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....

- Joe

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.

*New information*

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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