THE STORY OF VACUUM - PALAHALLI VISHWANTH

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This appeared in Deccan Herald spectrum supplement -August 4,2015
(if one googles vacuum and vishwanath, one can get it )




 

Vacuum - Pascal to Einstein

Palahalli R Vishwanath

" On September 19th, we went to the foothills. I had two instruments and both of them showed 71.1 mm of Mercury. I gave one of them to the pious priest Father Chaswin to keep recording the level of the mercury. I kept the other one with me and climbed the mountain hill with my associates. The mercury level was 62.7 mm at the top of the hill. Therefore the difference in the pressure at the two places is 84 mm of mercury. " This experiment conducted in 1628 on a mountain 400 km from Paris as per the instructions of the great Blaise Pascal was to unravel the mysteries of vacuum.

Aristotle ( BC 384-322) and others thought that there is no place without some form of matter or other and expessed it by the famous statement NATURE ABHORS VACUUM . Galileo (1564-1642), who disproved Aristotle's thinking about mechanics including falling bodies , was also the first one to ask " We have not registed the existence of vacuum to say whether it is there or not" . The next step was taken by Galileo's disciple Evangelisa Toricelli (1608- 1647) who invented the first mercury barometer. He poured mercury into a 4 foot long glass tube , closed the open end wth his finger and kept the tube in a trough filled with mercury. He observed that the mercury came down and a small space at the end of the tube remained empty. He measured the level of the mercury in the tube to be 76 cms. From this he proved that air has pressure and the empty space above mercury was what one could be termed vacuum !

The man who improved Toricelli's experiments was Blaise Pascal (1623-1662) , one of the great minds of the western world.He was a veritable polymath with interests in Mathematics, Physics, Computer Sciene, Logic and Theosophy. In his youth he designed a calculator and later he founded the field of Statistics. He also penned two philosphical classics in French - Penses and Letters - still held high for its language and style . Today a crater on the moon, acomputer langage and the unit of pressure are all named after Pascal

With his mountain experiment ( mentioned above ) Pascal concluded that air has weight and thus it exerts pressure which decreases with height. He further deduced that vacuum will be encountered above he atmosphere . In the present units, the pressure which is 760 tors at sea level drops to a microtor at 100 km above sea level. It is important to see that Pascal, like Galileo, stresses experimental methods : " You desciples of Aristotle. learn that truth of matter in physics can be ascertained only through experiments" . However Descartes a leading scientist of the time disagreed with Pascal and snidely remarked that Pascal's head is full of vacuum ..

It was only few years later another famous experiment on vacuum was conducted in Magdenburg by Otto Van Gureicke (1609-1696) , a politican - scientist of Germany. He joined two copper hemispheres and created vacuum inside by removing air out of it by a pump invented by him. Later he tried to show the strength of vacuum by trying to separate the two halves by using several horses on each side ( the number varied from one to 12) from either end. With time better and beter vacuum was achieved While a vacuum cleaner can provide a vacuum of 25 tors and ordinary pumps nanotors. some modern pumps can go down to the level of even get pico tar ( 1 in 10**12) . For very good vacuum , one needs to remove all types of matter from the enclosure. Even then few particles can remain. That is the best possible experimental vacuum. However vacuum has a totally different interpretation today

Edwin Hubble showed in 1930s that the universe is undergoing expansion. This remarkable result is the basis of modern cosmology. However the observation of distant supernovae in 1990s by two separate groups showed that the distances to the galaxies which hosted the supernovae were farther than would be consistent with a universe expanding at constant velocity. Thus it was shown that the expansion of the universe is going faster than expected . This acceleration of the universe fetched the Nobel Prize in 2011 for their discoverers. It was speculated that some repulsive energy - Dark Energy- is responsible for pulling the galaxies apart. The latest results from two satellite experiments show that this Dark Energy amounts to 76 % of all energy in the univese. With this repulsive energy getting dominant, the galaxies would drift apart from each other and the universe would end in a vast, dark, and cold state which could be called the 'Big Chill'

As to what constitues Dark Energy there are only speculations. One of the interpretations is that it could be due to strange behavior of particles in the micro world. Quantum mechanics allows energy and matter to appear out of nothingness, although only for a very brief moment. This continuous appearance and disappearance of matter could give energy to space that is otherwise empty. This is termed the vacuum energy that permeates all space. Virtual particle pairs could blink into existence and then annihilate in a very short timespan. These pairs would be particles with opposite charges like electrons and positrons. The well known phenomenon of Spontaneous Emission in an atom and several other effects like Casmir Effect and Lamb Effect are explained as due to fluctuations in the vacuum .

After proposing his General theory of Relativity in 1915, Albert Einstein realized that it could be applied to the state of the universe and thus proposed his cosmological equation in 1917. At that time since the universe was deemed to be static, he had used a constant called Cosmological Constant which represented a repulsive force to counter the effect of gravity . However when he learnt about the expansion of the universe , he dropped the Cosmological consant Considering it as his ' greatest blunder' . However when Dark Energy was discovered, it was realized that "repulsive" force reperesented by the Cosmological constant could explain the acceleration of the universe and thus the vacuum energy of space .In the early universe when all matter was packed much more densely, the vacuum energy played a much smaller role. However, now the gravitational pull between the galaxies is less and thus the vacuum energy plays a greater role

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Fig 1. A mercury barometer with vacuum at the top Fig 2. The famous Magdenburg experment