People often ask me: “What is spacetime? The easy answer is that it’s the "fabric of space.” That means little; space isn’t a fabric, and doesn’t act like one. But it does bend, and ripple, and stretch, and vibrate.
What is spacetime, really? It’s a coordinate system, a kind of conceptual grid. It’s a way of accounting for the dimensions of the Universe, where in addition to up and down, left and right, forward and backward, we also have time—the future and the past. The most revolutionary idea Einstein proposed was that physics only makes sense when space and time are inextricably linked together, where the way we travel through one changes the way we travel through the other. And gravity, a property of anything with mass or energy, changes both.
The most extreme example of this is a black hole. Approach a black hole and, aside from ultimately being torn limb from limb by the steeply climbing gravitational field, you won’t notice much changing. But relative to someone keeping a safe distance, your wristwatch, heart rate, thought processes, and every other aspect of your existence, will have slowed to a standstill.
By the time you cross the event horizon, marking the point of no return, you will appear to the rest of the Universe as a slowly fading, darkening image of yourself. And if you look out at the Universe you’ve left behind, you’ll see it whirling away in fast-forward, charging on into the future without you. The rest of your journey, into the singularity at the center of the abyss, will be yours alone to see.
While the interior of a black hole may be inaccessible to us, the existence of black holes is well established fact. Most of the time, we observe them when they pull nearby matter into themselves, creating glowing-hot whirlpools of gas known as accretion disks. Sometimes, we see them indirectly through the orbits of stars around them. And in some circumstances, their gravity and the whirling accretion disk can warp magnetic fields into such a tangle that they create massive jets of particles that extend light years from the centers of galaxies.
As of last year, however, there is a new way to see black holes: through how they alter spacetime itself.
Any gravitating body can be said to make a “dent” in spacetime, a region where the grid curves inward. When massive objects accelerate through the universe, that dent moves too, and creates a disturbance in the space around it. When two black holes orbit each other, the ripples their motion creates in space can radiate out across the universe, stretching and squeezing everything in their path.
These ripples are called gravitational waves, and last year, for the first time, scientists had a machine sensitive enough to detect them: the Laser Interferometer Gravitational-Wave Observatory, or LIGO, experiment. LIGO measures the distance between carefully suspended mirrors by bouncing lasers back and forth through vacuum tubes four kilometers long. When a gravitational wave passes through, the laser light goes very slightly out of phase. LIGO can detect this when the change in distance is only a thousandth the width of a proton.
LIGO’s specialty is the collision of black holes. When two black holes orbit too close to each other, they begin a fatal dance, spiraling inward until their event horizons touch, and they merge, becoming one. The mass of the final black hole is equal to the sum of the masses of the two—minus a bit: the mass that was converted entirely into gravitational waves in the final moment. The energy of that burst of gravitational radiation can, for that brief moment, add up to more energy than the combined light of every star in the entire observable Universe. The waves can travel for billions of years across the cosmos, spreading out in all directions, and still have enough energy to shift the mirrors of LIGO, allowing us to feel the vibrations from the black hole collision.
So far, LIGO has detected three black hole in-spiral events. As far as the basic physics of black holes and gravitational waves goes, all of the events are perfectly consistent with the predictions of Einstein’s general relativity, a theory of gravity set down more than 100 years ago. But not everything is as physicists expected. The ripples we’ve seen were produced by black holes more massive than we thought we would find.
Making a black hole is easy. Take a massive star (at least several times more massive than our Sun) and wait several million years for it to collapse. (Safety note: this generally happens in a rather messy way.) After the stardust clears, you’ll be left with a nice “stellar-mass” black hole, weighing somewhere in the vicinity of a few to 10 times the mass of the Sun. We see evidence for stellar-mass black holes all over the galaxy, because they have a bad habit of ripping apart other stars that happen to be orbiting around them. LIGO is built primarily to see black holes like this, if they are in binary orbits with each other and collide nearby.
There are also much, much bigger black holes out there, with masses millions of times that of the Sun, in the centers of galaxies. These seem to grow by pulling in a lot of gas and dust from their surroundings, though the formation process for them is still a bit unclear. These supermassive black holes should also collide sometimes, but the gravitational waves of those collisions are of too low a frequency to be detected by LIGO.
The black holes LIGO has seen, though, are somewhere in the middle—in the range of tens of solar masses. We’ve always assumed that black holes of this mass range should probably exist, but they’re hard to find evidence for in observations. So why has LIGO seen these, and not the smaller stellar-mass black holes everyone expected?
There are a few possible explanations. Some have to do with theories of how black holes form, some with how quickly black holes merge in different environments, and some with selection effects: the fact that bigger black hole mergers make a bigger “splash” in spacetime, and are therefore easier for LIGO to spot, even far away. What is definitely true is that these mid-size black holes are helping us to refine our understanding of how black holes form and grow.
While a few physicists have proposed that the explanation could be something really exotic, like a huge invisible population of black holes left over from the early universe, it’s far too early to say that the previous calculations were wrong or that we have to really rewrite everything. This is still definitely a case of what we in science call small-number statistics: until we have data from a lot of merger events, we can’t say anything definitive.
The good news is that LIGO will keep searching, and over time, new detectors will come online that will give us more data and help us pinpoint which direction each signal came from. This means we might be able to see bursts of light that could be produced by the events or figure out which galaxy they happened in, which will make figuring out how they happened even easier. But even with just these few detections, a new era of astronomy has begun.
Gravitational wave astronomy, where we view the Universe not in light or particles but by picking up the vibrations of spacetime itself, is a true revolution in our view of the cosmos. We can only guess at what it might show us as we continue to feel our way forward into the vast unknown.