Thursday, 20 May 2010
The Nature Of Space
Lambert Dolphin a retired physicist has some very useful insights into the nature of space which are well worth considering.
" Aristotle (384 - 322 BC) taught that the physical world was made up of four elements: air, earth, fire and water. Tying these all together so that the "elements" intercommunicated was a "subtle" medium, a fifth element: the aether -- later to be known as the vacuum. (The Latin root vacuus means "empty"). In a sense the aether was the substratum of the material world. The Greeks believed that "nature abhors a vacuum" so they could not imagine space as being totally empty.
The Greeks believed the stars were suspended from, or attached to, a rotating crystalline shell at a fixed distance from the earth. When some of the "stars" (planets) were observed to be moving with respect to the "fixed" stars, a series of rotating crystal spheres was postulated. The earth was believed to be fixed, immovable, and at the center of the creation. Not until the 16th Century were these Greek (Ptolemaic) ideas challenged by the Copernican revolution. One of the most mysterious concepts in western physics since Aristotle's day is the concept of the vacuum. Until Galileo Galilei (1564 - 1642) challenged the notion, the velocity of light was assumed by most everyone to be infinite, so the nature of the space between the earth and the crystal spheres was not of great concern.
Rene Descartes (1596-1640) championed the theory that the aether was a plenum, from the Greek word meaning "full." Because it was so difficult for the scientists of that era to understand "action at a distance," Descartes imagined that a very dense medium of very small particles pervaded everything. This medium was capable of transmitting force from one object to another by collisions. The aether "particles" were in constant motion and there were no spaces between the particles. In a sense the aether was more solid than matter, yet invisible. Descartes universe was a purely "mechanical universe" and his theories were soon superseded.
Galileo's former secretary, Evangelista Torricelli filled a long glass tube with mercury in 1644. Inverting the tube into a dish of mercury he observed that the mercury dropped some 30 inches at the closed upper end of the tube, thereby creating what was obviously a vacuum. Blaise Pascal (1623 -1662) took this work even further and soon everyone was convinced that the vacuum of space was empty after all.
If light were corpuscular in nature as some believed, it was not difficult to imagine that light "particles" (we now call them photons) could traverse a pure vacuum without the necessity of a real medium pervading all of space. But other experiments soon began to show that light was a wave phenomenon. Of course waves could travel through the plenum aether by collisions, however at the time only compressional waves were imagined. [Sound waves or seismic waves are compressional in nature, for instance, but light waves proved to be transverse]. In parallel with all these growing controversies, the velocity of light was finally measured by Olaf Roemer in 1675 and found to be finite, although the values he obtained were a few percent higher than the present value, 299,792.4358 km/sec.
By the time of Sir Isaac Newton (1642-1727) the aether was believed by many scientists to be "luminiferous." That is, the aether was said to be more fluid than solid, though it was elastic, and therefore it was a medium which would support waves. James Clerk Maxwell (1839 - 1879) enjoyed great success when he found a set of equations which beautifully described how light waves could travel through such a luminiferous aether. He showed that light waves are composed of oscillating electric and magnetic vectors in an x-y plane for a wave traveling in the z-direction. For a waves to exist at all, it is natural to suppose that there is some sort of supporting medium. Such a medium must possess elasticity (a spring like property) and also inertia, (a mass like like property). In fact, the velocity of a wave in any medium is equal to the square root of the stiffness divided by the density of the medium.
In the case of electromagnetic waves (gamma rays, x-rays, radio waves, heat, and light of various wavelengths), Maxwell found that the aether possessed an electric field scaling parameter, called "dielectric permittivity," and a magnetic field scaling parameter, called permeability, such that the velocity of light was equal to one over the square root of permeability times permittivity. In support of the notion that the aether was a real medium it was observed that empty space behaved like a transmission line with a "characteristic impedance" of 377 ohms, (which is the ratio of permeability to permittivity for "free space.")
This new theory also explained how light slows down in glass, in gases, in water -- because media other than the vacuum had a different permeability and permittivity. The aether was once again thought of as a very real medium which could be stretched or compressed -- it had resilience or compliance, and inertia. Yet no known physical substance had a stiffness to mass density ratio anywhere near 9 x 1016 which was required of the aether as a medium. The aether appeared to possess elasticity but negligible inertia