Science moves ahead, not at a steady rate, but by leaps and bounds. The surges forward are usually because of contributions of scientists with the ability to think differently than everyone else.
So it was when Sir Isaac Newton first formulated his laws, including the law of gravity. This states that every mass exerts a gravitational pull on everything else. The greater the mass, the greater the gravitational pull. It is this pull that is responsible for the planets remaining in orbits around their suns, or moons staying in orbit around planets. Newton’s laws explained a great many observed phenomenon, while at the same time opening the door to more questions and curiosities.
Albert Einstein made some of the next very great leaps in scientific theory, when he formulated his famous theory of relativity. Keep in mind that the equation we all know is the end product, and not the mathematics that led up to it. It is in the mathematics that things started getting very interesting indeed.
Among other things, Einstein’s math meant that space and time were related rather than separate, and while it explained many things that Newton’s laws could not, it also predicted some things that many scientists simply were not ready to accept, including Einstein himself.
Among the most exotic were black holes.
The theory of relativity predicts that a star the size of ours is doomed to explode, with the core collapsing under the force of gravity to the size of the moon, and to become a white dwarf, while maintaining a gravity around it that isn’t much less than the mass of the sun as it is now. At that point, it stabilizes as inner nuclear forces balance the gravitational forces. A tablespoon full of this material would weigh thousands of tons.
The theory then predicts that a larger star, a couple times larger in mass than our sun, would have a gravitational force so strong that the nuclear forces that keep atoms apart would no longer be enough to prevent further collapse. As electrons and protons inside of the atoms were crushed together, they would cancel each other out, forming neutrons. A neutron star would be far more compact than the white dwarf our sun is destined to become. A star the mass of our sun that became a neutron star (this is used for illustration, as this is insufficient mass to create a neutron star) would be roughly the size of Manhattan. Astronomers have detected dozens of neutron stars, including one that is at the center of the Crab Nebula. A tablespoon full of neutron star material would weigh millions of tons.
Einstein’s law went further, though. It predicted that a massive star, on the order of 3 to 10 times the mass of our sun, not even the forces that kept the neutrons apart would be enough to prevent ultimate collapse. (Astonomers know of many stars that are up to 200 times the mass of our sun.) The star would continue to collapse until it was trillions of times smaller than the nucleus of an atom, yet they would still have the gravitational pull of the original massive star. This would be a singularity, the center of a black hole. Not only would normal physics not apply, the object would appear to vanish from our space-time.
Can such an object exist? Not only can they, they do. We have detected several objects including two at the heart of our own galaxy that can only be black holes. Many scientists propose that all galaxies have a black hole at the center. In any event, scientists have located several black holes.
This isn’t at all as easy as it sounds. The gravitational pull of a singularity is so strong that not even particles of light, called photons, can escape. This means that for all intents and purposes, a black hole cannot be seen. So how do we know that they are there? This is where it becomes even more interesting.
Anything caught within a certain distance of the singularity, called the Swartzchild radius and named after the scientist who formulated it, is drawn in to the black hole. As gases and dust spiral in, they move faster and faster, and emit energy as they do so. That extreme energy, in the form of very energetic gamma radiation, is measurable and detectable. At least at our level of understanding, there is nothing else that can cause this sort of a telltale radiation.
There are so many things, mostly theoretical, that are interesting about black holes; the spin, their properties, discoveries made, the breakdown of normal physics, and even wilder thoughts of what lies at the other side of a black hole, but those are best dealt with in more specific articles. Suffice to say that black holes are unusual and fascinating, and they give us the glimmer of even more exotic discoveries that lay ahead.