The nature of light had been a great mystery for centuries. Scientists had trouble grasping the concept of its wave and particle nature.
If it was a wave what did it propagate through? Why did it appear to travel at the same speed in all directions?
It wasn't until Albert Einstein described this theory of special relativity in 1905 that it began to come into focus.
Einstein argued that space and time were relative and that the speed of light was the constant that connected the two.
What is the Speed of Light
It is often stated that the speed of light is constant and that nothing can travel faster than the speed of light. This isn't entirely accurate.
What they really mean is that the fastest that anything can travel is the speed of light in a vacuum. This value is 299,792,458 meters per second (186,282 miles per second).
But light actually slows down as it passes through different media. For instance when light passes through glass it slows down to about two thirds of its speed in a vacuum.
Even in air - which is nearly a vacuum - light slows down slightly.
This phenomenon has to do with the nature of light - an electromagnetic wave. As it propagates through a material its electric and magnetic fields "disturb" the charged particles that it comes in contact with. These disturbances then cause the particles to radiate light at the same frequency, but with a phase shift. The sum of all these waves produced by the "disturbances" will lead to an electromagnetic wave with the same frequency as the original light, but with a shorter wavelength and, hence a slower speed.
Interestingly, matter can travel faster than the speed of light in different media. In fact, charged particles from deep space (called cosmic rays) penetrate our atmosphere they are traveling faster than the speed of light in air. They create optical shockwaves known as Cherenkov radiation.
Light and Gravity
Current theories of physics predict that gravitational waves also travel at the speed of light, but this has yet to be experimentally confirmed. Otherwise there are no other objects are quantities that travel at the speed of light.
One exception to this may be space-time itself. It appears that distant galaxies are moving away from us faster than the speed of light. This is a current research issue that scientists are still trying to understand. However, one interesting consequence of this is that a travel system based on the idea of a warp drive, in which a space craft is at rest relative to space and it is space that moves - like a surfer riding a wave on the ocean - could allow for superluminal travel. Of course there are other practical and technological limitations that stand in the way, but interesting nonetheless.
Travel Times for Light
One of the questions that I get quite often as a professor physics and astronomy is "how long would it take light to go from...?" So here are a few of the common ones (all times approximate):