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Why gravitational waves truly are the “scientific breakthrough of the year”

True scientific breakthroughs are extremely rare. We had one in 2016.

LIGO/T. Pyle

2016 has been a rough year. Yet science in 2016 has remained a source of optimism.

To solve a scientific problem, you have to believe it is solvable. That makes scientific discovery inherently optimistic. Discoveries help us understand our world — the scarred, imperfect beautiful mess of it — just a bit better. The biggest breakthroughs in science, which don’t come around that often, also open doors for new questions to be answered.

This year, we had such a breakthrough.

“The discovery of gravitational waves has changed the scientific landscape,” the journal Science writes today, declaring them “the breakthrough of the year.”

In February, and then again in June, physicists announced they heard the subtle rumbling of a ripple in spacetime, the result of two black holes colliding into one another. These observations confirmed the existence of “gravitational waves,” which were predicted by Albert Einstein more than 100 years ago but never actually recorded until this year. The waves were picked up by two huge science experiments — one in Louisiana and one in Washington state — called LIGO, or the Laser Interferometer Gravitational-Wave Observatory.

The discovery is huge, and not just because it answered a 100-year-old question. It’s huge because it launched a whole new branch of science.

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.

The accolades for LIGO have been copious: A few days ago, the journal Nature named Gabriela Gonzales, one of the scientific leaders of the project, one of its 10 people “who mattered” in 2016. The magazine Physics World named LIGO a breakthrough of the year, and Foreign Policy magazine put LIGO scientists on a year-end list of top global thinkers. (The effort did not receive the Nobel Prize this year, but there’s a good chance it will in the future.)

LIGO and gravitational waves show that scientific progress still has momentum in a fractured world. Science summarized the importance of the discovery this way: “A new science beckons.”

Why gravitational waves matter

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, told me in June.

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

"Right now I’m being bathed in gravitational waves, you’re being bathed in gravitational waves," Dave Reitze, the executive director of the LIGO Lab, said. "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).

(*Scientists use the sound metaphor because the frequencies of the gravitational waves are comparable to the frequencies of the sound waves we hear.)

One of the waves LIGO heard (around Christmas last year) 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

LIGO, which is funded by the National Science Foundation, operates two massive experiments in Louisiana and Washington state. Both are giant 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.

And all of this took decades of work: The theory LIGO tested was developed in the early 20th century, the LIGO project began in the 1980s, it was first turned on in 2002, and it took an international effort to hear and confirm the waves that came through this year. It’s having a big moment now, but it goes to show that big breakthroughs have deep roots, and science is ultimately a collaborative, multigenerational effort.

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

For now, LIGO can’t be pointed at a region in the sky to 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 of 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). Next in line is a detector in Italy called VIRGO, which is slated to begin operations in 2017. With three detectors, scientists will be able to better pinpoint where in the sky these waves are coming from.

Here are some cool things the next era of 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 last 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 dark!), 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," Avi Loeb, a Harvard theoretical physicist, said. "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."


Watch: Understanding gravitational waves