Re: [escepticos] Luz que se mueve mas rapido que la luz

[ungil@...>]

Hola, hola.

Santiago Arteaga wrote:
>         Creo que he encontrado otro ejemplo de esos en los que a 
> un periodista le cuentan una cosa, no se entera, y escribe algo
> execrable.  http://www.larazon.es/hoy/sociedad10.htm
(...) 
> 	Me suena a cagada del periodista; apuesto a que le han 
> contado una historia y no se ha enterado. Las velocidades de grupo 
> desde siempre han podido ser mayores que la de la luz, pero es que 
> son algo ficticio.

Cierto es que lo de la velocidad de grupo es conocido, pero no es tan
ficticio. De alguna manera, un pulso de luz es una especie de "montaña" 
que se desplaza a la velocidad de la luz. En algún sitio lei que en
determinadas circunstancias podias quedarte solo con el principio del
pulso. Tienes entonces una "montañita" correspondiente a lo que era el pie
de la montaña original, mientras que el resto sale en otra dirección.
Entonces el pulso resultante (la "montañita") parece haberse movido de
repente hacia delante, superando por tanto la velocidad de la luz.
No va contra la relatividad ni nada, pero es curioso.
No sé si en este caso se trata de lo mismo, ni me hago responsable de que
la descripción que hago esté bien (lo lei hace varios años en
Investigación y Ciencia).

En cuanto a la noticia esta, no he leído lo publicado en La Razón, pero 
apareció un artículo en el New York Times.
(http://www.nytimes.com/library/national/science/053000sci-physics-light.html)
(Hace falta registrarse, así que lo copio al final)
No lo he leído con mucha atención, pero algo de interés tendrá el asunto
si lo van a publicar en Nature (al menos lo han enviado).

Saludos,
	Carlitos

Light Exceeds Its Own Speed Limit, or Does It?

By JAMES GLANZ

<p>The speed at which light travels through a 
vacuum, about 186,000 miles per second, is 
enshrined in physics lore as a universal 
speed limit. Nothing can travel faster than 
that speed, according freshman textbooks 
and conversation at sophisticated wine 
bars; Einstein's theory of relativity would 
crumble, theoretical physics would fall into 
disarray, if anything could.

<p> Two new experiments have demonstrated 
how wrong that comfortable wisdom is. 
Einstein's theory survives, physicists say, 
but the results of the experiments are so 
mind-bending and weird that the easily unnerved are advised--in all
seriousness--not 
to read beyond this point.

<p> In the most striking of the new experiments a pulse of light that
enters a transparent chamber filled with specially prepared cesium gas is
pushed to speeds of 300 
times the normal speed of light. That is so 
fast that, under these peculiar circumstances, the main part of the pulse
exits the 
far side of the chamber even before it enters 
at the near side.

<p> It is as if someone looking through a 
window from home were to see a man slip 
and fall on a patch of ice while crossing the 
street well before witnesses on the sidewalk 
saw the mishap occur--a preview of the 
future. But Einstein's theory, and at least a 
shred of common sense, seem to survive 
because the effect could never be used to 
signal back in time to change the past--avert 
the accident, in the example.

<p> A paper on the experiment, by Lijun 
Wang of the NEC Research Institute in 
Princeton, N.J., has been submitted to Nature and is currently undergoing
peer review. It is only the most spectacular example of  work by a wide
range of 
researchers recently who have produced superluminal speeds of propagation
in various materials, in hopes 
of finding a chink in Einstein's armor 
and using the effect in practical applications like speeding up electrical 
circuits.

<p> "It looks like a beautiful experiment," said Raymond Chiao, a
professor of physics at the University of 
California in Berkeley, who, like a 
number of physicists in the close-knit 
community of optics research, is 
knowledgeable about Dr. Wang's 
work.

<p>  Dr. Chiao, whose own research 
laid some of the groundwork for the 
experiment, added that "there's 
been a lot of controversy" over 
whether the finding means that actual information--like the news of an 
impending accident--could be sent 
faster than <I>c</I>, the velocity of light. 
But he said that he and most other 
physicists agreed that it could not.

<p> Though declining to provide details of his paper because it is under 
review, Dr. Wang said: "Our light 
pulses can indeed be made to travel 
faster than <I>c</I>. This is a special property of light itself, which is
different 
from a familiar object like a brick," 
since light is a wave with no mass. A 
brick  could not travel so fast without 
creating truly big problems for physics, not to mention humanity as a 
whole.

<p> A paper on the second new experiment, by Daniela Mugnai, Anedio 
Ranfagni and Rocco Ruggeri of the 
Italian National Research Council, 
described what appeared to be 
slightly faster-than-<I>c</I> propagation of 
microwaves through ordinary air, 
and was published in the May 22 
issue of Physical Review Letters.

<p> The kind of chamber in Dr. Wang's 
experiment is normally used to amplify waves of laser light, not speed 
them up, said Aephraim M. Steinberg, a physicist at the University of 
Toronto. In the usual arrangement, 
one beam of light is shone on the 
chamber, exciting the cesium atoms, 
and then a second beam passing 
thorugh the chamber soaks up some 
of that energy and gets amplified 
when it passes through them.

<p> But the amplification occurs only 
if the second beam is tuned to a 
certain precise wavelength, Dr. 
Steinberg said. By cleverly choosing 
a slightly different wavelength, Dr. 
Wang induced the cesium to speed up 
a light pulse without distorting it in 
any way. "If you look at the total 
pulse that comes out, it doesn't actually get amplified," Dr. Steinberg 
said.

<p> There is a further twist in the 
experiment, since only a particularly 
strange type of wave can propagate 
through the cesium. Waves Light signals, consisting of packets of waves, 
actually have two important speeds: 
the speed of the individual peaks and 
troughs of the light waves themselves, and the speed of the pulse or 
packet into which they are bunched. 
A pulse may contain billions or trillions of tiny peaks and troughs. In
air 
the two speeds are the same, but in 
the excited cesium they are not only 
different, but the pulses and the 
waves of which they are composed 
can travel in opposite directions, like 
a pocket of congestion on a highway, 
which can propagate back from a toll 
booth as rush hour begins, even as all 
the cars are still moving forward.

<p> These so-called backward modes 
are not new in themselves, having 
been routinely measured in other 
media like plasmas, or ionized gases. 
But in the cesium experiment, the 
outcome is particularly strange because backward light waves can, in 
effect, borrow energy from the excited cesium atoms before giving it 
back a short time later. The overall 
result is an outgoing wave exactly 
the same in shape and intensity as 
the incoming wave; the outgoing 
wave just leaves early, before the 
peak of the incoming wave even arrives.


<p> As most physicists interpret the 
experiment, it is a low-intensity precursor (sometimes called a tail, even 
when it comes first) of the incoming 
wave that clues the cesium chamber 
to the imminent arrival of a pulse. In 
a process whose details are poorly 
understood, but whose effect in Dr. 
Wang's experiment is striking, the 
cesium chamber reconstructs the entire pulse solely from information 
contained in the shape and size of the 
tail, and spits the pulse out early.

<p> If the side of the chamber facing 
the incoming wave is called the near 
side, and the other the far side, the 
sequence of events is something like 
the following. The incoming wave, its 
tail extending ahead of it, approaches the chamber. Before the incoming 
wave's peak gets to the near side of 
the chamber, a complete pulse is 
emitted from the far side, along with 
a backward wave inside the chamber that moves from the far to the 
near side.

<p> The backward wave, traveling at 
300 times c, arrives at the near side 
of the chamber just in time to meet 
the incoming wave. The peaks of one 
wave overlap the troughs of the other, so they cancel each other out and 
nothing remains. What has really 
happened is that the incoming wave 
has "paid back" the cesium atoms 
that lent energy on the other side of 
the chamber.

<p> Someone who looked only at the 
beginning and end of the experiment 
would see only a pulse of light that 
somehow jumped forward in time by 
moving faster than c.

<p> "The effect is really quite dramatic," Dr. Steinberg said. "For a 
first demonstration, I think this is 
beautiful."

<p> In Dr. Wang's experiment, the outgoing pulse had already traveled 
about 60 feet from the chamber before the incoming pulse had reached 
the chamber's near side. That distance corresponds to 60 billionths of 
a second of light travel time. But it 
really wouldn't allow anyone to send 
information faster than <I>c</I>, said Peter 
W. Milonni, a physicist at Los Alamos National Laboratory. While the 
peak of the pulse does get pushed 
forward by that amount, an early 
"nose" or faint precursor of the 
pulse has probably given a hint to the 
cesium of the pulse to come.

<p> "The information is already there 
in the leading edge of the pulse," Dr. 
Milonni said. "You can get the impression of sending information
superluminally even though you're not 
sending information."

<p> The cesium chamberhas  reconstructed the entire pulse shape, using 
only the shape of the precursor. So 
for most physicists, no fundamental 
principles have been smashed in the 
new work.

<p> Not all physicists agree that the 
question has been settled, though. 
"This problem is still open," said Dr. 
Ranfagni of the Italian group, which 
used an ingenious set of reflecting 
optics to create microwave pulses 
that seemed to travel as much as 
25% faster than <I>c</I> over short distances.

<p> At least one physicist, Dr. Guenter 
Nimtz [[umlaut over u]] of the University of Cologne, holds the opinion 
that a number of experiments, including those of the Italian group, 
have in fact sent information superluminally. But not even Dr. Nimtz 
believes that this trick would allow 
one to reach back in time. He says, in 
essence, that the time it takes to read 
any incoming information would fritter away any temporal advantage, 
making it impossible to signal back 
and change events in the past.

<p> However those debates end, however, Dr. Steinberg said that techniques
closely related to Dr. Wang's 
might someday be used to speed up 
signals that normally get slowed 
down by passing through all sorts of 
ordinary materials in circuits. A 
miniaturized version of Dr. Wang's 
setup "is exactly the kind of system 
you'd want for that application, Dr. 
Steinberg said.

<p> Sadly for those who would like to 
see a computer chip without a speed 
limit, the trick would help the signals 
travel closer to the speed of light, but 
not beyond it, he said.