People Are Asking Why Light Doesn’t Take On The Velocity Of Its Source

Over in the Facebook group Physics is Fun, one member stirred things up a bit by claiming “light takes on the velocity of its source”, meaning that if you were to cycle forwards and turn on your bike light, the light emitted from it would travel at light speed + whatever incredible speed you are traveling at on your bike.

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 different on Mars), and you can change it by altering the medium (e.g. by heating the atmosphere) but not by adding your own speed.

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 overtake sound if you want to, showing that you are not adding speed.

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 theory of relativity. Whether you’re sat on Earth, Mars, or Zoozve, if you measure the speed of light you’ll find it chugging along at a cool 299,792,458 meters per second (983,571,056.43 feet per second), the absolute speed limit of the universe.

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 “luminiferous ether” permeating all of space and time. According to the theory, the ether would have a direction to it, like wind in our atmosphere. As Earth makes its way through its orbit, if this theory were correct, the wind would blow us from different angles, and the speed of light propagating through it would be different depending on its angle through the ether wind. 

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. 

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