The Black Hole That Slows Down Time

Most people know black holes slow down time near their event horizons. What surprises people is that nothing speeds clocks up there—every effect near a black hole makes time run slower for a distant observer. Take the supermassive black hole at the center of our galaxy, Sagittarius A*. The star S2 whips around it at nearly 3% of the speed of light, and in 2018 astronomers measured its light being stretched to longer wavelengths: gravitational redshift, the signature of time running slower in a strong gravitational field. Its enormous speed slows its clock too, through relativistic time dilation. Gravity and motion team up to slow time itself—it doesn’t get much weirder than that.