Albert Einstein 
{ 1879 - 1955 }
A Selection of Quotes
 Human beings and their circle of compassion
- "A human being is part of the whole called by us universe , a part limited in time and space.
- We experience ourselves, our thoughts and feelings as something separate from the rest.
- A kind of optical delusion of consciousness.
- This delusion is a kind of prison for us, restricting us to our personal desires and to affection for a few persons nearest to us.
- Our task must be to free ourselves from the prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty...
- We shall require a substantially new manner of thinking if mankind is to survive."
The Tree of Life ...
- "All religions, arts and sciences are branches of the same tree.
- All these aspirations are direected toward ennobling man's life, lifting it from the sphere of mere physical existence and leading the individual towards freedom."
The mystic emotion, knowledge, and religious sentiment
- "The finest emotion of which we are capable is the mystic emotion.
- Herein lies the germ of all art and all true science.
- Anyone to whom this feeling is alien, who is no longer capable of wonderment and lives in a state of fear is a dead man.
- To know that what is impenatrable for us really exists and manifests itself as the highest wisdom and the most radiant beauty, whose gross forms alone are intelligible to our poor faculties - this knowledge, this feeling ... that is the core of the true religious sentiment.
- In this sense, and in this sense alone, I rank myself amoung profoundly religious men."
What is the Theory of Relativity?
article by Albert Einstein (1919)
(The London Times, November 28)
I GLADLY accede to the request of your
colleague to write something for The Times on relativity.
After the lamentable breakdown of the old active intercourse between
men of learning, I welcome this opportunity of expressing my feelings
of joy and gratitude toward the astronomers and physicists of
England. It is thoroughly in keeping with the great and proud
traditions of scientific work in your country that eminent scientists
should have spent much time and trouble, and your scientific institutions
have spared no expense, to test the implications of a theory which
was perfected and published during the war in the land of your
enemies. Even though the investigation of the influence of the
gravitational field of the sun on light rays is a purely objective
matter, I cannot forbear to express my personal thanks to my English
colleagues for their work; for without it I could hardly have
lived to see the most important implication of my theory tested.
We can distinguish various kinds of theories in physics.
Most of them are constructive. They attempt to build up a picture
of the more complex phenomena out of the materials of a relatively
simple formal scheme from which they start out. Thus the kinetic
theory of gases seeks to reduce mechanical, thermal, and diffusional
processes to movements of molecules -- i.e., to build them up
out of the hypothesis of molecular motion. When we say that we
have succeeded in understanding a group of natural processes,
we invariably mean that a constructive theory has been found which
covers the processes in question.
Along with this most important class of theories there exists
a second, which I will call "principle-theories." These
employ the analytic, not the synthetic, method. The elements which
form their basis and starting-point are not hypothetically constructed
but empirically diseovered ones, general characteristics of natural
processes, principles that give rise to mathematically formulated
criteria which the separate processes or the theoretical representations
of them have to satisfy. Thus the science of thermodynamics seeks
by analytical means to deduce necessary conditions, which separate
events have to satisfy, from the universally experienced fact
that perpetual motion is impossible.
The advantages of the constructive theory are completeness,
adaptability, and clearness, those of the principle theory are
logical perfection and security of the foundations.
The theory of relativity belongs to the latter class. In
order to grasp its nature, one needs first of all to become acquainted
with the principles on which it is based. Before I go into these,
however, I must observe that the theory of relativity resembles
a building consisting of two separate stories, the special theory
and the general theory. The special theory, on which the general
theory rests, applies to all physical phenomena with the exception
of gravitation; the general theory provides the law of gravitation
and its relations to the other forces of nature.
It has, of course, been known since the days of the ancient
Greeks that in order to describe the movement of a body, a second
body is needed to which the movement of the first is referred.
The movement of a vehicle is considered in reference to the earth's
surface, that of a planet to the totality of the visible fixed
stars. In physics the body to which events are spatially referred
is called the coordinate system. The laws of the mechanics of
Galileo and Newton, for instance, can only be formulated with
the aid of a coordinate system.
The state of motion of the coordinate system may not, however,
be arbitrarily chosen, if the laws of mechanics are to be valid
(it must be free from rotation and acceleration). A coordinate
system which is admitted in mechanics is called an "inertial
system." The state of motion of an inertial system is according
to mechanics not one that is determined uniquely by nature. On
the contrary, the following definition holds good: a coordinate
system that is moved uniformly and in a straight line relative
to an inertial system is likewise an inertial system.
By the "special principle of relativity" is meant the
generalization of this definition to include any natural event
whatever: thus, every universal law of nature which is valid in
relation to a coordinate system C, must also be valid,
as it stands, in relation to a coordinate system C',
which is in uniform translatory motion relatively to C.
The second principle, on which the special theory of relativity
rests, is the "principle of the constant velocity of light
in vacuo." This principle asserts that light in vacuo always
has a definite velocity of propagation (independent of the state
of motion of thc observer or of the source of the light). The
confidence which physicists place in this principle springs from
the successes achieved by the electrodynamics of Maxwell and Lorentz.
Both the above-mentioned principles are powerfully supported
by experience, but appear not to be logically reconcilable. The
special theory of relativity finally succeeded in reconciling
them logically by a modification of kinematics -- i.e., of the
doctrine of the laws relating to space and time (from the point
of view of physics). It became clear that to speak of the simultaneity
of two events had no meaning except in relation to a given coordinate
system, and that the shape of measuring devices and the speed
at which clocks move depend on their state of motion with respect
to the coordinate system.
But the old physics, including the laws of motion of Galileo
and Newton, did not fit in with the suggested relativist kinematics.
From the latter, general mathematical conditions issued, to which
natural laws had to conform, if the above-mentioned two principles
were really to apply. To these, physics had to be adapted. In
particular, scientists arrived at a new law of motion for (rapidly
moving) mass points, which was admirably confirmed in the case
of electrically charged particles. The most important upshot of
the special theory of relativity concerned the inert masses of
corporeal systems. It turned out that the inertia of a system
necessarily depends on its energy-content, and this led straight
to the notion that inert mass is simply latent energy. The principle
of the conservation of mass lost its independence and became fused
with that of the conservation of energy.
The special theory of relativity, which was simply a systematic
development of the electrodynamics of Maxwell and Lorentz, pointed
beyond itself, however. Should the independence of physical laws
of the state of motion of the coordinate system be restricted
to the uniform translatory motion of coordinate systems in respect
to each other? What has nature to do with our coordinate systems
and their state of motion? If it is nccessary for the purpose
of describing nature, to make use of a coordinate system arbitrarily
introduced by us, then the choice of its state of motion ought
to be subject to no restriction; the laws ought to be entirely
independent of this choice (general principle of relativity).
The establishment of this general principle of relativity
is made easier by a fact of experience that has long been known,
namely, that the weight and the inertia of a body are controlled
by the same constant (equality of inertial and gravitational mass).
Imagine a coordinate system which is rotating uniformly with respect
to an inertial system in the Newtonian manner.
The centrifugal forces which manifest themselves in relation to
this system must, according to Newton's teaching, be regarded
as effects of inertia. But these centrifugal forces are, exactly
like the forces of gravity, proportional to the masses of the
bodies.
Ought it not to be possible in this case to regard the coordinate
system as stationary and the centrifugal forces as gravitational
forces? This seems the obvious view, but classical mechanics forbid
it.
This hasty consideration suggests that a general theory
of relativity must supply the laws of gravitation, and the consistent
following up of the idea has justified our hopes.
But the path was thornier than one might suppose, because
it demanded the abandonment of Euclidean geometry. This is to
say, the laws according to which solid bodies may be arranged
in space do not completely accord with the spatial laws attributed
to bodies by Euclidean geometry. This is what we mean when we
talk of the "curvature of space." The fundamental concepts
of the "straight line," the "plane," etc.,
thereby lose their precise significance in physics.
In the general theory of relativity the doctrine of space
and time, or kinematics, no longer figures as a fundamental independent
of the rest of physics. The geometrical behaviour of bodies and
the motion of clocks rather depend on gravitational fields, which
in their turn are produced by matter.
The new theory of gravitation diverges considerably, as
regards principles, from Newton's theory. But its practical results
agree so nearly with those of Newton's theory that it is difficult
to find criteria for distinguishing them which are accessible
to experience. Such have been discovered so far:
- In the revolution of the ellipses of the planetary orbits
round the sun (confirmed in the case of Mercury).
- In the curving of light rays by the action of gravitational
fields (confirmed by the English photography of eclipses).
- In a displacement of the spectral lines
toward the red end of the spectrum in the case of light transmitted
to us from stars of considerable magnitude (unconfirmed so far).
*
The chief attraction of the theory lies in its logical completeness.
If a single one of the conclusions drawn from it proves wrong,
it must be given up; to modify it without destroying the whole
structure seems to be impossible.
Let no one suppose, however, that the mighty work of Newton
can really be superseded by this or any other theory. His great
and lucid ideas will retain their unique significance for all
time as the foundation of our whole modern conceptual structure
in the sphere of natural philosophy.
Note: Some of the statements in your paper
concerning my life and person owe their origin to the lively imagination
of the writer. Here is yet another application of the principle
of relativity for the delectation of the reader: today I am described
in Germany as a "German savant," and in England as a
"Swiss Jew." Should it ever be my fate to be represented
as a bête noire, I should, on the contrary, become
a "Swiss Jew" for the Germans and a "German savant"
for the English.
footnote:
* this criterion has
since been confirmed
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