Scientists have discovered many laws of nature. They have learned some basic rules about how objects appear to interact, how light appears to travel and interact with other light and matter. They have studied gravity, magnetism and electricity and have learned laws which govern each. Using this knowledge, we can predict eclipses, build bridges, and put a man on the moon.
But where did these laws come from? We know how the laws work, but what do we know about why they work? It has long been a tenet of science that it only deals with finding laws that describe how things work so that we can have a better world by applying them in inventions and technology. Science has no way to discover the ultimate cause behind the laws. It makes little difference to how your cell phone works to know whether the governing laws were written by a Creator, or whether they are self-existent truths. We mostly just want our high-tech devices to work, and the low-tech varieties also.
Thus, science has been content to adopt the equations that came from some scientist's mind as explaining things. Sir Isaac Newton produced some of the best known laws of all time, including his three laws of motion, and law of gravity. Maxwell later wrote the equations of electricity and magnetism, and Schroedinger wrote an equation for quantum wave mechanics. It is rarely asked why these equations work, we are just glad that someone discovered that they do indeed work.
When a scientist does ask "why" an equation works, the only answer allowed in science is to explain it with an even more fundamental law. For example, Johannes Kepler produced three laws which describe the motion of planets around the sun. He gave no reason why they should work, but only claimed that they do. Then Sir Isaac Newton came up with his laws of motion and gravity, and was able to derive all of Kepler's laws from them. Thus, Newton's appear to be more fundamental, and Kepler's are now an exercise for the student.
Scientists discovered many laws about how light travels and behaves. At first there was a big debate about whether light was a particle (like a little bullet) that could travel through a vacuum or a wave (like sound) that required a medium (like air) for transport. Newton proposed that it was a particle, because it didn't seem to go around corners like waves do. But then a host of wave properties of light were discovered, such as interference (when wave crests add), and diffraction (it does indeed go around corners). When Maxwell showed that light could be explained as an electromagnetic wave, that seemed to end the debate once and for all. Light was a wave. The medium through which light traveled was called the "luminiferous ether" or simply "ether." There were some famous, and now amusing, statements made at the end of the nineteenth century that physics had answered all the hard questions, and all that was left was to fill in some details.
Before continuing the science, let's look at how nineteenth century science was interpreted by some LDS scholars, who compared the light of Christ to the ether:
"The Spirit of God which emanates from Deity may be likened to electricity, or the universal ether, as explained in our manual, which fills the earth and the air, and is everywhere present. It is the power of God, the influence that he exerts throughout all his works by which he can effect his purposes and execute his will, in consonance with the laws of free agency which he has conferred upon man. By means of this Spirit every man is enlightened, the wicked as well as the good, the intelligent and the ignorant, the high and the low, each in accordance with his capacity to receive the light." — Joseph Fielding Smith
". . . from the presence of these Divine Beings proceeds an essence or substance (perhaps like unto ether) variously called 'spirit,' 'light,' 'light of truth,' 'light of Christ,' corresponding somewhat to what other teachers regard as 'vital force' or 'energy' which permeates all nature and constitutes the immanence of God in the universe; through which the purposes of the Divine Intelligence are impressed upon other minds and also upon matter, and hence the orderly creations and their maintenance—the cosmos." — B. H. Roberts
So at that time, it looked like "light" as understood by science and religion had something in common. Now let's get back to science, to see how its story changed. One of those minor details yet to be determined at the end of the nineteenth century concerned just what was the precise speed of the earth moving through the ether. After all, the earth is believed to orbit the sun, the sun is moving relative to the nearby stars, and then our whole galaxy is rotating, so the earth's "absolute speed" must be fairly large. Then in 1887 the famous Michelson-Morley experiment failed to measure any speed at all relative to the ether.
The young Albert Einstein took that result and used it to rewrite much of physics. He proposed as one of his two fundamental postulates that the speed of light is constant, independent of the motion of either the emitter or observer. He declared that no experiment could be done to detect the motion through the ether because of that property of light. Then the big switch came, so watch the magician's hands closely. Because science only studies the observable, and because even in theory no experiment could detect the ether, then as far as science is concerned the ether does not exist. Poof, the ether has vanished. The vocabulary was amended, and it is now said that light travels through a vacuum.
Light was then shown to have bullet-like properties, and Einstein received the Nobel Prize for explaining the "photoelectric effect." For a while, light seemed to behave both as a particle ("photon") and also as a wave. Later it was shown that all particles, such as electrons, also have wave-like properties. In the minds of most physicists, the apparently wave-particle duality of light was resolved on the side of the photon, with all of the wave properties being explained by the amazing theory of quantum electrodynamics.
