Superluminal Motion:
Fact or Fiction?
SUPERLUMINAL OBJECTS AND PARADOXES OF RELATIVITY THOU6HT EXPERIMENTS AND EXAMPLES
Since realizing the possibilities of superluminal travel. Scientists have been developing theories and thought experiments for the testing of faster-than-light phenomena. One such thought experiment concerns a pair of scissors. As the blades on these scissors are closed, the notch where the blades meet accelerates toward the blade tips. Consider now a pair of scissors with extremely long blades. Asthe blades close, they grow more and more parallel to each other and the notch velocity increases without limit, theoretically able to surpass the speed of light.
Now consider going to the matinee and seeing the marquee lights flashing and blinkg in sequence. Unless the lights are causally connected, that is unless light one triggers light two, light two triggers light three, and so forth, then the lights can be
Other phenomena also theoretically involve faster-than-light motion. These include the end of a search light beam, a comet's tail, the shadow of a planet, and an oscilloscope trace beam. Asthe massless end of the searchlight beam sweeps across the sky, the tip can theoretically move faster than light. As the comet head sweeps around the sun, the long tail sweeps out a larger orbit moving at superluminal velocities.Ashadow is like a negative search light beam. If a planet orbiting close to the sun, casts a shadow on a distant planet, the shadow flies across with superluminal speed. Closer to home, there is the Techtronix 7104 oscilloscope. The electron trace beam sweeps across the screen at a measured.be.If the display tube's length were doubled, it would have an effective writing speed of 1.2c.
POSSIBLE EVIDENCE
One last example was discovered around the turn of the century by pioneer radio engineers. They learned that the radio signals in the upper atmosphere undeniably traveled faster than light. The reason was that the radio waves were moving through ionized gas and not through normal air. These radio waves pulses have two different velocities, a group velocity, or the velocity of the pulse packet, and a phase velocity, the velocity of the individual waves within the group. It was the phase velocity of the radio waves that was moving faster than light, not the more physical group velocity. The above examples are all phenomena involving faster-than-light motion. The motion in the examples, however, is either not physically realizable, or is not causal in nature thereby not violating the principle of causality.
TACHYONS
The idea of faster-than-light particles was first conceived around the turn of the century by professor Arnold Sommerfeld in Munich, Germany. Gerald Feinberg first named the particles tachyons from the Greek tachys, meaning swift. Tachyons are still theoretical particles whose existence has not been verified.
PHYSICAL CHARACTERISTICS
Physical characteristics of the tachyon are highly unique. All tachyons move faster than light. As they lose energy, they speed up. As they gain energy, they slow down, with c as the lower limit on their velocity. This is mathematically acceptable, but very difficult to measure because tachyons have an imaginary mass. Tachyons are also difficult to measure because they violate time-order. In other words, two different people in two different places would see one tachyon do two different things, if they could see the tachyon at all. For example, if one person sees a tachyon coming out of a gun, the other person sees the tachyon going into the gun.
CREATION AND DETECTION
Since no particle can be accelerated through the light barrier, how can tachyons be created? According to quantum theory it is possible through particle collisions. When two particles collide, their kinetic energy is converted to the mass of a new particle. Any particle can, in theory, be made through a series of controlled
collisions. To make a tachyon, one need only concentrate enough energy in one place and time to create a particle moving, from the point of conception, faster than light.
Scientists have tried to detect tachyons which might be produced by cosmic ray collisions in the atmosphere. Cosmic rays are highly energetic particles that continually bombard the planet. They hit the upper atmosphere traveling at the speed of light. They react with particles there, creating a group of new particles moving slightly slower than c.These secondary particles interact with the air to create even more particles turning into a veritable "air shower." In 1973, using a large array of particle detectors in Australia, Roger Clay and Philip Crough successfully identified a faster-than-light precursor particle to the rest of the air shower. Unfortunately this experiment has never been successfully repeated, nor have other experiments detected similar faster-than-light particles.
DEFENSE OF TACHYON THEORY
According to a well-known supporter of tachyons, George Sadarshan of the University of Texas, there are three reasons to continue the search for tachyons. Sadarshan's first claim is that tachyons could answer a number of questions about particle physics, astrophysics, and cosmology.
His second claim is that there is no reason to believe that tachyons are not out there. Sadarshan says that the math and the theory are consistent and therefore experimental verification is possible. He cites a precedent: British physicist Paul Dirac accurately predicted how electrons should move. His equations though, pointed to the existence of positively charged, electron-like particles. Many scientists were quick to condemn this possibility, but in 1932 Carl Anderson discovered the positron, confirming Dirac's theory. Sadorshan believes tachyons, likewise, will someday be verified.
Finally, Sadarshan points out that tachyons continuously appear in the math and theories that physicists use every day. For example, in the early 1970's a theory known as string theory was abandoned by all but a handful of scientists. Those few scientists have subsequently proven string theory to be the most promising path to linking the four fundamental forces of nature: nuclear strong, electromagnetic, nuclear weak, and gravitational. String theory is now widely accepted, but tachyons repeatedly arise in this theory. Scientists react to this by hacking away at the theory until all references to tachyons are removed. Some believe that the theory would have more validity if the tachyons were included.
Although mathematics and theory point to the existence of faster-than-light particles, experimental evidence is inconclusive. Physicists generally consider these tachyons to be a purely mathematical result which is a physical impossibility.
QUANTUM MECHANICAL EVIDENCE FOR SUPERLUMINAL VELOCITIES
Where better to look for evidence of superluminal travel than in another area of science which doesn't sit well with intuitive reasoning? The world of quantum mechanics, with all of its oddities, offers theoretical and experimental verification of many of nature's laws, and superluminal velocities of quantum particles are no exception.
Though much in the field of Quantum Electrodynamics is based on the assertion that no information can travel faster than light. A simple model of quantum mechanics suggests the possibility of particles which can travel fast than light, albeit over a finite distance.
QUANTUM PARTICLES AND TUNNELING
Imagine a tennis ball being thrown at a brick wall. Upon impact, the ball will rebound and travel back in the opposite direction. This ball is exhibiting particulate properties, that is, it is behaving like a solid particle. Now imagine a rope tied to the same wall. If a wave travels down the rope toward the wall, most of it will be reflected at the wall, just as the tennis ball was. A portion of the wave, however, will be transmitted, importing its energy to the wall and setting up a vibration. There is always a portion of the wave transmitted, unless the wall is of infinite mass.
A quantum particle incident upon a barrier is neither like the tennis ball nor the wave in the rope. Rather, it exhibits properties of both particulate matter and waves. The quantum particle can be thought of as a wave packet, its width in space related to its velocity through the famous Heisenberg Uncertainty relation. A common interpretation of this wave packet is that it represents a probability distribution. That means that where the amplitude of the wave packet is the greatest corresponds to the position with the highest probability of finding, or measuring, the particle.
When the quantum wave packet is incident upon a barrier, or a region where a particle would be classically forbidden, it is partially reflected and partially transmitted. If the barrier is of finite width, the transmitted portion will leave the other side and resume its motion as a smaller, slightly distorted wave packet. Since this-packet is a portion of the original probability distribution, there is a small but finite probability of measuring the location of the quantum particle on the fa r side of the barrier. This phenomenon is referred to as quantum barrier penetration, or tunneling.