On September 19, 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,who continued recording the level of mercury and I kept the other one with me, while climbing the mountain 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.

In general, vacuum comes from the Latin word vacuus, which means vacant or void. So, vacuum is any space that is devoid of matter. The journey of vacuum starts around 384 BC, when Aristotle and others thought that there is no place without some form of matter or other and expressed it by the famous statement ‘nature abhors a vacuum’. Galileo (1564-1642), who disproved Aristotle’s thinking about mechanics including falling bodies, was also the first one to ask, “We have not registered 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 four-feet-long glass tube, closed the open end with 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. The level of mercury in the tube was 76 cm. This proved that air has pressure and the empty space above mercury was what one could call as vacuum. 

The man who improved upon Toricelli’s experiments was Blaise Pascal (1623-1662), a veritable polymath with interests in mathematics, physics, computer science, logic and theosophy. In his youth, he designed a calculator and later founded the field of statistics. He also penned a French classic – Penses the Provincial Letters – still held high for its language and style. Today a crater on the moon, a computer language and the unit of pressure are all named after Pascal.

Ongoing experiments
With his ingenious mountain experiment, Pascal concluded that air has weight and
exerts pressure which decreases with height. He further deduced that vacuum will be encountered above the atmosphere. In the present units, the pressure which is 760 torr at sea level drops to a micro torr at about 100 km above sea level.

It was only a few years later that another famous experiment on vacuum was
conducted in Magdeburg by Otto Van Gureicke (1609-1696), a German scientist. He joined two copper hemispheres and created vacuum inside by removing air out of it through 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 ends.

With time, better vacuum was achieved. While a vacuum cleaner can provide a
vacuum of 25 torr and ordinary pumps nano torr, some modern pumps can go down to the level of even a pico torr. For very good vacuum, one needs to remove all types of matter from the enclosure. Even then a few particles can remain. That is the best possible experimental vacuum. However, vacuum has a totally different interpretation today.  
Is space a perfect vacuum?

When you take the definition of vacuum literally, outer space should be the perfect example of a vacuum space due to its low density and pressure. But that is not the case. In 1930s, Edwin Hubble showed that the universe is undergoing expansion, which has been proved right by 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 proved that the expansion of the universe was 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 – was responsible for pulling the galaxies apart. The latest results from two satellite experiments show that this dark energy amounts to 76 per cent of all energy in the universe. 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 constitutes dark energy, there are only speculations. One of the interpretations is that it could be due to the strange behaviour 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 as 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 effects due to the vacuum fluctuations.

After proposing his General Theory of Relativity in 1915, Albert Einstein realised 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 constant, considering it as his ‘greatest blunder’.  However, when dark energy was discovered, it was realised that the repulsive force represented 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 consequently, the vacuum energy plays a greater role.