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Grave new world

Nature Physics volume 12, page 197 (2016) | Download Citation

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Physicists have finally detected gravitational waves, in a triumph of ingenuity and perseverance. And now we need to explain them to the general public.

On 14 September 2015, the upgraded Laser Interferometer Gravitational-Wave Observatory (advanced LIGO) made history: the two sites — one in Livingston, Louisiana and the other in Hanford, Washington — detected gravitational waves for the first time since they were predicted 100 years ago by Albert Einstein. The disturbance came from the merger of two black holes of roughly 36 and 29 solar masses, some 1.3 billion light-years away, spiralling inwards, creating a single black hole of 62 solar masses. Keen mathematicians will note that 3 solar masses are missing. That was the energy going into the gravitational waves (equivalent to the total energy from the light of all the stars in the Universe, for 0.02 seconds). As the signal was a brief 'chirp' in the audible frequency range, we can now hear the Universe, whereas previously we could only see it.

Of course, the announcement, made on 11 February 2016, immediately hit the headlines. “We did it”, stated David H. Reitze, executive director of LIGO. The excitement was palpable. Some of us cried. But the public's response was largely summed up by the satirical news source, The Daily Mash, with their headline: “Scientists completely fail to explain 'gravitational waves'”.

And it isn't difficult to explain. Every mass has a gravitational field, and whenever that mass accelerates, the gravitational field changes accordingly. Isaac Newton believed that the fields changed instantaneously on a global scale, but Einstein put a speed limit on the Universe. As information can only travel at the speed of light, information must propagate as a wave — a gravitational wave. These waves convey information on the motion of masses and are complementary to electromagnetic waves that convey information on the motion of charges.

Unfortunately, gravitational waves are weak. Advanced LIGO was able to measure a strain on the order of 10−21. This was only possible due to its recent US$200 million upgrade (involving additional input from the UK, Germany and Australia). But this first detection already tells us so much. Not only do we have a confirmation that black holes of masses greater than 25 solar masses exist, they can do so in a binary system and merge within the lifetime of the Universe. And let's not forget that it is a major, if unsurprising, confirmation of Einstein's general relativity. But what is truly mind-blowing is that not one of the telescopes operating at electromagnetic wavelengths has detected a counterpart event. A black hole merger emits no light, so only a gravitational wave detector was able to sense it.

And with the Virgo interferometer (in Cascina, Italy) coming online, and several other gravitational observatories at other wavelengths in the works, not to mention the Evolved Laser Interferometer Space Antenna (eLISA) (see Nature Phys. 11, 613–615; 2015), we may be able to 'hear' what happened just after the Big Bang, when the Universe was transparent to gravitational waves but not to electromagnetic ones. In the meantime, we should learn to explain the physics of these spectacular events to non-physicists.

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