Gravitational Waves: Why Are Scientists Celebrating?

There have been many scientific highlights of physics and astronomy in recent years: the Higgs Boson, landing a probe on a comet, and an amazing fly-by of Pluto. But all this is dwarfed by what has been announced this week. A new era of science has begun.
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The entire planet shook on September 14th, 2015. The very fabric of space itself rippled and distorted. Your own body was repeatedly squashed and stretched.

But you can be forgiven for not noticing: the amount of stretching was around one billionth the diameter of an atom.

So what's all the fuss about? What happened on September 14th, and why has it now got scientists so excited?

In 1915, Albert Einstein published his general theory of relativity, a visionary and revolutionary description of the nature of gravity. General relativity makes a host of bizarre but very specific predictions about how gravity affects matter and light. A huge number of these predictions have been exquisitely confirmed by experiments. So while we still don't know whether Einstein's theory is correct (and indeed we strongly suspect that general relativity isn't the whole story), it seems to be an excellent description of how the Universe works under almost every circumstance we can currently examine.

But there's always been one glaring gap in the story.

In 1916, Einstein pointed out that under many circumstances, a moving massive object will cause space itself to ripple, a little like the expanding ripples of water produced by a stone skipping over a pond. In particular, if two heavenly bodies are in orbit around each other, then the space around them will be filled with these "gravitational waves", generated as the two objects bend the space around them with their mass. The waves that result then race away from the orbiting system at the speed of light, carrying away energy and thus causing the two objects to gradually but inexorably spiral in towards each other.

In 1916 this was completely esoteric. The two objects in orbit would have to be incredibly massive, and the size of the expected rippling would be far more smaller than the smallest scales imaginable.

But in 1974, astronomers Joe Taylor and Russell Hulse discovered an extraordinary natural laboratory, which finally allowed us to put Einstein's idea to the test. Taylor and Hulse found two super-dense neutron stars, swinging around each other every eight hours. This is exactly the situation needed to test the predictions of general relativity. And indeed, the two stars are slowly coming together, their orbits shrinking as they presumably leak gravitational waves. The rate of shrinkage is precisely as predicted by Einstein's theory, a result for which Taylor and Hulse received the 1993 Nobel Prize in Physics. Since then, other pairs of neutron stars have been found, all also spiralling inwards as predicted.

As satisfying as this was to see, there was still wiggle room. Indirect evidence is not proof, and there are other theories that can explain why two stars decay in their orbits like this. Maybe it is not gravitational waves at play, but some other, even more exotic phenomenon?

To seal the deal, we need to directly detect gravitational waves. We need to actually see a wooden yardstick repeatedly shrink and stretch as gravitational waves from some distant binary system wash over the Earth.

The best hope of seeing such gravitational waves is right before the two objects finally spiral together, collide, and merge. In these final frenzied moments, the ripples in space become much larger.

Unfortunately, "much larger" is still not very big. Even for two stars about to collide, the amount of shrinking or stretching produced by their gravitational waves is unimaginably tiny. An actual yardstick sitting in front of you would oscillate in its length by around 0.000000000000001 millimeters, which is far smaller even than the diameter of a proton. This could be happening all the time, and we would never be able to tell.

But astronomers are not easily discouraged.

In Washington and Louisiana, a pair of very unusual scientific experiments have been constructed, at a cost of more than $600 million. Each site hosts a 5-mile tunnel, in the shape of an "L", pumped free of air and with powerful lasers shining through the vacuum. Together, the two facilities comprise the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), an extraordinarily complicated technological yardstick specifically designed to search for these tiny ripples.

Advanced LIGO was scheduled to begin its first search for gravitational waves on September 18th, 2015. But incredibly, four days before this, while the facility was undergoing its final engineering checks, a signal came in. A booming signal. A signal exactly as Einstein predicted. In this case, what Advanced LIGO saw was the apparent final death cry of two enormous black holes, spiralling into each other before catastrophically coalescing.

The result seems so clean, so beautiful, so textbook in its appearance, that even the most hardened skeptics should be satisfied. And the scientific community now deservedly celebrates.

Indeed there are not one, but two, reasons for joy and buoyancy over this stunning breakthrough.

First, what Advanced LIGO has seen is a genuine, robust, spectacular confirmation of Einstein's general relativity. An obscure, specific, but profound prediction, made exactly 100 years ago, has been exquisitely verified.

But that's not the most exciting thing. What's exciting is that we now have an entirely new way of studying the cosmos.

When Harriot and Galileo first turned telescopes to the skies in 1609, they immediately saw things that nobody had ever seen before, and their understanding of the Universe immediately changed.

The same thing is now happening here.

Things we should never be entitled to see - colliding black holes, merging neutron stars, gargantuan collisions of galaxies - can now be routinely revealed to us. We are poised to discover whole new types of phenomena, and we will now receive entirely new insights on familiar objects.

There have been many scientific highlights of physics and astronomy in recent years: the Higgs Boson, landing a probe on a comet, and an amazing fly-by of Pluto. But all this is dwarfed by what has been announced this week. A new era of science has begun.

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