Tachyons Robert Ehrlich

Ask most physicists about tachyons, however, and you'll be told that they belong strictly in the realm of science fiction. That skepticism is understandable, since nearly all experiments searching for tachyons have so far turned up negative. Even worse, according to some physicists, if tachyons exist, they could be used to send messages back in time. Nevertheless, over the years a few physicists have held out hope that tachyons might actually exist --possibly disguised as some other known particle.

In fact, in 1985 Alan Chodos, Avi Hauser, and Alan Kostelecky suggested that the elusive neutrino might actually be a tachyon. Over the years, neutrinos, which are produced in nuclear reactions such as those inside the sun, have been full of surprises. Just last year, for example, it was shown that one or more of the three types of neutrinos has a small but nonzero mass. If neutrinos really are tachyons, this would be their biggest surprise of all.

The direct approach to test this idea would be to measure the speed of neutrinos and see if it exceeds that of light. Such direct tests have proven inconclusive based on measurements of neutrinos reaching Earth in 1987 from a supernova in a nearby galaxy. The problem is that if neutrinos do have a speed in excess of light, the excess is miniscule -- perhaps only 0.00006 miles per hour. But, indirect tests of the hypothesis may be feasible. For example, one implication of neutrinos being tachyons is that normally stable particles such as the proton should disintegrate if traveling with sufficiently high speed or energy. The proton would disintegrate into three particles: neutron, positron (antiparticle of the electron) and neutrino. No such proton disintegrations have ever been seen at the highest energies available in particle accelerators.

But, in 1993 Alan Kostelecky wondered what about nature's own particle accelerators that produce the cosmic rays, some of which have been seen with 100 million times greater energy than man has produced. The number of cosmic ray particles falling in each energy interval is referred to as the cosmic ray spectrum. A well-known kink (known as the "knee") in the spectrum occurs at an energy of about four million billion electronvolts. Could the knee be explained by the onset of proton decay at that energy Kostelecky wondered? Ehrlich has recently extended Kostelecky's idea to explain a number of specific features of the cosmic ray spectrum besides the knee. Of course, the good agreement between Ehrlich's model and the observed spectrum is far from conclusive proof that neutrinos really are tachyons, since more conventional explanations might also explain the cosmic ray spectrum.

 But, Ehrlich's model does make one very specific prediction that could settle the issue: there should be a large number of neutrons in the cosmic rays in a very narrow energy interval (a "neutron spike"), which lies just above the knee. If neutrons should be observed in this narrow "window," no other model could explain their presence, because unstable neutrons, which live for only about 10 minutes before disintegrating, shouldn't be able to reach  Earth from distant cosmic ray sources, unless the neutron energies and speeds are very much higher. (At high energies or velocities approaching the speed of light Relativity theory extends the lifetime of unstable particles like neutrons.) Cosmic ray data from the 1970's and 1980's looking at the astronomical source Cygnus X-3 (an X-ray binary star) does appear to show exactly the neutron spike feature that Ehrlich's model predicts.

 One fly in the ointment, however, is that more recent and sensitive experiments fail to show any such feature. Either the earlier experiments were in error, or perhaps the source faded with time. Ehrlich believes that the latter is true, and that a neutron spike should still be detectible in the cosmic rays: just look at the arrival directions of those cosmic rays which fall in the narrow energy window, and see if they cluster about particular points in the sky. If so, we will have strong evidence that  neutrinos are tachyons. Data currently exists to check out this exciting possibility.

 If tachyons really do exist, don't expect that we will be building "warp drives" for space ships that go faster than the speed of light. Relativity still forbids any object initially going at sub-light speeds from breaking the "light barrier." Tachyons can manage to go faster than light only because they always travel at "superluminal" speeds from the very moment of their creation in subatomic collisions or disintegrations. Faster-than-light neutrinos are also unlikely to have any practical value in terms of communications, because they exceed light speed by only a very tiny amount. And what about that spooky business of sending messages backward in time? Ehrlich doesn't believe that would be possible even if tachyons really do exist. "Nature would have found some way to eliminate such absurd possibilities, and all the associated paradoxes," he notes.