Is the speed of light a constant?

As everyone must know, the speed of light is considered a constant (designated as C), namely 186,000 miles per second or to be precise, 299,792,458 meters per second. Einstein said that nothing could go faster than the speed of light. But, is this always true?

There are some situations where light goes slower than the value of C. That's when light propagates through a transparent medium like glass or plastic. In this case, light is refracted or bent from its in-line direction, which slows it up. This follows Snell's Law: n1 sin θ1= n2 sine θ2 where n1 is the index of refraction of the first medium and n2 is the index of refraction of the second medium, and θ1 is the angle between the ray and the surface of the first medium and θ2 is the angle between the ray and the surface of the second medium. A measure of this bending is the refractive index of a material, and the larger this number is is the slower light goes through it. This wave property of light is essential for us to see because lenses are created to take advantage of it and our eyes contain a lens to focus the light.

Many astronomical measurements depend on the speed of light being a constant. It's how the distance to nearby planets and stars can be determined. This follows from the inverse-square law that says that the Intensity is proportional to 1 divided by the square of the distance. Since this same speed limit applies to all electromagnetic emissions, the constant C is essential to GPS devices. It's also necessary for how lasers work.

There have been observations that indicated that light was going faster than C. These have to do with quantum effects, specifically the superposition property where two particles can have the same quantum state. There are other effects that involve particles tunneling through a barrier at the same speed regardless of how thick the barrier is. This results in an effective speed higher than C.

This is particularly seen when two photons become entangled and operate as if they are linked even at great distances. The effect happens almost instantaneously thereby effectively defeating the speed of light limit. What happens is that the polarization of the two photons will be synced so that if the one is polarized 'up' the other one will be 'down'. This effect has been termed the quantum speed of light. No one knows how fast this is, but some estimates put it at 10,000 times the value of C.

This concept could be used to communicate at vast distances, but don't look for it to show up soon. There's a lot more work needed to really utilize this concept.

When it comes to quantum particles, there are some that can go faster than light. Neutrinos are in this class. They appear to go faster than light, but the problem is that they pass through the Earth and us as if we weren't there, making them useless for a communication device. Apparently, scientists at CERN have recorded particles in the Hadron collider going faster than light.

Light being both a wave and a particle makes it very complicated and weird. It's hard to understand how light really works. How any of this would help us find a way to go faster than light is questionable.

Thanks for reading.