Such an exciting day! I started writing, but one could go on for hours trying to provide some background. If you're interested in this sort of thing there are going to be many descriptions of gravitational waves and General Relativity. In a nutshell gravity is really the warping of spacetime by mass.1 The amount of warping is almost the same as Newtonian gravity and you get things like planets orbiting the Sun. It is also just a bit different in more extreme cases and today reported on one of those tests.
If you've fallen down a flight of stairs or run off a roof with bed sheet wings mistakenly hoping to fly, you may be left with the impression that gravity must be very strong. In the scheme of things it isn't. It just seems strong because your mass is quite a lot and the Earth's mass is huge. A very small electrical charge can easily lift a surprisingly large mass against gravity.
Einstein predicted that shaking (accelerating) a mass would produce gravity waves - compressions and expansions in space-time something like the compressions and expansions you get with sound. Everything with mass that accelerates in the universe produces them. When you dance with a partner, you are sending ripples in space-time out to the Universe at the speed of light. But since gravity is so weak he couldn't imagine anything that would make them large (loud) enough to be detectable. But that was before black holes were known and amazing advanced in instrumentation physics had been made.
Black holes pack a huge amount of mass into a small space. If two of them are orbiting - dancing if you will - with each other, the effect can be strong. Strong enough that the energy carried away by the waves changes their orbits. In the mid 60s people started looking - not so much thinking they'd find anything, but rather refining experimental apparatus in preparation when the art had advanced to the point where detection might occur.
This part of experimental physics - making incredibly fine and novel measurements - has had a profound impact on society. Most medical instrumental comes from it. GPS and other 'impossible' wacky schemes. Today it is impossible not to thing of the interferometer ..
Around 1890 Albert Michelson and Edward Morley floated a block of granite in a tub of mercury. They needed to damp out vibrations and that was a good, albeit not healthy, technique. They built an interferometer to measure of the speed of light at different points of our orbit around the Sun. They wanted to measure the speed of the light with the Earth going into the luminiferous aether that supposedly filled space. They found that the speed of light was the same in either direction paving the way for Einstein and special relatively. Interferometers are found in many experiments, but also in industry where sub-micron precision is required - *all* integrated circuits made it through a variety of machines that had to know where they were.
The NY Times has produced an excellent video presenting the generalities.
and this from Caltech
They're measuring modulated displacements of a mirror less than a millionth of a billionth of a millimeter. Over a thousand times smaller than the diameter of a proton. The mind wobbles. The finest measurements ever made.
To astrophysicists this goes way beyond confirming a prediction of general relativity. It opens up a new window on the Universe Gravity wave astronomy. I can think of a half dozen really interesting questions.2 Much more sensitive instruments will be required, but that can be done in space. In theory these instruments are simple and elegant. In practice the signal is tiny compared to the noise and isolating it pushes technology. That push has traditionally made physics worthwhile to the societies that fund it. While it is unlikely that a company can reap the benefits from doing fundamental research these days, a society that funds it creates, among other things, dramatic advances in technology and measurement that can ignite new ideas and businesses.
About thirty years ago Kip Thorne wrote this in a serious journal:
If cosmic gravitational waves can be detected and studied, they will create a revolution of our view of the universe comparable to or greater than that which resulted from the discovery of radio waves.
Nature has a way of surprising you when you look at her in a completely new way. About twenty five years ago the American Superconducting Supercollider was killed by Congress. Given the success of the Hubble Space Telescope there was a sense that a bit should be invested in astronomy. Astronomers proposed some large land-based telescopes. The astrophysics and particle physics community proposed two large interferometers to seriously go after gravity waves. The astronomers fought it tooth and nail. LIGO won out and we finally see the results.
Such a day of wonder
1 OK - mass-energy. It gets deep fast, but the fine bits are incredibly important.
2 for starters
do gravity waves travel at the speed of light (if they don't physics gets really strange)?
what is the internal structure of neutron stars?
what are the details of supernova explosions?
just what is the rate of the expansion of the universe?
and many others...