* relativity of simultaneity ( simultaneous events in one reference frame are not simultaneous in almost all frames moving relative to the first ).

This theory has a wide range of consequences which have been experimentally verified, including counter-intuitive ones such as length contraction, time dilation and relativity of simultaneity, contradicting the classical notion that the duration of the time interval between two events is equal for all observers.

The problem can be understood in terms of the relativity of simultaneity in special relativity, which says that different inertial reference frames will disagree on whether two events at different locations happened " at the same time " or not, and they can also disagree on the order of the two events ( technically, these disagreements occur when spacetime interval between the events is ' space-like ', meaning that neither event lies in the future light cone of the other ).

The notion of simultaneity depends on the frame of reference ( see relativity of simultaneity ), so switching between frames requires an adjustment in the definition of the present.

The special theory of relativity can be viewed as the introduction of operational definitions for simultaneity of events and of distance, that is, as providing the operations needed to define these terms.

Special relativity suggests that the concept of simultaneity is not universal: according to the relativity of simultaneity, observers in different frames of reference can have different perceptions of whether a given pair of events happened at the same time or at different times, with there being no physical basis for preferring one frame's judgments over another's ( though in a case where one event A happens in the past light cone of another event B, all frames will agree that A happened in the past of B ).

However, there are some, such as Dean Zimmerman, who have argued that it is possible to accept the physical predictions of relativity while adopting an alternative interpretation of the theory ( For instance, see Lorentz ether theory ) in which there is a single privileged frame whose judgments about length, time and simultaneity are the " true " ones, even though there would be absolutely no empirical way to distinguish this frame from other frames, and no real experience could identify it.

Eternalism takes its inspiration from physics, especially the Rietdijk-Putnam argument, in which the relativity of simultaneity is used to show that each point in the universe can have a different set of events that are in its present moment.

To speak of " the shape of the universe ( at a point in time )" is ontologically naive from the point of view of special relativity alone: due to the relativity of simultaneity we cannot speak of different points in space as being " at the same point in time " nor, therefore, of " the shape of the universe at a point in time ".

The fact that simultaneity depends on relative velocity caused problems for many scientists and laymen trying to accept relativity in the early days.

The idea of absolute simultaneity survived until 1905, when the special theory of relativity and its supporting experiments discredited it.

Within the framework of the theory and its terminology there is a relativity of simultaneity that affects how the specified events are aligned with respect to each other by observers in relative motion.

In relativity, temporal coordinate systems are set up using a procedure for synchronizing clocks, discussed by Poincaré ( 1900 ) in relation to Lorentz's local time ( see relativity of simultaneity ).

The common denominator between the special relativistic notions — the lack an absolute reference frame, metric transformations of the Lorenzian type, the relativity of simultaneity, the incorporation of the time dimension with three spatial dimensions — and the Cubist idea of mobile perspective ( observing the subject from several view-points simultaneously ) published by Jean Metzinger and Albert Gleizes was, in effect, a descendant from the work of Poincaré and others, at least from the theoretical standpoint.

Eventually, Albert Einstein ( 1905 ) was the first who completely removed the ad-hoc character from the contraction hypothesis, by demonstrating that this contraction was no dynamical effect in the aether, but rather a kinematic effect due to the change in the notions of space, time and simultaneity brought about by special relativity.

Yet in relativity theory the constancy of light velocity in all inertial frames in connection with the relativity of simultaneity destroys this equality.

In the special case of an inertial observer in special relativity, the time is measured using the observer's clock and the observer's definition of simultaneity.

These results came at the beginning of the golden age of general relativity, which was marked by general relativity and black holes becoming mainstream subjects of research.

* Emission theory, a competing theory for the special theory of relativity, explaining the results of the Michelson-Morley experiment

The combination of this description with the laws of special relativity results in a heuristic derivation of general relativity.

All results are in agreement with general relativity.

When describing graviton interactions, the classical theory ( i. e., the tree diagrams ) and semiclassical corrections ( one-loop diagrams ) behave normally, but Feynman diagrams with two ( or more ) loops lead to ultraviolet divergences ; that is, infinite results that cannot be removed because the quantized general relativity is not renormalizable, unlike quantum electrodynamics.

These results have not gained much attention from mainstream science, since they are in contradiction to a large quantity of high-precision measurements, all of them confirming special relativity.

Such a theory of quantum gravity would yield the same experimental results as ordinary quantum mechanics in conditions of weak gravity ( gravitational potentials much less than c < sup > 2 </ sup >) and the same results as Einsteinian general relativity in phenomena at scales much larger than individual molecules ( action much larger than reduced Planck's constant ), but moreover be able to predict the outcome of situations where both quantum effects and strong-field gravity are important ( at the Planck scale, unless large extra dimension conjectures are correct ).

Levinson presented research results documenting rather significant linguistic relativity effects in the linguistic conceptualization of spatial categories between different languages.

The history of special relativity consists of many theoretical results and empirical findings obtained by Albert Michelson, Hendrik Lorentz, Henri Poincaré and others.

While the results were not surprising since gravity was known to act on everything, including light ( see tests of general relativity and the Pound-Rebka falling photon experiment ), the self-interference of the quantum mechanical wave of a massive fermion in a gravitational field had never been experimentally confirmed before.

Although Newton's theory has been superseded, most modern non-relativistic gravitational calculations are still made using Newton's theory because it is a much simpler theory to work with than general relativity, and gives sufficiently accurate results for most applications involving sufficiently small masses, speeds and energies.

The rotational symmetry between time and space coordinate axes ( when one is taken as imaginary, another as real ) results in Lorentz transformations which in turn result in special relativity theory.

Symmetry between inertial and gravitational mass results in general relativity.

Indeed, the conservation of 4-momentum in inertial motion via curved space-time results in what we call gravitational force in general relativity theory.

The negative results are generally considered to be the first strong evidence against the then prevalent aether theory, and initiated a line of research that eventually led to special relativity, in which the stationary aether concept has no role.

There has been some historical controversy over whether Albert Einstein was aware of the Michelson-Morley results when he developed his theory of special relativity, which pronounced the aether to be " superfluous ".

General relativity, however, incorrectly predicts the results of a broad body of scientific experiments where quantum mechanics proves its sufficiency.

Originally developed in order to quantize vacuum general relativity in 3 + 1 dimensions, the formalism can accommodate arbitrary spacetime dimensionalities, fermions, an arbitrary gauge group ( or even quantum group ), and supersymmetry, and results in a quantization of the kinematics of the corresponding diffeomorphism-invariant gauge theory.

His papers on relativity produced the first exact solutions to the Einstein field equations, and a minor modification of these results gives the well-known solution that now bears his name: the Schwarzschild metric.

In 1935, Gustaf Wilhelm Hammar disproved a theoretical challenge to special relativity that attempted to explain away the null results of Michelson – Morley – type experiments as being a mere artifact of aether-dragging, using an odd-reflection Sagnac interferometer.

This results in the counterintuitive results of the theory of relativity and quantum mechanics becoming obvious in everyday life.

Furthermore, it was subject to relativity and thus was not constant for all observers, therefore, in 2012, the IAU redefined it again to just simply be.

It thus satisfies a more stringent general principle of relativity, namely that the laws of physics are the same for all observers.

In physics, the Lorentz transformation or Lorentz-Fitzgerald transformation describes how, according to the theory of special relativity, different measurements of space and time by two observers can be converted into the measurements observed in either frame of reference.

Since relativity postulates that the speed of light is the same for all observers, the Lorentz transformation must preserve the spacetime interval between any two events in Minkowski space.

Special relativity incorporates the principle that the speed of light is the same for all inertial observers regardless of the state of motion of the source.

# The laws of physics are the same for all observers in uniform motion relative to one another ( principle of relativity ).

Special relativity does not claim that all observers are equivalent, only that all observers at rest in inertial reference frames are equivalent.

The starting point for general relativity is the equivalence principle, which equates free fall with inertial motion, and describes free-falling inertial objects as being accelerated relative to non-inertial observers on the ground.

The essential idea behind relational quantum mechanics, following the precedent of special relativity, is that different observers may give different accounts of the same series of events: for example, to one observer at a given point in time, a system may be in a single, " collapsed " eigenstate, while to another observer at the same time, it may be in a superposition of two or more states.

It is important to the definition of both comoving distance and proper distance in the cosmological sense ( as opposed to proper length in special relativity ) that all observers have the same cosmological age.

Even light itself does not have a " velocity " of c in this sense ; the total velocity of any object can be expressed as the sum where is the recession velocity due to the expansion of the universe ( the velocity given by Hubble's law ) and is the " peculiar velocity " measured by local observers ( with and, the dots indicating a first derivative ), so for light is equal to c (- c if the light is emitted towards our position at the origin and + c if emitted away from us ) but the total velocity is generally different than c .( Davis and Lineweaver 2003, p. 19 ) Even in special relativity the coordinate speed of light is only guaranteed to be c in an inertial frame, in a non-inertial frame the coordinate speed may be different than c ; in general relativity no coordinate system on a large region of curved spacetime is " inertial ", but in the local neighborhood of any point in curved spacetime we can define a " local inertial frame " and the local speed of light will be c in this frame, with massive objects such as stars and galaxies always having a local speed smaller than c. The cosmological definitions used to define the velocities of distant objects are coordinate-dependent-there is no general coordinate-independent definition of velocity between distant objects in general relativity ( Baez and Bunn, 2006 ).

In relativity, different observers may disagree as to the particular value of the mass of a given system, but each observer will agree that this value does not change over time as long as the system is isolated ( totally closed to everything ).

If special relativity is to hold up exactly to this scale, different observers would observe quantum gravity effects at different scales, due to the Lorentz-FitzGerald contraction, in contradiction to the principle that all inertial observers should be able to describe phenomena by the same physical laws.

# The principle of relativity holds, i. e. equivalence of all inertial observers.

In general relativity, negative mass is generalized to refer to any region of space in which for some observers the mass density is measured to be negative.

* Frame fields in general relativity ( Lemaître observers in the Schwarzschild vacuum )

In the " membrane paradigm ", the black hole is described as it should be seen by an array of these stationary, suspended noninertial observers, and since their shared coordinate system ends at the event horizon ( because an observer cannot legally hover at or below the event horizon under general relativity ), this conventional-looking radiation is described as being emitted by an arbitrarily-thin shell of " hot " material at or just above the event horizon, where this coordinate system fails.

Wigner postulated that for the transition probability between states to be the same to all observers related by a transformation of special relativity.