In the last piece we introduced the idea that gravity and light are two fundamental forces of nature. In general, these forces govern the interactions of the very large. In this piece we will discuss the strong and weak nuclear forces which govern the very small. We will also use these forces to explain a principle of complementarity in physics called wave particle duality.
The strong interaction
Let us begin with the humble atom. For those who do not know, atoms are the building blocks of matter and an atomic nucleus contains two types of blocks called nuclei. The first nucleon is the proton which has a positive charge, the second is the neutron which has no net charge and the final component of a complete atom is the electron which orbits the atomic nucleus much like a planet orbits a star. One might ask: if a neutron has no charge and a proton has a positive charge, how are they held together? The answer is the strong nuclear force; whose associated particle is conveniently named the gluon and causes the neutron and proton of an atomic nucleus to adhere to one another. The strong nuclear force is so named because it is the strongest force in the universe, orders of magnitude stronger than gravity but it only acts over a very small distance; about one attometer which is about one quadrillion times smaller than the diameter of a human hair, while in theory gravity has an infinite range.
A diagram of an atom of Helium
The weak interaction
So far we have identified that the nucleus of an atom contains both protons and neutrons. Now, what if I said that protons and neutrons are made of the same substance and that a proton can become a neutron and visa versa? Well, believe it or not it's true and we call this the weak interaction and it's responsible for radioactive decay and nuclear fission. There are numerous types of radiation in the universe, in particular the conversion of a neutron into a proton or visa versa is called beta decay and occurs by the exchange of bozons, which are the associated particles of the weak field, just as the gluon is the associated particle of the strong field. But curiously, radiation can come in either the form of a particle or in the form of a wave. Alpha decay (a type of radioactive decay) involves emission of an alpha particle. Beta decay (which we have already discussed) causes the emission of an electron, which can exist as either a wave or particle and finally gamma decay, which propagates as gamma radiation in the form of a wave.
A diagram of alpha, beta & gamma decay
Wave particle duality
This leads me to a discussion about complementarity. As we have seen from radioactive decay, matter can exist as a wave or a particle. In general we describe things in terms of its wave like properties or its particle like properties for the sake of convenience and it marks an inadequacy in the English language to describe the true nature of matter. It becomes necessary to introduce a new term to the problem of the nature of matter and that term is superposition. Superposition is a term which allows us to expand on the concept that something can exist as a wave or a particle by introducing what I can only describe as a riddle: For a binary operation in A and B, if the result is not A and not B, not both and not either, then what is it? This is the riddle of superposition and underpins the limitations of our understanding of the nature of our reality. Every observable in the universe is in a state of superposition (so it is not a wave, it is not a particle, it is not both and it is not neither). An observable will only leave superposition upon observation. The only means we have as humans to make observations is to shed light on the problem, literally and figuratively. So if there is no light falling on an object you wish to observe then objects are in a superposition of states and have no duty to remain bound in any particular position. This has many counterpoints with a theory called Heisenberg's uncertainty principle, where he states that an object has either known momentum or known position but never both simultaneously. Like a footballer, when the footballer is in motion his position is unknown but his momentum is well understood because he is in motion. When he stops running his position becomes certain again, but he's stopped moving so we have no idea what his momentum is. So what does this mean for us, the noble observer? Well it means that objects in the very room you sit in are stationary only because you can observe them to be. The act of observation keeps them from entering a superposition of states. Turn off the lights and they enter superposition, meaning they no longer have any obligation to remain in a fixed position because you have no means of observing otherwise. Verification of the concept of superposition was achieved with Young's double slit experiment in 1803, however intimations of this unusual effect come even earlier than this. I would like to finish with a quote from Werner Heisenberg.
"The two mental pictures which experiments lead us to form one of particles, one of waves are both incomplete and have only the validity of analogies which are accurate only in limiting cases [...] the apparent duality arises in the limitations of our language."
Gravity is one of the four fundamental forces of nature, it is responsible for holding galaxies together and for keeping our planet in a stable orbit around the sun, another such force is the electromagnetic force and governs the radiation of light in the cosmos. In this entry we will discuss the incongruence of gravity and its ability to distort electromagnetic fields, thus putting into question our perceptions of the universe as seen through Gravity's lens, a theory first proposed by Albert Einstein in 1915.
What is electromagnetism?
Electromagnetism has two components. The first is the magnetic force, which I'm sure you've encountered when trying to overcome the force of repulsion or attraction between two magnets. Second is the electric force, sometimes called voltage or more correctly electromotive force (EMF) and is responsible for making current flow through electrical circuitry and gives us such wonders as the electric kettle and the humble toaster.
So the electromagnetic force is the combination of electric and magnetic components and gives us a thing called light, which I'm sure you're all quite familiar with. The light in this room is in the visible part of electromagnetic spectrum but not all light is visible. Your mobile phone is emitting and absorbing light in the form of radio waves to provide you with good coverage, a microwave emits light to heat your food and an x ray machine shoots light at you so you can see your skeletal structure. In short, light is pretty incredible. What is even more incredible is that no matter how much energy a light source has, that is, no matter how much energy a packet of light has, it will always travel at the same speed. That's because light is our cosmic speed limit and only one method of exceeding this limit exists, called the Alcubierre drive, which would only be possible with the invention of a device capable of controlling gravity (plus it's not really faster than light, it's kind of cheating to be honest.) It works like a jet engine, contracting air in front of it and expanding air behind it to generate thrust. The only difference is the Alcubierre drive contracts and expands space time to generate thrust.
The components of the electromagnetic wave
So why is gravity so special?
Gravity is incredibly weak, how do I know that? Well, stand up. So then you might say "Well hang on, you just told me that gravity holds the cosmos together" and it does, because gravity depends entirely on mass. Anything that has a mass has gravity. The bigger the mass, the bigger it's gravity. What's peculiar about gravity is that it has no associated particle, for example in the electromagnetic spectrum the associated particle is the photon and as discussed it can come in a number of shapes and sizes, but gravity has no such particle. In fact of the four fundamental forces of nature, it is the only one without an associated particle, at least not one we have proven to exist. More questions arise when you think "well, if there was an associated particle, would it have a mass and thus be affected by the very force that governs it?" To that question I have no answer and is the reason gravity has remained an elusive and mysterious force in nature.
So what does gravity have to do with light?
To understand how light can be distorted by gravity we will use black hole theory. So, every mass that exists has gravity, that gravity distorts time space. To visualize this imagine you have a blanket held rigidly at four corners. This represents time space and a few pieces of fruit that represent mass in the universe. As you add fruit to the blanket the shape of the blanket begins to distort and the amount of distortion depends on the mass of the fruit, this is in essence what gravity is, the amount of distortion in the shape of time and space. So what is a black hole? Well, when a star of a sufficient size reaches the end of its life it usually undergoes a massive coronal mass ejection called a supernova and if it meets certain criteria it can implode on itself and start to form a very small, very dense ball that we call a singularity. This singularity has an infinite mass, and thus, creates an infinitely large distortion of time space called a black hole. So a black hole isn't actually a hole, it's an infinite bend in space and time. So what does that mean for our friend light? Well, nothing can escape the massive gravity created by a black hole, not even light, and thus black holes are almost entirely invisible. So let's think about something, if black holes can absorb light, and a black hole is in essence just a large mass, then how do we know that the light from a distant star is not in fact being bent out of shape by the presence of massive stars and planets before it reaches our eyes? This creates a massive uncertainty in the position of celestial objects. It's akin to walking through a mirror maze and colliding with all the walls before finally finding the path you were supposed to be following. Thus the term gravity's lens.
The effect of gravity on space time
The black hole camera
Another wonderful thing about black holes is that should a telescope survive the trip inside the event horizon of a black hole, all the light in the universe would be falling on the singularity and like a giant celestial lens, would allow the telescope to observe the entire universe. The only problem then would be getting that information back out of a black hole for interpretation, but as nothing can escape the massive gravity, the information would be trapped inside. Thus spawning an idea. If one could move a black hole to the site of another black hole thus creating a binary system, should one black hole have larger gravity than the other, quantum information could be extracted from the weaker black hole as it was consumed by the larger one. There are problems with this idea; as discussed the large mass creates a bend in space and time, so time would travel much more slowly near the black hole, meaning it would take thousands of years to record a few moments worth of data, not to mention moving black holes across the universe would be a logistical nightmare.
To conclude
I hope this piece (like all my work) has made you think about our perception of the world around us. I hope it puts more questions than answers into your head, I hope it provokes thought and I hope you have enjoyed reading it. Gravity and light are only two of the fundamental forces. There exist two more, the strong and weak nuclear forces. These govern the very very small, while the others govern the very very large. In my next piece I will talk about these other forces and their implications on the world around us. Until then, may the force be with you.
A video of me delivering this piece at Cave writings in the Ink Factory in Dublin City
