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6 reasons why scientists are so excited about gravitational waves

LIGO just found gravitational waves ... again. The real excitement is what happens next. 

LIGO/Caltech/MIT/Sonoma State

Around 3 billion years ago, a black hole 32 times the mass of our sun crashed into another black hole 19 times the mass of our sun. The resulting cataclysm created a new monster black hole, and literally warped space and time. If you were near the collision, everything around you would stretch and squash as though you were looking into a funhouse mirror.

And we know this happened, because two machines — one in Louisiana and one in Washington state — detected a tiny trace of that ripple in spacetime passing through Earth on January 4, 2017.

It was the third time scientists at LIGO — the Laser Interferometer Gravitational-Wave Observatory — have detected the collision of two black holes. The first time, last year, was a monumental breakthrough in science, proving a 100-year-old Einstein prediction and ushering in a new age of astronomy. Before, we could only observe the heavens by looking at light or other forms of radiation. Now we can “see” gravity.

“These are the most powerful astronomical events witnessed by human beings,” Mike Landry, a LIGO director at Caltech, said on a recent press call. “Two times the mass of the sun were converted into deformations in the shape of space.”

This latest discovery, published Thursday in Physical Review Letters, helps scientists better understand black holes: how many there are, how big they can grow, and how often and why they collide. But at the same time, these discoveries are starting to become routine. LIGO scientists estimate that when the observatory is upgraded and becomes more sensitive in the coming years, they’ll detect around one black hole merger a day.

LIGO has discovered a new population of black holes with masses that are larger than what had been seen before with X-ray studies alone (purple).
LIGO/Caltech/MIT/Sonoma State

LIGO is only going to grow more powerful, over the next few decades. The most exciting discoveries really are yet to come. Here’s what could happen in this new astronomical age.

(For a fuller explanation of how LIGO works, check out this article.)

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.

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," Chad Hanna, a Penn State physicist who works on LIGO, said in an interview last year. "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 last year. "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 the universe 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 that 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.

3) Combining gravitational wave observations with electromagnetic observations

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 LIGO picks up on a neutron star — or perhaps two neutron stars colliding with each other — it can then point traditional telescopes at them to watch the light show. “That will tell us about the extreme states of nuclear matter,” Landry says. It will also “mark a new era of cooperative astronomy,” he says. We’ve never before been able to measure an object in terms of both gravity and radiation. Seeing them in tandem allows us to learn more about how they are related.

4) Learning how common it is for black holes to orbit each other

Before gravitational wave astronomy, no scientist had observational proof that two black holes could orbit each other. Now we’ve seen three pairs of them doing it. LIGO scientists predict that once they upgrade the sensors to be more sensitive, they’ll be detecting these collisions at a rate of one per day. And the more they detect, the more scientists can hypothesize about the number of black holes 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 in an interview last year. "That would be the most exciting."

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

“I want [to capture a gravitational wave] signal that no one has predicted and for which no one has an explanation,” Caltech’s Landry says. “I want to really surprise the world with a discovery that requires astrophysicists to go back from scratch and think of new ideas.”

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