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<h1 class="css-19v093x">There's no way to measure the
speed of light in a single direction</h1>
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<div class="css-7kp13n">By</div>
<div class="css-7ol5x1"><span class="css-1q5ec3n">Brian
Koberlein</span></div>
<div class="css-8rl9b7">phys.org</div>
<div class="css-zskk6u">3 min</div>
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<figcaption>How to measure the
round-trip speed of light. Credit:
Wikipedia user Krishnavedala</figcaption></figure>
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<p>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.</p>
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<p>When Einstein proposed the theory of
relativity, it was to explain why light
always had the same <a rel="tag"
href="https://phys.org/tags/speed/">speed</a>.
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.</p>
<p>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.</p>
<p>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 <a
href="https://briankoberlein.com/blog/burden-of-proof/">forbids
you from measuring the time it takes light
to travel from point A to point B.</a> 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.</p>
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<figcaption>A Milne universe with
anisotropic light would look uniform.
Credit: Wikipedia user BenRG</figcaption></figure>
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<p>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.</p>
<p>If the speed of light varies with its
direction of motion, then we would see the <a
rel="tag"
href="https://phys.org/tags/universe/">universe</a>
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.</p>
<p>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 <a rel="tag"
href="https://phys.org/tags/relativity/">relativity</a>
effects in <a rel="tag"
href="https://phys.org/tags/time/">time</a>
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 <a rel="tag"
href="https://phys.org/tags/light/">light</a>,
it would still appear homogeneous.</p>
<p>So it seems simple cosmology isn't able to
prove Einstein's assumption about the <a
rel="tag"
href="https://phys.org/tags/speed+of+light/">speed
of light</a> either. Sometimes, the most
basic ideas in science are the most
difficult to prove.</p>
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