Binary Neutron Stars

One amazing feature of general relativity is that extremely massive objects can inspiral into one another and collide. This is possible because of gravitational waves, a key prediction of Einstein's theory that has since been observed directly by detectors on Earth. When black holes collide, they can produce massive gravitational waves, but produce no light. Neutron Stars, on the other hand, potentially can produce extremely bright explosions; if we can see these mergers with both light and gravitational waves, the information we can gather is much more powerful than what we could learn from either by itself.

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The mergers of binary neutron stars are incredibly complicated, the extreme gravitational environment must be solved for at the same time as the properties of the extremely dense matter in the neutron stars. This is an extremely challenging computational tasks and simulating even a single second of a neutron star merger could take a thousand years on a laptop. That's why these things are usually done on supercomputers made up of thousands of individal CPUs. To make things even more complicated, we don't fully understand the behavior of matter inside of these simulations, because it's a hundred trillion times denser than water, and experiments with matter like this are also extremely difficult. Therefore we actually can use the mergers of neutron stars to learn about matter in the most extreme conditions. This may someday help to answer some of the most important questions of physics, such as how well our current theories explain how the universe works at a fundamnental level.