Physicists have built the first ever black hole bomb, a long-theorised phenomenon where energy is boosted by a black hole and trapped by surrounding mirrors until an explosion occurs. Thankfully, this version is just a safe toy model rather than using a real black hole in space, but as the physical principles are identical, studying it could help researchers better understand how real black holes spin.
The idea of extracting energy from a black hole was first proposed in 1969 by physicist Roger Penrose. He noted that a particle flying extremely close to a spinning black hole will gain energy due to a curious effect of general relativity, which sees the black hole drag and accelerate space-time around it.
Two years later another physicist, Yakov Zeldovich, realised that a similar process could occur in other scenarios, like light moving around a rapidly-spinning metal cylinder. He calculated that this 鈥渟uperradiance鈥� effect should occur as long as the cylinder spins at the same frequency as the light 鈥� but this is incredibly fast. 鈥淚t鈥檚 impossible to rotate anything [made] of matter at these kinds of speeds,鈥� says at the University of Southampton, UK.
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Zeldovich also suggested that, by surrounding the rotating cylinder with a cylindrical mirror, the amplified energy could be reflected and built up in a positive feedback loop, until the energy is either vented out or it explodes. Applying this idea to black holes, one could be used to produce a 鈥渂lack hole bomb鈥�, releasing as much energy as a supernova. This would also work even without an external energy source, with the black hole amplifying tiny electromagnetic fluctuations in the vacuum of space itself, effectively producing energy from noise.
All of this remained theoretical, but now Ulbricht and his colleagues have found a way to demonstrate Zeldovich鈥檚 feedback loop using a rotating aluminium cylinder and magnetic fields. Ulbricht built the first prototype during the UK鈥檚 first covid-19 lockdown in 2020. 鈥淓verything was closed, and I was really bored and I wanted to do something, so I built the setup and started to do these experiments, and I saw amplification. I was so super excited that, actually, you could say it rescued me during covid.鈥�
He soon recruited colleagues to build a more robust experimental setup, which consists of a rotating aluminium cylinder powered by an electric motor, surrounded by three layers of metal coils producing a magnetic field that also rotates around the cylinder at a similar speed. In this setup, the coils act as the mirror and the magnetic field as light and, as Zeldovich predicted, this produced an even larger magnetic field emanating from the cylinder.
鈥淵ou throw a low-frequency electromagnetic wave against a spinning cylinder, who would think that you get back more than what you threw in? It鈥檚 totally mind boggling,鈥� says at the University of Lisbon in Portugal.
Ulbricht and his team then showed that even without the coils producing an external magnetic field to begin with, the setup would still generate a runaway signal in the surrounding coils, just like the theoretical example of a black hole without an external energy source. 鈥淲e鈥檙e basically generating a signal from noise, and that is the same thing that happens in the black hole bomb proposal,鈥� says Ulbricht.
鈥淗aving accurate measurements in the laboratory of this process really allows you to confidently say, 鈥榊es, this must happen in black hole physics as well鈥�,鈥� says Cardoso.
While the lab version is only an analogue, it could help physicists understand how real black holes give energy to particles around them. This could help test theoretical ideas about as-yet unseen particle fields, such as one giving rise to dark matter.
鈥淚f new fields exist, we should be seeing, for instance, gravitational waves being emitted from this cloud around black holes, or we should see black holes spinning down because they鈥檙e giving their energy away to these new particles,鈥� says Cardoso. 鈥淪o superradiance is turning black holes into particle detectors, and much better particle detectors than [the Large Hadron Collider at] CERN can be for this type of dark matter.鈥�
arXiv
Mysteries of the universe: Cheshire, England
Spend a weekend with some of the brightest minds in science, as you explore the mysteries of the universe in an exciting programme that includes an excursion to see the iconic Lovell Telescope.
