How can we see black holes if they’re invisible?

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The Shadow of a Black Hole

From NOVA:

The invisible manifests itself through the visible: so say many of the great works of philosophy, poetry, and religion. It’s also true in physics: we can’t see atoms or electrons directly and dark matter seems to be entirely transparent, yet this invisible stuff makes and shapes the universe as we know it.

Then there are black holes: though they are the most extreme gravitational powerhouses in the cosmos, they are invisible to our telescopes. Black holes are the unseen hand steering the evolution of galaxies, sometimes encouraging new star formation, sometimes throttling it. The material they send jetting away changes the chemistry of entire galaxies. When they take the form of quasars and blazars, black holes are some of the brightest single objects in the universe, visible billions of light-years away. The biggest supermassive black holes are billions of times as massive as the Sun. They are engines of creation and destruction that put the known laws of physics to their most extreme test. Yet, we can’t actually see them. [read the rest at NOVA…]

This piece, which emphasizes the great science coming from the Event Horizon Telescope (EHT), is a  companion to my earlier NOVA essay, “Do we need to rewrite general relativity?”

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The long jet of gas in the galaxy M87, which is driven by the supermassive black hole at the galaxy’s center. New observations have revealed the structure of the gas disk near the black hole.

A collection of four big telescopes in Arizona, California, and Hawaii have banded together to examine one of the biggest black holes we know: the beast at the heart of the galaxy M87. What they found: the disk of gas driving M87’s huge jet rotates the same direction as the black hole that made it.

New observations from the Event Horizon Telescope (actually an array of four millimeter-wave telescopes working in concert) have revealed the best view so far of the supermassive black hole in the galaxy M87. As described in a Science paper, astronomers measured the motion of gas to a distance approximately 5.5 times the event horizon radius. That is close enough to confirm the gas circles in the same direction the black hole itself rotates. These observations help clarify the origin of the powerful jet of gas streaming from the galaxy’s center at a high fraction of the speed of light: it is likely driven by the swirling matter near the black hole’s boundary. [Read more….]

High-resolution image of supermassive black hole shows engine of destruction