By Alex Kimani
A year ago, House representative Alexandria Ocasio-Cortez of New York and Senator Edward J. Markey of Massachusetts, both Democrats, proposed the Green New Deal, a nonbinding congressional resolution that lays out a grand plan for tackling climate change by meeting 100% of the power demand in the United States through clean, renewable, and zero-emission energy sources.
While the future of the clean energy proposal remains uncertain, the majority of Americans have been reading from the same page regarding what needs to be done: Dramatically cutting down the country’s reliance on fossil fuels over the next two decades is critical to lowering greenhouse gas (GHG) emissions and address climate change, with six in 10 U.S. adults saying they would favor policies with this energy goal. Thankfully, scientists have been researching alternative energy solutions like wind and solar power for decades, including lesser-known sources that may seem a little unusual or even downright ridiculous and unrealistic.
You can chalk up harvesting energy from blackholes to the latter category.
Fifty years ago, British mathematical physicist, Roger Penrose, proposed a seemingly absurd idea how an alien society (or future humans) could harvest energy from a rotating black hole by dropping an object just outside its sphere of influence also known as the ergosphere where it could gain negative energy. Since then, nobody has been able to verify the viability of this seemingly bizarre idea–that is until now.
Half a century later, researchers have finally proven that Mr. Penrose really was onto something big.
The Penrose Process
Blackholes have captured the imagination of astrophysicists ever since the first one was discovered in 1971–and for good reason. After all, they are some of the strangest and most fascinating objects in outer space.
Blackholes represent the end-stage of the life cycle of stars so massive that, once it’s gone supernova, the core can no longer withstand its own gravity and collapses totally into a singularity–a single one-dimensional point of infinite density. This singularity sits inside a region called the event horizon–the point at which the gravity around the black hole is so strong, not even light-speed is sufficient to achieve escape velocity.
Back in 1969, Roger Penrose proposed that if an object splits into two and one part falls into the black hole, the other part can be retrieved with more energy than the original object, because it has lost negative energy–basically saying a minus of a minus makes a plus!
Two years later, Russian physicist Yakov Zel’dovich adapted this idea and translated it to other rotating systems that we could test back on Earth. Zel’dovich predicted that waves with angular momentum–also known as “twisted waves”–reflected from a rotating metal cylinder will be amplified. The cylinder sees these twisted waves to be rotationally Doppler shifted , that is their frequency changes depending on the rotation speed. This is similar to the linear Doppler shift where the pitch of a police siren changes as it speeds past.
If Zel’dovich’s cylinder rotates fast enough then something very odd happens. The Doppler shifted frequency becomes negative. These negative frequency waves just appear to us as normal positive frequency waves. But remarkably this changes the way the waves interact with the cylinder. The metal usually absorbs the waves. But when the Doppler frequency goes negative, the absorption becomes amplification and the reflected wave has more energy than when it went in. Like with Penrose’s aliens and the rotating black hole, the twisted waves have taken energy from the rotating absorbing cylinder.
There was just one problem with Zel’dovich’s original proposal: The speed of the rotating cylinder needs to be at least 1 billion rotations per second–well beyond anything we can achieve mechanically without centrifugal pieces ripping the whole thing apart.
So there was no was no way to physically validate the theory–until a team of physicists from the University of Glasgow’s School of Physics and Astronomy in Scotland came along. They devised an experiment based on Zel’dovich’s work but used sound waves instead of lightwaves. Sound waves are a million times slower than light waves, meaning the rotating disc does not need to spin nearly as fast.
Their experiment consisted of a ring of speakers set up to introduce a twist in the sound waves, analogous to the twisted light in Zel’dovich’s experiment. A rotating sound absorber made out of a foam disc was analogous to the rotating blackhole, the rotation of which would speed up as the sound waves hit it. They used a ring of speakers to create a twisted sound wave on the other side of the disc and directed it towards a rotating sound absorber and placed microphones behind the foam that also rotate, to measure the Doppler shifted waves that pass through the disc as it spins. They could play back the sound recorded by the microphones as the rotation speed increases.
The results were nothing short of amazing. As the disc’s rotation accelerated, the pitch of the sound hitting the microphones lowered until it was inaudible, then began to rise again back to the original pitch – but a good 30% louder than the sound coming from the speakers. In essence, the sound waves were picking up additional energy from the rotating disc.
What you actually hear is the frequency of the twisted wave being Doppler shifted to zero, where it becomes too low to hear, then it gets louder again with increasing negative frequency.
If you plot the amplitude of the sound signal of the twisted wave, you notice that the sound is distinctly louder after the Doppler frequency has gone from positive to negative. In other words, the amplitude of the negative frequency waves is much larger when the absorber is rotating,
The area shaded in red is the region where the waves are taking energy from the absorber’s rotational energy. This amazing amplification effect lies at the heart of Penrose’s proposal to extract energy from black holes and could guide new experiments towards amplification of electromagnetic waves, and maybe even quantum fluctuations.
Obviously, this experiment does not not explicitly verify that Penrose’s idea for energy extraction will actually work for a black hole. Rather, it proves something that seems counter-intuitive: That shifting wave frequencies from positive to negative actually results in the waves gaining, rather than losing, energy.
We are still a long way off extracting energy from rotating black holes. Still, the latest experiment by the University of Glasgow’s School of Physics and Astronomy provides a nice proof of concept that could open up new and exciting avenues of scientific exploration.