How can a theory describe everything? Surely we already have theories for most things? And anyway, what actually is everything?
In this post, we will be explaining the concept of a Theory of Everything. What does it consist of, and what are the main challenges surrounding the formation of one? In future posts, we will be looking at the main candidates which could, one day, become the Theory of Everything.
So lets start with the definition of a “Theory of Everything”. If you search it on dictionary.com, you get “a theory intended to show that the electroweak, strong, and gravitational forces are components of a single quantized force.” and Wikipedia says “A Theory of Everything (ToE) or final theory is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle.” So which is right? Well it turns out that both mean the same thing. There are only four basic interactions in the universe, these we briefly touched on in our post on the standard model particles. That means that everything that ever happens, is due to these forces. The reason you are pulled towards the floor is gravity, the reason you don’t actually sink through the floor is electromagnetism, the reason the atoms inside of you don’t break apart is the strong force, and, though slightly less relatable, the reason that we can carbon-date objects, or that the sun itself exists in the way it does, is the weak force. Therefore, the result of any experiment can, in theory, be predicted, if we take into account all four of these, as well as the properties of the particles involved. A ToE will unite all these interactions in a way that means that this sort of prediction can be made.
A youtuber called MinutePhysics, who does brilliant videos along the same lines as our posts, recently posted this video. We recommend checking out his channel.
So what are the fundamental interactions that need to be described? We have already covered Electromagnetism and the strong and weak force briefly, and a few weeks ago we posted two posts going into depth on gravity. So lets recap: There are four main forces, or interactions, which I listed above and they all do different things. The one that we can relate to the most is gravity. It is the force that pulls us to earth, and causes us to orbit the sun. Electromagnetism is what causes electricity, the grouping of atoms into molecules, so every material we see, touch and use every day, and also, why magnets work (credit to Steven Spencer on twitter for the link). The other two forces, the strong and weak force, are only applicable at the atomic level. The strong force holds protons (and the quarks inside of them) together in an atom, where their like charges should cause them to repel each other, and the weak force causes quarks to change flavour through the emitting and absorbing of W and Z bosons. If the references to particles are confusing, you may want to check back to our post “What are the subatomic particles?”.
It was found that, at high energy states, like right after the big bang, electromagnetism combines with the weak force, to make the electroweak force.
The electroweak force, and the strong force are described in a part of quantum mechanics called the Standard Model. They are described at a very small scale, using fields and carrier particles. However, problems arise when you try to integrate gravity into the Standard Model. At these small scales, gravity is so weak that it is practically irrelevant. Our most accurate theory of gravity is Einstein’s theory of relativity. However, Einstein describes gravity as twisted space-time, not as a field with a carrier particle. Therefore, for gravity to be united with the standard model, we need to radically re-think what gravity is. What we just talked about is shown in this diagram:
|Theory of Everything|
|Gravitation||Electronuclear force (GUT)|
|Strong force||Electroweak force|
As we can see, the next stage in finding the Theory of Everything is trying to unify the strong force with the electroweak force to create the electronuclear force. This would be a Grand Unified Theory (GUT), where all three interactions of the standard model merge. You can think of it as one step down from a ToE, and many of the theories that aim to lead to a ToE, would also lead to a GUT. The GUT would only set in at very high energies, around 16 GeV (Gigaelectronvolts). This is a lot larger than it is possible for us to reach at the moment, so a GUT remains theoretical at least for now.
Overall the main objective, at the moment, is to obtain a theory of gravity, that describes it in terms of quantum mechanics. However, with many new strains of particle physics emerging, like supersymmetry and dark matter, a completed ToE could be very different to what we think.
But there is more to this than just unifying the forces. Could we, in theory, if we had a TOE, unlimited time with unlimited computing power, create a model of the universe, from start to end? If every little interaction could be explained in one theory, why can’t the life of the universe, which can be broken down to a large number of these interactions, be described by this theory? If it was the case that everything was determined at the very beginning, does this mean that free will doesn’t exist? It is true that for every interaction between particles we would be able to predict the results, however, the problem arrises when we actually have to predict when these interactions would happen. For this we have to look at bit into quantum mechanics.
When we look at the movement of particles, we can only ever predict the probability of it being in one place. When an electron moves from one position to another, it does so in anyway that it possibly can. This means that whilst the electron is moving, it is everywhere in the whole universe at once. This is an incredibly foreign concept, but one that we have to accept. This means that we cannot predict the course the universe will take, but only the probability of each particle being in any position. This then leads onto free will, and there are two positions on this. Either we do not have free will and our actions, and the events around us occur because of these probabilities, or we do have free will. At our macro-scale, we are able to influence the probabilities of things going on around us. The philosophical debate about free will has raged for millennia, and it seems, will continue for many more, even once we have a Theory of Everything.
In a couple of weeks we will be talking about The Theory of Everything, but then considering the leading theories, and the problems they face, that could become the ToE.