January 14, 2016: This week there's been a new storm of rumors suggesting scientists might have detected gravity waves, finally. But this time around, everyone is taking the announcement with a big grain of salt. Back in May 2014, we published this interview with Rana Adhikari about why the chase is so difficult and fraught with false starts.

So what exactly are gravitational waves?

I think it's easiest to explain by analogy. For an ocean wave, the wave is just the movement of the water. Water is the medium. And in the case of sound waves, air is the medium. But for gravitational waves, there is no mediumat least not in the sense of some matter. What's moving is space itself. If you imagine space as having grid-lines, gravitational waves are the fluctuation of those grid-lines. They're the warping of the actual coordinates of space.

We're all making gravitational waves all the time. They're caused by the acceleration of [virtually any] mass, and they travel at the speed of light. Things like biking down the street or the flight of a bumblebee makes these waves, albeit exceedingly weak ones.

But space is extremely stiff, and so the amount of energy it takes to make a wave which we could conceivably measure is [monumental]. All practical schemes for detecting gravitational waves are aimed at detecting those from super-massive sourcesthings which are roughly the size of the sun or thousands of times bigger than thatthat are moving at a good fraction of the speed of light.

And what's our interest in these waves?

Our hope is to do astronomy with them. We want to use them as a tool to hear (the information is a lot more like sound than light) astrophysical phenomena all over the universe, such as the merging of pairs of neutron stars or black holes. And ultimately, we'd like to use these waves to test Einstein's theory of gravity.

And unlike light or radio waves where you have to worry about obstructions along your line of sight, gravitational waves basically travel unimpeded through matter and space. So you're able to get a picture of an event happening anywhere in the universe with virtually no distortion. It takes something like a black hole to significantly scatter these waves.

Have we ever detected them before?

Well the answer is kind of semantic. Although gravitational waves were first predicted by Einstein in 1916, our first observational evidence of them wasn't found until 1974.

In 1974 there was a binary star system that was discovered by astronomers Russell Hulse and Joseph Taylor. The orbit of these stars decayed at just the precise rate that Hulse and Taylor calculated that energy had to be exiting in the form of gravitational waves. More [recently], last March the BICEP2 instrument announced it'd found the fingerprint of gravitational waves from immediately after the big bang in the cosmic microwave backgroundwhich is the oldest light we can see in the universe.

But we've never directly measured the waveform of gravitational waves on Earth. The difference is almost like [seeing proof] that sound was made, versus actually hearing and recording that sound. But the other methods are just as legitimatethese are all detection techniques.

But we're trying to detect them now with the instrument you're working with, the Laser Interferometer Gravitational Wave Observatory (LIGO). Can you explain what LIGO is and does?

Well, LIGO is basically a very powerful and very stable laser [measuring device]. We have a laser beam which is split in two and is bounced off two of the most exquisitely well polished mirrors in the world. One half of the laser beam goes 2.5 miles north to one mirror, and the other goes 2.5 miles east to the other. After the light bounces off those mirrors and comes back, we can measure how far each has traveled. If a gravitational wave comes by, the physical space in between the laser and the mirror can be distorted, and so the laser beams will travel different distances.

The Washington State LIGO detector.

But the change in distance we expect from a gravitational wave is a ridiculously smalla billion times smaller than the size of an atom. It's so faint that everything on the Earth seems to conspire to cover up these waves. Anything you can imagine is (probably correctly) a problem for us: the acoustic noise of a plane flying overhead, electromagnetic changes from lightning strikes anywhere in the country, the ambient seismic vibrations of the Earth.

So much of our effort has been focused on the humongous engineering challenge of shielding ourselves from this ambient noise. And what we can't shield, we have all types of detectors so we can subtract any additional noise from our system. But there are limits, which is why we have two devices at opposite ends of the countryin Louisiana and Washington state. In order for us to claim a gravitational wave detection, the wave must be the same at both sites.

You recently said that we're on the cusp of not only directly detecting these waves, but of measuring them and finally doing astrophysics with them. Why is that?

Although people have made LIGO-type detectors since the '70s, the first large scale ones only came into operation around 2000. Our first generation of LIGO ran from 2002 to 2010. We knew that that detector didn't have a great chance at detecting signals, but we wanted experience with the technology and the engineering challenges. We searched, but we didn't find anything.

But we are just now finishing up the instillation of the second generation detector. And we expect that, starting late next year, we'll be able to directly detect gravitational waves for the first time. We estimate that we'll have at least several detections per year, and that there will be some that will come in with enough signal amplitude that we'll be able to use those gravitational waves to do some very interesting astrophysics. But I think no matter what I say, whatever number I predict, I'll end up being wrong. There are many unknowns, and nature is bound to surprise us.

Really though, my claim was mostly focused on what we'll be able to do with the following generation of detectors, the thirdwhich I hope is between 5 to 10 years out. We're now starting to explore tricks like using cryogenic techniques to cool down our mirrors, and using so-called squeezed light with our lasers.

When we do finally detect these waves, what's next?

There are plans for space-based detectors, and those have benefits. For one, you don't have to pay for real estate, and the detectors can be extremely big. You also don't have all the noise issuesfrom earthquakes, and lighting and manmade sourcesthat you have with ground-based detection.

As for LIGO, will there just be an endless number of generations? I don't know, but in some way that's really the beauty of astronomy. You keep making more sensitive instruments, and whenever you have a chance to observe something about the universe in a different way, you do it. Because there will always be more to look for.

I see this as being similar to the beginning of the 20th century, when all astronomers had were optical telescopes. When you look through those telescopes, the universe seems pretty steady, like there's not a lot going on. But as people learned observe the sky in things like radio waves, x-rays, gamma rays, they saw that it's really quite the opposite. You look out into space and there are all kinds of exciting phenomena: stars are exploding, galaxies are emerging, black holes are at the center of galaxies. It's a violent and vibrant universe.

That change of our understanding of space came about because we utilized new types of observational tools. Our hope for gravitational waves is not just that they'll be useful as an observational tool, but they'll once again radically change how we look at the universe.

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