Fundamentals of Physics Part 2
Vacuum
The concept of vacuum is inherent to modern physics theory, whether at the sub-atomic, the atomic or the cosmic dimensions, and every aspect of modern physics theory was developed from, and is still based upon, a 2500 year old theory of the ultimate structure of matter.
This structure was one of matter in the form of distinct, indestructible particles that were separated by a non-material, empty space, a perfect vacuum. The theory is generally attributed to Leucippus or Democritus, who proposed that matter was ultimately indivisible and took the form of indestructible particles – atoms.
And it is important to understand the reasoning at that time for this assumption of a space completely devoid of matter.
Greek philosophers were well aware that there were three states of matter, the solid, liquid and gaseous states. And such atoms could logically explain the general rigidity of solid macroscopic matter as being composed of atoms in close proximity, such as a pile of sand or oranges. But to explain the fluidity of the liquid and the gaseous states required the atoms to be separated, in order that they could be ‘moved aside’ so that a macroscopic, solid body could, potentially, move through these states. And of course, with distinct particles, even in solid matter there would be a proportion of separating empty space.
Such a separation by a non-material space however raised a problem, in that in gases the atoms would naturally subside to the Earth’s surface under the influence of gravity, so further assumptions were necessary.
These were that these atoms were in eternal, kinetic motion at high velocities within an ’empty space’ and were colliding with one another, and further that these collisions were perfectly elastic, meaning that no energy was lost as a result of such interactions. And it was also necessary to assume that this separating ’empty space’ could not in any way inhibit the motions of atoms due to any frictional effects.
Thus it was concluded that this empty space was a perfect vacuum, the state of non-existence of matter, and this ancient concept of Greek philosophers remains firmly in place today, accepted by theoretical physicists as the core premise of the ultimate structure of matter today.
And it is this theory of ‘atoms and space’, i.e. the kinetic atomic theory of gases, essentially one of tangible, indestructible, material particles and ’empty space’, that was the basis from which all currently accepted theories have been developed, and are completely dependent. And not only those concerning the structure of macroscopic matter at sub-microscopic dimensions, but those relating to the wider structure of the universe.
Aristotle famously rejected the invention of the vacuum and, largely due to the subsequent rise to political domination of Western Europe by the Church of Rome, his views on this and the theory of matter suggested by him (along with a geocentric universe) was effectively enforced until the fragmentation and the dissipation of the church’s power during the Renaissance.
At this time, over two millenia later, Galileo was intrigued by the fact that water cannot be drawn up by suction above a height of more than around 10 metres and he suggested to a pupil, Torricelli, that he investigate this effect.
In 1643 Torricelli experimented with mercury instead and, filling an 80cm long, glass tube, closed at one end, with this liquid and, while holding a finger over the open end, inverted the tube into a bowl containing the same metal. On removing his finger the level of the mercury fell to a height of about 76cm leaving a space above it empty of the liquid.
The space thus produced at the top of the tube above the column of mercury was immediately accepted as being completely empty of matter, and so Torricelli was generally believed to have created a perfect vacuum.
This experimental result had wide repercussions, one of which was that it seriously undermined the, then widely accepted, authority of Aristotle.
And, it was of course no coincidence that, just four years later, in 1647, Pierre Gassendi resurrected Greek kinetic atomic theory, which was of course dependent on the ‘existence’ of vacua.
“As for the containing space in which atoms reside, Gassendi offers empirical and apriori arguments on behalf of a void. He takes the barometric experiments of his day, including his own at Toulon, to demonstrate the existence of at least a partial, disseminated void. Moving beyond such empirical argument, he echoes ancient atomists’ reasoning. Thus he rehearses the classical arguments that without a disseminated void between the parts of bodies, one cannot explain division and separation of matter at the level of basic particles, and that without the inane coacervatum (inter-particulate void) one cannot explain the motion of bodies through space.”1
Blaise Pascal’s subsequent experiments, by taking Torricelli’s apparatus up Mont Blanc in the Alps, demonstrated its value as a gauge of atmospheric pressure, the barometer. It was believed that the column of mercury was held up solely by atmospheric pressure acting on the external surface of the mercury, which led to the belief that the Earth’s atmosphere only extended to a certain altitude, whereafter the perfect vacuum of space began.
But it is now common knowledge that mercury freely evaporates at Standard Temperature and Pressure and, if it is subjected to lower and lower pressures, evaporation accelerates so, in the conditions of low pressure induced by the weight of the column of mercury, it is obvious that a rapid evaporation will occur at the top surface.
The so called ’empty space’ created is not a true vacuum and is a volume of low pressure containing mercury vapour, in other words it is full of mercury in its gaseous state.
Incidentally, this metal is extremely toxic, and today in laboratories where it is exposed, highly sophisticated breathing apparatus is employed. But Newton was unaware of this and at one period suffered a serious debilitating illness, which was no doubt mercury poisoning resulting from his many experiments with it, this was recently confirmed by an analysis of hairs taken from his body, which contain fifteen times normal levels.
However, as a result of Torricelli’s experiments it was generally believed not only that the vacuum occupied all of space above the Earth’s atmosphere, but that in line with Greek atomic theory it permeated down into these gases and into the liquid and solid matter at the surface.
“Torricelli’s invention did more than demonstrate the vacuum at the top of the tube. It supported the belief that the atmosphere is only a thin layer surrounding the Earth, and that outer space is empty”2
It was accordingly assumed that the supposed vacuum in the tube exerted no force, attractive or repulsive, on the upper surface of the mercury, and that the vacuum in the apparatus corresponded to the vacuum that occupied the celestial regions above the atmosphere.
That this view was widely accepted is indicated by Newton’s comment in one of his notebooks – “The height of the atmosphere may bee known from Torricellius his experiment”.
Thus it was assumed that above the earth’s thin cover of material atmosphere (extending, it was assumed, to an altitude of around 80-100 km) no gaseous matter was present and that, apart from the odd solid particle, a perfect vacuum occupied all the spaces separating the planets and the stars, and this belief remained embedded in scientific consciousness until the 1960’s, when near space exploration showed this to be false.
“The NASA Space Shuttle at 250- 300 Km altitudes ‘in space’ was found to be in air, with the same proportions of oxygen and nitrogen as at sea level, at a concentration of 1 billion atoms per m3 compared to 3 x 10¹⁹ per cc at sea level. – Thus in no sense could it be called a vacuum”.
“The sun too has an atmosphere, and because the sun accounts for more than nine tenths of the total mass of the solar system, its atmosphere is much larger than that of any planet. – The solar atmosphere extends far beyond the orbit of the earth, and at 80,000 Km our atmosphere merges imperceptibly with that of the sun”.3
But today it is still taught at secondary level physics that a barometer of this type contains a vacuum, for example a textbook states :-
“A mercury barometer works by stabilizing the level of mercury in the sealed glass tube with the atmospheric pressure on the open mercury chamber. The top of the barometer is filled with nothing (vacuum) – – – ”
Also if an online search engine is used to obtain images of barometers, in each case the volume above the column of mercury is described as a ‘vacuum’.
At this point it is necessary to define a ‘vacuum’, the word comes from Latin, meaning “an empty space, void”, derived from ‘vacuus’, meaning “empty”.
These are definitions from a scientific dictionary:-
Vacuum
1. A region containing no matter; free space.
2. A region in which gas is present at a low pressure.
3. The degree of exhaustion of gas within an enclosed space.
This is ambiguous and only the first is literally correct with respect to the Latin origin of the word. A true vacuum is the state of total non-existence of matter, in other words the non-existence of matter in any region/defined volume. I.e. ‘non-existence’, ‘nothingness’.
And while the word may be in wide colloquial use, as in ‘vacuum cleaner’, for professionals such as physicists and educators using the word ‘vacuum’ to describe compartments or regions of low or very densities, where matter is undoubtedly present, is misleading, low or very low pressure, would be more accurate.
As a result of Torricelli’s experiments, it was generally believed that a perfect vacuum was an integral part, not only of macroscopic matter, but was the dominant component of the wider universe. But this belief in a universal vacuum was, and still is, an insurmountable problem, an intangible barrier, for the transmission of forces, as articulated by Newton (in a letter to Richard Bentley at Cambridge):-
“That gravity should be innate, inherent and essential to matter, so that one body may act upon another at a distance through a vacuum without the mediation of any thing else by and through which their action or force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters any competent faculty of thinking can ever fall into it.” (Or, in less polite language, he would be stupid)
Newton was born in 1643, just prior to Gassendi’s revival of Greek theory, and around this time Descartes introduced his concept of an aether, a very dense medium of very small particles that pervaded all space. Later Hooke described light as a mechanical wave and then Huygens revised and expanded Hooke’s theory and hypothesised that light is a wave propagating through an aether, believing that it penetrated all matter and space.
Thus the purpose of this ‘luminiferous’ aetherial medium was to provide, or to ‘fill’ the vacuum with qualities that can transfer light, as a wave, between material entities or bodies, and this medium is discussed in the following paper.
1 http://plato.stanford.edu/entries/gassendi/
2 The History and Philosophy of Science, LWH Hull, Longmans, 1959
3 Air – The Nature of Atmosphere and Climate, Michael Allaby, Facts on File, 1992