Jun 18, 2015
Our world is made up of atoms: i.e. nuclei and electrons held together by electromagnetism. At the same time, though not all particles have electric charge, they all have a gravitational “charge” (i.e. their mass). And gravity is universal and attractive. So it is only natural to ask: why don’t we observe any "atoms" bound by gravity? The answer is that gravity is weak: a small magnet can lift a nail against the gravitational pull of the entire earth. The weakness of gravity means that a gravitational “hydrogen atom” would have a radius larger than the size of the observable universe. Thus to make a gravitational atom, we need to look to places where gravity is strong—like around black holes!