Last week, NOVA presented an episode on black holes, these most mysterious and mind-boggling physical objects.
Hosted by astrophysicist Janna Levin, from Barnard College in New York, the episode was truly fantastic. Very clear science and stunning graphics. Levin was gracious and fun, the kind of benevolent teacher you want for your kids. The text was tight, with the narration closely following the quotes from the many invited guests. And the science, of course, was nothing short of amazing. As Levin said, black holes are places where reality beats fiction. Hands down.
Go into an elementary school classroom and mention black holes. Eyes will light up, a mix of awe and fear. To a child, the image of a hole in space that swallows everything that comes near is pretty awesome. "A place to send off your worse enemies," I joke when I'm lecturing them.
That gravity alone can do that, create a chunk of space that is so curved that it closes up upon itself, clearly makes it the weirdest of the four forces we know. We all know that gravity is what makes things fall down. It never rests. The other day, my 6-year-old asked me if in space gravity is zero. Before I could open my mouth to answer, my 11-year-old jumped in: "Of course not! There is always some gravity, unless you move infinitely far-away from an object, which is impossible."
Everything is connected, everything attracts everything else. Unless — unless you are free falling. When you fall, you don't feel your own weight and things work as if gravity didn't exist for you. This is a strange idea, but if you recall what happens when you go down on a fast elevator (you feel lighter the faster the elevator goes down; that is, gravity seems weaker to you), you know it makes sense.
Albert Einstein figured this out sometime between 1905 and 1910, when he was trying hard to expand his theory of relativity to objects that could accelerate. His original theory, the special, only considered objects moving at constant speeds. When Einstein saw the link between gravity and acceleration, he was stunned. But it makes sense. If you go up an elevator, you feel heavier the faster the elevator goes up. Gravity and acceleration are deeply linked. (My co-blogger Adam Frank explored gravity and elevators here some time ago.)
Einstein's General Theory of Relativity became a new theory of gravity, superseding Issac Newton's. Not that Newton's theory was wrong, far from it. We use it all the time, including when we send rockets to outer space. But Einstein's works better in strong gravity. In science, "works better" means gives better results, closer to observations. And it does so by making an amazing conceptual leap: that gravity can be interpreted as the curvature of space around an object; the more massive the object, the more curved the space around it. So, close to a star, space is more curved than close to you. Also, it's not just space; time is affected, too, ticking slower the stronger gravity is.
Quiz question: If you could put a clock on the surface of the sun, would it tick faster or slower than on the surface of the Earth?
If you answered slower, you are correct!
In black holes, this effect becomes extreme. Someone safely on the outside, watching a clock fall into the hole would see the time pass slower and slower and slower. How is that possible?
Stars are kind of like living creatures in that they also have a life cycle: They are born (in regions we call stellar nurseries, huge gas clouds in outer space), live dramatically, fusing hydrogen into helium for a long time until they "die," when they run out of hydrogen. Without the energy released from nuclear fusion to balance the imploding pressure from gravity, the star collapses onto itself, releasing a huge amount of energy in the process. Large stars, at least eight times more massive than the sun, become supernovas. Their core, what's left over after the explosion, is what could become a black hole.
There are two possibilities: Roughly, stars with masses between eight and 20 times that of the sun become what's called a neutron star, a star made of neutrons, the particles that share the atomic nucleus with protons. Why? Well, in the frenzy of collapse, as gravity becomes tighter and tighter, protons and electrons get so squeezed together that they essentially fuse into neutrons. (The fancy name of this process is inverse beta decay.) These neutrons can get squeezed a lot and still resist gravity's pressure: A typical neutron star can have the mass of the sun and the size of a mountain!
But if the stellar core is too heavy, neutrons can't stop gravity. The neutrons in the core will keep getting squeezed, and gravity will keep getting stronger. With no brakes, the process just keeps going on until gravity is so strong that not even light can escape from its neighborhood: A black hole is born.
Black holes come in different sizes, a point well-explained in the NOVA episode. Even tiny ones could, at least in principle, be created on Earth, in particle accelerators such as the Large Hadron Collider in Geneva, Switzerland, where the Higgs particle was discovered in 2012. But don't worry, these tiny guys are harmless, evaporating away in a fraction of a second; they won't grow to swallow Earth.
The big ones are the ones that wreak havoc in the cosmos. We now know they exist, something that Einstein was not pleased about. To him, an object that, at its very center, had a "singularity," a point in space where gravity becomes infinitely strong and the laws of Nature break down, was just unacceptable.
Sorry Einstein, but black holes are here to stay. We now know that at the heart of pretty much every galaxy there is a giant black hole. In our own, the Milky Way, there is a four-million solar mass behemoth, capable of swallowing stars whole! At Andromeda, our neighbor galaxy, the giant black hole at the center has an estimated 100-million solar masses.
We have also "seen" the gravitational waves generated when black holes collide with one another, a discovery celebrated with the Nobel prize last year.
What remains a complete mystery is what's inside them. Once we cross the point of no return (also known as the horizon), things change in a curious way. In our reality, we are free to move in space, but time only goes forward. We can control where we go, but not how time goes. Once inside a black hole, this role is somewhat reversed. Now, there is only one place to go, the center of the hole, a one-way track to oblivion. Time, however, may be lose. Possibly, as we dive into the center of the hole, we will see all of time at once, as in Jorge Luis Borges magnificent short story The Aleph. It is also quite possible that there is no real singularity at the center, but something else. Some theories speculate that a black hole is really a kind of tunnel to another spot in the universe (known as an Einstein-Rosen bridge, think a subway tunnel), or even to another universe. We don't know.
Another possibility is that some new kind of physics goes into play when gravity becomes too intense, and the concept of a singularity is just a temporary band-aid until we figure things out.
In any case, we still have a lot to learn about gravity. It's ironic that this most familiar of forces is also the most mysterious to us. It brings to mind one of my favorite Einstein's quotes, one that we would do well to remember:
"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