‘Why I believe in a cosmic speed limit’

Speeding fine: Characters in Star Wars used hyperdrive to travel faster than light.
by Sean Carroll

A theoretical physicist at the California Institute of Technology. He is author of ‘From Eternity to Here’ and ‘The Particle at the End of the Universe’.

Stephen Hawking is working on a small spaceship which will travel at one-fifth of the universal ‘speed limit’ known as the speed of light. This limit is a wonderful thing, says the author.

I want to give thanks for a feature of the physical world that many people grumble about rather than celebrating, but is undeniably central to how nature works at a deep level: the speed of light.

The speed of light in vacuum, traditionally denoted by c, is exactly 299,792,458 metres per second.

There are a few great things about the speed of light. One is that it is a fixed, universal constant, as measured by inertial observers, in vacuum. Light can slow down if it propagates through a medium, but that is hardly surprising.

It takes just over a second for light to travel from the Earth to the Moon and eight minutes to get to the Sun

The other is that it is an upper limit. The fact that the speed of light is such an insuperable barrier to the speed of travel really bugs people. On everyday-life scales, c is incredibly fast; but once we start contemplating astrophysical distances, suddenly it seems maddeningly slow.

It takes just over a second for light to travel from the Earth to the Moon; eight minutes to get to the Sun; over five hours to get to Pluto; four years to get to the nearest star; twenty-six thousand years to get to the galactic centre; and two and a half million years to get to the Andromeda galaxy.

That is why almost all good space-opera science fiction takes the easy way out and imagines faster-than-light travel.

I think we should do better than just be grumpy about the finite speed of light.

Like it or not, it is a crucial part of the nature of reality. It did not have to be, in the sense of all possible worlds; the Newtonian universe is a relatively sensible set of laws of physics, in which there is no speed-of-light barrier at all.

That would be a very different world indeed. In Newton’s cosmos, when a planet moves around the Sun, its (admittedly feeble) gravitational field changes instantly throughout all of space.

In principle, in pre-relativistic laws of physics it would be possible to imagine communication or transportation devices that took you from here to billions of light years away, in as short a time as you can imagine.

That seems like fun, but think about what you are giving up. The speed of light enforces what physicists think of as locality — what happens at one point in spacetime influences what happens nearby, and those influences gradually spread out.

A universe without the speed of light would be one where different parts of space were potentially connected in dramatic ways. That would be convenient for some purposes — but so utterly different from the real world that it is hard to think through all of the consequences consistently.

What matters is not that light travels at a certain speed — it is that the universe has an ultimate upper speed limit.

It just so happens that massless particles travel at that speed. But even in a world without any massless particles, there would still be a speed limit. We would not call it the ‘speed of light’ in such a world, but something else, like the ‘Einstein speed’ or some such.

We live in a world where it inevitably takes time for signals to travel from one place to another, and I for one am thankful for it.

This is an extract from a post originally written on Sean Carroll’s personal blog, ‘Preposterous Universe’, reprinted with kind permission. See the link under Become An Expert to read the article in full.

You Decide

  1. Are you glad the universe has an upper speed limit?

Activities

  1. Why is the speed of light the universal speed limit? Do some research and prepare to explain your answer to your class in a maximum of one minute.

Word Watch

Vacuum
A part of space with nothing in it at all.
c
This letter may be used because it stands for the Latin word celeritas, which means speed; it may also stand for constant.
Upper limit
Physical particles, as far as we know, always move at speeds less than or equal to c.
Andromeda
The closest large galaxy to the Milky Way (where Earth is).
Science fiction
For example, the creators of Star Trek used a system called Warp Drive; The Hitchhiker’s Guide to the Galaxy featured the Infinite Improbability Drive.
Cosmos
The universe when seen as a whole, with systems and rules which apply throughout. The laws Newton established have been used to calculate distances and orbits, for example of planets, moons, comets and spacecraft.
Spacetime
A four-dimensional combination of width, height, depth and time. All events occur at a unique point in spacetime, as nothing else is happening in the same space at the same point in time.
Massless particles
Particles which have, theoretically, no mass when at rest. Photons and gravitons — whose existence is not confirmed — are believed to be massless.

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