# The Speed Of Light Is A Measure of Distance, Not Time

One of the most revolutionary concepts that we learned in the 20th century is that time is not a universal measurement. Time is relative — time slows down for objects approaching the speed of light.

Humans have not come anywhere close to approaching the speed of light, so we have not had the opportunity to experience this phenomenon directly.

It doesn’t matter how much our lives are governed by the same seconds, minutes, hours, days, and weeks, regardless of where we live on the globe, time will never be absolute. The rate at which it passes depends entirely on your speed and acceleration at any given moment.

Time is relative to speed and gravity (time dilation), and so is space (length contraction). Light speed is constant for all observers, so time and space can’t be.

Einstein announced his work in November 1915 and on March 20, 1916 published an academic paper, the first consolidated view of the final version of General Relativity, in Annalen der Physik. Bending of light rays observed during the solar eclipse of 1919 provided the first experimental evidence, catapulting Einstein to global stardom.

In Einstein’s theory of relativity, time dilation describes a difference of elapsed time between two events, as measured by observers that are either moving relative to each other, or differently, depending on their proximity to a gravitational mass. Basically, it states that the faster we go, the more the time is affected. But if time is as relative as this suggests, it can seem a little contradictory.

General relativity generalizes special relativity and Newton’s law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the curvature of spacetimeis directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations.

The Einstein field equations are nonlinear and very difficult to solve.

Einstein used approximation methods in working out initial predictions of the theory. But as early as 1916, the astrophysicist Karl Schwarzschild found the first non-trivial exact solution to the Einstein field equations, the Schwarzschild metric. This solution laid the groundwork for the description of the final stages of gravitational collapse, and the objects known today as black holes.

Concepts introduced by the theories of relativity include:
1. Measurements of various quantities are relative to the velocities of observers. In particular, space contracts and time dilates.
2. Spacetime: space and time should be considered together and in relation to each other.
3. The speed of light is nonetheless invariant, the same for all observers.

There are only a few universal constants in the physical universe (including the constant speed of light in a vacuum and the constant effect of gravitational mass). Everything else, (including time, space, and motion), is relative to a given frame of reference (coordinates in spacetime accounting for speed and gravity).

All observers will always record light speed in a vacuum as 299,792,458 m/s regardless of their own speed (velocity) or gravity (proximity to mass).

When an object speeds up, or becomes more massive (if for example it approaches a massive planet or blackhole and is effected by its gravity), its clock runs more slowly relative to a slower moving and/or less massive observer, and it also appears to shrink relative to that observer.

The opposite is true for the faster moving and/or more massive observer, the clock of the slower and less massive objet moves more quickly relative to that observer’s frame of reference, and, in terms of size, the object appears to enlarge relative to the observer.

This is true for both speed and gravity, because in both cases a type of mass is being gained. The quicker object is getting closer to light speed, and thus it gains a type of mass called ‘relativistic mass’. The more massive object has increased its proximity to mass, and thus its gravitational mass has increased. Likewise, slowing down or moving away from the mass has the opposite effect, it sheds some relativistic mass.

Mass by any name is always the same general thing, the curving of spacetime around an object (so velocity and proximity to mass have the same effect, and generally any type of mass added or subtracted from a system will have the same effect).

No matter how close to light speed an object is, it will always record light speed as the same, so time and space (not the speed of light) have to change from an observer’s frame of reference to account for this. This is what relativity means, it describes how things are relative to a frame instead of constant in all frames (like light speed).

This doesn’t imply that time and space aren’t real, or that motion doesn’t happen, it just implies that we can’t determine exact time, space, or motion from our frame of reference without accounting for relativity.

Even though we can’t get accurate measurements of time or space that work for all observers from a single frame without accounting for relativity, we can find fundamental truths like the order of causality by measuring light and by applying a little relativity related math and physics (so, for example, the end result is we can build satellites that work despite the relative differences).

Time dilation is when time speeds up or slows down relative to an observer due to speed or gravity. Time dilation means that the time between two events is different for observers moving at different speeds.

It could help to think of time as a ‘system effect’. That is, time is not an absolute and it is not really like a river flowing from past to present.

Looking at the figure above, the observer on the left is stationary. This observer has made a simple clock that sends a pulse of light up to a mirror and it reflects back down to a detector.

The moving observer does not experience time slowing down. To the moving observer everything on the ship, the clocks, computers, chemistry, brain signals all seem to be moving along normally at one second per second.

The stationary observer looking at the moving observer sees that the moving observer has slowed down. The clock on the space ship moves slower, the astronaut will talk more slowly and he/she will also age more slowly.

Length contraction is when space shrinks or grows relative to an observer.

General relativity is Einstein’s theory that gravity is mass curving spacetime.

Special relativity is Einstein’s theory that speed increases relativistic mass (energy acting as mass and curving spacetime).

Mass-Energy mass and energy can be considered as mass-energy. They don’t convert into each other, but they are conserved as each other. Mass-energy is a measurable property of the four forces and elementary particles. All matter is made from mass energy including light and everything in the universe.

Light Speed is the constant speed of light (pure energy) in a true vacuum. Since the speed of light is constant, and both we and light are made of mass-energy, most everything else ends up being relative.

The Constants are the only things in the universe that aren’t constant. There are a lot of constants, but only a few underlying constants. We can consider the important constants light speed (max speed of light) and the gravitational constant (the constant force between two bodies).

Spacetime is 3 dimensions of space and 1 dimension of time. Together they are a constant.

A Frame of Reference is coordinates in spacetime, this accounts for velocity and speed.

Gravitational time dilation, is when time speeds up or slows down due to gravity. The more massive an object is relative to the observer, the slower the observer sees the object’s clock run.

Gravitational length contraction, is when length shrinks or expands due to gravity.The more massive an object is relative to the observer, the smaller the observer sees the object get.

Velocity time dilation is when time speeds up or slows down due to speed. The faster an object is moving relative to the observer, the slower the observer sees the objects clock run.

Velocity length contraction is length contracting due to velocity. The faster an object goes relative to an observer, the more length appears to shrink. The slower an object goes, the more length appears to grow. Your twin flying in a rocket ship away from you through space is younger.

### Time, Space and Motion are relative to frame of reference

Time, space, and motion are relative to frame of reference (point of view), while light speed (the maximum, and only, speed of pure energy in a true vacuum) and the gravitational constant (the constant gravitational force between two bodies with mass, not earth’s local gravity) aren’t. This is because everything that exists in spacetime is made out of mass-energy.

Mass-energy is, in simple terms, light speed particles either moving in a single direction at light speed (energy), or not (mass). Given that everything is made out of the same star stuff, and that stuff is light moving at light speed (laymen for electromagnetic energy and the related forces), the rest of the universe has to be relative to that.

### The only real constant is the nature of light itself

As Einstein noticed, ‘light speed is constant, so either time and space can’t be’. As it turns out, neither time and space are constant. The only real constant is the nature of light itself. The effects of this are time dilation and length contraction.

From the observer’s frame of reference (point of view), objects that are moving faster, or are closer to mass, have time move slower and space contract. Meanwhile, to the observer, objects that are moving slower, or are further away from mass, have time move faster and space expand. (see a visual here).

This may seem strange, but remember, the only thing ‘actually’ happening in the universe is light moving at light speed or not moving at light speed, relativity just explains the physics of light not moving at light speed and instead acting as mass. To what extent light is moving in a single direction at max velocity, or is not (mass curbing spacetime and acting as gravity), determines how other observers see time dilate and length contract relative to each other, from their point of view.

If we put together every single one of the theories related to relativity including Maxwell, Einstein, Lorentz, and others, then the obvious conclusion is time and space are relative.

It is important to note that time dilation is relative to the perspective of the observer. In other words, if you were to travel in the spaceship with the clock, you would notice no time dilation. Everything would seem normal to you. From your perspective, the beams of light would be traveling back and forth between the mirrors just as if you were sitting still. However, from a stationary perspective, time slows down. In other words, only the stationary observer observes time dilation in the moving ship.

### The speed of light is a measure of distance, not time

On the other side of the ‘ruler’ is ‘Planck length’. That is the smallest distance between two ‘frames of reference’. When motion happens, particles jump from Planck frame to Planck frame in relative time (or exist in-between frames in a state of quantum super position).

We can measure the distance of those jumps at ‘light speed’. Thus, the Planck length and light speed give us a measurement system for causality. That is as close as we get to a universal ‘clock’, and it’s really just a universal measuring stick (keep in mind that length contracts, so a ruler gets wonky just like a clock when considering relativity).

The Planck constant (Planck’s constant) links the amount of energy a photon carries with the frequency of its electromagnetic wave. It is named after the physicist Max Planck. It is an important quantity in quantum physics.

The equation that defines Planck’s constant is called the Planck-Einstein relation, and it looks like this: E = hf. Here, E is the energy of each packet (or quanta) of light, measured in Joules, f is the frequency of light, measured in hertz, and h is of course Planck’s constant.

Photons: The Quanta of Light. According to the Planck hypothesis, all electromagnetic radiation is quantized and occurs in finite “bundles” of energy which we call photons. The quantum of energy for a photon is not Planck’s constant h itself, but the product of h and the frequency.

### Theoretical Physicist Max Planck made many contributions to theoretical physics

The Planck postulate, one of the fundamental principles of quantum mechanics, is the postulate that the energy of oscillators in a black body is quantized, and is given by E=nhv.

E is energy, n is the quantum number, h is the Planck constant, v is the oscillation frequency.

The Planck constant (denoted h, also called Planck’s constant) is a physical constant that is the quantum of action, central in quantum mechanics.

Along with light speed c and the gravitational constant (G), the Planck constant is one of a few important universal constants that can be used as a sort of natural “measuring stick”, due to its representation of a constant limit in the physical universe.

E=mc2 and E=hf are fundamental statements about the constant nature of energy within our physical universe.

The Planck constant can be used to measure most increments of energy effectively such as electron energy states in an atom (and the photonic energy emitted and absorbed by electrons).

The reduced Planck constant, derived from the Planck constant, is used to measure the smallest values quantum particles quantize to, such as the quantization of angular momentum (spin). Using the Planck and reduced Planck we can construct ‘mechanical’ Planck units (units based on the Planck constant) for space, time, energy, momentum, mass, acceleration, force, impulse, pressure, frequency, and generally all quantifiable mass-energy effects.

Time, from our perspective, stands still at light speed and time stands still in black holes. So time, relative to a frame of reference in spacetime, can slow down, speed up, or stop altogether.

‘All matter originates and exists only by virtue of a force which brings the particle of an atom to vibration and holds this most minute solar system of the atom together. We must assume behind this force the existence of a conscious and intelligent mind. This mind is the matrix of all matter.’

Max Planck

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