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Black holes are remarkable for a lot of things, especially their simplicity. These are just … holes. They are “blacks”. This simplicity allows us to draw surprising parallels between black holes and other branches of physics. For example, a team of researchers showed that a special type of particle can exist around a pair of black holes in the same way that an electron can exist around a pair of hydrogen atoms – the first example of a “gravitational molecule”. This strange object can give us clues to the identity of black matter and the ultimate nature of space-time.
Plow the field
To understand how the new research, which was published in September in the Pre-Print Database arXiv, explains the existence of a gravitational molecule, we must first explore one of the most fundamental – and unfortunately almost never mentioned – aspects of modern physics: the field.
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A field is a mathematical tool that tells you what you might expect to find while traveling from one place to another in the universe. For example, if you’ve ever seen a televised weather report on temperatures in your area, you’ll watch a friendly depiction of a field: as you move around your city or state, you’ll know what type of temperatures you are. likely to find, and where (and whether you should bring a jacket).
This type of field is known as a “scalar” field because “scalar” is the sophisticated mathematical way of saying “just a single number”. There are other types of fields in physics, such as “vector” fields and “tensor” fields, which provide more than one number for each location in spacetime. (For example, if you see a map of splashed wind speed and direction on your screen, you are looking at a vector field.) But for the purposes of this research paper, we only need to know the scalar type.
The atomic power couple
In the heyday of the mid-twentieth century, physicists adopted the concept of the field – which had been around for centuries by then and was absolutely obsolete for mathematicians – and went to town with it.
They realized that fields aren’t just handy math gadgets – they actually describe something super-fundamental about the inner workings of reality. They discovered, basically, that everything in the universe is really a field.
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Take the humble electron. We know from quantum mechanics that it’s pretty difficult to determine exactly where an electron is at any given time. When quantum mechanics first appeared, it was a pretty nasty mess to figure out and unravel, until the realm came along.
In modern physics, we represent the electron as a field – a mathematical object that tells us where we are likely to spot the electron the next time we look. This field reacts to the world around it – say, because of the electrical influence of a nearby atomic nucleus – and changes to change where we should see the electron.
The end result is that electrons can only appear in certain regions around an atomic nucleus, giving rise to the whole field of chemistry (I’m simplifying it a bit, but you see my point).
Black hole buddies
And now the black hole part. In atomic physics, you can completely describe a elementary particle (like an electron) in terms of three numbers: its mass, its spin, and its electric charge. And in gravitational physics, we can completely describe a black hole in three numbers: its mass, its spin and its electronic charge.
Coincidence? The jury is out on that one, but for now, we can exploit that similarity to better understand black holes.
In the jargon of particle physics we just explored, you can describe a atom like a small nucleus surrounded by the electron field. This electronic field responds to the presence of the nucleus and only allows the electron to appear in certain regions. The same goes for electrons around two nuclei, for example in a diatomic molecule like hydrogen (H2.)
You can describe the environment of a black hole in the same way. Imagine the tiny singularity of a black heart that looks a bit like the nucleus of an atom, while the surrounding environment – a generic scalar field – is similar to that which describes a subatomic particle. This scalar field responds to the presence of the black hole and allows its corresponding particle to appear only in certain regions. And just like in diatomic molecules, you can also describe scalar fields around two black holes, like in a binary system of black holes.
The study’s authors found that scalar fields can indeed exist around binary black holes. Additionally, they can form in certain patterns that resemble the way electron fields organize themselves into molecules. So, the behavior of scalar fields in this scenario mimics the way electrons behave in diatomic molecules, hence the nickname “gravitational molecules”.
Why the interest in scalar fields? Well on the one hand, we don’t understand the nature of dark matter or dark energy, and it is possible both dark energy and dark matter could consist of one or more scalar fields), just as electrons are made up of the electronic field.
If dark matter is indeed composed of some sort of scalar field, then this result means that dark matter would exist in a very strange state around binary black holes – the mysterious dark particles should exist in very specific orbits, just like electrons do so in atoms. But binary black holes don’t last forever; they emit gravitational radiation and eventually collide and merge into a single black hole. These dark matter scalar fields would affect any gravitational waves emitted in such collisions, as they would filter, deflect, and reshape waves passing through regions of increased dark matter density. This means that we might be able to detect this kind of dark matter with sufficient sensitivity in existing gravitational wave detectors.
In short: we may soon be able to confirm the existence of gravitational molecules, and through this open a window to the hidden dark sector of our cosmos.
Originally posted on Live Science.
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