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Why gravitational wave astronomy has physicists so damn excited

This illustration shows the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other.
LIGO/T. Pyle

This story starts 1.4 billion years ago, in a far-flung corner of the universe, with a catastrophe. Two black holes — the densest, most destructive forces known to nature — collided with one another.

Black holes are so massive that when they collide they disrupt the very fabric of space and time itself. And like a stone cast into still water, that disruption rippled outward, warping space and time as it grew, throughout the endless pond that is the universe.

This past Christmas, that wave of gravity finally reached Earth. In its travels it grew very quiet: If you measured its amplitude — the distance from the top to the bottom — it would measure much less than the width of an atom. Until recently, we had no capability to record waves this faint. But this Christmas, we had just the right "microphone" up and running.

On June 15, scientists announced that they have — for the second time — observed the gravitational waves produced by orbiting black holes with the help of the Laser Interferometer Gravitational-Wave Observatory (LIGO). The results appear in the journal Physical Review Letters.

As with the first detection, which was confirmed back in February, the latest gravitational waves were produced by the collision of two black holes. That’s cool.

But there’s something even more important here to consider: We’ve entered a new age of astronomy. And it may fundamentally change our understanding of the universe.

"This second [discovery] should convince anyone who was skeptical of the first discovery that this isn’t a fluke," Dave Reitze, the executive director of the LIGO Lab, run out of MIT and Caltech, tells me. "We’ve now seen two of these binary black hole collisions emerging to form a new black hole — this is something no other type of observatory has been able to see before. … Gravitational wave astronomy is for real."

An illustration of how the two orbiting black holes produced gravitational waves. In this latest discovery, scientists "heard" 55 of these ripples as they passed through the Earth. The masses of the black holes were 14 and eight times the mass of the sun. They merged into a single black hole, with a mass of 21 of our suns. The very last part, where the black holes merge and release tons of waves, is what LIGO heard.
LIGO/T. Pyle

Why gravitational waves matter

Right now, our telescopes can only see objects that emit electromagnetic radiation — visible light, X-rays, gamma rays, and so on. But some objects — like colliding black holes or the smoking gun of the Big Bang — don't emit any electromagnetic radiation. Instead, they emit gravity.

And that's why, with gravitational wave astronomy, invisible objects in the universe may soon become visible. Another bonus: Gravitational waves are unchanged by the matter they move through. (Visible light, on the other hand, can get absorbed or reflected by cosmic bodies or dust before it reaches our telescopes, leaving us with a cruddy view of things.)

Just as sound waves disturb the air to make noise, gravitational waves disturb the fabric of spacetime to push and pull matter as if it existed in a funhouse mirror. If a gravitational wave passed through you, you’d see one of your arms grow longer than the other. If you were wearing a watch on each wrist, you'd see them tick out of sync.

Two-dimensional representation of gravitational waves generated by two neutron stars orbiting each other.
Wikimedia

Gravitational waves are generated by any movement of mass. "For instance, if I wave my arms really crazily, I would generate gravitational waves," Sarah Caudill, a physicist at the University of Wisconsin Milwaukee and one of the authors of today’s discovery, tells me.

But there’s no way to detect gravitational waves that faint. For now, our sensors need a really, really loud source — like the the collision of two black holes.

"Right now, I’m being bathed in gravitational waves, you’re being bathed in gravitational waves," says Reitze. "The reason why our interferometers [i.e., detectors] don’t sense it is because the amplitude of those waves — the size of the signal they are creating — is much less than our detectors are capable of detecting."

Two black holes colliding unleash a loud* thunderclap of gravity. But by the time they reach Earth 1.4 billion years later, they’ve become very faint (like how pond ripples become less frenzied farther away from a dropped stone).

(*The correct metaphor is "listening," because the frequencies of the gravitational waves are comparable to the frequencies of the sound waves we hear.)

The wave LIGO heard on Christmas was around 0.7 attometers tall. An attometer is 10^-18 meters. That’s much smaller than an atom. The following GIF starts out showing the width of an atom, and then zooms down to 10^-18. It’s amazing that we were able to hear something that small.

LIGO

How do we "hear" these really "loud" but tiny gravitational waves?

LIGO, which is funded by the National Science Foundation, consists of two enormous science experiments: One is in Louisiana; the other is located in Washington state. Both are massive L-shaped tubes. Each arm of the tube is 2.5 miles long.

During the experiments, a laser beam is equally split between the two arms. At the end of each arm is a mirror, which reflects the laser back to the starting point. What LIGO is looking for is evidence that gravitational waves are distorting spacetime enough that one of the arms becomes temporarily longer than the other.

These changes can be incredibly tiny, since LIGO is sensitive enough to detect a change in distance smaller than the width of a proton.

"Each detector is like a violin string, and we’re waiting for a gravitational wave to ‘pluck’ each detector string," Reitze explains.

The basic setup of the LIGO interferometer.
LIGO

If a wave is detected in one of the sites, it has to be corroborated with the other site (to make sure it's not just a false signal from local automobile traffic or other disturbances).

LIGO was first set up in 2002, and for years it found nothing. In 2010 it was shut down for an upgrade, which extended its range. When it was turned back on in 2015, it almost immediately began hearing the waves.

By listening in on these loud waves, the scientists are able to reconstruct the cataclysmic events that created them. Just from the two detectors, scientists can determine the mass of the black holes and how far away they are, roughly map where in the sky they are, and distill some information about the shape of their orbits.

Take a listen to what LIGO heard on December 25 here:

LIGO

This simple audio recording requires a huge international effort. The paper in Physical Review Letters has hundreds of authors.

(Note: The official publication notes the detection occurred on December 26 in UTC — universal time — but it was still Christmas when the sensors went off in the United States.)

What other cool things can we learn from gravitational wave astronomy?

For now, LIGO can’t be pointed at a region in the sky and search for gravitational waves. Rather, it just hears the gravitational waves that are passing through Earth at any particular moment. And it currently doesn’t do a great job at pinpointing where these waves are coming from.

This is the approximate location of the gravitational wave event detected on December 26, 2015. It’s a pretty big, broad area of the sky. LIGO explains in a press release: "The colored lines represent different probabilities for where the signal originated: the outer purple line defines the region where the signal is predicted to have come from with a 90 percent confidence level; the inner yellow line defines the target region at a 10 percent confidence level."
LIGO/Axel Mellinger

Luckily, in the coming decades as many as five detectors will come online across the world (as well as some space-based detectors). And this is when gravitational wave astronomy will truly take off.

"We’re really at the beginning of this field," says Chad Hanna, a Penn State physicist who works on LIGO. "What’s tremendous and exciting about it is that it’s a completely new way of discovering things that we don’t yet know."

Here are some cool things gravitational wave astronomy could accomplish.

1) Seeing further back in time

One problem with our current fleet of telescopes is that they can’t see back to the very early universe.

"If you look with visible light as far as we can look in the universe, the universe is no longer transparent; it becomes opaque," Cliff Burgess, a particle physicist at McMaster University, told me in February. "Almost nothing is opaque to gravity." With LIGO, we could potentially listen in on the gravitational waves emanating from the early universe, or even the Big Bang, and gain a better understand of how it formed.

2) Improving on Einstein’s theory of general relativity

A century ago, Einstein published his theory of general relativity. And it has dominated our understanding of gravity ever since. But physicists (and Einstein himself) have long speculated the theory isn’t complete, as it doesn’t play well with the laws of quantum mechanics. Gravitational waves could help physicists put general relativity to harder and harder tests to see where it fails.

"We’ve found that these black holes are completely consistent with Einstein’s theory that he formed 100 years ago," Caudill says. "So that’s cool, but as we get more and more detections, we can probe his theory even deeper, and maybe expose holes in it."

3) Discovering new neutron stars

Neutron stars are the extremely dense cores of collapsed stars that can emit large amounts of gravity. What’s cool about them is that they also produce light. "If you can see an event like a neutron stars colliding, or a black hole and a neutron star colliding" with LIGO, Caudill says, you can then point traditional telescopes at them to watch the light show.

4) Learning how common it is for black holes to orbit one another

Before the February announcement, no scientist had observational proof that two black holes could orbit each other. Now we’ve seen two pairs of them doing it. Gravitational wave astronomy will help us understand how many of these pairs exist in the universe.

5) Finding the source of dark matter

Dark matter is theorized to make up 27 percent of all the matter in the universe. But we’ve never seen dark matter (it’s obfuscous!), and we don’t know where it comes from.

Matter creates gravity. Perhaps gravitational waves can help us trace the origins of dark matter. It could exist in the form of many tiny black holes. It could be the remnants of "primordial" black holes created at the beginning of the universe. We don’t know.

6) Finding new, weird celestial objects

The universe is a big, dark place.

"We might find sources [of gravity] we were not expecting," says Avi Loeb, a Harvard theoretical physicist. "That would be the most exciting."

Perhaps we’ll find evidence of "cosmic strings," hypothesized weird wrinkles in spacetime containing a massive amount of energy. And the chances of finding these strange new objects only increases as the power of LIGO increases and its counterparts come online.

It will be like "going from simple Galileo telescopes to the types of telescopes you put on top of mountains," Reitze says. "For the next 50 years, this is going to be a really exciting field."


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