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(Source: NASA)

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The physics of gravitational waves

Tuesday 8th March 2016 11:27 am

Gravitational waves distort the fabric of space-time. How? Gravity is geometry, explains Dr Karl.

Last time, I talked about how in September 2015, a series of eight gravitational waves (caused by two black holes colliding) had been measured rippling through our planet as easily as light passes through glass.

These gravitational waves briefly changed both the shape, and the size, of our planet. Most of us didn’t notice, because the change in size was very small — around one hundredth of a trillionth of a metre. But the astronomers did notice, thanks to a revolutionary telescope.

But what are gravitational waves?

That’s a very deep question, so let’s go back to basics. We are all familiar with the three-dimensional nature of the world around us. You can travel back-and-forth (the first dimension), you can travel left-and-right (the second dimension) and you travel up-and-down (the third dimension).

If you studied geometry at school, you might remember the x-, y- and z-axes of a graph. But there’s a fourth dimension as well — time (which normally travels forward at one second every second). Put all together, they make the imaginatively named space-time continuum or fabric of the universe.

Electromagnetic waves (such as light, microwaves, X-rays and so on) travel across this space-time fabric of the universe.

So now it’s time for a deep and fundamental insight — and in just three words. Here it comes: gravity is geometry. That was it.

Think of a bowling ball sitting on a trampoline — it makes a dent. Let’s pretend that our Sun is that bowling ball. So our Sun makes a local distortion in the fabric of space-time.

Along comes a totally innocent comet. If there were no Sun, and no dent in the fabric of space-time, that comet would continue in a straight line. But there is a dent in shape of space-time, and so the path of the comet is deflected. Depending on its original path and speed, it might smash into the Sun, or it might come very close to the Sun and loop around it, or it might just very slightly have its course changed.

So gravity is just geometry — it’s just a local distortion in the fabric of space-time.

And a gravitational wave? That’s a moving distortion in the fabric of space-time — it travels through space-time, not on top of it.

About a century ago, Albert Einstein came up with his Theory of General Relativity, which is really a theory of gravity.

One of Einstein’s many great insights was to treat both space and time as real things. And he said that any accelerating mass would give off gravitational waves, which would then ripple through the background fabric of space-time at the speed of light.

So while electromagnetic waves travel across the space-time continuum of the universe, gravitational waves are a travelling distortion of the actual space-time continuum of the universe.

For electromagnetic waves, space and time make the stage and backdrop upon which these waves dance. But for gravitational waves, space and time are changes in the stage and backdrop.

Gravitational waves are not energy travelling across space-time — they are the actual rippling and changing of the shape of space-time itself.

If I move my hand at constant velocity, it does not emit gravitational waves. But if I accelerate my hand (make it speed up and then slow down), it will emit gravitational waves. It’s not the velocity, but the fact that the velocity changes.

But because gravity is very weak (compared to the electromagnetic force, or the strong or weak nuclear forces), the amount of power in the gravitational waves from my accelerating hand is microscopic.

Even if you go for something a lot bigger, like the Earth orbiting the Sun, the gravitational waves emitted amount to only about 200 watts. That’s still very small — a decent car engine can generate a thousand times more. (Just as an aside, in response to continually losing 200 Watts, the Earth’s orbit shrinks – but only by about one three-hundredth of the diameter of a hydrogen atom each year, which is about a third of a trillionth of a metre.)

You need truly cataclysmic events, like a pair of black holes colliding into each other, to emit the very powerful gravitational waves that we can detect here on Earth.

So speaking of detection, just how did the astronomers pick up these faint gravitational waves as they distorted the fabric of space-time, and they rippled through our planet. I’ll talk more about that, next time …

 

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This blog first appeared on Dr Karl's Great Moments in Science

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