Gravitational Waves from Black Hole Binary Detected by LIGO
Pictured: Representation of two black holes spinning into each other and emitting gravitational waves. Found on http://ryanmarciniak.com/archives/1672
By now you've all heard the news of LIGO's announcement today, with the discovery of gravitational waves. The gravitational waves originated from an astrophysical source, being a black hole binary, two black holes orbiting each other, whose acceleration as the two whirl into each other produce gravitational waves, until the point of merging. The signal associated with this known as a "chirp" signal, one whose frequency increases and increases until BOOM—the black holes merge and the signal just stops. This is the signal that was observed, which both confirms the discovery of gravitational waves as well as the existence of black hole binaries! And the plot of the actual signals match so beautifully the theoretical ones. The two 1.3-billion-light-year-distant black holes that generated the signal had masses of 29 and 36 times that of the Sun, and (very briefly!) 50 times as much energy as all the stars in the observable universe... Think about that for a second!! WOW! It's also worth noting that a light year is the distance it takes light to travel in one year, which is about 9.5 trillion km, so this pair is pretty far away. After merging, the two black holes combined into one 62-solar-mass black hole, releasing the remaining 3 solar masses as energy in the process.
Pictured: The plots show the data of the strain produced by the gravitational waves plotted with respect to time, taken at both LIGO observatories, the first representing the data taken in Hanford (Washington), and the second one, in Livingston (Louisiana). The time delay of 7 milliseconds in the signal detected in Hanford can be seen here, with the signal shifted slightly to the right. The smoother curve that goes through both the plots is the theoretically predicted one, and agrees very well. The final plot superimposes both curves (after accounting for the time delay) and shows the two agree beautifully, confirming that the signal detected was from the same source. Credit: LIGO
It would make sense to talk a little bit about what LIGO is and how it works before explaining how the LIGO detection places constraints on the mass of the graviton, a hypothetical particle that acts as the mediator for the gravitational force. The LIGO observatory consists of two giant interferometers separated across the United States, with one in Louisiana, and the other in Washington. The setup is basically that of a sophisticated and giant Michelson interferometer (I've included a link below that describes how it works pretty well). The general idea is this: each interferometer sends out a laser beam that goes through a beam splitter that splits the laser beam into two and sends them across what we call "arms", each 4-km in length (whereas the Michelson interferometers we use in physics labs are tiny in comparison, with arms only tens of centimeters apart, yet still difficult to tune). Mirrors then reflect the beam back, and with no interference, the light waves from the laser beams cancel out (that is to say, when the two are added together, you get nothing, as crests and troughs of the waves cancel each other out), and nothing is detected. If, however, a gravitational wave passes through, the strain in the gravitational wave causes the light to "wiggle", or shift, and then a signal is recorded. In the case of the signal detected, that shift was 1/1000th the diameter of a proton. The diameter of a proton is ~10^-15 meters (that's a decimal followed by 14 zeros, and then a 1). Tiny, right? Well the signal was 10^-18 meters, so a decimal followed by 17 zeros followed by a 1 (my mind was blown by this)! This is how sensitive LIGO is.
Now, the reason there are two interferometers, spread at a distance of some 3000 km apart, is that, as you can imagine, these interferometers are so sensitive that tiny local interferences can also cause false signals to appear. So if a false signal is detected at one interferometer, but not the other, then they know it's not a detection. The two work in conjunction to both confirm the detection of a signal. In this case, there was a time delay between the detection of the signals at each interferometer of 7 milliseconds (the one in Louisiana saw the signal first, followed by the one in Washington). This indicates that the signal originated from the direction of the sky towards the Southern Hemisphere, and also tells us that gravitational waves do travel at the speed of light. This then places a constraint on the mass of the graviton at 1.2*10^-22 eV/c^2, which in familiar units, corresponds to an upper limit of 2.1*10^-58 kg... Thus, the graviton must be very nearly massless indeed. And, since we can say gravity propagates at the speed of light, then we can also say that the gravitational waves were emitted 1.3 billion years ago, since the binary (now merged) is located 1.3 billion light-years away.
I just thought this was a discovery so important that it really deserved some explaining. Hence I've also put together a nice list of links below that give you more details on the revolutionary discovery of gravitational waves, confirming yet another prediction of Einstein's theory of General Relativity, a prediction made a century ago. What a great time it is to be alive!
Links:
LIGO's Press Conference (begins at about the 27:10 mark; a great demonstration of LIGO's interferometer from Rainer Weiss can be found at the 54:10 mark): https://youtu.be/aEPIwEJmZyE?t=27m13s
Demonstration of LIGO's interferometer: https://www.ligo.caltech.edu/video/IFO-response
Great explanation from Perimeter Institute for Theoretical Astrophysics: https://www.perimeterinstitute.ca/news/gravitational-waves-101
Summary of the results from astronomer Phil Plait: http://www.slate.com/blogs/bad_astronomy/2016/02/11/gravitational_waves_finally_detected_at_ligo.html
A good article on the discovery with great media: http://www.dailygalaxy.com/my_weblog/2016/02/epic-discovery-gravitational-waves-detected-from-colliding-black-holes-nasaligo.html
A great explanation of gravitational waves, posted by fellow Astronomy Club member Matthew Scatterty (thanks Matthew!): https://www.youtube.com/watch?v=gw-i_VKd6Wo&feature=youtu.be A good explination of gravitational waves for the layperson, care of our Media Coordinator Susan Chen (thanks Susan!): https://www.youtube.com/watch?v=4GbWfNHtHRg
For those of you inclined, the actual paper: http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102
LIGO Document P150914-v14 (Observation of Gravitational Waves from a Binary Black Hole Merger): https://dcc.ligo.org/LIGO-P150914/public