Sabine Hossenfelder has written an interesting post about which problems might be fruitful for research in the foundations of physics. Since our blog is about foundations of physics, let us comment on her program.
Lubos Motl wrote a harsh criticism of Hossenfelder.
The thesis of Hossenfelder is that when experiments conflict theoretical predictions, that is a fruitful experiment-led problem.
If theory itself is inconsistent, that makes a fruitful theory-led problem.
Hossenfelder analyzes 12 problems, if they are fruitful and if they are experiment-led or theory-led. Our blog has touched several of those problems. Let us go through the list of problems.
Dark matter
There is no principle in particle physics which prohibits weakly interacting particles. A priori, the existence of dark matter is more probable than its non-existence. The competing hypotheses, like MOND, suffer from the fact that it is hard to modify newtonian mechanics or general relativity without breaking conservation of energy, momentum, and angular momentum, or equivalence principles. We have not seen anyone developing a MOND model where conservation laws would hold.
Dark energy
Dark energy can be accommodated to general relativity through a cosmological constant. It does not break anything. But what is the origin of the cosmological constant? In our blog, we hold the view that empty space is truly empty - it does not contain energy or vacuum fluctuations. We would not explain dark energy by "vacuum energy" which is present in empty space. Dark energy might be an unknown force.
Hierarchy problem
Why is gravity much weaker than other forces? An anthropic argument is that a strong gravity would make everything collapse into black holes, and humans would not exist.
The weakness itself is not mysterious about gravity. But gravity does have mysterious properties: why does it affect all mass-energy, why does it appear to modify spacetime geometry, why the force is always attractive, and why the gravitating mass is equivalent to the inertial mass?
Grand unification
There is nothing in particle physics that requires the electroweak and strong interactions to be unified at high energies. However, the unification of electromagnetism with the weak interaction hints at that possibility.
Quantum gravity
One of the goals of our optical gravity model, and also our renormalization/regularization study, is to find a way to integrate gravity into ordinary quantum mechanics.
There are problems with the geometry of black holes in classical general relativity. We do not know if the Kerr solution is stable.
We do not know at what speed does information about mass-energy distribution spread in general relativity.
To build a model of quantum gravity, we need to clarify classical general relativity.
Black hole information loss
Our view is that Hawking radiation probably does not exist. Therefore, there is no information loss problem.
In quantum mechanics, systems develop in a unitary way and there is no information loss. The fact that the hypothetical Hawking radiation would break this principle is one of the symptoms which show that Hawking used flawed quantum field theory. Other symptoms include problems with energy conservation, momentum conservation, and the classical limit of his hypothesis.
We do not understand why some physicists hold a religious view that Hawking radiation "must" exist. The derivations of Hawking radiation rest on a very shaky, and probably flawed, use of quantum field theory.
Particle masses
There is no principle in particle physics that requires particle rest masses to have a deeper explanation. But there may exist a model, a string model, for example, which might cast more light on the problem.
Quantum field theory
Our hypothesis is that both the infrared and ultraviolet divergences of Feynman integrals are a result of a wrong integration order. We will study that hypothesis in spring 2019.
The Landau pole means that higher order Feynman diagrams will contribute more to the process than lower order diagrams. It is a complexity explosion. The energy is so high that a black hole would form before Landau pole energies are reached. The black hole may save us from a Landau pole.
The measurement problem
Our view is that the many worlds interpretation, where the "branch" for an observing "subject" is chosen with the Bohmian hidden variable method, is the most sensible interpretation of quantum mechanics.
It is not clear if we can ever devise experiments which would differentiate between interpretations. The problem may remain a philosophical one.
The flatness problem
If empty space is truly empty of energy, then flatness is expected.
In optical gravity, we have a hypothesis that the true geometry of spacetime is the flat Minkowskian geometry. But we would need a model to explain the Big Bang. If spacetime is flat, why does the universe appear to expand?
Magnetic monopoles
Some GUTs imply the existence of magnetic monopoles. However, in ordinary particle physics there is no principle that would dictate that they should exist.
A deeper understanding of quantum electrodynamics may resolve this problem. An electron is a source of the electric field. Why there is no source particle for the magnetic field?
Baryon asymmetry
We pointed out that if there are superheavy particles and antiparticles, then a single particle might decay into a whole visible universe which contains just matter. The asymmetry is probably just a local phenomenon.
Why is the cosmic microwave background so isotropic?
Why is the temperature so uniform in areas which are not causally connected in a standard Big Bang model? There may be unknown laws of physics which create a nearly uniform energy distribution in a phase change of the universe. There is no need for the patches to be causally connected if the same mechanism creates the mass-energy in each patch.
The inflation hypothesis explains the uniformity, but Paul Steinhardt has criticized it because it requires fine-tuning which may be even harder than the problem it tries to explain.
Another explanation would be a Big Bounce model. But we do not know laws which would cause the universe to contract after a Big Bang.
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