The speed of light in a vacuum is constant and any experiment suggesting otherwise is viewed with suspicion. The speed of light is variable in different mediums, with some having a high refractive index that can slow or even stop light. However, the slowdown is illusory as the speed of light never actually slows down. Einstein-Bose condensates and hot rubidium gas can completely stop light.
The speed of light in vacuum is 299,792,458 meters per second or 670,615,343 miles per hour. This is represented by the variable c, which stands for the Latin celeritas, which means speed. There is a consensus among physicists that accepting the theory of general relativity, Einstein’s prevailing theory of physics, involves accepting the speed of light in a vacuum as constant. Therefore, any experiment that proposes that the speed of light in a vacuum changes with time is viewed with great suspicion in the physics community.
It is widely known, however, that the speed of light is variable when it’s not moving in a vacuum. The ratio of the speed of light through a given medium to through a vacuum is called the refractive index of the medium or optical density. Some media have such a high refractive index that they can slow light to the speed of a walking person, or even bring it to a temporary stop.
For example, the speed of light in air is very close to its speed in a vacuum. Depending on the density of the transparent medium, it can slow down the light to a greater or lesser extent. Water and glass can slow it to 3/4 and 2/3 c respectively. Different wavelengths also travel at different speeds through different mediums. For example, blue light travels at a different speed than red light when it passes through a prism, causing the two to separate in a process called dispersion.
In reality, the speed of light never slows down. It is only delayed as the photons are absorbed and re-emitted by the atoms in the space between. When a ray of light exits a transparent medium in a vacuum, it continues traveling at the same speed as when it originally entered, without any additional energy. This proves that the slowdown is simply illusory.
At least two mediums are capable of slowing light tremendously: Einstein-Bose condensates and hot rubidium gas. Both were used to completely stop light. This was first achieved on an interim basis in experiments conducted in 2001.
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