In the recent years, the Holy Grail of modern physics has been, the formulation of one master theory that is capable of describing every other theory ever described. It's like the ultimate general theory of the universe, which can be applied to every scenario we can ever dream of, and whose predictions would sufficiently match the observed phenomena.
This is, to state broadly and roughly, the Theory of Everything.
Why is such a theory needed?
If you're into philosophy, you have most certainly read about Montaigne, whose famous argument regarding subjective or scenario-specific theories, as opposed to perfect generalisations, is infamous. Montaigne argued that, instead of trying to find theories that apply to every scenario and situation, we should rather focus on building smaller, more focused theories that apply to specific situations only.
However, when it comes to physics, such an argument doesn't apply, very well. This is because of the sheer anomalies our current theories face, while trying to solve certain problems.
We have, usually speaking, two broad theories which explain pretty much everything, namely:
1. The General Theory of Relativity
and
2. Quantum Mechanics (in the broader sense)
The former applies to the universe at the macroscopic level, where we do calculations on planets, stars and whole galaxies, and even the universe as a whole! The latter is more microscopic, i.e it revolves around the atomic and sub-atomic levels of reality.
It is known that the Newtonian mechanics, that we still use for terrestrial calculations, can actually be derived from both these theories. Thus, it forms a kind of neutral zone between the two giant theories.
Anomalies: General Relativity vs Quantum Mechanics
The problem arises where, to paraphrase Brian Greene, we try to solve problems involving bodies which lies in the domains of both the theories. For example, a black hole is supposedly a large mass of matter concentrated within a very small volume.So, both general relativity and quantum mechanics can claim to solve it, since the black hole is simultaneously very small and very massive.
The problem arises when we apply both these theories. Neither yields answers that matches the observed phenomena, and there's no way both these theories can be correct.
Therefore, one of these theories must be more **wrong* than the other, and both these theories must be the approximations of *effective theories* of some underlying theory*. This supposed theory, that unites general relativity with quantum physics (as one of its unique features), is commonly called the _Theory of Everything*.
The grand unified theory
As we all know, there are currently 4 distinct forces in nature, which shape everything in the universe. There's a fifth one that's also hypothesized, namely an antigravitational interaction, but for our basic purpose, the first four would serve here.
These forces shape and control everything, and they are currently the components of the standard model of particle physics. These forces are:
1. Strong nuclear interaction, the strongest of the forces.
2. Electromagnetic interaction
3. Weak nuclear interaction
4. Gravity
The problem arises when we try to unite all these forces. The primary proposition of the GUTs (Grand Unified Theories) is that, these forces must be the broken forms of some more primary force. The current GUTs try to unite the first three forces into a single force.
This gives us another way of constructing a theory of everything, by building a framework that unifies all the forces. The unknown superforce which would be the combination and source of all the four forces, would perhaps have been the force that shaped the early universe, at least before T + 10^(-43) seconds (the current threshold of our knowledge). It's also intricately linked to the Einsteinian concept ofSingularity.
Current contestants for the Theory of Everything title
Currently, these are two major theories, which attempt to relate relativity and quantum physics, and provide a quantum theory of gravity. These are:
1. The M Theory (higher form of the Superstring theory)
2. Loop Quantum Gravity
3. Causal Sets proposition
M Theory revolves around the concept of strings, which are actually vibrating filaments of energy, which, like strands, give rise to all particles that we see in thestandard model.
On the other hand, LQG primarily proposes that space is granular, and not continuous. This is similar to the quantization of energy in Planck's theory, which allows only certainty definite values of the energy that can be released or absorbed by an excited atom.
The third title, Causal Sets proposition, is an indirect theory that has been developed very recently. Instead of directly relating quantum physics (mainly the standard model) and general relativity, this new theory attempts to build a quantum theory of gravity, from which general relativity can be then worked out (we like this approach very much, since it attempts to do something fascinating, by making use of something so basic as the set theory to attempt to explain something so grand).
Final words
Currently, no single theory has been able to achieve either of the goals. And as such, there's no ToE as of now. However, we're confident that by mid-century (to paraphrase Michio Kaku mentions in his book Physics of the future), this dilemma would be resolved.
References
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.59.521 (Spacetime as a causal set)
The elegant universe (Brian Greene)
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