Place two identical alarm clocks on a table and set them off at the same time. If one is on a slippery surface and starts to slowly slide, its timing will be thrown off, and within a minute they'll ring out of sync. That's roughly how the new method proposed by astrophysicists for supermassive black holes works. Except instead of alarm clocks, we have X-ray flashes, and instead of a slippery table, we have space itself, twisted by the hole's rotation.
The authors calculated how light rays emitted from a single point near the hole reach a distant observer. Some rays loop around the hole in the direction of its spin, like running on a moving walkway, your speed adds up. Other rays travel against the spin, and space slows them down. As a result, light arriving from the left and right of the spin axis is delayed differently. This difference is called mirror asymmetry in arrival time, or [formula]$A_t$[/formula].
The faster the hole spins and the closer the flash is to it, the stronger the skew. For typical supermassive holes at the centers of galaxies, the time shift can reach several hours. This isn't just a curious fact: by measuring such asymmetry, astronomers can directly "feel" the hole's spin even without seeing it. The method promises to become a new tool in X-ray mapping of black hole vicinities — where space and time behave like stretched and twisted rubber.
If the supermassive black hole at the center of the Milky Way were spinning at nearly its maximum rate, the difference in arrival time of light from a single flare could be about an hour — enough time to bake a pie.
In the movie Interstellar, the characters experience time dilation near a black hole — the new method captures a similar effect, but not for travelers themselves, rather for the light they might see.