A previous post on this subject is *here*. One reference for this post is V. A. Ugarov’s *Special Theory of Relativity* (Mir, 1979).

A light clock is a device with an emission-reflection-reception cycle of light that registers the current time and stance in units of cycle length and duration. Consider two identical light clocks, at first in their reference frames at rest, *K, K*´. Then, as the light clock in *K*´ moves transversely relative to *K* with uniform motion at velocity *v* (right), from *K* one observes the following:

The illustration above shows one cycle length of the light path (i.e., wavelength), *X*, on the left and one cycle duration (i.e., period), *T*, on the right at rest in reference frames *K, K*´. For the reference frame *K*´, in motion relative to reference frame *K*, call the arc length of one cycle of the light path *x**.* Call the distance between the beginning and ending points of one cycle *x*_{⊥}*. *For the reference frame *K*´ relative to reference frame *K*, call the arc time of one cycle of the light path *t**.* Call the *distime* between the beginning and ending instants of one cycle *t*_{⊥}*.*

*Following Ugarov*: Observing clock time rates in the two frames *K* and *K*´ moving relative to each other, one can only compare the reading of one clock time from one frame with readings of several clock times from another frame, because two clock times from different reference frames occur at the same point in space only once. In one of the frames there must be at least two clock times which are supposed to be synchronized. For the sake of definiteness we shall be comparing one clock time, *t**,* from the frame *K*´ with two clock times from the frame *K*, at the point in the beginning and end of a cycle.

Let a clock and a light source be located at the origin *O*´ of the frame *K*´. A mirror is set perpendicular to the *L* axis at the distance *X*/2 from the light source (and the clock). A light signal is transmitted from the source to the mirror from which it is reflected back and returns to the origin point *O*´ with the period *T* = *X*/*c*. Both the light source and the mirror are at rest in the frame *K*´ and the signal travels there and back along the same straight line.

Now let us consider the propagation of the same signal in the frame *K* relative to which the source and the mirror move to the right together with the frame *K*´ at velocity *v*. Although the signal was sent from the two coincident origins, *O* and *O*´, the reflection from the mirror will occur at another point *x*_{⊥}/2 of the frame *K* and the reception of the reflected signal at the point *x*_{⊥} of the axis. In this way the path of the signal in the frame *K* traces out two sides of an isosceles triangle.

As the path traveled by light in the frame *K* is greater than that in the frame *K*´, one can expect that the period *T* between the sending and reception of the signal, when measured in the frame *K*, will be greater than *t*. Indeed, the observer from the frame *K* will certify that the two events, i.e. the emitting of light from the origin *O*´ and its return to the origin *O*´, occur at the two different points of space. The period *T* between these two events in the frame *K* will be measured in this case by the two clocks removed from each other by the distance *vt*_{⊥} along the direction of motion. The velocity of light is equal to *c* in all reference frames. With Euclidean space we obtain:

(*x*/2)² = (*ct*/2)² = (*vt*_{⊥}/2)² + (*X*/2)².

Given *t* = *t*_{⊥} and collecting *t* from this equation, we get

*t² *(1 − *v*²/*c*²) = (*X*/*c*)²,

* t *= (*X*/*c*)/√(1 − *v*²/*c*²)* =* *γ* (*X*/*c*)

where *γ* = 1/√(1 − *v*²/*c*²).

Considering that *X*/*c* = *T*, then

*t = γ T*.

Since *γ* > 1, in spacetime a clock (or any body) moving transversely relative to an observer in an inertial frame will run slower (dilate) than at rest in its frame of reference.

*Following Ugarov but with Euclidean time*: Observing clock *stance* rates in the two frames *K* and *K*´ moving relative to each other, one can only compare the reading of one clock* stance* from one frame with readings of several clock *stances* from another frame, because two clock *stances* from different reference frames occur at the same instant in *time* only once. In one of the frames there must be at least two clock *stances* which are supposed to be *synstancized*. For the sake of definiteness we shall be comparing one clock* stance* *x*_{⊥} from the frame *K*´ with two clock *stances* from the frame *K*, at the instant of the first and last points of a cycle.

Let a clock and a light source be *chronated* at the origin instant *O*´ of the frame *K*´. A mirroring event occurs parallel to the *t*_{⊥} axis at the distime *T*/2 from the light source (and the clock) perpendicular to the relative motion. A light signal is transmitted from the source to the mirror from which it is reflected back and returns to the origin instant *O*´ with the cycle length *X* = *cT*. Both the light source and the mirror are at rest in the frame *K*´ and the signal travels there and back along the same straight line.

Now let us consider the propagation of the same signal in the frame *K* relative to which the source and mirror move to the right together with the frame *K*´ at velocity *v*. Although the signal was sent from the two coincident origin instants, *O* and *O*´, the reflection from the mirror will occur at another instant *t*_{⊥}/2 of the frame *K* and the reception of the reflected signal at the instant *t*_{⊥}. In this way the path of the signal in the frame *K* traces out two sides of an isosceles triangle.

As the *time* path travelled by light in the frame *K* is greater than that in the frame *K*´, one can expect that the cycle length *L* between the sending and reception of the signal, when measured in the frame *K*, will be greater than *x*. Indeed, the observer from the frame *K* will certify that the two events, i.e. the emitting of light from the origin *O*´ and its return to the origin *O*´, occur at the two different instants of time. The cycle length *X* between these two events in the frame *K* will be measured in this case by the two clock* stance*s removed from each other by the *distime* *x*_{⊥}/*v* along the direction of motion. The velocity of light is equal to *c* in all reference frames. With Euclidean time we obtain:

(*t*/2)² = (*x*/2*v*)² = (*x*_{⊥}/2*c*)² + (*T*/2)².

Given *x* = *x*_{⊥} and collecting *x* from this equation, we get

*x² *(1 − *v*²/*c*²) = (*cT*)²,

*x *= (*cT*)²/√(1 − v²/*c*²)* =* *γ* (*cT*),

where *γ* = 1/√(1 − *v*²/*c*²).

Considering that *cT* = *X*, then

*x = **γ* *X*.

Since *γ* > 1, in *timespace* a body or wave moving transversely relative to an observer in an inertial frame will be longer (expand) than at rest in its frame of reference.