Over in the Facebook group Physics is Fun, one member stirred things up a bit by
This, as several annoyed commenters pointed out, is incorrect. Light propagates at light speed, regardless of the speed of its source. But that doesn’t mean it’s a completely out there suggestion. After all, it took the actual Einstein to realize this in the first place.
The confusion comes from expecting light to act like matter. If you were to ride a bike forward and fire a gun in the same direction, you would instinctively expect that the bullet would travel at the usual speed of that bullet plus your cycling velocity. And you’d be correct.
Sound is different from light, but is a better analogy for explaining light than a bullet. As you probably remember from school, sound is a vibration propagating as an acoustic wave through a medium, be it liquid, solid, or gas. The speed of sound is dictated by the medium it is traveling through (meaning that it is
If you were to travel on a bike and shout forwards, you would probably instinctively understand that the sounds you produced would not travel at the speed of sound plus the speed of you on your bike. And you’d be right. In fact, you can increase your speed and
Of course, sound is different to light, but it helps to think of something other than firing a bullet. The speed of light is a universal constant, meaning it is the same wherever you measure it in the universe, according to Einstein’s special
Though Einstein gets credit (rightly) for the thought experiments that led him to realize the speed of light was a constant, it had actually been proven experimentally before that.
Before Einstein, there was a theory that light was carried through space by a frictionless and physically undetectable “
Physicists A.A. Michelson and Edward W. Morley, performing their respective experiments in 1881 and 1887, aimed to measure the speed of light in respect to the ether.
The idea was to send a light source at a beam splitter, sending it off in two directions. One would be in the direction that Earth is headed, another at a right angle to that. The beams then traveled toward two mirrors, which sent the light back at a detector. If the luminiferous ether theory was correct, then the light would be detected at different times, affected by its journey through the ether.
What the experiments found, however, was that light was the same speed in all directions, evidence that the speed of light was constant, and that the ether theory was incorrect.