When a black hole is active, we tend to focus on the effect it has on the material it absorbs. It makes sense to do that; black holes themselves are difficult to investigate. But the interaction between the black hole and the material should also have an effect on the black hole – as the material gains, it should also gain mass.
Such small feedback responses – especially the ones previously ignored as trivial – are known as back-responses, and scientists have just observed an analog of one specific to black holes that can be seen in water swirling down a drain.
It is a detection that could help study black hole phenomena that are too subtle for our current instruments, such as the Hawking radiation believed to be emitted by black holes. This is a theoretical type of radiation from a black body that would eventually – after a very, very long time – see a black hole completely evaporate, provided it didn’t grow at all.
To study cosmic objects in greater detail than we can over the vast distances of space, scaled down versions or analogues can be made in a laboratory. Like, for example, a recent experiment to replicate the white dwarf’s nuclear pressure.
Black hole analogues are an excellent way to learn more about these enigmatic objects, and different types can help reveal their secrets in different ways.
Optical fibers and Bose-Einstein condensates have both been used to learn more about Hawking radiation. But one of the simplest has to do with how black holes feed: the bathtub drain.
The growth of a black hole can be compared to water swirling down a drain. By treating matter as a ripple in a field, the water can be in space itself, or a field rippling with quantum activity.
Measuring the ripple reactions as the water disappears through a swirling drain may have something to say about energy waves disappearing in a black hole.
An analog of a black hole in the bathtub. (The University of Nottingham)
From such analogues we have learned a lot about the effect of black holes on space and the material around it. But with an external water pump keeping the background of the system stable, it was unclear whether a water black hole analog would have the freedom to respond to waves.
This series of experiments marks the first time that a draining bathtub has an effect on the black hole itself.
“We have shown that analog black holes, like their gravitational counterparts, are intrinsically reactive systems,” said physicist Sam Patrick of the University of Nottingham in the UK.
“We have shown that waves moving in a draining bathtub push water through the plug hole, significantly altering the rate of drainage and thus changing the effective gravity of the analog black hole.”
When waves rippled the system toward the drain, they pushed additional water in, accelerating the “growth process” so significantly that the water level in the tub dropped noticeably, even if a pump held the same level of water.
This change in the water level is consistent with a change in the properties of the black hole, the researchers said.
This can be extremely useful information, in part because an increase in mass changes the gravity of a black hole – it changes the way the black hole deforms its surrounding spacetime, as well as the effect the black hole has on the accretion disk. In addition, it offers a new way to study how waves can influence the dynamics of black holes.
“What really stood out to us is that the back response is strong enough to cause the water level across the system to drop so far that you can see it with the eye! This was really unexpected,” said Patrick.
“Our study paves the way to experimentally investigate interactions between waves and the spacetimes through which they move. This type of interaction, for example, will be crucial for examining the evaporation of black holes in the laboratory.”
The team’s research is published in Physical Review Letters.