Enlightenment

Two videos came out (Veritasium, Dialect) about measuring the one-way speed of light. Both are the usual fare for science videos, though Dialect is on the spicy side. For a bit of primer, the one-way speed of light is the distance traveled per time taken between two points. The two-way speed of light is the round trip distance traveled per time taken between two points. The aforementioned videos claim we cannot actually measure the one-way speed of light.

Nevertheless, I want to argue that this does not matter in modern physics theory. For light, predictions of physics would not change; i.e., the phenomenon of one-way light is not measurable. Modern physics is based on sophisticated math, but when does that pile of math ever mean this is a measurable phenomenon? This is important because physics theories have such non-measurable concepts. That is why, in physics jargon, there is a delineation between non-measurable and measurable.

The main category of non-measurable phenomena in physics is called gauges. I believe the one-way speed of light counts as a gauge.

What is a Gauge?

Physics theories are not 100% efficient in their descriptions. In a sense, they overshoot. There is a redundancy in the number of parameters really needed in general. For example, from Newtonian mechanics, the absolute total energy is redundant. Case in point, I could state that an apple that is motionless ¹ on some table has tens of millions of Joules of energy. I could also say that the same apple has zero Joules of energy. In both cases, if we were to quickly remove the table, the apple’s motion would be described in the same way in Newtonian physics.

How does this make sense?

What does it then mean for bombs and bonds to have tremendous energies compared to the lethargic motionless apple with “no energy”?

This makes sense because the motion of objects in Newtonian mechanics only cares about the change/difference of energy. The absolute value of energy is thus redundant. Our apple starts out motionless and then begins to fall because of the pull of gravity. The gravity is said to do work on the apple, causing its energy to increase! The apple’s kinetic energy kinetic-energy increases.

It is meaningful to say something has “lots of energy” because (with a small perturbation) it can naturally evolve to a state where it has a lot of kinetic energy. Things that move fast compared to you hit hard! Kinetic energy has a special property in physics: it has a lower bound. Motionless objects (with zero kinetic energy) can only be pushed or pulled into motion, increasing their kinetic energy.

Absolute energy in Newtonian mechanics is an example of a gauge in physics. A particular value of absolute energy does not change observations, so it’s said to be unobservable.

One-Way Speed of Light

I would claim this choice of gauge applies to the two-way speed of light as well. Just like the apple, we care about the trajectories of analogous bodies in special relativity. One can construct cases where light speed is variable, but if these changes in light speed do not end up as changes in the predictions of the theory (changes in trajectories), I would say such one-way speed of light is a gauge.

For convenience, I would say to choose the one-way light-speed gauge to make looking at the physics easier and more insightful. Nevertheless, parting ways from the assumptions of special relativity could be useful.

Further Reading

1. The gauge (phase) does not matter (Gauge Theory)
2. Introduction to Gauge Theory
3. Gauge Invariance
4. One-way speed of light article
5. Example where one-way speed of light matters