quarta-feira, 16 de dezembro de 2009

Signal Travels Farther and Faster Than Light

By Malcolm W. Browne - July 22, 1997

It was as if some ghostly bridge across the city of Geneva, Switzerland, had permitted two photons of light nearly seven miles apart to respond simultaneously to a stimulus applied to just one of them.

The twin-photon experiment by Dr. Nicolas Gisin (http://www.gap-optique.unige.ch/Members/Nicolas/Resume.htm) of the University of Geneva and his colleagues last month was the most spectacular demonstration yet of the mysterious long-range connections that exist between quantum events, connections created from nothing at all, which in theory can reach instantaneously from one end of the universe to the other.

In essence, Gisin sent pairs of photons in opposite directions to villages north and south of Geneva along optical fibers of the kind used to transmit telephone calls. Reaching the ends of these fibers, the two photons were forced to make random choices between alternative, equally possible pathways.

Since there was no way for the photons to communicate with each other, "classical" physics would predict that their independent choices would bear no relationship to each other. But when the paths of the two photons were properly adjusted and the results compared, the independent decisions by the paired photons always matched, even though there was no physical way for them to communicate with each other.

Albert Einstein sneered at the very possibility of such a thing, calling it "spooky action at a distance." Scientists still (somewhat shamefacedly) speak of the "magic" of "quantum weirdness." And yet all experiments in recent years have shown that Einstein was wrong and that action at a distance is real.

The idea behind Gisin's experiment was not new. Since the 1970s, physicists have been testing a prediction of quantum theory that "entangled" particles continue to communicate with each other instantaneously even when very far apart.

Entangled particles are identical entities that share common origins and properties, and remain in instantaneous touch with each other, no matter how wide the gap between them.

Past experiments on entangled particles were carried out over distances of 100 yards or less. By showing that the link between two entangled particles survives even when they are seven miles apart, Gisin set a dramatic distance record.

"In principle, it should make no difference whether the correlation between twin particles occurs when they are separated by a few meters or by the entire universe," he said in an interview.

"This research is interesting not only from a scientific and philosophical point of view, but because of a very practical consequence: we can now create a completely secure code. A quantum key, which is now within reach, would allow banks to carry out transactions with each other over optical fibers, completely safe from all possible code-breaking methods and from eavesdropping or interference."

The idea for such a system, he said, originated with Dr. Artur D. Eckert at Oxford University in England.

Details of the Swiss experiment will be described in a forthcoming technical paper, Gisin said, and he is working with the Swiss telecommunications agency to develop a cryptographic system based on entangled particle "twins." Identical random-number sequences generated simultaneously by pairs of widely separated twins would serve as cipher keys equivalent to the "one-time pads" used by spies and governments to encode and decode ultra-secret messages.

The receiver and sender of a secret message based on a one-time pad each must have a copy of the pad, which contains a random sequence of numbers. The sequence defines a series of mathematical operations used to encipher the message, and the reverse sequence is used to decipher it. The key pads of sender and receiver are used for only one message and then destroyed; this means that every letter of every message is enciphered by its own unique key and is therefore completely immune to cryptanalysis.

One of the leading experimentalists in quantum optics, Dr. Raymond Y. Chiao (*) of the University of California, Berkeley, hailed the Geneva experiment as "wonderful."

But an underlying enigma of quantum mechanics remains unfathomed.

The connections that persist between distant but entangled particles are "one of the deep mysteries of quantum mechanics," Chiao said in an interview. "These connections are a fact of nature proven by experiments, but to try to explain them philosophically is very difficult," he said.
(*) Chiao has become well known in the field of quantum optics due to several important experiments. He was first to measure the quantum tunnelling time (**), which was found to be between 1.5 to 1.7 times the speed of light. He also was the first to measure the topological Berry's Phase (Geometric phase).
(**) The time experienced by a phenomenon when passing through a tunneling barrier. In superluminal (***) phenomena, the tunneling time is expected to be zero in some theories. But in others, it may have a nonzero value.
Portions of this entry contributed by Waldyr A. Rodrigues, Jr.
A superluminal phenomenon is a frame of reference traveling with a speed greater than the speed of light c. There is a putative class of particles dubbed tachyons which are able to travel faster than light. Faster-than-light phenomena violate the usual understanding of the "flow" of time, a state of affairs which is known as the causality problem (and also called the "Shalimar Treaty").
It should be noted that while Einstein's theory of special relativity prevents (real) mass, energy, or information from traveling faster than the speed of light c (Lorentz et al. 1952, Brillouin and Sommerfeld 1960, Born and Wolf 1999, Landau and Lifschitz 1997), there is nothing preventing "apparent" motion faster than c (or, in fact, with negative speeds, implying arrival at a destination before leaving the origin). For example, the phase velocity and group velocity of a wave may exceed the speed of light, but in such cases, no energy or information actually travels faster than c. Experiments showing group velocities greater than c include that of Wang et al. (2000), who produced a laser pulse in atomic cesium gas with a group velocity of (-310+/-5)c. In each case, the observed superluminal propagation is not at odds with causality, and is instead a consequence of classical interference between its constituent frequency components in a region of anomalous dispersion (Wang et al. 2000).
It turns out that all relativistic wave equations possesses infinity families of formal solutions with arbitrary speeds raging from zero to infinity, called undistorted progressive waves (UPWs) by Rodrigues and Lu (1997). However, like the arbitrary-speed plane wave solutions, UPWs have infinite energy and therefore cannot be produced in the physical world. However, approximations to these waves with finite energy, called finite aperture approximations (FAA), can be produced and observed experimentally (Maiorino and Rodrigues 1999). Among the infinite family of exact superluminal solutions of the homogeneous wave equation and Maxwell equations are waves known as X-waves. X-waves do not violate special relativity because all superluminal X-waves have wavefronts that travel with the speed parameter c (the speed of light) that appears in the corresponding wave equation. The superluminal motion of the peak is therefore a transitory phenomenon similar to the reshaping phenomenon that occurs (under very special conditions) for waves in dispersive media with absorption or gain and which is in this case responsible for superluminal (or even negative) group velocities (Maiorino and Rodrigues 1999).
Several authors have published theories claiming that the speed-of-light barrier imposed by relativity is illusionary. While these "theories" continue to be rejected by the physics community as ill-informed speculation, their proponents continue to promulgate them in rather obscure journals. An example of this kind is the Smarandache hypothesis, which states that there is no such thing as a speed limit in the universe (Smarandache 1998). Similarly Shan (1999ab) has concluded that the superluminal communication must exist in the universe and that they do not result in the casual loop paradox.

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