
The fundamental reason for merging space and time into spacetime is that space and time are separately not invariant, which is to say that, under the proper conditions, different
observers will disagree on the length of time between two events (because of time dilation) or the distance between the two events (because of length contraction). 
When the event considered is infinitesimally close to each other, then we may write In a different inertial frame, say with coordinates , the spacetime interval can be written
in a same form as above. 
Many counterintuitive consequences emerge: in addition to being independent of the motion of the light source, the speed of light is constant regardless of the frame of reference
in which it is measured; the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity); and the linear additivity of velocities no longer
holds true. 
In special relativity, an observer will, in most cases, mean a frame of reference from which a set of objects or events is being measured.

[3] In the context of special relativity, time cannot be separated from the three dimensions of space, because the observed rate at which time passes for an object depends
on the object’s velocity relative to the observer. 
Spacetime in special relativity Spacetime interval[edit] See also: Causal structure In three dimensions, the distance between two points can be defined using the Pythagorean
theorem: Although two viewers may measure the x, y, and z position of the two points using different coordinate systems, the distance between the points will be the same for both (assuming that they are measuring using the same units). 
A position in spacetime is called an event, and requires four numbers to be specified: the threedimensional location in space, plus the position in time (Fig.

The magnitude of this scale factor (nearly 300,000 kilometres or 190,000 miles in space being equivalent to one second in time), along with the fact that spacetime is a manifold,
implies that at ordinary, nonrelativistic speeds and at ordinary, humanscale distances, there is little that humans might observe which is noticeably different from what they might observe if the world were Euclidean. 
the true distance Likewise, a timelike spacetime interval gives the same measure of time as would be presented by the cumulative ticking of a clock that moves along a given
world line. 
Suppose an observer measures two events as being separated in time by and a spatial distance Then the spacetime interval between the two events that are separated by a distance
in space and by in the coordinate is: or for three space dimensions, [32] The constant the speed of light, converts time units (like seconds) into space units (like meters). 
In special relativity, however, the distance between two points is no longer the same if measured by two different observers when one of the observers is moving, because of
Lorentz contraction. 
In ordinary space, a position is specified by three numbers, known as dimensions.

In particular, one notes that if two events are simultaneous in a particular reference frame, they are necessarily separated by a spacelike interval and thus are noncausally
related. 
Each location in spacetime is marked by four numbers defined by a frame of reference: the position in space, and the time (which can be visualized as the reading of a clock
located at each position in space). 
[citation needed] General relativity also provides an explanation of how gravitational fields can slow the passage of time for an object as seen by an observer outside the
field. 
Although measurements of distance and time between events differ for measurements made in different reference frames, the spacetime interval is independent of the inertial
frame of reference in which they are recorded. 
Spacetime intervals are equal to zero when In other words, the spacetime interval between two events on the world line of something moving at the speed of light is zero.

These two lines form what is called the light cone of the event O, since adding a second spatial dimension (Fig.

Until the 20th century, it was assumed that the threedimensional geometry of the universe (its spatial expression in terms of coordinates, distances, and directions) was
independent of onedimensional time. 
All observers who measure the time and distance between any two events will end up computing the same spacetime interval.

The series of events can be linked together to form a line which represents a particle’s progress through spacetime.

Unlike the analogies used in popular writings to explain events, such as firecrackers or sparks, mathematical events have zero duration and represent a single point in spacetime.

However, in 1905, Einstein based his work on special relativity on two postulates: • The laws of physics are invariant (i.e., identical) in all inertial systems (i.e., nonaccelerating
frames of reference)[citation needed] • The speed of light in vacuum is the same for all inertial observers, regardless of the motion of the light source. 
The squared interval is a measure of separation between events A and B that are time separated and in addition space separated either because there are two separate objects
undergoing events, or because a single object in space is moving inertially between its events. 
[18][15] In 1905, Einstein introduced special relativity (even though without using the techniques of the spacetime formalism) in its modern understanding as a theory of space
and time. 
14), and included a remarkable demonstration that the concept of the invariant interval (discussed below), along with the empirical observation that the speed of light is
finite, allows derivation of the entirety of special relativity. 
The latticework of clocks is used to determine the time and position of events taking place within the whole frame.

Animation illustrating relativity of simultaneity All observers will agree that for any given event, an event within the given event’s future light cone occurs after the given
event. 
For example, if one observer sees two events occur at the same place, but at different times, a person moving with respect to the first observer will see the two events occurring
at different places, because (from their point of view) they are stationary, and the position of the event is receding or approaching. 
[citation needed] The logical consequence of taking these postulates together is the inseparable joining of the four dimensions—hitherto assumed as independent—of space and
time. 
[19][note 2] In 1900, he recognized that Lorentz’s “local time” is actually what is indicated by moving clocks by applying an explicitly operational definition of clock synchronization
assuming constant light speed. 
Although time comes in as a fourth dimension, it is treated differently than the spatial dimensions.

Thus, a different measure must be used to measure the effective “distance” between two events.

The magenta hyperbolae, which cross the x axis, are timelike curves, which is to say that these hyperbolae represent actual paths that can be traversed by (constantly accelerating)
particles in spacetime: Between any two events on one hyperbola a causality relation is possible, because the inverse of the slope—representing the necessary speed—for all secants is less than . 
[33]: 2 The invariance of interval of any event between all intertial frames of reference is one of the fundamental results of special theory of relativity.

Since spatial distance traversed by any massive object is always less than distance traveled by the light for the same time interval, real intervals are always timelike.

However, it is not at all clear when Minkowski began to formulate the geometric formulation of special relativity that was to bear his name, or to which extent he was influenced
by Poincaré’s fourdimensional interpretation of the Lorentz transformation. 
But special relativity provides a new invariant, called the spacetime interval, which combines distances in space and in time.

Einstein performed his analysis in terms of kinematics (the study of moving bodies without reference to forces) rather than dynamics.

A spacetime diagram is typically drawn with only a single space and a single time coordinate.

The interior of the future light cone consists of all events that are separated from the apex by more time (temporal distance) than necessary to cross their spatial distance
at lightspeed; these events comprise the timelike future of the event O. 
[21][22] He did not pursue the 4dimensional formalism in subsequent papers, however, stating that this line of research seemed to “entail great pain for limited profit”,
ultimately concluding “that threedimensional language seems the best suited to the description of our world”. 
By using the mass–energy equivalence, Einstein showed, in addition, that the gravitational mass of a body is proportional to its energy content, which was one of the early
results in developing general relativity. 
2–4, event O is at the origin of a spacetime diagram, and the two diagonal lines represent all events that have zero spacetime interval with respect to the origin event.

Spacetime diagram illustrating two photons, A and B, originating at the same event, and a slowerthanlightspeed object, C The equation above is similar to the Pythagorean
theorem, except with a minus sign between the and the terms. 
The past light cone contains all the events that could have a causal influence on O.

(a) Galilean diagram of two frames of reference in standard configuration, (b) spacetime diagram of two frames of reference, (c) spacetime diagram showing the path of a reflected
light pulse To gain insight in how spacetime coordinates measured by observers in different reference frames compare with each other, it is useful to work with a simplified setup with frames in a standard configuration. 
From the reference frame of an observer moving at, the events appear to occur in the order A, B, C. The white line represents a plane of simultaneity being moved from the
past of the observer to the future of the observer, highlighting events residing on it. 
He obtained all of his results by recognizing that the entire theory can be built upon two postulates: The principle of relativity and the principle of the constancy of light
speed. 
[18][15] While his results are mathematically equivalent to those of Lorentz and Poincaré, Einstein showed that the Lorentz transformations are not the result of interactions
between matter and aether, but rather concern the nature of space and time itself. 

We are always concerned with differences of spatial or temporal coordinate values belonging to two events, and since there is no preferred origin, single coordinate values
have no essential meaning. 
Space and Time included the first public presentation of spacetime diagrams (Fig.

In 1908, Hermann Minkowski—once one of the math professors of a young Einstein in Zürich—presented a geometric interpretation of special relativity that fused time and the
three spatial dimensions of space into a single fourdimensional continuum now known as Minkowski space. 
A minor variation is to place the time coordinate last rather than first.

[8]: 17–22 In this idealized case, every point in space has a clock associated with it, and thus the clocks register each event instantly, with no time delay between an event
and its recording. 
However, Lorentz considered local time to be only an auxiliary mathematical tool, a trick as it were, to simplify the transformation from one system into another.

Introduction Definitions[edit] Nonrelativistic classical mechanics treats time as a universal quantity of measurement which is uniform throughout space, and separate from
space. 
11, imagine that the frame under consideration is equipped with a dense lattice of clocks, synchronized within this reference frame, that extends indefinitely throughout
the three dimensions of space. 
Failure to understand the difference between what one measures/observes versus what one sees is the source of much error among beginning students of relativity.

By the mid1800s, various experiments such as the observation of the Arago spot and differential measurements of the speed of light in air versus water were considered to
have proven the wave nature of light as opposed to a corpuscular theory. 
While discussing various hypotheses on Lorentz invariant gravitation, he introduced the innovative concept of a 4dimensional spacetime by defining various four vectors, namely
fourposition, fourvelocity, and fourforce. 
In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single fourdimensional manifold.

[…] He told me later that it came to him as a great shock when Einstein published his paper in which the equivalence of the different local times of observers moving relative
to each other was pronounced; for he had reached the same conclusions independently but did not publish them because he wished first to work out the mathematical structure in all its splendor. 
In addition, C illustrates the world line of a slowerthanlightspeed object.

In dimensional Minkowski spacetime (having one temporal and one spatial dimension), the points at some constant spacetime interval away from the origin (using the Minkowski
metric) form curves given by the two equations with some positive real constant. 
Although these theories included equations identical to those that Einstein introduced (e.g., the Lorentz transformation), they were essentially ad hoc models proposed to
explain the results of various experiments—including the famous Michelson–Morley interferometer experiment—that were extremely difficult to fit into existing paradigms. 
The opening words of Space and Time include Minkowski’s famous statement that “Henceforth, space for itself, and time for itself shall completely reduce to a mere shadow,
and only some sort of union of the two shall preserve independence.” 
Because of the invariance of the spacetime interval, all observers will assign the same light cone to any given event, and thus will agree on this division of spacetime.

It is hence possible for event O to have a causal influence on event D. The future light cone contains all the events that could be causally influenced by O.

Light cone in 2D space plus a time dimension A light (double) cone divides spacetime into separate regions with respect to its apex.
Works Cited
[‘luminiferous from the Latin lumen, light, + ferens, carrying; aether from the Greek αἰθήρ (aithēr), pure air, clear sky
2. ^ By stating that simultaneity is a matter of convention, Poincaré meant that to talk about time at all, one must have synchronized
clocks, and the synchronization of clocks must be established by a specified, operational procedure (convention). This stance represented a fundamental philosophical break from Newton, who conceived of an absolute, true time that was independent of
the workings of the inaccurate clocks of his day. This stance also represented a direct attack against the influential philosopher Henri Bergson, who argued that time, simultaneity, and duration were matters of intuitive understanding.[19]
3. ^
The operational procedure adopted by Poincaré was essentially identical to what is known as Einstein synchronization, even though a variant of it was already a widely used procedure by telegraphers in the middle 19th century. Basically, to synchronize
two clocks, one flashes a light signal from one to the other, and adjusts for the time that the flash takes to arrive.[19]
4. ^ A hallmark of Einstein’s career, in fact, was his use of visualized thought experiments (Gedanken–Experimente) as a
fundamental tool for understanding physical issues. For special relativity, he employed moving trains and flashes of lightning for his most penetrating insights. For curved spacetime, he considered a painter falling off a roof, accelerating elevators,
blind beetles crawling on curved surfaces and the like. In his great Solvay Debates with Bohr on the nature of reality (1927 and 1930), he devised multiple imaginary contraptions intended to show, at least in concept, means whereby the Heisenberg
uncertainty principle might be evaded. Finally, in a profound contribution to the literature on quantum mechanics, Einstein considered two particles briefly interacting and then flying apart so that their states are correlated, anticipating the phenomenon
known as quantum entanglement.[24]
5. ^ In the original version of this lecture, Minkowski continued to use such obsolescent terms as the ether, but the posthumous publication in 1915 of this lecture in the Annals of Physics (Annalen der Physik)
was edited by Sommerfeld to remove this term. Sommerfeld also edited the published form of this lecture to revise Minkowski’s judgement of Einstein from being a mere clarifier of the principle of relativity, to being its chief expositor.[26]
6. ^
(In the following, the group G∞ is the Galilean group and the group Gc the Lorentz group.) “With respect to this it is clear that the group Gc in the limit for c = ∞, i.e. as group G∞, exactly becomes the full group belonging to Newtonian Mechanics.
In this state of affairs, and since Gc is mathematically more intelligible than G∞, a mathematician may, by a free play of imagination, hit upon the thought that natural phenomena actually possess an invariance, not for the group G∞, but rather for
a group Gc, where c is definitely finite, and only exceedingly large using the ordinary measuring units.”[28]
7. ^ For instance, the Lorentz group is a subgroup of the conformal group in four dimensions.[29]: 41–42 The Lorentz group is
isomorphic to the Laguerre group transforming planes into planes,[29]: 39–42 it is isomorphic to the Möbius group of the plane,[30]: 22 and is isomorphic to the group of isometries in hyperbolic space which is often expressed
in terms of the hyperboloid model.[31]: 3.2.3
8. ^ In a Cartesian plane, ordinary rotation leaves a circle unchanged. In spacetime, hyperbolic rotation preserves the hyperbolic metric.
9. ^ Even with no (de)acceleration i.e. using one
inertial frame O for constant, highvelocity outward journey and another inertial frame I for constant, highvelocity inward journey – the sum of the elapsed time in those frames (O and I) is shorter than the elapsed time in the stationary inertial
frame S. Thus acceleration and deceleration is not the cause of shorter elapsed time during the outward and inward journey. Instead the use of two different constant, highvelocity inertial frames for outward and inward journey is really the cause
of shorter elapsed time total. Granted, if the same twin has to travel outward and inward leg of the journey and safely switch from outward to inward leg of the journey, the acceleration and deceleration is required. If the travelling twin could ride
the highvelocity outward inertial frame and instantaneously switch to highvelocity inward inertial frame the example would still work. The point is that real reason should be stated clearly. The asymmetry is because of the comparison of sum of elapsed
times in two different inertial frames (O and I) to the elapsed time in a single inertial frame S.
10. ^ The ease of analyzing a relativistic scenario often depends on the frame in which one chooses to perform the analysis. In this linked image,
we present alternative views of the transverse Doppler shift scenario where source and receiver are at their closest approach to each other. (a) If we analyze the scenario in the frame of the receiver, we find that the analysis is more complicated
than it should be. The apparent position of a celestial object is displaced from its true position (or geometric position) because of the object’s motion during the time it takes its light to reach an observer. The source would be timedilated relative
to the receiver, but the redshift implied by this time dilation would be offset by a blueshift due to the longitudinal component of the relative motion between the receiver and the apparent position of the source. (b) It is much easier if, instead,
we analyze the scenario from the frame of the source. An observer situated at the source knows, from the problem statement, that the receiver is at its closest point to him. That means that the receiver has no longitudinal component of motion to complicate
the analysis. Since the receiver’s clocks are timedilated relative to the source, the light that the receiver receives is therefore blueshifted by a factor of gamma.
11. ^ Not all experiments characterize the effect in terms of a redshift. For
example, the Kündig experiment was set up to measure transverse blueshift using a Mössbauer source setup at the center of a centrifuge rotor and an absorber at the rim.
12. ^ Rapidity arises naturally as a coordinates on the pure boost generators
inside the Lie algebra algebra of the Lorentz group. Likewise, rotation angles arise naturally as coordinates (modulo 2π) on the pure rotation generators in the Lie algebra. (Together they coordinatize the whole Lie algebra.) A notable difference
is that the resulting rotations are periodic in the rotation angle, while the resulting boosts are not periodic in rapidity (but rather onetoone). The similarity between boosts and rotations is formal resemblance.
13. ^ In relativity theory, proper
acceleration is the physical acceleration (i.e., measurable acceleration as by an accelerometer) experienced by an object. It is thus acceleration relative to a freefall, or inertial, observer who is momentarily at rest relative to the object being
measured.
14. ^ Newton himself was acutely aware of the inherent difficulties with these assumptions, but as a practical matter, making these assumptions was the only way that he could make progress. In 1692, he wrote to his friend Richard Bentley:
“That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro’ a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to
another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it.”
15. ^ More precisely, the gravitational field couples to itself. In Newtonian gravity, the potential
due to two point masses is simply the sum of the potentials of the two masses, but this does not apply to GR. This can be thought of as the result of the equivalence principle: If gravitation did not couple to itself, two particles bound by their
mutual gravitational attraction would not have the same inertial mass (due to negative binding energy) as their gravitational mass.[56]: 112–113
16. ^ This is because the law of gravitation (or any other inversesquare law) follows from
the concept of flux and the proportional relationship of flux density and field strength. If N = 3, then 3dimensional solid objects have surface areas proportional to the square of their size in any selected spatial dimension. In particular, a sphere
of radius r has a surface area of 4πr2. More generally, in a space of N dimensions, the strength of the gravitational attraction between two bodies separated by a distance of r would be inversely proportional to rN−1.
17. Different reporters viewing
the scenarios presented in this figure interpret the scenarios differently depending on their knowledge of the situation. (i) A first reporter, at the center of mass of particles 2 and 3 but unaware of the large mass 1, concludes that a force of repulsion
exists between the particles in scenario A while a force of attraction exists between the particles in scenario B. (ii) A second reporter, aware of the large mass 1, smiles at the first reporter’s naiveté. This second reporter knows that in reality,
the apparent forces between particles 2 and 3 really represent tidal effects resulting from their differential attraction by mass 1. (iii) A third reporter, trained in general relativity, knows that there are, in fact, no forces at all acting between
the three objects. Rather, all three objects move along geodesics in spacetime.
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Photo credit: https://www.flickr.com/photos/jms2/14103897845/’]