Black holes, gravitational waves, and time travel

In February 2016, two-and-a-half years ago, scientists announced the first direct detection of gravitational waves from elsewhere in the universe. The LIGO Observatory, the Laser Interferometric Gravitational Wave Observatory, announced that they had seen these signals of black holes 30 times the mass of the sun spiralling into each other a billion years ago, giving off gravitational waves.

They were actually detected back in September of 2015, and then announced in February 2016. It’s a truly groundbreaking discovery, even though it had been anticipated for years. It’s one of those things which will go down in the textbooks and in the history books as a real cornerstone of how we think about the universe.

These events are quite rare on a galaxy by galaxy basis. In our galaxy, the rate at which two 30 solar-mass black holes would coalesce is one in a millions years. Luckilty there are a lot of galaxies out there.

A gravitational wave can be described as a ripple in the fabric or the shape of space or of space and time, that is produced, in the case of the LIGO observation, by two colliding black holes, travelling across the universe, bringing information about its source.

The word ripple is meant to evoke the idea of a ripple on the surface of a pond. If you throw a pebble into the pond, and it’s a very quiet pond, you see the waves propagating out. In fact, these waves are quite different, where as the surface of the pond is disturbed, it goes up and down and up and down in a wave. Here, what happens is space is stretched and then scrolls in one direction, perpendicular to the direction the wave is propagating. And then in the other perpendicular direction, it scrolls and it stretches. It’s a stretch on one direction, and a squeeze on the other.

What does it mean for space to be stretched, and squeezed? We could just think of particles or little tiny asteroids out there floating in space as the wave goes by, and they’re at rest with respect to each other initially, and they each ride on the stretching and squeezing space, getting pushed apart, and then together, and so on, with the distance between them changing.

Einstein’s opinion about whether or not these were even a real thing changed over time. There were times when he lost faith in gravitational waves, but very quickly realised that they ought to be real. But it was controversial among theorists who worked in relativity theory, at least among some fraction of the community, all the way up into the 1980s, until the physics community got it sorted out.

There is a famous thought experiment by Richard Feynman, known as the Sticky bead argument, trying to show that gravitational waves are physically real.

Feynman’s thought experiment went as follows. Take a stick and put some beads on it. There’s a little bit of friction between the beads and the stick, and when the gravitational waves go by, they move the beads back and forth because they can slide. The stick is stiff and it resists being stretched and squeezed, so it hardly moves at all move because of its resistance. The beads don’t resist, so the beads go back and forth, rubbing on the stick, and they heat the stick up. If you have strong enough waves, they might even start a fire.

People hadn’t identified the right mathematical way to discuss waves until Feynman gave this beautiful description. Back in those days there was skepticism about gravitational waves, and there was arguably even more skepticism about black holes.

LIGO is a set of instruments called “gravitational wave detectors” or “gravitational wave interferometers” that are designed to detect gravitational waves coming from the distant universe and extract the information they carry; information that can be used to learn about the universe. Each of these instruments is something that measures the stretching and squeezing of space, monitors the stretching and squeezing of space, monitors the pattern of each stretch and squeeze. There are four mirrors, each of weighing about 40 kilograms. They’re perfect mirrors and they hang from overhead supports by Quartz fibres.

There are two mirrors along one arm of an L, and the other two along the other arm, so there are two arms that are perpendicular with mirrors at each end of an arm. These mirrors are 4 kilometres apart, and when the gravitational wave comes along, it pushes the mirrors on one arm together, while it’s pushing the ones on the other arm apart. The amount of the push and squeeze is the relevant thing, measured by laser beams. What was finally detected by LIGO was 1/100th the diameter of a proton or the nucleus of an hydrogen atom. That is roughly a trillion times smaller than the wavelength of the light that is used to make the measurement.

We have learned that black holes do collide and merge, and that they do form binaries as expected, similar in neutron stars.

One of the most interesting things is the fact that when two black holes collide, two objects are colliding that aren’t made from matter. They have no solid surface. They have nothing solid in them at all. They’re made only from warped space and warped time. And so when they collide and merge, they create a veritable storm in the shape of space in the rate of flow of time while they’re oscillating rate of flow of time while they’re oscillating shape of space like the surface of the ocean in a huge storm out at sea. 

Just before the gravitational waves were discovered, solutions to Einstein’s equations on supercomputers began to tell us about these storms. We have now seen the waves from these storms. The agreement between the predictions of the simulations and the observations is absolutely remarkable. And so for the first time now, we have both a theoretical understanding and an observational verification of storms in the fabric of space and time.

Sticking out of each spinning black hole is a twisting vortex of space. It’s very much like the twist of a tornado, so two tornados sticking out of a black hole. One of them, at the North Pole of the black hole, it has a counterclockwise twist of space; the South Pole, a clockwise twist of space. And these vortices, when two black holes collide and merge, wind up with four vortices sticking out of a merged black hole plus two more vortices that were created by the orbital angular momentum. So you can have as many as six vortices sticking out. 

But black holes don’t like to have any more than two vortices, so these vortices have to fight with each other in some manner and do a shakedown to two vortices. This is very interesting behaviour of empty space.

The option that is most likely is that we begin with smaller black holes that form inside globular clusters, or big clusters of stars. And they sink to the bottom of the cluster through gravitational interactions with the smaller stars, with less massive stars. They sink to the bottom, they find each other, they collide and merge, and then that merged hole merges with another merged hole, that builds up fairly quickly to two 30 solar mass black holes.

These are very different than the supermassive black holes we have at the centres of galaxies. The ones at the centre of our own galaxy is about four million times the mass of the sun. The one at the centre of the Andromeda galaxy, the nearest big galaxy to our own, is more than 100 million times the mass of the sun. Completely different kinds of beasts.

Black holes really exist. Wormholes, not only are hypothetical, but the chances that they exist naturally are exceedingly small. The chances that they can be made by a very advanced civilisation, are bigger, but still small. And if they get made by an advanced civilisation, they probably implode before you can travel through them. The chances that the civilisation can stabilise them so you can travel through, again, are small, but not zero.

The laws of physics allow them, but unless you did something very strange with them, they would implode and self-destruct. The work that has been done points rather strongly to a conclusion that they probably can’t exist. And if they can exist they very probably can’t exist naturally.

Wormholes, or Einstein–Rosen bridges as they are also known, were conceived independently by Einstein and his colleague, Nathan Rosen in 1936, and previously proposed by Hermann Weyl in 1928. But it was John Wheeler who really pushed hard to understand these initially, because he believed that on very small scales, known as the plank length, the scale where space and time as we know them must become probabilistic, wormholes would fluctuate, just like everything else in the universe, due to quantum physics. On those very small scales, John Wheeler argued that you would likely find a froth of fluctuating wormholes.

So Einstein gives us this equation for general relativity. There’s a left-hand side which says spacetime is curved, and there’s a right-hand side which says there’s stuff in the universe, matter and energy, causing space time to curve. And the left-hand side with the curvature is very pretty and understandable, and the right-hand side with stuff is kind of a mess, which is probably why we don’t understand wormholes very well.

Although wormholes are things that are made from warp space time, without matter, if you have them made from warp space time without matter, then they self-destruct. You would have to put some kind of matter in them to hold them open, and that’s where it becomes tough and messy.

The issue Wheeler had regarding wormholes, was that when you get down to these very tiny length scales associated with quantum effects, you don’t even know the correct laws of physics at all well. And so then you have to start speculating. That’s where he gave some pretty plausible arguments that you would have this quantum foam of fluctuating wormholes. But as mentioned previously, the problem is you don’t understand the laws of physics for a big wormhole. You don’t understand the matter well enough to be sure that whether you can hold the wormhole open with it.

It has been conjectured that one could use a wormhole as a sort of time machine. As mentioned earlier, a wormhole is a hypothetical object that is somewhat similar to a black hole in the sense that it has a spherical mouth, but it’s sort of like the horizon of a black hole, except you can travel two ways through this mouth. You can travel in and back out, and you go through it, and it leads to another place in the universe.

Perhaps, if you have a big wormhole and you can use it to travel across the galaxy very quickly, don’t you worry because special relativity says, it would be like going backward in time. Well, you can go faster than the speed of light, but not locally.

And the most distant part of the Universe is moving away from us faster than the speed of light, so we can’t see it ’cause light can’t get to us from it, but it’s doing it. And so what the speed limit really says is that if you have two objects that are close enough to each other that there is no significant warping of space and time between them, then they can’t move faster than the speed of light with respect to each other. But when you’ve got a wormhole, you’ve got lots of warping with space and time, and all bets are off.

If I wanted to use a wormhole as a time machine, I could take one mouth and I put my daughter in one mouth. She carries that mouth out through the Universe in her spaceship and comes back at high speed and time for her slows down, as seen in the external universe. And I sit on Earth, it doesn’t slow down. But as seen through the wormhole, our clocks run at the same rate. And so you begin. The clocks always running at the same rate, hers and mine as seen through the wormhole. When she comes back, she’s very young, and I’m very old, as seen through the exterior. As seen through the interior of the wormhole however, we have the same age difference as we have now. And so there’s something crazy going on. The craziness is that you have created a time machine.

What do the laws of physics say about this? Could we really do it? If you’re an engineer in a very advanced civilisation and you can do anything that is allowed by the laws of physics, you got to look at more than the laws of relativity. The laws of relativity say, “Sure, you can make a time machine.”

But you also got to look at the laws of quantum physics and the behaviour of matter. There’s always going to be some matter present because with quantum physics, there’s always at least a little bit of fluctuating matter present. Now you’ve got a wormhole you’ve turned into a time machine – relativity allowed it – what did quantum physics do? What did it say? And the answer is that quantum physics says, with high probability, that the wormhole is going to self-destruct the moment you turn it into this time machine.

It seems like from various different perspectives, the universe is kind of reluctant to let you build a time machine.

In February 2016, gravitational waves were discovered by a specially designed observatory tuned for just this purpose. These waves, predicted by Einstein, are ripples moving at the speed of light across the fabric of space-time, and are generated by severe gravitational disturbances, such as the collision of two black holes. And that’s exactly what was observed.

The gravitational waves of the first detection were generated by a collision of black holes in a galaxy 1.3 billion light-years away, and at a time when Earth was teeming with simple, single-celled organisms. While the ripple moved through space in all directions, Earth would, after another 800 million years, evolve complex life, including flowers and dinosaurs and flying creatures, as well as a branch of vertebrates called mammals. Among the mammals, a sub-branch would evolve frontal lobes and complex thought to accompany them. We call them primates. A single branch of these primates would develop a genetic mutation that allowed speech, and that branch—Homo sapiens—would invent agriculture and civilization and philosophy and art and science. All in the last ten thousand years.

Ultimately, one of its twentieth-century scientists would invent relativity out of his head, and predict the existence of gravitational waves. A century later, technology capable of seeing these waves would finally catch up with the prediction, just days before that gravity wave, which had been traveling for 1.3 billion years, washed over Earth and was detected.