# Speed of light

The speed of light (fixed at: 299,792,458 m/s in 1983 by the International office of the weights and measures, by redefining the Mètre) are a Constante physics whose value specifies is obtained in experiments as of the 17th century by the Astronome Danish Ole Christensen Rømer: in 1676, it proposes a solution with a problem encountered by Cassini, which observes a delay of fifteen minutes in the Occultation predicted Io, a satellite of Jupiter. Rømer allots this delay to a lengthening of the distance Ground-Io about the diameter of the terrestrial orbit, a sufficient distance to influence the course of the light significantly. Speed of light was then estimated at: 200,000 kilometers a second, approximately 35% in lower part of its true value because of uncertainties of the time on the size of the orbit of the Ground. However, Cassini expressed doubts on the validity of the results of his/her colleague. James Bradley proposed then an estimate with: 300,000 km/s (the Expérience of Fizeau is the first measures nonastronomical and gives a result of the same order: : 315,000 km/s). These first experimental tests rested moreover on the standard-meter, an additional source of error. Later, the problems will be reversed when the Mètre is defined to leave the Célérité C (speed of light in the vacuum), which was not possible that when a sufficient precision in the determination of C had been reached. Today, speed of light constitutes one of the pillars of the contemporary Theoretical physics.

## Speed of light in the vacuum

According to the theories of the Physical modern, and in particular the Maxwell's equations, the visible Light and all the electromagnetic waves have a constant speed in the Vide, the speed of light .

One thus regards it as a Constante physics noted C (of the Latin celeritas , “speed”). But it is not only constant (one thinks) in all the places (and at all the ages) of the universe (cosmological principles weak and strong, respectively); it is it also of an inertial reference mark to another (restricted Principe of equivalence). In other words: whatever the inertial reference mark of reference of an observer or the speed of the object emitting the light, any observer will obtain same measurement.

No material object nor no signal can travel more quickly than C within the framework of the existing theories. Only can “travel” more quickly than C (at speed known as Supraluminique) of the virtual faces (shadow at long distance from an object in rotation, for example), and one cannot, of course, make use of it to transmit a signal, nor of energy. It is not even in fact not objects strictly speaking.

the experiment of Alain Aspect watch who an observer can be informed instantaneously, by a measurement on a close particle, state of a remote particle, but there is not there either real a transmission of signal.

The speed of light in the vacuum is noted C :

C =: 299,792,458 meters a second

This value is “exact” by definition. Indeed, since 1983, the Mètre is defined starting from speed of light in the vacuum in the international Système of units, as being the length of the way traversed in the vacuum by the light throughout one 1/299792458 of second. With the result that the meter is defined today by the second, via the speed fixed for the light.

## Interaction of the light with the matter

• speed of light is always lower than C in a medium which contains matter, that more especially as the matter is denser;
• In a medium known as birefringent, speed of light also depends on its plan of polarization;
• the difference in speed of light propagation in different mediums is at the origin of the phenomenon of Réfraction.

However, the speed of light , without another precision, generally gets along for speed of light in the vacuum. To note that if no object can exceed speed of light in the vacuum in some medium that it is, to exceed speed of light in the same medium is possible: for example in water the neutrinos can go more quickly than the light.

## Why is this more possible high speed?

Speed of light is not a speed limits to the conventional direction. We are accustomed to adding speeds, for example we will estimate normal that two cars running to 60 kilometers per hour in opposite directions see one and the other like approaching at 60 km/h + 60 km/h = 120 km/h. And this approximate formula is perfectly legitimate for speeds of this kind (60 km/h = 16,67 m/s).

But, when one speeds is close to that of the light, such a traditional calculation deviates too much from the results observed; indeed, as of the end of the 19th century, various experiments (in particular, that of Michelson) and observations let appear a speed of light in the identical vacuum in all the inertial reference marks.

Minkowski, Lorentz, Poincaré and Einstein introduced this question into the theory galiléenne, and realized need for replacing an implicit and inaccurate principle by another compatible with the observations:

• it was necessary to give up additivity speeds (allowed by Galileo without demonstration) for the light;

• to introduce a new concept, the constancy of C (noted by the experiment).

After calculative working, it was released that the new formula of composition comprised a corrective term into 1 (1+vw/c ²), about 2,7×10-10 only with the Speed of sound.

The effect becomes more visible when speeds exceed c/10, and spectacular as v/c approaches 1: two spaceships travelling one towards the other at the speed of 0,8× C (compared to an observer between the two), will not perceive a speed of approach (or relative speed ) equal to 1,6× C , but only 0,96× C actually (see table opposite).

This result is given by the Transformation of Lorentz:

$u = \left\{v + W \ over 1 + v W/c^2\right\}$

where v and W are speeds of the spaceships, and U the speed perceived of a vessel since the other.

Thus, whatever the speed to which moves an object compared to another, each one will measure the speed of the luminous impulse received like having the same value: speed of light; on the other hand, the frequency of an electromagnetic radiation transmitted between two objects in relative displacement will be modified by Effect Doppler.

Albert Einstein unified work of his/her three colleagues in a homogeneous theory of relativity, applying these strange consequences to the traditional Mécanique. The experimental confirmations of the theory of relativity were with go, with the precision of measurements of the time near.

Within the framework of the theory of relativity, the particles are classified in three groups:

• the Baryon S , particles of Rest mass real and positive, move at speeds lower than C ;
• the Luxon S , particles of null rest mass, move only at the speed C in the vacuum;
• the Tachyon S , hypothetical particles of rest mass imaginary, would move with high speeds with C ; the majority of the physicists consider that these particles do not exist (for reasons of Causalité), although the question is still not closed.

Rest masses combined with the multiplicative factor $\ gamma = 1/\ sqrt \left\{1 - v^2/c^2\right\}$ gives a real energy for each group defined previously.
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