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Negative-energy particles may extract energy from black holes

4 February 2019

Simulations of the particles in plasma jets provide a fuller picture of how a rotating black hole interacts with matter in its vicinity.

An artist's depiction of a plasma jet
Credit: NASA/JPL-Caltech

How do rotating black holes lose energy? One possible mechanism is the Blandford–Znajek process, whereby a rotating black hole with a magnetic field along its axis of rotation will lose rotational energy to that field and create relativistic jets of plasma. Simulations typically treat those emanating plasma jets as a continuous fluid and capture the energy loss due to the Blandford–Znajek process reasonably well. However, a fluid model overlooks both the role of individual particles in the plasma and the creation of electron–positron pairs. Now Kyle Parfrey of NASA’s Goddard Space Flight Center and his colleagues have created a simulation that treats the plasma around a black hole as a collection of charged particles. Their calculation of a rotating black hole in a uniform magnetic field provides evidence that another process—Penrose—may play a significant role in the extraction of energy from a black hole.

Average particle energy at infinity of electrons (left) and positrons (right) for high- (top) and low-density (bottom) plasmas in the vicinity of a black hole. Blue indicates a negative particle energy.
Average particle energy at infinity of electrons (left) and positrons (right) for high- (top) and low-density (bottom) plasmas in the vicinity of a black hole. Blue indicates a negative particle energy.

To simplify the complicated physics of electron–positron pair creation in their simulation, Parfrey and his colleagues used a rule: For each point in space and time, when the component of the electric field parallel to the magnetic field was above a certain threshold, an electron–positron pair would be injected. The selected threshold would set the density of the resulting plasma. The researchers tested a higher and lower threshold to achieve a lower- and higher-density plasma, respectively. They observed plasma dynamics similar to what they’d seen in previous simulations, with one important addition: particles with negative average energy, as measured by a distant observer, with some of those particles passing through the horizon into the black hole. Those negative-energy particles, indicated by blue in the figure, lowered the energy of the black hole in the Penrose process. In the higher-density plasma, the net energy into the black hole was positive. However, in the lower-density plasma, the energy lost through the Penrose process was comparable to the energy lost through the Blandford–Znajek process.

The result demonstrates the importance of considering both the Blandford–Znajek and Penrose processes in theoretical studies and the necessity of reevaluating previous analysis of experimental observations. However, improvements to the simulation are still needed before any direct comparison can be made with experimental data, such as forthcoming images from the Event Horizon Telescope of the black hole at the center of our galaxy (see the Quick Study by Dimitrios Psaltis and Feryal Özel, Physics Today, April 2018, page 70). Parfrey and his colleagues say future simulations will require a more realistic treatment of electron–positron creation to correctly model the photon emission. (K. Parfrey et al., Phys. Rev. Lett. 122, 035101, 2019.)

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