How to See Black Holes + Kugelblitz Challenge Answer

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Matt O'Dowd (0:48)

We've been talking a bit about black holes lately and we'll continue to do so. We tend to be pretty theoretical in how we think of them, partly because the theory predicts some fun stuff that no human will likely ever experience. In fact, the second part of this episode will be the answer to our Escape the Kugelblitz challenge, which is highly theoretical and in fact a little implausible. So to ground us a little bit first, I want to talk about actual real black holes. In particular, how we see these things. There's been no reasonable doubt about the reality of black holes for some time. Although they don't emit any light themselves, they can have a very visible effect on their surroundings. The most spectacular effect is when black holes feed. Matter falling into the extreme gravitational well of a black hole will reach incredible speeds and temperatures, causing the region around black holes to shine. This gives us things like quasars, supermassive black holes in galaxy cores that feed on a superheated whirlpool of gas. That accretion disk shines so brightly that we see them to the ends of the universe. Then there are X-ray binaries. Sometimes the motion of a visible star reveals it to be in orbit around a companion that is dark in visible light, but bright in fluctuating X-ray emission. This happens when the substance of a visible star is accreting onto a companion neutron star or black hole. The most famous and the closest is the Cygnus X1 black hole, 6,000 light years away.

At around 15 times the mass of the sun, the dark object in this system can't be anything but a black hole.

The other famous black hole in the Milky Way is, of course, its own supermassive black hole. By the way, almost all galaxies have these lurking at their cores. From our perspective, that places it in the constellation of Sagittarius. We call our supermassive black hole Sagittarius A star. Sag A star is visible in X-rays, which occasionally flash brighter as it gobbles up a wisp of gas. But more compellingly, we've tracked the motion of stars near the galactic core for many years. They show crazy slingshot orbits around an empty patch of space. These orbits tell us that a dark something of around 4 million solar masses lurks in the center.

The recent observations of gravitational waves from a pair of merging black holes by LIGO could be considered our first direct detection of black holes. However, some upcoming studies are expected to lead to even better understanding. The Event Horizon Telescope is right now in the process of mapping space around the Milky Way's SAGE star black hole. The EHT isn't a single telescope. It's a collaboration of currently nine and eventually 12 or more radio telescopes distributed across the planet. They use very long baseline interferometry, VLBI, to synthesize observations at millimeter and submillimeter wavelengths. The effect is a telescope thousands of kilometers in diameter, at least in terms of its spatial resolution.

EHT could currently detect an orange on the surface of the moon, if oranges were bright in microwaves. This has enabled EHT to map the strange magnetic field structures around the SADJ star black hole. Now, the actual event horizon is even smaller than this insane resolution, but EHT isn't finished yet. It's expected that over the next year or so, as EHT brings more and more telescopes online, it will actually see the dark, circular shadow of the SADJ star event horizon. Interferometry is going to be used to study much smaller black holes in our galaxy, the remnants of dead stars. These black holes occasionally pass in front of more distant background stars, gravitationally lensing the star's light. At visible wavelengths, this should look like a brightening of the star, an effect called microlensing.

But interferometry will enable incredibly high res mapping and the distant star should appear to split into two or four images as its light passes around the gravitational field of the black hole.