20 November 2021 | New Scientist | 45ANDRIY^ ONUFRIYE
NK
O/GETTY
IM
AGEST
IME, various wags supposedly said,
is nature’s way of stopping everything
happening at once. That might not be
the most useful way of thinking about things,
however, not least given our confusion about
how time works (see “Why does time only
move forwards?”, page 39). Take a long, hard
look at physics today and it isn’t time that stops
everything happening at once – it is light.
The idea that light always travels at the
same speed, and that nothing can travel faster
than that, is hard-baked into modern physics.
It is still difficult to get your head around the
mind-boggling consequences. Think of
travelling in a spaceship with the beam of your
headlights zinging off in front of you into the
vacuum of space. A stationary observer outside
your ship would see those photons travelling
at light speed – 299,792,458 metres per second,
for those taking notes. The crux is that so
would you, no matter how fast your ship was
travelling in the same direction.
According to Albert Einstein’s theories
of relativity, which he developed in the early
years of the 20th century, space and time
themselves warp to accommodate the
otherwise insurmountable contradictions
that arise from light’s absolute speed.
His special theory of relativity gives a
mathematical explanation for the cosmic
speed cap: as objects with mass accelerate to
higher speeds, they require more and more
energy to keep them accelerating. To attain
light speed, you need infinite energy – an
impossibility. Light only gets a free pass as it
has no mass, as in fact do other masslessthings, such as the ripples in space-time
known as gravitational waves.
But why all that? One answer is that light
speed – or let’s just say, a cosmic speed limit –
acts as a brake on the rate at which influences
can propagate in the universe. If anything
went faster, it would open the door to effects
preceding causes. “If you can travel faster than
light, you get all kinds of problems with
causality,” says Claudia de Rham at Imperial
College London. The universe’s past, present
and future would conceivably occur in a
jumbled mess all at once – and we wouldn’t
be here to wonder about it. In those terms,
a cosmic speed limit might be one of those
things that is “just so” in a universe with
intelligent observers (see “Why is the universe
just right?”, page 41, and “Why is the universe
intelligible?”, page 48).
There remains the question of why that
speed limit – why not twice as fast, say?
Cosmologist João Magueijo, also at Imperial
College, has spent years exploring the idea that
the speed of light might have started off much
higher, perhaps infinite, at the big bang and
slowed down since. This would explain certain
puzzling features of the universe today such as
its strange uniformity. “This is actually a very
minor tweak to relativity,” he says.
A speed of light that evolves over time
wouldn’t itself be that dramatic, says de Rham,
but she adds that current observations give no
reason to believe it does. There would still need
to be some other parameter behind the scenes
that sets how the speed of light changes, she
says – so such considerations don’t help much
with the “why?”.
For his part, Magueijo suspects that the
speed of light is bound up with very deep
matters of physics that we don’t fully
understand yet, relating to the nature of
space and time themselves and whether
they emerge from some deeper layer of
reality. Get a glimpse of what is pulling the
strings, and the cosmic speed limit isn’t
the only thing we might see in a whole
new light. Joshua Howgego.Light’s finite speed
is what separates
causes from effectsWhy is
there a
cosmic
speed
limit?