For the first time, physicists have created a “black hole bomb” in the lab, providing evidence for the “Zel’dovich effect” proposed half a century ago.
The idea behind the Zel’dovich effect came from an unusual place. In 1969, British physicist and mathematician Roger Penrose suggested that energy could be extracted from black holes by lowering an object into the ergosphere (the region just outside of the event horizon) and allowing it to accelerate the object, stealing some of the black hole’s energy. The idea, known as the Penrose process, requires negative energy to be acquired by the object in order for it to be recovered from the black hole – otherwise, all you’d be doing is feeding the black hole.
“Let’s imagine that we launch a particle from very far away into the ergosphere of a Kerr black hole, following a retrograde orbit, that is, a trajectory directed against the black hole’s rotational direction,” Jorge Pinochet, professor in the physics department of the Universidad Metropolitana de Ciencias de la Educación, explains of the process in a recent preprint paper. “Suppose we calculate the trajectory so that upon entering the ergosphere, the particle fragments into two pieces, one of which is absorbed by the black hole, and the other escapes outward, moving an arbitrarily large distance away.”
“Due to the extreme intensity of gravity inside a black hole, general relativity allows the absorbed fragment to have negative energy.”
Under these circumstances, Penrose showed that the escaping fragment would have more energy than the fragment absorbed by the black hole. That may sound like conservation of energy laws are being broken, but according to what we know of general relativity, that is not the case.
“The trick to obtain this result is that the black hole absorbs negative energy, which leads to a reduction in its mass-energy, which translates into a decrease in its rotational speed,” Pinochet continues. “In other words, we have extracted rotational energy from the black hole.”
We don’t have any conveniently close black holes to play with (probably thankfully), but a few years later, Belarusian physicist Yakov Zel’dovich came up with a far more practical way to test the concept of stealing extra energy from a rotating system. The idea has links to the Doppler effect, which can make light appear red or blue shifted depending on how the emitting object is moving relative to us, as well as the rotational Doppler effect.
“The linear version of the doppler effect is familiar to most people as the phenomenon that occurs as the pitch of an ambulance siren appears to rise as it approaches the listener but drops as it heads away. It appears to rise because the sound waves are reaching the listener more frequently as the ambulance nears, then less frequently as it passes,” study lead author Dr Marion Cromb, now Research Fellow at the University of Southampton, explained in a statement concerning a previous study.
“The rotational doppler effect is similar, but the effect is confined to a circular space. The twisted sound waves change their pitch when measured from the point of view of the rotating surface. If the surface rotates fast enough then the sound frequency can do something very strange – it can go from a positive frequency to a negative one, and in doing so steal some energy from the rotation of the surface.”
To test the idea, the team previously bounced sound waves off a spinning disc, and listened for a shift in frequency that indicated energy had been gained from the disc’s rotation. Then, the team conducted the experiment using electromagnetic waves.
“The Zel’dovich effect works on the principle that waves with angular momentum, that would usually be absorbed by an object, actually become amplified by that object instead, if it is rotating at a fast enough angular velocity. In this case, the object is an aluminium cylinder and it must rotate faster than the frequency of the incoming radiation,” Cromb said in a statement about that previous study.
“Colleagues and I successfully tested this theory in sound waves a few years ago, but until this most recent experiment it hadn’t been proven with electromagnetic waves. Using relatively simple equipment – a resonant circuit interacting with a spinning metal cylinder – and by creating the specific conditions required, we have now been able to do this.”
To conduct the experiment, the team needed to rotate the aluminum cylinder so fast that, from its perspective, it sees a twisted wave shifted in angular rotation with a “negative frequency”.
“The condition for amplification is from the rotating perspective of the object,” Cromb explained. “Twisting electromagnetic fields hitting it have become rotationally Doppler shifted, so much (or so low) that they’ve gone through zero and into a ‘negative’ angular frequency. Negative frequency then means negative absorption, and this means amplification.”
Following that study, the team got even more ambitious, attempting to make a so-called “black hole bomb” analogue. In a “black hole bomb”, energy is reflected back at the black hole and amplified, and reflected back again, leading to runaway signal amplification from where there was only noise.
The team managed to create such an analogue black hole bomb, using a reflective aluminum cylinder rotating slower than a surrounding electromagnetic field.
“Here, we demonstrate experimentally that a mechanically rotating metallic cylinder not only definitively acts as an amplifier of a rotating electromagnetic field mode but also, when paired with a low-loss resonator, becomes unstable and acts as a generator, seeded only by noise. The system exhibits an exponential runaway amplification of spontaneously generated electromagnetic modes thus demonstrating the electromagnetic analogue of Press and Teukolsky’s ‘black hole bomb’,” the team writes in their paper, which has not yet been peer reviewed.
While this isn’t exactly stealing energy from a black hole levels of cool, the analogue suggests that black holes could amplify energy in this way.
“A challenge for the future remains the observation of spontaneous [electromagnetic] wave generation and runaway amplification seeded from the vacuum,” the team writes in their discussion. “However, based on the results presented here, this now remains a purely technological (even if very hard) feat.”
The study is posted to the pre-print server arXiv and is yet to be peer-reviewed.
Source Link: "Black Hole Bomb": Energy-Stealing Zel’dovich Effect Confirmed In The Lab
