Enlightenment

Two videos came out (Veritasium, Dialect) about measuring the one-way speed of light. Both are the usual fair 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 acutally 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 ie. the phenomenon of one-way light is not measurable. Modern is physics is based on sophisticated maths, but when does that pile of math ever mean this is a measurable phenomenon. This is important be cause physics theories have such non-measurable concepts. That is why in physics jargon there is a delineation between non-measurable vs measurable.

The main category of non-measurable phenomenon in physics are 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 over shoot. 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 and 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 removed 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 then for bombs and bonds to have tremdenous energies compared to the lethargic motionless apple with “no energy”?

This makes sense because the motion of objects in Newtonian mechanics only cases about the change/difference of energy. The absolute value of energy is than redundant. Our apple starts out motionless and then starts to falls because of the pull of gravity. The gravity is said to do work on the apple, causing its energy to increase! The apples 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 spetial property in physics: it has a lower bound. Motionless objects (with zero kinetic energy), can on be pushed or pulled into motion, increasing in kinetic energy.

Absolute energy in newtonian mechanics is an example of a gauge in physics. A particlar value of absolute energy does change observervations, 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 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 does end up as changes in the predictions of the theory (change in trajectories), I would say such one-way speed of light is a gauge.

For convenience I would say choose the one-way light-speed guage 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 matter