Physics Journal Club seminar explains gravitational waves
Josh Hartman / Sun Star
A long time ago in a galaxy far far away, two orbiting black holes collided with each other and the gravitational shockwave it created was felt here on Earth on Sep. 14 at 9:50 a.m. UTC. This discovery was published in “Physical Review Letters” (PRL) on Feb. 11.
This was the topic of a Physics Journal Club seminar given by David Newman, a professor of physics and member of the Geophysical Institute. The seminar also addressed Einstein’s Theory of Relativity, which had it’s 100th year anniversary in 2015.
“Last semester I said that we should talk about the theory of relativity because of the anniversary, but I thought we should wait for an announcement because I felt that one might come,” Newman said. “It has come.”
The room quickly filled up with people attending the seminar, eventually leading to several people standing in the hallway to listen in. This is fairly common. The room is usually full, according to Newman.
The paper published in the journal PLR was titled: “Observation of Gravitational Waves from a Binary Black Hole Merger.” It detailed how, with gravitational waves, scientists observed two black holes merge together 1.3 billion light-years away.
Gravitational waves are ripples in the fabric of space-time, much like how dropping a pebble into a calm pond will create small waves on the water’s surface. Some cosmic events—like black holes colliding or stars exploding—will create ripples in the gravitational field that travel at the speed of light throughout the universe.
The discovery of these waves is difficult because of how small they are, according to Newman.
“Trying to measure things a thousandth of the size of a proton is absolutely absurd,” Newman said. “But they did it.”
The observation was made by researchers at the Laser Interferometer Gravitational-wave Observatories (LIGO) in Hanford, Wash. and Livingston, La. Over $1.5 billion from the National Science Foundation was used to fund this project, which was used over several years of research.
“People have been looking for gravitational waves since I was a grad student,” Robert McCoy, director of UAF’s Geophysical Institute, said at the seminar.
There have been several false positives in the past. It was thought that gravitational waves were discovered earlier in 2015, however, scientists soon learned that the signal was thrown off by galactic dust: gas and particles residing in space.
The interferometers at the LIGO facility that discovered the signal work by shooting a laser with the equivalent power of 100 kilowatts down two long, perpendicular hallways. The laser then bounces back-and-forth between mirrors along the length of the hallway many times. Once the lasers meet back up they should have the same amplitude if there was no change in the length of the hallways.
Gravitational waves slightly bend the matter that they pass through. When the wave passes through the interferometer one of the hallways is slightly shorter and one is slightly longer leading to the amplitudes of the lasers being slightly off.
The interferometers at the LIGO facilities are very sensitive. In order to make sure the signal they get is a real gravitational wave, and not a mistake at one of the facilities, the two LIGOs at Hanford and Livingston were calibrated together and had been for sixteen days before discovering the signal that indicated the merging of 2 black holes.
The signal that they discovered was nearly two orders of magnitude above the threshold that they needed to identify a gravitational wave. It was also the exact signal that the researchers would expect to be given off by the merging to 2 black holes.
The researchers think that they will be able to detect up to 100 of these signals a year at the LIGO facilities, according to Newman.
There are more LIGO facilities being built in India, Italy and Australia.
This research has implications for astronomy because scientists can use gravitational waves to observe the universe around us much better than with the telescopes we currently use, according to Newman.
This is especially effective because gravitational waves pass through most objects undisturbed. so they can provide a very accurate map of what the universe looks like, Newman added.
“All of us have an innate desire to know about our universe,” Newman said. “You don’t have to be an astronomer to look up at the sky and wonder how the stars and the moon works.
The Physics Journal Club gives seminars every week on Friday, starting at 3:45 p.m. in the Globe Room of the Elvey Building.
“In a lot of universities, this would be called physics seminar, we call it the journal club because we try to keep it more informal,” Newman said. “Anyone from students on up can come and give presentations.”
The seminars are organized and sometimes given, by Newman and Chung-Sang Ng, a professor of Physics.
The Physics Journal Club seminars have been running for over 20 years, probably much longer, according to Newman.
At the original journal club meetings beer was served at the back of the room because it was not yet illegal, Newman said.
“I think it’s really really important for people to come together and hear new things,” Newman said. “If you’re just stuck in your own world that’s a really bad thing.”
The next Physics Journal Club seminar will be given by Bill Bristow, Professor of Electrical and Computer Engineering, about the High Frequency Active Auroral Research Program (HAARP). Details about this and future seminars can be found on the Physics Journal Club website.