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THE SCIENCES What is known about tachyons, theoretical particles that travel faster than light and move backward in time? Is there scientific reason to think they really exist? October 21, 1999 Raymond Y. Chiao is professor of physics at the University of California, Berkeley. He replies: "Briefly, tachyons are theoretically postulated particles that travel faster than light and have 'imaginary' masses. Editor's note: imaginary mass is a bizarre theoretical concept that comes from taking the square root of a negative number; in this case, it roughly means that a particle's mass is only physically meaningful at speeds greater than light.] "The name 'tachyon' (from the Greek 'tachys,' meaning swift) was coined by the late Gerald Feinberg of Columbia University. Tachyons have never been found in experiments as real particles traveling through the vacuum, but we predict theoretically that tachyon-like objects exist as faster-than-light 'quasiparticles' moving through laser-like media. (That is, they exist as particle-like excitations, similar to other quasiparticles called phonons and polaritons that are found in solids. 'Laser-like media' is a technical term referring to those media that have inverted atomic populations, the conditions prevailing inside a laser.) ADVERTISEMENT "We are beginning an experiment at Berkeley to detect tachyon-like quasiparticles. There are strong scientific reasons to believe that such quasiparticles really exist, because Maxwell's equations, when coupled to inverted atomic media, lead inexorably to tachyon-like solutions. "Quantum optical effects can produce a different kind of 'faster than light' effect (see "Faster than light?" by R. Y. Chiao, P. G. Kwiat, and A. M. Steinberg in Scientific American, August 1993). There are actually two different kinds of 'faster-than-light' effects that we have found in quantum optics experiments. (The tachyon-like quasiparticle in inverted media described above is yet a third kind of faster-than-light effect.) "First, we have discovered that photons which tunnel through a quantum barrier can apparently travel faster than light (see "Measurement of the Single-Photon Tunneling Time" by A. M. Steinberg, P. G. Kwiat, and R. Y. Chiao, Physical Review Letters, Vol. 71, page 708; 1993). Because of the uncertainty principle, the photon has a small but very real chance of appearing suddenly on the far side of the barrier, through a quantum effect (the 'tunnel effect') which would seem impossible according to classical physics. The tunnel effect is so fast that it seems to occur faster than light. "Second, we have found an effect related to the famous Einstein-Podolsky-Rosen phenomenon, in which two distantly separated photons can apparently influence one anothers' behaviors at two distantly separated detectors (see "High-Visibility Interference in a Bell-Inequality Experiment for Energy and Time," by P. G. Kwiat, A. M. Steinberg, and R. Y. Chiao, Physical Review A, Vol. 47, page R2472; 1993). This effect was first predicted theoretically by Prof. J. D. Franson of Johns Hopkins University. We have found experimentally that twin photons emitted from a common source (a down-conversion crystal) behave in a correlated fashion when they arrive at two distant interferometers. This phenomenon can be described as a 'faster-than-light influence' of one photon upon its twin. Because of the intrinsic randomness of quantum phenomena, however, one cannot control whether a given photon tunnels or not, nor can one control whether a given photon is transmitted or not at the final beam splitter. Hence it is impossible to send true signals in faster-than-light communications. "I refer interested readers to our paper 'Tachyonlike Excitations in Inverted Two-Level Media' by R. Y. Chiao, A. E. Kozhekin, and G. Kurizki, Physical Review Letters, Vol. 77, page 1254; 1996, and references therein.