Billions of years ago, two black holes merged in a violent explosion that rippled the fabric of our universe.
Those cosmic ripples known as gravitational waves produced by this collision spread far and wide in all directions, carrying with them information about the black holes that brought them into being.
In September 2015, that information made it to Earth. While these weren’t the first gravitational waves to reach our planet, they were the first we could observe.
Two powerful tools known as theLaser Interferometer Gravitational-Wave Observatories (LIGO) were able to directly observe the gravitational waves sent out by the two black holes, opening up a new way for scientists to study the inner-workings of some of the most extreme objects in the universe.
Until now, scientists studying the cosmos were limited to just staring at our universe using different wavelengths of light.
While this type of investigation has completely transformed our understanding of how stars, galaxies, planets and other objects work, it also has left us in the dark when trying to understand the inner lives of black holes and other exotic objects.
All of that is changing now, however.
In the not too distant future, scientists should be able to peer into the hearts of exploding stars, figure out how matter is changed within the hot, high-pressure center of a neutron star, and better characterize what a black hole really is all thanks to barely-detectable waves sent out to the far ends of the observable universe.
Being an astronomer right now, as gravitational wave science begins in earnest is “kind of the equivalent of being there when Galileo put together his first telescope,” scientist Edo Berger, who is involved in LIGO-related research, said in an interview.
A light turning on
The entire history of astronomy has hinged on studying the universe with light, but now, we have an entirely different way to peer out into the cosmos. It’s as if astronomy as we know it has gained a new sense.
Instead of trying to look directly at something like a black hole that doesn’t give off light, astronomers can now piece apart the “chirps” of gravitational waves to learn more about the masses, sizes and lives of the objects that created them.
“… Using gravitational waves we can probe environments that are enshrouded with a lot of matter which blocks our view,” Harvard University astronomer Avi Loeb said via email.
“For example, when a massive star collapses or when a neutron star gets swallowed by a stellar-mass black hole, or when two massive black holes coalesce while being surrounded by gas duringthe merger of two galaxies, we cannot easily probe the center of the action because it is hidden behind a veil of matter,” Loeb added.
“But gravitational waves can penetrate easily through matter and reveal the inner working of such engines. “
You can’t feel or see gravitational waves move through Earth’s part of space, but they do affect us nonetheless.
In fact, the signal discovered in September warped all of the matter on Earth including all of the matter in our bodies by just a fraction of a proton.
And that’s what LIGO had to measure. Both observatories one located in Louisiana, another in Washington recorded the moment the gravitational waves passed through Earth’s part of space at the same time.
The twin “L”-shaped observatories both have a laser that runs down each arm of the L to mirrors located at the end of the arms. If no gravitational waves pass through Earth, the lasers should each bounce back to the middle at precisely the same time, but if a wave were to pass through, that timing would be off.
This is because the matter around the laser stretches ever so slightly as the wave passes through, changing the length of the arms but not affecting the light itself.
“What LIGO had to do to detect the waves was to measure the motion of mirrors (due to the passing gravitational wave) that was smaller than a single proton,” LIGO researcher Nergis Mavalvala said.
“Imagine that, put mirrors 4km (2.5 miles) apart and watch them get closer or farther to each other by a distance one-one-thousandth the size of a proton.”
Discoveries already pouring in
Scientists have already analyzed data brought to Earth by the gravitational waves discovered in September, characterizing the black holes that created those ripples like never before.
A study published in June 2016 found that the two black holes which gave rise to the gravitational waves actually began their lives as massive stars orbiting one another.
Eventually, after millions of years in orbit around one another, the stars collapsed, forming two black holes about 30 times the mass of our sun. And one day, those black holes merged, rippling the fabric of space and time like a bowling ball spinning around on a bed sheet.
The authors of the study used data gathered by LIGO to create a computer model of the universe that would have given rise to the gravitational waves detected here on Earth billions of years after the black hole merger.
“The black holes were monsters, and the results show that their progenitor stars would have been some of the brightest and most massive in the universe,” physicist J.J. Eldridge wrote in a piece accompanying the study at the time.
Neutron stars and a new state of matter
LIGO should eventually do even more than reveal the secret lives of black holes as well.
In the future, astrophysicists are hoping to use gravitational wave tools to figure out what’s going on in the intensely hot, high pressure middle of a very mysterious class of stars known as neutron stars.
“You build an instrument for things you want to measure, and then you see things that you didn’t expect to see”
Neutron stars are more massive than the sun but packed down into an area the size of the city of Boston. These types of stars form when stars about four to eight times the size of our sun die.
The hearts of these stars might actually be so dense and high pressure, that they warp molecules into a totally different state of matter than what can be observed in labs on Earth.
“In this case, of course, it [the matter in a neutron star] exists in a state that we’re not familiar with from our own personal experience because we’ve never witnessed those kinds of pressures,” Berger said.
At the moment, LIGO isn’t able to easily detect neutron star mergers as they are somewhat less energetic than black hole collisions, but in the future it should be able to as its sensitivity advances, revealing the hearts of those dense objects.
Gravitational wave science also has the ability to add to the already rich tapestry of science done by looking at light in the universe.
Some astronomers are already attempting to pinpoint the optical sources of gravitational waves to see if there’s any kind of light signal that goes along with mergers of black holes.
At the moment, LIGO isn’t very good at pinpointing exactly where a signal is coming from in the sky, so other technologies could be further developed to meet that challenge in the future, allowing scientists to gather precise data on the sources of gravitational waves, LIGO’s Mavalvala said.
And one day, LIGO and the host of new technology that will be produced around it may even hear a new signal that scientists can’t even imagine now.
“You build an instrument for things you want to measure, and then you see things that you didn’t expect to see,” Mavalvala said.
“I think that’s the true promise of these instruments.”