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Tag Archives: Space & Time

Matters relating to length and duration in physics and transportation

Reflected motion

This post was inspired by Chandru Iyer’s post here.

Consider a light ray sent a certain distance s that is immediately reflected back. According to Newtonian mechanics if a light ray travels at speed c, then for a body moving at speed v relative to the stationary frame, the light ray should travel at the speed c − v one way and at speed c + v the other way.

The total distance of the light ray is 2s. The total time of the light ray is

\frac{s}{c-v}+\frac{s}{c+v} = \frac{s(c+v)+s(c-v)}{c^2-v^2}=\frac{2sc}{c^2-v^2}=2s \gamma^2 /c

Then the mean speed is

\frac{2s}{\frac{s}{c-v}+\frac{s}{c+v}}=\frac{2s}{\frac{2sc}{c^2-v^2}}=\frac{c^2-v^2}{c}=c(1-(v^2/c^2))= c/\gamma^2

However, according to Einstein’s relativity theory, the mean speed of light is a constant, c. So the above speed needs to be multiplied by the gamma factor squared, γ². As Iyer notes, this is accomplished by contracting the moving rulers by the factor (1/γ) and dilating the moving clocks by the factor γ.

But that is not the correct approach.

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Space and time reciprocity

This post is related to others, such as here. Consider an analogue clock:

Analogue clock

The movement of the hand clockwise relative to the dial is equivalent to the movement of the dial couter-clockwise relative to the hand. That is, the motion of the hand relative to the dial corresponds to the opposite motion of the dial relative to the hand. This is the reciprocity principle of relative motion.

This means that motion in the space frame and the time frame are opposites. The displacement direction and the dischronment direction move in opposite directions. The velocity of A relative to B in space corresponds to the velocity of B relative to A in time, and vice versa.

The interchange of space and time needs to take this into account. Their vectors are in opposite directions.

Inverse units, inverse arithmetic

The use of space (stance) as an independent variable and time as a dependent variable leads to inverse ratios. There is pace instead of speed, that is, change in time per unit of distance instead of change in length per unit of time. But a faster pace is a smaller number, which is counterintuitive and contrary to speed, for which faster speeds are larger numbers. There are two approaches to dealing with this:

  1. Use two kinds of arithmetic: the usual one, in which zero signifies the smallest quantity, and an arithmetic in which zero signifies the largest quantity (or infinity). Then they are isomorphic, with their extremes corresponding inversely. For example, speed and pace both measure how fast a body is going, but they use different arithmetics. A large number for speed corresponds to a small number for pace, and vice versa.

  2. Use the same arithmetic for both but invert one when making a comparison. For example, speed and pace are effectively inverses (apart from which is the independent and which the dependent variable). Given a speed and a pace for some body, to compare them requires inverting either the speed or the pace. An arithmetic means of one corresponds to the harmonic mean of the other.

The first approach inverts the arithmetic, whereas the second approach inverts the units. The second approach is preferable because we are so accustomed to ordinary arithmetic that introducing an alternative would be unnecessarily difficult. It’s much easier to change units than to change arithmetic.

The second approach is found in the comparison of time mean speed and space mean speed: the time mean speed is the arithmetic mean of speeds with a common time unit, and the space mean speed is the harmonic mean of speeds with a common length unit. The space mean speed is essentially the mean pace inverted, which is a subcontrary speed.

Relativity uses a quotient, β, which is the speed of a body divided by the speed of light. Since the speed of light is the highest speed, β is always between zero and one, inclusive, in which zero signifies rest and one signifies the speed of light. What corresponds to β for pace? At first it seems to be the pace of a body divided by the pace of light. But such a quotient is the inverse of β, which would require an inverse arithmetic. The second approach is to take the inverse of this, which equals β. This is consistent with the inverse correspondence between space and time.

Lorentz transformation derivation

The first derivation is similar to here.

Lorentz transformations for space with time

Let unprimed x and t be from inertial frame K and primed x′ and t′ be from inertial frame K′. Since space is assumed to be homogeneous, the transformation must be linear. The most general linear relationship is obtained with four constant coefficients, A, B, C, and D:

x′ = Ax + Bt
t′ = Ct + Dx

A body at rest in the K′ frame at position x′ = 0 moves with constant velocity v in the K frame. Hence the transformation must yield x′ = 0 if x = vt. Therefore, B = −Av and the first equation becomes

x′ = A (x – vt).

Using the principle of relativity

According to the principle of relativity, there is no privileged Galilean frame of reference: therefore the inverse transformation for the position from frame K′ to frame K should have the same form as the original but with the velocity in the opposite direction, i.e., replacing v with −v:

x = A (x′ − (−vt′)) = A (x′ + vt′).

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Equivalence principle

Inertia is the property of a body that resists changes in its motion. Inertial mass of a body is the ratio of the applied force divided by the body’s acceleration. Gravitational mass is the mass of a body as measured by its gravitational attraction to other bodies.

The Equivalence Principle takes several forms. The Newtonian version of the equivalence principle:

The simplest way to state the equivalence principle is this: inertial mass and gravitational mass are the same thing. Then, gravitational force is proportional to inertial mass, and the proportionality is independent of the kind of matter. Ref.

The Einsteinian version of the Equivalence Principle:

All objects fall the same way under the influence of gravity; therefore, locally, one cannot tell the difference between an accelerated frame and an unaccelerated frame. Ref.

These can be expressed in terms of facial vass and levitational vass as follows:

Facilia is the property of a body that does not resist changes in its motion. Facilial vass of a body is the ratio of the applied release divided by the body’s retardation. Levitational vass is the vass of a body as measured by its levitational attraction to other bodies.

The Equivalence Principle then states:

Facilial vass and levitational vass are the same thing. Levitational release is proportional to facilial vass, and the proportionality is independent of the kind of matter.

All bodies rise the same way under the influence of levity; therefore, locally, one cannot tell the difference between a retardated frame and an unretardated frame.

Galilean transformations derived

This derivation of the Galilean transformations is similar to that of the Lorentz transformations here.

Since space and time are assumed to be homogeneous, the transformations must be linear. The most general linear relationship is obtained with four constant coefficients: A, B, C, and D:

x′ = Ax − Bt
t′ = Ct − Dx

Without loss of generality, if t = 0, let A = 1, and if x = 0, let C = 1. Then

x′ = x − Bt
t′ = t − Dx

The inverse transformation for the position from frame R′ to frame R should have the same form as the original but with its motion in the opposite direction, as is confirmed by algebra:

x = x′ + Bt′
t = t′ + Dx′

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Independent and dependent motion variables

Independent variables are measured first, independent of other variables. They may be either set to a fixed value or allowed to change at a fixed rate. An example of the former is a race in which the distance is the independent variable set for the race, and of the latter is a time variable, which increases with each tick of a clock.

Dependent variables are functionally dependent on an independent variable. A dependent variable may or may not be causally dependent on the independent variable. Dependent variables are measured relative to the independent variable. For example, given a time of four minutes (the independent variable), how far can someone run (the dependent variable)? Dependent variables are known by many names, including target variable.

The extent of motion

The extent of a motion is measured by its time intervals (“times”) and its space intervals (“spaces”). Let T represent an interval of time, and let S represent an interval of space. Uniform motion may be stated as a proportion in either of two ways:

(1) Given the ratio of two elapsed times, the corresponding ratio of two traversed spaces are in the proportion:

S1 : S2 :: T1 : T2

(2) Given the ratio of two elapsed spaces, the corresponding ratio of two traversed times are in the proportion:

T1 : T2 :: S1 : S2

In case (1) time is the independent variable and space is the dependent variable (the time speed). In case (2) space is the independent variable and time is the dependent variable (the space speed). Let us adopt the convention that the variable on the right side of the proportion is independent, and the variable on the left side is the dependent variable.

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Interchangeability of space and time

The extent of a motion is measured in two ways: by its time (duration) and by its space (length). The relation between these two measures is the subject here.

Although a definition of uniform motion was given by Archimedes, Galileo was the first to give a complete definition:

Equal or uniform motion I understand to be that of which the parts run through by the moveable in any equal times whatever are equal to one another. (Galileo, Two New Sciences, “On Equable Motion,” tr. by Stillman Drake, p. 148)

Archimedes first stated a proportion for uniform motion:

If a point move at a uniform rate along any line, and two lengths be taken on it, they will be proportional to the times of describing them. (Archimedes, The Works of Archimedes, ed. by T. L. Heath, Dover, p.155.)

In other words, given a uniform motion and on it any two lengths, L and M, then the lengths and the corresponding times of motion, T and U, will satisfy the proportion L : M :: T : U.

Galileo gave the converse proportion:

If a moveable equably carried [latum] with the same speed passes through two spaces, the times of motion will be to one another as the spaces passed through. (Galileo, Two New Sciences, “On Equable Motion,” tr. by Stillman Drake, p. 149)

In other words, given a uniform motion and on it any two spaces, S1 and S2, then the times of motion, T1 and T2, for the corresponding lengths will satisfy the proportion T1 : T2 :: S1 : S2.

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Classical dynamics

The following presents the spatio-temporal and temporo-spatial versions of Newton’s laws based on the book Classical Dynamics of Particles and Systems by Thornton and Marion (Fifth Edition, 2008).

Start with page 49, section 2.2:

2.2 Newton’s [Spatio-temporal] Laws

We begin by simply stating in conventional form Newton’s laws of mechanics:

I. A body remains at rest [in space] or in uniform motion unless acted upon by a force.

II. A body acted upon by a force moves in such a manner that the time rate of change of momentum equals the force.

III. If two bodies exert forces on each other, these forces are equal in magnitude and opposite in direction.

To demonstrate the significance of Newton’s Third Law, let us paraphrase it in the following way, which incorporates the appropriate definition of mass:

III′. If two bodies constitute an ideal, isolated system [in space], then the accelerations of these bodies are always in opposite directions, and the ratio of the magnitudes of the accelerations is constant. This constant ratio is the inverse ratio of the masses of the bodies.

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Ballistic table based on launching from a height and angle with coasting ascent and descent (no drag, no thrust). Note the handy trigonometry identity for range: 2 sin θ cos θ = sin 2θ. This table is in pdf form here.



Initial space angle = θ Initial time angle = φ
Initial height distance = y0 Initial height distime = b0
Elapsed time interval = t Elapsed stance interval = s
Distance downrange or horizontal location = x Distime downrange or horizontal chronation = a
Altitude distance or vertical location = y Altitude distime or vertical chronation = b
Gravitational acceleration = g Levitational retardation = h
Initial velocity = v₀ Initial lenticity = w₀
Initial horizontal velocity = v0x = v0 cos θ Initial horizontal lenticity = w0a = w0 cos φ
Initial vertical velocity = v0y = v0 sin θ Initial vertical lenticity = w0b = w0 sin φ
Horizontal velocity = vx = v0x Horizontal lenticity = wa = w0a
Vertical velocity = vy = v0y – gt Vertical lenticity = wb = w0b – hs
Velocity at apex point: vy = 0 Lenticity at apex instant: wb = 0
Horizontal location x = v0x t Horizontal chronation a = w0a s
Vertical location y = v0yt – ½ gt2 Vertical chronation b = w0bs – ½ hs2
Vertical location at impact point: y = 0 Vertical chronation at impact instant: b = 0
Time of flight to apex tapex = v0y/g Stance of flight to apex sapex = w0b/h
Total time of flight ttotal = 2tapex = 2v0y/g Total stance of flight stotal = 2sapex = 2w0b/h
Distance range to apex xapex = vox voy/g Distime range to apex aapex = woa wob/h
Total distance range xtotal = 2vox voy/g Total distime range atotal = 2woa wob/h
Max altitude distance yapex = ½ v0y2/g Max altitude distime bapex = ½ w0b2/h
Trajectory formula: y = y0 + x tan θ − ½ gx²/v0x2 Trajectory formula: b = b0 + a tan φ − ½ ha²/w0a2