 Greetings and welcome to the Introduction to Astronomy. In this video we are going to discuss gravitational waves which are a relatively new discovery although they were predicted long ago by Einstein's theory of general relativity. So let's take a look at what we have here and what we know is that first of all the gravitational waves were a prediction of general relativity so this was back in 1916 that they were predicted. It took about a hundred years for them to actually be detected. What it is is that any accelerating object with mass will produce gravitational waves. That can mean you, that can mean me, that can mean the earth, the moon, the sun, anything that is accelerating will produce these gravitational waves so they're all over the place. The problem is that the gravitational force is so weak that gravitational waves are really hard to detect because they are so weak so the gravitational waves that I give off are so small that they could not possibly be detected or distinguished from other background gravitational waves. So there were some thoughts on them. They were believed to exist and they were a prediction of general relativity that had not yet been discovered. Now they are an analog to electromagnetic waves. Electromagnetic waves are caused by accelerating charged particles and they are related more to the electromagnetic force so they are much stronger. That's why we can see electromagnetic waves from accelerating charged particles. What we see as visible light x-rays or radio waves are much easier to see than these gravitational waves. Now, early on there was an indirect detection in terms of a binary pulsar to neutron stars orbiting each other and we can determine their orbits and their orbit is slowing because they are giving off gravitational waves. Remember that energy has to be conserved so if gravitational waves are being given off the energy has to be coming from some place in that system and in this case the orbit was found to slow by the exact amount that general relativity described. However, it was only recently that we've been able to directly detect these gravitational waves. So let's take a look at how we could do that and what we find is that in order to detect them first of all we need a very strong source of gravitational waves. The moon moving, the sun moving, the earth moving are not going to be near strong enough. So what we look for is coalescing black holes or neutron stars. So these are objects with very large mass that as they coalesce together would form would give off massive amounts of gravitational waves and in fact these two would actually implode and collapse in on each other forming a black hole. So you could take two neutron stars forming a black hole or of course black holes forming a much larger black hole. Now the way to detect this the observatory that was built was called LIGO and that was the laser interferometer gravitational wave observatory. There were set up in multiple sites originally two in the United States one down in Louisiana and the other in Washington State and that was done for a very specific reason. Gravitational waves even from things like colliding black holes were still very weak and you could get measurements by local effects such as somebody slamming a door or a car or a truck driving nearby could cause some problems in the measurements. So by looking at them at two sites and now three I believe there are ways to eliminate any local effects. If you see if you see an event in one at one observatory but not at the other then you know that it was something local and you can throw that out. Now what LIGO does is to use these very long arms and to measure variations in the length of the very long arms. So essentially what happens is we send a laser signal down and back across here a number of times and then split that signal now it goes two different directions there is one that goes down this one that goes down here and then we combine those signals together and we look for minute changes in the path that the length of the path that the light had to travel and that would be caused by passing gravitational waves and the amount of course we're looking for is incredibly tiny we're talking about less than atomic size shifts in the amount of the paths that are traveled and these paths are kilometers long but however in 1915 sorry in 2015 they were able to make the first detection of these gravitational waves so what was found and what have we seen so far well something quite like this and in fact gw150914 was the first detection the detection was actually made on September the 14th of 2015 and was not announced until it had been confirmed and verified a few months later in February of 2016 the visibility was seen here this was in Hanford Washington and this is in Livingston Louisiana and then when we look at them together we see really the same signals and when we subtract out the theoretical signal what we should have gotten from the observed signal we find we could get a match with two black holes merging together one of them being 20 solar masses the other about 36 solar masses so we get the signal here we get the observation here and when we subtract them we don't see any patterns left behind which gives us good confidence that our model fits the observations these would have merged 1.3 billion years ago so not just recently but this was in a galaxy over one point over a billion light years away so the earth was of course quite different at that time but those signals those gravitational waves which would travel at the speed of light have taken that long to get here there have now been a number of more detections that have been made over the last few years and more will continue to be made and we will start to learn more about these gravitational waves so most of what has been detected so far is from black hole mergers but we have also been able to now detect neutron star mergers as well so let's finish up here as we do with our summary and what we find is that gravitational waves are produced by the motion of any massive object any object with mass they were predicted by einstein in 1916 and finally detected by lego almost a century later in 2015 and there have been more detection since then of course we are just getting started with this type of astronomy so we can only detect very massive objects colliding now but likely future improvements will continue this and allow for detection of less massive objects over time so that concludes our lecture on gravitational waves we'll be back again next time for another topic in astronomy so until then have a great day everyone and i will see you in class