The no relativity of the speed of light

6 Jun


 The relativistyc physics sustains that both space and time have values which depends of the reference system where they are meassured; but the speed light in the vacuum doesn´t change, it always remains the same over any reference system.

This “dogma” of the relativity theory is based in speed of light measure physics experiments, so it has a consistent prove, besides, it becomes to other physics consequences of this theory such as the difference in clocks time measures and the light curvature due the gravity force.

However, for the people common sense, it is hard to accept that the speed of light doesn´t change in several reference systems while the space and time do, they apparently have absolute values; it means that many of us should prefer to accept that the light can have different values of speed if we would have measured in reference systems which they are with different relative velocities, than to accept that time pass in different way for one person that another when they are in different reference systems.

The this essay goal is to show, in a single way, that certainly it make sense to say about the absolute feature of the speed of light, and why, in a different way that the material bodies behavior, the speed of light doesn’t change in different inertial reference systems.


The bodies velocity relativity

 Time ago I was surprised to see the great velocity which an helicopter crossed in the air that was coming in opposite direction that the helicopter in which I was traveling; we are accustomed to see an helicopter in the sky going to a moderate velocity, slower that an airplane; if we are flying in an helicopter we sense a slow velocity too because objects in the ground pass slow; however the helicopter velocity is relativity high comparing with the cars for example. When we have this rare experience of crossing with another helicopter in the air in the opposite direction we newly realize the high velocity we are traveling. But besides, the velocity at which we see to come to the other helicopter is the double that it really has, because we are adding the our own helicopter velocity. If our helicopter goes to 300 Km/hr and the other one comes to 250 Km/hr, we see that the helicopter comes to us to 550 Km/hr.

Any experience of this kind show us that the objects velocities are relatives, they depends of the reference system that we have to measure.

However, why the speed of light breaks out this rule?, why when we measure the speed of light going to the light source it has the same result that when we measure going out the light source?


Measuring the speed of the object that moves over the floor

We have the following experiment:

A man measures the velocity of a cart which departs from a lamppost which is located at a “d” distance of the person, the time is measured by two synchronized clocks they mark the cart departure and its arrival where man there is, the first clock in the lamppost registers the departure time, the second clock close the man registers the arrival time.


In accordance with the figure, the person will measure the speed of the cart dividing the space covered between the lamppost (starting point) and he itself, between the time t that is the difference of time of the clocks 2 and 1 that are synchronized

t = Clock 2 Time – Clock 1 Time

v = d/t

If we do that the person is over a skate with a v1 velocity:


In this case the person will measure a v + v1 cart velocity


The person assumes that, however he is under movement, he is the reference, so the floor is moving but he is static, he will take the departure time from the Clock 1 register when he was at the initial position ( to “d” mts from the lamppost), after t1 time he will meet with the cart, it is to d1 distance from his initial position; meanwhile cart should run a d2 distance in the same t1 time.

The man skate velocity is:

v1 = d1/t1

But for the man the distance travelled by the cart is “d” that assumes that the floor is what moves and, therefore the speed measuring to the cart is:

v2 = d/t1

It means:

v2 = (d2 + d1)/t1 = d2/t1 + d1/t1

But: d2/t1 is the cart speed that we know it is “v”.


v2 = v + v1

It means the man will measure an apparent cart speed as v+v1.

In order to explain better this fact we can modify our draw:


Instead of that person is skating on a platform nearing the pole of which left the cart, we do make the platform which is close to the person with a velocity of v1 and the person to take as reference the floor that lies beneath the platform over it the platform is running (and the person is static with respect to the arrow drawn on the floor).


In this case it is easy to see that the person will measure that the cart goes with a speed greater that “v”, because he is over a platform which is moving with a v1 velocity, so the cart will move with v+v1 velocity.


The light is not dragged on the floor

 If we change this experiment replacing the cart for a tiny light wave.


If we take account that the emission light source was in the lamppost whose time was registered by the Clock 1, we will see that it not necessary to add the “v1” platform velocity with “v”, because the light in not dragged on the floor, so it is not influenced for the platform velocity. The light speed, in this case, will be the same as the person has measured in the system where he was without skate.

A projectile has no absolute speed

 With the example above it is clear that the speed of light does not vary to measure it in two references that has a speed v1 between each other. But we could refuse this experiment conclussions and say that if the light does not drag over the floor a fired projectile does not neither, in this case the speed of the projectile (recall the case of helicopters that cross) nor should change at different inertial reference systems.

In the following example, we despise the vertical component of free-fall body and assume a horizontal movement.


Certainly, in this case, if the “v” velocity is that the projectile had in the reference system which the person has no skate, in this new reference system the velocity will be v + v1, because when the fire happens its natural impulse is “v”, but as we are considering that the lamppost is moving with a “v1” velocity so it adds a “v1” impulse to the projectile making move to v+v1. Indeed, we know that the observer is running over the skate going to the lamppost and the projectile was not fired but only with “v”, but as we are considering that the observer is inert in other reference system, he will see that the mechanic laws about movement quantity transmission or kinetic energy must be obeyed for the lamppost and the projectile, so he will see a “v+v1” projectile speed because the lamppost gave it an additional impulse.


Then, why we can not discuss additional impetus also to the emission light?. It is in this case the light is not a mechanical object, it is rather a wave, therefore we don’t have to consider  that the lamppost is going to add speed to the wave of light has.

Not all waves have absolute speed

While it is true that light being a wave, therefore not varies its speed in different reference systems, however we can not say that any further wave has absolute speed; it is because the mechanic waves as a guitar cord, water waves, the sound, and so on, they displace in a physic medium, therefore they can be influenced by the movement of these such as a dragging cart; otherwise when we talk about the absolute light speed it is referred in the vacuum displacement, such is a way to affirm the no existence of ether, the vacuum is the space itself, and this has relative characteristics causing it to stretch and shrink fitting to the absolute speed of light.

Indeed, Michelson and Morley Interferometer experiment tried to prove the existence of the ether, however met with surprise that the light not varied from speed before the influence of the movement of the Earth’s rotation, therefore there could be no such ether.

Another way to manifest itself does not influence the speed of the physical environment on the displacement of the light is with the concept of momentum, the momentum for a physical body is

p= mv


in such “m” is the mass of the body and “v” its speed, the collision of two bodies produce an alteration of the momentum of the bodies but remains the sum of momentums total before and after the collision, i.e.:


p1 + p2 = p3 + p4

m1 x v1 + m2 x v2 = m1 x v3 + m2 x v4

In such m1 and m2 are the masses of the bodies, v1 and v2 their speeds before the clash, v3 and v4 their velocities after the collision.

But when we talk about light or photon momentum the formula is:

p = h/λ

In such h is the Plank constant and λ the light wavelenght, it means the light speed is not present.

This last illustrated that the transmission of mechanical movement is not fulfilled for light because its momentum has another nature.

The following figure shows the effect of Compton, the clash between an electron and a photon of light.




End, to establish the laws of physics, we arbitrarily take points of departure that gives coherence to the explanations of observations made, we say that light is absolute speed and there is nothing greater than this because it is the way most coherent to explain macro and microscopic cosmos phenomena.

A video about this theme is in the following link:




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