Gravity Chapter
5: Vacuum
As discussed earlier a space had to be assumed by the Greek philosophers
to exist between the solid atoms of air to explain its fluidity and
this space clearly could not, in any way, inhibit the assumed motion
of the atoms.
Open debate on the merits of Greek atomic theory effectively
ceased due to the Dark Ages, however at the time of Galileo
the situation changed and the following extract describes the position
clearly.
Galileo was struck by the observation that water could
not be lifted more than 32 feet, by what was then supposed
to be suction, from above. His pupil Torricelli made the
crucial experiment in 1643. Torricelli filled a glass tube, closed
at one end, with mercury. He held this vertically, with its open
end beneath the surface of mercury in a trough. He found that, if
the (height of the end of the tube) was less than about 30 inches,
the tube remained full of mercury. This could be explained by the
Aristotelian principal that the mercury had to remain in the tube
because nature would not allow a vacuum at the top. But when the
tube was raised so that (the height) was more than 30 inches, the
mercury rose to a height of 30 inches only. A space appeared at the
top: and this was presumably quite empty, as there was no way for
anything to enter it. Here, then, was a vacuum that nature refused
to fill. Some cause, other than the supposed impossibility of a vacuum,
was needed to explain how the mercury was supported in the tube.
Torricelli concluded that the mercury was kept in the tube
by the weight of the atmosphere, pressing down the mercury
in the trough. In order that the mercury might run out
of the tube, the level of mercury in the trough would have
to rise. This was prevented by the pressure of air. Water will rise
about 32 feet before the vacuum appears. According to Torricelli’s
theory this is to be expected; because 32-foot column of water and
the 30-inch column of mercury, having the same cross-section, are
equally heavy, and therefore need the same amount of support.
This apparatus of Torricelli was the barometer. The height
of the mercury column was found to fluctuate; and the practical
value of the barometer, as a storm glass, was quickly recognised.
But the interesting fact, from a scientific point of view,
was that the barometer fell steadily as its height above
sea level increased. This suggested that the atmosphere
extended only to a limited distance from the earth’s surface. When the
barometer was on a hill, there was less pressure on the mercury in the
trough, because there was less air above it to be supported. Thus 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.12 (My
emphasis)
It was assumed accordingly, and correctly, that if such
a barometer were taken to progressively higher and higher
altitudes, then at some point the external pressure would
be so low that the mercury inside the tube would be at
the same level as that outside the tube. It was thus assumed
that at this, unspecified, altitude the earth’s atmosphere
ended and the vacuum of outer space began. Thus it was believed at
this time that a perfect vacuum could be created and also that outer
space was a vacuum.
As discussed earlier Gassendi, in 1647, then resurrected
Greek atomic theory, which of course depended upon the
existence of an empty space or a vacuum between the atoms
in air.
Analysis of Current Vacuum Beliefs
With the benefit of modern knowledge
let us consider the ‘vacuums’ of Galileo and Torricelli.
As it was difficult practically to construct apparatus
extending to over 32 feet high to consider Galileo’s problem
with water, Torricelli carried out his experiments with
mercury, a much heavier liquid.
But if we look at the characteristics of water in this
respect and constructed a water barometer, clearly this
would when taken to the top of a mountain, by a reduction
in the height of the column of water indicate a lower atmospheric
pressure here than at sea level, in the same way as a mercury barometer
would.
It has also been demonstrated that water boils at a lower
temperature at altitude, an Englishman on an early expedition
to the Himalayas was said to complain, “One couldn’t get
a decent cup of tea on Everest”.
What we call boiling is a change of state from the liquid
to the gaseous state, thus at a lower pressure, water,
as with any liquid, changes from the liquid state to the
gaseous state at a proportionately lower temperature.
And if we subject of quantity of water to progressively
lower pressures, at some point it will spontaneously boil,
or change from the liquid to the gas state.
This is precisely what happens if we were to construct
a water barometer extending over 32 feet into the air.
What is happening here is that the reduced pressure at
the top of the column of water, caused by the gravitational
attraction of the mass of the whole column of water to
the earth, is sufficient to boil, or change the state of
the water at this point from the liquid to the gaseous
state.
Therefore the vacuum at the top of a water barometer is
not a perfect vacuum but a partial vacuum, and this space
is filled with water vapour at low pressure.
This is precisely what occurs at the top of a column of
mercury in a mercury barometer, the pressure is so low
that the mercury boils and an atmosphere of mercury vapour
is created at the top of the column.13
Therefore the assumption of Torricelli that he had created
a perfect vacuum was not true, however this assumption
was generally believed at this time and this, of course,
reinforced the beliefs of those who were promoting the
kinetic-atomic theory.
In 1684 Von Guericke carried out his famous experiment,
which involved evacuating air from a copper sphere made
in two semi-spherical parts and, attaching two teams of
horses to each half, attempted to pull the sphere apart.
This they could not do and it was assumed that the pressure
of the atmosphere on the outside of the semi-spheres was
sufficient to prevent their separation.
Michelson & Morley’s Experiment.
During the 1800s there was strong
philosophical opposition to the concept of a vacuum existing,
reinforced by the (it has to be said) logical assumption that light
or other radiant energy could not travel through a vacuum. However
at the same time it was the general belief that the atmosphere of the
earth only extended to a certain distance and then space began. However
as to what the characteristics of space were and at what altitude it
began, there was no means of finding out.
However for those scientists, and this included Clerk Maxwell,
who rejected the idea of a vacuum, there was the problem
of explaining how the earth, travelling at some considerable
velocity through space in its orbit around the sun, did not progressively
lose its atmosphere, or suffer a loss of momentum, as a result of
frictional forces acting on the upper surface of the atmosphere.
To address this problem the idea of a medium or substance,
that was somehow different from the atmosphere surrounding
earth and that permeated all of space was proposed, and
this was the ‘luminiferous (or light producing) aether
or ether’.
It was assumed firstly that this ether was the medium by
which light traversed ‘space’, and also that it was stationary
in relation to the motion of the earth through ‘space’.
If this were the case, it was argued, then the speed of
light measured in the direction of the earth’s motion through
space would differ, i.e. would be greater than, the speed
of light measured at 90° to this direction as in the diagram
below.
Figure 16
An experiment was carried out in 1887 by Michelson and Morley in an
attempt to show this differential, but the results were not as they
anticipated and indicated that the speed of light was the same in both
directions.
This ‘failure’ was ultimately considered proof that the
‘aether’ did not exist and that outer space was in fact
a vacuum and a confirmation that light did travel through a vacuum.
This of course was seen to reinforce the arguments of the protagonists
of kinetic atomic theory and led to the 1905 assertion by Einstein
that ‘the concept of an aether is superfluous’ and the general acceptance
of the ‘discontinuity of matter’.
With respect to space, from the time of Torricelli and
up to the middle of the 20th century it was the assumption
of most people, including most scientists, that the earth
was surrounded by an envelope of atmosphere which ended at some,
undefined, point, whereupon ‘space’ began.
In the sixties spectrographic analysis of space was carried
out that indicated that a volume of 1 cubic metre of inter-stellar
space contained an average of 3 million atoms of hydrogen
(plus smaller proportions of nitrogen and oxygen) and that
this density appeared to be consistent.14
Penzias and Wilson’s Experiment
In 1964 two scientists, Penzias and
Wilson, carried out an experiment to measure the background
radiation emanating from space. It was discovered that this radiation
was consistent in all directions from apparently empty space, which
indicated that the temperature here was 2.7° K, or in other words 2.7
degrees above absolute zero.
This confirmed that inter-galactic space was not completely
empty and contained a consistent distribution of matter.
Atmosphere of Earth
In the last century, as a result of high altitude
flights, balloons and latterly space exploration, our knowledge
of the extent of the earth’s atmosphere has improved considerably and
it is now known that there is no definitive point or border
that divides the atmosphere of earth from the matter that exists in
the space between the earth and the moon and the earth and the sun.
The essential difference between the gas at the earth’s surface and
the gas in space 100 or more kilometres from the surface is one of
density only, which of course progressively decreases with altitude.
It is now accepted that the atmosphere of the earth extends well out
into space and merges with the atmosphere of the sun.
‘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’. 15
Thus it is now known that outer space is not a vacuum,
but by our definition below, only a partial vacuum and,
it is currently assumed that, the difference between the
gaseous matter at the surface of the earth and the gaseous
matter in outer space, apart from composition, is the interval
of ‘empty space’ between them.
Continued >
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