Swarthmore Professor Explains
Significance to Science and Man
The report was stunning, even if the event took place a billion years ago when two black holes collided. News of the crash just reached Earth, in the form of gravitational waves which were detected last September by scientists working with the U.S.-led Laser Interferometer Gravitational-Wave Observatory — LIGO.
The discovery was announced this month by joyous researchers, who shared audio transcriptions of a “chirp” picked up on the massive antennae at LIGO stations in Louisiana and Washington state. The chirp was emitted as gravitational waves [GW] produced by the quickening mutual orbit of this pair of black holes during the one-quarter of a second just before and as they converged. The waves generated during that period were within a frequency range (35 – 250 cycles per second) detectable by the antennae as they reached and passed through Earth.
Thinkers about science have called this one of the greatest discoveries of the past century, and as fate would have it, the discovery was made almost exactly 100 years after Albert Einstein published his General Theory of Relativity, which predicted the existence of GW, and through later inference by others, black holes.
Tristan Smith, assistant professor of physics at Swarthmore College, greeted the news with joy. “My work involves gravitational waves, as well as other areas of space physics from a fraction of a fraction of a second after the Big Bang.” He took time this week to explain and interpret the meaning of this momentous discovery.
Question: What are black holes?
Black holes are regions of space where the gravity is so large that light itself can’t escape. They’re a basic prediction of Einstein’s theory of gravity. It’s true that there were aspects and consequences of his theory where he and other scientists of the 20th century felt that black holes might be just a mathematical result, but not actually existing in the real world. The same with GW — there were several papers where he tried to show that they existed, or that they didn’t exist… This discovery confirms, for the first time, that GW do exist and provides further and more direct evidence that black holes exist.
Q: How many black holes are there?
There’s evidence that black holes exist at the center of all galaxies, and there are 100 billion galaxies in the observable universe. That includes the Milky Way, the galaxy that we live in. There are really beautiful movies that show stars orbiting an invisible object in the Milky Way. You can use the shape of their orbits to figure out that the gravity that is pulling them is of a mass 4 million times the mass of our sun. That’s a huge black hole.
Q: So are black holes holding the galaxies together?
Even though black holes are very massive, the galaxies themselves are much more massive. It’s not clear how much black holes affect the galaxies. They come in different sizes. The black holes that were indirectly seen through these GW, their masses are much less than the black holes at the centers of galaxies. These were about 30 times the mass of our sun.
Q: What led these relatively small black holes to collide?
How did they find each other? Why were they even close to each other? One of the exciting things is we’re trying to figure out why the BH formed in this binary system. The best guess now is that both of them started out as massive stars in a binary system, formed out of the same cloud of gas and dust. If stars are massive enough, the end stage of their lives is to become black holes. They’re close to one another; they’ve both survived these violent explosive deaths of stars… it’s very romantic and monogamous.
Q: There must have been great belief in the GW theory to justify building this observatory. Was LIGO funded by taxpayer dollars?
Yes, the main form of funding is the National Science Foundation, which is taxpayer funded. It’s a testament to the kind of risk-taking that’s necessary to have a breakthrough in technological and intellectual boundaries that define us as human beings. It’s really a huge success story of many people at all levels of government and academia coming together, having a vision, and seeing it through to the end… actually, to the beginning!
Q: Why are GW so important?
My focus has been on GW from a fraction of a fraction of a second after the big bang. We can’t get information from that epoch in any other way besides GW.
Q: Why are GW such good messengers of this information?
Let’s say you are on one side of a wall and I am on another. Light that I produce will not get through that wall, but if I drop something, you can hear the sound. The universe is transparent to GW; they pass through everything nearly unaffected, which is also why they’re so hard to detect. It happens that these GW’ frequencies are in the audible range, so we can actually listen to them. Up until about 400,000 years after the Big Bang, the universe was opaque to light. We can never get any light from that era, but we can get GW all the way back.
Q: Was LIGO oriented to scan a certain part of the sky?
No, the antennae are pretty omnidirectional. The two colliding black holes just happened to be the ones that were detected. In fact, the researchers had turned the instrument on for engineering analysis a day or two before; they didn’t expect to see GW at a particular time. Since the events that produce GW produce very little light, there is no way to anticipate their arrival.
Q: How frequently can we now expect to be able to detect GW?
Now that we’ve actually seen a GW, the expectation is a few times per year. If all works the way we expect it to, LIGO should soon announce more.
Q: How should this momentous discovery change the way laymen think about the universe?
I think this should change how we all think about the universe in several ways. We usually think of space and time as static and inert. GW are at their core challenging that relationship with space and time. Space itself is rippling and wrinkling and changing and propagating. Two ancient, distant black holes colliding and their waves washing over the earth, that’s pretty profound. And this detection that we are celebrating is the first direct evidence that black holes – this exotic region of the universe from which light cannot escape — exist and they behave the way Einstein predicted they should. We can now say that the universe has these extreme objects, and that, for me, just fuels imagination and creativity across the board.