Black holes are the Universe’s most jealously guarded secrets. We all want to know what’s going on inside such exclusive space clubs, yet the best we can do is stand outside and listen to the beat.
To do this, scientists host their own parties. Sure, these aren’t as fun as twisted pits of spacetime, but they’re as close as we’ll get to getting VIP treatment on a black hole‘s dance floor.
The Black Hole Laboratory at the University of Nottingham in the UK contains no bona fide black holes. What it does have is a rippling tank of water coloured with green dye and a hole to send it down.
Physicists from the lab and the Universidade Federal do ABC in Brazil recently used this setup to identify wave patterns in water circling a drain, which could help us understand the dulcet tones of a screaming newborn hole.
When black holes merge and form bigger black holes, the whole Universe hears about it: space hums with a tune called a quasinormal mode that holds clues about the new black hole’s characteristics, such as its mass and angular momentum.
These modes have become all the fashion in physics with ongoing advances in gravitational wave astronomy, and physicists are keen to extract as much detail as they can from the way space shakes after these cosmic crashes.
Pulling information out of a quasinormal mode requires knowing a few things about how energy dissipates in a field, and how certain features in the wave’s patterns change or persist with time.
One thing that potentially affects a wave’s characteristics is vorticity – the curving flow of its underlying medium as it’s dragged by competing forces.
For most simple models describing a black hole’s oscillations, it’s assumed space is little more than a quiet background through which waves ripple, meaning its vorticity isn’t normally taken into account.
This might not make all that much of a difference. Or it could be significant. It’s not like we can get a close look at these whirlpools of spacetime to find out.
But we can examine quasinormal modes in other media and hunt for signs of interference.
Water isn’t necessarily the perfect metaphor for spacetime, sure, but the fundamentals work just fine. The analogy even has its grounding in theoretical work conducted by Canadian physicist William “Bill” Unruh nearly 40 years ago, who proved the hydrodynamic equations are a close match for describing gravity around sufficiently sized masses.
Away from a drain, waves on a surface tend to move faster than the current, allowing them to ripple in virtually any direction. But as water flows towards the equivalent of a black hole, it picks up speed, taking its mess of wave patterns with it.
“Fluid velocity is much higher than wave velocity, so the waves are dragged down by the water flow even when they’re propagating in the opposite direction,” physicist Maurício Richartz told José Tadeu Arantes from Agência FAPESP.
Measuring the oscillations revealed patterns that stuck around for extended periods near the edge of the vortex, states that reflected characteristics of the hole, such as its size and angles.
“Our main finding was that some oscillations decayed very slowly, or in other words remained active for a long time, and were located spatially in the vicinity of the drain,” Richartz told Agência FAPESP.
“These oscillations were no longer quasinormal modes, but a different pattern known as quasi-bound states.”
The researchers hope to intentionally create more of these long-lived ‘quasi-bound’ energy states, and under different conditions, to study their implications for rotating black holes.
It’s been more than a century since Einstein’s field equations were shown to result in curious objects known as singularities: strange distortions of space that give birth to the monsters of gravity we call black holes.
Yet, in spite of decades of research, we’re little closer to understanding the physics black holes exhibit.
Luckily for us, black holes aren’t as silent as they are dark. We just need to learn how they play their music.
This research was published in Physical Review Letters.