Derek Muller has a habit of making claims that sound ridiculous at first — then proving them. In this piece, he argues that despite everything we think we know about the speed of light, no one has actually measured it directly. The claim isn't just provocative; it's built on a century-old argument from Einstein himself.
The Convention Problem
Muller opens with what sounds like clickbait: "I will prove to you that light may never actually travel at this speed and I can say that because no one has actually measured it." But he quickly pivots to something more substantive. The core of his argument is that we measure the two-way speed of light — a round trip from point A to B and back — but not the one-way speed.
As Muller explains, to measure one-way speed you need two synchronized clocks: one at the laser, one at the finish line. But synchronizing those clocks requires knowing how fast light travels in that direction — which is exactly what you're trying to measure. It's a vicious loop.
We need synchronized clocks to measure the one-way speed of light but we need to know the one-way speed of light in order to synchronize our clocks.
This is the piece's central insight, and it's genuinely hard to argue with. The logic is circular in a way that makes the one-way speed fundamentally unknowable.
Einstein's Convention
Muller draws on Einstein's 1905 paper on electrodynamics of moving bodies to make his sharpest point: "he says there is no way that we can meaningfully compare the times that measure unless we establish by definition that the time required by light to travel from A to B equals the time it requires to travel from B to A."
This is where Muller lands hardest. The speed of light being the same in opposite directions isn't a physical discovery — it's a convention. He quotes Einstein directly: "it's a stipulation that I can make of my own free will to arrive at a definition of simultaneity." This is bold language, and it deserves emphasis. The word 'convention' appears multiple times throughout the piece, and Muller wants you to sit with its implications.
The historical example he uses — Fizeau's 1849 experiment measuring light between rapidly spinning gear teeth — shows that what we call the speed of light was actually a round-trip measurement. Every experimental method Muller discusses, from high-speed cameras to fiber optic cables to synchronized clocks moved apart at equal speeds, ultimately measures two-way speed.
What We Can't Test
The most provocative thought comes late in the piece: if the speed of light is different in each direction — say, c/2 going forward and instantaneous on return — no physics would break. The round trip still works out to c. Muller puts it this way:
None of physics breaks and that's the crazy thing.
This is genuinely unsettling. The laws of physics are constructed so that they work regardless of whether light travels at different speeds in different directions. We simply can't tell the difference.
The Mars communication example is particularly effective. If light takes 20 minutes to reach Mars but returns instantaneously, two parties on Earth and Mars would experience exactly the same local time delays while their clocks drift out of sync by 10 minutes. They'd never know it.
Counterpoints
Critics might note that Muller's argument, while intellectually sound, borders on philosophical navel-gazing. The speed of light being a convention doesn't change the fact that every practical application — GPS satellites, interferometry, fiber optics — works perfectly with our current definitions. We've built an entire technological infrastructure on this 'convention' and it hasn't failed.
Some physicists would argue that the round-trip measurement is sufficient for all physical predictions. The one-way speed may be unknowable, but it doesn't matter for anything we actually do.
It's known as the Einstein synchronization convention — so the idea that the speed of light is the same in opposite directions is neither a supposition nor a hypothesis about the physical nature of light but a stipulation.
Bottom Line
Muller's strongest move is the historical and logical framework he builds around Einstein's original 1905 paper. The weakest part is the practical implication — this is an interesting thought experiment, but there's no experimental evidence that the speed of light actually varies by direction. The piece works because it makes you question something you've accepted as fundamental: that light travels at a constant speed in all directions. The answer may not matter for physics, but the question is genuinely unsettling.