The theory of → gravitation developed by Albert Einstein
(1916) that describes the gravitation as the → space-time
curvature caused by the presence of matter or energy.
Mass creates a → gravitational field which
distorts the space and changes the flow of time. In other words, mass
causes a deviation of the → metric of
space-time continuum from that of the "flat" space-time structure described by the
→ Euclidean geometry and treated in
→ special relativity.
General relativity developed from the
→ principle of equivalence between
gravitational and inertial forces.
According to general relativity, photons follow a curved path
in a gravitational field. This prediction was confirmed by the
measurements of star positions near the solar limb during the
total eclipse of 1919. The same effect is seen in the
delay of radio signals coming from distant space probes when
grazing the Sun's surface. Moreover, the space curvature caused by
the Sun makes the → perihelion
of Mercury's orbit advance by 43'' per century more than that predicted by
Newton's theory of gravitation. The → perihelion advance
can reach several degrees per year for
→ binary pulsar orbits.
Another effect predicted by general
relativity is the → gravitational reddening.
This effect is verified in the → redshift
of spectral lines in the
solar spectrum and, even more obviously, in
→ white dwarfs. Other predictions of the theory include
→ gravitational lensing,
→ gravitational waves, and the
invariance of Newton's → gravitational constant.

The Newton's equations of motion, if they hold in any
→ reference frame,
they are valid also in any other reference frame moving with uniform
velocity relative to the first.

The first postulate in Einstein's theory of
→ special relativity whereby all the laws of physics
are the same in every → inertial reference frame.
In other words, no physical measurement can distinguish one
inertial reference frame from another.
See also → principle of constancy.

A basic concept of → special relativity
whereby → events that are simultaneous in one
→ reference frame are not simultaneous in another
reference frame moving with respect to the first.

The requirement employed by Einstein in his relativity theories, that the
equations describing the laws of physics are the same in all frames of reference.
This statement and that of the constancy of
the speed of light constitute the founding principles of special
relativity.

The theory formulated by A. Einstein in 1905, which is based on the following
two → postulates:
1) → Principle of relativity:
The laws of physical phenomena are the same when studied
in terms of two reference systems moving at a constant velocity
relative to each other.
2) → Principle of constancy:
The → velocity of light in free space is the same for all
observers and is independent of the relative velocity of the source of
light and the observer.
The term "special theory of relativity" refers to the restriction in the
first postulate to reference systems moving at a constant velocity relative
to each other (→ inertial reference frame).
See also → general relativity.