Scope
The theory of relativity transformed physics and astronomy during the 20th century. When first published, relativity superseded a 200-year-old theory of mechanics elucidated by Isaac Newton. It changed perceptions.
However, Einstein denied that Newton could ever be superseded by his own work.
The theory of relativity overturned the concept of motion from Newton's day, by positing that all motion is relative. Time
was no longer uniform and absolute. Physics could no longer be
understood as space by itself, and time by itself. Instead, an added
dimension had to be taken into account with curved spacetime. Time now depended on velocity, and contraction became a fundamental consequence at appropriate speeds.
In the field of physics, relativity catalyzed and added an essential depth of knowledge to the science of elementary particles and their fundamental interactions, along with ushering in the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.
Two-theory view
The theory of relativity was representative of more than a single new physical theory.
It affected the theories and methodologies across all the physical
sciences. However, as stated above, this is more likely perceived as two
separate theories. There are some explanations for this. First, special relativity was published in 1905, and the final form of general relativity was published in 1916.
Second, special relativity fits with and solves for elementary particles and their interactions, whereas general relativity solves for the cosmological and astrophysical realm (including astronomy).
Third, special relativity was widely accepted in the physics
community by 1920. This theory rapidly became a significant and
necessary tool for theorists and experimentalists in the new fields of atomic physics, nuclear physics, and quantum mechanics.
Conversely, general relativity did not appear to be as useful. There
appeared to be little applicability for experimentalists as most
applications were for astronomical scales. It seemed limited to only
making minor corrections to predictions of Newtonian gravitation theory.
Its impact was not apparent until the 1930s.
Finally, the mathematics of general relativity
appeared to be incomprehensibly dense. Consequently, only a small
number of people in the world, at that time, could fully understand the
theory in detail. This remained the case for the next 40 years. Then, at
around 1960 a critical resurgence in interest occurred which has
resulted in making general relativity central to physics and astronomy.
New mathematical techniques applicable to the study of general
relativity substantially streamlined calculations. From this physically
discernible concepts were isolated from the mathematical complexity.
Also, the discovery of exotic astronomical phenomena in which general relativity was crucially relevant, helped to catalyze this resurgence. The astronomical phenomena included quasars (1963), the 3-kelvin microwave background radiation (1965), pulsars (1967), and the discovery of the first black hole candidates (1971).
On the theory of relativity
Einstein stated that the theory of relativity belongs to the class of
"principle-theories". As such it employs an analytic method. This means
that the elements which comprise this theory are not based on
hypothesis but on empirical discovery. The empirical discovery leads to
understanding the general characteristics of natural processes.
Mathematical models are then developed which separate the natural
processes into theoretical-mathematical descriptions. Therefore, by
analytical means the necessary conditions that have to be satisfied are
deduced. Separate events must satisfy these conditions. Experience
should then match the conclusions.
The special theory of relativity and the general theory of relativity
are connected. As stated below, special theory of relativity applies to
all inertial physical phenomena except gravity. The general theory
provides the law of gravitation, and its relation to other forces of
nature.
Special relativity
Main article: Special relativity
USSR stamp dedicated to Albert Einstein
Special relativity is a theory of the structure of spacetime. It was introduced in Einstein's 1905 paper "On the Electrodynamics of Moving Bodies" (for the contributions of many other physicists see History of special relativity). Special relativity is based on two postulates which are contradictory in classical mechanics:
- The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity).
- The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.
The resultant theory agrees with experiment better than classical mechanics, e.g. in the Michelson-Morley experiment that supports postulate 2, but also has many surprising consequences. Some of these are:
- Relativity of simultaneity:
Two events, simultaneous for one observer, may not be simultaneous for
another observer if the observers are in relative motion.
- Time dilation: Moving clocks are measured to tick more slowly than an observer's "stationary" clock.
- Length contraction: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
- Mass–energy equivalence: E = mc2, energy and mass are equivalent and transmutable.
- Maximum speed is finite: No physical object, message or field line can travel faster than the speed of light in a vacuum.
The defining feature of special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations. (See Maxwell's equations of electromagnetism and introduction to special relativity).
General relativity
General relativity is a theory of gravitation developed by Einstein in the years 1907–1915. The development of
general relativity began with the equivalence principle, under which the states of
accelerated motion and being at rest in a gravitational field (for example when standing on the surface of the Earth) are physically identical. The upshot of this is that free fall is inertial motion; an object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics. This is incompatible with classical mechanics and special relativity
because in those theories inertially moving objects cannot accelerate
with respect to each other, but objects in free fall do so. To resolve
this difficulty Einstein first proposed that spacetime is curved. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.
Some of the consequences of general relativity are:
- Clocks run more slowly in deeper gravitational wells. This is called gravitational time dilation.
- Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars).
- Rays of light bend in the presence of a gravitational field.
- Rotating masses "drag along" the spacetime around them; a phenomenon termed "frame-dragging".
- The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light.
Technically, general relativity is a metric theory of gravitation whose defining feature is its use of the Einstein field equations. The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move inertially
