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Gravitational waves discovered: the universe has spoken

When two black holes collide, the resulting gravitational ripples can be felt across the cosmos. Henze, NASA

Gravitational waves discovered: the universe has spoken

When two black holes collide, the resulting gravitational ripples can be felt across the cosmos. Henze, NASA

Up until now, humanity has been deaf to the gravitational “sounds” of the universe. At last, we have heard our first message. On September 14, 2015, the first data arrived!

Our observation of the gravitational waves from the merger of two black holes simultaneously represents our first glimpse of the first stars in the universe, and a direct observation of the final end point of stellar evolution. We have seen the vibrations of the shimmering event horizon of a newly formed black hole, where time comes to an end.

It is hard to overstate the significance of this discovery. It is our first direct contact with our first stellar ancestors. It is our first direct view of a place in the universe where matter loses all its identity and time comes to an end. It is the first of many messages that will tell us how many black holes are out there and how much of the mass of the universe they can account for.

A long time coming

The first detection occurred 99 years after Albert Einstein predicted the existence of gravitational waves. Two Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) laser interferometers simultaneously detected a signal characteristic of a pair black holes – 29 and 36 times the mass of the sun – merging into one.

Gravitational waves are akin to sound waves: they make things vibrate. Our detectors are our bionic ears that allow us to listen to the universe. The signal from the pair of black holes started two octaves below middle C, and rose up to middle C in one tenth of a second. The signal itself was detected as a vibration of the distance between mirrors four kilometres apart. They changed their spacing by about a billionth of the diameter of an atom.

The Sound of Two Black Holes Colliding. LIGO163 KB (download)

Even so tiny, the signal stood well above the noise, and arrived at the two LIGO detectors, which are 3,002km apart, just 6.9 milliseconds apart, which is characteristic of a wave travelling at the speed of light and coming in at an angle of about 45 degrees.

Black hole binary merger signals encode their distance, their masses and their spins. The signals matched predictions very closely, showing no sign of any deviation from Einstein’s theory.

The aLIGO detector was sensitive enough to pick up the minute perturbations in space-time. University of Birmingham Gravitational Waves Group, Christopher Berry

Good vibrations

The ringing at the end of the waveform is the dying vibration of the new formed black hole, which is 62 times the mass of the sun. The estimated distance of the event was more than one billion light years. The surface area of the new black hole is four times the area of Tasmania.

The total power output in gravitational waves over the event duration was more than 10²² times the power output of the sun, or 10⁴⁹ Watts, peaking to 10⁵⁰W in the last milliseconds.

The total energy output was almost 10⁴⁸ Joules. The sun would have to emit steadily for ten thousand times the age of the universe to give out that much energy. The black holes gave it out in one tenth of a second!

The gravitational signal detected had an energy density about one hundred thousand times greater than starlight from Sirius, the brightest star in the sky. And it was from a distance where even the world’s biggest telescopes are unable to detect an individual star.

The power of the wave exceeds the brightness of any historically recorded supernovae in our own galaxy, such as the supernova of 1006 CE which was reported to have been almost as bright as Venus.

It also greatly exceeds the energy output of gamma ray bursts, which were the previous record holders for explosions in the universe. Because the signal was pure gravitational energy and not electromagnetic energy, only super-sensitive laser interferometer gravitational wave detectors are capable of their detection.

In terms of superlatives, this gravitational wave burst is momentous. It is not only the first direct detection of gravitational waves, it also proves that gravitational waves propagate near the speed of light, and interact with detectors as predicted.

This source proves that black holes have size and mass in agreement with predictions made by Karl Schwarzschild exactly 100 years ago in 1916. For the first time, nature has provided signals that allow us to confirm that the black holes discovered by astrophysicists are consistent with the black holes theorised by general relativity. General relativistic black holes are the only objects capable of creating such an energy flux.

From hoping to knowing

The first detection turns hypotheticals into observational reality. Suddenly the idea of space-time as medium able to ripple and curve is inescapable. Suddenly we know that the black holes predicted by Einstein’s theory are really out there drifting through the universe.

The first detection of gravitational waves confirms the physics of the interaction of gravitational waves with laser interferometers.

This relates to a conundrum, raised by skeptics time after time: if gravitational waves move all the mirrors and the laser beams by the same amount, signals might never show up. This discovery has put paid to this conundrum, one which gravitational wave physicists had never doubted. Our gravity radios work and they detect signals.

I predict that the next big breakthrough will be detection of the first binary neutron star coalescence. This signal will give us a direct probe of the mysterious nuclear matter inside neutron stars – matter as dense as an atomic nucleus, where one teaspoon of matter can weigh one billion tonnes.

The merger of binary neutron stars would also create gravitational waves that we ought to be able to detect. Author provided

For these types of signals, radio, optical and X-ray telescopes will be needed to work together with the gravitational wave detectors to observe the accompanying electromagnetic waves.

Because laser interferometer gravitational wave detectors operate according to the laws of quantum mechanics, the successful detection of gravitational waves demonstrates the universality of the two foundational theories of physics: quantum mechanics and general relativity. The tension of their well-known incompatibility adds special interest to the new discoveries.

Gravitational wave discovery provides both an enormous opportunity, and an enormous responsibility for educating the public on the discoveries that will forever change the way we view the universe.

It provides an opportunity to modernise science education, to teach our youngest students concepts such as curved space that underpin the theory of gravity, and the concept of photons that underpins our quantum understanding of the world.