When a black hole is active, we tend to concentrate on the effect it has on the material it swallows. It makes sense to do so; black holes themselves are difficult to investigate. But the interaction between the black hole and the material must also have an effect on the black hole – as it acquires material, it must also increase in mass.
Such small feedback reactions – especially reactions previously ignored as trivial – are known as reactions, and scientists have just observed an analogue specific for black holes, which can be seen in water running in a drain .
This is a detection that can help investigate black hole phenomena that are too subtle for our current instruments, such as the Hawking radiation that is thought to be emitted through black holes. It is a theoretical kind of black body radiation that will eventually – after a very, very long time – see a black hole evaporate completely, provided it does not grow at all.
To study cosmic objects more finely than we can over the great distances of space, scaled-down versions or analogues can be created in a laboratory. Such as a recent experiment to replicate white dwarf core pressure.
Analogues of black holes are a great way to learn more about these enigmatic objects, and different types can help reveal their secrets in several ways.
Fiber optic and Bose-Einstein condensates were both used to learn more about Hawking radiation. But one of the simplest has to do with how black holes feed: the draining bathtub.
Growth of black holes can be compared to water turning in a drain. If you treat matter as a ripple in a field, the water can stand on its own during space-time, or a field that ripples with quantum activity.
Measuring the ripple reactions as the water disappears into a swirling drain can say something about the waves of energy disappearing into a black hole.
A bathtub-black hole analog. (The University of Nottingham)
From such analogues we learned a lot about the effect of black holes on the space and materials around them. But with an external water pump holding the background of the system steady, it was unclear whether an analog in the black hole would have the freedom to respond to waves.
This set of experiments is the first time that a draining bathtub shows an effect on the black hole itself.
“We have shown that analogous black holes, like their gravitational counterparts, are intrinsic reaction systems,” said physicist Sam Patrick of the University of Nottingham in the United Kingdom.
“We have shown that waves moving in a draining bath push water through the plug hole, which significantly changes the drain speed and consequently changes the effective gravity of the analog black hole.”
When waves were sent in the system in the direction of the drain, they pushed in extra water, which accelerated the “accretion” process so significantly that the water levels in the bath dropped noticeably, even while a pump held the same water level.
This change in water level is consistent with a change in the characteristics 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 distorts its surrounding space-time, as well as the effect the black hole has on the growth disk. In addition, it offers a new way to investigate how waves can affect the dynamics of black holes.
“What was striking to us was that the backlash was large enough so that it dropped the water level over the whole system so that you could see it with the eye! It was really unexpected,” Patrick said.
“Our study paves the way for experimentally investigating interactions between waves and the space-times through which they pass. This type of interaction, for example, would be crucial in investigating the evaporation of black holes in the laboratory.”
The team’s research was published in Physical overview letters.