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The awesome origins of gravitational waves

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When two black holes collide, the resulting gravitational ripples can be felt across the cosmos (Source: Henze/Nasa)

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The awesome origins of gravitational waves

Tuesday 1st March 2016 11:35 am

The recently discovered gravitational waves were created under mind-boggling circumstances. Dr Karl goes into the beautiful and awe-inspiring story of their creation.

Back on September 14, 2015, at 0951 Universal Time (which we used to call Greenwich Mean Time), strange waves of energy stormed at our Earth from somewhere in the southern sky.

They rippled straight through our entire planet. In response, our planet changed shape eight times in a very short window of time — about one fifth of a second. With each of those eight impulses, the entire planet shrank in one direction, and simultaneously expanded in the other direction.

Each impulse was bigger than the one before it. The eighth one was the biggest. And then, after just one fifth of a second, they were gone — travelling away from us at the speed of light.

They were the first gravitational waves that we humans had ever observed. And they were caused by two black holes crashing together to make one bigger black hole.

These two black holes were about 1.3 billion light years away from us. They were significantly heavier than our Sun. One was 29 times heavier — what the astronomers describe as having 29 solar masses. The other had 36 solar masses. They had been orbiting each other for (literally) billions of years.

According to the mathematics given to us by Albert Einstein in his Theory of General Relativity back in 1916, these black holes had all that time been emitting energy, in the form of gravitational waves. Because they were losing this energy, their orbits around each other were shrinking.

At first, billions of years ago, they were a long way apart. At this early stage, they were losing only small amounts of energy, and their orbits were shrinking only very slowly. But Einstein’s maths told us that as they got closer, they would lose energy much more quickly.

It was inevitable — they were definitely going to slam into each other. It was going to be big. And our instruments here on Earth were just sensitive enough to pick up the gravitational wave energy that they pumped out in their last eight orbits, before their final collision.

At that very last instant immediately before they collided, they were travelling at about half the speed of light — roughly 150,000 kilometres/second. They were orbiting each other at 75 times in each second. And remember — these were not inconsequential objects. No, these were black holes, with masses 29 and 36 times that of our Sun!

In that cataclysmic impact, they somehow merged into just one black hole, weighing in at 62 solar masses. But if you add together the initial black holes with 29 and 36 solar masses, you get 65 solar masses, not 62. You can see that three times the mass of the Sun went missing. Where did it go? That mass got turned into gravitational waves — following another of Einstein’s equations, E = mc2, or basically, mass is congealed energy, and energy is just mass that has been unleashed.

Now let’s just slow down and think about that. Each second, our Sun burns about 620 million tonnes of hydrogen to make energy. Our Sun is halfway through its life, but over its entire existence of about 10 billion years, it will burn up less than one thousandth of its mass. Now consider these two colliding black holes. They turned into energy, not just one thousandth of the mass of our Sun — but three times! And they did this, not over 10 billion years, but in less than one fifth of a second.

In that instant, they temporarily generated over 50 times the power output of all the billion trillion stars in our entire universe put together.

If you were close to that collision, with all that power let loose, everything would have got very messy. But we’re 1.3 billion light years away — and so, that power got diluted over a huge volume of space, about a billion cubic light years.

And as the biggest of those gravitational waves rippled through our planet, in response our planet changed. Earth is about 12,750 kilometres across. It shrank, and then expanded, by a very tiny amount — about 10 times the diameter of a proton, or one hundredth of a millionth of a millionth of a metre.

Each of these eight gravitational waves came through with a higher frequency than the one before it. In one fifth of a second, they shifted from 30 Hz up to 150 Hz, or from the third lowest note on a grand piano up to the D below middle C.

But what were these waves made of? They weren’t audio waves, they weren’t electromagnetic waves — but they were astonishing waves of change that had the power to reshape Planet Earth. They were gravitational waves, and I’ll talk more about them, next time …


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  1. andrew hadj says:


    that is a brilliant article.

    thank you for the time/effort/knowledge


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