Doppler Effect - The history and its Uses in astronomy - Palahalli Vishwanath - in Deccan Herald of 15 sep 2015

 in Deccan Herald of 15 sep 2015

http://www.deccanherald.com/content/500952/doppler-effect-origin-findings-evolution.html

The Sky and the Train

Palahalli R Vishwanath

(An experiment on a moving train in mid 19th century provided the proof

fo Doppler Effect which was used in some of the greatest discoveies in modern astronomy. )

Even today the coming of a train is a source of excitement in small towns . It must have been more so on 3rd June, 1845 when Maarssen , a small town on the Utrecht - Amsterdam line , was to play witness to an important experiment in science. Since no proper accounts of the expriment exist, one can only try to reconstruct the event : Apart from many people on the the platform of the station, there were six trumpeteers playing some musical notes . They were awaiting a special train from Utrecht which was 12 km away. Slowly people started hearing some musical notes from far but they were very high pitched. As the train approached they could see an an open carrriage in which there were six trumpeteers playing the same musical notes . And as the train came and went away the notes again differed, with the ones in the train being of low pitch . There were repeat runs of the train as though to check and recheck the differing musical notes

In 1842 Christiain Doppler (1803-1853) from the east European city of Prague proposed a new theoy in which a person watching a source of waves would find an increase in frequency as it comes near him and a decrease as it goes away from him. It was akin to the situation where A , and B are playing ball in which A is standing still B is moving towards him. A recives the ball sooner and sooner as B approaches him and receives it later and later as B goes away from him. Thus the frequency of the event increases at first and then decreases. However Doppler believed that his theory would not be proved in his lifetime since he could not imagine a fast moving source. But one man felt that an experiment could be done in trains which had already achieved a speed of about 60 km per hour. He was Mr Buys Ballot (1817-1890) a Dutch meteorologist who mamged to get the above experiment done on the train and thus provided the first proof what came to be known as Doppler effect. Several experiments in the past like this and those of Galileo, Guericke, Pascal, etc conducted on a grand scale and and in front of many people played important part in popularizing science.

Armond Fizeau (1819-1896) , a Frenchman , applied the Doppler effect to the

electromagnetic spectrum in 1948. Light has different colours each signifying a different frequency; for eg, Red and blue colours have lowest and highest frequency respectively. Therefore a light source going away from /coming towards us will show a spectrum with a shift to the red /blue. He also showed that one could find the speed of the source with knowledge of the redshift. (z~v/c where z is the red shift, v is the velocity of he object and c is the velocity of light). Fizeau was also the first one to make an accurate determination of the velocity of light. Later in early 20th century Albert Einstein showed that red shift could also arise from the gravity of an object.

Doppler had made a prescient remark : It is almost to be accepted with certainty that this will in the not too distant future offer astronomers a welcome means to determine the movements and distances of ..stars .The three important applications of Doppler Effect in modern astronomy are 1) the expansion of the universe, 2) the quasars and 3) dark matter

In the beginning of the 20th century the stage was set for great strides in cosmology with bigger and better telescopes.Harrow Shapley( 1885-1972) obtained in 1919 the correct shape and size of our galaxy with the sun placed quiet far from the center. Edwin Hubble(1889-1953) found that many nebulae including the well known Andromeda were not nearby and were actually external galaxies. Thus Hubble showed that the universe is much bigger than what was imagined. Later Hubble improved upon Slipher's observations that some celestial objects were receding very fast from us . Using Doppler Shift measurements Hubble collected data from 46 external galaxies and foudn that while all galaxies were indeed going away very fast, farther galaxies were going faster that the nearer ones. Thus he found a simple relation between the distance of the galaxy and its velocity with a constant( eventually called Hubble's Constant) connecting them. This showed clearly that the univese is expanding which can be envisioned thus : if one starts putting air into a b balloon the distance between the dots on the balloon keeps on increasing . The age of the unvierse can also be determined from the Hubble's consatant. This discovry of expansion of the universe is considered as one of the greatest observations of modern science and was also one of the proofs for The Big Bang theory.

In the early 1960s there were surprises in the relatively infant field of radio astronomy. While only big galaxies were expected to give copious radio emission , it was found that some very small regions of the sky also gave out lot of radio waves. When optical astronomers looked at light from these objects, they found enormous red shift which confounded them because they implied very large distances. The distance of one of these objects was about 2.4 billion light years, about 2000 times farther than Andromeda galaxy. Its intrinsic brightness was that of a trillion suns. These came to be known as Quasars (Quasi Stellar radio sources), the most powerful and distant astronomical sources known to man. The farthest quasar known at present is 12.9 billion light yeras away and must have been fromed in the infancy of our universe. Today they areconsidered as as members of a still larger group called Active Galctic Nuclei (AGN) which contain very huge ( a billion solar masses) black holes at their center.

There was a big surprise in the early 1970s when the speed of stars in galaxy was determined with the help of Doppler Shift. It was found that outer stars of the galaxy did not travel at lower speeds compared to those near the center. In the case of the solar system the speed of outer planets is much lesseer than that of the inner ones (e.g. the speed of earth/saturn round the sun is 30 / 10 kilometers per hour). The way to speed up the outer planets would be to add more mass to the solar system between the planets. By the same argument each galaxy is surrounded by significant amounts of unknown matter. Such ' galactic rotation curves' found by Vera Rubin and others have shown the existence of Dark Matter in the universe which is still not understood.

Apart from these ' celstial' uses of doppler effect, it finds great use in terestrial matters also like weather patterns , radar, Echocardipgram, Doppler scans for the health of the foetus and possible problems of blood vessels. So let us remember Christian doppler next time we hear the varying pitch of the siren of an ambulance !

Pic 1 - Doppler effect in light

Pic 2 - Expanding Universe 

 

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