Are there other phenomena besides light whose speed is invariant?

Answer 1

It's all about quantum mechanics. Molecules can have a temporarially set speed at which they rotate, expand and contract, or move, but they can not go just a little bit slower or a little bit faster, their speeds are constant unless there is not enough energy to stay at a speed, or enough to "jump" speeds. I believe Philip Ball can describe it best:

When a molecule absorbs light, it's energy increases- it gets "hotter," by which I mean that the molecule may shake, jiggle, or spin more vigorously. It is then said to have been "excited" by absorption. The reason why molecules absorb photons of light at some frequencies but not at others is that their motions are determined by the rules of quantum mechanics, which place restrictions on which motions are allowed. We saw in chapter one (My note: I'm typing this straight from the book, but I can't show all of chapter one) that the electrons in atoms and molecules are not free to adopt any old orbit around the nuclei, with correspondingly arbitrary energies; instead, they are confined to a ladder of specific energy levels, with the energies in between the rungs being forbidden. The electronic energy levels are said to be quantized. In the same way, the energies of the motions that the molecules execute in space may also be quantized. The two-atom oxegyn molecule O2 for example, both rotates (tumbles end over end) and vibrates (the bond between the two atoms stretching and contracting) in free space. The energies associated with these motions may take only certain values, forming a ladder of energy levels like those for the electron energies. As the energies of rotation and vibration depend respectively on the speed of rotation (the number of revolutions per second, say) and the frequency of vibration (the number of in-and-out oscillations per second), the latter two quantities are quantized too. The o2 molecule, terefore, can vibrate only at specific frequencies and spin only at specific speeds.

Phew, that was alot of typing! Yes, I did get that straight out of the book, I didn't copy-paste. He then goes on to explain how the larger an object is, the colser the energy levels are, so there is no real "forbidden zone", and therefore the object can accelerate gradually.

Edit : Sorry about the typos, I'll pick them out as I find them.

Answer 2

Yes.

All zero rest mass particles travel at the invariant speed of light in vacuum. Speed of light is a total misnomer.

Answer 3

Everything in the Electro-magnetic spectrum moves at the speed of light. So, radio, infrared,ultra-violet, x-rays...... all have (the same) invariant speed.

Answer 4

Electricity.

Answer 5

Maybe gravity, but that has not been tested experimentally yet.