Part 1

About 3 billion years ago, if there was intelligent life on the galaxy we call PG 1302-102, it should have known it was in serious trouble.

Our galaxy has a supermassive black hole in the middle. But that galaxy had two. One was about ten times as big as the other. Taken together, they weighed a billion times as much as our Sun.

They gradually spiraled in towards each other… and then, suddenly, one fine morning, they collided. The resulting explosion was 10 million times more powerful than a supernova—more powerful than anything astronomers here on Earth have ever seen! It was probably enough to wipe out all life in that galaxy.

We haven’t actually seen this yet. The light and gravitational waves from the disaster are still speeding towards us. They should reach us in roughly 100,000 years. We’re not sure when.

Right now, we see the smaller black hole still orbiting the big one, once every 5 years. In fact it’s orbiting once every 4 years! But thanks to the expansion of the universe, PG 1302-102 is moving away from us so fast that time on that distant galaxy looks significantly slowed down to us.

Orbiting once every 4 years: that doesn’t sound so fast. But the smaller black hole is about 2000 times more distant from its more massive companion than Pluto is from our Sun! So in fact it’s moving at very high speed – about 1% of the speed of light. We can actually see it getting redshifted and then blueshifted as it zips around. And it will continue to speed up as it spirals in.

What exactly will happen when these black holes collide? It’s too bad we won’t live to see it. We’re far enough that it will be perfectly safe to watch from here! But the human race knows enough about physics to say quite a lot about what it will be like. And we’ve built some amazing machines to detect the gravitational waves created by collisions like this—so as time goes on, we’ll know even more.

Part 2

Even before the black holes at the heart of PG 1302-102 collided, life in that galaxy would have had a quasar to contend with!

This is a picture of Centaurus A, a much closer galaxy with a quasar in it. A quasar is huge black hole in the middle of a galaxy—a black hole that’s eating lots of stars, which rip apart and form a disk of hot gas as they spiral in. ‘Hot’ is an understatement, since this gas moves near the speed of light. It gets so hot that it pumps out intense jets of particles – from its north and south poles. Some of these particles even make it to Earth.

Any solar system in Centaurus A that gets in the way of those jets is toast.

And these jets create lots of radiation, from radio waves to X-rays. That’s how we can see quasars from billions of light years away. Quasars are the brightest objects in the universe, except for short-lived catastrophic events like the black hole collisions and gamma-ray bursts from huge dying stars.

It’s hard to grasp the size and power of such things, but let’s try. You can’t see the black hole in the middle of this picture, but it weighs 55 million times as much as our Sun. The blue glow of the jets in this picture is actually X rays. The jet at upper left is 13,000 light years long, made of particles moving at half the speed of light.

A typical quasar puts out a power of roughly 1040 watts. They vary a lot, but let’s pick this number as our ‘standard quasar’.

But what does 1040 watts actually mean? For comparison, the Sun puts out 4 x 1026 watts. So, we’re talking 30 trillion Suns. But even that’s too big a number to comprehend!

Maybe it would help to say that the whole Milky Way puts out 5 x 1036 watts. So a single quasar, at the center of one galaxy, can have the power of 2000 galaxies like ours.

Or, we can work out how much energy would be produced if the entire mass of the Moon were converted into energy. I’m getting 6 x 1039 joules. That’s a lot! But our standard quasar is putting out a bit more power than if it were converting one Moon into energy each second.

But you can’t just turn matter completely into energy: you need an equal amount of antimatter, and there’s not that much around. A quasar gets its power the old-fashioned way: by letting things fall down. In this case, fall down into a black hole.

To power our standard quasar, 10 stars need to fall into the black hole every year. The biggest quasars eat 1000 stars a year. The black hole in our galaxy gets very little to eat, so we don’t have a quasar.

There are short-lived events much more powerful than a quasar. For example, a gamma-ray burst, formed as a hypergiant star collapses into a black hole. A powerful gamma-ray burst can put out 10^44 watts for a few seconds. That’s equal to 10,000 quasars! But quasars last a long, long time.

So this was life in PG 1302-102 before things got really intense – before its two black holes spiraled into each other and collided. What was that collision like? I’ll talk about that next time.

The above picture of Centaurus A was actually made from images taken by three separate telescopes. The orange glow is submillimeter radiation – between infrared and microwaves—detected by the Atacama Pathfinder Experiment (APEX) telescope in Chile. The blue glow is X-rays seen by the Chandra X-ray Observatory. The rest is a photo taken in visible light by the Wide Field Imager on the Max-Planck/ESO 2.2 meter telescope, also located in Chile. This shows the dust lanes in the galaxy and background stars.

Part 3

What happened at the instant the supermassive black holes in the galaxy PG 1302-102 finally collided?

We’re not sure yet, because the light and gravitational waves will take time to get here. But physicists are using computers to figure out what happens when black hole collide!

Here you see some results. The red blobs are the event horizons of two black holes.

First the black holes orbit each other, closer and closer, as they lose energy by emitting gravitational radiation. This is called the ‘inspiral’ phase.

Then comes the ‘plunge’ and ‘merger’. They plunge towards each other. A thin bridge forms between them, which you see here. Then they completely merge.

Finally you get a single black hole, which oscillates and then calms down. This is called the ‘ringdown’, because it’s like a bell ringing, loudly at first and then more quietly. But instead of emitting sound, it’s emitting gravitational waves—ripples in the shape of space!

In the top picture, the black holes have the same mass: one looks smaller, but that’s because it’s farther away. In the bottom picture, the black hole at left is twice as massive.

Here’s one cool discovery. An earlier paper had argued there could be two bridges, except in very symmetrical situations. If that were true, a black hole could have the topology of a torus for a little while. But these calculations showed that – at least in the cases they looked at—there’s just one bridge.

So, you can’t have black hole doughnuts. At least not yet.

These calculations were done using free software called SpEC. But before you try to run it at home: the team that puts out this software says:

Because of the steep learning curve and complexity of SpEC, new users are typically introduced to SpEC through a collaboration with experienced SpEC users.

It probably requires a lot of computer power, too. These calculations are very hard. We know the equations; they’re just tough to solve. The first complete simulation of an inspiral, merger and ringdown was done in 2005.

The reason people want to simulate colliding black holes is not mainly to create pretty pictures, or even understand what happens to the event horizon. It’s to understand the gravitational waves they will produce! People are building better and better gravitational wave detectors—more on that later—but we still haven’t seen gravitational waves. This is not surprising: they’re very weak. To find them, we need to filter out noise. So, we need to know what to look for.

The pictures are from here:

• Michael I. Cohen and Jeffrey D. Kaplan and Mark A. Scheel, On toroidal horizons in binary black hole inspirals, Phys. Rev. D 85 (2012), 024031.

Part 4

Let’s imagine an old, advanced civilization in the doomed galaxy PG 1302-102.

Long ago they had mastered space travel. Thus, they were able to survive when their galaxy collided with another—just as ours will collide with Andromeda four billion years from now. They had a lot of warning—and so do we. The picture here shows what Andromeda will look like 250 million years before it hits.

They knew everything we do about astronomy—and more. So they knew that when galaxies collide, almost all stars sail past each other unharmed. A few planets get knocked out of orbit. Colliding clouds of gas and dust form new stars, often blue giants that live short, dramatic lives, going supernova after just 10 million years.

All this could be handled by not being in the wrong place at the wrong time. They knew the real danger came from the sleeping monsters at the heart of the colliding galaxies.

Namely, the supermassive black holes!

Almost every galaxy has a huge black hole at its center. This black hole is quiet when not being fed. But when galaxies collide, lots of gas and dust and even stars get caught by the gravity and pulled in. This material form a huge flat disk as it spirals down and heats up. The result is an active galactic nucleus.

In the worst case, the central black holes can eat thousands of stars a year. Then we get a quasar, which easily pumps out the power of 2000 ordinary galaxies.

Much of this power comes out in huge jets of X-rays. These jets keep growing, eventually stretching for hundreds of thousands of light years. The whole galaxy becomes bathed in X-rays—killing all life that’s not prepared.

Let’s imagine a civilization that was prepared. Natural selection has ways of weeding out civilizations that are bad at long-term planning. If you’re prepared, and you have the right technology, a quasar could actually be a good source of power.

But the quasar was just the start of the problem. The combined galaxy had two black holes at its center. The big one was at least 400 million times the mass of our Sun. The smaller one was about a tenth as big—but still huge.

They eventually met and started to orbit each other. By flinging stars out the way, they gradually came closer. It was slow at first, but the closer they got, the faster they circled each other, and the more gravitational waves they pumped out. This carried away more energy—so they moved closer, and circled even faster, in a dance with an insane, deadly climax.

Right now—here on Earth, where it takes a long time for the news to reach us—we see that in 100,000 years the two black holes will spiral down completely, collide and merge. When this happens, a huge pulse of gravitational waves, electromagnetic radiation, matter and even antimatter will blast through the galaxy called PG 1302-102.

I don’t know exactly what this will be like. I haven’t found papers describing this kind of event in detail.

One expert told the New York Times that the energy of this explosion will equal 100 million supernovae. I don’t think he was exaggerating. A supernova is a giant star whose core collapses as it runs out of fuel, easily turning several Earth masses of hydrogen into iron before you can say “Jack Robinson”. When it does this, it can easily pump out 1044 joules of energy. So, 100 millon supernovae is 1052 joules. By contrast, if we could convert all the mass of the black holes in PG 1302-102. into energy, we’d get about 1056 joules. So, our expert was just saying that their merger will turns 0.01% of their combined mass into energy. That seems quite reasonable to me.

But I want to know what happens then! What will the explosion do to the galaxy? Most of the energy comes out as gravitational radiation. Gravitational waves don’t interact very strongly with matter. But when they’re this strong, who knows? And of course there will be plenty of ordinary radiation, as the accretion disk gets shredded and sucked into the new combined black hole.

The civilization I’m imagining was smart enough not to stick around. They decided to simply leave the galaxy.

After all, they could tell the disaster was coming, at least a million years in advance. Some may have decided to stay and rough it out, or die a noble death. But most left.

And then what?

It takes a long time to reach another galaxy. Right now, travelling at 1% the speed of light, it would take 250 million years to reach Andromeda from here.

But they wouldn’t have to go to another galaxy. They could just back off, wait for the fireworks to die down, and move back in.

So don’t feel bad for them. I imagine they’re doing fine.

By the way, the expert I mentioned is S. George Djorgovski of Caltech, mentioned here:

• Dennis Overbye, Black holes inch ahead to violent cosmic union, New York Times, 7 January 2015.

Part 5

When distant black holes collide, they emit a burst of gravitational radiation: a ripple in the shape of space, spreading out at the speed of light. Can we detect that here on Earth? We haven’t yet. But with luck we will soon, thanks to LIGO.

LIGO stands for Laser Interferometer Gravitational Wave Observatory. The idea is simple. You shine a laser beam down two very long tubes and let it bounce back and forth between mirrors at the ends. You use this compare the length of these tubes. When a gravitational wave comes by, it stretches space in one direction and squashes it in another direction. So, we can detect it.

Sounds easy, eh? Not when you run the numbers! We’re trying to see gravitational waves that stretch space just a tiny bit: about one part in 1023. At LIGO, the tubes are 4 kilometers long. So, we need to see their length change by an absurdly small amount: one-thousandth the diameter of a proton!

It’s amazing to me that people can even contemplate doing this, much less succeed. They use lots of tricks:

• They bounce the light back and forth many times, effectively increasing the length of the tubes to 1800 kilometers.

• There’s no air in the tubes—just a very good vacuum.

• They hang the mirrors on quartz fibers, making each mirror part of a pendulum with very little friction. This means it vibrates very well at one particular frequency, and very badly at frequencies far from that. This damps out the shaking of the ground, which is a real problem.

• This pendulum is hung on another pendulum.

• That pendulum is hung on a third pendulum.

• That pendulum is hung on a fourth pendulum.

• The whole chain of pendulums is sitting on a device that detects vibrations and moves in a way to counteract them, sort of like noise-cancelling headphones.

• There are 2 of these facilities, one in Livingston, Louisiana and another in Hanford, Washington. Only if both detect a gravitational wave do we get excited.

I visited the LIGO facility in Louisiana in 2006. It was really cool! Back then, the sensitivity was good enough to see collisions of black holes and neutron stars up to 50 million light years away.

Here I’m not talking about supermassive black holes like the ones in the doomed galaxy of my story here! I’m talking about the much more common black holes and neutron stars that form when stars go supernova. Sometimes a pair of stars orbiting each other will both blow up, and form two black holes—or two neutron stars, or a black hole and neutron star. And eventually these will spiral into each other and emit lots of gravitational waves right before they collide.

50 million light years is big enough that LIGO could see about half the galaxies in the Virgo Cluster. Unfortunately, with that many galaxies, we only expect to see one neutron star collision every 50 years or so.

They never saw anything. So they kept improving the machines, and now we’ve got Advanced LIGO! This should now be able to see collisions up to 225 million light years away… and after a while, three times further.

They turned it on September 18th. Soon we should see more than one gravitational wave burst each year.

In fact, there’s a rumor that they’ve already seen one! But they’re still testing the device, and there’s a team whose job is to inject fake signals, just to see if they’re detected. Davide Castelvecchi writes:

LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team. Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.” The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.

Aargh! The disappointment would be crushing.

But with luck, Advanced LIGO will soon detect real gravitational waves. And I hope life here in the Milky Way thrives for a long time – so that when the gravitational waves from the doomed galaxy PG 1302-102 reach us, hundreds of thousands of years in the future, we can study them in exquisite detail.

For Castelvecchi’s whole story, see:

• Davide Castelvecchi Has giant LIGO experiment seen gravitational waves?, Nature, 30 September 2015.

For pictures of my visit to LIGO, see:

• John Baez, This week’s finds in mathematical physics (week 241), 20 November 2006.

For how Advanced LIGO works, see:

• The LIGO Scientific Collaboration Advanced LIGO, 17 November 2014.

References

To see where the pictures are from, click on them. For more, try this:

• Ravi Mandalia, Black hole binary entangled by gravity progressing towards deadly merge.

The picture of Andromeda in the nighttime sky 3.75 billion years from now was made by NASA. You can see a whole series of these pictures here:

• NASA, NASA’s Hubble shows Milky Way is destined for head-on collision, 31 March 2012.

Let’s get ready! For starters, let’s deal with global warming.

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