Why is light fast?

Answer 1

Compared to what ? You might just as well ask: "Why are we slow ?"

You're not going to like it, but here's your answer:

During the 1800's, Coulomb developed our knowledge of how static electric charges

work, Ampere developed our knowledge of how electric currents (moving charges) work,

and Maxwell put the whole thing together and showed that waves could be made

out of this stuff. Then he developed the differential equation for how the wave would

behave. Other physicists zeroed in on the constant coefficient of one of the terms in

the differential equation. That was the number that tells the speed of the wave, and it

was made out of two characteristics of space that Coulomb and Ampere had already

measured. Putting together the "static permittivity" and the "magnetic permeability"

of vacuum, they came up with a number that would be the speed of these waves

if they existed. It was a while after that before the speed of heat and light could

be measured, but when they were, it was so close to that number in Maxwell's

differential equation that we knew that those must be the waves that it predicted.

And every improved measurement of the permittivity, the permeability, and the speed

of light, brings them all into better agreement.

So the answer to your question is: The speed of a wave depends on the properties

and characteristics of the stuff it's moving through. That's true of all waves, whether

sound or light, whether moving through vacuum, air, water, or jello. The physical properties

of the medium determine the speed of physical waves, the electrical properties of the medium

determine the speed of electromagnetic waves, and the reciprocal of the square root of the

product of the static permittivity and the magnetic permeability of free space is equal to

299,792,458 meters (186,282 miles) per second.