he?d wanted to. As far as the atoms in the
interferometer were concerned, they had been caught in the act. They
were forced to take on the guise of particles, as quantum mechanics
says they should. As a result, the interference pattern vanished.
Next, Pritchard moved the laser much closer to the first diffraction
grating, so that it passed across the two beams before they had
diverged so much. As if by magic, the interference pattern
reappeared. Why? In the act of observing, the details you can make
out are limited by the wavelength of the light--or X-rays, or
whatever type of electromagnetic radiation you happen to be using.
Thus it is impossible to build a light microscope powerful enough to
see an atom, no matter how large the lenses, because the wavelength
of visible light is a thousand times larger than an atom. Likewise,
when Pritchard moved the laser to its new position, the separation
between the beams was less than the wavelength of the laser light,
which made the two beams indistinguishable. In effect, even though
the atoms passing through the interferometer had been struck by
photons, their positions had not been observed because the picture,
so to speak, was too fuzzy. That restored the uncertainty about
which arm they were in, and the two components of the atom beam once
again began to interfere with each other.
Pritchard then gradually moved the laser back toward the second
grating, so that the beams were farther and farther apart when the
laser light hit them, making it possible to tell with more and more
certainty which beam an atom was in. As he did, the interference
pattern gradually faded. At a point where the wavelength of the
photons was exactly twice the separation between the beams--the
minimum wavelength needed to tell with certainty which beam the atom
was in--the interference pattern disappeared completely. The atom in
principle could have been located, and it once again dropped any
pretense of acting like a wave.
Such splitting of atomic hairs may one day actually turn out to have
some practical significance. It?s been suggested that quantum
computers would carry out calculations with particles that are in
two or more quantum states at once. This would allow them, in
theory, to perform calculations that are unthinkable for present-day
computers, whose electrons exist in only one place at a time.
Whatever form quantum computers ultimately take, the need to control
the transition from particle to wave and back again will be crucial
to their feasibility. ?We?ll need to reverse it, protect it, distill
it, error-correct it, and so on,? says Pritchard. His experiment is
an ambitious first step in that direction. And even in the strange,
counterintuitive world of quantum mechanics, there needs to be a
first step.