Ripples in space time, predicted by Einstein's general theory of relativity 100 years ago, were first detected on Sept. 14, 2015 using the U.S.-based Laser Interferometer Gravitational-Wave Observatory. Since then, three more have been found.
The discoveries have had scientists around the world buzzing with excitement. Cornell University physicist Saul Teukolsky described the first detection as "one of the greatest scientific discoveries of the past 50 years."
The discovery was so significant that, on Oct. 3, 2017, the Nobel Prize in Physics was awarded to three scientists involved in the research.
Here's what you need to know about gravitational waves and why their discovery is such a big deal.
What are gravitational waves?
Albert Einstein's general theory of relativity predicts that ripples in space time will be produced by massive objects when they change shape in time — for example, when neutron stars or black holes collide.
Those ripples, called gravitational waves, will propagate through space time at the speed of light.
What are they like?
Normally, space time is "flat" like the surface of a lake when there's no wind, says Pfeiffer, a Canada research chair in numerical relativity and gravitational wave astrophysics at the University of Toronto's Canadian Institute for Theoretical Astrophysics.
In flat space-time, two light beams travelling parallel to each other will never intersect.
If you drew a straight line on the surface of a lake and then threw a stone into the water, producing waves, that might cause the line to curve.
The same thing happens with space-time when a gravitational wave passes through, Pfeiffer says.
"It also curves and has … wiggles that affect how light moves through space and time and everything else, for that matter."
That could cause two light beams that were originally parallel to cross paths. Observing things like that could provide information about the event that produced the gravitational waves, the way water waves tell you about the size and shape of the object that was thrown into the lake.
For example, the first gravitational waves detected by LIGO tell us about the size of the colliding black holes, roughly how fast they're spinning, and how long it took them to merge into a single black hole.
Why are scientists so excited about them?
We've been able to learn a tremendous amount about the universe from electromagnetic waves or light. We can see things far away in space using telescopes and detect things we can't see using invisible light ranging from radio waves to X-rays.
But there are amazing phenomena we can't normally detect because they don't give off light, such as two black holes colliding, says Luis Lehner, a researcher who studies such systems at the Perimeter Institute for Theoretical Physics in Waterloo, Ont.
As Bob McDonald, host of CBC Radio's Quirks & Quarks puts it, "gravitational waves could open an entirely new window on the universe that could be as profound as the one opened by Galileo more than 400 years ago" when he first used a telescope to look at objects in the night sky.
Another big advantage of gravitational waves is they aren't blocked or scattered by objects in their path the way light is. That makes them "pristine or exquisite carriers of information" about the event that produced them, Lehner says.
Finally, Einstein's theory of general relativity predicts what kind of gravitational waves will be produced by different types of phenomena. Pfeiffer says being able to detect them allows scientists to finally test the theory under "really violent conditions." And so far, it seems to be holding up.
Why are gravitational waves so hard to detect?
Scientists have been trying to detect gravitational waves for about 30 years, but it's been difficult because they don't interact with matter the way light does.
"They go through us, so to speak, without being disturbed," Lehner said.
Depending on what their source is, their frequency also varies. And any given detector can only detect certain frequencies of gravity waves, just as our eyes can only see visible light and not ultraviolet light or X-rays.
How can gravitational waves be detected?
As gravity waves pass by, they perturb space time so that light takes a slightly longer time to travel between two objects, says Pfeiffer.
To us, that makes it look like the distance between the two objects is changing slightly.
LIGO is a ground-based detector that uses lasers to measure and look for tiny changes in the distance between two mirrors several kilometres apart.
The LISA Pathfinder will use lasers to measure precisely and compare the motion of two different masses in free fall.
Once it has been tested, that technology will be put into three satellites in a triangle formation a million kilometres from each other called the eLISA mission. It will measure small changes in the distance between the satellites to detect gravitational waves. However, eLISA itself won't be launched until 2034.
What kind of gravitational waves could LIGO and eLISA detect?
LIGO and eLISA are designed to detect different frequencies of gravitational waves.
LIGO should be able to detect waves made by smaller objects like neutron stars and smaller black holes up to 500 times the mass of our sun. The first gravitational waves it detected were from two colliding black holes that are about 30 times the mass of the sun.
More massive objects such as colliding super-massive black holes, up to a million times the mass of the sun, will be detectable with eLISA.
Pfeiffer said Canadian scientists are also trying to detect gravitational waves by their effect on objects called pulsars.