The Event Horizon Telescope (EHT) recently unveiled the first images of the environment around a black hole. Since then, strides have been made in increasing the resolution and adding details to the images of this region dominated by the most intense gravity.
What type of gravity are we referring to? Due to the conflicts between general relativity and quantum mechanics, as well as the lack of explanation for dark matter, various enhanced theories of gravity have been proposed to address physics' current gaps. The extreme conditions near a black hole might highlight differences between these theories. Consequently, a group of physicists has embarked on investigating whether any notable differences could be discerned by future telescopic advancements, potentially ruling out some alternative theories to relativity.
Searching for Subtlety
Any substitute for general relativity faces a significant challenge. General relativity successfully explains a wide range of phenomena, from cosmic structures to earthly measurements. Any alternative must, therefore, display variations that are incredibly subtle and difficult to detect.
However, a black hole’s environment may be intense enough to accentuate these minor differences. The gravitational pull in such regions is so extreme that a rotating black hole can influence surrounding spacetime – an occurrence known as frame dragging – forcing light to bend on its journey to Earth. This effect is highly sensitivity to gravity. According to the researchers, general relativity predicts that images from such a region will show a series of nested ring-like patterns, each defined by the number of half-orbits the photons complete before reaching observers. Accurately measuring these photon rings and their positions presents a compelling method to probe gravity in areas of strong, yet stationary, curvature.