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Matters relating to space and time in physics and transportation

Space, time and causality

If we drop a stone into a calm body of water, it sends out circular waves. As the waves move outward, the clock is ticking and we say the dropped stone caused the waves, which are an irreversible process in time. But we would also say the waves are moving in space, so why isn’t it an irreversible process in space? It is, we just don’t ordinarily speak that way.

What is the difference between the “now” and the “here”? The now is the present, which seems to move with us in time. But the here moves with us in space, like a webcam that follows us everywhere.

Is the past where we were or what we were? It’s both. Is the future where we will be or what we will be? Again, it’s both. Events in times past can cause events in the present time, and events in places past can cause events in the present place. There is an exact parallel.

Causality is transmitted through time and space. We’ve heard this in relativity theory but we don’t need relativity to realize it’s true. The world line of an object in space and time is subject to causality in space and time.

But just as space has three dimensions (directions), so does time. So causality has what — six dimensions? No, causality has three dimensions because they are the same three dimensions. This is no different from saying that force has three dimensions, which are the same three dimensions as space. Dimensions are a property that applies to vector quantities.

We’re so used to thinking that dimensions are spatial but they are just as much temporal — and dynamical (having to do with force and torque) and a property of every other vector quantity in physics. Dimensions are an abstraction that applies to many physical quantities.

A dual of the second law of thermodynamics

This is a continuation of the series of posts on the duality of space and time. Consider an isolated system of particles over a period of time. The system covers a specific distance in space and a specific duration in time. Consider only one dimension of space and one dimension of time with an origin point.

The second law of thermodynamics says that the entropy of the distribution of particles at each instant of time over the space tends to increase with increasing duration. Call this the s-entropy since the distribution is over space. What about the distribution of particles at each point of space over the time period? Call this the t-entropy (time entropy).

Consider different scenarios. If the system is at equilibrium, there will be no change over time and the distribution will be constant, which would be the minimum t-entropy, that is, zero. If the system is near equilibrium, there will be little change over time and the distribution will be near constant, which would be a low t-entropy. If the system is far from equilibrium, i.e., the particles are bunched up together, the system will change toward equilibrium.

The tendency is for small intervals of time with many particles to end up with fewer particles, and small intervals of time with few particles to end up with more particles. If the origin is near the concentration of particles, the t-entropy of the distributions of particles in time over space will tend to decrease. If the origin is away from the concentration of particles, the t-entropy of the distributions of particles in time over space will tend to increase.

Mechanics in multidimensional time

As (spatial) velocity and acceleration are vectors, so are their temporal analogues. This perspective makes sense because of the multidimensionality of time. There is an implicit recognition that time has directionality since time is commonly considered as any real number, and not a non-negative real number, which it would be if time were merely a magnitude. This “reverse time” is an example of time’s directionality — which however has nothing to do with reverse causal sequences.

The (spatial) length (especially of an object) is a magnitude that is used to represent physical space. Similarly, the duration (or length of time) is a magnitude that is used to represent physical time. We speak of the location or position in space of an object or a place. Similarly, we speak of the point in time or temporal position of an action or event.

A point in space or time is “that which has no part” (Euclid) whose location is represented by a position vector. A point itself is an abstraction that is zero dimensional but makes up all multidimensional abstract ‘spaces’ (which may represent space or time or whatever). If s is the distance of a point from a specified origin point in space, then its position may be represented by a position vector whose magnitude equals s. If t is the duration of a point in time from a specified origin point in time, then its position may be represented by a temporal position vector whose magnitude equals t.

The movement of a point through space in time may be represented by a vector function of temporal position t whose value is the spatial position at each temporal position t. The movement of a point through time in space may be represented by a vector function of spatial position s whose value is the temporal position at each spatial position s.

During the time interval (duration) Δt = t2 – t1, the position vector of an object changes from r1 = r(t1) to r2 = r(t2), with a displacement vector Δr = r2r1 (boldface represents vectors). The rate of change of the displacement vector is the average (time) velocity vector over the time interval, vavg = Δr / Δt. The rate of change of the average velocity vector is the average acceleration vector aavg = Δv / Δt.

Similarly, while traversing the space interval (length) Δs = s2 – s1, the position vector of an object changes from p1 = p(s1) to p2 = p(s2). The rate of change of the displacement vector is the average space velocity vector over the length of space, uavg = Δp / Δs. The rate of change of the average space velocity vector is the average space acceleration vector bavg = Δu / Δs.

Instantaneous velocity is considered to be measured over a differential of time (duration), dt. In that case the instantaneous (temporal) velocity is defined as v(t) = ds/dt and the instantaneous (temporal) acceleration as a(t) = dv/dt = d2s/dt2.

Similarly, the coincidental spatial velocity may be measured over a differential of space (length), ds. The coincidental spatial velocity is defined as u(s) = dt/ds and the coincidental spatial acceleration as b(s) = du/ds = d2t/ds2.

Measures of speed and velocity

The speed of an object is the ratio of distance (or length) traveled and the duration of travel. It is derived from the distance traveled during a given duration. It is expressed as the measured distance divided by the given duration, that is, distance relative to duration in units of distance over duration, e.g., m/s, km/hr, etc.

For example, the speeds of vehicles passing a fixed point along a roadway may be measured over a given duration by loop detectors and other fixed-location speed detection equipment. These are called spot speeds. The (arithmetic) average of such speeds is called the time mean speed since they are measured during a given period of time. Accordingly, each speed could be called a time speed.

But there is another, complementary way of determining speed. One can select a distance and measure the duration of travel while traversing that distance. Then the measured duration should be in the numerator to show the duration relative to distance, with units s/m, hr/km, etc. Unless the speed is constant, this is not the inverse of the time speed because the distances and durations will not match. It is called the pace, which means the change in time per change in position.

For example, probe vehicles may be in the traffic stream which measure their distance during a set period of time. Or these may be sampled using automatic vehicle location (AVL) data. The harmonic average of such speeds is called the space mean speed since it is measured over a given segment length. Accordingly, each speed could be called a space speed.

Why the harmonic average? Consider each space speed as an inverse speed: put the measured duration of travel in the numerator and the segment length in the denominator, so that the given segment length provides the units for this pace.

Now the average speed may be related to the average pace as follows: invert each speed to put the duration in the numerator and the length in the denominator, take their (arithmetic) average, and invert again to get the average speed. This is the harmonic mean of the space speeds.

Velocity is a vector of speed with the direction of movement. A time velocity may be defined as a velocity whose magnitude is a time speed, and a space velocity as a velocity whose magnitude is a space speed. If its magnitude is a pace, the components are duration divided by length, which is not velocity. It could be called legerity. Why might one use legerity instead of velocity? If the duration is measured for a given length, the legerity gives the appropriate measure: duration relative to length.

What does the direction of the legerity mean? Since it measures duration (relative to a given length), its direction is the temporal direction of movement. This shows again that the same three dimensions may be associated with time (duration) as well as space (length).

Homogeneity and isotropy of time

The homogeneity and isotropy of space are well-known. The homogeneity of time is partly known but is confused by an “arrow of time” concept that is not applicable to space and time. The isotropy of time is unknown (and usually denied) also because of confusion with an inapplicable “arrow of time” concept.

I previously wrote about the Multidimensionality of time. As space has three dimensions, so does time and they are the same three dimensions.

As space is homogeneous in each dimension, so is time. For example, it does not matter whether an experiment takes place “here” or 10 minutes north and 5 minutes east of “here” (if they are both inertial reference frames). The translational invariance of time is exactly like the translational invariance of space.

As space is isotropic, i.e., the same in all directions, so is time. For example, the duration measured by a clock is the same whether it is facing north, south, east, west, up, or down. And the duration is the same whether it is oriented horizontally, longitudinally, or transversely.

It is said that in classical mechanics time is reversible. This is a confused statement. What can be shown is that if a classical particle moves in one direction, its movement in the opposite (“reverse”) direction is also classical. Since both space and time are directional, that would equally well be true of space as of time but no-one says that space is reversible. It is best to leave questions of (ir)reversibility to thermodynamics, causality, etc.

Noether’s theorem shows that the homogeneity of space leads to the conservation of momentum, the homogeneity of time leads to the conservation of energy, and the isotropy of space leads to the conservation of angular momentum. I haven’t checked it yet but it is natural to expect that the isotropy of time leads to the conservation of rotational energy.

Multidimensionality of time

This post is another in a series on the duality of space and time. I have emphasized that the basis for space is length and the basis for time is duration. What, then, about direction? Does direction apply to both space and time? Yes, and in the same manner.

If someone says, “The hotel is 10 minutes away by car” how is that different from saying “The hotel is 5 miles away by car”? One provides a duration and the other a distance. Neither provides a direction. Both require movement to measure. They are exactly parallel.

If someone says, “The hotel is 10 minutes north by car” how is that different from saying “The hotel is 5 miles north by car”? One provides a duration and the other a length, each with a direction. Both require movement to measure. They are again exactly parallel.

Is the direction “north” part of space in one case but not in the other case? Then what does “north” mean in the sentence “The hotel is 10 minutes north by car”? It means that the direction “north” and the duration “10 minutes” are combined, just as we combined the direction “north” and the length “5 miles”. It would be arbitrary to say that direction applies to space (length) and not to time (duration).

So what is direction? It is something independent of length and duration, that is, it is independent of space and time but can be applied to either space or time. Direction is what makes the scalar “lengths” into a vector of directed lengths, often called a displacement. In the same way, direction is what makes the scalar “duration” into a vector of directed duration, which could be called a temporal displacement.

Is the concept angle only related to space? Look at the hand of a clock. Is it measuring an angle of space or of time? Both. We read a clock directly as time, a duration measured by revolving hands. But we recognize the spatial angles, too, and can use clock numbers to indicate space, as in “10 o’clock high” for a direction in space.

But if there are three dimensions for direction in space, does that mean there are three dimensions for direction in time? Yes, and they are the same three dimensions. For example, an isochrone map shows contour lines (isolines) for durations in two dimensions. It is like an isodistance map which shows travel lengths in two dimensions. The only difference between these maps is whether durations or lengths are shown; the two dimensions are the same.

So when we say that looking into outer space is looking back in time, that includes the three dimensions we see.

Space, time, and arrows

This post is a continuation on the duality of space and time. The basis of space is distance (or length) and the basis of time is duration. It must be emphasized that both distance and duration are scalars, i.e., they have magnitude but no direction. They are not one-dimensional because that would entail direction, represented by a positive and negative quantity. So scalars are non-negative real numbers (zero is a degenerate case).

Consider two sentences: “The Arcade building was a block long.” “They were stuck in traffic on Lake Shore Drive for 12 hours.” The first sentence expresses a distance and the second expresses a duration. Note that both sentences use the past tense. In the present tense the Arcade building in Chicago’s Pullman district doesn’t exist because it was demolished in 1926, and the mammoth traffic jam on Chicago’s Lake Shore Drive in February 2011 is over. This parallel shows that anyone who wants to say the past time is in the opposite direction from the present time could equally well say that past space is, too.

The problem is Arthur Eddington’s “arrow of time” which says time is one-way or asymmetric. But time in this sense has to do with tense, not duration, and has no application to space and time. Note that more recent work on “arrows of time” has focused on thermodynamics, causality, etc., and not on space and time.

What, then, is the meaning of a time line that goes from negative to zero to positive? If “now” is at the zero point, isn’t the negative part in the past? In fact, this is no different from a “space line” that goes from negative to zero to positive with “here” at the zero point. The location of a point in the past would be negative, but that does not lead us to say that space is one-way or asymmetric.

Putting “-t” into an equation of physics does not change the tense or make the present precede the past. It simply reverses the direction of the duration. If “+t” is to the right, then “-t” is to the left.

Arrow of tense

The arrow of time is a concept developed by Arthur Eddington in 1927. It is an arrow that points from the past through the present into the future. One problem with this concept is that multiple futures are possible; it would have to be a many-headed arrow. Another problem is that it could just as well be pointing from the future through the present to the past. The choice is arbitrary and may simply reflect a progressive bias.

One could as well speak of an arrow of place that points from there (where one was) to here (where one is) to there (where one is going). So both space and time have their arrows.

A deeper problem with the concept is that it’s really about tense in language. Different languages have different ways of indicating the time when an action or event occurs, or when a state or process holds. The past, present, and future tenses are one means of doing this. But there are other tenses such as the still sense, indicating that that a state is still the case. And some languages such as Chinese are tenseless.

So the arrow of time would be better called the arrow of tense and understood as a property of language. If the arrow of time is used at all, it should be paired with the arrow of place.

Duality of space and time

Several dualities of space and time are known, but there are thought to be exceptions for the dimensions of space and the arrow of time. It turns out these are not exceptions; space and time are fully dual. To understand this first note that movement is required for the measurement of time and space, and then compare the various meanings of the words time with the parallel meanings of space (or place):

(1) Time as duration, a period of time, is a length of time, analogous to a length of space. Duration and length are both scalar quantities.

(2) Time as points in time, instants of time, associated with specific actions or events. This is analogous to points in space, locations, which may also be associated with actions or events. Duration is a difference between two points in time as length is a difference between two points in space.

(3) Time as tense, a grammatical sense which expresses how an action or event relates to the present time, usually relative to the moment of speaking. This corresponds to language which expresses how an action or event is oriented toward the present location, usually relative to the place of speaking.

(4) Adverbs of time are relative to the speaker and include now, yesterday, tomorrow, later, etc. These correspond to adverbs of place, which are relative to the speaker and include here, there, down here, over there, etc.

(5) Time as the arrow of time is the forward flow from past times to the present time to future times. There is a corresponding flow from past places to the present place to future places which could be called the arrow of place. These are one-dimensional views of time and space which could be reversed by looking backwards.

(6) The speed of an object is a scalar measure of its rate of movement, expressed either as the travel length divided by the time taken (average speed) or the rate of change of position with respect to time at a particular point (instantaneous speed). To examine the relation of speed with time and space, consider highway traffic flow measurement which distinguishes two types of average speed:

The time-mean speed is the arithmetic mean of the vehicle speeds measured at one roadside location. The space-mean speed is the harmonic mean of speeds measured by the travel times collected between roadside locations (or on probe vehicles between two locations). Why the harmonic mean? Because the units are in the numerator, so it is a kind of inverse speed (the inverse of the duration of travel divided by the unit of travel length).

From this we may define the time speed as the travel length divided by the unit of travel duration. If measured at a point or “spot” it is called a spot speed, which is the instantaneous speed of a vehicle at a specified location. The space speed may be defined as the inverse of the duration of travel divided by the unit of travel length. For constant speeds, these values are equal but they are conceptually different.

The time speed has spatial direction but no time direction because the temporal denominator is a scalar. For the space speed the measurement of duration has temporal direction but no spatial direction because the spatial denominator is a scalar.

Rectilinear motion is along a straight line, with the distance from a point in that line varying with the time. Angular motion is the rotation of an object about a fixed point or fixed axis in a given time period.

Velocity is the rate at which an object changes its position. A time velocity may be defined as the vector of travel through space divided by the scalar unit of travel duration. The space velocity may be defined as the inverse of the vector of travel through time divided by the scalar unit of travel length.

(7) Direction is a vector of orientation or movement whose magnitude may be a length or a duration. A movement from here to there is also a movement from now to then which may be expressed as a vector. We tend to think of this in spatial terms but it may equally well be thought of in temporal terms.

There will be more to come on this topic but the bottom line is that length and duration are both scalars that may become dimensioned or tensed in an appropriate context. Space and time are dual.