# what propels light

light has a top speed of “light speed”. however experiments have shown it can be slowed down. my question is what propels light, how does it remain constant?

The speed of light in a vacuum is constant. Light in a vacuum must always move at c. “Slowing down” light involves causing it to move through a medium in which it must interact with the medium material. The most common result of this is simple refraction.

The simple answer to your question is that nothing propels light, things emit light and that light will move forever at c until it interacts with something.

ETA: Here is the Wikion this.

Newtons first law of motion:
Everything persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.

I hit the post button before I had finished, there. :o I am aware that, technically, Newton’s laws only apply to matter. However, the general point remains: in post 17th century physics (Newtonian and Einsteinian alike), motion is not something that requires constant propulsion to be maintained. Motion is something that continues along the same, unless something stops it from doing so. The notion that motion requires constant propulsion is part of the Aristotelian physics that Galileo and Newton are famous for having refuted and replaced.

In classical physics there needs to be something that causes the initial propulsion. That’s what I’m curious about.

As far as what “propels” light…

Light is an EM field.

1. A changing electric field creates a changing magnetic field.

2. A changing magnetic field creates a changing electric field.

3. Goto 1.

That depends on how the light was originally generated. There are many ways for this to happen. If it comes from a star, nuclear fusion.

That’s pretty much why classical physics no longer works, in a nutshell.

Although it is not understood by the general public, under special relativity all objects have a constant velocity four-vector that describes the speed of an object not only through three orthogonal spatial dimensions but also through the temporal dimension, and that singular speed is C. For an object in motion (relative to a specified coordinate frame) the speed through time for the observer in motion is reduced by the Lorentz factor, giving rise to the phenomenon we know as relativistic time dilation. It should be understood, however, that the reason the observer sees dilation effects is because he is in motion through space. In other words, he is trading his time for a change in position. The faster he goes, the more time he gives up, e.g. the external world moves faster around him.

Objects with rest mass require propulsion in order to change velocity. This occurs by transferring the equivalent complementary momentum to a “propellant” of some kind such that global momentum remains unchanged. (The propellant can be an actual propellant, like fuel and oxidizer in the case of a rocket, water in the case of a powerboat, or the Earth itself in the case of a horse or runner.) However, while light has momentum proportional to its wavelength, it has no rest mass and is never at rest. If you could ride on a “beam of light” like the path of a single photon and experience motion in time they way it does, you wouldn’t see individual steps where the photon is in motion. Instead, you would see one long continuous path from inception to termination. For a photon, all points along its world-line occur simultaneously and there is no apparent motion whatsoever.

So to answer the question of the o.p., nothing propels light. By its very nature, it just goes and cannot ever stop. It can be accelerated, but this just results in a change of direction or wavelength, not speed.

Stranger

According to relativity could light be said to be motionless with everything else travelling at the speed of light?

No. For an object moving at any speed at all short of c, you can do this. But for photons and other massless objects (if there are any) that travel at exactly c, things don’t make sense if you try to do this.