Measuring the Spin of Black Holes: A Space-Based Approach
Black holes, often depicted as cosmic monsters that devour everything, including light, are not as simple as their popular image suggests. They do spin, and at incredibly high speeds. Determining the exact velocity of these spins is crucial for understanding their impact on their surroundings, from the immediate vicinity to the entire galaxy. A recent study by Tegan Thomas and her team from the University of Virginia offers both good and bad news in this regard.
The bad news is that, despite recent advancements, we still cannot precisely measure the spin of black holes. The good news is that a potential solution is on the horizon. The study highlights two competing theories regarding the maximum spin velocity of black holes. The first, proposed by Kip Thorne in the 1970s, suggests that black holes can spin at up to 99.8% of the speed of light, limited by the resistance of photons emitted from their accretion disc. The second theory, introduced by Charles Gammie in 2004, posits that highly magnetized jets act as brakes, capping the spin at 93.75% of the speed of light.
For decades, scientists have debated which of these theories is correct. However, the Event Horizon Telescope (EHT), which captured the first direct image of a black hole a decade ago, has limitations. Its resolution of 20 microarcseconds is insufficient to distinguish between the two spin theories. To address this, the authors employed advanced 3D General Relativistic Magnetohydrodynamics (GRMHD) simulations, creating synthetic radio images that could be analyzed by the EHT.
Surprisingly, the EHT cannot differentiate between the two spin models. The overall accretion rate and relativistic jets produced by the black hole are nearly identical in both scenarios. Crucially, at the EHT's resolution, light curves, linear polarization, and circular polarizations of the signals are almost indistinguishable.
The solution may lie in the black hole's photon ring, a thin but extremely bright circle of light within the plasma ring. This ring is formed by light rays that have been trapped by the black hole's gravity, making multiple rotations before escaping towards Earth. However, current Earth-based sensors lack the sensitivity to detect this ring, which is only 5 microarcseconds wide.
The Black Hole Explorer (BHEX) mission, currently in the planning stages as a NASA Small Explorer mission, aims to address this issue. By placing a radio telescope in Earth's orbit and working in conjunction with the EHT's components, BHEX will create an interferometer capable of directly observing the photon ring of Sgr A*. This will enable scientists to determine the precise shape of the ring and potentially resolve the decades-long debate about the maximum spin velocity of black holes.
In conclusion, while we cannot yet measure the spin of black holes precisely, the development of space-based tools like BHEX offers a promising future for unraveling the mysteries of these cosmic phenomena.
