# Spacetime

Mathematical model combining space and time From Wikipedia, the free encyclopedia

Mathematical model combining space and time From Wikipedia, the free encyclopedia

In physics, **spacetime**, also called the **space-time continuum**, is a mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams are useful in visualizing and understanding relativistic effects, such as how different observers perceive *where* and *when* events occur.

Until the turn of the 20th century, the assumption had been that the three-dimensional geometry of the universe (its description in terms of locations, shapes, distances, and directions) was distinct from time (the measurement of when events occur within the universe). However, space and time took on new meanings with the Lorentz transformation and special theory of relativity.

In 1908, Hermann Minkowski presented a geometric interpretation of special relativity that fused time and the three spatial dimensions of space into a single four-dimensional continuum now known as Minkowski space. This interpretation proved vital to the general theory of relativity, wherein spacetime is curved by mass and energy.

Non-relativistic classical mechanics treats time as a universal quantity of measurement that is uniform throughout, is separate from space, and is agreed on by all observers. Classical mechanics assumes that time has a constant rate of passage, independent of the observer's state of motion, or anything external.^{[1]} It assumes that space is Euclidean: it assumes that space follows the geometry of common sense.^{[2]}

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.^{[3]}^{: 214–217 } General relativity provides an explanation of how gravitational fields can slow the passage of time for an object as seen by an observer outside the field.

In ordinary space, a position is specified by three numbers, known as dimensions. In the Cartesian coordinate system, these are often called *x*, *y* and *z*. A point in spacetime is called an *event*, and requires four numbers to be specified: the three-dimensional location in space, plus the position in time (Fig. 1). An event is represented by a set of coordinates *x*, *y*, *z* and *t*.^{[4]} Spacetime is thus four-dimensional.

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.^{[5]} Although it is possible to be in motion relative to the popping of a firecracker or a spark, it is not possible for an observer to be in motion relative to an event.

The path of a particle through spacetime can be considered to be a sequence of events. The series of events can be linked together to form a curve that represents the particle's progress through spacetime. That path is called the particle's *world line*.^{[6]}^{: 105 }

Mathematically, spacetime is a *manifold*, which is to say, it appears locally "flat" near each point in the same way that, at small enough scales, the surface of a globe appears to be flat.^{[7]} A scale factor, (conventionally called the *speed-of-light*) relates distances measured in space to distances measured in time. 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, non-relativistic speeds and at ordinary, human-scale distances, there is little that humans might observe that is noticeably different from what they might observe if the world were Euclidean. It was only with the advent of sensitive scientific measurements in the mid-1800s, such as the Fizeau experiment and the Michelson–Morley experiment, that puzzling discrepancies began to be noted between observation versus predictions based on the implicit assumption of Euclidean space.^{[8]}

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. This usage differs significantly from the ordinary English meaning of the term. Reference frames are inherently nonlocal constructs, and according to this usage of the term, it does not make sense to speak of an observer as having a location.^{[9]}

In Fig. 1-1, 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. Any specific location within the lattice is not important. The latticework of clocks is used to determine the time and position of events taking place within the whole frame. The term *observer* refers to the whole ensemble of clocks associated with one inertial frame of reference.^{[9]}^{: 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. A real observer, will see a delay between the emission of a signal and its detection due to the speed of light. To synchronize the clocks, in the data reduction following an experiment, the time when a signal is received will be corrected to reflect its actual time were it to have been recorded by an idealized lattice of clocks.^{[9]}^{: 17–22 }

In many books on special relativity, especially older ones, the word "observer" is used in the more ordinary sense of the word. It is usually clear from context which meaning has been adopted.

Physicists distinguish between what one *measures* or *observes*, after one has factored out signal propagation delays, versus what one visually sees without such corrections. Failing to understand the difference between what one measures and what one sees is the source of much confusion among students of relativity.^{[10]}

By the mid-1800s, 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.^{[11]} Propagation of waves was then assumed to require the existence of a *waving* medium; in the case of light waves, this was considered to be a hypothetical luminiferous aether.^{[note 1]} The various attempts to establish the properties of this hypothetical medium yielded contradictory results. For example, the Fizeau experiment of 1851, conducted by French physicist Hippolyte Fizeau, demonstrated that the speed of light in flowing water was less than the sum of the speed of light in air plus the speed of the water by an amount dependent on the water's index of refraction.^{[12]}

Among other issues, the dependence of the partial aether-dragging implied by this experiment on the index of refraction (which is dependent on wavelength) led to the unpalatable conclusion that aether *simultaneously* flows at different speeds for different colors of light.^{[13]} The Michelson–Morley experiment of 1887 (Fig. 1-2) showed no differential influence of Earth's motions through the hypothetical aether on the speed of light, and the most likely explanation, complete aether dragging, was in conflict with the observation of stellar aberration.^{[8]}

George Francis FitzGerald in 1889,^{[14]} and Hendrik Lorentz in 1892, independently proposed that material bodies traveling through the fixed aether were physically affected by their passage, contracting in the direction of motion by an amount that was exactly what was necessary to explain the negative results of the Michelson–Morley experiment. No length changes occur in directions transverse to the direction of motion.

By 1904, Lorentz had expanded his theory such that he had arrived at equations formally identical with those that Einstein was to derive later, i.e. the Lorentz transformation.^{[15]} As a theory of dynamics (the study of forces and torques and their effect on motion), his theory assumed actual physical deformations of the physical constituents of matter.^{[16]}^{: 163–174 } Lorentz's equations predicted a quantity that he called *local time*, with which he could explain the aberration of light, the Fizeau experiment and other phenomena.

Henri Poincaré was the first to combine space and time into spacetime.^{[17]}^{[18]}^{: 73–80, 93–95 } He argued in 1898 that the simultaneity of two events is a matter of convention.^{[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.^{[note 3]} In 1900 and 1904, he suggested the inherent undetectability of the aether by emphasizing the validity of what he called the principle of relativity. In 1905/1906^{[20]} he mathematically perfected Lorentz's theory of electrons in order to bring it into accordance with the postulate of relativity.

While discussing various hypotheses on Lorentz invariant gravitation, he introduced the innovative concept of a 4-dimensional spacetime by defining various four vectors, namely four-position, four-velocity, and four-force.^{[21]}^{[22]} He did not pursue the 4-dimensional formalism in subsequent papers, however, stating that this line of research seemed to "entail great pain for limited profit", ultimately concluding "that three-dimensional language seems the best suited to the description of our world".^{[22]} Even as late as 1909, Poincaré continued to describe the dynamical interpretation of the Lorentz transform.^{[16]}^{: 163–174 }

In 1905, Albert Einstein analyzed special relativity in terms of kinematics (the study of moving bodies without reference to forces) rather than dynamics. His results were mathematically equivalent to those of Lorentz and Poincaré. He obtained them 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. His work was filled with vivid imagery involving the exchange of light signals between clocks in motion, careful measurements of the lengths of moving rods, and other such examples.^{[23]}^{[note 4]}

Einstein in 1905 superseded previous attempts of an electromagnetic mass–energy relation by introducing the general equivalence of mass and energy, which was instrumental for his subsequent formulation of the equivalence principle in 1907, which declares the equivalence of inertial and gravitational mass. By using the mass–energy equivalence, Einstein showed that the gravitational mass of a body is proportional to its energy content, which was one of the early results in developing general relativity. While it would appear that he did not at first think geometrically about spacetime,^{[3]}^{: 219 } in the further development of general relativity, Einstein fully incorporated the spacetime formalism.

When Einstein published in 1905, another of his competitors, his former mathematics professor Hermann Minkowski, had also arrived at most of the basic elements of special relativity. Max Born recounted a meeting he had made with Minkowski, seeking to be Minkowski's student/collaborator:^{[25]}

I went to Cologne, met Minkowski and heard his celebrated lecture 'Space and Time' delivered on 2 September 1908. [...] 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. He never made a priority claim and always gave Einstein his full share in the great discovery.

Minkowski had been concerned with the state of electrodynamics after Michelson's disruptive experiments at least since the summer of 1905, when Minkowski and David Hilbert led an advanced seminar attended by notable physicists of the time to study the papers of Lorentz, Poincaré et al. Minkowski saw Einstein's work as an extension of Lorentz's, and was most directly influenced by Poincaré.^{[26]}

On 5 November 1907 (a little more than a year before his death), Minkowski introduced his geometric interpretation of spacetime in a lecture to the Göttingen Mathematical society with the title, *The Relativity Principle* (*Das Relativitätsprinzip*).^{[note 5]} On 21 September 1908, Minkowski presented his talk, *Space and Time* (*Raum und Zeit*),^{[27]} to the German Society of Scientists and Physicians. The opening words of *Space and Time* include Minkowski's 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." *Space and Time* included the first public presentation of spacetime diagrams (Fig. 1-4), 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.^{[note 6]}

The spacetime concept and the Lorentz group are closely connected to certain types of sphere, hyperbolic, or conformal geometries and their transformation groups already developed in the 19th century, in which invariant intervals analogous to the spacetime interval are used.^{[note 7]}

Einstein, for his part, was initially dismissive of Minkowski's geometric interpretation of special relativity, regarding it as *überflüssige Gelehrsamkeit* (superfluous learnedness). However, in order to complete his search for general relativity that started in 1907, the geometric interpretation of relativity proved to be vital. In 1916, Einstein fully acknowledged his indebtedness to Minkowski, whose interpretation greatly facilitated the transition to general relativity.^{[16]}^{: 151–152 } Since there are other types of spacetime, such as the curved spacetime of general relativity, the spacetime of special relativity is today known as *Minkowski spacetime.*

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. The distance is "invariant".

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. The situation is even more complicated if the two points are separated in time as well as in space. 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 the moving point of view sees itself as stationary, and the position of the event as receding or approaching. Thus, a different measure must be used to measure the effective "distance" between two events.^{[31]}^{: 48–50, 100–102 }

In four-dimensional spacetime, the analog to distance is the interval. Although time comes in as a fourth dimension, it is treated differently than the spatial dimensions. Minkowski space hence differs in important respects from four-dimensional Euclidean space. 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). Special relativity provides a new invariant, called the **spacetime interval**, which combines distances in space and in time. All observers who measure the time and distance between any two events will end up computing the same spacetime interval. Suppose an observer measures two events as being separated in time by and a spatial distance Then the squared spacetime interval between the two events that are separated by a distance in space and by in the -coordinate is:^{[32]}

or for three space dimensions,

The constant the speed of light, converts time units (like seconds) into space units (like meters). 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. The separation interval is the difference between the square of the spatial distance separating event B from event A and the square of the spatial distance traveled by a light signal in that same time interval . If the event separation is due to a light signal, then this difference vanishes and .

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. Because of the constancy of speed of light, the light events in all inertial frames belong to zero interval, . For any other infinitesimal event where , one can prove that
which in turn upon integration leads to .^{[33]}^{: 2 } The invariance of the spacetime interval between the same events for all inertial frames of reference is one of the fundamental results of special theory of relativity.

Although for brevity, one frequently sees interval expressions expressed without deltas, including in most of the following discussion, it should be understood that in general, means , etc. 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.

The equation above is similar to the Pythagorean theorem, except with a minus sign between the and the terms. The spacetime interval is the quantity not itself. The reason is that unlike distances in Euclidean geometry, intervals in Minkowski spacetime can be negative. Rather than deal with square roots of negative numbers, physicists customarily regard as a distinct symbol in itself, rather than the square of something.^{[3]}^{: 217 }

**Note:**There are two sign conventions in use in the relativity literature:- and
- These sign conventions are associated with the metric signatures (+−−−) and (−+++). A minor variation is to place the time coordinate last rather than first. Both conventions are widely used within the field of study.
^{[34]} - In the following discussion, we use the first convention.

In general can assume any real number value. If is positive, the spacetime interval is referred to as **timelike**. Since spatial distance traversed by any massive object is always less than distance traveled by the light for the same time interval, positive intervals are always timelike. If is negative, the spacetime interval is said to be **spacelike**. 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. Such an interval is termed **lightlike** or **null**. A photon arriving in our eye from a distant star will not have aged, despite having (from our perspective) spent years in its passage.^{[31]}^{: 48–50 }

A spacetime diagram is typically drawn with only a single space and a single time coordinate. Fig. 2-1 presents a spacetime diagram illustrating the world lines (i.e. paths in spacetime) of two photons, A and B, originating from the same event and going in opposite directions. In addition, C illustrates the world line of a slower-than-light-speed object. The vertical time coordinate is scaled by so that it has the same units (meters) as the horizontal space coordinate. Since photons travel at the speed of light, their world lines have a slope of ±1.^{[31]}^{: 23–25 } In other words, every meter that a photon travels to the left or right requires approximately 3.3 nanoseconds of time.

This section needs additional citations for verification. (March 2024) |

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.* With care, this allows simplification of the math with no loss of generality in the conclusions that are reached. In Fig. 2-2, two Galilean reference frames (i.e. conventional 3-space frames) are displayed in relative motion. Frame S belongs to a first observer O, and frame S′ (pronounced "S prime") belongs to a second observer O′.

- The
*x*,*y*,*z*axes of frame S are oriented parallel to the respective primed axes of frame S′. - Frame S′ moves in the
*x*-direction of frame S with a constant velocity*v*as measured in frame S. - The origins of frames S and S′ are coincident when time
*t*= 0 for frame S and*t*′ = 0 for frame S′.^{[6]}^{: 107 }

Fig. 2-3a redraws Fig. 2-2 in a different orientation. Fig. 2-3b illustrates a *relativistic* spacetime diagram from the viewpoint of observer O. Since S and S′ are in standard configuration, their origins coincide at times *t* = 0 in frame S and *t*′ = 0 in frame S′. The *ct*′ axis passes through the events in frame S′ which have *x*′ = 0. But the points with *x*′ = 0 are moving in the *x*-direction of frame S with velocity *v*, so that they are not coincident with the *ct* axis at any time other than zero. Therefore, the *ct*′ axis is tilted with respect to the *ct* axis by an angle *θ* given by^{[31]}^{: 23–31 }

The *x*′ axis is also tilted with respect to the *x* axis. To determine the angle of this tilt, we recall that the slope of the world line of a light pulse is always ±1. Fig. 2-3c presents a spacetime diagram from the viewpoint of observer O′. Event P represents the emission of a light pulse at *x*′ = 0, *ct*′ = −*a*. The pulse is reflected from a mirror situated a distance *a* from the light source (event Q), and returns to the light source at *x*′ = 0, *ct*′ = *a* (event R).

The same events P, Q, R are plotted in Fig. 2-3b in the frame of observer O. The light paths have slopes = 1 and −1, so that △PQR forms a right triangle with PQ and QR both at 45 degrees to the *x* and *ct* axes. Since OP = OQ = OR, the angle between *x*′ and *x* must also be *θ*.^{[6]}^{: 113–118 }

While the rest frame has space and time axes that meet at right angles, the moving frame is drawn with axes that meet at an acute angle. The frames are actually equivalent.^{[31]}^{: 23–31 } The asymmetry is due to unavoidable distortions in how spacetime coordinates can map onto a Cartesian plane, and should be considered no stranger than the manner in which, on a Mercator projection of the Earth, the relative sizes of land masses near the poles (Greenland and Antarctica) are highly exaggerated relative to land masses near the Equator.

In Fig. 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. These two lines form what is called the *light cone* of the event O, since adding a second spatial dimension (Fig. 2-5) makes the appearance that of two right circular cones meeting with their apices at O. One cone extends into the future (t>0), the other into the past (t<0).

A light (double) cone divides spacetime into separate regions with respect to its apex. 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. Likewise, the *timelike past* comprises the interior events of the past light cone. So in *timelike intervals* Δ*ct* is greater than Δ*x*, making timelike intervals positive.^{[3]}^{: 220 }

The region exterior to the light cone consists of events that are separated from the event O by more *space* than can be crossed at lightspeed in the given *time*. These events comprise the so-called *spacelike* region of the event O, denoted "Elsewhere" in Fig. 2-4. Events on the light cone itself are said to be *lightlike* (or *null separated*) from O. 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.^{[3]}^{: 220 }

The light cone has an essential role within the concept of causality. It is possible for a not-faster-than-light-speed signal to travel from the position and time of O to the position and time of D (Fig. 2-4). 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. Likewise, it is possible for a not-faster-than-light-speed signal to travel from the position and time of A, to the position and time of O. The past light cone contains all the events that could have a causal influence on O. In contrast, assuming that signals cannot travel faster than the speed of light, any event, like e.g. B or C, in the spacelike region (Elsewhere), cannot either affect event O, nor can they be affected by event O employing such signalling. Under this assumption any causal relationship between event O and any events in the spacelike region of a light cone is excluded.^{[35]}

All observers will agree that for any given event, an event within the given event's future light cone occurs *after* the given event. Likewise, for any given event, an event within the given event's past light cone occurs *before* the given event. The before–after relationship observed for timelike-separated events remains unchanged no matter what the reference frame of the observer, i.e. no matter how the observer may be moving. The situation is quite different for spacelike-separated events. **Fig. 2-4** was drawn from the reference frame of an observer moving at *v* = 0. From this reference frame, event C is observed to occur after event O, and event B is observed to occur before event O.^{[36]}

From a different reference frame, the orderings of these non-causally-related events can be reversed. 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. The observation that simultaneity is not absolute, but depends on the observer's reference frame, is termed the relativity of simultaneity.^{[36]}

Fig. 2-6 illustrates the use of spacetime diagrams in the analysis of the relativity of simultaneity. The events in spacetime are invariant, but the coordinate frames transform as discussed above for Fig. 2-3. The three events (A, B, C) are simultaneous from the reference frame of an observer moving at *v* = 0. From the reference frame of an observer moving at *v* = 0.3*c*, the events appear to occur in the order C, B, A. From the reference frame of an observer moving at *v* = −0.5*c*, 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. The gray area is the light cone of the observer, which remains invariant.

A spacelike spacetime interval gives the same distance that an observer would measure if the events being measured were simultaneous to the observer. A spacelike spacetime interval hence provides a measure of *proper distance*, i.e. 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. A timelike spacetime interval hence provides a measure of the *proper time* = ^{[3]}^{: 220–221 }

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In Euclidean space (having spatial dimensions only), the set of points equidistant (using the Euclidean metric) from some point form a circle (in two dimensions) or a sphere (in three dimensions). In (1+1)-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. These equations describe two families of hyperbolae in an *x*–*ct* spacetime diagram, which are termed *invariant hyperbolae*.

In Fig. 2-7a, each magenta hyperbola connects all events having some fixed spacelike separation from the origin, while the green hyperbolae connect events of equal timelike separation.

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 . On the other hand, the green hyperbolae, which cross the *ct* axis, are spacelike curves because all intervals *along* these hyperbolae are spacelike intervals: No causality is possible between any two points on one of these hyperbolae, because all secants represent speeds larger than .

Fig. 2-7b reflects the situation in (1+2)-dimensional Minkowski spacetime (one temporal and two spatial dimensions) with the corresponding hyperboloids. The invariant hyperbolae displaced by spacelike intervals from the origin generate hyperboloids of one sheet, while the invariant hyperbolae displaced by timelike intervals from the origin generate hyperboloids of two sheets.

The (1+2)-dimensional boundary between space- and time-like hyperboloids, established by the events forming a zero spacetime interval to the origin, is made up by degenerating the hyperboloids to the light cone. In (1+1)-dimensions the hyperbolae degenerate to the two grey 45°-lines depicted in Fig. 2-7a.

Fig. 2-8 illustrates the invariant hyperbola for all events that can be reached from the origin in a proper time of 5 meters (approximately 1.67×10^{−8} s). Different world lines represent clocks moving at different speeds. A clock that is stationary with respect to the observer has a world line that is vertical, and the elapsed time measured by the observer is the same as the proper time. For a clock traveling at 0.3 *c*, the elapsed time measured by the observer is 5.24 meters (1.75×10^{−8} s), while for a clock traveling at 0.7 *c*, the elapsed time measured by the observer is 7.00 meters (2.34×10^{−8} s).^{[3]}^{: 220–221 }

This illustrates the phenomenon known as time dilation. Clocks that travel faster take longer (in the observer frame) to tick out the same amount of proper time, and they travel further along the x–axis within that proper time than they would have without time dilation.^{[3]}^{: 220–221 } The measurement of time dilation by two observers in different inertial reference frames is mutual. If observer O measures the clocks of observer O′ as running slower in his frame, observer O′ in turn will measure the clocks of observer O as running slower.

Length contraction, like time dilation, is a manifestation of the relativity of simultaneity. Measurement of length requires measurement of the spacetime interval between two events that are simultaneous in one's frame of reference. But events that are simultaneous in one frame of reference are, in general, not simultaneous in other frames of reference.

Fig. 2-9 illustrates the motions of a 1 m rod that is traveling at 0.5 *c* along the *x* axis. The edges of the blue band represent the world lines of the rod's two endpoints. The invariant hyperbola illustrates events separated from the origin by a spacelike interval of 1 m. The endpoints O and B measured when *t*′ = 0 are simultaneous events in the S′ frame. But to an observer in frame S, events O and B are not simultaneous. To measure length, the observer in frame S measures the endpoints of the rod as projected onto the *x*-axis along their world lines. The projection of the rod's *world sheet* onto the *x* axis yields the foreshortened length OC.^{[6]}^{: 125 }

(not illustrated) Drawing a vertical line through A so that it intersects the *x*′ axis demonstrates that, even as OB is foreshortened from the point of view of observer O, OA is likewise foreshortened from the point of view of observer O′. In the same way that each observer measures the other's clocks as running slow, each observer measures the other's rulers as being contracted.

In regards to mutual length contraction, **Fig. 2-9** illustrates that the primed and unprimed frames are mutually rotated by a hyperbolic angle (analogous to ordinary angles in Euclidean geometry).^{[note 8]} Because of this rotation, the projection of a primed meter-stick onto the unprimed *x*-axis is foreshortened, while the projection of an unprimed meter-stick onto the primed x′-axis is likewise foreshortened.

Mutual time dilation and length contraction tend to strike beginners as inherently self-contradictory concepts. If an observer in frame S measures a clock, at rest in frame S', as running slower than his', while S' is moving at speed *v* in S, then the principle of relativity requires that an observer in frame S' likewise measures a clock in frame S, moving at speed −*v* in S', as running slower than hers. How two clocks can run *both slower* than the other, is an important question that "goes to the heart of understanding special relativity."^{[3]}^{: 198 }

This apparent contradiction stems from not correctly taking into account the different settings of the necessary, related measurements. These settings allow for a consistent explanation of the *only apparent* contradiction. It is not about the abstract ticking of two identical clocks, but about how to measure in one frame the temporal distance of two ticks of a moving clock. It turns out that in mutually observing the duration between ticks of clocks, each moving in the respective frame, different sets of clocks must be involved. In order to measure in frame S the tick duration of a moving clock W′ (at rest in S′), one uses *two* additional, synchronized clocks W_{1} and W_{2} at rest in two arbitrarily fixed points in S with the spatial distance *d*.

- Two events can be defined by the condition "two clocks are simultaneously at one place", i.e., when W′ passes each W
_{1}and W_{2}. For both events the two readings of the collocated clocks are recorded. The difference of the two readings of W_{1}and W_{2}is the temporal distance of the two events in S, and their spatial distance is*d*. The difference of the two readings of W′ is the temporal distance of the two events in S′. In S′ these events are only separated in time, they happen at the same place in S′. Because of the invariance of the spacetime interval spanned by these two events, and the nonzero spatial separation*d*in S, the temporal distance in S′ must be smaller than the one in S: the*smaller*temporal distance between the two events, resulting from the readings of the moving clock W′, belongs to the*slower*running clock W′.

Conversely, for judging in frame S′ the temporal distance of two events on a moving clock W (at rest in S), one needs two clocks at rest in S′.

- In this comparison the clock W is moving by with velocity −
*v*. Recording again the four readings for the events, defined by "two clocks simultaneously at one place", results in the analogous temporal distances of the two events, now temporally and spatially separated in S′, and only temporally separated but collocated in S. To keep the spacetime interval invariant, the temporal distance in S must be smaller than in S′, because of the spatial separation of the events in S′: now clock W is observed to run slower.

The necessary recordings for the two judgements, with "one moving clock" and "two clocks at rest" in respectively S or S′, involves two different sets, each with three clocks. Since there are different sets of clocks involved in the measurements, there is no inherent necessity that the measurements be reciprocally "consistent" such that, if one observer measures the moving clock to be slow, the other observer measures the one's clock to be fast.^{[3]}^{: 198–199 }

Fig. 2-10 illustrates the previous discussion of mutual time dilation with Minkowski diagrams. The upper picture reflects the measurements as seen from frame S "at rest" with unprimed, rectangular axes, and frame S′ "moving with *v* > 0", coordinatized by primed, oblique axes, slanted to the right; the lower picture shows frame S′ "at rest" with primed, rectangular coordinates, and frame S "moving with −*v* < 0", with unprimed, oblique axes, slanted to the left.

Each line drawn parallel to a spatial axis (*x*, *x*′) represents a line of simultaneity. All events on such a line have the same time value (*ct*, *ct*′). Likewise, each line drawn parallel to a temporal axis (*ct*, *ct′*) represents a line of equal spatial coordinate values (*x*, *x*′).

- One may designate in both pictures the origin
*O*(=*O*′) as the event, where the respective "moving clock" is collocated with the "first clock at rest" in both comparisons. Obviously, for this event the readings on both clocks in both comparisons are zero. As a consequence, the worldlines of the moving clocks are the slanted to the right*ct*′-axis (upper pictures, clock W′) and the slanted to the left*ct*-axes (lower pictures, clock W). The worldlines of W_{1}and W′_{1}are the corresponding vertical time axes (*ct*in the upper pictures, and*ct*′ in the lower pictures). - In the upper picture the place for W
_{2}is taken to be*A*> 0, and thus the worldline (not shown in the pictures) of this clock intersects the worldline of the moving clock (the_{x}*ct*′-axis) in the event labelled*A*, where "two clocks are simultaneously at one place". In the lower picture the place for W′_{2}is taken to be*C*_{x′}< 0, and so in this measurement the moving clock W passes W′_{2}in the event*C*. - In the upper picture the
*ct*-coordinate*A*of the event_{t}*A*(the reading of W_{2}) is labeled*B*, thus giving the elapsed time between the two events, measured with W_{1}and W_{2}, as*OB*. For a comparison, the length of the time interval*OA*, measured with W′, must be transformed to the scale of the*ct*-axis. This is done by the invariant hyperbola (see also Fig. 2-8) through*A*, connecting all events with the same spacetime interval from the origin as*A*. This yields the event*C*on the*ct*-axis, and obviously:*OC*<*OB*, the "moving" clock W′ runs slower.

To show the mutual time dilation immediately in the upper picture, the event *D* may be constructed as the event at *x*′ = 0 (the location of clock W′ in S′), that is simultaneous to *C* (*OC* has equal spacetime interval as *OA*) in S′. This shows that the time interval *OD* is longer than *OA*, showing that the "moving" clock runs slower.^{[6]}^{: 124 }

In the lower picture the frame S is moving with velocity −*v* in the frame S′ at rest. The worldline of clock W is the *ct*-axis (slanted to the left), the worldline of W′_{1} is the vertical *ct*′-axis, and the worldline of W′_{2} is the vertical through event *C*, with *ct*′-coordinate *D*. The invariant hyperbola through event *C* scales the time interval *OC* to *OA*, which is shorter than *OD*; also, *B* is constructed (similar to *D* in the upper pictures) as simultaneous to *A* in S, at *x* = 0. The result *OB* > *OC* corresponds again to above.

The word "measure" is important. In classical physics an observer cannot affect an observed object, but the object's state of motion *can* affect the observer's *observations* of the object.

Many introductions to special relativity illustrate the differences between Galilean relativity and special relativity by posing a series of "paradoxes". These paradoxes are, in fact, ill-posed problems, resulting from our unfamiliarity with velocities comparable to the speed of light. The remedy is to solve many problems in special relativity and to become familiar with its so-called counter-intuitive predictions. The geometrical approach to studying spacetime is considered one of the best methods for developing a modern intuition.^{[37]}

The twin paradox is a thought experiment involving identical twins, one of whom makes a journey into space in a high-speed rocket, returning home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin observes the other twin as moving, and so at first glance, it would appear that each should find the other to have aged less. The twin paradox sidesteps the justification for mutual time dilation presented above by avoiding the requirement for a third clock.^{[3]}^{: 207 } Nevertheless, the *twin paradox* is not a true paradox because it is easily understood within the context of special relativity.

The impression that a paradox exists stems from a misunderstanding of what special relativity states. Special relativity does not declare all frames of reference to be equivalent, only inertial frames. The traveling twin's frame is not inertial during periods when she is accelerating. Furthermore, the difference between the twins is observationally detectable: the traveling twin needs to fire her rockets to be able to return home, while the stay-at-home twin does not.^{[38]}^{[note 9]}

These distinctions should result in a difference in the twins' ages. The spacetime diagram of Fig. 2-11 presents the simple case of a twin going straight out along the x axis and immediately turning back. From the standpoint of the stay-at-home twin, there is nothing puzzling about the twin paradox at all. The proper time measured along the traveling twin's world line from O to C, plus the proper time measured from C to B, is less than the stay-at-home twin's proper time measured from O to A to B. More complex trajectories require integrating the proper time between the respective events along the curve (i.e. the path integral) to calculate the total amount of proper time experienced by the traveling twin.^{[38]}

Complications arise if the twin paradox is analyzed from the traveling twin's point of view.

Weiss's nomenclature, designating the stay-at-home twin as Terence and the traveling twin as Stella, is hereafter used.^{[38]}

Stella is not in an inertial frame. Given this fact, it is sometimes incorrectly stated that full resolution of the twin paradox requires general relativity:^{[38]}

A pure SR analysis would be as follows: Analyzed in Stella's rest frame, she is motionless for the entire trip. When she fires her rockets for the turnaround, she experiences a pseudo force which resembles a gravitational force.

^{[38]}Figs. 2-6and 2-11 illustrate the concept of lines (planes) of simultaneity: Lines parallel to the observer'sx-axis (xy-plane) represent sets of events that are simultaneous in the observer frame. In Fig. 2-11, the blue lines connect events on Terence's world line which,from Stella's point of view, are simultaneous with events on her world line. (Terence, in turn, would observe a set of horizontal lines of simultaneity.) Throughout both the outbound and the inbound legs of Stella's journey, she measures Terence's clocks as running slower than her own.But during the turnaround(i.e. between the bold blue lines in the figure), a shift takes place in the angle of her lines of simultaneity, corresponding to a rapid skip-over of the events in Terence's world line that Stella considers to be simultaneous with her own. Therefore, at the end of her trip, Stella finds that Terence has aged more than she has.^{[38]}

Although general relativity is not required to analyze the twin paradox, application of the Equivalence Principle of general relativity does provide some additional insight into the subject. Stella is not stationary in an inertial frame. Analyzed in Stella's rest frame, she is motionless for the entire trip. When she is coasting her rest frame is inertial, and Terence's clock will appear to run slow. But when she fires her rockets for the turnaround, her rest frame is an accelerated frame and she experiences a force which is pushing her as if she were in a gravitational field. Terence will appear to be high up in that field and because of gravitational time dilation, his clock will appear to run fast, so much so that the net result will be that Terence has aged more than Stella when they are back together.^{[38]} The theoretical arguments predicting gravitational time dilation are not exclusive to general relativity. Any theory of gravity will predict gravitational time dilation if it respects the principle of equivalence, including Newton's theory.^{[3]}^{: 16 }

This introductory section has focused on the spacetime of special relativity, since it is the easiest to describe. Minkowski spacetime is flat, takes no account of gravity, is uniform throughout, and serves as nothing more than a static background for the events that take place in it. The presence of gravity greatly complicates the description of spacetime. In general relativity, spacetime is no longer a static background, but actively interacts with the physical systems that it contains. Spacetime curves in the presence of matter, can propagate waves, bends light, and exhibits a host of other phenomena.^{[3]}^{: 221 } A few of these phenomena are described in the later sections of this article.

A basic goal is to be able to compare measurements made by observers in relative motion. If there is an observer O in frame S who has measured the time and space coordinates of an event, assigning this event three Cartesian coordinates and the time as measured on his lattice of synchronized clocks (*x*, *y*, *z*, *t*) (see **Fig. 1-1**). A second observer O′ in a different frame S′ measures the same event in her coordinate system and her lattice of synchronized clocks (*x*′, *y*′, *z*′, *t*′). With inertial frames, neither observer is under acceleration, and a simple set of equations allows us to relate coordinates (*x*, *y*, *z*, *t*) to (*x*′, *y*′, *z*′, *t*′). Given that the two coordinate systems are in standard configuration, meaning that they are aligned with parallel (*x*, *y*, *z*) coordinates and that *t* = 0 when *t*′ = 0, the coordinate transformation is as follows:^{[39]}^{[40]}

Fig. 3-1 illustrates that in Newton's theory, time is universal, not the velocity of light.^{[41]}^{: 36–37 } Consider the following thought experiment: The red arrow illustrates a train that is moving at 0.4 c with respect to the platform. Within the train, a passenger shoots a bullet with a speed of 0.4 c in the frame of the train. The blue arrow illustrates that a person standing on the train tracks measures the bullet as traveling at 0.8 c. This is in accordance with our naive expectations.

More generally, assuming that frame S′ is moving at velocity *v* with respect to frame S, then within frame S′, observer O′ measures an object moving with velocity *u*′. Velocity *u* with respect to frame S, since *x* = *ut*, *x*′ = *x* − *vt*, and *t* = *t*′, can be written as *x*′ = *ut* − *vt* = (*u* − *v*)*t* = (*u* − *v*)*t*′. This leads to *u*′ = *x*′/*t*′ and ultimately

- or

which is the common-sense **Galilean law for the addition of velocities**.

The composition of velocities is quite different in relativistic spacetime. To reduce the complexity of the equations slightly, we introduce a common shorthand for the ratio of the speed of an object relative to light,

Fig. 3-2a illustrates a red train that is moving forward at a speed given by *v*/*c* = *β* = *s*/*a*. From the primed frame of the train, a passenger shoots a bullet with a speed given by *u*′/*c* = *β*′ = *n*/*m*, where the distance is measured along a line parallel to the red *x*′ axis rather than parallel to the black *x* axis. What is the composite velocity *u* of the bullet relative to the platform, as represented by the blue arrow? Referring to Fig. 3-2b:

- From the platform, the composite speed of the bullet is given by
*u*=*c*(*s*+*r*)/(*a*+*b*). - The two yellow triangles are similar because they are right triangles that share a common angle
*α*. In the large yellow triangle, the ratio*s*/*a*=*v*/*c*=*β*. - The ratios of corresponding sides of the two yellow triangles are constant, so that
*r*/*a*=*b*/*s*=*n*/*m*=*β*′. So*b*=*u*′*s*/*c*and*r*=*u*′*a*/*c*. - Substitute the expressions for
*b*and*r*into the expression for*u*in step 1 to yield Einstein's formula for the addition of velocities:^{[41]}^{: 42–48 }

The relativistic formula for addition of velocities presented above exhibits several important features:

- If
*u*′ and*v*are both very small compared with the speed of light, then the product*vu*′/*c*^{2}becomes vanishingly small, and the overall result becomes indistinguishable from the Galilean formula (Newton's formula) for the addition of velocities:*u*=*u*′ +*v*. The Galilean formula is a special case of the relativistic formula applicable to low velocities. - If
*u*′ is set equal to*c*, then the formula yields*u*=*c*regardless of the starting value of*v*. The velocity of light is the same for all observers regardless their motions relative to the emitting source.^{[41]}^{: 49 }

This section needs additional citations for verification. (March 2024) |

It is straightforward to obtain quantitative expressions for time dilation and length contraction. Fig. 3-3 is a composite image containing individual frames taken from two previous animations, simplified and relabeled for the purposes of this section.

To reduce the complexity of the equations slightly, there are a variety of different shorthand notations for *ct*:

- and are common.
- One also sees very frequently the use of the convention

In Fig. 3-3a, segments *OA* and *OK* represent equal spacetime intervals. Time dilation is represented by the ratio *OB*/*OK*. The invariant hyperbola has the equation *w* = √*x*^{2} + *k*^{2} where *k* = *OK*, and the red line representing the world line of a particle in motion has the equation *w* = *x*/*β* = *xc*/*v*. A bit of algebraic manipulation yields

The expression involving the square root symbol appears very frequently in relativity, and one over the expression is called the Lorentz factor, denoted by the Greek letter gamma :^{[42]}

If *v* is greater than or equal to *c*, the expression for becomes physically meaningless, implying that *c* is the maximum possible speed in nature. For any *v* greater than zero, the Lorentz factor will be greater than one, although the shape of the curve is such that for low speeds, the Lorentz factor is extremely close to one.

In Fig. 3-3b, segments *OA* and *OK* represent equal spacetime intervals. Length contraction is represented by the ratio *OB*/*OK*. The invariant hyperbola has the equation *x* = √*w*^{2} + *k*^{2}, where *k* = *OK*, and the edges of the blue band representing the world lines of the endpoints of a rod in motion have slope 1/*β* = *c*/*v*. Event A has coordinates
(*x*, *w*) = (*γk*, *γβk*). Since the tangent line through A and B has the equation *w* = (*x* − *OB*)/*β*, we have *γβk* = (*γk* − *OB*)/*β* and

The Galilean transformations and their consequent commonsense law of addition of velocities work well in our ordinary low-speed world of planes, cars and balls. Beginning in the mid-1800s, however, sensitive scientific instrumentation began finding anomalies that did not fit well with the ordinary addition of velocities.

Lorentz transformations are used to transform the coordinates of an event from one frame to another in special relativity.

The Lorentz factor appears in the Lorentz transformations:

The inverse Lorentz transformations are:

When *v* ≪ *c* and *x* is small enough, the *v*^{2}/*c*^{2} and *vx*/*c*^{2} terms approach zero, and the Lorentz transformations approximate to the Galilean transformations.

etc., most often really mean etc. Although for brevity the Lorentz transformation equations are written without deltas, *x* means Δ*x*, etc. We are, in general, always concerned with the space and time *differences* between events.

Calling one set of transformations the normal Lorentz transformations and the other the inverse transformations is misleading, since there is no intrinsic difference between the frames. Different authors call one or the other set of transformations the "inverse" set. The forwards and inverse transformations are trivially related to each other, since the *S* frame can only be moving forwards or reverse with respect to *S*′. So inverting the equations simply entails switching the primed and unprimed variables and replacing *v* with −*v*.^{[43]}^{: 71–79 }

**Example:** Terence and Stella are at an Earth-to-Mars space race. Terence is an official at the starting line, while Stella is a participant. At time *t* = *t*′ = 0, Stella's spaceship accelerates instantaneously to a speed of 0.5 *c*. The distance from Earth to Mars is 300 light-seconds (about 90.0×10^{6} km). Terence observes Stella crossing the finish-line clock at *t* = 600.00 s. But Stella observes the time on her ship chronometer to be as she passes the finish line, and she calculates the distance between the starting and finish lines, as measured in her frame, to be 259.81 light-seconds (about 77.9×10^{6} km).
1).

There have been many dozens of derivations of the Lorentz transformations since Einstein's original work in 1905, each with its particular focus. Although Einstein's derivation was based on the invariance of the speed of light, there are other physical principles that may serve as starting points. Ultimately, these alternative starting points can be considered different expressions of the underlying principle of locality, which states that the influence that one particle exerts on another can not be transmitted instantaneously.^{[44]}

The derivation given here and illustrated in Fig. 3-5 is based on one presented by Bais^{[41]}^{: 64–66 } and makes use of previous results from the Relativistic Composition of Velocities, Time Dilation, and Length Contraction sections. Event P has coordinates (*w*, *x*) in the black "rest system" and coordinates (*w*′, *x*′) in the red frame that is moving with velocity parameter *β* = *v*/*c*. To determine *w*′ and *x*′ in terms of *w* and *x* (or the other way around) it is easier at first to derive the *inverse* Lorentz transformation.

- There can be no such thing as length expansion/contraction in the transverse directions.
*y'*must equal*y*and*z*′ must equal*z*, otherwise whether a fast moving 1 m ball could fit through a 1 m circular hole would depend on the observer. The first postulate of relativity states that all inertial frames are equivalent, and transverse expansion/contraction would violate this law.^{[43]}^{: 27–28 } - From the drawing,
*w*=*a*+*b*and*x*=*r*+*s* - From previous results using similar triangles, we know that
*s*/*a*=*b*/*r*=*v*/*c*=*β*. - Because of time dilation,
*a*=*γw′* - Substituting equation (4) into
*s*/*a*=*β*yields*s*=*γw′β*. - Length contraction and similar triangles give us
*r*=*γx′*and*b*=*βr*=*βγx′* - Substituting the expressions for
*s*,*a*,*r*and*b*into the equations in Step 2 immediately yield

The above equations are alternate expressions for the t and x equations of the inverse Lorentz transformation, as can be seen by substituting *ct* for *w*, *ct*′ for *w*′, and *v*/*c* for *β*. From the inverse transformation, the equations of the forwards transformation can be derived by solving for *t*′ and *x*′.

The Lorentz transformations have a mathematical property called linearity, since *x*′ and *t*′ are obtained as linear combinations of *x* and *t*, with no higher powers involved. The linearity of the transformation reflects a fundamental property of spacetime that was tacitly assumed in the derivation, namely, that the properties of inertial frames of reference are independent of location and time. In the absence of gravity, spacetime looks the same everywhere.^{[41]}^{: 67 } All inertial observers will agree on what constitutes accelerating and non-accelerating motion.^{[43]}^{: 72–73 } Any one observer can use her own measurements of space and time, but there is nothing absolute about them. Another observer's conventions will do just as well.^{[3]}^{: 190 }

A result of linearity is that if two Lorentz transformations are applied sequentially, the result is also a Lorentz transformation.

**Example:** Terence observes Stella speeding away from him at 0.500 *c*, and he can use the Lorentz transformations with *β* = 0.500 to relate Stella's measurements to his own. Stella, in her frame, observes Ursula traveling away from her at 0.250 *c*, and she can use the Lorentz transformations with *β* = 0.250 to relate Ursula's measurements with her own. Because of the linearity of the transformations and the relativistic composition of velocities, Terence can use the Lorentz transformations with *β* = 0.666 to relate Ursula's measurements with his own.

The Doppler effect is the change in frequency or wavelength of a wave for a receiver and source in relative motion. For simplicity, we consider here two basic scenarios: (1) The motions of the source and/or receiver are exactly along the line connecting them (longitudinal Doppler effect), and (2) the motions are at right angles to the said line (transverse Doppler effect). We are ignoring scenarios where they move along intermediate angles.

The classical Doppler analysis deals with waves that are propagating in a medium, such as sound waves or water ripples, and which are transmitted between sources and receivers that are moving towards or away from each other. The analysis of such waves depends on whether the source, the receiver, or both are moving relative to the medium. Given the scenario where the receiver is stationary with respect to the medium, and the source is moving directly away from the receiver at a speed of *v _{s}* for a velocity parameter of

On the other hand, given the scenario where source is stationary, and the receiver is moving directly away from the source at a speed of *v _{r}* for a velocity parameter of

Light, unlike sound or water ripples, does not propagate through a medium, and there is no distinction between a source moving away from the receiver or a receiver moving away from the source. Fig. 3-6 illustrates a relativistic spacetime diagram showing a source separating from the receiver with a velocity parameter so that the separation between source and receiver at time is . Because of time dilation, Since the slope of the green light ray is −1, Hence, the relativistic Doppler effect is given by^{[41]}^{: 58–59 }

Suppose that a source and a receiver, both approaching each other in uniform inertial motion along non-intersecting lines, are at their closest approach to each other. It would appear that the classical analysis predicts that the receiver detects no Doppler shift. Due to subtleties in the analysis, that expectation is not necessarily true. Nevertheless, when appropriately defined, transverse Doppler shift is a relativistic effect that has no classical analog. The subtleties are these:^{[45]}^{: 541–543 }

- Fig. 3-7a. What is the frequency measurement when the receiver is geometrically at its closest approach to the source? This scenario is most easily analyzed from the frame S′ of the source.
^{[note 10]} - Fig. 3-7b. What is the frequency measurement when the receiver
*sees*the source as being closest to it? This scenario is most easily analyzed from the frame S of the receiver.

Two other scenarios are commonly examined in discussions of transverse Doppler shift:

- Fig. 3-7c. If the receiver is moving in a circle around the source, what frequency does the receiver measure?
- Fig. 3-7d. If the source is moving in a circle around the receiver, what frequency does the receiver measure?

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In scenario (a), the point of closest approach is frame-independent and represents the moment where there is no change in distance versus time (i.e. dr/dt = 0 where *r* is the distance between receiver and source) and hence no longitudinal Doppler shift. The source observes the receiver as being illuminated by light of frequency *f*′, but also observes the receiver as having a time-dilated clock. In frame S, the receiver is therefore illuminated by blueshifted light of frequency

In scenario (b) the illustration shows the receiver being illuminated by light from when the source was closest to the receiver, even though the source has moved on. Because the source's clocks are time dilated as measured in frame S, and since dr/dt was equal to zero at this point, the light from the source, emitted from this closest point, is redshifted with frequency