A small crystal device can raise gravity wave detectors to detect the birth call of black holes

In 2017, astronomers witnessed the birth of a black hole for the first time. Gravitational wave detectors picked up the ripples in space-time, caused by two neutron stars colliding to form the black hole, and other telescopes then observed the explosion.

But the true nitty-gritty of how the black hole formed, the movements of matter in the moments before it was locked away within the black hole’s event horizon, did not materialize. This is because the gravitational waves that have been dropped in these last moments have had such a high frequency that our current detectors cannot catch them.

If you could observe ordinary matter as it turns into a black hole, you would see something similar to the big bang play backwards. The scientists who design gravity wave detectors have been working hard to figure out how to improve our detectors to make this possible.

Our team is publishing a paper today that shows how this can be done. Our proposal can make detectors 40 times more sensitive to the high frequencies we need, allowing astronomers to listen to matter as it forms a black hole.

It involves creating strange new packets of energy (or “quantum”) that are a mixture of two kinds of quantum vibrations. Devices based on this technology can be added to existing gravity wave detectors to obtain the extra sensitivity.

Quantum Problems

Gravitational wave detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States use lasers to measure incredibly small changes in the distance between two mirrors. Because they measure changes a thousand times smaller than the size of a single proton, the effects of quantum mechanics – the physics of individual particles or quantum energy – play an important role in the way these detectors work.

There are two different types of quantum energy packs involved, predicted by Albert Einstein. In 1905 he predicted that light would come in packets of energy that we call photon; two years later he predicted that heat and sound energy would come in packets of energy called phoneme.

Photons are widely used in modern technology, but phonons are much more difficult to use. Individual sounds are usually overwhelmed by a large number of random sounds that are the heat of their environment. In gravitational wave detectors, phonons reflect inside the mirrors of the detector, degrading their sensitivity.

Five years ago, physicists realized that you can solve the problem of insufficient high-frequency sensitivity with devices that combine phonons with photons. They have shown that devices that transport energy in quantum packets that share the properties of both phonons and photons can have striking properties.

These devices involve a radical change in a well-known concept called ‘resonant amplification’. Resonant amplification is what you do when you hit a playground’s swing: if you hit at the right time, all your little pushes create big swings.

The new device, called a “whitish cavity”, amplifies all frequencies equally. It’s like a swing that you can push at any old time and still end up with great results.

However, no one has yet worked out how to make one of these devices because the phonons in it will be overwhelmed by random vibrations caused by heat.

Quantum Solutions

In our article, published in Communication Physics, we show how two different projects currently underway can do the job.

The Niels Bohr Institute in Copenhagen has developed devices called phononic crystals, in which thermal vibrations are controlled by a crystalline structure cut into a thin membrane. The Australian Center of Excellence for Engineering Quantum Systems has also demonstrated an alternative system in which phonons are captured in an ultra-pure quartz lens.

We show that both of these systems meet the requirements to create the ‘negative distribution’ – which distributes light frequencies in an inverted rainbow pattern – required for whitish cavities.

Both systems, when added to the back of existing gravity wave detectors, would improve the sensitivity at a few kilohertz by more than 40 times to listen to the birth of a black hole.

What’s next?

Our research is not an immediate solution to the improvement of gravity wave detectors. There are enormous experimental challenges in making such devices practical tools. But it does provide a route to the 40-fold improvement of gravity wave detectors needed to detect births in black holes.

Astrophysicists have predicted complex gravitational waveforms that result from the convulsions of neutron stars as they form black holes. These gravitational waves can enable us to listen to the nuclear physics of a collapsing neutron star.

For example, it has been shown that they can clearly reveal whether the neutrons in the star remain as neutrons and whether they break up into a sea of ​​quarks, the smallest subatomic particles of all. If we could observe neutrons that turn into quarks and then disappear into the black hole singularity, it would be the exact reversal of the big bang, where the particles emerged from the singularity that created our universe.

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