# Time Supplement

This supplement answers a series of questions designed to reveal more about what science requires of physical time, and to provide background information about other topics discussed in the Time article.

## 1. What Are Instants and Durations?

A duration is an amount of time. The duration of Earth’s existence is about five billion years; the duration of a flash of lightning is 0.0002 seconds. Years and seconds are not durations; they are measures of durations. The second is the standard unit for the measurement of time in the SI system (the International Systems of Units, that is, Le Système International d’Unités). In informal conversation, an instant is a very short duration. In physics, however, an instant is instantaneous; it is not a finite duration but rather a “point” in time, or “a time.” The day begins at the instant called “midnight.” It is an interesting question whether a finite duration of a real event is always a linear continuum of instants, and, if so, how we know this.

## 2. What Is an Event?

In ordinary discourse, an event is a happening lasting a finite duration during which some object changes its properties. For example, this morning’s event of buttering the toast is the toast’s changing from unbuttered to buttered. Without mentioning objects and properties, an event might instead be defined simply as whatever is temporally before or after anything else. In ordinary discourse, unlike in physics, events are not basic, but rather are defined in terms of objects and their properties. In physics it is the other way round. Events are basic, and objects are defined in terms of them.

In physics the word “event” is a technical term that differs from the way we use it in ordinary language. An event for a physicist can last no time at all and not involve any change in a property and not involve more than a single point of space; these events are called “point events” and all other events are said to be composed of point events. When we speak of a point in a spacetime diagram being a spacetime event, that’s a non-standard use of the word “event” because no change needs to be taking place there then.

A mathematical space is a collection of points, and the points might represent anything, for example, dollars. But the points of a real space, that is, a physical space, are locations. For example, the place called “New York City” at one time is composed of the actual point locations which occur within the city’s boundary at that time.  A point event in a Minkowski diagram is merely a tiniest place in space where an event might or might not be known happen at that instant. That is, at some location in the past, such as at all the point locations constituting the Ford Theater in Washington DC on April 14, 1865, we might know that this is the event of U.S. President Abraham Lincoln’s assassination, but at other locations in the past we may have no idea whether any matter was present there then. For the future point events in the diagram, we usually know now hardly anything about what will happen there.

The physicists’ notion of point events is metaphysically unacceptable to many philosophers, in part because it deviates so much from the way “event” is used in ordinary language. In 1936, in order to avoid point events, Bertrand Russell and A. N. Whitehead developed a theory of time based on the assumption that all events in spacetime have a finite, non-zero duration. However, they had to assume that any finite part of an event is an event, and this assumption is no closer to common sense than the physicist’s assumption that all events are composed of point events. The encyclopedia article on Zeno’s Paradoxes mentions that Michael Dummett and Frank Arntzenius have continued in the 21st century to develop Russell’s and Whitehead’s idea that any event must have a non-zero duration.

McTaggart argued early in the twentieth century that events change. For example, the event of Queen Anne’s death is changing because it is receding ever farther into the past as time goes on. It is an open question in philosophy as to whether the passage of time is a feature of the world to be explained by noting how events change, such as their changing by growing deeper into the past. Many philosophers believe it is improper to consider an event to be something that can change.

For a more detailed discussion of what an event is, see the article Events.

## 3. What Is a Reference Frame?

A reference frame for a space is a coordinate system, namely a standard point of view or a perspective for making observations, measurements and judgments that assigns unique values to each point of space.

Choosing a good reference frame can make a situation much easier to describe. If you are trying to describe the motion of a car down a straight highway, you would not want to choose a reference frame that is fixed to a spinning carousel. Instead, choose a reference frame fixed to the highway. The motion of a planet is very complex as seen from Earth over many months. However, the motion is very simple in a frame of reference at rest relative to the Sun. Inertial frames are very special reference frames, as we shall see below.

A reference frame is often specified by selecting a solid object that doesn’t change its size and by saying that the reference frame is fixed to the object. We might select a reference frame fixed to the Rock of Gibraltar. Another object is said to be at rest in the reference frame if it remains at a constant distance in a fixed direction from the reference body used to define the frame. For example, your house is at rest in a reference frame fixed to the Rock of Gibraltar [not counting your house's vibrating when a truck drives by, nor the house's speed due to plate tectonics]. When we say the Sun rose this morning, we are implicitly choosing a reference frame fixed to the Earth’s surface. The Sun is not at rest in this reference frame.

The reference frame or coordinate system must specify locations, and this is normally done by assigning numbers to points of space. In a flat (that is, Euclidean) three-dimensional space, the analyst needs to specify four distinct points on the reference body, or four objects mutually at rest somewhere in the frame. One point is the origin, and the other three can be used to define three independent, perpendicular axes, the familiar x, y and z directions, assuming a Cartesian (that is, flat or rectangular) coordinate system were to be used. Two point objects are at the same place if they have the same x-value, the same y-value and the same z-value. To keep track of events, you will also need a time axis, a “t” axis, and so you will expand your three-dimensional mathematical space to a four-dimensional mathematical space. Two point events are simultaneous if they occur at the same place and also at the same time. In this way, the analyst is placing a four-dimensional coordinate system on the space and time. The coordinates could have been letters instead of numbers, but numbers are the best choice because we want to use them for measurement, not just for naming places.

The fact that physical spacetime has curvature implies that no single rigid (or Cartesian) coordinate system is capable of covering the entire spacetime. To cover all of spacetime in that case, we must make do with covering different regions of spacetime with different coordinate patches that are “knitted together” where one patch meets another. No single coordinate system can cover the surface of a sphere without creating a singularity, but the sphere can be covered by patching together two coordinate systems. Nevertheless if we can live with non-rigid curvilinear coordinates, then any curved spacetime can be covered with a global four-dimensional coordinate system in which every point being uniquely identified with a set of four numbers in a continuous way. That is, we use a curved coordinate system on curved spacetime.

That we use four numbers per point indicates the space is four-dimensional. And in creating coordinate systems for spaces, the usual assumption is that we should supply n independent numbers to specify a place in an n-dimensional space, where n is an integer. This is usual but not required; instead we could exploit the idea that there are space-filling curves which permit a single continuous curve to completely fill, and thus coordinatize, a region of dimension higher than one, e.g., a plane. For this reason (namely, that each point in n-dimensional space doesn’t always need n numbers to name the point), the contemporary definition of “dimension” is rather exotic.

## 4. What Is an Inertial Frame?

An inertial frame is either a non-accelerating frame, or, less generally, a reference frame in which Newton’s laws of motion hold. Any spacetime obeying the laws of Special Relativity can have an inertial frame across the whole universe. In General Relativity any very small region of spacetime can have an inertial frame.

Suppose you’ve pre-selected your frame. How do you tell if you are in an inertial frame? The answer is that you check that objects accelerate only when acted on by forces. That is, you check that any object’s acceleration is zero if no net force acts on the object. If no unbalanced external forces are acting on a moving object, then the object moves in a straight line. It doesn’t curve; it coasts. And it travels equal distances in equal amounts of time.

Any frame of reference moving at constant velocity relative to an inertial frame is also an inertial frame. A reference frame spinning relative to an inertial frame is never an inertial frame.

Einstein’s theory of special relativity is intended to apply only to inertial frames. According to the theory, the speed of light in a vacuum is the same when observed from any inertial frame of reference. Unlike the speed of a spaceship, the speed of light in a vacuum isn’t affected by which inertial reference frame is used for the measurement. If you have two relatively stationary, synchronized clocks in an inertial frame, then they will read the same time, but if one moves relative to the other, then they will get out of synchrony. This loss of synchrony due to relative motion is called “time dilation.”

The presence of gravitation normally destroys any possibility of finding a perfect inertial frame. Nevertheless, any spacetime obeying the general theory of relativity and thus accounting for gravitation will be locally Minkowskian in the sense that any infinitesimal region of spacetime is an inertial frame obeying the principles of special relativity.

## 5. What Is Spacetime?

Spacetime is where events are located, or, depending on your theory of spacetime, it’s all possible events. The dimensions of spacetime include the time dimension of before-after and the three ordinary space dimensions of, say, up-down, left-right, and forward-backward. That is, spacetime is usually represented with a four-dimensional mathematical space, one of whose dimensions represents time and three of whose dimensions represent space.

Spacetime is the intended model of the general theory of relativity. This requires it to be a differentiable space in which physical objects obey the equations of motion of the theory. Minkowski space (that is, Minkowski spacetime) is the model of special relativity. It’s a certain 4-dimensional real vector space. General relativity theory requires that spacetime be locally like Minkowski spacetime.

Hermann Minkowski, in 1908, was the first person to say that spacetime is fundamental and that space and time are just aspects of spacetime. Minkowski meant it is fundamental in the sense that the spacetime interval between any two events is intrinsic to spacetime and does not vary with the reference frame, unlike a spatial distance or temporal duration.

Spacetime is a continuum in which we can define points and straight lines. However, these points and lines do not satisfy the principles of Euclidean geometry when gravity is present. Einstein showed that the presence of gravity affects geometry by warping space and time. Einstein’s principal equation in his general theory of relativity implies that the curvature of spacetime is directly proportional to the density of mass in the spacetime. That is, Einstein says the structure of spacetime changes as matter moves because the gravitational field from matter actually curves spacetime. Black holes are a sign of radical curvature. The Earth’s curving of spacetime is very slight but still significant enough that it must be accounted for in clocks of the Global Positioning Satellites (GPS) along with the other time dilation effect that is caused by speed. The GPS satellites are launched with their clocks adjusted so that when they reach orbit they mark time the same as Earth-based clocks

There have been serious attempts over the last few decades to construct theories of physics in which spacetime is a product of more basic entities. The primary aim of these new theories is to unify relativity with quantum theory. So far these theories have not stood up to any empirical observations or experiments that could show them to be superior to the presently accepted theories. So, for the present, the concept of spacetime remains fundamental.

The metaphysical question of whether spacetime is a substantial object or a relationship among events, or neither, is considered in the discussion of the relational theory of time.

## 6. What Is a Minkowski Diagram?

A spacetime diagram is a representation of the point-events in spacetime. A Minkowski diagram is a representation of a spacetime obeying the laws of special relativity. In a Minkowski spacetime diagram, normally a rectangular coordinate system is used, the time axis is shown vertically, one or two of the spatial axes are suppressed (that is, not included), and an object’s inertial motion (its coasting) produces a series of events that are represented by a straight line. Here is an example with only one space dimension included. The above diagram shows a point sized Einstein standing still midway between the two places at which there is a flash of light. The directed arrows represent the path of light rays from the flash. In a Minkowski diagram, a physical (point) object is not represented as occupying a point but as occupying a line containing all the spacetime points at which it exists. That line, which usually isn’t straight, is called the worldline of the object. In the above diagram, Einstein’s worldline is a vertical straight line. If an object’s worldline intersects or meets another object’s worldline, then the two objects collide at the point of intersection. The units along the vertical time axis are customarily chosen to be the product of time and the speed of light so that worldlines of light rays make a forty-five degree angles with each axis. The set of all light speed worldlines going through an event defines the two light cones of that event: the past light cone and the future light cone.

Inertial motion produces a straight worldline, and accelerated motion produces a curved worldline. If at some time Einstein were to jump on a train moving by at constant speed, then his worldline would, from that time upward, tilt away from the vertical and form some angle less than 45 degrees with the time axis. Events on the same horizontal line of the Minkowski diagram are simultaneous in that reference frame. A moving observer is added to this diagram to produce the diagram below in the discussion about the relativity of simultaneity.

Not all spacetimes can be given Minkowski diagrams, but any spacetime satisfying Einstein’s Special Theory of Relativity can. Minkowski diagrams are diagrams of a Minkowski space, which is a spacetime satisfying the Special Theory of Relativity and having zero vacuum energy. Einstein’s Special Theory falsely presupposes that physical processes, such as gravitational processes, have no effect on the structure of spacetime. When attention needs to be given to the real effect of these processes on the structure of spacetime, that is, when general relativity needs to be used, then Minkowski diagrams become inappropriate for spacetime. General relativity assumes that the geometry of spacetime is locally Minkowskian but not globally. That is, spacetime is locally flat in the sense that in any very small region one always finds spacetime to be 4-D Minkowskian (but not 4-D Euclidean). Special relativity holds in infinitesimally small region of spacetime that satisfies general relativity, and so any such region can be fitted with an inertial reference frame. When we say spacetime is “really curved” and not flat, we mean it really deviates from 4-D Minkowskian geometry.

To repeat a point made earlier, when we speak of a point in these diagrams being a spacetime event, that’s a non-standard use of the word “event.” A point event in a Minkowski diagram is merely a location in spacetime where an event might or might not happen. The point exists even if no object is actually there.

## 7. What Are the Metric and the Interval?

A space is simply a collection of points. How far is it between any two points? The metric is the answer to this question. A metric on a space provides a definition of distance (or length) by giving a function from each pair of points to a real number, called the distance between the points.  In Euclidean space, the distance between two points is the length of the straight line connecting them. The metric of a space determines its geometry, and this metric and geometry are intrinsic in the sense that they do not change as we change the reference frame.

The natural metric for any two points in a one-dimensional Euclidean space, such as time, is the numerical difference between the coordinates of the two points. For example, the duration between an event with the coordinate 5 AM and an event with the coordinate 7 AM is two hours (assuming the events occur on the same day). If we select a standard clock, then that clock implicitly defines the metric of time since it yields the correct answer for the duration between any two point events. When we define the unit of time, the second, to be so many ticks of the standard clock, we are implicitly specifying the metric. For example, to speak simplistically, if you want to know how much time has passed between the birth of Jesus and the death of Abraham Lincoln, then you find the tick on the standard clock that is simultaneous with the birth of Jesus and you count how many ticks occurred until the tick that is simultaneous with the death of Abraham Lincoln.

Points of space are located by being assigned a coordinate. For doing quantitative science rather than merely qualitative science we want the coordinate to be a real number, not, say, a letter of the alphabet. A coordinate for a point in two-dimensional space requires two numbers; a coordinate for a point in n-dimensional space requires n numbers.

In a 2-dimensional space, the metric for the distance between the point (x,y) with Cartesian coordinates x and y and the point (x’,y’) with coordinates x’ and y’ is defined to be the square root of (x’ – x)2 + (y’ – y)2 when the space is flat, that is, Euclidean. If the space is not flat, then a more sophisticated definition of the metric is required. Note the application of the Pythagorean Theorem for Euclidean space.

Our intuitive idea of what a distance is tells us that, however we define distance for a space, we want it to have the following distance-like properties. Letting d(p,q) stand for the distance between any two points p and q in the space, the following four conditions must be satisfied:

1. d(p,p) = 0, and d(p,q) is greater than or equal to 0
2. If d(p,q) = 0, then p = q
3. d(p,q) = d(q,p)
4. d(p,q) + d(q,r) is greater than or equal to d(p,r)

Notice that there is no mention of the path the distance is taken across; all the attention is on the point pairs themselves.

Do these conditions capture your idea of distance? If you were to check, you’d find that the 2-D metric defined above, namely the square root of (x’ – x)2 + (y’ – y)2, does satisfy these four conditions. In 3-D Euclidean space, the metric that is defined to be the square root of (x’ – x)2 + (y’ – y)2 + (z’ – z)2 works very well.

Consider the 4-D mathematical space that is used to represent the spacetime obeying the laws of special relativity theory, namely Minkowski spacetime. What’s an appropriate metric for this space? Well, if we were just interested in the space part of this spacetime, then the above 3-D Euclidean metric is fine. But we’ve asked a delicate question because the fourth dimension of this mathematical space is really a time dimension and not a space dimension. Here is the so-called Lorentzian metric or Minkowski metric for any pair of point events at (x’,y’,z’,t’) and (x,y,z,t) in Minkowskian 4-D spacetime:

Δs2 = – (x’ – x)2 – (y’ – y)2 – (z’ – z)2 + c2(t’ – t)2

Δs is called the interval of Minkowski spacetime. The interval corresponds to what clocks measure between a pair of timelike events [that is, between a pair of events separated enough in time that one could have had a causal effect on the other] and what rulers measure between a pair of spacelike events. One other happy feature of this metric is that the value of the interval is unaffected by changing to a new reference frame provided the new reference frame is not accelerating relative to the first. Changing from a first frame to a new, unaccelerated reference frame on the spacetime will change the values of all the coordinates of the points of the spacetime, but some relations between all pairs of points won’t be affected, namely the intervals between pairs of points. Take any two observers who use a reference frame in which they, themselves are fixed, and assume they are moving at constant velocity relative to each other. Now consider some single event with a finite duration. The two observers won’t agree on how long the event lasts, but they will always agree on the interval between the beginning and end of the event.

The interval of spacetime is complicated because its square can be negative, unlike with the space intervals we’ve discussed so far. If Δs2 is negative, the two points have a space-like separation, meaning these events have a greater separation in space than they do in time. If Δs2 is positive, then the two have a time-like separation, meaning enough time has passed that one event could have had a causal effect on the other. If Δs2 is zero, the two events might be identical, or they might have occurred millions of miles apart. In ordinary space, if the space interval between two events is zero, then the two events happened at the same time and place, but in spacetime, if the spacetime interval between two events is zero, this means only that there could be a light ray connecting them. All the events that have a zero spacetime interval from some event e constitute e’s two light cones because they have the shape of cones when represented in a Minkowski diagram, one cone for events in e’s future and one cone for events in e’s past. It is because the spacetime interval between two events can be zero even when the events are far apart in distance that the term “interval” is very unlike what we normally mean by the term “distance.”

Because true metrics are always positive, the Lorentzian metric that we used above for Minskowski spacetime is not a true metric, nor even a pseudometric; but it is customary for physicists to refer to it loosely as a “metric” because Δs retains enough other features of distance.

Adding space and time dependence to each term of the Lorentzian metric produces the metric for general relativity.

## 8. Does the Theory of Relativity Imply Time Is Partly Space?

In 1908, Minkowski remarked that “Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.” Many people took this to mean that time is partly space, and vice versa. C. D. Broad countered that the discovery of spacetime did not break down the distinction between time and space but only their independence or isolation. He argued that their lack of independence does not imply a lack of reality. The Broad-Minkowski disagreement is still an issue in philosophy, but if Broad is correct, then time is time; it’s not space at all.

Nevertheless, there is a deep sense in which time and space are “mixed up” or linked. This is evident from the Lorentz transformations of special relativity that connect the time t in one inertial frame with the time t’ in another frame that is moving in the x direction at a constant speed v. In this equation, t’ is dependent upon the space coordinate x and the speed. In this way, time is not independent of either space or speed. It follows that the time between two events could be zero in one frame but not zero in another. Each frame has its own way of splitting up spacetime into its space part and its time part.

The reason why time is not partly space is that, within a single frame, time is distinct from space. Time is not simply an arbitrary one-dimensional sub-space of spacetime; it is a distinguished sub-space. That is, time is a distinguished dimension of spacetime, not an arbitrary dimension. What being distinguished amounts to is that when you set up a rectangular coordinate system on spacetime with an origin at, say, the event of Mohammed’s birth, you may point the x-axis east or toward Mecca or away from the center of Earth, but you may not point it forward in time—you may do that only with the t-axis, the time axis.

## 9. Is Time the Fourth Dimension?

Yes and no; it depends on what you are talking about. Time is the fourth dimension of 4-d spacetime, but time is not the fourth dimension of space, the space of places.

Mathematicians have a broader notion of the term “space” than the average person; and in their sense a space need not consist of places, that is, geographical locations. Not paying attention to the two meanings of the term “space” is the source of all the confusion about whether time is the fourth dimension. The mathematical space used by mathematical physicists to represent physical spacetime is four dimensional and in that space, the space of places is a 3-d sub-space and time is another 1-d sub-space. Minkowski was the first person to construct such a mathematical space, although in 1895 H. G. Wells treated time as a fourth dimension in his novel The time Machine. Spacetime is represented mathematically by Minkowski as a space of events, not as a space of ordinary geographical places.

In any coordinate system on spacetime, it takes at least four independent numbers to determine a spacetime location. In any coordinate system on the space of places, it takes at least three. That’s why spacetime is four dimensional but the space of places is three dimensional. Actually this 19th century definition of dimensionality, which is due to Bernhard Riemann, is not quite adequate because mathematicians have subsequently discovered how to assign each point on the plane to a point on the line without any two points on the plane being assigned to the same point on the line. The idea comes from Georg Cantor. Because of this one-to-one correspondence, the points on a plane could be specified with just one number. If so, then the line and plane must have the same dimensions according to the Riemann definition. To avoid this problem and to keep the plane being a 2-d object, the notion of dimensionality of a space has been given a new, but rather complex, definition.

## 10. Is There More Than One Kind of Physical Time?

Every reference frame has its own physical time, but the question is intended in another sense. At present, physicists measure time electromagnetically. They define a standard atomic clock using periodic electromagnetic processes in atoms, then use electromagnetic signals (light) to synchronize clocks that are far from the standard clock. In doing this, are physicists measuring ‘”electromagnetic time” but not other kinds of physical time?

In the 1930s, the physicists Arthur Milne and Paul Dirac worried about this question. Independently, they suggested there may be very many time scales. For example, there could be the time of atomic processes and light, which is measured best by atomic clocks. There also could be the time of gravitation and large-scale physical processes, which is measured best by the rotation of a pulsar (pulsating star). The two physicists worried that the atomic clock and the astronomical clock might drift out of synchrony after being initially synchronized, yet there would be no reasonable explanation for why they don’t stay in synchrony. Ditto for clocks based on the pendulum, on superconducting resonators, on the spread of electromagnetic radiation through space, and on other physical principles. Just imagine the difficulty for physicists if they had to work with electromagnetic time, gravitational time, nuclear time, neutrino time, and so forth. Current physics, however, has found no reason to assume there is more than one kind of time for physical processes.

In 1967, physicists did reject the astronomical standard for the atomic standard because the deviation between known atomic and gravitation periodic processes could be explained better assuming that the atomic processes were the more regular of the two. Physicists had no reason to believe that a gravitational periodic process, that is just as regular initially as the atomic process and that is not affected by friction or impacts or other forces, would ever drift out of synchrony with the atomic process, yet this is the possibility that worried Milne and Dirac.

## 11. How is Time Relative to the Observer?

Physical time is not relative to any observer’s state of mind. Wishing time will pass does not affect the rate at which the observed clock ticks. On the other hand, physical time is relative to the observer’s reference system–in trivial ways and in a deep way discovered by Albert Einstein.

In a trivial way, time is relative to the chosen coordinate system on the reference frame, though not to the reference frame itself. For example, it depends on the units chosen as when the duration of some event is 34 seconds if seconds are defined to be a certain number of ticks of the standard clock, but is 24 seconds if seconds are defined to be a different number of ticks of that standard clock. Similarly, the difference between the Christian calendar and the Jewish calendar for the date of some event is due to a different unit and origin. Also trivially, time depends on the coordinate system when a change is made from Eastern Standard Time to Pacific Standard Time. These dependencies are taken into account by scientists but usually never mentioned. For example, if a pendulum’s approximately one-second swing is measured in a physics laboratory during the autumn night when the society changes from Daylight Savings Time back to Standard Time, the scientists do not note that one unusual swing of the pendulum that evening took a negative fifty-nine minutes and fifty-nine seconds instead of the usual one second.

Isn’t time relative to the observer’s coordinate system in the sense that in some reference frames there could be fifty-nine seconds in a minute? No, due to scientific convention, it is absolutely certain that there are sixty seconds in any minute in any reference frame. How long an event lasts is relative to the reference frame used to measure the time elapsed, but in any reference frame there are exactly sixty seconds in a minute because this is true by definition. Similarly, you do not need to worry that in some reference frame there might be two gallons in a quart.

In a deeper sense, time is relative, not just to the coordinate system, but to the reference frame itself. That is Einstein’s principal original idea about time.

Einstein’s idea is that without reference to the frame, there is no fixed time interval between two events, no ‘actual’ duration between them. Einstein illustrated his idea for two observers, one on a moving train in the middle of the train, and a second observer standing on the embankment next to the train tracks. If the observer sitting in the middle of the rapidly moving train receives signals simultaneously from lightning flashes at the front and back of the train, then in his reference frame the two lightning strikes were simultaneous. But the strikes were not simultaneous in a frame fixed to an observer on the ground. This outside observer will say that the flash from the back had farther to travel because the observer on the train was moving away from the flash. If one flash had farther to travel, then it must have left before the other one, assuming that both flashes moved at the same speed. Therefore, the lightning struck the back of the train before the lightning struck the front of the train in the reference frame fixed to the tracks.

Let’s assume that a number of observers are moving with various constant speeds in various directions. Consider the inertial frame of reference in which each observer is at rest in his or her own frame. Which of these observers will agree on their time measurements? Only observers with zero relative speed will agree. Observers with different relative speeds will not, even if they agree on how to define the second and agree on some event occurring at time zero (the origin of the time axis). If two observers are moving relative to each other, but each makes judgments from a reference frame fixed to themselves, then the assigned times to the event will disagree more, the faster their relative speed. All observers will be observing the same objective reality, the same event in the same spacetime, but their different frames of reference will require disagreement about how spacetime divides up into its space part and its time part.

This relativity of time to reference frame implies that there be no such thing as The Past in the sense of a past independent of reference frame. This is because a past event in one reference frame might not be past in another reference frame.

In some reference frame, was Adolf Hitler born before George Washington? No, because the two events are causally connectible. That is, one event could in principle have affected the other since light would have had time to travel from one to the other. We can select a reference frame to reverse the usual Earth-based order of two events only if they are not causally connectible, that is, only if one event is in the absolute elsewhere of the other. Despite the relativity of time to a reference frame, any two observers in any two reference frames should agree about which of two causally connectible events happened first.

## 12. What Are the Relativity and Conventionality of Simultaneity?

Leibniz suggested that simultaneous events are those that in principle could not have causally affected each other. The suggestion is unacceptable in relativity theory which implies that causal influence moves no faster than the speed of light and that simultaneity is relative to reference frame. If the universe obeys relativistic physics, then events that occur simultaneously with respect to one reference frame will not occur simultaneously in another reference frame that is moving with respect to the first frame. This is called the “relativity of simultaneity.” It applies only to pairs of events in each other’s absolute elsewhere because, if the two aren’t in each other’s absolute elsewhere, then there is a reference-frame-free answer to which of the two occurred first. In the diagram below, the two light flashes are in each other’s absolute elsewhere. This Minkowski diagram represents Einstein sitting still in the reference frame while Lorentz is traveling rapidly away from him and toward the source of flash 2. Because Lorentz’s timeline is a straight line we can tell that he is moving at a constant speed. The two flashes of light arrive at Einstein’s location simultaneously, creating spacetime event B. However, Lorentz sees flash 2 before flash 1. That is, the event A of Lorentz seeing flash 2 occurs before event C of Lorentz seeing flash 1. So, Einstein will readily say the flashes are simultaneous, but Lorentz will have to do some computing to figure out that the flashes are simultaneous in the frame because they won’t “look” simultaneous. However, if we’d chosen a different reference frame from the one above, one in which Lorentz is not moving but Einstein is, then Lorentz would be correct to say flash 2 occurs before flash 1 in that new frame. So, whether the flashes are or are not simultaneous depends on which reference frame is used in making the judgment. It’s all relative.

This relativity of simultaneity is philosophically less controversial than the conventionality of simultaneity. To appreciate the difference, consider what is involved in making a determination regarding simultaneity. Given two events that happen essentially at the same place, physicists assume they can tell by direct observation whether the events happened simultaneously. If we don’t see one of them happening first, then we say they happened simultaneously, and we assign them the same time coordinate. The determination of simultaneity is more difficult if the two happen at separate places, especially if they are very far apart. One way to measure (operationally define) simultaneity at a distance is to say that two events are simultaneous in a reference frame if unobstructed light signals from the two events would reach us simultaneously when we are midway between the two places where they occur, as judged in that frame. This is the operational definition of simultaneity used by Einstein in his theory of relativity. Instead of using the midway method, we could take the distant clock and send a signal home to our master clock, one already synchronized with our standard clock; the master clock immediately sends a signal back to the distant clock with the information about what time it was when the signal arrived. We at the distant clock notice that the total travel time is t and that the master clock’s signal says its time is, say, noon, so we immediately set our clock to be noon plus half of t.

The “midway” method described above of operationally defining simultaneity in one reference frame for two distant signals causally connected to us has a significant presumption: that the light beams travel at the same speed regardless of direction. Einstein, Reichenbach and Grünbaum have called this a reasonable “convention” because any attempt to experimentally confirm it presupposes that we already know how to determine simultaneity at a distance. This is the conventionality, rather than relativity, of simultaneity. To pursue the point, suppose the two original events are in each other’s absolute elsewhere; they couldn’t have affected each other. Einstein noticed that there is no physical basis for judging the simultaneity or lack of simultaneity between these two events, and for that reason said we rely on a convention when we define distant simultaneity as we do. Hillary Putnam, Michael Friedman, and Graham Nerlich object to calling it a convention–on the grounds that to make any other assumption about light’s speed would unnecessarily complicate our description of nature, and we often make choices about how nature is on the basis of simplification of our description. They would say there is less conventionality in the choice than Einstein supposed.

The “midway” method isn’t the only way to define simultaneity. Consider a second method, the “mirror reflection” method. Select an Earth-based frame of reference, and send a flash of light from Earth to Mars where it hits a mirror and is reflected back to its source. The flash occurred at 12:00, let’s say, and its reflection arrived back on Earth 20 minutes later. The light traveled the same empty, undisturbed path coming and going. At what time did the light flash hit the mirror? The answer involves the so-called conventionality of simultaneity. All physicists agree one should say the reflection event occurred at 12:10. The controversial philosophical question is whether this is really a convention. Einstein pointed out that there would be no inconsistency in our saying that it hit the mirror at 12:17, provided we live with the awkward consequence that light was relatively slow getting to the mirror, but then traveled back to Earth at a faster speed. If we picked the impact time to be 12:05, we’d have to live with the fact that light traveled slower coming back. There is a physical basis for not picking the impact time to be less than noon nor later than 12:20, because doing so would violate the physical principle that causes precede their effects. One requirement we place on the concept of simultaneity is that distant events which are simultaneous could not be in causal contact with each other. We can satisfy that requirement for any choice of impact time from 12:00 to 12:20.

## 13. What Is the Difference between the Past and the Absolute Past? A single point’s absolute elsewhere, absolute future, and absolute past partition all of spacetime beyond the point into three disjoint regions. If point A is in point B’s absolute elsewhere, the two events are said to be “spacelike related.” If the two are in each other’s forward or backward light cones they are said to be “timelike related” or “causally connectible.”

## 14. What is Time Dilation?

According to special relativity, two properly functioning clocks next to each other will stay synchronized. Even if they were to be far away from each other, they’d stay synchronized if they didn’t move relative to each other. But if one clock moves away from the other, the moving clock will tick slower than the stationary clock, as measured in the inertial reference frame of the stationary clock. This slowing due to motion is called “time dilation.” If you move at 99% of the speed of light, then your time slows by a factor of 7 relative to stationary clocks. In addition, you are 7 times thinner than when you are stationary, and you are 7 times heavier. If you move at 99.9%, then you slow by a factor of 22.

Time dilation is about two synchronized clocks getting out of synchrony due either to their relative motion or due to their being in different gravitational fields. Time dilation due to difference in constant speeds is described by Einstein’s special theory of relativity. The general theory of relativity describes a second kind of time dilation, one due to different accelerations and different gravitational influences. Suppose your twin’s spaceship travels to and from a star one light year away. It takes light from your Earth-based flashlight two years to go there and back. But if the spaceship is fast, your twin can make the trip in less than two years, according to his own clock. Does he travel the distance in less time than it takes light to travel that distance? No, according to your clock he takes more than two years, and so is slower than light.

We sometimes speak of time dilation by saying time itself is “slower,” but time isn’t going slower in any absolute sense, only relative to some other frame of reference. Does time have a rate? Well, time in a reference frame has no rate in that frame, but time in a reference frame can have a rate as measured in a different frame, such as in a frame moving relative to the first frame.

Time dilation is not an illusion of perception; and it is not a matter of the second having different definitions in different reference frames.

Newton’s physics describes duration as an absolute property, implying it is not relative to the reference frame. However, in Newton’s physics the speed of light is relative to the frame. Einstein’s special theory of relativity reverses both of these aspects of time. For inertial frames, it implies the speed of light is not relative to the frame, but duration is relative to the frame. In general relativity, however, the speed of light can vary within one reference frame if matter and energy are present.

Time dilation due to motion is relative in the sense that if your spaceship moves past mine so fast that I measure your clock to be running at half speed, then you will measure my clock to be running at half speed also, provided both of us are in inertial frames. If one of us is affected by a gravitational field or undergoes acceleration, then that person isn’t in an inertial frame and the results are different.

Both types of time dilation play a significant role in time-sensitive satellite navigation systems such as the Global Positioning System. The atomic clocks on the satellites must be programmed to compensate for the relativistic dilation effects of both gravity and motion.

For more on general relativistic dilation, see the discussion of gravity and black holes.

## 15. How Does Gravity Affect Time?

Einstein’s general theory of relativity (1915) is a generalization of his special theory of special relativity (1905). It is not restricted to inertial frames, and it encompasses a broader range of phenomena, namely gravity and accelerated motions. According to general relativity, gravitational differences affect time by dilating it. Observers in a less intense gravitational potential find that clocks in a more intense gravitational potential run slow relative to their own clocks. People live longer in basements than in attics, all other things being equal. Basement flashlights will be shifted toward the red end of the visible spectrum compared to the flashlights in attics. This effect is known as the gravitational red shift. Even the speed of light is slower in the presence of higher gravity.

Informally one speaks of gravity bending light rays around massive objects, but more accurately it is the space that bends, and as a consequence the light is bent, too. The light simply follows the shortest path through spacetime, and when space curves the shortest paths are no longer Euclidean straight lines.

## 16. What Happens to Time Near a Black Hole?

A black hole is a volume of very high gravitational field and thus of severe warp in the spacetime continuum, so much so that light from the inside cannot shine out beyond the hole’s horizon. Time slows down for a person falling toward the surface (horizon) of a black hole. Time goes infinitely slowly at the surface-horizon of the hole, where velocities slow to zero and temperatures becomes infinite, as judged by us from far away. But from the perspective of a person falling through the horizon of the hole, time is normal, temperature is normal, velocity is normal but increasing, and there’s no noticeable surface or boundary.

As in most other galaxies, there is a massive black hole at the center of our galaxy. It is in the direction of the constellation Sagittarius. Astrophysicists believe black holes are commonly formed by the inward collapse of stars that have burned out, stars that are at least two and a half times larger than our Sun. The center of a black hole (sometimes called “the singularity”) is extremely dense—infinitely dense according to relativity theory, but not that dense according to theories of quantum gravity. The hole is surrounded by an event horizon, marking the point of no return. Anything getting that close could never escape the inward pull, even if it had an unlimited fuel supply and could travel at near the speed of light. Anything falling into the hole by crossing the event horizon will quickly crash into the center of the black hole and be crushed to an extremely small volume. If you were to fall into a black hole with a billion times the mass of our Sun, then you’d have a half hour on your clock before you reached the center. If you were to fall into a black hole that weighed the same as the Sun, then you’d have ten microseconds before you reached the center. Very little of the mass of the universe is in the form of black holes.

The first black hole solution to Einstein’s equations of general relativity was discovered by Schwarzschild in 1916. This black hole’s horizon is perfectly spherical. Because even light itself could not escape from inside a black hole, John Wheeler chose the name “black hole.” The radius of the black hole is directly proportional to its mass; if the mass doubles, so does the radius of the horizon.  The fact that real black holes are made from star dust rather than tomatoes or gold is irrelevant to their behavior outside their event horizon. All that counts is their total mass, how fast they spin, and whether they have any electric charge. Most black holes have no electric charge, but most spin very rapidly.

An astronaut flying into a distant black hole will take an infinite coordinate time but only a finite amount of proper time to cross the horizon. In relativity theory, the proper time between two events along a worldline is the time that would be shown on a clock whose path in spacetime is that worldline between the events. The proper time of any event on a particle’s worldline depends on the history of the particle and is not the same as the coordinate time of the same event. Coordinate time is time along the worldline of an ideal clock at the origin of the coordinate system. The coordinate time between the two point events is the time separation of the events given by an observer at rest in the frame. Proper time is independent of coordinate time, although the usual convention is to measure both times in the same units, namely seconds. As judged by a clock on Earth in an Earth-based frame of reference, an astronaut flying into a distant black hole will take an infinite coordinate time to reach the event horizon of the black hole. That is, if we could see the astronaut’s clock, the clock would appear to us to slow to a halt. But as judged by the astronaut, it will take only a few microseconds of the astronaut’s proper time to pass through the event horizon and crash into the center of the black hole. If you, the person falling toward the event horizon, were to successfully escape being pulled through the horizon and were to return home to Earth, you’d discover that you were younger than your Earth-bound twin and that your initially synchronized clocks showed that yours had fallen behind. It is in this sense that you would have experienced a time warp, a warp in the time component of spacetime.

## 17. What Is the Solution to the Twin Paradox?

This paradox is also called the clock paradox and the twins paradox. It is an argument about time dilation that uses the special theory of relativity to produce a contradiction.  Consider two twins at rest on Earth with their clocks synchronized. One twin climbs into a spaceship and flies far away at a high, constant speed, then reverses course and flies back at the same speed. When they reunite, will the twins still be the same age? An application of the equations of special relativity theory implies that the twin on the spaceship will return and be younger than the Earth-based twin. Here is the argument for the twin paradox. It’s all relative, isn’t it? That is, either twin could regard the other as the traveler. Let’s consider the spaceship to be stationary. Wouldn’t relativity theory then imply that the Earth-based twin could race off (while attached to the Earth) and return to be the younger of the two twins? If so, we have a contradiction because, when the twins reunite, each will be younger than the other.

Herbert Dingle famously argued in the 1960s that the paradox reveals an inconsistency in special relativity. Almost all philosophers and scientists now agree that it is not a true paradox, in the sense of revealing a logical inconsistency within relativity theory, but is merely a complex puzzle that can be adequately solved within relativity theory, although there is dispute about whether the solution can occur in special relativity or only in general relativity, especially if we try to account for which twin is “really” in an inertial frame and why inertial frames are special. Einstein said the solution to the paradox requires general relativity. Max Born said, “the clock paradox is due to a false application of the special theory of relativity, namely, to a case in which the methods of the general theory should be applied.” In 1921, Wolfgang Pauli said, “Of course, a complete explanation of the problem can only be given within the framework of the general theory of relativity.”

There have been a variety of suggestions in the relativity textbooks on how to solve the paradox. Here is one. Each twin can consider the other twin to be the one who moves, but their situations are not symmetric. Regardless of which twin is considered to be stationary, only one twin feels the acceleration at the turnaround point, so it should not be surprising that the two situation have different implications about time. And when the gravitational fields are taken into considerations, the equations o general relativity do imply that the younger twin is the one who feels the acceleration. However, the force felt by the spaceship twin is not what “forces” that twin to be younger. Nothing is forcing the twin to be younger anymore than something is forcing the speed of light to remain constant. A second suggestion for how to solve the paradox is to say that only the Earthbound twin can move at a constant velocity in a single inertial frame. If the spaceship twin is to be considered in an inertial frame and moving at a constant velocity, as required by special relativity, then there must be a different frame for the Earthbound twin’s return trip than the frame for the outgoing trip. But changing frames in the middle of the presentation is an improper equivocation and shows that the argument of the paradox breaks down.

A third suggestion for solving the paradox is to apply general relativity and then note that there is an asymmetry in the two spacetime intervals taken by the twins. Sitting still on Earth is a way of maximizing the proper time during the trip, and flying near light speed in a spaceship away from Earth and then back again is a way of minimizing the proper time. The reasoning in the paradox makes the mistake of supposing that the twin on Earth could somehow have the shorter interval.

These three suggestions tend to leave many people unsatisfied, probably because they think of the following situation. If we remove the stars and planets and other material from the universe and simply have two twins, isn’t it clear that it would be inappropriate to say “there is an observable difference” due to one twin feeling an acceleration while the other does not? Won’t both twins feel the same forces, and wouldn’t relativity theory be incorrect if it implied that one twin returned to be younger than the other? Therefore, why does attaching the Earth to one of the twins force that twin to be the older one upon reunion? The answer to these questions requires appealing to general relativity. Notice that it is not just the Earth that is attached to the one twin. It is the Earth in tandem with all the stars and galaxies. When the spaceship-twin is considered to be at rest, then the the stars also rush away and back. Because of all this movement of mass, the turnaround isn’t felt by the Earthbound twin who moves in tandem with those stars, but is felt by the spaceship twin, according to general relativity. So, regardless of which twin is considered to be at rest, it is only the spaceship twin who feels any acceleration. Explaining this failure of the Earthbound twin to feel the force at the turnaround when the spaceship twin is at rest shows that a solution to the paradox ultimately requires a theory of the origin of inertia. But the point remains that the asymmetry in the experience of the two twins accounts for the aging difference and for the error in the argument of the twin paradox.

It would be interesting to consider the twin paradox in the situation where the universe contains only four objects: the twins, a heavy spaceship, and an Earth that weighs the same as the spaceship. Would the spaceship twin still be younger after the trip?

If you are the twin in the spaceship, then by flying fast and returning to Earth you do gradually advance into your twin’s future, but your twin does not go to your past.

## 18. What Is the Solution to Zeno’s Paradoxes?

See the article “Zeno’s Paradoxes” in this encyclopedia.

## 19. How Do Time Coordinates Get Assigned to Points of Spacetime?

A space is a collection of points. In a space that is supposed to be time, these points are the instants. Our question is how we assign time numbers to these points. Before discussing time coordinates specifically, let’s consider what is meant by assigning coordinates to a mathematical space, one that might represent either physical space, or physical time, or spacetime, or something else. In a one-dimensional space, such as a curved line, we assign unique coordinate numbers to points along the line and we make sure that no point fails to have a coordinate. For a 2-dimensional space, we assign pairs of numbers to points. For a 3-d space, we assign triples of numbers. If we assign letters instead of numbers, we can’t use the tools of mathematics to describe the space. But we can’t assign any coordinate numbers we please. There are restrictions. For example, if the space has a certain geometry, then we have to assign numbers that reflect this geometry. Here is the fundamental method of analytic geometry:

Consider a space as a class of fundamental entities: points. The class of points has “structure” imposed upon it, constituting it a geometry–say the full structure of space as described by Euclidean geometry. [By assigning coordinates] we associate another class of entities with the class of points, for example a class of ordered n-tuples of real numbers [for a n-dimensional space], and by means of this “mapping” associate structural features of the space described by the geometry with structural features generated by the relations that may hold among the new class of entities–say functional relations among the reals. We can then study the geometry by studying, instead, the structure of the new associated system [of coordinates]. (Sklar, 1976, p. 28)

The goal in assigning coordinates to a space is to create a reference system for the space. A reference system is a reference frame plus either a coordinate system or an atlas of coordinate systems placed by the analyst upon the space to uniquely name the points. These names or coordinates are frame dependent in that a point can get new coordinates when the reference frame is changed. For 4-d spacetime obeying special relativity and its Lorentzian geometry, a coordinate system is a grid of smooth timelike and spacelike curves on the spacetime that assigns to each point three space coordinate numbers and one time coordinate number. No two distinct points can have the same set of four coordinate numbers. Inertial frames can have global coordinate systems, but in general we have to make due with atlases. If we are working with general relativity where spacetime can curve and we cannot assume inertial frames, then the best we can do is to assign a coordinate system to a small region of spacetime where the laws of special relativity hold to a good approximation. General relativity requires special relativity to hold locally, and thus for spacetime to be Euclidean locally. That means that locally the 4-d spacetime is correctly described by 4-d Euclidean solid geometry. Consider two coordinate systems on adjacent regions. For the adjacent regions we make sure that the ‘edges’ of the two coordinate systems match up in the sense that each point near the intersection of the two coordinate systems gets a unique set of four coordinates and that nearby points get nearby coordinate numbers. The result is an “atlas” on spacetime.

For small regions of spacetime, we create a coordinate system by choosing a style of grid, say rectangular coordinates, fixing a point as being the origin, selecting one timelike and three spacelike lines to be the axes, and defining a unit of distance for each dimension. We cannot use letters for coordinates. The alphabet’s structure is too simple. Integers won’t do either; but real numbers are adequate to the task. The definition of “coordinate system” requires us to assign our real numbers in such a way that numerical betweenness among the coordinate numbers reflects the betweenness relation among points. For example, if we assign numbers 17, pi, and 101.3 to instants, then every interval of time that contains the pi instant and the 101.3 instant had better contain the 17 instant. When this feature holds, the coordinate assignment is said to be monotonic. There is no way to select one point of spacetime and call it the origin of the coordinate system except by reference to actual events. In practice, we make the origin be the location of a special event, and one popular choice is the birth of Jesus. Negative time coordinates are assigned to events occuring before the birth of Jesus.

The choice of the unit presupposes we have defined what “distance” means. The metric for a space specifies what is meant by distance in that space. The natural metric between any two points in a one-dimensional space, such as the time sub-space of our spacetime, is the numerical difference between the coordinates of the two points. Using this metric, the duration between the 11:00 instant and the 11:05 instant is five minutes. The metric for spacetime defines the spacetime interval between two spacetime locations, and it is more complicated than the metric for time alone. The spacetime interval between any two events is invariant or unchanged by a change to any other reference frame, although the spatial distances and durations do vary. More accurately, in the general theory, the infinitesimal spacetime interval between two neighboring points is invariant. The units of the spacetime interval are seconds squared.

In this discussion, there is no need to worry about the distinction between change in metric and change in coordinates. For a space that is topologically equivalent to the real line and for metrics that are consistent with that topology, each coordinate system determines a metric and each metric determines a coordinate system. More precisely, once you decide on a positive direction in the one-dimensional space and a zero-point for the coordinates, then the possible coordinate systems and the possible metrics are in one-to-one correspondence.

There are still other restrictions on the assignments of coordinate numbers. The restriction that we called the “conventionality of simultaneity” fixes what time-slices of spacetime can be counted as collections of simultaneous events. An even more complicated restriction is that coordinate assignments satisfy the demands of general relativity. The metric of spacetime in general relativity is not global but varies from place to place due to the presence of matter and gravitation. Spacetime cannot be given its coordinate numbers without our knowing the distribution of matter and energy.

The features that a space has without its points being assigned any coordinates whatsoever are its topological features. These are its dimensionality, whether it goes on forever or has a boundary, how many points there are, and so forth.

## 20. How Do Dates Get Assigned to Actual Events?

Our purpose in choosing a coordinate system or atlas to assign real numbers to all spacetime points is to express relationships among actual and possible events. The relationships we are interested in are order relationships (Did this event occur between those two?) and magnitude relationships (How long after A did B occur?). The date of a (point) event is the time coordinate number of the spacetime location where the event occurs. We expect all these assignments of dates to events to satisfy the requirement that event A happens before event B iff t(A) < t(B), where t(A) is the time coordinate of A. The assignments of dates to events also must satisfy the demands of our physical theories, and in this case we face serious problems involving inconsistency as when a geologist gives one date for the birth of Earth and an astronomer gives a different date.

It is a big step from assigning numbers to points to assigning them to real events. Here are some of the questions that need answers. How do we determine whether a nearby event and a distant event occurred simultaneously? Assuming we want the second to be the standard unit for measuring the time interval between two events, how do we operationally define the second so we can measure whether one event occurred exactly one second later than another event? How do we know whether the clock we have is accurate? Less fundamentally, attention must also be paid to the dependency of dates due to shifting from Standard Time to Daylight Savings Time, to crossing the International Date Line, and to switching from the Julian to the Gregorian Calendar.

Let’s design a coordinate system. Suppose we have already assigned a date of zero to the event that we choose to be at the origin of our coordinate system. To assign dates to other events, we first must define a standard clock and declare that the time intervals between any two consecutive ticks of that clock are the same. The second, our conventional unit of time measurement, will be defined to be so many ticks of the standard clock. We then synchronize other clocks with the standard clock so the clocks show equal readings at the same time. The time at which a point event occurs is the number reading on the clock at rest there. If there is no clock there, the assignment process is more complicated.

We want to use clocks to assign a time even to distant events, not just to events in the immediate vicinity of the clock. To do this correctly requires some appreciation of Einstein’s theory of relativity. A major difficulty is that two nearby synchronized clocks, namely clocks that have been calibrated and set to show the same time when they are next to each other, will not in general stay synchronized if one is transported somewhere else. If they undergo the same motions and gravitational influences, they will stay synchronized; otherwise, they won’t. For more on how to assign dates to distant events, see the discussion of the relativity and conventionality of simultaneity.

As a practical matter, dates are assigned to events in a wide variety of ways. The date of the birth of the Sun is measured very differently from dates assigned to two successive crests of a light wave. For example, there are lasers whose successive crests of visible light waves pass by a given location every 10 to the minus 15 seconds. This short time isn’t measured with a stopwatch. It is computed from measurements of the light’s wavelength. We rely on electromagnetic theory for the equation connecting the periodic time of the wave to its wavelength and speed. Dates for other kinds of events also are often computed rather than directly measured with a clock.

## 21. What Is Essential to Being a clock?

Clocks record numerical information about time. They measure the quantity of time, the duration. Every clock has two essential functions. One function is to generate a sequence of events that are nearly all of the same duration. In a wall clock, the events are pairs of successive ticks. In a pendulum clock, the events are swings of the pendulum. A second essential function is to display a count of these periodic events. This count measures the duration of the event that the clock is used to measure, and the count is normally converted into seconds or some other units of time. This counting can be especially difficult if the ticks are occurring a trillion times a second. It is an arbitrary convention that we design clocks to count up to higher numbers rather than down to lower numbers as time goes on. It is also a convention that we re-set a clock as we cross a time-zone.

Another goal for a clock is that it be accurate.

## 22. What Does It Mean For a Clock To Be Accurate?

An accurate clock is in synchrony with the standard clock. When the time measurements of the clock agree with those of the standard clock, we say the clock is accurate or properly calibrated or synchronized with the standard clock or simply correct. An accurate clock shows that it is time t just when the standard clock shows that it is time t, for all t. Accuracy is different from precision. If four clocks read exactly thirteen minutes slow compared to the standard clock, then the four are very precise, but they all are inaccurate by thirteen minutes.

A clock isn’t really measuring the time in a reference frame other than one fixed to the clock. In other words, a clock primarily measures the elapsed proper time between events that occur along its own worldline. Technically, a clock is a device that measures the spacetime interval along its own worldline. If the clock is at rest in an inertial frame, then it measures the “coordinate time.” If the spacetime has no inertial frame, then that spacetime can’t have a coordinate time. Because clocks are intended to be used to measure events external to themselves, a third goal in clock building is to ensure there is no difficulty in telling which clock tick is simultaneous with which events to be measured that are occuring away from the clock. For some situations and clocks, the sound made by the ticking helps us make this determination. We hear the tick just as we see the event occur that we desire to measure. For another example, we might want to determine when the Sun comes up in the morning at some particular place where we and our clock are located.  In this situation, the time measurement is made by our seeing the Sun rise just when we see the digital clock face show a specific time of day. More accuracy in the measurement process requires less reliance on human judgment.

In our discussion so far, we have assumed that the clock is very small, that it can count any part of a second and that it can count high enough to be a calendar. These aren’t always good assumptions. Despite those practical problems, there is the theoretical problem of there being a physical limit to the shortest duration measurable by a given clock because no clock can measure events whose duration is shorter than the time it takes light to travel between the components of that clock, the components in the part that generates the sequence of regular ticks. This theoretical limit places a lower limit on the error margin of the measurement.

Is the standard clock itself accurate in the sense of showing the correct time? That is an odd question that will be discussed below.

## 23. What Is Our Standard Clock?

By current convention [in 1964 by ratification by the General Conference of Weights and Measures for the International System of Units, which replaced what was called the "metric system"], the standard clock is the clock we agree to use for defining the standard second. Since 1999, the standard second in the United States has been defined to be the duration of 9,192,631,770 periods (cycles, oscillations, vibrations) of a certain kind of microwave radiation in the standard clock. More specifically, the second is defined to be the duration of 9,192,631,770 periods of the microwave radiation required to produce the maximum fluorescence of cesium 133 atoms (that is, their radiating a specific color of light) as the atoms make a transition between two specific hyperfine energy levels of the ground state of the atoms. This is the internationally agreed upon unit for atomic time [the T.A.I. system]. The old astronomical system [Universal Time 1] defined a second to be 1/86,400 of an Earth day.

For atomic time, the atoms of cesium with a uniform energy are sent through a chamber that is being irradiated with these microwaves. The frequency of these microwaves is tuned until the maximum number of cesium atoms flip from one energy to the other, showing that the microwave radiation frequency is now precisely tuned to be 9,192,631,770 vibrations per second. Because this frequency for maximum fluorescence is so stable from one experiment to the next, the vibration number is accurate to so many significant digits. The National Institute of Standards and Technology’s F-1 atomic fountain clock, which was adopted in late 1999 as the primary time standard of the United States, is so accurate that it drifts by less than one second every 20 million years. We know there is this drift because it is implied by the laws of physics, not because we have a better clock from which to make the judgment.

The standard clock is used to fix the units of all lengths because the unit of length depends on the unit of time. The meter depends on the second. It does not follow from this, though, that time is more basic than space. All that follows is that time measurement is more basic than space measurement. And this has to do with convention and with the fact that current science is capable of measuring time more precisely than space.

Thanks to the regularity of light propagation in a vacuum, the meter is defined in terms of the second. The meter is defined in terms of the pre-defined second as being the distance light travels in exactly 0.000000003335640952 seconds or 1/299,792,458 seconds. That number is picked so that the new meter will be nearly the same distance as the old meter, which was the distance between two marks on a platinum bar that was kept in the Paris Observatory. Why is the meter defined in terms of the second, instead of having the second defined in terms of the meter as, say, how long it takes light to travel a certain distance? The answer is that distance can’t be measured as accurately as time. Time can be more accurately measured than distance, voltage, temperature, mass, or anything else.

These standard definitions of the second and the meter amount to defining or fixing the speed of light in a vacuum in all inertial frames. The speed is exactly one meter per 0.000000003335640952 seconds or 299,792,458 meters per second, or approximately 186,282 miles per second or about a foot per nanosecond. There can no longer be any direct measurement to see if that is how fast light REALLY moves in an inertial frame; it is simply defined to be moving that fast. Any measurement that produced a different value for the speed of light would be presumed initially to have an error in, say, its measurements of lengths and durations, or in its assumptions about the influence of gravitation and acceleration, or in its assumption that the light was moving in a vacuum. This initial presumption comes from a deep reliance by scientists on Einstein’s theory of relativity. However, if it were eventually decided by the community of scientists that the theory of relativity is incorrect and that the speed of light shouldn’t have been fixed as it was, then the scientists would call for a new world convention to re-define the second. Some physicists believe that a better system of units would first define the speed of light, then define the second, and then make the meter be a computed consequence of these.

Although a microwave atomic clock is currently used for our standard unit of time, it is expected that in the first quarter of the 21st century, physicists will agree to use an optical atomic clock, and then the definition of the second will be changed to refer to an optical frequency, rather than to a microwave frequency.

## 24. Why are Some Standard Clocks Better Than Others?

Other clocks ideally are calibrated by being synchronized to the standard clock, but some choices of standard clock are better than others. Let’s consider the various goals we want to achieve in choosing one standard clock rather than another.

One goal is to choose a clock that doesn’t drift very much. That is, we want a clock that has a very regular period–the durations between ticks are congruent. Throughout history, scientists have detected that their current standard clock seemed to be drifting. In about 1700, scientists discovered that their best watches and clocks showed that the time from one day to the next, as determined by sunrises, varied throughout the year. Therefore, they decided to define durations in terms of the mean day throughout the year. Before the 1950s, the standard clock was defined astronomically in terms of the mean rotation of the Earth upon its axis [solar time]. For a short period in the 1950s and 1960s, it was defined in terms of the revolution of the Earth about the Sun [ephemeris time]. The second was defined to be 1/86,400 of the mean solar day, the average throughout the year of the rotational period of the Earth with respect to the Sun. Now we’ve found a better standard clock, a certain kind of atomic clock [atomic time]. All atomic clocks measure time in terms of the natural resonant frequencies of various atoms and molecules. The periodic behavior of a super-cooled cesium atomic clock is the best practical standard clock we have so far discovered. [The dates of adoption of these standard clocks was left vague in the previous sentences because different international organizations adopted different standards in different years.]

To achieve the goal of restricting drift, we isolate the clock from outside effects. That is, a practical goal in selecting a standard clock is to find a clock that can be well insulated from environmental impact such as comet strikes upon Earth, earthquakes, stray electric fields or the presence of dust. If not insulation, then we pursue the goal of compensation. If there is some theoretically predictable effect of the impact upon the standard clock, then the clock can be regularly adjusted to compensate for this effect. Why is choosing the cesium atomic clock as our standard better than choosing an astronomical process such as the mean yearly motion of the Earth around the Sun? Can’t we compensate for all the relevant effects on the motion of the Earth around the Sun? Not easily. The problem is that the Earth’s rate of spin varies in a practically unpredictable manner. Meanwhile, we believe that the relevant effects such as comet bombardment, earthquakes, the ocean’s tides, the sloshing of Earth’s molten core, and other processes, are affecting the revolution of the Earth, but not affecting the atomic clock.

We add leap years and seconds in order to keep our atomic-based calendar in synchrony with the rotations and revolutions of the Earth. We want to keep atomic-noons occurring on astronomical-noons and ultimately to prevent Northern hemisphere winters from occurring in some future July, so we systematically add leap years and leap seconds and leap microseconds in the counting process. These changes do not affect the duration of a second, but they do affect the duration of a year because, with leap years, not all years last the same number of seconds. In this way, we compensate for the Earth-Sun clocks falling out of synchrony (step) with our standard clock.

Another desirable feature of a standard clock is that reproductions of it stay in synchrony with each other when environmental conditions are the same. Otherwise we may be limited to relying on a specifically-located standard clock that can’t be trusted elsewhere.

One goal in choosing a standard clock is that its readings should be consistent with the accepted laws of motion. For instance, Newton’s first law of motion says that a body in motion should cover the same distance during the same time interval unless acted upon by an external force. If we used our standard clock to run a series of tests of the time intervals as a body moved along its path and found that the law was violated and we couldn’t account for this violation and we were sure that there was no problem with the law, then the problem would be with the clock. Leonhard Euler [1707-1783] was the first person to suggest this requirement on our choice of a standard clock.

A related goal in selecting a standard clock is to reduce mystery in physics by finding a periodic process that, if adopted as our standard, makes the resulting system of physical laws simpler and more useful. Choosing the atomic clock as standard is much better for this purpose than choosing the periodic dripping of water from our goat skin bag or even the periodic revolutions of the Earth about the Sun. If scientists retain the Earth-Sun clock as the standard and say that by definition the Earth does not slow down in any rotation or in any revolution, then when a comet collides with Earth which might tempt the scientists to say the Earth slowed down a bit, the scientists would be forced instead to alter their atomic theory and say the frequency of light emitted from cesium atoms increases for seemingly no apparent reason when comets collide with Earth. By switching to the cesium atomic standard, these alterations are unnecessary, the mysteries vanish, and scientists can readily explain the non-uniform wobbling of the Earth’s daily rotations and yearly revolutions by reference to the tides on the Earth, the movement of the liquid metal within the Earth, the gravitational pull of other planets, dust between planets, and collisions with comets.

Do we pursue the goal of selecting the standard clock that is accurate? To some philosophers, that is a bad question. What it means for the standard clock to be accurate depends on your philosophy of time. If you are a conventionalist, then once you select the standard clock it can not fail to be accurate in the sense of being correct. On the other hand, if you are an objectivist, you will say the standard clock can be inaccurate. According to the objectivist, time is marching on independent of all events, and an accurate standard clock will be in sync with this “march.” If it is out of sync, then our standard clock won’t be telling the true time. But since our civilization does not have a direct method of detecting this march, we must simply say that best of all possible choices for a standard clock is one that is the closest to being correct.

Let’s say more about this. When we choose a standard clock we are choosing a metric. We are making a choice about how to measure the duration between any two events and are making a choice abut how to compare any two durations in order to decide whether they are of equal duration. Philosophers of time continue to dispute the extent to which the choice of metric is conventional rather than forced by nature. The objectivist says the choice is forced and that the success of the standard atomic clock over the standard solar clock shows that we were more accurate in our choice of the standard clock. An objectivist believes that whether two intervals of time are really equivalent is an intrinsic feature of nature, and choosing the standard clock isn’t any more conventional than is choosing to say the Earth is round rather than flat. Taking the conventional side on this issue, Adolf Grünbaum argues that time is metrically amorphous. It has no intrinsic metric in the sense of its structure determining the measure of durations. Instead, we choose the metric we do in order to achieve the goals mentioned above, but satisfying those goals is no sign of being correct.

The conventionalist as opposed to the objectivist would say that if we were to require by convention that the instant at which Jesus was born and the instant at which Abraham Lincoln was assassinated occurred only 24 seconds apart, whereas the duration between Lincoln’s assassination and his burial is 24 billion seconds, then we could not be mistaken. It’s up to us to say what is correct when we first create our conventions about measuring duration. We can consistently assign any numerical time coordinates we wish, subject only to the condition that the assignment properly reflect the betweenness relations of the events that occur at those instants. That is, if event J (birth of Jesus) occurs before event L (Lincoln’s assassination) and this in turn occurs before event B (burial of Lincoln), then the time assigned to J must be numerically less than the time assigned to L, and both must be less than the time assigned to B so that t(J) < t(L) < t(B). A simple requirement. It is other features of nature that lead us to reject the above convention about 24 seconds and 24 billion seconds. What features? There are many periodic processes in nature that have a special relationship to each other; their periods are very nearly constant multiples of each other; and this constant stays the same over a long time. For example, the period of the rotation of the Earth is a fairly constant multiple of the period of the revolution of the Earth around the Sun, and both these periods are a constant multiple of the periods of swinging pendulums. The class of these periodic processes is very large, so the world will be easier to describe if we choose our standard clock from one of these periodic processes. A good convention for what is regular will make it easier for scientists to find simple laws of nature and to explain what causes other events to be irregular. It is the search for regularity and simplicity and removal of mystery that leads us to adopt the conventions we do for numerical time coordinate assignments.