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Note: This is the last article in the three-article series on particle and fundamental physics. Here are the first and second articles.
While the Standard Model is a hugely successful theory, it is not the last word on the matter. There is much to be explained, much more to be discovered, which currently is not within the scope of explanation of this theory. Explaining them would take us closer to an actual Theory of Everything – a theory which can explain all the occurrences of the Universe.
This sounds very far-reaching, so let us be clear what a Theory of Everything is. It is not something that can predict everything that can happen, down to the minute level. Nor is it a way to develop explanations for every possible physical process. It is simply a way to understand the rules, if you could call them so, on which this Universe operates.
If this is disappointing to know that a Theory of Everything cannot do so much, consider an example. The rules of any game, for example cricket, are the same everywhere. They are usually pretty easy to remember and explain all we need to know about the game. This does not mean that we can predict the outcome of a game beforehand. Nor can we know why a certain player has a good day sometimes and a bad day on others.
A Theory of Everything is similar. It explains the rules, not the outcomes of the Universe. And even when it explains the rules, it only lays down the bare essentials, not all possibilities. Let us use cricket as an example again. The rule for an LBW dismissal is the same everywhere. But the umpire’s choice matters too, and makes a difference.
This article will delve into some areas where the Standard Model falls short or fails to explain, and then will progress towards how a Theory of Everything could take shape.
Index
There are some physical phenomena which the Standard Model fails to explain, despite some of the occurrences having a lot of support experimentally. Here are the most prominent ones.
As we’ve seen in the introduction to the standard model, there are many quantities such as mass, charge, etc. associated with every particle and so on. Several have definite values that can only be known from experiments. The Standard Model fails to offer any reason for these specific values or at least a way to calculate them without resorting to experiments.
This makes the Standard Model seem incomplete in this sense, that experiments are the only way to know certain facts about the universe.
The force mediating particle for the gravity is the hypothetical graviton, which has not been experimentally observed so far. This in itself is not too serious a problem as the weakness of gravity means that detecting such a problem will require sensors much more sensitive than the ones we have now.
A more serious problem is that General Relativity (which deals with gravity) and the quantum mechanical applications of the Standard Model give different answers in some situations. This is troublesome because both these theories are independently successful in their respective areas, but fail to agree in their common ground. Which is wrong, and why? Or do both need modifications? History might suggest the latter.
As we have seen in previous articles in this series, every particle in the Standard Model has its corresponding antiparticle. This made antimatter sound like a real possibility. Yet, we rarely observe or hear of antimatter in nature, and most of it is lab-made.
From a living perspective, this is in fact quite lucky, because matter and antimatter react very dangerously and explode into pure energy if they come into contact. This process is called annihilation and is currently uncontrollable and very hazardous. This would be deadly to all life because everything would end up getting converted to energy. There would be no matter, no “stuff” to make things from.
That still doesn’t answer the original question – why isn’t there an abundance of matter over antimatter? From a theoretical view, equal amounts of matter and antimatter should have come into existence with the Big Bang. Yet, there is an overwhelming amount of matter in the Universe, and trace amounts of antimatter. It is almost like a painter mixes equal amounts of black and white paints, and instead of getting grey, ends up with a nearly white paint.
Sometime after the Big Bang, there must have been a shifting of the balance in favor of matter. It is also possible that our understanding of the Big Bang is flawed. Either way, there is no clear final word on the matter – pun intended.
Antimatter, though somewhat of a mystery, is still part of the Standard Model. Dark matter is something else entirely.
Logically, to observe something, the observer has to interact in some way with whatever they are observing. When it comes to interactions in physics, there are four fundamental ones: electromagnetic, strong, weak and gravitational interactions. The case with dark matter is that hardly interacts via any of the forces. The term “dark” refers to the fact that it doesn’t interact with photons (light particles) and hence cannot be seen. Since photons deal with electromagnetic forces, this rules out any such interaction between dark matter and matter.
In fact, it is strongly suggested that dark matter doesn’t interact with the strong force, and possibly even the weak force either. So what does it interact with, and why bother?
Well, the only possibility left is the weakest of the fundamental forces, gravity. And this is what gives conclusive evidence for the existence of dark matter. While there are many observations that suggest it, the simplest one is the process of galaxy formation.
Galaxies are basically collections of stars that need gravity to form. If most the universe was made mostly of normal matter, galaxies as we know them simply couldn’t exist. Only the most gigantic galaxies and stars would have enough mass to clump together by the effects of gravity. And the remaining lighter ones would float away, or not even form in the first place. For example, according to an estimate, stars would have to be around 10 times as heavy as the sun on average, in a Universe without dark matter, for the usual processes of galaxy formation and so on to occur. Yet, most stars in our Universe are actually only 40% or so of the mass of the sun on average.
If you were a scientist, these observations would feel something like this. Think of a room full of light ping pong balls. You would expect them to bounce around a lot and not really stick together. But in some places, they actually do stick together, because there is some transparent glue holding them in place. Frustratingly, you cannot see this glue, or interact with it – it sticks only to the ping pong balls, nothing else. You know the glue exists, but there is nothing you can do right now to study the glue or how it works.
This is just the real-life case – there doesn’t seem any other obvious way, except gravity, to detect dark matter – as we’ve seen, it barely interacts with any of the forces. Despite this, it is something that literally holds the Universe together. Solving this mystery would take us a step closer to understanding our Universe better.
If dark matter holds much of the Universe’s contents together, dark energy keeps the universe expanding, at an increasing rate. This discovery initially shocked researchers, because everyone expected the universe to be slowing down in its expansion. The Universe is basically a balloon that gets easier to fill, instead of harder, as it grows larger. Ridiculous though this sounds, the fact is established experimentally. Theoretically, though, there is no firm explanation about why dark energy exists or what its nature is. All that physicists have now is the mathematics behind dark energy, but there isn’t a way it fits into the Standard Model right now.
While on the topics of dark matter and dark energy, it is interesting to note that only 5% of the Universe’s energy content comes from normal matter – the sort that is explained by the Standard Model. 27% or so is dark matter, and the rest, astoundingly 68%, is dark energy. It is humbling to know that the Standard Model explains only 5% of the Universe’s energy content. The rest, a whopping 95%, is yet unknown or unexplained.
Now that we have seen some, not all, of the Standard Model’s limitations, it is natural to wonder where the road ahead leads. While we yet cannot suggest solutions to the problems above, we can roughly chart out what we might expect from a Theory of Everything.
The path to the Standard Model was highlighted by unification. This was initially seen in the unification of electricity and magnetism, in a combined field called electromagnetism, in the 19th century. Next, this was succeeded by the unification of optics and electromagnetism when it was understood that light was simply electromagnetic radiation. More recently in the 20th century, the electromagnetic and weak forces were explained as aspects of a single electroweak force.
A trend which we observe is that each step of unification requires higher and higher levels of energy, at which point the various forces can be explained as parts of one whole. As a bizarre analogy, consider an alien simply observing our world. They might see many cities, full of confusing structures. If they use some energy and roam the cities, they will see that all cities consist of common elements – roads and buildings. To understand better, they use more energy and break the roads and buildings. It makes them see that both have some materials in common – say concrete. For increasing their understanding, they had to increase the energy they put into their observations. They were rewarded with a kind of unification – they first observed that cities have common elements, namely roads and buildings. On the next level of energy, they noticed that roads and buildings share common materials such as concrete.
Right now, the most pressing unifications required are those between the electroweak force and the strong force, followed by gravity. While physicists have estimates for the energy levels required for such a unification to happen, these are currently well beyond the reach of current technological development. The hope is that the theory is well established first before we could perform confirming experiments.
We have seen quite a few unexplained phenomena in the beginning of this article. Any successful Theory of Everything must provide reasonable explanations for these that are consistent with the rest of the theory. This does mean that there could be a significant waiting period, for the sake of definitive experimental data to support these phenomena and guide us towards a better understanding of the said occurrences.
There are several candidates for the claim of the Theory of Everything. String theory and M-theory, two very advanced theories, are the most prominent amongst them. However, both of these have many predictions that are yet to be verified experimentally.
Scientists choosing this field have an interesting challenge to face: they must provide theoretical explanations for a lot of facts that have not much experimental evidence in the present, or in the near future. Furthermore, they have to live with the knowledge that many scientists before them, even Einstein, have spent their lives trying to achieve the same results, and yet failed. There are some in the scientific community who reasonably suggest that a Theory of Everything is even impossible or at least not understandable for humans.
This might sound incredibly uninspiring, yet the harshness of the challenge is what drives many-particle and theoretical physicists to this field. After all, it is part of human curiosity to try and understand our Universe. The Theory of Everything would be the last word on the matter. Everything to know about the rules of the Universe, summed up in one single explanation. It is the beauty of such an explanation that spurs us forwards.
Whether or not we get there anytime soon is unknown. What remains for sure is that, for those dedicated to it, this is a wonderfully rewarding field of study. And if we actually arrive at a solution, it would probably rank as one of humanity’s greatest scientific achievements. Not the end, yes, but a great milestone in the march of science.
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