Is the speed of light a constant?

Originally, the second was defined as 1/84,600 day, and the meter was defined as 1/40,000 the polar circumference of Earth. The speed of light was measured by those two standards and determined to be 299,792,458 metres per second. Finally, the definitions were changed so that the meter is now defined as 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum, and the speed of light is defined as 299,792,458 metres per second. In other words, the second is now defined as the time it takes for light to travel 299,792,458 metres.

Now that the speed of light in vacuum is constant by definition, it is impossible for it to change except by changing the defintions. Also, by today's definitions, the speed of light has always been exactly what it is today. Everything else that is not constant by definition may change, but not the speed of light in meters per second.

As mentioned earlier, light is slower in media other than vacuum. In water, it is .75 times as fast as in vacuum. In diamond, it is .413 times as fast as in vacuum.

There is no evidence to indicate any changing speed of light. If the speed of light were changing for whatever reason, any reference for it would be distorted into giving us a measurement that it weren't changing.

That is the speed at which light itself DOES TRAVEL if enabled to travel in empty space.

In non-empty regions of space, light effectively travels slower. BUT, light itself ISN'T travelling slower. What REALLY IS happening, is that the constituent atoms are acting as "stop signs" rather than "roadway". Empty space between them are the roadway...the atoms are just the stop signs, making light be absorbed and re-emitted, "wasting time" in the optically dense media.

No there is NOT anything faster than the speed of light.

If you ever could conclusively prove something to be faster, you would disrupt all laws of physics that we presently know, and be responsible to re-write them to adapt for your discovery.

It is NOT a question of aerodynamics OR structural integrity OR anything else that can be solved by throwing money and engineering at the problem (like solving supersonic travel was). It is FUNDAMENTALS of inertia and space/time themselves that forbid superluminal travel OF ANYTHING.

yes the speed of light is a constant

nothing with mass is faster

nothing is faster then the speed of light. and yes its a constant speed

The speed of light in a vacuum is constant. Light may be slower than 3x10^8m/s in other mediums e.g. glass, water etc.