Measuring the speed of light from space, shown below, can be performed by any better equipped astronomical observatory. It needs just the existing equipment of the observatory.
Selection of the measuring instrument
Light is an electromagnetic wave. A light wave has a specific wavelength, just as each water wave has its own wavelength.
We can measure the wavelength of light. This time we choose a Fabry–Pérot interferometer to measure the wavelength. The light is directed into the instrument, and the measured wavelength is read on the screen. The Fabry–Pérot interferometer is chosen because it measures the wavelength of light, but is not sensitive to the frequency of light.
A look at the already known measurement
The Sun rotates on its axis, similar to the Earth. The Fabry–Pérot interferometer does not detect different wavelengths of light in light from the Sun as a result of the different orbital velocities of the Sun.
The speed of the Sun’s rotation can be measured with a wavelength meter, where the light passes through the grid before entering the instrument.
This measurement shows unequivocally that the grating changes the wavelength of light when the light comes from a moving light source.
We repeat the measurement
The circumstances of this measurement can be investigated by taking this measurement again. We pay attention to all measured results.
In the repetition of the measurement, we measure the wavelength of light coming from a moving light source in space. The central element of this measurement is a grid (mesh), which in physics is called a diffraction grating. We direct the light into the grid so that it travels through it.
The wavelength of the light is measured for the first time before the light reaches the grid (λ1) and for the second time after the grid, after the light has already passed the grid (λ2).
Physical science points out that the wavelength of light does not change when it passes through a grid, but many already known measurements do not confirm this opinion.
Circumstance of measurement
The Fabry–Pérot interferometer is sometimes supplemented with one or another grid. For the purposes of this measurement, the Fabry–Pérot interferometer must not contain any grating. We are measuring the effect of the grid on light, so any grid built into the measuring instrument would spoil the measurement results.
In a Fabry–Pérot interferometer, the ‘grid’ sometimes cannot be completely avoided. Each bracket or gauge edge responds similarly to the grid.
Measuring the speed of light in front of the grid
We measure that between the grid and the sink, light moves at a speed of c. Grid and sink do not move. From the known speed of light behind the grid and the measured wavelength of light (λ2), we identify the frequency of light (f=c/λ2). From the known frequency and the measured wavelength in front of the grid (λ1), we recognize the speed of light in front of the grid (f.λ1).
Why this record?
The question arises, why write an article about the measurement before repeating the measurements. It would make sense to do the measurement first and then publish the measured results of the measurement.
When I think about it, I realize that only the measurement is undemanding, but it is not simple to write and publish an article about the results.
Consequences of publication of measurement results
On the diffraction grating, the wavelength of light changes, but its frequency does not. This means that the speed of light changes on the diffraction grating. The measurement shows that the speed of light in a vacuum is not always the same.
In physics, several theories are based on the assumption that the speed of light is always the same in a vacuum. The different speeds of light therefore cast doubt on the theory of relativity, the big bang, dark matter, and more.
A sociological-psychological view of measurement
Physical theories are not without emotional charge. Perhaps the measurement will require a change in today’s views on physics. It may even require changes in the very foundations of physics. The measurement may be sobering.
The measurement is not technologically demanding, but it is stressful. We perceive the speed of light as the foundation of science. Any different thought excites us.
When we realize that the speed of light from space can be measured, we are tempted. Either measure the speed of light, or insist on an agreed but unmeasured hypothesis.
If we finally decide to measure the speed of light, we encounter another problem. How to disseminate measurement results that are different from their expectations among physicists. How will they react to such news?
Therefore, for this measurement, a suitable time will have to come, which will be ripe for facing the results of the measurement.
When measuring the wavelength of light, the authors of the measurements often measure that the wavelength of light does not change with the speed of the light source. If we were to delve deeper into the measurements, they would find that the Fabry–Pérot interferometer does not detect changes in wavelength in the case of different velocities of the light source. Such measurement results would not be rejected with the assessment that the measurement failed.
There is no solution in changing the type of measuring instrument without thinking about the cause of such or different measurement results.