[D66] There's no way to measure the speed of light in a single direction

[R.O.]

  There's no way to measure the speed of light in a single direction

By
Brian Koberlein
phys.org
3 min
View Original 
<https://getpocket.com/redirect?url=https%3A%2F%2Fphys.org%2Fnews%2F2021-01-there-no-way-to-measure.html>

How to measure the round-trip speed of light. Credit: Wikipedia user 
Krishnavedala

Special relativity is one of the most strongly validated theories 
humanity has ever devised. It is central to everything from space travel 
and GPS to our electrical power grid. Central to relativity is the fact 
that the speed of light in a vacuum is an absolute constant. The problem 
is, that fact has never been proven.

When Einstein proposed the theory of relativity, it was to explain why 
light always had the same speed <https://phys.org/tags/speed/>. In the 
late 1800s, it was thought that since light travels as a wave, it must 
be carried by some kind of invisible material known as the "luminiferous 
aether." The reasoning was that waves require a medium, such as sound in 
air or water waves in water. But if the aether exists, then the observed 
speed of light must change as the Earth moves through the aether. But 
measurements to observe aether drift came up null. The speed of light 
appeared to be constant.

Einstein found that the problem was in assuming that space and time were 
absolute and the speed of light could vary. If instead, you assumed the 
speed of light was absolute, space and time must be affected by relative 
motion. It's a radical idea, but it's supported by every measurement of 
light's constant speed.

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. 
<https://briankoberlein.com/blog/burden-of-proof/> 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.

A Milne universe with anisotropic light would look uniform. Credit: 
Wikipedia user BenRG

Since all the round-trip speed of light measurements give a constant 
result, you might figure you can just divide the time by two and call it 
a day. This is exactly what Einstein did. He assumed the time there and 
back was the same. Our experiments agree with that assumption, but they 
also agree with the idea that the speed of light coming toward us is 10 
times faster than its speed going away from us. Light doesn't have to 
have a constant speed in all directions, it just has to have a constant 
"average" round-trip speed. Relativity still holds if the speed of light 
is anisotropic.

If the speed of light varies with its direction of motion, then we would 
see the universe <https://phys.org/tags/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. Since we observe a uniform cosmos in all directions, surely 
that shows the speed of light is constant.

Well, not quite, as a new study shows. 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 <https://phys.org/tags/relativity/> effects 
in time <https://phys.org/tags/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 <https://phys.org/tags/light/>, it would still 
appear homogeneous.

So it seems simple cosmology isn't able to prove Einstein's assumption 
about the speed of light <https://phys.org/tags/speed+of+light/> either. 
Sometimes, the most basic ideas in science are the most difficult to prove.


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