Usually a lighthouse has a very narrow beam of rotating light; the narrower the beam, the higher the energy within it and the narrower the flash time compared to the entire cycle of the beacon. The lighthouse that Alexander Kaplan of Johns Hopkins University (Baltimore, MD) and Peter Shkolnikov of the State University of New York at Stony Brook have conceptualized is a relativistic one. The cycling beacon is actually an electron orbiting at almost the speed of light in a tight circle of 3000 Å in diameter.1 An electron passing through any point in that orbit would appear to radiate out of the orbit along a tangential line in a very narrow cone. An observer glancing at the orbit along that tangential line would see that radiation as a very short flash, like the beacon from a relativistic lighthouse.
Rather than guide submicroscopic ships into submicroscopic harbors, however, Kaplan and Shkolnikov want to make relativistic flash bulbs so they can take stop action photographs of fission and fusion processes in atomic nuclei. Viewing such processes would require flashes so short they would be measured in zeptoseconds (10-21 s). And the theorists propose creating them by using a "lasetron," which would use circularly polarized light from petawatt lasers to induce relativistically short blasts of synchrotron-like radiation.
"People know of synchrotrons, in which they run a relativistic electron beam around a huge circle," Kaplan said. "Remember the supercollider? They wanted to have a 60-mile radius. The electron goes around for many seconds, and you need a huge magnetic field. In our case, the magnetic field is replaced by the electric field of the laser beam. The frequency is on the order of 1015 Hz, so the ratio [between the lasetron and synchrotron] is 16 orders of magnitude. It's a huge difference."
The proposed lasetron would actually conduct the electrons through a thin wire oriented perpendicular to the incident laser beam. Similar to an antenna, the wire would emit zeptosecond flashes twice in each laser cycle and perpendicular to both the laser beam and the wire (see figure). Converting the theory into a practical working device may present some difficulties, however. "The pulses are great," Kaplan said. "They look very sexy and sound very sexy, but nobody knows how or when they can be observed." Current technology doesn't allow measurement of pulses on the order of 0.1 fs, much less five orders of magnitude smaller, he explained.
An answer to this problem may lie in the huge magnetic fields that the lasetron is expected to create. "What we have is a current moving in a circle," Kaplan said. "We have a coil. And what would you expect from a coil with a current running through it? There's a magnetic field inside. We looked at it on paper and in ten minutes we got a result and didn't believe it. We got a magnetic field that no one has ever been able to obtain in a laboratory. It is about one million Tesla."
A magnetic field that large, existing for femtoseconds at a time, would scatter a beam of neutrons, thereby providing an indirect indicator of the zeptosecond light flashes. The availability in the laboratory of such a large magnetic field also presents exciting prospects for basic research.
"It's a huge magnetic field that does not exist on Earth," Kaplan said. "It doesn't even exist on the Sun. On an astrophysical scale, it can be seen in the vicinity of very highly compressed white-dwarf stars. Whenever we can create a condition in the laboratory that only exists in space, the astrophysical people are extremely happy. This is always the beginning of new work."
REFERENCE
1. A. E. Kaplan, P. L. Shkolnikov, Phy. Rev. Lett. 88(7) 4801 (FEB. 18, 2002).