Gravitational waves? Ask an expert

February 23, 2016

News, Expert

On Feb. 11, a team of scientists revealed that they had recorded the sound of two black holes colliding, the first direct evidence of gravitational waves that were predicted by Einstein’s general theory of relativity. What's the significance? We asked physics professor and department chair Ted Bunn, whose research focuses on Big Bang cosmology, to explain this discovery and share his thoughts on its impact.
Interview by Andrea johnson Almoite, '99

What was your reaction to the news?
I knew about this experiment, which has been under development for a long time, and I knew that the experiment was getting close to the point where they could reasonably expect to see something, but I didn’t expect anything this soon. There had been rumors going around among physicists for a few days, maybe even a few weeks, that the LIGO [Laser Interferometer Gravitational-Wave Observatory] people were going to have a big announcement, but I didn’t really believe it. I thought it would be a couple of years until they actually saw something.

I also thought the first time that anyone saw something like this, that it would be controversial about whether they had seen it or not, that the signal would be so weak that people would argue about whether it was really there or not. This was not that way. When they showed their graphs of the signal, it was totally clear that what they saw was there. I think they deserve a lot of credit. They were very careful, and they seemed to have done their homework.

Explain in lay terms what the scientists discovered.
Part of Einstein’s big idea, the idea that came to be known as general relativity, is that space is dynamic and that the geometrical properties of space can change over time and respond to the matter around it.

Essentially, any time mass moves, according to Einstein’s theory, it disturbs the space around it. That disturbance spreads out like ripples away from the source, but for those disturbances to have measurable effects, you need a very large source causing them; you need mass moving around in a violent way. The events that can cause detectable waves are very rare, and even these very massive disturbances cause very tiny ripples, distortions, so you need an incredibly sensitive instrument, and you need to look for some of these violent events in the universe.

Every time we've gotten a new way of mapping the universe, we've learned the answers to questions we didn't even know to ask.

What was actually seen was the distortions in space, caused by two black holes that orbited each other, spiraled around each other, merged, and formed one black hole. That’s one of the most violent energetic events in the universe, and it’s one of the few events large enough to cause ripples that are still detectable from even a great distance. This event was a billion light years away, so that means the event that we saw didn’t just happen now. It happened a billion years ago, and these waves traveling at the speed of light are only just now reaching us.

There are now rumors that the same instrument has seen more events like this since then that they’re still working on analyzing, but the researchers are not saying until they know for sure, which is the right thing.

But if that’s true, maybe these events are more common than people actually think they are, which would also be interesting.

What impact do you think this will have on the field?
For our understanding of the universe, it’s very important.

The first thing is that it directly confirms that space is dynamic, and that’s one of the most important ideas in physics. Part of Einstein’s idea is that space is not the background or arena on which everything else happens, but that space is, in fact, a player in everything. Space can move and wiggle around and produce these waves, and then it responds to the stuff that’s around it. On a philosophical level, this means that space is different than people thought it was, and that’s something we physicists have more or less believed for a while now. There’s more evidence that Einstein’s theory is right, but this is direct evidence that these ripples in space can happen. On a fundamental level, this one important ingredient in our understanding of the universe is confirmed, and it’s been waiting to be confirmed for 100 years. That doesn’t happen every day.

Images courtesy of NASA

It also means that these predicted events of black hole mergers really do happen. When we study black holes in other ways, we don’t really see close to the edge of the black hole itself. We see the stuff that’s well outside the black hole and try to infer what the black hole is like. This event is evidence that black holes have horizons and that they behave the way we expect them to, right up to the edge of the black hole.

Over time, with this instrument, and then ideally with more advanced instruments based on the same idea, we should be able to start observing these things systematically. This event, we know roughly how far away it was, and we know in very broad terms in what direction it happened, but we don’t know exactly where it happened. With a better version of the instrument, you could pinpoint where these things happened. You could go look for evidence of them in other ways.

Eventually, I think this will be a tool like other astronomical tools that will allow us to survey the universe and make maps of where and when these things happen. Every time we’ve gotten a new way of mapping the universe, we’ve learned the answers to questions we didn’t even know to ask yet about how the universe is structured and how it changed over time.

In the coming decades, I hope that this will be a very rich set of information about times and places where some of these very violent events happen in the universe. What that will tell us I don’t even know yet. I don’t know what questions we’ll want to ask yet.