Why is there a cosmic speed limit? It could even be why we're here
Nothing in the cosmos can travel faster than light speed. By distinguishing cause and effect and stopping everything happening in a jumbled mess, our existence depends on it
PHYSICS 17 November 2021
By
Joshua Howgego
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TIME, 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?“). 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.
Read more: 13 of the most profound questions about the cosmos and ourselves
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 massless things, 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 “W
hy is the universe just right?“, and “
Why is the universe intelligible?“).
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.
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