TL;DR

• The classic problem: you can’t time a one‑way light trip unless your two clocks agree first. Most ways to make them agree secretly use light.

• Our rule: no light to set the clocks.

• Our move: build a home‑made time standard from sound sent through three very different materials (water, plastic/PMMA, and aluminum). We call this the Sound‑3 gauge.

• Because each material responds differently to wind, temperature, and stress, we can separate and cancel those effects, then lock our two clocks to that shared standard.

• Only after that do we fire one optical pulse from A to B and read the travel time. The number we publish is one‑way light speed in the Sound‑3 gauge. It’s not “absolute,” but it’s precise, reproducible, and doesn’t use light to measure light.

Gauge first. Measure second.

Why this is a big deal

Everyone agrees on the average, there‑and‑back speed of light. That’s easy: one clock can do it. The fight starts when you try to say how fast light goes one way. To do that you need two clocks that already agree, and the usual ways to make them agree all smuggle in assumptions about light.

Our answer is simple: don’t use light. Use sound instead—and not just in one pipe, but in three very different ones. If those three channels tell the same timing story after we correct for their known quirks, we’ve earned a clean, declared way to keep our clocks in step. That declared way is our gauge.

The core idea

Picture a long, straight frame with three parallel tracks: a water tube, a plastic rod, and an aluminum bar. We “ping” each track from both ends and listen at the other side. Because water, plastic, and metal react differently to air currents, temperature shifts, and mechanical strain, the three timing readings don’t drift the same way. That difference is our leverage: it lets us solve for the nuisance (like a faint background breeze) and cancel it.

When the dust settles, we lock our clocks to the best common story told by all three tracks. That shared story is our Sound‑3 simultaneity. Now we finally send a single flash of light from A to B and read off the time difference. That gives us “one‑way light speed in Sound‑3.”

What the photochemical tag is (and isn’t)

We add a tiny cell containing a light‑sensitive molecule (stilbene). When we trigger it with UV, it responds almost instantly. We use that response as a local timestamp to check for hidden delays in our electronics and optics. It’s a latency sentinel, not a second clock at the far end. It keeps us honest about cables, detectors, and triggers—without helping to synchronize the two ends.

What counts as success

• No light used for synchronization. Sound only.

• A declared, repeatable gauge. We say exactly how the Sound‑3 timing standard is built so anyone can copy it.

• A full uncertainty budget. We show how much error comes from path length, timing electronics, the sound‑based gauge itself, and local instrument delays.

• Realistic precision. With today’s acquisition gear we expect tens‑of‑ppm‑class results on kilometer‑scale runs, with a clear path to better timing and length metrology if we want to push further.

How we convince skeptics

• Swap the ends. Flip which side launches and which side listens to make sure we’re not fooling ourselves with a fixed hardware bias.

• Rotate the whole rig. Turn it 180° and repeat; if the lab’s background (like a tiny rotational effect) matters, it will show up.

• Heat and cool in steps. Nudge temperature and watch each material respond the way it should. If the model and reality match, confidence goes up.

• Run blind schedules. Randomize the sound pings so we can’t “help” the outcome.

• Separate what changes with direction from what doesn’t. Direction‑dependent effects get treated as physics to be modeled; direction‑independent offsets get treated as instrument latency.

What this is—and what it isn’t

• This is: a precise, documented way to measure the one‑way speed of light in a clearly named time standard (Sound‑3) that does not rely on light for synchronization.

• This is not: a claim that we’ve found the one true, convention‑free number. We’re explicit: our result depends on the declared gauge. That’s the point. We make the convention transparent, reproducible, and independent of light.

Instrument‑absolute, convention‑relative.

Replicate it yourself (high‑level checklist)

• A long, straight baseline with three parallel channels: water tube, plastic rod, aluminum bar.

• A central hub that schedules and records all pings and sensors.

• Temperature, pressure, and strain sensors to keep tabs on the environment.

• A UV trigger, the small photochemical cell, and a detector for local latency checks.

• Software that compares the three sound channels, cancels shared drift, and sets the Sound‑3 clock standard before any light measurement happens.

FAQ

Are you “breaking” relativity? No. We’re choosing a different way to decide what “at the same time” means across space, and then reporting our result honestly inside that choice.

Why three materials? Diversity. Water, plastic, and metal drift for different reasons and by different amounts. That diversity lets us separate real environmental sway from pure timing.

Why not use GPS? GPS timing is excellent—but it depends on assumptions about light and radio we are deliberately avoiding here.

Can I get the math anyway? Sure, but it’s not required to understand the idea. The short version is: three different sensors give you enough clues to remove the background and set a clean standard.