Black holes are notable for several things, especially their simplicity. They’re just holes that are “black”. This simplicity allows us to draw surprising parallels between black holes & other branches of physics. For instance, a team of researchers has shown that a special particle can exist around a pair of black holes in a similar way as an electron can exist around a pair of hydrogen atoms, the first example of a “gravitational molecule“. This strange object may give us hints to the identity of dark matter & the ultimate nature of space-time.
Ploughing The Field
To understand, how the new research which was published in September to the preprint database arXiv, explains the existence of a gravitational molecule. We first need to explore one among the most fundamental and yet sadly almost never talked about, aspects of recent physics: the field.
A field is a mathematical tool that tells you what you possibly expect to seek out as you travel from place to place in the universe. For instance, if you have ever seen weather report of temperatures in your local area, you are looking at a viewer-friendly representation of a field: As you travel around your town or state, you’ll know what type of temperatures you’re likely to seek out & where ( whether you would like to bring a jacket).
This kind of field is referred as a “scalar field“ because “scalar“ is the fancy mathematical way of saying “just single number”. There are other forms of fields out there in physics-land like “vector fields” & “tensor fields” which give more than one number for every location in space-time. (e.g. if you see a map of wind speed & direction splashed on your screen, you are looking at a vector field.). But for the purposes of this research paper, we only got to know the scalar kind.
The Atomic Power Couple
In the heydays of mid-20th century, physicists took the concept of field which had been around for hundreds of years at that time and was absolutely old-hat to the mathematicians and visited town with it.
They realized that fields aren’t just handy mathematical gimmicks, they describe something super-fundamental about the inner workings of reality. They discovered basically that everything in the universe is really a field.
Take humble electron. We all know from quantum physics that it’s pretty tough to pin down exactly where an electron is at any given moment. When quantum physics first emerged, this was a reasonably nasty mess to know & untangle until the field came along.
In modern physics, we represent the electron as a field, a mathematical object that tells us where we’re likely to spot the electron the next time we glance. This field reacts to the world around it due to the electrical influence of a close-by atomic nucleus and modifies itself to vary where we need to see the electron.
The end result’s that electrons can appear only in certain regions around an atomic nucleus giving rise to the whole field of chemistry.
Black hole buddies
And now the black hole part. In atomic physics, you’ll completely describe a fundamental particle (like an electron) in terms of three numbers: its mass, its spin & its charge. And in gravitational physics, you’ll completely describe a region in terms of three numbers: its mass, its spin & its electron charge.
Coincidence? The jury’s out on that one but for the time, we will exploit that similarity to understand black holes.
In jargon-filled language of particle physics that we just explored, you’ll describe an atom as a small nucleus surrounded by the electron field. That electron field responds to the presence of the nucleus & allows the electron to seem only in certain regions. Same is true for electrons around 2 nuclei, for instance, during a diatomic molecule like hydrogen (H2).
Similarly, you can describe the environment of black hole. Imagine the small singularity at a black heart somewhat like the nucleus of an atom while the encompassing environment, a generic field is analogous to the one that describes an elementary particle. That field responds to the presence of the black hole & allows its corresponding particle to seem only in certain regions. And even as in diatomic molecules, you can describe scalar fields around two black holes like in a binary black hole system.
The authors of the study found that scalar fields can exist around binary black holes. What’s more, they can form themselves into certain patterns that resemble how electron-fields arrange themselves in molecules. So, the behavior of scalar fields therein scenario mimics how electrons behave in diatomic molecules, hence the moniker “gravitational molecules”.
Why the interest in scalar fields? Well for one, we don’t understand the character of dark matter or dark energy and it’s possible both dark energy & dark matter might be made from one or more scalar fields, a bit like electrons are made from the electron field.
If dark matter is indeed composed of some kind of field, then this result means that dark matter would exist in a very strange state around binary black holes, the mysterious dark particles would need to exist in very specific orbits, a bit like electrons do in atoms. But binary black holes don’t last forever; they emit gravitational radiation & eventually collide & coalesce into one black hole. These dark matter scalar fields would affect any gravitational waves emitted during such collisions because they might filter, deflect and reshape any waves passing through regions of increased dark matter density. This means that we’d be ready to detect this type of dark matter with enough sensitivity in existing gravitational wave detectors.