Your daughter’s question is a cute one. I like it because it highlights the distinction between the speed of light and all other speeds. The speed of light is unimaginably special in our universe. Strange though it may sound, even if light didn’t exist there would still be the speed of light and it would still have the same value. The speed of light is part of the geometry of space-time and the fact that light travels at “the speed of light” is almost a cosmic afterthought. Gravity and the so-called “strong force” also travel at that speed. OK, so there is actually a multi-way tie for first place in the speed rankings. Your daughter’s question is what comes next? The actual answer is that it’s a many-way tie between everything else. With enough energy, you can get anything moving at just under the speed of light, at least in principle. For example, subatomic particles such as electrons, protons, and even atomic nuclei are routinely accelerated to just under the speed of light in sophisticated machines
around the world. The universe itself has natural accelerators that whip subatomic particles up until they are traveling so close to the speed of light that it’s hard to tell that they aren’t quite at the speed of light. Nonetheless, I assure you that they’re not. The speed of light is so special that nothing that has any mass at all can possibly travel at the speed of light. Only the ephemeral non-massive particles such as light particles (photons), gravity particles (gravitons), and strong force particles (gluons) can actually travel at the speed of light. In fact, once photons, gravitons, and gluons begin to interact with matter, they don’t travel at the speed of light either. It’s sort of a guilt-by-association: as soon as these massless particles leave the essential emptiness of the vacuum and begin to interact with matter, even they can’t travel at the speed of light anymore. That said, I can still offer the likely second place finisher on the speed list. I’m going to skip over light, gravity, and the strong force traveling in extremely dilute matter because that’s sort of cheating — if you take something that naturally travels at the speed of light and slow it down the very, very slightest bit, of course it will come ridiculously close to the speed of light. In real second place are almost certainly cosmic ray particles. These cosmic rays are actually subatomic particles that are accelerated to fantastic energies by natural processes in the cosmos. How such accelerators
work is still largely a mystery but some of the cosmic ray particles that reach our atmosphere have truly astonishing energies — once in a while a single cosmic ray particle that is smaller than an atom will carry enough energy with it that it is capable of moving small ordinary objects around. Even if it carries the energy of a fly, that’s a stupendous amount of energy for an atomic fragment. Those cosmic ray particles are traveling so close to the speed of light that it would be a photo-finish with light itself.