dietz%USC-CSE@ECLA.ECLnet (01/22/84)
I just read an interesting paper in the Journal of the British Interplanetary Society (JBIS, 1983, pages 507-508) on the production of antimatter (for use in interstellar propulsion systems) using concentrated laser beams. The idea is to focus enough light into a small volume so that the electric field becomes strong enough to create particle/antiparticle pairs out of the vacuum. The field necessary for electron/positron production is about 2.4x10^18 volts/meter. A light pulse with an energy of 2 megajoules lasting 3 femtoseconds focused on a volume .2 microns across does the trick. (Light waves with a wavelength of .2 microns oscillate 4.5 times in 3 femtoseconds.) The energy delivered to the interaction area should be converted to electrons/positrons with high efficiency (> 90% if helium nuclei are present to separate the particles). The intensity and energy density of the pulse are truly impressive: over 10^34 watts/m^2 and 5x10^25 joules/m^3 (or, a matter density of 6x10^5 grams/cm^3). That high energy density suggests that it shouldn't be too hard to get a very narrow laser pulse in which the energy density approaches that of normal matter. To reach a density of 1 gr/cm^3, a light pulse 1 cm long and .2 microns across must have an energy of (4x10^-13 kg) x (3x10^8 m/sec)^2 = 36 kilojoules. The Livermore SHIVA laser produces 10 kilojoule pulses, with a pulse length of 100 picoseconds, or about 3 cm (they are much wider than .2 microns, though). That much light should change the refractive index of the vacuum, leading to self focusing. If the energy density of the pulse decreases from the front to the rear the refractive index would decrease going back along the pulse, so the photons in the back would move faster than those in the front, causing the pulse to shorten. This optical soliton would not disperse with distance -- a real "photon torpedo". How would one create such a pulse? You'd need a laser cavity that's very narrow, and you'd have to pump lots of energy into it. A laser cavity 1 meter long and .2 microns across has a volume of about 4x10^-8 cm^3, or at most about 4x10^-7 grams of lasing material. So, on the order of 10^11 joules of energy per gram of lasing material would be needed. That's enough to accelerate the matter to 5% of the speed of light, if it was converted into kinetic energy. More energy would be needed to overcome laser inefficiencies. Most of the outer electrons will be stripped away at these energies, so lasing will probably occur in the far UV or X-ray region. This is beginning to sound like the rumors about nuclear pumped X-ray lasers. Could those beams be self-focusing? Perhaps that's why Teller is so up on the idea.