(E21) Waves in Space
Allegedly it was Galileo who one night tried to measure the velocity of light, using two hooded lanterns and a helper. The helper with one lantern was posted at a far-away spot and was instructed to uncover his lantern the moment he saw light from Galileo's. When Galieo uncovered his lamp, the returning flash came almost instantaneously. He then told his helper to repeat the experiment at a much greater distance. The delay was just as short as before and Galileo concluded that is was mostly due to the reaction time of the helper, and that light spread very rapidly, maybe instantaneously.
The first actual estimate of the velocity of light in empty space was made in 1676 by Ole Roemer, a young Danish astronomer at the observatory of Paris, as a by-product of a method to determine longitude in mapping the Earth. It was a clever feat of deduction, but since it was already described elsewhere in this collection, the reader is encouraged to look there
By the 1800s, a lot was known about light. Eyeglasses, telescopes and microscopes were in use--all based on the property that light slowed down in transparent materials. It was known to spread as a wave, propagating in empty space at about 300,000 km per second--a velocity nowadays denoted by the letter c--and slightly more slowly in air. A wave is a spreading disturbance associated with an oscillating phenomenon (mathematics has a more precise definition), giving it a wavelength λ (lambda, Greek lower case L), and a frequency ν = c / λ, the number of up/down oscillations per second when the wave passes any point in space (ν is nu, the Greek lower case N).
Sound is such a wave, an oscillation which can spread through air, water, walls and solids. Its wavelength can be inferred from the size of the instrument producing it, e.g. a plucked string--and the bigger the source, the longer the wavelength (centimeters to many meters) and deeper the sound (Big dog: woof, woof! Little dog: yip, yip!). Its velocity in air, demonstrated by the time lag between lightning and thunder, is about 330 meters per second.
Light also has its wavelength, but it is very small, as indicated by the sharpness of visible images. Only with very small objects, of a size not too much larger than a wavelength, is the image fuzzed out. Looking at a bright light with half-closed eyes, we see "
Microscopes and telescopes similarly have limited powers of magnification depending on wavelength, beyond which one only gets magnified blurs. Patterns of light that has passed a thin slit, the colors of patches of kerosene floating on water (whose thickness is not too many times the wavelength λ, dependent on color) or (these days) the colors reflected by the narrow grooves of a computer's compact memory disk--all these are related to the wave nature of light.
But what sort of wave? Sound does not spread in vacuum, but light does. Scientists in the 1800s therefore speculated that all space was filled with a "lumineferous (light carrying) aether" (will come back to that). A great achievement of 19th century was to show that light was linked to electricity--that it was an electromagnetic wave, part of a large family of such waves which grew with new discoveries to include infra red, ultra-violet, radio, microwaves, X-rays, gamma-rays and more.
Units of Electricity
The study of electricity in the 1800s uncovered an interesting relationship. The electrostatic law found by Coulomb, giving the force between electric charges q1 and q2 a distance r apart, was
The choice of the constant k1 allows one to define a "natural" unit of electricity, linked to fundamental measurables like (meter, kilogram, second). By choosing k1=1, for instance, a unit of electric charge is defined as the amount attracting or repelling an equal charge at a distance of 1 meter with a force of one Newton (about the weight of 100 gram or 3.5 ounces)--defined from the fundamentals by Newton's laws of motion.
Magnetic forces, on the other hand, allow one to define a unit of electric current i, by the force formula
giving the force between parallel wire elements of length s1 and s2 separated by a distance r. Ignore now the ratio in the square brackets--whatever units length is measured in, it remains the same (e.g. convert from inches to centimeters--any distance is multiplied by a factor 2.54, which then cancels out). If here F is again in Newtons and we choose k2=1, this defines a natural unit of electric current i.
However, electric current and electric charge are related! A unit of electrical current should also correspond to a unit of electric charge moving at some velocity! Hidden somewhere in this haystack of units is a velocity which is a natural constant of nature.
It turns out to be the velocity of light c--possibly with factors like the ratio of the circumference of a circle to the diameter (π=3.141526...) which would actually be part of the formula. This may carry two kinds of message:
(2) The velocity c is somehow "built into" the structure of
nature, just as the number π is built into mathematics.
It turns out both hold true. Light is indeed an electromagnetic phenomenon, while c appears unexpectedly in formulas like Einstein's E = mc2
Sound in air is a pressure wave, advancing in the same direction as the force it produces. Light is altogether different, a "transverse wave" with effects in the directions perpendicular to its advancing front--like a sideways jiggling of a bowl of jelly pudding, or certain types of earthquake waves. Ordinary light vibrates in all directions perpendicular to its rays, but certain crystals (known to Faraday) can separate vibrations into components polarized in two directions, perpendicular to the advance of light and to each other. The human eye can barely detect any difference, but Polaroid eyeglasses with crystals embedded in mutually perpendicular directions can separate two differently polarized images, and thus give us 3-D film images. Faraday also knew that light reflection from transparent surfaces (but not from ordinary mirrors with thin metal coating!) depended on the polarization of its components (in reference to the direction of the source), and that the blue color of the sky, from scattered sunlight, also had a preferred polarization, depending on the angle between the line of sight and the direction of the Sun.
Polarized waves are somewhat like waves propagating on a long rope when one end is vigorously shaken. Shake the end up and down, and the waves are all in a vertical plane. Shake it sideways, and the waves are horizontal. And when the shaking is in random directions, so are the waves, though if the rope passes a fixed vertical slit (which acts like a polarizing crystal) only vertical waves pass (or the vertical part of waves with non-vertical oscillations).With a horizontal slit, only horizontal waves pass.
Faraday wondered if magnetic field lines filling space could oscillate like such ropes, and whether light, with its polarizations, consists of such oscillations. In April 1846 he gave an impromptu lecture Thoughts on Ray Vibrations on his speculations. In his report on the lecture, he wrote:
This beginning of the electromagnetic theory of light occurred in somewhat unusual circumstances. Faraday was scheduled to introduce a Friday lecture at the Royal Institution by Charles Wheatstone, remembered nowadays for his "Wheatstone bridge" for measuring electrical resistance (his talk however was on a timing device). Wheatstone came to the lecture hall, but grew anxious and left again before the talk began (allegedly, since then it has been customary at the Royal Institution to lock speakers in a room half an hour before their presentations).
With an audience but no speaker, Faraday gave Wheatstone's talk (he knew the subject), but with a lot of free time left at the end, added a talk about his own ideas. A hesitant speculation, not a polished talk, but that is what brand-new scientific ideas often look like. Once you know the answer, it may look deceptively simple and obvious; Faraday recaptures the uncertain mix of guess and hypothesis which precedes knowledge. He later ended a letter about the ideas he had described with these words:
But a key ingredient was still missing: because of magnetic induction, (discussed in what follows) there also existed an oscillating electric field--a possibility of sensing an electric force in the region where the wave is spreading.
The complete idea of an "electromagnetic wave," propagating in what is otherwise empty space, was later correctly worked out by James Clerk Maxwell, and it fit all known properties of light, e.g. the partial polarization of light reflected from glass. The velocity of propagation in a vacuum is a natural consequence of the two basic laws, cited above. And we no longer claim that the waves propagate in some "aether" filling space, because that would be a most unusual substance, one whose motion was unobservable. We just call the medium of propagation "space"--sometimes "space-time", or simply "the vacuum."
Next Stop: E22. Electromagnetic Fields|
Author and Curator: Dr. David P. Stern