Black holes are mysterious objects - there's a lot we don't know about them. One longstanding question has been whether rotating black holes, which are so powerful they drag space-time along with them, could be used as an energy source.
Author
Marion Cromb
Research Fellow in Physics, University of Southampton
The physicist Roger Penrose suggested that, if an object fell into a rotating black hole in such a way that it split - with one part escaping - the part that left should effectively gain energy from the black hole.
So if we sent objects or light towards a rotating black hole, we may be able to get energy back. It's difficult to directly prove all this, however. But we have recently published our second study, in Nature Communications, experimentally verifying a more general theory behind it. This theory concerns all rotating objects that can absorb matter or radiation, and a black hole is, in essence, just a very big and effective absorber.
The idea dates back to 1971 and the Soviet physicist Yakov Zel'dovich. Generalising Penrose's idea, he predicted something very simple. If you take a cylinder that absorbs energy from waves, and you spin it, then it should actually spend its own energy to amplify some waves (boosting their energy).
This would apply to waves that possessed their own inherent rotation (known as angular momentum) in the same direction as the cylinder and had a low enough frequency with respect to the cylinder's rotation rate.
Zel'dovich's proposal in turn inspired Stephen Hawking's famous idea that black holes should slowly radiate their energy away by amplifying photons from the quantum vacuum.
Tricky experiment
Despite the simplicity of the Zel'dovich effect and its key relation to fundamental physics, this effect had not been directly tested until recently.
Zel'dovich's condition for amplification was general, but his description of a hypothetical system that could show such an effect was quite specific. It involved waves travelling in free space (at the speed of light) with a type of angular momentum known as OAM, short for orbital angular momentum, (meaning the light beams were twisted) and hitting a rapidly rotating cylinder.
But this suggested that the amplification effect would be tiny, because unless the cylinder could rotate at a speed comparable to that of light - a construction that would be mechanically impossible today - the OAM waves that could meet the condition would be spread over an area so large that the cylinder would be in (what Zel'dovich termed) a "non-wave zone" - it would barely interact with the waves at all.
Due to this, it was wrongly thought to be basically unobservable in experiments.
Hard proof
That is until we realised that the effect should also occur in sound waves, which travel much slower than the speed of light. Using sound waves with orbital angular momentum, in 2020 we showed Zel'dovich amplification for the first time in an experiment.
After showing the effect existed in one system, we thought an electromagnetic version might not be so hard after all. We were able to remove the previous limitations by trapping the electromagnetic wave in a resonant circuit, rather than in free space. The oscillating waves in our single circuit didn't have orbital angular momentum, but contained another type of angular momentum, termed "spin".
With this circuit, we could funnel the oscillating magnetic part of the wave through a small area where we placed a rotating aluminium cylinder. We then measured how the power in the circuit changed with the cylinder rotation speed. If the cylinder was absorbing the field, it acted as a normal positive resistance in the circuit, draining the power. If it was amplifying the field, it acted as a negative resistance - as a power source.
We found that the amplification of the field by the cylinder was exactly as predicted by Zel'dovich's condition - meaning we had proven the effect in electromagnetic waves for the first time.
In trying this experiment we also found something unexpected. The way this cylinder creates a negative resistance and amplifies the surrounding circuit when it spins fast enough is very similar to the way that wind turbines generate energy.
Inside a wind turbine is an induction generator, where an alternating current is sent in to create a rotating magnetic field around the rotor. And when the rotor blades spin faster than the surrounding rotating magnetic field, the current is amplified, and energy is generated.
While there is also other physics involved in modern induction generators, it is still astonishing that all the ingredients for proving the Zeldovich effect with electromagnetism were hiding in plain sight for so long.
This link we discovered to induction generators will enable us to optimise these electromagnetic experiments to test the Zel'dovich effect further, leaning on the many years of engineering that have gone in to making motor and generator technology better.
Perhaps the knowledge of this link to the Zel'dovich effect will also go the other way, providing engineers with a new physics perspective to harness for energy generation.
This experiment, showing the Zel'dovich effect is present in electromagnetism, also unlocks the potential to see the effect on a quantum level. Quantum theory tells us empty space is not empty - it has some fluctuations.
Any amplification effect should also be able to amplify such energy fluctuations into real photons - creating matter out of a quantum field. This would mean that a rotating cylinder, even in the absence of all other forces, would gradually slow down due to this process.
As for black holes, the implications are exciting. Perhaps in the future, harnessing the rotation of black holes could be used to power technology or spaceships.
Some have proposed conditions that would create a runaway energy generation effect termed a "black hole bomb". With improvements to our experiment, we hope to also test this runaway amplification for the Zel'dovich effect.
Marion Cromb receives funding from EPSRC grant EP/W007444/1.
Hendrik Ulbricht receives funding from EPSRC (grant EP/W007444/1).
