https://wiki.tfes.org/Universal_AccelerationAs the article states, there is no definition of a "resting" observer within Special Relativity. However, we do have a definition of a moving observer. In fact, there is a moving observer who always has the same velocity:

Photons.

The velocity of light is a constant and no matter into which moving coordinate-system you transform the movement of the photon, it always moves at the speed of light.

Why is this interesting? Because it allows us to measure how fast the Universally Accelerating Flat Earth is right now, measured as a percentage of the speed of light, which is a hard, well-known number.

The experiment is fairly easy. You need:

- 1x laser-pointer

- 1x sheet of paper

- 1x ruler for measuring distances

If Flat Earth experiences a constant acceleration of 9.81 m/s², after 1 year = 31,557,600 seconds it has a velocity of approximately 3*10^8 m/s. That's light-speed. Now, of course we are not moving at light-speed, but this simple estimate makes it reasonable to assume that Flat Earth has been moving at a velocity close to the speed of light since a few years after its creation.

What happens if you throw a ball horizontally at a constant velocity? It will fall down and its trajectory is a parable. The same would happen to a photon in a Universally Accelerating Flat Earth.

The photon is moving horizontally at a constant velocity, while the frame of reference is moving upwards with an acceleration. That means, to an observer within the frame of reference the photon is moving downwards.

The experiment is simple:

1. You hold a laserpointer horizontally and activate it.

2. By holding the sheet of paper at various distances to the laserpointer, you can make the trajectory of the photons visible.

3. As you know the speed of light, you can calculate the present velocity of Universally Accelerated Flat Earth based on how the beam of light drops down to Flat Earth.

The formula for evaluation is simple:

1. The photon reaches a paper held at a distance of 1 meter after a time t = 1 meter / c

2. We assume that 1 meter is an such an insignificant length for an object moving at the speed of light, that we can estimate for Flat Earth to have a constant velocity during that short interval of time.

3. We measure the drop-off d of the beam of light, compared to the horizontal axis defined by our laser-pointer.

4. We calculate d/t, and now we know how fast Flat Earth has been on average during our experiment.

For example:

If the beam of light has a drop-off of 0,1 m over a distance of 1 m, then Flat Earth had an average velocity of 10% of speed of light during the measurement.

What drop-off do you see when pointing a laserpointer sideways?