Spotting black holes

Black holes remain among the more mysterious objects in the universe. Though John Michell and Pierre-Simon LaPlace first posited their existence back in the 18th century, nobody has ever actually seen a black hole. Dr. Sean O’Neill, visiting assistant professor in the Department of Physics at Pacific Lutheran University in Tacoma, attempted to shed some light on these objects that don’t emit any during a talk at this month’s meeting of the Seattle Astronomical Society.

Dr. Steve O’Neill of PLU spoke about black
holes at the December meeting of the Seattle
Astronomical Society. Photo: Greg Scheiderer.

O’Neill’s talk was titled “If We Can’t See Black Holes, How Do We Know They Exist?” His answer to the question boiled down to the notion that scientists have not yet come up with any other plausible explanation for some of the phenomena that they have seen.

The professor noted that traditional methods of observing astronomical objects simply are not practical for viewing black holes.

It would not work to send a spacecraft for a look. O’Neill pointed out that the nearest likely black hole is some 1,300 light years away from Earth. It would take a craft like Voyager about 25 million years to get there, and then, even if it arrived with its power source and transmitter intact, you would still have to wait 1,300 years to receive any messages about its findings.

“Traveling there is a terrible option,” O’Neill understated. “The direct visit option is bad even for things in the outer solar system, let alone things outside of our solar system.”

Imaging is also well nigh impossible, O’Neill said, and not just because a black hole, by definition, does not emit any light. Black holes, though incredibly massive, are also dense and quite small. Today’s telescopes don’t offer adequate resolution for a visual or photographic look; it would take a scope about ten thousand times the size of Hubble to spot the supermassive black hole at the center of the Milky Way.

Other methods offer some hope. O’Neill says we might well be able to spot the gravitational effects of a black hole, especially one circling another or dancing gravitationally with another massive object. In such cases general relativity predicts gravitational waves in space-time, and these might be observed directly. The approach is to use laser interferometry to detect changes in light wavelength. O’Neill says it’s a complicated process from which it is difficult to separate observational noise.

“In practice, there have been no detections of this phenomenon happening yet, even though most people think it probably does happen,” O’Neill said.

O’Neill says gravitational lensing also holds some promise, especially as observing equipment gets better.

“It’s tough to pick out the individual little black holes, though,” he said, noting that the method is used to look at distant, large, massive objects that lens other distant objects.

Though we haven’t yet seen a black hole, there’s plenty of evidence that infers that they exist. O’Neill shared data from observations of stars orbiting the center of our galaxy, seen in the infrared to cut through the dust blocking our direct visual view. Using Newton’s laws on the data from a number of years to reconstruct the orbits of the stars suggests they’re going around something that is about 3.7 million times more massive than our Sun. Whatever it is, we can’t see it because it doesn’t emit any light of its own.

“It’s tough to come up with a good alternative of what this could be,” O’Neill said. “It’s tough to imagine that gravity just goes wrong at this one point, for some reason, at the center of our galaxy.”

“That’s where we get a lot of direct evidence for what we think is the black hole at the center of our own Milky Way,” he concluded.

Looking at other objects leads to similar conclusions. Cygnus X-1 is a huge source of x-rays that is pulling material from a donor star nearby. The material holds a great deal of potential energy because of the high gravity of the system.

“All of that energy has to be converted into some form,” O’Neill explained. “Some of it is certainly kinetic, because stuff will speed up, but some of it is also going to be thermal energy. It will hit other little particles of gas, all of this will heat up to the point that it starts emitting x-rays, and that’s the stuff that we think we can see.”

One of O’Neill’s research interests is computer modeling of the jets of material often spotted shooting out of the centers of galaxies, such as Centaurus A. He shared a number of these simulations, in which material plummets toward a presumed black hole, doesn’t quite fall in, and then shoots away at great velocity. The models can be rotated to simulate views from various angles and compare the results to actual observations. While it’s an active area of research, O’Neill says most scientists are on the same page with their thinking.

“The reigning theoretical model for these jets by far—there’s essentially no viable alternative—is that fundamentally they’re powered by black hole gravity at the source,” he said.

While O’Neill notes that computer simulations like the ones he creates are way cheaper than observing, he expects that actual observations of gravitational waves from merging black holes are not far off. He also thinks that high-resolution x-ray and radio observations will allow us to see the disks of material around black holes within his lifetime.