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Tag Archives: Physics

Immediate motion

I recently wrote about rest in space and time here. This post is about the opposite: immediate motion, arriving at a destination instantly.

Immediate motion means an infinite speed in space. An infinite speed results in an immediate change of place: something moves from one location to another in an instant. It’s here and there at the same time. The departure and arrival are simultaneous.

A body at infinite speed is at two places at the same time, but a speed ratio has a finite time interval. If it’s the same time, how can there be a finite time interval?

For speed the time interval is fixed as the length changes. If the speed approaches infinity, then the travel length in the numerator approaches infinity, so the time interval in the denominator becomes a smaller and smaller proportion and the ratio approaches infinity. The body is at two places sinultaneously.

Immediate motion also means a zero pace in time. A zero pace results in an immediate change of time: something moves from one time to another in an instant. It’s now and then at the same location. The departure time and arrival time are at the same location. I’m calling this simulocus.

But wait, two times at the same location seems like no motion at all. What gives?

For pace the length interval is fixed as the time changes. If the pace approaches zero, then the travel time in the numerator approaches zero, so the length interval in the denominator becomes a larger and larger proportion and the ratio approaches zero. The body is at two times simulocusly.

Does immediate motion exist? Not under the Lorentz transformation, in which there is a finite maximum speed. But the Galilean transformation implicitly uses an infinite speed of light. And the co-Galilean transformation implicitly uses a zero pace of light.

Three relativity transformations

Two transformations of inertial reference frames are well-known: the Galilean and the Lorentz transformations. There is a third transformation as well, which will be called the co-Galilean transformation. Below is a derivation of all three transformations, closely following the paper Getting the Lorentz transformations without requiring an invariant speed by Andrea Pelissetto and Massimo Testa (American Journal of Physics 83 (2015), p.338-340). Their approach is based on the work of von Ignatowsky in the early 20th century.

We wish to characterize the transformations that relate two different inertial frames. Let us consider two inertial observers K and K′. Let r = (x, x2, x3) and w = (t t2, t3) be space and time coordinates for K and = (x´, x2´, x3)´ and = (t´, t2´, t3´) be the corresponding quantities for K′.

In order to simplify the argument, we will restrict our considerations to the subgroup of transformations involving x and t only, setting x2´= x2, x3´ = x3, t2´ = t2, and t3´ = t3. This is equivalent to choosing coordinates so that K and K′ are in relative motion along the x and t directions in K and the x′ and t´ directions in K´.

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Simultaneity without clocks

Watches didn’t always exist. Neither did clocks that were transportable or manufactured in large quantities. I mention this because one way to determine the simultaneity of events is to have synchronized clocks transported to multiple locations – even an endless number of locations in theory.

How can an observer determine the simultaneous events from their frame of reference? Answer: simultaneous events are observed simultaneously by an observer. But how can this be reconciled with other observers who may observe the same events as non-simultaneous?

That is the point of relativity: applying transformations to coordinates from different frames of reference so that the equations of physics are the same in all reference frames. But relativity requires a convention of simultaneity (or a demonstration of what events are simultaneous events). Since I have defined time in terms of stopwatches rather than clocks, how can simultaneity be determined?

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Space-time exchange postulate

Rates of motion are almost always expressed as a ratio with respect to time. For example, the average speed of a body is the travel distance of the body divided by the travel time. This makes the independent variable time and distance the dependent variable.

However, there is no physical dependency of motion on time rather than distance. One could just as well express the average rate of motion as the travel time of the body divided by the travel distance. The ratios are equally valid.

This is a general result. There is a binary symmetry between space and time. Travel distance and travel time are interchangeable as far as the equations of physics are concerned. J. H. Field has expressed this as a postulate for space-time exchange (STE):

(I) The equations describing the laws of physics are invariant with respect to the exchange of space and time coordinates, or, more generally, to the exchange of the spatial and temporal components of four vectors. (A four-vector has three components of length and one of distime.)

He avoids the question of 3D time by limiting the STE to the direction of inertial motion. Here we generalize the STE postulate to include 3D time:

(II) The equations describing the laws of physics are invariant with respect to the exchange of space and time coordinates, or, more generally, to the exchange of the spatial and temporal components of six-vectors. (A six-vector has three components of length and three of time.)

Field found the STE to violate Galilean symmetry, but this is incorrect because time is three dimensional, and there is a co-Galilean transformation symmetric to the Galilean transformation.

The STE postulate affirms the complete symmetry of space and time, which is built on the symmetry of length and duration. As distance is the metric of space, a kind of length, so distime is the metric of time, a kind of duration. The metric of space or time may be used to organize events linearly, with equivalence classes defined for events at the same position in the order.

Rest in space and time

Rest means no motion, or at least no motion detected by an observer.

We know what rest in space means: staying in the same place. That is, rest means no change of position, no travel distance, no length of motion. So at rest the numerator of the speed is zero.

Yet clocks tick on. The denominator of speed is not zero. The speed of rest then equals zero, that is, a zero length of motion divided by a non-zero quantity of time. Speed v = Δxt = 0/Δt = 0.

What is rest in time? It means staying at the same time. That is, rest means no duration of motion, no travel time. So at rest the numerator of the pace is zero.

In this case, is the length of motion zero, too? No. For pace the length is the independent quantity. It doesn’t depend on the motion. It depends on the given length or unit of length. So the pace of rest is zero, that is, a time of zero divided by a non-zero length. Pace u = Δtx = 0/Δx = 0.

Yet a zero pace seems to say one gets a change of place with no lapse of time. What gives?

Length of motion in the pace ratio is the independent variable. Whether length is conceived to be continually increasing, as if it were a clock, or just a quantity of length for comparison, it is independent of the motion measured. The numerator, the time, is what is measured and compared with a quantity of length to determine the pace.

It is similar with speed. Whether or not there is a clock ticking away, the denominator is a quantity of time compared with a quantity of length. All the clocks in the world could be broken, yet the denominator of speed, the change in time, would still be non-zero.

Consider a vehicle with an odometer and a stopwatch that is running whenever the vehicle is in motion. Both the odometer and the stopwatch would record no additional time for a vehicle at rest. This could not be represented as a ratio since 0/0 is not a valid ratio. Such a state has an indeterminate rate of motion.

Relativity alone

In a paper titled Nothing but Relativity (Eur. J. Phys. 24 (2003) 315-319) Palash B. Pal derived a formula for transformations between observers that is based on the relativity postulate but not a speed of light postulate. In a paper titled Nothing but Relativity, Redux (Eur. J. Phys. 28 (2007) 1145-1150) Joel W. Gannett presented an alternate derivation with fewer implicit assumptions. Here we’ll use Pal’s approach to derive the time-space version.

Consider two inertial timeframes S and , where the second one moves with legerity u, along the t-axis, with respect to the first one. There are two other time axes. The coordinates and radial distance in the S-timeframe will be denoted by t and x, and in the timeframe will be denoted with a prime. The time-space transformation equations have the form:

= T(t, x, u) and = X(t, x, u),

and out task is to determine these functions. A few properties of these functions can readily be observed. First, the principle of relativity tells us that if we invert the legerity in these equations, we must obtain the same functional forms:

t = T(t´, x´, –u) and x = X(t´, x´, –u).

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Circular orbits

*** Revised from June 2017 ***

With circular motion there is a radius and circumference that may be measured as distance or duration. Call the spatial circumference S, and the temporal circumference T, which is known as the period. Distinguish the spatial and temporal versions of the radius: R, for space, and Q, for time. Then S = 2πR and T = 2πQ. Also, R = Qv, and Q = Ru, with speed, v, and pace, u.

Circular orbits are a convenient entry into Kepler’s and Newton’s laws of planetary motion. Copernicus thought the orbits were circular, and most planetary orbits are in fact nearly circular. We have then three propositions from the perspective of the Sun toward each orbiting planet:

  1. Each planet orbits the Sun in a circular path with radius R in 3D space.
  2. The Sun is at the center of mass of each planet’s orbit.
  3. The speed of each planet is a constant, v.

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Galilean transformation expanded

The Galilean transformation is typically presented for motion in direction of the x-axis, with the other axes unchanged:

x´ = xvt, y´ = y, z´ = z, and t´ = t,

where v is the relative velocity of the observers. This is incompatible with the Lorentz transformation, but more than that, it is inconsistent with the two-way (round-trip) speed of light in a vacuum.

The Lorentz transformation can be made compatible with the round-trip speed of light if light is considered to travel instantaneously to its observer, which is usually the final leg. The speed of light for the other part of the round trip can be inferred so that their harmonic mean equals c, which is the most that is known (see One-way speed of light).

That is, if the speed of light in the Lorentz transformation is allowed to approach infinity, then the transformation will approach the Galilean transformation. Here the Galilean transformation arises as a limit of the Lorentz transformation by the speed of light approaching infinity, rather than usual the relative velocity approaching zero.

The Lorentz transformation for motion in the direction of the x-axis is:

x´ = γ (xvt), y´ = y, z´ = z, and t´ = γ (tvx/c²), with γ = (1 – v²/c²)–1/2,

where γ (gamma) is the Lorentz factor. As c → ∞, γ → 1 and t´ → t. This can only be the case for one part of the light trip, which we’re taking as the last part of the trip.

Why the last part? Because that’s what is observed, directly or reflected in a mirror. And in everyday conversation the place where something is observed to be is spoken of as where it is now. Even with the convention of a constant speed of light, one has to be very pedantic to keep correcting others and oneself by saying that where something is seen to be is in fact where it was in the past.

For a round trip, the speed of light for the part not directly observed can be inferred from the empirical result that the round trip speed equals the constant, c:

x´ = γ (xvt), y´ = y, z´ = z, and t´ = γ (t – 4vx/c²), with γ = (1 – 4v²/c²)–1/2.

The speed of light for the unobserved part is inferred from the necessity that the harmonic mean equals c:

(1/c1 + 1/c2) = 2/c,

where c1c/2 as c2 → ∞. This harmonic mean of speeds is the arithmetic mean of paces. What is actually measured is the pace of light from the independent length traversed in the dependent time.

Length and time parallels

This post continues the parallelism between length and time, and includes some new terms.

Length and time both have base units in SI metric: the meter (or better: metre to distinguish it from a device) and the second. They can both be associated with direction. Length in a direction is from or toward an event place. Time in a direction is from or toward an event time.

Multiple dimensions of length are called space. However, space can mean merely the space between two points. To designate 3D space, let’s use the Latin spatium (space). Analogously, let’s use the term tempium to designate 3D time (cf. Latin tempus, time).

Events ordered by time are in time order. Events ordered by length are in length order. Events ordered by importance could be said to be in magna order.

Things are persistent events. Things have length. Things have three dimensions of length. Events have duration. Events have three dimensions of duration. The extent of space between things is called distance. The extent of tempium between events could be called temstance.

Relative space is divided into here and there; “here I am, there I was, there I will be.” The present tense of space is here. The past or future tense of space is there. Here I am. Some places were traversed in the past. Some places will be traversed in the future.

Relative tempium is divided into now and then; “now I am, then I was, then I will be.” The present tense of tempium is now. The past or future tense of tempium is then. Now I am. Some times were traversed in the past. Some times will be traversed in the future.

Matter is a spatial substance. Figure is a temporal substance. Matter has mass, solidity. Figure has vass, lightness. Many sports move matter, such as a baseball pitcher throwing the ball. A figure skater traces out figures in space and time, that is, spatium and tempium.

Observers and participants

Observers detect objects and events with objects. These objects are essentially passive; they must be made to do things by force and work.

Participants are subjects among subjects, actively engaging in events and making them happen. Subjects participate and participants are subjects.

The perspectives of an observer and a participant are inverses of each other. They are different attitudes. An observer has an attitude of standing apart from the world. A participant has an attitude of being part of the world. The world is placed in different contexts because of the attitude of the contact.

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