[MUSIC PLAYING] Now that gravitational waves are definitely a thing, it's time to think about some of the crazy things we can figure out with them.

In some cases, we're going to need a gravitational wave observatory the size of a galaxy.

In fact, we've already built one.

[MUSIC PLAYING] 11 00:00:27,860 --> 00:00:30,860 We are at the cusp of a golden age of gravitational wave astronomy.

We've already talked about the Laser Interferometer Gravitational-Wave Observatory, LIGO, and the first discovery of gravitational waves here.

Those videos came out almost two years ago.

And a lot has happened since then.

But first, an update on LIGO.

In its two and a half years of operation, LIGO has observed five certain black hole-black hole mergers.

These events have been consistently surprising.

The black holes in question were enormous.

In three cases, both members of the black hole binary pair were well over 20 times the mass of the sun.

And in the other cases, they were 30 plus solar masses.

But if these black holes formed in the deaths of massive stars, which we think they must, then they should weigh in at between 5 and 15 solid masses, 20 at most.

It's hard to imagine black holes forming bigger than this or both of a binary pair growing this large after formation.

Researchers are spending lots of brainpower trying to come up with new and creative solutions to explain this anomaly.

Some are trying to adjust stellar evolution models to allow for the formation of more massive black holes.

Others are calculating whether these black holes may have grown inside globular clusters, where the stellar density is so high that we expect lots of black hole mergers.

There's also some more creative speculation.

Perhaps we've observed the merging of primordial black holes formed in the instant after the Big Bang.

Or perhaps they grew after being swept up in a quasar accretion disk.

I actually did some work on that last one.

Yet, everyone wants in on the gravitational wave game.

A more recent paper has a very different explanation for the apparent large black hole masses.

Perhaps these gravitational waves signals were amplified by another phenomenon predicted by Einstein's general relativity, gravitational lensing.

The paths of gravitational waves should also be warped by intervening gravitational fields which can amplify the signal and stretch out the wavelengths.

That could make a binary merger in the 5 to 15 solar mass range look like a much more massive merger.

Of course, the really big recent news was the observation of a neutron star-neutron star merger.

We covered that in detail here when the detection was still a rumor.

Yet, that one turned out to be true.

For the first time, the event behind a gravitational wave signal was also seen in light.

First, as a gamma ray burst.

But then also across the electromagnetic spectrum.

Observatories across the planet and in orbit around the planet swiveled to watch the afterglow of this collision.

Besides what they tell us about neutron stars, more observations like this should allow us to figure out where the gravitational wave signals are often also gravitationally lensed.

Virgo, the Italian-based gravitational wave observatory, was online for the neutron star merger, and was extremely important in narrowing down its origin.

More are coming.

LIGO-India, or IndIGO, will join the LIGO network, aiming to begin operations in 2024.

So, LIGO is great and all.

But with its four kilometer arm length, it's only sensitive to a frequency range from around 1 hertz to 10 kilohertz.

That roughly corresponds to the frequency of the binary orbits just before merger, which for black holes and neutron stars clocks in at a few to maybe 1,000 orbits per second in the last instant.

The more massive the binary, the lower the frequency.

To catch the merger of the million to billion solar mass black holes, supermassive black holes that live in the centers of galaxies, we need to observe gravitational waves in the 0.1 million hertz to 0.1 hertz range.

That would require an interferometer with arms of millions of kilometers in length.

Sure, why not?

That's LISA, the Laser Interferometer Space Antenna.

This spacecraft by the European Space Agency is scheduled for a 2034 launch.

The three components of this craft will trail behind the earth in an orbit around the sun.

Its arms will be 2.5 million kilometers in length.

That's roughly 10 times the distance from the Earth to the moon.

It has to be that big to listen to the 0.1 millihertz to 0.1 hertz range.

It will see those merging supermassive black holes, as well as the faint hum of thousands of binary pairs of white dwarfs, neutron stars, and black holes long before they merger.

LISA Pathfinder, launched at the end of 2015, was a mission design as a proof of concept for the technology involved in the LISA mission.

And it was a rousing success, demonstrating the feasibility of the mission.

The universe is flooded with space-time ripples.

We expect a faint gravitational wave background buzz from an earlier epoch of the universe in which binary supermassive black holes were common, or from cosmic strings, if they exist.

Or even from the instant after the Big Bang, when the speculative graviton decoupled from the other fundamental forces.

LISA may detect some of this.

But much of this gravitational wave background will have wavelengths as long as many light years.

That's beyond any gravitational wave interferometer that we could ever physically construct.

We'd need a network of perfect timing devices scattered across the galaxy.

Oh, wait.

We already have one.

Remember pulsars, neutron stars with jets that sweep past the Earth as the star processes, resulting in a sequence of flashes more regular than an atomic clock.

We're already using these to study the gravitational wave background at the 1 to 100 nanohertz range.

The international pulsar timing array is a massive effort spanning many universities and radio observatories around the world.

It monitors dozens of the fastest rotating pulsars, millisecond pulsars, spanning many thousands of light years.

It looks for shifts in the time of arrival of their signals.

Such shifts could indicate tiny fluctuations in the fabric of space within the pulsar array volume due to the passage of impossibly vast gravitational waves.

This galaxy scale observatory is already in operation and has placed valuable limits on the amplitude of the gravitational wave background.

More time and more data will hopefully yield a detection.

This may be the only way we can look directly at the instant after the Big Bang.

Some scientists are even trying to see how gravitational waves should interact with stars.

Gravitational radiation should have physical effects as shown by the sticky bead argument first presented by Richard Feynman.

He came up with a thought experiment of a simple gravitational wave detector, a rod with two sliding beads.

As a gravitational wave passes by, the beads are free to follow the expansion and contraction of space while the rod resists due to the atomic forces between its atoms.

But as they move along the rod, these beads generate friction and thus heat.

That heat energy comes from the gravitational wave.

This thought experiment isn't practical, but it demonstrates that in the right circumstances gravitational waves should be able to dump some of their energy into matter, for example, into stars.

Stars oscillate at particular frequencies.

We spoke about this in our episode on asteroseismology.

If a gravitation wave frequency matches the natural resonant frequency of a star, oscillations can be set up inside the star.

This generates an internal friction, heating up the stellar interior.

The star should then brighten.

And it may be possible to observe this effect in the dense star fields of galactic cores if those galaxies also contain binary supermassive black holes that are generating gravitational waves.

A similar effect may cause white dwarf stars in binary orbits to explode as they absorb gravitational radiation from their own orbits.

Links to all of the programs and papers I mentioned are in the description.

We couldn't talk about everything.

There's a lot happening.

Gravitational wave astronomy is currently in a gold rush.

Questions long unanswered are now being resolved, and new mysteries open up.

There are going to be some extremely important and also entirely unexpected discoveries as we learn to decode the incredible wealth of information carried to us in the rippling fabric of space-time.