Are there other phenomena (besides light) whose speed is invariant?

[quantumphilosopher]

Are there any other phenomena (optical or not) whose speed is constant and does not depend on whether the source is moving or not?

[pervect]

Are there any other phenomena (optical or not) whose speed is constant and does not depend on whether the source is moving or not?

Since the neutrino has been found to have mass, it is expected to travel (very slightly) slower than the speed of light.

This leaves gravitational radiation as a form of radiation expected to travel at 'c'. The strong force carriers (gluons) may also be a candidate for moving at 'c' - the major problem is they can't really be isolated as I understand it (not very well, unfortunately), which makes it difficult to measure their speed.

Note that ANY massive particle, with a sufficiently high velocity, will appear to travel at a nearly constant speed independent of the motion of the source. This is how the neutrino, for instance, was thought to be massless for a very long time.

While the speed of an ultra-relativistic massive particle won't be exactly constant, it can be easily shown that the motion of the source does not directly add to the speed. For instance, suppose an object moving with a velocity v1 emits a massive particle with a velocity v2, where v2 is very close to 'c'. It's quite easy to make v1+v2>c, and it is equally easy to perform an experiment to show that the massive particle moves with a velocity v<c by emitting a light beam and showing that it beats the massive particle. This is explainable by how velocities add in SR (they don't add).