Measuring Black Hole Spin: The Challenge and Future Solutions

• Astronomers are currently unable to measure the precise spin rates of black holes using existing technology.
• Two competing theories suggest maximum spin limits of 99.8% and 93.75% of the speed of light.
• The proposed Black Hole Explorer (BHEX) mission could provide the necessary resolution to resolve this long-standing debate.

Black Hole Spin Theories

• Kip Thorne's 1970s theory: Limits maximum spin to 99.8% of light speed, restricted by photons from the accretion disc.
• Charles Gammie's 2004 theory: Limits maximum spin to 93.75% of light speed, restricted by the braking effect of highly magnetized jets.

Limitations of Current Technology

• The Event Horizon Telescope (EHT) remains limited to a resolution of 20 microarcseconds (µas).
• 3D General Relativistic Magnetohydrodynamics (GRMHD) simulations show that, at EHT resolution, black holes spinning at these two different limits appear identical.
• Key indicators like accretion rates, jet structure, light curves, and polarizations overlap significantly at current resolution levels.

Future Prospects: The BHEX Mission

• Scientists are targeting the "photon ring," a thin, bright circle of light trapped by gravity and orbiting the black hole, to distinguish between spin models.
• Detecting this ring requires 5µas sensitivity, beyond the reach of Earth-based sensors.
• NASA's Black Hole Explorer (BHEX) is currently being developed to orbit a radio telescope that works in tandem with Earth-based arrays like GBT and ALMA, creating a massive interferometer capable of resolving the photon ring.