Making waves that travel like beams.

Making waves that travel like beams

"Fire the photon torpedoes," Captain Kirk commands with cool leadership. Helmsman Sulu responds, and two twinkling blobs of light shoot out from the Enterprise. On their journey toward a Klingon warship, the blobs hold their shape. They do not spread out. The energy they contain does not dissipate.

This is science fiction. Evidence is mounting, though, that th ings like photon torpedoes might be creeping toward reality.

Three scientists report in the Jan. 9 PHYSICAL REVIEW LETTERS that they can create complex ultrasonic pulses that do not spread, or diffract, when traveling short distances in water. The researchers call these unusual waves "accoustic directed-energy pulse trains," or ADEPTS. Other researchers have worked on related phenomena with names like electromagnetic directed-energy pulse trains and electromagnetic missiles.

Within the mathematical expressions describing the properties of propagating waves are hints that specially constructed waves -- ADEPTs, for example -- might be good for the "localized, slowly decaying transmission of energy in space-time,c the researchers say. Such waves would not spread out, according to Richard W. Ziolkowski, a physicist at the Lawrence Livermore (Calif.) NAtional Laboratory and one of the authors of the report. Livermore colleague D. Kent Lewis and Bill D. Cook (both mechanical engineers) of the University of Houston are coauthors.

The physical existence of such waves could make for a host of exotic applications, Ziolkowski told SCIENCE NEWS. For example, a satellite might gather solar energy and then send it in pencil-like beams to small receivers at or near the Earth's surface. Point-to-point radio communication could be as private and tap-obvious as are the conversation of two children talking via string-joined paper cups. Energy-bearing beams that don't dissipate even after traveling thousands of miles would be of great interest to the Department of Defense. Its Innovative Science and Technology Office -- part of the Strategic Defense Initiative Organization -- has been supporting research in this area, including earlier theoretical studies by Ziolkowski.

As normal waves propagate, they spread out according to the laws of diffractions. Millions of widely separated listeners can hear the same radio program because the electromagnetic signals carrying the show spread out in a growing spherical wavefront starting at the broadcast tower. The same holds for almost any type of electromagnetic or acoustic wave that people might observe as, say, visible light, X-rays or sound.

But what's gained in transmission coverage is lost in power. The energy traveling within a typical wave distributes over increasingly larger areas and becomes less intense. That's why AM and FM radio shows fade as you get farther from their origins. It's also why baseball fans in the balcony have to scream louder than fans in box seats if they want the players to hear them. Even needle-thin laser beams get wider and less energy-packed as they travel. Not so for ADEPTs.

Unlike typical acoustic or electromagnetic waves that radiate from, say, vibrating vocal cords or oscillating electrons in an antenna, ADEPTs emerge from a complex interplay of acoustic waves made by an underwater "array" of electronically driven crystal that vibrate at ultrasonic frequencies. The researchers drive each element of the array so that it creates a somewhat different pulse, but these sum into a pencil-like ultrasonic beam, Ziolkoski says. Since ADEPTs remain confined to small regions, the energy they contain also remains concentrated.

So far, the researchers have launched ADEPTs that travel only a few feet before they die out. The challenge now is to build arras that can launch them much farther, Ziolkoski says.