I want your inputs to my document History of Light. Hermann Bondi, in his book Relativity and Common Sense describes light in this way: "Light is ageless. There is no passage of time for light." It seems ironic to have a title History of Light if light is ageless. Actually the document is about the people who made discoveries about light, and their discoveries. Please give references (book titles or URLs). I especially want the recent discoveries. Here is what I have now:

History of Light

Early philosophers thought that light originated in the eyes, reaching out like the beam of a lighthouse to 'feel' the nature of the world. Empedocles (5^th century BC) realized that the darkness of night is caused by the earth getting in the way of the light from the sun. Epicurus (3^rd century BC) had similar ideas. His ideas were summarized by Roman author Lucretius who wrote On the Nature of the Universe in 55 BC. He said "The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." This is a remarkably accurate description for the time, but is not what most people thought at that time.

Abu Ali al-Hassan ibn al-Haytham (known as Alhazen) was the greatest scientist of the middle ages. His book Opticae Thesaurus (The Treasury of Optics) was only published in Europe (still in Latin) in 1572. He used several arguments that sight is a result of light entering the eye from the outside world. One of these was the phenomenon of after-images. He talked about the way images are formed in 'camera obscura', which means a darkened chamber or room. He realized that light travels in straight lines. He thought of light as being made up of a stream of tiny particles, which bounced off objects that they struck. He realized that light rays bending in water is a result of the light traveling at different speeds in water and in air.

Johannes Kepler (1571-1630) described the human eye in the same term as a pinhole camera, with light entering through the pupil, and forming an image on the back of the eye. Rene Descartes (1596-1650) proved that the image is upside down by taking an eye from a dead ox, scraping the back to make it transparent, and looking at the image on the retina.

Galileo Galilei (1564-1642), hearing about the invention of the telescope, made his own version and discovered the four largest moons of Jupiter in 1610. The Danish astronomer Ole Romer observed that the timing of the eclipses of the moons seemed to be affected by whether the Earth was on the same side of the sun as Jupiter was, or on the opposite side. He explained the differences in the eclipse timings as due to the extra time for the light from the moons to reach Earth from the farther distance.

Christian Huygens (1629- ) had a fully worked out wave theory of light in 1778. It was published in 1890. His theory made a significant prediction: light should travel more slowly in a denser medium (such as water) than in a less dense medium (such as air). This explained the way light was refracted (bent) when it went through water.

Isaac Newton (1642-1727) though, was stuck on the corpuscles (particle) theory. He had a theory for why a glass prism turns white light into a rainbow of colors. He found that a second prism could put all the colors back together into white light again, but did not tell anybody right away. He used this knowledge to build a telescope with a curved mirror so that it would not suffer from chromatic abberation. His first physics paper was published in 1672 on his theory of colors. Robert Hooke (1635-1703) was the Curator of Experiments at the Royal Society. He responded to Newton's paper in patronizing terms, causing Newton to keep his complete theory of optics to himself until Hooke died. Newton is credited with the 'Scientific Method', which means that a theory is wrong if it doesn't match up with experiments.

Francesco Grimaldi (1618-1663) studied the behavior of sunlight let into a dark room through a small hole. He found that the spot of light was larger that the hole, and had colored fringes. He also found out that a small obstruction would have a shadow with colored edges, where light had leaked into the fringes of the shadow. It had spread in to the shadow, and different colors had spread in by different amounts. Grimaldi gave this phenomenon the name diffraction. His work was published two years after his death, and he was not around to argue with Newton. Hooke also saw that light leaks into a shadow.

The Swiss mathematician Leonhard Euler (1707-) supported the wave theory of light. His theory was published in 1746. He pointed out all problems of light traveling as a stream of particles (including diffraction), and made the analogy between vibration of light and vibration of sound waves.

Thomas Young (1773-1829) carried out experiments on light interference. He explained some of Newton's own experiments in terms of the wave theory. He calculated the wavelength of Red and Violet light to good accuracy. Young failed to convince his contemporaries.

Augustin Fresnel (1788-1827) qualified as an engineer in 1809. Optics was a hobby. In 1817 the French Academy of Sciences offered a prize to anyone who could provide the best experimental study of the phenomenon of diffraction, and supply a theory that satisfactorily explained the experiment. The judges included the mathematician Simeon Poisson, the physicist Jean Biot, and the astronomer Pierre Laplace, all strong supporters of the Newtonian theory. Fresnel had calculus equations that he had trouble solving completely. Poisson jumped on one of Fresnel's examples, solved the relevant equations, and presented them, thinking that they would disprove Fresnel's theory. Poisson's calculation showed that there should be a tiny bright spot directly behind a small round object in the center of the shadow. The experiment was carried out, and the spot was there. Fresnel had won. Fresnel is noted for his development of a lens with concentric rings, which was named after him.

Michael Faraday (1791-1867) joined the City Philosophical Society at the age of 19, where he heard lectures on scientific subjects. He was hired by the Royal Institution (London?), at 21. In 1831 he proved that you could generate electricity with a moving magnet. Faraday's greatest contribution to science was his work on electricity and magnetism. He is most noted for his concept of the field of force, which he described as 'lines of force.' With his theory, you didn't need the 'aether.'

James Clerk Maxwell (1831-1879) was the greatest theoretical physicist between Newton and Einstein. He became professor of Natural Philosophy at Marischal College in Aberdeen in 1856, and the first Professor of Experimental Physics at Cambridge University in 1874. He invented color photography using red, green & blue filters with black & white film. His greatest work was four equations that summed up everything there was to say about electricity and magnetism. In a paper he wrote "...light consists of the transverse undulations of the same medium which is the cause of electric and magnetic phenomena."

History of Light

The Frenchman Jean Foucault (1819-1868), who is known for his famous pendulum, measured the speed of light in water and air with a rotating mirror, and showing that it moves more slowly in water and travels as a wave. Albert Michelson (1852-1931) refined the method and made many accurate measurements in air. He won the Nobel Prize in 1907. He is known for trying to measure the motion of the earth relative to the aether with a light interferometer of his own design along with Edward Morley (1838-1923). Their experiments left no doubt that the speed of light is the same regardless of how the earth is moving. The Dutch physicist Hendrik Lorentz showed that the earth could not drag the aether with it, so the aether theory was dead.

The Irish physicist George Fitzgerald (1851-) correctly predicted the way in which an oscillating electric current should produce what we now know as radio waves. In 1889 he offered an explanation of the results of the Michelson-Morley experiment. The same idea was put forth by Lorentz in the 1890s, and later he developed a set of equations known as the Lorentz transformations that describe how the length and other properties 'transform' when viewed by observers moving at different velocities.

Albert Einstein (1879-1955) showed that the same transformation equations apply to mechanical systems, and published his Special Theory of Relativity in 1905. Basically it says that an object moving close to the speed of light would get shorter in the direction of motion and have more mass, relative to an observer, and that nothing (with mass ?) can travel faster than light. His famous equation E = mc² is part of that theory.

The German physicist Max Plank (1858-) made a heroic effort to explain the way electromagnetic radiation (including light) is radiated by hot objects. After several trips up blind alleys, Plank realized that the nature of black body radiation could be explained if hot objects emit (or absorb) energy in packets of a definite size, which he called quanta. Each packet of waves has an energy, which depends on its frequency. The energy is equal to the frequency times a number now known as Plank's constant. He announced the discovery in December 1900, and was awarded the Nobel Prize in 1918. Einstein explained, in a paper published in 1905, the way in which electrons are knocked out of a metal surface by light (the photoelectric effect) as due to the impact of particles of light (quanta) with the electrons in the metal. He received the Nobel Prize for this in 1921.

Robert Millikan (1868-1953) thought that the suggestion that light was made of particles was nonsense, and set out to prove Einstein wrong. By 1915 his experiments forced him to admit Einstein was right. Along the way he got the first accurate measurement of Plank's constant. He also measured the charge of the electron with great precision. He received his Nobel Prize in 1923. In 1926 the physicist Gilbert Lewis named the light quanta 'photons' from the Greek word for light, (photos).

The Indian Physicist Satyendra Nath Bose (1894-1974) paved the way for quantum mechanics and a theory of light and matter. He tied all the pieces together with a mathematical description of light quanta. Bose found that he could derive Plank's formula by treating photons as particles, which have to be counted in a certain way. Photons are not conserved. Photons are being sent out in vast numbers, and absorbed by the earth and other bodies. These processes are not in balance; the number of photons in the universe is constantly changing. Electrons, on the other hand, are conserved; the number is always the same. A different kind of statistics apply to 'particles' like photons.

Einstein took up the idea of the new statistics, and applied it to other problems in three papers that were his last major contribution to quantum theory. He showed that, under the right circumstances, molecules ought to behave as waves. Late in 1924 he was sent a paper by Louis De Broglie claiming that particles such as electrons could behave as waves. Erwin Schrodinger developed a complete description of the quantum world, wave mechanics. 'Particles' such as photons that obey Bose-Einstein statistics are called 'Bosons.' 'Particles' such as electrons that obey Fermi-Dirac statistics (after Enrico Fermi and Paul Dirac) are called 'Fermions.'

Richard Feynman (1918-1988) was the greatest theoretical physicist of his generation. His greatest achievement was what he called "the strange theory of light and matter," quantum electrodynamics or QED for short. QED is the most successful and accurate scientific theory there has ever been. It tells how electrons and electromagnetic radiation interact with one another. It explains everything in the physical world except gravity and the behavior of atomic nuclei. It deals with probability waves. The probabilities tell you where you are most likely to find particles. There are only three things that matter about how light and matter interact says Feynman. First, there is the probability of a photon going from one place to another. Second, there is the probability of an electron going from one place to another. Third, there is the probability that an electron either absorbs or emits a photon.

One of Feynman's key contributions to QED was his realization that we have to take account of every possible path from A to B, not just straight line routes, but the most complicated routes imaginable. See the experiment with two holes in my document Quantum Mysteries. Light should bounce off a mirror at all kinds of angles, not just the "the angle of reflection = the angle of incidence." This is supposed to explain why we see rainbow colors on a CD rom disk. We see the rainbow colors at all angles, not just the "=" angle.

The physicist Paul Dirac, in 1929, developed a description of the way photons and electrons interact that took account of the special theory of relativity. He used the probabilities of interaction to calculate the magnetic moment of an electron. Dirac found that the number should have a value of 1, but experiments gave the value 1.00116. In 1948, Julian Schwinger found a way to improve Dirac's calculation. Schwinger realized that while an electron was on its way from one place to another, there was nothing to stop it from emitting a photon, and then reabsorbing it. This introduces a complication into the probability calculations, and the result is to make the calculated magnetic moment a little bit bigger.

By the middle of the 1980s, physicists had extended the calculation to include the effects of three 'extra' photons. Each complication is less probable than the one before, and makes a smaller correction to the calculation (and is much harder to calculate). At this level the calculation of the magnetic moment is 1.00115965246, with an uncertainty of 20 in the last two digits. By then the experimental value was 1.00115965221 with an uncertainty of 4 in the last digit. This is the best agreement between theory and measurement that has ever been.

A bizarre feature of QED is how two electrons exchange a photon. One emits the photon, and later the other absorbs it. But we are equally entitled to say that the second electron absorbs the photon before the first one emits it! Time has no meaning for a photon. When a photon has enough energy, it can turn itself into a pair of electron-like particles. One has a positive charge, and is called a positron. For this to happen, the E in the photon must be more than the mc² of the two electrons. When an electron and a positron meet, they reverse the process, and annihilate each other to form an energetic photon. Feynman realized that this interaction can be described in terms of just one electron. The interaction sends the electron traveling backwards in time until it meets another energetic photon, when it is 'turned around' again and travels forward in time.

QED is not a perfect theory. The layers of complex interactions result in an endless addition of probabilities. This causes the calculation of the charge or mass of electrons to come out as infinity, which is clearly nonsense. Feynman, Schwinger, and Sinitiro Tomonaga found out how to get rid of the infinities – they divided both sides of the equations by infinity. This is called renormalization, and gives the answers that match experiments. For this the three were awarded the Nobel Prize in 1965.

Maxwell's equations are completely symmetrical as far as time is concerned. When you solve the equations to describe the way a wave propagates, you always get two answers, one corresponding to a wave moving forwards in time and the other to a wave moving backwards in time. In the revised version of what has become known as the Wheeler-Feynman absorber theory, when an electron jiggles about, it sends out both a wave into the future, and a wave into the past. The result is an overlapping sea of interacting electromagnetic waves filling the entire universe. This was just what was needed to explain a phenomenon known as radiation resistance. The wave going forward in time takes the same amount of time as the wave going backward in time, so the reaction is instantaneous regardless of the distance traveled.

The Wheeler-Feynman absorber theory only gives the right answers if the Universe is a closed box from which no energy can escape (finite or 'closed'). Astronomers have come up with compelling evidence that it is closed.

In 1992 Japanese researchers, carrying out an experiment devised by an Indian team, showed that individual photons exhibit wave-like properties and particle-like properties at the same time. The experiment involved two prisms separated by a small air gap, and two detectors (one for the reflected light and one for the transmitted light). The light could only get across the gap by tunneling, yet the counters never clicked at the same time. Tunneling means the light was behaving as waves, and the counters proved that the light was behaving as particles.