A couple of weeks ago, after the release of the first image of a black hole, we asked our social media followers to submit their questions about the phenomenon. Here are some answers, supplied by members of the 91̽»¨ physics community.<\/p>\n
@sauteractual asked via Instagram, how frequently do black holes occur and do they ever occur within our own galaxy or just neighboring galaxies?<\/strong><\/p>\n Professor of physics and dean emeritus\u00a0Tom Helliwell answers:<\/p>\n There is a supermassive black hole at the center of our galaxy as well, with a mass of about 4 million times that of our Sun. This is a large mass, but not nearly so large as that of the recently imaged black hole at the center of M87, which has a mass of 6.5 billion solar masses.<\/p>\n Our central black hole, which is located in the direction of the constellation Sagittarius, is also not nearly as active as the one in M87, in that ours has less hot gas and fewer stars swirling around it. We believe there are also a very large number of black holes scattered through our Milky Way galaxy that originated from the cores of large stars which collapsed when the stars underwent supernova explosions. The outer portion of such a star blows up, forming a supernova that can be as bright as the rest of the galaxy put together, at least for a few days, after which it quickly dims. In the meanwhile, the core of such a star collapses, to become a so-called “neutron star\u201d if it is not too massive, or a black hole if it is sufficiently heavy.<\/p>\n We have not observed a supernova explosion in the Milky Way galaxy itself since the year 1604, but when our galaxy was younger, billions of years ago, there would have been more frequent supernova explosions, some of which undoubtedly led to black holes. These black holes would still be circulating around in our galaxy. There are likely thousands or even millions of such black holes in our galaxy, with masses of a few times that of our Sun; the number of them is unknown. In any case, it is extremely unlikely that our solar system will meet up with one in the foreseeable future, at least we can sincerely hope so.<\/p>\n Another Instagram follower, @Ptowncracker, asks these questions answered by physics professor Brian Shuve<\/a>:<\/strong><\/p>\n Is there a way around it to possibly look at the other side?<\/strong><\/p>\n You can certainly see \u201caround\u201d the black hole\u2014any light that passes directly through the black hole will get stuck in it and won\u2019t come out, but light from sources behind the black hole might be bent around the black hole due to the curvature of spacetime: This is an effect called gravitational lensing. The actual \u201chole\u201d on M87 is relatively small compared to the size of the galaxy, so we can see most objects behind the black hole.<\/p>\n What happens when you go into the black hole?<\/strong><\/p>\n The point of no return is called the \u201cevent horizon\u201d\u2014any light, or other matter, that gets closer than the event horizon can\u2019t come out again. However, according to general relativity, nothing special actually happens at the event horizon\u2014if you were to pass through, you wouldn\u2019t necessarily know you had gone through. This is because gravity is a manifestation of curved space, but if you zoom into a curved space, it appears to be flat. For example, if you look out your window, you might think the Earth is flat because you are so close (and small compared to the size of the planet) that it looks flat, even though if you zoom out enough you can see that it is curved. Your body is small compared to the typical curvature of space, and so if you are freely falling you don\u2019t feel anything different.<\/p>\n As you get closer to the center of the black hole, however, the space becomes so curved that the space near your feet is curved differently from the space near your head. This leads to what are called \u201ctidal forces\u201d (because they are responsible for the ocean\u2019s tides, among other things) that will essentially rip you apart. Your body would be elongated until your tissues are no longer able to keep your legs connected to your body. Gruesome perhaps, but this only happens once you get very close to the black hole center.<\/p>\n What is on the other side? Where does the hole go\u2014to another dimension?<\/strong><\/p>\n The answer is: We don\u2019t quite know. According to the theory of general relativity, there is no \u201cother\u201d side\u2014things just get crushed at the center of the black hole. However, the curvature of space time at the center of the black hole is technically infinite, and the underlying assumptions of our theory break down at this point. It means it\u2019s possible that something very different happens near the very center than what I\u2019ve just described. What we do know (and that this photo confirms) is that nothing comes out of the black hole, so whatever happens to objects once they cross the event horizon, there\u2019s no way for us to know what happens for sure!<\/p>\n