Dielectric Material Slowing Light
Most recent answer: 10/22/2007
Q:
Does a dielectric material slows down the ultimate speed at which information can travel or it just slows down the phase velocity of light.
- Azeem (age 25)
Pakistan
- Azeem (age 25)
Pakistan
A:
A dielectric material does slow down the rate at which information is
transmitted by light, which is given by its ’group velocity’. Usually
light’s ’phase velocity’, which determines the refraction of a
continuous plane wave when it encouters regions of different refractive
index, is also reduced below the vacuum value, c, but not always. The
phase velocity can be bigger than c. /mike w.
"Information" can still travel at speeds up to the speed of light, however. In particular, high-energy particles can be thrown at dielectrics, and before they slow down, they are still traveling at close to the speed of light (in vacuum, not in the material). Cosmic-ray muons are constantly flying through materials, dielectric or no, it makes no difference, and it takes a lot of material to slow them down. In the mean time, because they are charged, they carry with them the electromagnetic field of a moving, charged object. Since the muons are moving faster than the phase velocity of light in a material, they create a "shock wave" of Cherenkov radiation as they traverse a dielectric. Some physicists use this radiation, which gets emitted in a cone around the muon flight path, to detect muons in big tanks of water or oil.
Tom
"Information" can still travel at speeds up to the speed of light, however. In particular, high-energy particles can be thrown at dielectrics, and before they slow down, they are still traveling at close to the speed of light (in vacuum, not in the material). Cosmic-ray muons are constantly flying through materials, dielectric or no, it makes no difference, and it takes a lot of material to slow them down. In the mean time, because they are charged, they carry with them the electromagnetic field of a moving, charged object. Since the muons are moving faster than the phase velocity of light in a material, they create a "shock wave" of Cherenkov radiation as they traverse a dielectric. Some physicists use this radiation, which gets emitted in a cone around the muon flight path, to detect muons in big tanks of water or oil.
Tom
(published on 10/22/2007)