A D V E N T U R E S in C Y B E R S O U N DInvestigations with Light : 1704 - 1887Prior to the mid 17th century, light was presumed to move instantaneously across any distance. Astronomers at the French Academy showed (by comparing the times they expected to see the moons of Jupiter appear from behind that planet with the times they actually observed (about a 15 minute difference)) that light does travel with a definite speed (their calculations were actually fairly accurate, @ 150,000 miles per second). That finding, generated new interest in the "physics" of light, what is it, how does it propagate, etc. The dominant theory through the end of the 17th century was the undulatory (wave) theory of light. Newton's Opticks of 1704 persuaded almost everybody that light is a stream of corpuscles (particles) and he established the corpuscular theory of light. That held until the early 19th century when the French (in trying to save the corpuscular theory) established a wave theory of light. That lasted until the early 20th century when quantum mechanics told us light can be either a wave or a particle depending on what experiments we do. All that is confusing, but here's an easy mnemonic: In the 17th century light was a wave: in the 18th, it became a particle; in the 19th, it turned back into a wave; and finally, in the 20th we've compromised. Under quantum theory and the Uncertainty Principle, we describe light as either a wave or a particle depending on the experiments we choose to make.
1704 Newton's Opticks gains immediate acceptance (in contrast to his Principia) and dominates 18th century discussions of light. Very little novel experimentation; virtually no efforts to challenge Newton's corpuscular theory (at least, none that I know of). When the French accepted newtonianism, they became the great champions of his corpuscular theory of light as well as the theory of universal gravitation and celestial dynamics. 1800 Thomas Young (1773-1829) an English physician who had studied medicine in Germany, where he came under the influence of vitalistic nature philosophy, sets out to challenge the Newtonian corpuscular theory. Young offered a new wave theory of light based on analogies to the propagation of sound waves (he'd done his MD dissertation, had to do them in those days, but MD's don't anymore, on the physiology of hearing). He proposed a theory based on longitudinal waves, the kind of waves a slinky will do if you hold the two ends and let it "slosh" back and forth. A good physical analogy can be had in imagining the "back and forth wave motion" of rush hour traffic on an expressway if you watch from a helicopter. As the cars move along, packs will develop and then thin out, develop and thin out, etc. The physical phenomenon driving Young's theory was interference. That's what will happen if you put up a baffle, shine a strong light on it, and let the light shine through two pinholes. In order to understand this, however, you have to know what happens if you let a light shine through just one pinhole. On the screen behind the baffle you'll see a circle of light: 1805 The French struck back right away. Etienne Louis Malus (1775-1812) discovered the phenomenon of polarization. If you use a polarizing filter, it cuts down glare (as with polarized sunglasses) and gives the light a "grain." You can tell there is a "grain" because if you place a second polarizing filter behind the first, but turned at 90o no light passes the two filters. It seemed to work in a way that called venetian blinds to mind: Young had established interference as the phenomenon that any theory of light had to explain. Corpuscles won't create interference patterns. But Young's longitudinal waves should not show the effects of polarization. The push-pull waves should move through "holes" as easily as they move through "slats." If anything, the polarizing filters would cause longitudinal waves to scatter, but not to react as if they have "grain." c.1820 Auguste Jean Fresnel (1788-1827), another French scientist, comes to the rescue of Young's basic "wave" theory by postulating transverse waves to replace the older longitudinal waves. Instead of moving in the push-pull, slinky-like patterns, transverse waves are more like the vibrations of a guitar string, they move in all directions (-|-) but never in a push pull. With these waves, the single polarizing filter would eliminate all but one axis of motion (---). The second filter would remove that axis (). Fresnel's (pronounced Fre'nell) transverse waves would explain both interference patterns (Young's phenomenon) and polarization (Malus's phenomenon), but they created their own new problem: the need for a luminiferous ether. Waves won't propagate in a vacuum, they have to vibrate something. Longitudinal waves will propagate in a fluid, and Young's waves were, thus, not much of a problem. You just need to postulate that there is a very subtle fluid pervading all space. Transverse waves, however, are a real problem. They won't propagate in a fluid--only in rigid, actually semi-rigid, bodies. You can't make a guitar string "twang" unless you stretch it tight. For the remainder of the 19th century, postulating how light can propagate in a rigid or semi-rigid (jello/jelly) luminiferous ether became the major preoccupation for those studying light. Eventually, physicists settled on a kind of jello-like subtle ether that allowed all solids to pass. This problem of how light propagates, moves, through the ether is, of course, the problem that started Albert Einstein thinking about the movement of light. That problem is the one that led him first to the Special Theory of Relativity (1905) and then to the General Theory of Relativity (1915). But, before we get there, we have to look at James Clerk Maxwell again as well as the most famous experiment, or set of experiments, in the history of science. That, of course, is the Michelson-Morley experiment trying to measure the ether wind. 1873 After dealing with Faraday's lines of force in the 1850s James Clerk Maxwell (1831-1879) moved on to other related matters. Actually, he did lots of things, and it is worthwhile mentioning a few of his activities as a theoretical physicist:
a. based on newtonian calculations, theorized that the rings of saturn most be composed of particles of matter floating in space, denied the contemporary theory of astronomers that they were solid ringsThe work we are really concerned about with the history of light, however, is Maxwell's 1873 Treatise on Electricity and Magnetism. This is the work in which he provided a comprehensive scheme for explaining electromagnetic radiation, the behavior of fields of force. The most important point here is found in the assumption of light as an electromagnetic wave, and the statement that it is only one form of electromagnetic radiation, its form determined by the wavelength of the radiation. Following Maxwell's analysis, we can postulate a spectrum of electromagnetic radiations:
very short wavelength / high energy1880s Culminating in 1887 at the Case Institute of Technology in Cleveland, Albert Michelson (1852-1931) carried out a series of experiments to measure the ether wind, or ether drag. (Michelson started at the U.S. Naval Academy, went to Berlin, finally to Case where Morley joined his effort). Michelson invented the interferometer to discover the effects of ether drag on the speed of light. Basically, the problem is this: The ether was assumed to be stationary in space, co-terminus with the fixed stars. The earth was thought to move through the ether both as it travels around the sun and as it spins on its axis. Thus, it moves through the stationary ether. Nominally, there should then be an ether "wind" blowing over the surface of the earth at all times. Now, if light moves as a wave through the ether, it should move at different speeds, or appear to move at different speeds, as it goes "with" the wind, "against" the wind, or "across" the wind. Think of swimmers racing 50 yards "with," "against," or "across" the current of a river. The interferometer was a device for measuring these effects, by racing lightbeams "with," "against," and "across" the ether as it streams over the surface of the earth. The interferometer combines "races" across the ether wind with each other and races with and against the wind with each other. In such races, the swimmers moving back and forth across the current have the advantage. Their times should be better. The same for the light beams. The ones moving across the ether wind should return faster. With the light source on and the shutter rotating (basically it's a fan) pulses, or blips, of light travel up the source arm to the angled mirror in the middle of the interferometer. The center mirror is half-silvered, i.e., it lets half the light go on in a straight line and sends the other half the light down the arm to the right. The mirrors on the three destination arms of the interferometer are set so they can bounce the "blips" back and forth several times before they are reconverged by striking the half-silvered mirror once again. If the light beams require different times to complete their bouncing back and forth on the crossed arms, they will be out of phase when they reconverge at the half-silvered mirror. If so, when that mirror sends them back down the source arm, there will be no light, remember interference (if the waves are out of synch, they cancel each other out.) The operator simply watches the half-silvered mirror by looking over the top of the light source and shutter. If the "blips" going through the shutter are out of synch coming back, the operator won't see them. If they're in synch, taking the same time to make the trip up and down or across, they'll be visible. Every time Michelson or Michelson and Morley did this experiment, it failed! They never detected any ether drag! By all calculations, the instrument was sensitive enough for the job. The Case model was mounted on a block of sandstone floating in a pool of mercury that acted as a shock absorber. They had to be able to rotate it as they watched for blips in order to be able to see if different orientations to the ether wind had any effect. In fact, the way they calibrated it was simply to get the blips coming back strong by adjusting the mirrors to set their distances. Then, as they rotated in any direction, the blips should have gotten weaker, and then disappeared, it never happened. The failure of the Michelson / Morley experiment is its significance. There were many attempts to explain the failure. The most popular were put forth by an Irish physicist, George Fitzgerald, an English physicist, Joseph Larmor, and a Dutch physicist, Hendrik Lorentz. All said basically the same thing and the theory became known as the Lorentz-Fitzgerald Contraction. Under this theory, the arms carrying the beams back and forth, with and against, the wind contracted just enough to shorten that race course to allow those light beams to finish the race at exactly the same time as those that moved across the ether wind. It sounds silly when stated so simply, but a lot of sophisticated theory went into the Lorentz-Fitzgerald contraction. These are the equations that you sometimes hear people say express the theory of Special Relativity in a different mathematical form.
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