What is currently stopping us from having a theory of everything? i.e. what mathematical barriers, or others, are stopping us from unifying GR and QM? I have read that string theory is a means to unify both, so in this case, is it a lack of evidence stopping us, but is the theory mathematically sound?
Answer
One thing that stops us from having a theory of everything is actually quite simple. Gravity as we understand it, thanks to the strong equivalence principle, is not a force. It is entirely geometrizable because there is actually no coupling constant between a physical object and the "gravitational field".
This means that there is no a priori way to discriminate the action of "gravity" on different objects: it acts the same for everybody (obviously, I'm not speaking about the interaction of EM with gravity and stuff here).
On the contrary, quantum fields as we know them are defined on space-time, and therein exist coupling constants that tell you how the dynamics of an object are influenced by the value of the field on a given space-time point.
In this respect, one can easily see that the question "if usual fields with coupling constants happen on space-time, where does space-time interaction happen?" hardly makes sense. This shows that a theory of everything has to treat space-time as something else than just an usual quantum field.
Let's stick to Newtonian mechanics in order to understand what I mean by "no coupling constant". Let me remind you that in some inertial frame, the second law is $F = m_I a$, for some object of inertial mass $m_I$. Now, call $\phi(x,t)$ some potential. A physical object is said to interact with $\phi$ with a coupling constant $q_\phi$ if $F = - q_\phi \nabla \phi$.
Now, what happens if the quotient $m_I/q_\phi = G$ is the same constant for all physical objects? Newton's second law shows the acceleration of an object that interacts with such potential is the same for everyone, that is, $G a(t) = -\nabla \phi(x,t)$. This means that there's no way to discriminate physical objects by looking, only at how they interact with $\phi$. Hence, we are always free to follow a "generalized" strong equivalence principle, which would stipulate that to be inertial is to be in "free fall" in the potential $\phi$. This would lead us to a geometric formulation of $\phi$ as a metric theory of space-time. There is therefore no need to introduce a coupling constant $q_\phi$ and to see the $\phi$-interaction as a force. Now, notice that this is exactly what happens for gravitation.
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