Speed limit

A speed limits , indicated C (for celerity ), applies to any body, in any point of the Univers and whatever the relative Speed of the observant . This ultimate speed , identified by the restricted Relativity, is impossible to be exceeded by any object having a Masse; the particles of null mass can only move exactly with this Speed limit. Thus the Light and the Waves radio, carried by the Photon S, move in the vacuum precisely with this limit, that is to say approximately: 299792 Km/s.

Description

The restricted Relativité showed that neither an object nor physical information can move at an high speed with C . Moreover, only the particles of null mass can move at a speed equal to C (it is the case of the Photon for example), reason for which the speed limit coincides with speed of light (at the point to sometimes generate confusions as for their respective roles). Consequently, according to restricted relativity, the instantaneous exchange of information is impossible, any information travelling at a finished speed.

The speed limit, which is that of the light in the vacuum, is equal to $\ scriptstyle C \; = \; 299 \; 792 \; 458$ m/s. For reasons of simplicity the approximation $\ scriptstyle C \; \ approx \; 300 \; 000$ Km/s is often used.

Objects only defined by the thought (the such face of a wave or a shadow on a distant object) can have an high speed with C , but that does not contradict the restricted relativity which prohibits speeds Supraluminique S only with the carriers of information or energy.

Paradox

Albert Einstein wondered what he would see, if he caught up with a '' ray of light '', i.e. what he would see, if he moved with 300.000 km/s. He would still see a luminous ray moving at the same speed compared to him! And it is its own theory, the restricted Relativité, which it will build a few years later which returns the assumption to see a ray of light at impossible rest .

Indeed, speed of light is an absolute constant whatever the reference frame of study. In the vacuum, it is never neither larger nor smaller than C . Thus a luminous ray emitted by a lamp inside a car rolling to 90 km/h, still moves at the speed C in this car. And if Einstein catches up with the luminous ray, he would still see it moving at the speed C .

The reason of this paradox is that the Law of composition speeds galiléenne is not applicable any more as soon as speeds concerned are of about size of that of the light. In the relativistic case, speeds are not added any more.

Popularized interpretation

A picturesque way to explain this speed limit is to revisit the concept of mass inert. It is the mass of a body which is opposed to its actuation, and more the body is massive plus its actuation and its acceleration is difficult. In relativity, it is possible to consider the fact that this mass (which then any more does not have completely the same direction only in traditional mechanics) increases with the speed of the body. Thus, more the body goes quickly, less it is easy to accelerate it, until it reaches speed limits C and an infinite “mass”.

Case of the tachyons

Although the existence of the Tachyon S, particles which would move at an high speed with C , collects the votes of few physicists, it is interesting to note that they are also concerned with this speed limit: indeed, contrary to the traditional particles, their speed is always strictly higher than C .

For the question of the mass, the rest mass of a tachyon (even if it is a fantastic notion) is a number pure Imaginaire, from where the finished energy of the tachyon in spite of its supraluminic speed.

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