in Deccan Herald of 15 sep 2015
http://www.deccanherald.com/content/500952/doppler-effect-origin-findings-evolution.html
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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