Could rotating black holes be the wind turbines powering the distant future? Premium

Black holes are mysterious objects. 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.

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.

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.

Despite the simplicity of the Zel’dovich effect and its key relation to fundamental physics, this effect had not been directly tested until recently.