FW: Clear message for causality: Experiment confirms that
information cannot be transmitted faster than the speed of light
Joseph S. Barrera III
joe at barrera.org
Thu Dec 18 09:22:46 PST 2003
I'm really not that fond of causality, myself.
And the stars are *so* far away....
P.S. This is Eugene's cue to call us "monkeys" again :-)
Clear message for causality
Physics in Action: December 2003
*Experiment confirms that information cannot be transmitted faster than
the speed of light*
Ever since Einstein stated that nothing can travel faster than light,
physicists have delighted in finding exceptions. One after another,
observations of such "superluminal" propagation have been made. However,
while some image or pattern- such as the motion of a spotlight projected
on a distant wall - might have appeared to travel faster than light, it
seemed that there was no way to use the superluminal effect to transmit
energy or information.
In recent years, the superluminal propagation of light pulses through
certain media has led to renewed controversy. In 1995, for example,
Günther Nimtz of the University of Cologne encoded Mozart's 40th
Symphony on a microwave beam, which he claimed to have transmitted at a
speed faster than light. Others maintain that such a violation of
Einstein's speed limit would wreak havoc on our most fundamental ideas
about causality, allowing an effect to precede its cause. Relativity
teaches us that sending a signal faster than light would be equivalent
to sending it backwards in time.
Now an experiment by Michael Stenner and Daniel Gauthier at Duke
University in North Carolina and Mark Neifeld at the University of
Arizona promises to shed fresh light on this century-old conundrum. The
Duke-Arizona team has attempted to directly measure the "speed of
information" by sending a message through a superluminal medium (M
Stenner et al. 2003 Nature 425 695). However, the work could merely fan
the flames of the dispute over faster-than-light propagation.
*The speed of light*
Special relativity tells us that no matter how much kinetic energy a
massive object gains, its speed will never exceed the speed of light in
a vacuum, c. This, at least, is generally accepted. However, the issue
for light - which is an electromagnetic wave composed of massless
photons - has never been quite as clear-cut. Special relativity says
that massless particles always travel exactly at c, but one of the first
things we are taught about light is that it travels slower in glass or
water than it does in a vacuum.
This delay is due to the absorption and re-emission of photons by
particles in the medium. When light hits an atom, the electrons begin
vibrating at the optical frequency, and the electromagnetic fields that
they re-radiate interfere with the original fields. Whether this
interference delays or advances the phase of the waves depends on the
properties of the atom and the frequency of the wave.
The fact that light might travel faster than it does in a vacuum caused
great bewilderment at the beginning of the 20th century. But the issue
was eventually clarified by Arnold Sommerfeld and Léon Brillouin, who
showed that the "phase velocity" - the speed of the individual ripples
on an idealized wave that extends to infinity in both directions - does
not directly describe the motion of the energy in a light pulse.
Instead, Sommerfeld and Brillouin realized that the propagation of a
photon should be described by the speed at which the peak of localized
packets of ripples moves - the "group velocity". Even when the phase
velocity is faster than light, the group velocity should be slower.
Despite this careful redefinition of the velocity of a wave, there are,
in fact, still exotic situations where the group velocity can exceed c.
However, these situations are characterized by a great deal of
absorption and distortion, which led Sommerfeld and Brillouin to argue
that the group velocity itself loses any meaning in such regions. They
showed that any abrupt change in the pulse shape - which the person
receiving the signal would not expect, and which would therefore carry
new information - would travel not at this superluminal group velocity
but only at c.
*The speed of information*
Experiments in the 1980s, however, showed that it was possible for the
peak of a wave packet to arrive sooner than it would had it travelled at
c. Most researchers agreed that the peaks of such smooth, predictable
pulses carried no new information, since one could foresee the arrival
of the peak from the shape of the earlier portion of the pulse. The
modern version of Einstein's law that no signal can travel faster than
light was therefore left intact. Some researchers, on the other hand,
pointed out that these "superluminal" pulses could well be used to
trigger a practical detection system earlier than pulses that had
travelled at c.
One obstacle to resolving the issue was that all known examples of
superluminality involved the loss of most of the incident pulse, either
through absorption or reflection. This loss degrades the quality of any
signal, leaving open the argument that it could still take longer to
accumulate information than in the case of slower, but lossless,
transmission through the vacuum.
However, 10 years ago Raymond Chiao of the University of California at
Berkeley proposed that under the right conditions an optical pulse might
pass through a transparent (lossless) medium faster than light.
Together, we suggested an experiment in which this could be observed.
The idea was to pump a sample of atoms into an inverted state in which
their optical properties are essentially opposite to those in normal
matter, and then to tune the signal probe to a particular frequency that
allowed it to propagate superluminally.
In 2000 Lijun Wang and co-workers at NEC in Princeton demonstrated this
effect, proving that a pulse peak could exit a small vapour cell even
before it should have had time to enter (see "No thing goes faster than
light" and "Taming light with cold atoms"). This rekindled the embers of
controversy, and much theoretical work has followed. However, until now
no experiment has come any closer to closing the book on this issue.
The work of Stenner and colleagues extends the NEC work in two ways.
First the researchers devised an improved way to pump a sample of
potassium vapour into the special superluminal state, which allowed them
to increase the relative size of the effect by a factor of five. Instead
of an advance of only 1/50 of the original pulse width, as Wang and
co-workers had achieved, their pulse leaped an astounding 1/10 of a
pulse width ahead of one travelling at c.
Second, not content to work with infinitely smooth, predictable pulses,
the researchers encoded a realistic "signal" on top of their light beams
(see figure). This signal can represent either a "1" or a "0", and the
goal is to find the earliest moment at which a receiver would be able to
determine whether a "0" or a "1" had been sent. Stenner and co-workers
found that although the smooth pulse arrives noticeably earlier through
the superluminal medium, the instant at which the "1s" and "0s" begin to
differ does not seem to be accelerated. In fact, carrying out a very
careful analysis of signal and noise, and eliminating nearly all
spurious delays that the equipment itself might introduce, the team
found that "new" information actually arrives somewhat slower than light.
This result will be welcomed by mainstream physicists, who believe that
Einstein's speed limit will always be respected. But it is worth
pointing out that in this experiment, as in any real-world situation,
the detection of information has to be defined statistically in terms of
how long it takes to reach a certain level of confidence about the
content of the message. In reality, there is always some delay between
the decision to send a message and the point at which the first photon
leaves the transmitter. Similarly, it takes a finite time after the
arrival of this first photon before any level of confidence can be achieved.
It is therefore easy to mistake a reduction in these latency times for
true faster-than-light propagation. For example, a system that filters
out some of the noise would make it easier to extract the message
quickly. While the current experiment has done an excellent job of
eliminating spurious effects, it would be easy to construct similar
experiments that erroneously indicate superluminal information transfer.
Furthermore, one could argue that such information transfer could have
occurred but still be masked from view by mere technical noise.
Perhaps more important is the continuing uncertainty about how to truly
pin down, even in theory, where this "new information" resides. Clearly,
a signal cannot be spread out over all of history in a perfectly smooth
pulse. On the other hand, truly discontinuous pulses are never observed
in the real world and they are unpalatable even in theory. While
theorists focus on geometric points where something unpredictable
happens, experimentalists counter that no energy is contained in an
idealized point, and that no information can be obtained until at least
one photon is detected. Experiments like this force us - in this
information-dominated age - to come to terms with the fact that we still
do not truly know how to describe a task as simple as saying "yes" or "no".
Aephraim M Steinberg is at the Institute for Experimental Physics,
University of Vienna, Austria
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