Relativity is one of the best established theories in physics, Koberlein says, but
several physicists have pointed out that while relativity assumes the vacuum speed of light is a universal constant, it also shows the speed can never be measured. Specifically, relativity forbids you from measuring the time it takes light to travel from point A to point B. To measure the speed of light in one direction, you’d need a synchronized stopwatch at each end, but relative motion affects the rate of your clocks relative to the speed of light. You can’t synchronize them without knowing the speed of light, which you can’t know without measuring. What you can do is use a single stopwatch to measure the round trip time from A to B back to A, and this is what every measurement of the speed of light does.
Einstein simply measured the round trip speed and divided by half. Simple enough. But that conclusion includes an assumption that can never be proved for the reasons given above.
What if the speed of light varied depending on whether it was approaching us or leaving us? Lewis and Barnes investigated this in a paper on arXiv. Koberlein summarizes what they concluded about anisotropic light (i.e., light that varies according to direction).
It turns out that if the speed of light varies with direction, so does length contraction and time dilation. The team considered the effects of anisotropic light on a simple relativistic model known as the Milne universe. It’s basically a toy universe similar in structure to the observed universe, but without all the matter and energy. They found that the anisotropy of light would cause anisotropic relativity effects in time dilation and cosmic expansion. These effects would cancel out the observable aspects of a varying light speed. In other words, even if the universe was anisotropic due to a varied speed of light, it would still appear homogeneous.
In short, there’s no way to tell whether it is anisotropic because the effects always cancel out. Say that light approached us faster than it left us. How would we perceive the universe? It would look the same it does now, except for one thing:
If the speed of light varies with its direction of motion, then
we would see the universe in a different way. When we look at distant galaxies, we are looking back in time because light takes time to reach us. If distant light reached us quickly in some direction, we would see the universe in that direction as older and more expanded. The faster light reaches us, the less “back in time” we would see.
Lewis and Barnes state the conclusion this way in the paper dated Dec 18, 2020:
The conclusion is that the presence of an anisotropic speed of light leads to anisotropic time dilation effects, and hence observers in the Milne universe would be presented with an isotropic view of the distant cosmos.
These same authors wrote a fascinating book together, A Fortunate Universe (Cambridge, 2016), in which they investigate many of the fine-tuning parameters of the universe and calculate each one’s ramifications for life. After having looked into a number of parameters, and having determined the life-permitting spaces within them (most of them extremely, extremely limited), they engage in a fascinating discussion. Barnes proposes the God hypothesis as the best explanation for the fine-tuning, and Lewis, his foil, plays the role of doubting Thomas.
They reason together about all possible refutations of fine-tuning, along with comeback arguments, and leave ample room for the obvious conclusion: there must be a Creator God." CEH