Cheap Astronomy - Dear CA # 133: Why It's Always C

The episode tackles a profound physics paradox: why does light move at the speed of light? Steve Nerlick reveals that 'c'—often called the speed of light—is actually the universe's ultimate speed limit, not a property of light itself. Contrary to intuition, light doesn’t define 'c'; rather, 'c' defines the fundamental relationship between space and time. At the Planck scale, space and time are indivisible, and this structure enforces that no distance can be crossed without time passing—making 'c' a constant of spacetime geometry, not motion. The episode debunks the myth that light moves at 'c' because it has no mass, instead explaining that massless particles like photons must move at 'c' in a vacuum, while massive ones can never reach it. Crucially, from a photon’s perspective, time and distance collapse—meaning it experiences no travel, effectively being everywhere at once. Yet to us, it takes time and distance. The real answer? 'c' isn’t a speed—it’s a law of spacetime. The universe prevents instantaneous travel not by stopping things, but by making time and space inseparable. This deep dive reframes 'c' as a geometric constant, not a velocity. It explains why gravity (via gravitational waves) and hypothetical gravitons also move at 'c', and why gluons—though massless—don’t always move at 'c' because they’re confined within particles. The episode concludes with a poetic twist: we can’t reach 'c', so we never experience the photon’s reality.