Doppler effect: origin, findings and evolution
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Perception Doppler effect in light.
In 1842, Christian Doppler proposed a new theory in which a person watching a source of waves would find an increase in frequency as it comes near him and a decrease in frequency 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 and ‘B’ is moving towards him.
‘A’ receives 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, Christian Doppler believed that his theory would not be proved in his lifetime since he could not imagine a fast-moving source to demonstrate his theory.
After a few years, one man felt that an experiment could be done in trains which had achieved a speed of about 60 km per hour. He was Buys Ballot, a Dutch meteorologist who managed to get the above experiment done on a train, thus providing the first proof what came to be known as the Doppler effect. Armand Fizeau, a Frenchman, applied the Doppler effect to the electromagnetic spectrum in 1948.
Light comprises different colours, each of them signifying a different frequency. Therefore, a light source going away from or coming towards us will show a spectrum with a shift to the red or blue respectively.
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). Armand 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 theory 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 the Doppler effect in modern astronomy are — in establishing the expansion of the universe, the quasars and the dark matter.
In the beginning of 20th century, the stage was set for great strides in cosmology with bigger and better telescopes. Harlow Shapley, in 1919, obtained the correct shape and size of our galaxy with the Sun placed quite far from the centre. Edwin Hubble 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.
Celestial applications
Later, Hubble improved upon Shapely’s observations that some celestial objects were receding very fast from us. Using Doppler effect measurements, Hubble collected data from 46 external galaxies and found that while all galaxies were indeed going away very fast, farther galaxies were going faster than 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 universe is expanding. The age of the universe can also be determined from the Hubble’s constant. This discovery of the 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 emit copious radio emission, it was found that some very small regions of the sky also gave out a lot of radio waves. When optical astronomers looked at light from these objects, they found enormous red shifts which confounded them because they implied very large distances.
The distance of one of these objects was about 2.4 billion light years, about 2,000 times farther than Andromeda galaxy. Its intrinsic brightness was that of 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 years away and must have been formed in the infancy of our universe. Today they are considered as members of a still larger group called Active Galactic Nuclei (AGN) which contain very huge (a billion solar masses) black holes at their centre.
There was a big surprise in the early 1970s when the speed of stars in the galaxy was determined with the help of Doppler effect. It was found that outer stars of the galaxy did not travel at lower speeds compared with those near the centre. In the case of the solar system, the speed of outer planets is much lesser than that of the inner ones.
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 has still not been understood.
Apart from these “celestial” uses of doppler effect, it finds great use in terrestial matters like understanding weather patterns, radar, echocardiogram, Doppler scans for the health of the foetus and possible problems of blood vessels. So the next time we hear the varying pitch of the siren of an ambulance, let us remember Christian Doppler and his contributions.
‘A’ receives 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, Christian Doppler believed that his theory would not be proved in his lifetime since he could not imagine a fast-moving source to demonstrate his theory.
After a few years, one man felt that an experiment could be done in trains which had achieved a speed of about 60 km per hour. He was Buys Ballot, a Dutch meteorologist who managed to get the above experiment done on a train, thus providing the first proof what came to be known as the Doppler effect. Armand Fizeau, a Frenchman, applied the Doppler effect to the electromagnetic spectrum in 1948.
Light comprises different colours, each of them signifying a different frequency. Therefore, a light source going away from or coming towards us will show a spectrum with a shift to the red or blue respectively.
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). Armand 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 theory 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 the Doppler effect in modern astronomy are — in establishing the expansion of the universe, the quasars and the dark matter.
In the beginning of 20th century, the stage was set for great strides in cosmology with bigger and better telescopes. Harlow Shapley, in 1919, obtained the correct shape and size of our galaxy with the Sun placed quite far from the centre. Edwin Hubble 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.
Celestial applications
Later, Hubble improved upon Shapely’s observations that some celestial objects were receding very fast from us. Using Doppler effect measurements, Hubble collected data from 46 external galaxies and found that while all galaxies were indeed going away very fast, farther galaxies were going faster than 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 universe is expanding. The age of the universe can also be determined from the Hubble’s constant. This discovery of the 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 emit copious radio emission, it was found that some very small regions of the sky also gave out a lot of radio waves. When optical astronomers looked at light from these objects, they found enormous red shifts which confounded them because they implied very large distances.
The distance of one of these objects was about 2.4 billion light years, about 2,000 times farther than Andromeda galaxy. Its intrinsic brightness was that of 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 years away and must have been formed in the infancy of our universe. Today they are considered as members of a still larger group called Active Galactic Nuclei (AGN) which contain very huge (a billion solar masses) black holes at their centre.
There was a big surprise in the early 1970s when the speed of stars in the galaxy was determined with the help of Doppler effect. It was found that outer stars of the galaxy did not travel at lower speeds compared with those near the centre. In the case of the solar system, the speed of outer planets is much lesser than that of the inner ones.
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 has still not been understood.
Apart from these “celestial” uses of doppler effect, it finds great use in terrestial matters like understanding weather patterns, radar, echocardiogram, Doppler scans for the health of the foetus and possible problems of blood vessels. So the next time we hear the varying pitch of the siren of an ambulance, let us remember Christian Doppler and his contributions.