Peering At The Edge Of Time: The Enigmatic Black Holes : 13.7: Cosmos And Culture The recent discovery of the youngest ever black holes inspires us to reflect upon these most mysterious of Nature's creations.
NPR logo Peering At The Edge Of Time: The Enigmatic Black Holes

Peering At The Edge Of Time: The Enigmatic Black Holes

Recently, scientists announced a remarkable discovery: using the NASA X-ray Observatory Chandra, they were able to find a young black hole, the youngest ever found, still reeling from its birth throes. On a personal note, it’s nice to see that the study is led by Daniel Patnaude from the Harvard-Smithsonian Center for Astrophysics, who is one of our own freshly-minted Ph.Ds from Dartmouth College: young astrophysicist finds his peer in the sky.

The object belongs to galaxy M100, at approximately 50 million light years from Earth and hence not exactly around the corner. This means that the light (or better, X-ray radiation) collected by Chandra’s antennas left the black hole 50 million years ago, about the same time that dinosaurs became extinct here on Earth. (That happened about 65 million years ago.) As aptly explained in NASA’s web site, it’s like cabling a photo of a baby to a far away place. The photo shows the baby, but it took a while to get to its destination.

The black hole, it’s inferred, is the remnant of a giant star, some 20 times more massive than the Sun. To see one just forming is incredibly exciting; no object is more mysterious than a black hole, where our much beloved notions of space and time cease to make sense.

Black holes challenge the boundaries of the reasonable. You can picture them as a secret protected by a spherical cocoon, which we call the “event horizon.” Just like with a beach, where the horizon delineates the limit of what you can see, the event horizon marks the point of no return: if you pass beyond it, you cannot come back out. Not even light, which explains the famous name, black hole.

Now, what does go on inside the horizon? Good question. Black holes come in different kinds, classified by their mass and rotation. A simple, non-rotating black hole has a point singularity in its center, a point where, yes, the laws of physics as we know them break down. This explains the excitement: inside the horizon there is a world beyond this one where what we know of reality doesn’t make sense any longer. And we want to see more than we can.

Picture this: you shoot your worst enemy down a non-rotating, spherical black hole. He travels in a spaceship that has a blue blinking light that blinks at every second. What do you see, safely from a distance? As your enemy’s spaceship approaches the horizon, the color of the blinking light changes gradually down the colors of the rainbow, from blue to red. Eventually, you won’t see the light any longer, as it gets stretched into longer wavelengths: infrared, microwave, radio. Also, the interval between the blinking increases. Once the spaceship crosses the horizon, the blinking essentially freezes: for you, time at the spaceship has stopped! Kind of frustrating, because you won’t see your enemy’s fate within the horizon. But you do know that the gravitational pull is so intense that just the difference from the tip to the bottom of the spaceship will force it to thin out like spaghetti. You feel a tinge of remorse. (Readers who want to know more can consult my book The Prophet and the Astronomer, where I discuss dying stars and black holes in great detail.)

The shifting of light to longer wavelengths is called “gravitational redshift.” (Not to be confused with the cosmological redshift from the expansion of the universe.) The slowing down of the blinking illustrates “time dilation”: the stronger the gravitational pull, the slower a clock ticks. At the horizon, a clock wouldn’t tick any longer. Black holes are indeed quite weird.

And what would your enemy see? Well, for him, all would proceed normally; as he is freefalling into the hole, he won’t feel time dilation or light redshift. But he will feel the differential pull of gravity and be stretched into oblivion. The only other curious thing he will feel as he crosses the horizon is that he won’t be able to escape the singularity. Just as in our lives time marches forward inexorably while we have the freedom to move in any direction of space, once you cross the horizon the roles of time and space are reversed: there’s only one direction of space, directly toward the crushing singularity at the center, where gravity becomes infinitely strong.

Or so we believe. The problem is that as we approach the singularity, the theory we use to describe black holes, Einstein’s general theory of relativity, breaks down. We need a different theory, capable of dealing with absurdly huge gravitational attraction in extremely small distances. That is, we need a theory of quantum gravity to understand how strong gravity behaves at very small distances, a theory we still don’t have.

What we do know, from the first intimations of such theory, is what Stephen Hawking called black hole evaporation: it turns out that black holes are not eternal. They evaporate slowly, losing their masses due to quantum effects. In fact, a black hole is not really black for this reason: as it evaporates, it emits radiation of a certain temperature, the Hawking temperature, roughly T = (1023kg/M)K, where M is the mass of the black hole in kilograms and K is the temperature in the Kelvin scale. (1 Kelvin = -273 Celsius. Sorry for the formula, but I’m sure some readers will appreciate it.) For comparison, the Sun’s mass is roughly 1030kg. So, a black hole with the Sun’s mass would “shine” with a temperature of 10-7K: very very cold! But note that the temperature increases inversely with the mass: a small black hole closer to the end of its life would brighten up!

Which leaves us with an intriguing question. As a black hole evaporates, its horizon shrinks. What happens as we reach the final singularity? Would the horizon disappear revealing a point where space and time are balled-up to infinite density? This does sound a lot like the Aleph of Jorge Luis Borges’ brilliant homonymous short story. We don’t know. There is something called the cosmic censorship conjecture, which says that a “naked singularity” is impossible: something will happen before things get to that pornographic all-revealing point where physics breaks down. But what? Good question. Some, including this author, have conjectured that as the black hole approaches the singularity, it will excite the extra spatial dimensions where superstrings are supposed to live and the singularity will actually disappear in a puff of strings. Wild stuff. But this was in the days when I was much more convinced of the reality of such unifying theories than I am now, as readers of my book A Tear at the Edge of Creation know well. Meanwhile, black holes remain puzzling and fascinating to scientists and non-scientists. Any new information we have on them, such as the news from Chandra, is most welcome.