Albert Einstein didn't like them.
To him, black holes were a bit of an embarrassment, as they compromised his dream of a "rational" nature, that is, natural phenomena that we could describe and quantify with the usual methods of science. According to this view, good scientific theories shouldn't generate absurd (read: "irrational") results.
The problem with black holes is that they do exactly that: They represent the extreme of having gravity pull so hard that the whole of space sort of curls up on itself, closing like a clamshell. If you are trapped in, you can't get out. Worse, at the very center of one, a point called singularity, gravity becomes infinitely powerful. Infinities aren't a good thing in physics, as they usually represent the breakdown of a theory.
Einstein must have thought that his general theory of relativity, one of the greatest intellectual achievements in the history of civilization, was too beautiful to predict such crazy stuff.
But black holes are here to stay, Einstein liking them or not. And they are truly remarkable objects, allowing us to study nature at its most extreme. As 13.7 blogger Adam Frank wrote here last week, we may be close to being able to actually see them, or at least their contorted light signature. Why contorted light? It turns out that as light from a distant source travels past a black hole, we see it stretched and bent as if passing through a lens. This effect, appropriately, is called gravitational lensing, and it's due to the bent space around a black hole: Light always takes the shortest path from Point A to Point B, and if space between these two points is curved, light follows it like a child going down a slide. What we see is the blurred image of the trip, as in this image from the Hubble Space Telescope of the Abell 2218 Galaxy Cluster.
We have known for some time now that pretty much all galaxies harbor a large black hole in their center. Our own, the Milky Way, has a 4 million solar-mass behemoth in its core. The giant sucks gas and anything that gets sufficiently close to it, perturbing the orbits of stars around it. (That's how we found out about its existence.) Three weeks ago, a group of astronomers from Japan working with the powerful Alma telescope in the Atacama desert in Chile announced the discovery of another black hole near the center of our galaxy, with "only" 100,000 solar masses. The giant is at about 200 light-years from the center. Our location at 25,000 light-years away. (Fortunately, otherwise we wouldn't be here to tell the story.) This is the first detection of a "midsize" back hole.
Current estimates suggest that there are about 100 million black holes in our galaxy alone. And possibly many pairs, or even more of them, like to nest together at the center. Just this week, a pair of monster black holes (called supermassive) separated by only about one light-year with a joint mass of 40 million suns were discovered at the beautiful and far-away spiral galaxy NGC 7674. This is the second pair of behemoths found at the center of a distant galaxy; another was announced in 2006, with the astonishing combined mass of about 15 billion suns and at 24 light-years apart.
Just as two kids jumping around each other on a trampoline send off waves rippling outwards on the fabric, as black holes orbit around each other (around their center of mass to be precise), they also distort space, sending off waves that propagate outwards at the speed of light. These waves register their choreography, which the gravitational wave detector LIGO has captured for pairs of smaller (stellar mass) black holes in our galaxy. Spiraling giant black holes in distant galaxies make space wave and quiver even more dramatically. And if for now we can only catch some of this dance indirectly, the situation will change when the next big gravitational wave observatory is operational, the Laser Interferometer Space Antenna, is launched into space in the 2030s. A long wait, I know, but the show will be worth it.
I wonder how Einstein would feel about all this?
Clearly, his intuition was wrong, and black holes are very much part of nature, producing spectacular effects and deepening our understanding of gravity and of the formation and structure of galaxies. I imagine he would take it humbly, accepting that in the game of science, nature always has the upper hand.
After all, as he once wrote: "What I see in Nature is a magnificent structure that we can comprehend only very imperfectly and that must fill a thinking person with a feeling of humility."
Marcelo Gleiser is a theoretical physicist and writer — and a professor of natural philosophy, physics and astronomy at Dartmouth College. He is the director of the Institute for Cross-Disciplinary Engagement at Dartmouth, co-founder of 13.7 and an active promoter of science to the general public. His latest book is The Simple Beauty of the Unexpected: A Natural Philosopher's Quest for Trout and the Meaning of Everything. You can keep up with Marcelo on Facebook and Twitter: @mgleiser