Which observation tests the predictions of general relativity near a supermassive black hole?

• A. The shadow size and photon ring observed by EHT match GR predictions.
• B. Detecting radio jets only.
• C. Measuring X-ray variability unrelated to GR.
• D. The presence of a luminous accretion disk alone.

Answer: (B)

Light bending in the strong gravity near a supermassive black hole provides a direct test of general relativity. In GR, photons that get very close to the black hole are bent dramatically, with some photons captured to form a dark shadow while others skim past the horizon and create a bright photon ring due to extreme lensing. The exact size and shape of this shadow, plus the detailed appearance of the ring, depend on the black hole’s mass and distance in a way GR precisely predicts.

Observed images of the shadow and photon ring around a supermassive black hole—such as those from the Event Horizon Telescope—show a ring-like structure with a diameter that matches GR’s predictions for the inferred mass and distance. That direct correspondence tests the spacetime geometry in the strong-field regime, something other observations don’t do as directly.

By contrast, detecting radio jets mainly informs us about jet launching and the physics of magnetic fields and plasma, not the spacetime curvature. X-ray variability unrelated to GR and the mere presence of a luminous accretion disk don’t probe how light propagates in the warped spacetime near the horizon.