The Daily Tar Heel

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Monday September 20th

Q&A With Charles Evans about gravitational waves


The first direct detection of gravitational waves marks a major advancement for physics and astronomy. Predicted by physicist Albert Einstein 100 years ago, gravitational waves were finally detected by the highly sensitive instruments known as Laser Interferometer Gravitational-Wave Observatory (LIGO).

Professor Charles Evans is a gravitational theorist in the UNC Department of Physics and Astronomy. Staff Writer CJ Farris spoke with Evans about the discovery.

Daily Tar Heel: What are gravitational waves?

Charles Evans: It’s a traveling disturbance in space and time. It’s a local warping and stretching and compression of space. We put a word on it in physics, we call it strain.

DTH: So why is this the first time we’ve directly detected gravitational waves?

CE: It just goes back to how incredibly weak gravity is. The gravitational force is really weak, the only reason why we are stuck to the Earth is just because there’s this huge amount of mass below us ... So what was required was to just reach the technological level. The technology in this experiment is extraordinary.

DTH: Can you talk about the origins of these gravitational waves?

CE: So we’ve seen black holes, but this is the first time where we’ve, in essence, seen two black holes orbiting around each other and catching them right at the end when their orbit is so tight that they are orbiting around each other 75 times a second, and then the last bit of energy is taken out of the orbit and the two merge together and become one large black hole.

DTH: Was this find expected?

CE: The LIGO experiment ran in an earlier version, called LIGO 1, for close to 10 years, but they never detected a signal like this ... The plan was always to upgrade the detector as technology improved ... So the expectation was even if they didn’t see something this time in this three-month period last fall, the expectation was that either in the next time or the time after that they would.

DTH: What is significant about this discovery?

CE: I’ll say personally, working in this area as a theorist, it’s something that I’ve awaited for 30 years ... As far as significance overall, the main thing about this is that once LIGO is operating regularly and it is joined by other detectors, we basically have got a new form of astronomy, gravitational wave astronomy. This was the dawn of the era of gravitational wave astronomy ... This event is an example of something that probably you’re not going to see in any way other than gravitational waves ... So if we are looking at it with a regular telescope or a satellite, even an x-ray or gamma ray satellite, we don’t see it, but now we can hear it.

DTH: What can we detect now that we understand gravitational wave astronomy?

CE: So the kinds of things that are going to be strong gravitational wave signatures are the most violent events that involve the most massive objects moving close to the speed of light. You just get to see cosmic violence. So take for example this: you have two black holes, each of which weighed about 30 times the mass of the sun, and they’re moving relative to each other at a good fraction of the speed of light. And these objects, they are about 200 kilometers in diameter — so that’s here to Charlotte and beyond or something like that — and there are two of them, and they’re separated by about the same amount of distance as their size, and they’re going around each other 75 times a second. And they’re throwing 60 times the mass of the sun around in circles 75 times a second — it’s just mindboggling.


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