Lauren Fuge
It’s hard to resist any physics story that starts with Albert Einstein. More than a century ago – in 1916, to be precise – the great theorist suggested that gravitational waves could be a natural outcome of his general theory of relativity.
He was right: gravitational waves are ripples in the fabric of spacetime itself. They can journey billions of light-years across the universe before they wash over the Earth – where we have only recently built the technology capable of spotting them, a century after Einstein’s prediction.
One such piece of technology is sitting in a lab in Perth, Western Australia, in a very cold fridge.
Inside the fridge, a quartz crystal detector is waiting to discover gravitational waves as we’ve never seen them before. In its 153 days of observations so far, it’s already spotted two rare signals lasting about a second each, which might be high-frequency gravitational waves – or dark matter, cosmic rays, or some other physics entirely.
“It’s a pulse of energy being deposited into the crystal,” explains William Campbell, a PhD student involved in the project. “The cause of that energy is the question.”
Specifically designed to work on the challenging frontier of detecting high-frequency gravitational waves, the detector was built by researchers at the ARC Centre of Excellence for Dark Matter Particle Physics (CDM) and the University of Western Australia (UWA).
“It’s a very new way to think about detecting high-frequency gravitational waves – one of the only detectors to actually be sensitive to these frequencies at all,” Campbell says.
“It’s a pulse of energy being deposited into the crystal – the cause of that energy is the question.”
Gravitational waves were first detected in 2015 to the delight of physicists around the world, but so far we’ve only spotted low-frequency events. Now, researchers are on the hunt for different types – produced by smaller but no less interesting cosmic processes.
“For traditional, low-frequency gravitational waves, we know what the sources are – like big black holes or neutron stars,” explains Maxim Goryachev from CDM and UWA, as well as the ARC Centre of Excellence for Engineered Quantum Systems (EQUS). “But the question is what the sources for high-frequency gravitational waves are.”
Theory suggests that they could be created by primordial (extremely light) black holes merging together, or potentially clouds of particles around black holes that slow down their rotation, emitting gravitational waves in a process known as “black hole superradiance”.
These are hypothetical sources, but the current generation of detectors can’t spot these types of waves anyway – they’re only sensitive to low-frequency signals. The world’s largest gravitational wave observatory, the Laser Interferometer Gravitational-wave Observatory (LIGO), is most sensitive around 100Hz, and it can only detect events up to 10,000Hz.
That means no one has yet detected a high-frequency gravitational wave. “Mostly because no one has tried – it’s hard,” Goryachev says.
But now, this new detector is pushing gravitational wave technology into higher frequencies – megahertz (1,000,000Hz) and above.
The technology is actually a throwback to the first generation of gravitational detectors, explains Campbell, also from CDM, UWA and EQUS.
“They’re called resonant bar detectors, so you have a big lump mass and you monitor its vibration when a wave passes through. And this method went stale,” he says. “People found a new and more sensitive way, which is with laser interferometry.”
Laser interferometry is used by the most successful gravitational wave detectors, like LIGO and Virgo, and involves using massive, kilometre-scale set-ups of lasers and mirrors to measure minute changes in the laser beams, caused by the influence of gravitational waves.
But this method doesn’t have the capability to detect high-frequency gravitational waves, so Goryachev, Campbell and team are now resurrecting the old method – on a smaller scale.
“Instead of using a big chunk of metal and measuring its vibrations, we’ve used a really small quartz disc, and it can vibrate at a much higher frequency,” Campbell says. “Through developments in modern technology of what these quartz crystals are traditionally used for – as timing sources – we’ve been able to get the required sensitivity to measure its change of state of vibration from passing gravitational waves.”
The quartz device is so small that you can hold it in your hands. It must then be placed in another device that shields it from unwanted radiation, and then the whole thing is put in a specialised fridge to be cooled to an extremely low temperature – 4 degrees Kelvin, or -269.15 degrees Celsius.
The whole set-up still fits comfortably in a lab, which is a strong advantage over kilometre-scale traditional detectors.
The detector first went online in 2019, and during the intervening time it has collected 153 days’ worth of continuous observations – and spotted two rare events.
“It’s exciting that this event has shown that the new detector is sensitive and giving us results, but now we have to determine exactly what those results mean,” says Campbell.
“If you’re looking for dark matter, you’re looking for new physics, basically – beyond the Standard Model.”
They’ve already ruled out interference, like white noise or other vibrations such as earthquakes or violent weather effects.
“It’s not like an error in a power surge or somebody bumping the equipment – it’s due to genuine physics from inside the crystal,” Campbell says.
The signals could possibly be high-frequency gravitational waves, but could also be meteor showers, internal processes in the crystal itself, charged particles (like muons) from cosmic rays, or some other much more energetic particles.
“We can speculate that those might be some highly massive dark matter particles like quark nuggets or WIMPzillas that have been proposed by some theorists,” Goryachev says. “Typical dark matter detectors might not be sensitive to these candidates as they are looking at lower energies.”
So, how will the team figure out what the signals really are?
There is one other high-frequency gravitational wave detector in the world, the Fermilab Holometer, in the US. However, it’s a small-scale interferometer that takes and processes its data quite differently, so the team can’t use it to confirm their own signals.
But in the lab in Perth, they’ve already built a second, identical detector that will soon come online to narrow down future signals. They’re also hoping to add a muon detector that is sensitive to cosmic rays to the mix.
“The idea is that you’ll have the cosmic ray detector, and your gravitational wave detector, and if you see an event in both detection systems, then you can confidently say that it is most likely a cosmic ray,” Campbell explains.
“These are much more common,” adds Goryachev. “They must happen every hour or so, depending on the energy.”
But if a signal is present in the quartz detector but absent in the cosmic ray detector, then things get a bit more exciting – it could mean they’ve snagged the very first detection of superheavy dark matter particles, or the very first detection of a high-frequency gravitational wave, potentially from primordial black holes.
And the latter, Goryachev points out, could also simultaneously be the first observation of dark matter.
“If you’re looking for dark matter, you’re looking for new physics, basically – beyond the Standard Model,” he explains. “If we confirm that [a signal] is from primordial black holes, that will be dark matter, because very small black holes would make up a lot of mass in our galaxy, and they’re just not accounted for during normal observations.
“Primordial black holes will be dark matter.”
Even though the hunts for dark matter and gravitational waves seem like two separate realms of physics, Campbell says that they have a lot in common, and that the ARC Centre of Excellence for Dark Matter Particle Physics can also contribute meaningfully to gravitational wave science.
“Even though the hunts for dark matter and gravitational waves seem like two separate realms of physics, they have a lot in common.”
“A lot of the technologies overlap quite nicely,” he says. “We are well-positioned to quickly build an experiment that tests for various new physics – developing the technologies required for dark matter detection sometimes allows us to quickly build an experiment that tests a different area of physics, because of this overlap.”
This technology largely involves precision timing, metrology and the ability to eliminate environmental noise – all of which, Campbell says, their lab does well.
And soon, they’ll shoot even higher.
“We have plans to extend our reach to even higher frequencies, where no other experiments have looked before,” he says. “Currently, our maximum frequency is limited by our hardware, and this is a small technical detail that we will overcome in the next generation of the experiment.”
Using the same technology, they can detect frequencies of about 700MHz (70,000 times higher than the maximum frequency detected by LIGO).
“Beyond that there must be some other approach,” Goryachev says. “We are in a process of a collaboration with other institutions, mostly from the EU, to build such detectors. Our vision is that it has to be built based on superconducting technology that can cover the 100MHz-50GHz range.” (For reference, a gigahertz is equal to one billion hertz.)
“With the quartz technology, we can go as low as 100,000Hz, but that will require serious investment,” he adds.
“We are also looking into going for lower temperatures. Now our detector works at four degrees above absolute zero, but we can go to 0.02 degrees and potentially get better sensitivity.”