WIRED: Why is it easier to simulate a black hole than a star?
Miguel Alcubierre: It sounds paradoxical because one imagines that black holes are tremendously exotic things and the stars are there, like the Sun: we see them. But stars are very complicated objects. They have gases, plasmas, nuclear reactions, there is convection in the moving gas, there is radiation, photons are being created, the photons have to leave, nuclear reactions produce neutrinos, which are even more complex… A black hole, in By comparison, it is a very simple object. There is nothing there. The only thing there is is gravity. It is a very intense gravity field. There is no matter, there are no gases, there are no fluids, there are no neutrinos, there is no radiation, there is nothing. It’s just very intense gravity. In that sense, it is a much purer system. Therefore, they are easier to study. Of course there are certain complications… A black hole has in the center what we call the singularitywhere gravity is infinite. In a certain region of the black hole, not even light can escape and that causes mathematical problems… But despite these problems, it is still easier to model a black hole than a star. There is another joke that people who dedicate themselves to these things make. We say that people who study stars do not do “astrophysics”, but “gastrophysics”, because stars are as complex as if you were entering a stomach: there is chemistry, reactions and a lot of movement.
WIRED: How was it possible to simulate the collision of black holes?
Miguel Alcubierre: The model of the collision of two black holes was a problem that took us forty years to solve. Well, not me. I wasn’t born when they started to solve it. When two black holes are in orbit they gradually get closer because they lose energy. They get closer and eventually collide. When they collide, a very intense signal of gravitational waves is produced and that is what we wanted to detect. The detectors were finally built and managed to detect them in 2015. Before 2015, although we had not detected them, we knew that it was a major problem because, eventually, we were going to want to detect them. It was first attempted to be resolved in 1964, the year I was born. It is a problem so complicated that it cannot be solved with pencil and paper. So you have to solve it with simulations on a computer. It was tried in ’64, but imagine the computers of that time. The mathematics involved was not well understood either. It just didn’t work. It continued to be tried for decades. It could more or less be simulated when two black holes collide head-on, but that was not going to happen in nature (in reality they collide in a spiral). We moved forward little by little. We didn’t understand how to write such complicated equations so that the computer could solve them efficiently. Inside black holes, the gravitational field is infinite and this cannot be processed by the computer. You put an infinity into the computer and it just stops. So we had to do something to work with these infinities. I worked a lot on this, but the final contribution was not mine. When we were working on this, what we did, in practice, was consider that there was an infinity and factorize. That is, remove the infinity by hand and work it on the outside with other techniques. Finally, what was left over, what was finite, we left to the computer. We removed the infinities at specific points. Here’s one, here’s another, we remove them, and we evolve everything else. But those infinities were still there. Then we had to put them back because they were there. All of this resulted in the black holes being unable to move in the simulation. We had nailed them to the simulation. In 2005, someone had a great idea. What if we told the computer that this is not an infinity but an immense number and let it do what it wants? What happens if you don’t remove the infinities by hand to see what it does? And so it worked (series). Suddenly, the black holes in the simulation started spinning and everything worked. One of those times when you are smarter than you should be. We tried to be very smart thinking that the computer was not going to be able to handle infinity and someone simply decided not to remove it. The reason a posteriori is that, of course, nothing physically escapes from the black hole. Not even light escapes from a physical black hole and since everything travels slower than light, nothing escapes. But when you upload it on the computer, there is a side effect. In this almost infinite place, your errors in calculation are enormous, but those errors do not escape the black hole for the same reason that light does not escape. If you are simulating it well, this error, even though it is huge, does not come out, it does not affect what is happening outside the black hole. That was the great eureka moment: not even error escapes a black hole. I did not participate in that discovery, but I had participated in other things in previous years: writing the equations in a more adequate way, in finding how to establish more adequate coordinate systems, which were also necessary ingredients. But the last thing was missing. From there the problem was solved, ten years before gravitational waves could be detected in 2015. There were people who had been working on this for 40 years. I, in particular, had been working on this topic for 15 years. After so many years, seeing that something you have worked so hard for can be done is very exciting.
#Miguel #Alcubierre #science #honest #admit #didnt #work #dont #follow #path
