After graduation Einstein was unable to obtain a regular position
for two years and did occasional tutoring and substitute teaching,
until he was appointed an examiner in the Swiss Patent Office at
Berne. The seven years Einstein spent at this job, with only evenings
and Sundays free for his own scientific work, were years in which
he laid the foundations of large parts of twentieth-century physics.
They were probably also the happiest years of his life. He liked
the fact that his job was quite separate from his thoughts about
physics, so that he could pursue these freely and independently,
and he often recommended such an arrangement to others later on.
In 1903 Einstein married Mileva Maric, a Serbian girl who had been
a fellow student in Zurich. Their two sons were born in Switzerland.
Einstein received his doctorate in 1905 from the
University of Zurich for a dissertation entitled, “Eine neue
Bestimmung der Moleküldimensionen” (“A New Determination
of Molecular Dimensions”), a work closely related to his studies
of Brownian motion. It took only a few years until he received academic
recognition for his work, and then he had a wide choice of positions.
His first appointment, in 1909, was as associate professor (extraordinarius)
of physics at the University of Zurich. This was followed quickly
by professorships at the German University in Prague, in 1911, and
at the Polytechnic in Zurich, in 1912. Then, in the spring of 1914,
Einstein moved to Berlin as a member of the Prussian Academy of
Sciences and director of the Kaiser Wilhelm Institute for Physics,
free to lecture at the university or not as he chose. As it turned
out, he found the scientific atmosphere in Berlin very stimulating,
and he greatly enjoyed having colleagues like Max Planck, Walther
Nernst, and, later, Erwin Schödinger and Max von Laue. During
World War 1, Einstein’s scientific work reached a culmination
in the general theory.of’relativity, but in most other ways
his life did not go well.
Mileva Einstein and their two sons spent the war
years in Switzerland and the Einsteins were divorced soon after
the end of the war. Einstein then married his cousin Elsa, a widow
with two daughters. Einstein’s health suffered, too. One of
his few consolations was his continued correspondence and occasional
visits with his friends in the Netherlands-Paul Ehrenfest and H.
A. Lorentz, especially the latter, whom Einstein described as having
“meant more to me personally than anybody else I have met
in my lifetime” and as “the greatest and noblest man
of our times.”
Einstein became suddenly famous to the world at
large when the deviation of light passing near the sun, as predicted
by his general theory of relativity, was observed during the solar
eclipse of 1919. His name and the term relativity became household
words. The publicity, even notoriety, that ensued changed the pattern
of Einstein’s life.
In 1933 Einstein was considering an arrangement
that would have allowed him to divide his time between Berlin and
the new Institute for Advanced Study at Princeton. But when Hitler
came to power in Germany, he promptly resigned his position at the
Prussian Academy and joined the Institute. Princeton became his
home for the remaining twenty-two years of his life. He became an
American citizen in 1940.
During the 1930’s Einstein was convinced that the menace to
civilization embodied in Hitler’s regime could be put down
only by force. In 1939, at the request of Leo Szilard, Edward Teller,
and Eugene Wigner, he wrote a letter to President Franklin D. Roosevelt
pointing out the dangerous military potentialities offered by nuclear
fission and warning him of the possibility that Germany might be
developing nuclear weapons. This letter helped to initiate the American
efforts that eventually produced the nuclear reactor and the fission
bomb, but Einstein neither participated in nor knew anything about
Einstein received a variety of honours in his
lifetime – from the 1921 Nobel Prize in physics to an offer
(which he did not accept) of the presidency of Israel after Chaim
Weizmann’s death in 1952.
One of Einstein’s last acts was his signing
of a plea, initiated by Bertrand Russell, for the renunciation of
nuclear weapons and the abolition of war. He was drafting a speech
on the current tensions between Israel and Egypt when he suffered
an attack due to an aortic aneurysm; he died a few days later. But
despite his concern with world problems and his willingness to do
whatever he could to alleviate them, his ultimate loyalty was to
his science. As he said once with a sigh to an assistant during
a discussion of political activities: “Yes, time has to be
divided this way, between politics and our equations. But our equations
are much more important to me, because politics is for the present,
but an equation like that is something for eternity.”
Einstein’s early interests lay in statistical
mechanics and intermolecular forces. However, his predominant concern
throughout the career was the search for a unified foundation for
all of physics. The disparity between the discrete particles of
matter and the continuously distributed electromagnetic field came
out most clearly in Lorentz’ (1853-1928) electron theory,
where matter and field were sharply separated for the first time.
This theory strongly influenced Einstein. The problems generated
by the incompatibility between mechanics and electromagnetic theory
at several crucial points claimed his attention. His strengths with
these problems led to his most important early work – the
special theory of relativity and the theory of quanta in 1905.
The discovery of X-rays, radioactivity, the electron
and the quantum theory brought about a sea change in our ideas and
understanding of phenomena at the atomic level. The world of Physics
was, however, changing in far reaching ways - with ramifications
for our understanding of the very shape of time, space and the universe.
This part of the revolution was brought about Albert Einstein, a
brilliant and creative theorist and the only thinker ever to be
ranked in the same class as Newton. To understand this part of the
revolution, we shall need to go back to James Clerk Maxwell (1831-1879)
and his ideas about light.
Ether – Unbroken from star to star
Maxwell had introduced a revolutionary set of equations that predicted
the existence of electromagnetic fields and established that magnetism,
electricity and light were a part of the same spectrum: the electromagnetic
spectrum. Light, he maintained, was a wave, not a particle, and
he thought that it travelled through an invisible medium he called
“the ether”, which filled all space. But physicists
began to see a problem, not with Maxwell’s electromagnetic
field equations, but with his ideas about the ether.
Maxwell wasn’t the first to come up with
this idea that some invisible medium called the ether must fill
the vastness of space, extending “unbroken from star to star”.
It dated back to the time of ancient Greeks. “There can be
no doubt,” Maxwell said in a lecture in 1873, “that
the interplanetary and interstellar spaces are not empty but are
occupied by a material substance or body, which is certainly the
largest, and probably the most uniform, body of which we have any
knowledge”. The idea of the ether seemed necessary because,
if light was a wave, it seemed obvious that it had to be a wave
travelling in some medium. But accepting what “seems obvious”
is not the way to do good science; if the ether existed, it should
be possible to find some proof of its existence.
The most famous “failed” experiment
Albert Michelson (1852-1931), an American Physicist,
had an idea . If the ether that filled the universe were stationary,
then the planet Earth would meet resistance as it moved through
the ether, creating a current, a sort of “wind”, in
the ether. So it followed that a light beam moving with the current
ought to be carried along by it, whereas a light beam travelling
against the current should be slowed. While studying with Hermann
von Helmholtz (1821-1894) in Germany, in 1881 Michelson built an
instrument called an interferometer, which could split a beam of
light, running the two halves perpendicular to each other, and then
rejoin the split beam in a way that made it possible to measure
differences in the speeds with great precision.
Michelson ran his experiment, but he was puzzled
by his results. They showed no differences in light velocity for
the two halves of the light beam. He concluded, “The result
of the hypothesis of a stationary ether is …. shown to be
incorrect, and the necessary conclusion follows that the hypothesis
But may be his results were wrong. He tried his
experiment again and again, each time trying to correct for any
possible error. Finally, in 1887, joined by Edward Morley, Michelson
tried a test in Cleveland, Ohio. Using improved equipment, and taking
every imaginable precaution against inaccuracy, this time surely
they would succeed in detecting the ether. But the experiment failed
again. Let us briefly describe the salient features of this momentous
If there is an ether pervading space, we move through it with at
least the 3x104 m/sec speed of the earth’s orbital motion
about the sun; if the sun is also in motion, our speed through the
ether is even greater (Motions of the Earth through a hypothetical
ether). From the point of view of an observer on the earth,
the ether is moving past the earth. To detect this motion, we can
use the pair of light beams formed by a half silvered mirror (The
Michelson - Morley experiment). One of these light beams
is directed to a mirror along a path perpendicular to the ether
current, while the other goes to a mirror along a path parallel
to the ether current. The optical arrangement is such that both
beams return to the same viewing screen. The purpose of the clear
glass plate is to ensure that both beams pass through the same thickness
of air and glass.
If the path lengths of the two beams are exactly
the same, they will arrive at the screen in phase and will interfere
constructively to yield a bright field of view. The presence of
an ether current in the direction shown, however, would cause the
beams to have different transit times in going from the half silvered
mirror to the screen, so that they would no longer arrive at the
screen in phase but would interfere destructively. In essence this
is the famous experiment performed in 1887 by Michelson and Morley.
In the actual experiment the two mirrors are not
perfectly perpendicular, with the result that the viewing screen
appears crossed with a series of bright and dark interference fringes
due to differences in path length between adjacent light waves (Fringe
Pattern observed in Michelson - Morley experiment). If
either of the optical paths in the apparatus is varied in length,
the fringes appear to move across the screen as reinforcement and
cancellation of the waves succeed one another at each point. The
stationary apparatus, then, can tell us nothing about any time difference
between the two paths. When the apparatus is rotated by 90°,
however, the two paths change their orientation relative to the
hypothetical ether stream, so that the beam formerly requiring the
time tA (along parth A) for the round trip now required tB (along
path B) and vice versa. If these times are different, the fringes
will move across the screen during the rotation.
This information can be used to calculate the fringe
shift expected on the basis of the ether theory. The expected fringe
shift ‘n’ in each path when the apparatus is rotated
by 90° is given by
n = Dv2 / ?c2 ;
Here, D is the distance between half silvered
mirror and each of the other mirrors (made about 10 metres using
multiple reflections), v is the ether speed - which is the Earth’s
orbital speed 3x104 (m/s), c is the speed of the light = 3x108 m/sec,
and l is the wave length of light used, about 5000Å (1Å=10-10m),
one then obtains n=0.2 fringe.
Since both paths experience this fringe shift,
the total shift should amount to 2n or 0.4 fringe. A shift of this
magnitude is readily observable, and therefore, Michelson and Morley
looked forward to establishing directly the existence of the ether.
To everybody’s surprise, no fringe shift whatever was found.
When the experiment was performed at different seasons of the year
and in different locations, and when experiments of other kinds
were tried for the same purpose, the conclusions were always identical:
no motion through the ether was detected.
The negative result of the Michelson-Morley
experiment had two consequences. First, it rendered untenable
the hypothesis of the ether by demonstrating that the ether has
no measurable properties – an ignominious end for what had
once been a respected idea. Second, it suggested a new physical
principle: the speed of light in free space is the same everywhere,
regardless of any motion of source or observer. As a result, the
Michelson-Morley experiment has become the most famous “failed”
experiment in the history of science. They had started out to study
the ether, only to conclude that the ether did not exist. But if
this were true, how could light move in “waves” without
a medium to carry it? What’s more, the experiment indicated
that the velocity of light is always constant.
It was a completely unexpected conclusion. But
the experiment was meticulous and the results irrefutable. Lord
Kelvin (1824-1907), said in a lecture in 1900 at the Royal Institution
that Michelson and Morley’s experiment had been “carried
out with most searching care to secure a trustworthy result,”
casting “a nineteenth century cloud over the dynamic theory
of light”. The conclusion troubled physicists everywhere,
though. Apparently, they were wrong about the existence of the ether
– and if they were wrong, then light was a wave that somehow
could travel without a medium to travel through. What’s more,
the Michelson - Morley results seemed to call into question the
kind of Newtonian relativity that had been around for a couple of
centuries and by this time was well tested; the idea that the speed
of an object can differ, depending upon the reference frame of the
observer. Suppose two cars are travelling along on a road. (There
weren’t many cars or roads in 1887, but one gets the idea.).
One car is going 80 kms per hour, the other 75 kms per hour. To
the driver of the slower car, the faster car would be gaining ground
at a rate of 5 kms per hour. Why would light be any different?
But that’s just what the Michelson and Morley
experiment had shown; Light does behave differently. The velocity
of light is always constant – no matter what. Astronauts travelling
in their spaceship at a speed of 2,90,000 km/sec alongside a beam
of light (which travels at 3,00,000 km/sec) would not perceive the
light gaining on them by 10,000 km/sec. They would see light travelling
at a constant 3,00,000 km/sec. The speed of light is a universal
According to Einstein's views, space and
time are more intimately connected with one another than it
was supposed before and with in certain limits, the notion
of space may be substituted by the notion of time and vice
versa. To make this statement more clear, let us consider
a passenger in a train having his meal in the dining car.
The waiter serving him will know that the passenger ate his
soup, meals and dessert in. the same place, that is, at the
same able in the dining car. But, from the point of view of
a person on the ground, the same passenger consumed the three
courses at points along the track separated by many kilometres.
We Can hence make the following trivial statement: Events
taking place in the same place but at different times in a
moving system will be considered by a ground observer as taking
place at different places.
Now, following Einstein's idea concerning
the reciprocity of space and time, let us replace in the above
statement the word "place" by the word "time"
and vice versa. The statement will now read: Events taking
place at the same time but In different places in a moving
system will be considered by a ground observer as taking place
at different times. This statement is far from being trivial.
It means that if, for example, two passengers at the far ends
of the dining car had their after-dinner coffee sipped simultaneously
from the point of view of the dining-car waiter, the person
standing on the ground will insist that the coffee was sipped
at different times! Since according to the principle of relativity,
neither Of the two reference systems should be 'preferred
to the other (the train moves relative to the ground or the
ground moves relative to the train), we do not have any reason
to take the waiter's impression as being true and ground observer's
impression as being wrong or vice versa. Of course, this would
not be apparent to you If you were the ground observer. This
is so because the distance of, say, 30 metres between two
passengers sipping their after dinner coffee at opposite ends
of the dinning car translates into a time interval of only
10-8 seconds, and there is no wonder that this is not apparent
to our senses. It would become appreciable when the train
travels close to the speed of light.
The transformation of time intervals into
space Intervals and vice versa was given a simple geometrical
interpretation by the German mathematician H. Minkowski. He
proposed that time or duration be considered as the fourth
dimension supplementing the three spatial dimensions (x, y,
z) and that transformation from one system of reference to
another be considered as a rotation of co-ordinates systems
in this four dimensional space. A point in these four dimensional
space is called an event. Relativistic effects like the length
contraction and the time dilation then become consequences
of the rotation of these space-time coordinates.
These effects being relative, each of the
two observers moving with respect to one another will see
the other fellow as somewhat flattened in the direction of
his motion and will consider his watch to be slow!
The Special Theory of Relativity:
Surprisingly, Einstein never received a Nobel
prize for the most important paper that he published in 1905, the
one that dealt with a theory that came to be known as the special
theory of relativity.
He also tossed out the idea of the ether, which
Michelson and Morley had called into question. Maxwell needed it
because he thought light travelled in waves, and if that were so,
he thought, it needed some medium in which to travel. But what if,
as Max Planck’s (1858-1947) quantum theory stated, light travels
in discrete packets or quanta? Then it would act more like particles
and wouldn’t require any medium to travel in.
By making these assumptions — that the velocity
of light is a constant, that there is no ether, that light travels
in quanta and that motion is relative — he was able to show
why the Michelson - Morley experiment came out as it did, without
calling the validity of Maxwell’s electromagnetic equations
into question. But, where does “relativity” enter?
We mentioned earlier the role of the ether as
a universal frame of reference with respect to which light waves
were supposed to propagate. Whenever we speak of “motion”,
of course, we really mean motion relative to a “frame of reference”.
The frame of reference may be a road, the earth’s surface,
the sun, the center of our galaxy; but in every case we must specify
it. Stones dropped in New Delhi and in Washington both fall “down”,
and yet the two move in opposite directions relative to the earth’s
center. Which is the correct location of the frame of reference
in this situation, the earth’s surface or its center? The
answer is that all frames of reference are equally correct, although
one may be more convenient to use in a specific case. If there were
an ether pervading all space, we could refer all motion to it, and
the inhabitants of New Delhi and Washington would escape from their
quandary. The absence of an ether, then, implies that there is no
universal frame of reference, so that all motion exists solely relative
to the person or instrument observing it.
The theory of relativity resulted from an analysis
of the physical consequences implied by the absence of a universal
frame of reference. The special theory of relativity treats problems
involving the motion of frames of reference at constant velocity
(that is, both constant speed and constant direction) with respect
to one another; the general theory of relativity, proposed by Einstein
a decade later, treats problems involving frames of reference accelerated
with respect to one another. The special theory has had a profound
influence on all of physics.
The paper in which the young Albert Einstein in
1905 set out the special theory of relativity confronted common
sense with several new and disquieting ideas. It abolished the ether,
and it showed that matter and energy are equivalent. The new ideas
derive from the central conception of relativity: that time does
not run at the same pace for every observer. This bold conception
lies at the heart of modern physics, all the way from the atomic
to the cosmic scale. Yet it is still hard to grasp, and the paradoxes
it pose continue to puzzle and to stimulate each generation of physicists.
The special theory of relativity is based upon
two axioms. The first states that the laws of physics may be expressed
in equations having the same form in all frames of reference moving
at constant velocity with respect to one another. This axiom expresses
the absence of a universal frame of reference. If the laws of physics
had different forms for different observers in relative motion,
it could be determined from these differences which objects are
“stationary” in space and which are “moving”.
But because there is no universal frame of reference, this distinction
does not exist in nature; hence the above axiom. Consequently, this
axiom implies that two observers, each of whom appears to the other
to be moving with a constant speed in a straightline, cannot tell
which of them is moving.
The second axiom of special relativity states
that the speed of light in free space has the same value for all
observers, regardless of their state of motion. This axiom follows
directly from the result of the Michelson - Morley experiment, and
implies that when both observers measure the speed of light, they
will get the same answer.
Neither of these axioms was new in itself. The
first axiom had long been implicit in the accepted laws of mechanics.
The second one was beginning to be accepted as the natural interpretation
of Michelson and Morley’s experiment in 1887. What was new,
then, in Einstein’s analysis was not one axiom or the other
but the confrontation of the two. They form the two principles of
relativity not singly but together. This is how Einstein presented
them jointly at the beginning of his paper.
So basically, in the special theory of relativity
Einstein revamped Newtonian physics such that when he worked out
the formulas, the relative speed of light always stayed the same.
It never changes relative to anything else, even though other things
change relative to each other. Mass, space and time all vary depending
upon how fast you move. As observed by others, the faster you move,
the greater your mass, the less space you take up and the more slowly
time passes for you! The more closely you approach the speed of
light, the more pronounced these effects become. Let us have a look
at some of the consequences of the theory of relativity.
It follows at once from the two axioms combined
that we have to revise the traditional idea of time. By tradition
we take it for granted that time is the same everywhere and for
everyone. Why not? It seems natural to assume that time is a universal
“now” for every traveller anywhere in the universe.
But, according to the theory of special relativity, time cannot
run at the same pace for two observers, one of whom is moving relative
to the other, if they are to get the same speed (that is for light)
when they time a beam of light that is moving with one of them.
Consider this example.
If you were an astronaut travelling at 90 percent
of the speed of light (about 2,70,000 kms per second), you could
travel for five years (according to your calendar watch) and you’d
return to Earth to find that 10 years had passed for the friends
you’d left behind. Or, if you could rev up your engines to
help you travel at 99.99 percent of the speed of light, after traveling
for only 6 months you’d find that 50 years had sped by our
Earth during your absence!
Clocks moving with respect to an observer appear
to tick less rapidly than they do when at rest with respect to him.
If we, in the S frame, or the stationary frame of reference,