Does light always travel at the same speed?

The speed of light in the vacuum is 299 792 458 meters per second or 670 615 343 miles per hour. This is represented by a variable c , which means Latin celeritas , which means speed. There is a consensus between physics that accepting the general theory of relativity, Einstein's predominant physics theory means accepting the speed of light in the vacuum as a constant. Any experiment that suggests that the speed of light in the vacuum over time perceived in the physical community with great suspicion.

However, it is generally known that the speed of light is variable when it does not move the vacuum. The speed ratio of light and vacuum is called the refractive index or the optical medium density. Some media have such a high refractive index index that they can slow down the light to a person's speed or even stop them temporarily.

For example, the speed of the IS air is very close to its velocity in the vacuum. Depending on how dense the transparent media is to slow down the light intogreater or smaller. Water and glass can slow down to 3/4 and 2/3 c . Different wavelengths also travel at different speeds by different media. For example, blue light travels at a different speed than the red when passing through the prism, causing both to separate in a process called dispersion.

In fact, the speed of light actually slows down. It is only delayed because the photons are absorbed and re -emitted atoms in the interventing space. When the light beam leaves the transparent medium to the vacuum, it continues to travel at the same speed as in the originally input, without added energy. This shows that slowing is only illusory.

At least two media are able to slow the light tremendously: Einstein-Bose condensates, D Hot Rubidium Gas. Both were used to complete the light. This was first achieved temporarily in experiments performed in 2001.