This episode is sponsored by The Ridge.

Go to ridge.com/space and use the promo code “space” to get 10% off your next order. [ ♪ Intro ]. Telescopes and observatories come in all shapes and sizes, from long tubes in your backyard, to domes and giant satellite dishes.

And until recently, they had one thing in common: In some way, they all pointed up. Okay, I know that sounds super obvious. Telescopes need to collect light and radiation from space, so of course they point up.

But that's also changing. Right now, researchers are working on two underground telescopes, and despite that sounding like a really silly idea, it's not silly! They have the potential to revolutionize the way we think about the universe.

Unlike other telescopes, these telescopes aren't really looking for light. Instead, they're trying to measure gravitational waves, the bending and stretching of spacetime. Gravitational waves come from things like merging black holes and colliding neutron stars.

As the waves move through space, they stretch and compress everything in their path, including stars, the Earth, and us. And by measuring those changes, we can study things that would otherwise be invisible to us. The problem is, these signals are weak.

Even if some of them start off strong, the disruptions they make are usually smaller than an atomic nucleus by the time they reach Earth. To study them, you need a specialized machine. And right now, the two biggest are LIGO in the U.

S., and Virgo in Italy. They aren't underground, but they have been hugely successful, and they've set the standard for future telescopes. LIGO has two locations, each with one pair of four-kilometer arms arranged at right angles.

And Virgo has a similar setup, but with one location, and with one pair of three-kilometer arms. These designs are so similar because both LIGO and Virgo detect gravitational waves using the same method: laser interferometry. Kind of like the name suggests, the technique relies on interference.

First, the system produces a laser beam, which gets split into two separate beams. One goes straight, and the other shoots off at a ninety-degree angle. Three or four kilometers later, and each of these lasers hits a mirror and travels back toward the same central point.

When they get there, they normally interfere with each other and cancel out. But if a gravitational wave passes through this system, that changes. The wave warps the space between the mirrors and that central point, meaning the lasers arrive at different times and don't cancel out.

Instead, a detector picks up a blip of light. And the interference pattern of that light can give astronomers data about what just happened. Unfortunately, because these instruments are so sensitive, a lot can interfere with them.

Anything from passing trucks to the tiniest shudder of the Earth can throw off results. And that's where underground telescopes come in. Because unlike light, gravitational waves can pass unaltered straight through the Earth.

So if your signals can pass through the planet, why not build your detectors underground, where things like traffic and seismic noise are muted? And that's exactly what a few teams are working on. One of these projects is called KAGRA.

It's being built by a team in Japan, and is an improved version of LIGO and Virgo. It's also two hundred meters underground and underneath a mountain. It's expected to come online in late 2019, and when it does, it will be the first underground gravitational wave observatory.

But while KAGRA will probably teach us a lot, it has nothing on a 2011 proposal made by researchers in the European Union for another detector. Their proposal is something called the Einstein Telescope, and it's impressive. Right now, a lot of the specific details about where it would be, or when it would be made, are still up in the air, but the technical details have all been worked out.

The Einstein Telescope would be based on the same laser interferometry technology as LIGO,. Virgo, and KAGRA. And like KAGRA, it would also be at least a hundred meters underground.

But the first big difference is that it would be way longer. Each arm would be about 10 kilometers long, as opposed to 3 or 4 for those other projects. And this, joined with the fact that it's underground, would help it be more sensitive to faint signals.

The second big difference is that it would have three arms in a triangle shape, made of three nested, two-pronged detectors. The design kind of looks like three overlapping Vs, as opposed to the old L-shape. And this triangle design would give the Einstein Telescope a few distinct advantages.

One is that the multiple detectors would add a layer of redundancy, making it easier to distinguish real detection events from false positives. Another is that having three detectors also allows you to detect a property of gravitational waves called their polarization. A bit like the polarization of light in sunglasses, this is what it looks like when the waves distort spacetime.

Einstein's theory of gravity, called general relativity, predicts that gravitational waves should have two possible polarization states. But other theories predict there should be more. So this telescope could either confirm its namesake's ideas, or point scientists in the direction of a more complete, fundamental theory of gravity.

Plus, this telescope has one more trick up its sleeve:. Each of the 3 detectors would have 2 measuring devices in it, each designed to detect different frequencies of gravitational waves. Gravitational waves are usually talked about like they're all the same.

But, like electromagnetic waves, gravitational waves actually come in a range of frequencies, each with different properties. And different astronomical events are expected to produce different frequencies. For instance, scientists predict that supernovas produce much higher frequency waves than supermassive black hole mergers do.

So if we're able to study a wide range of frequencies, we can study a wide range of objects. LIGO, Virgo, and KAGRA were all designed to detect gravitational waves in the 10 to 100,000 Hertz range. And the Einstein Telescope would have an instrument to do that, too.

But it would also have an additional instrument designed to detect waves from 1 to 250 Hertz. And that's really the most exciting part of all of this, because this is the frequency range where it's predicted that gravitational waves from the earliest days of the universe may be lurking. We can't see further back in time than about four hundred thousand years after the Big Bang, because that's when the earliest light in the universe was emitted.

But scientists predict that, if we can detect gravitational waves at these low frequencies, we might be able to see waves from the first nanoseconds after the Big Bang, if not even earlier. And this could help to resolve a huge number of outstanding questions about the birth and evolution of, you know, the whole universe. And it might even lead us to discover new questions we didn't even know we wanted to ask.

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