Leading modern theoretical physicists can tell you:
- The objective of quantum mechanics is to predict outcomes of (quantum mechanical) experiments.
You may find this a bit strange. Isn't the objective of theoretical physics to understand physical processes. In the case of quantum mechanics, that would be to understand the microscopic physics of atoms, electrons, protons, neutrons and more. But this is not possible in the case of quantum mechanics, since in the words of Richard Feynman:
- Nobody understands quantum mechanics.
So what is left is the to predict outcomes of experiments, which seems a bit like betting on a horse race. Or is the theoretical physicist simply mocking with you, to avoid further questions?
Ok, if the experiment agrees with the prediction, then you may view that as support to an idea that the theory is correct, in that specific case. But to confirm a theory by experiments requires massive experiments. It is not enough, in general, to make just one experiment and then say that the theory is correct. Maybe that experiment was very special?
In any case this practised a lot: To confirm Einstein's General Theory of Relativity GR, it was enough with one observation of a very slight change of the apparent position of a star during the solar eclipse on May 29 in 1919 (the perihelion shift of Mercury was not a prediction). For a second confirmation, we had to wait until the LIGO gravitational wave detection at 09.51 UTC on 14 September 2015 of two ~30 solar mass black holes merging about 1.3 billion light-years from Earth. But that was also a very special case. In any case, GR is now considered to be fully confirmed by two very special/extreme (and questionable) observations.
Similarly, the Standard Model of fundamental physics is supposed to now be fully confirmed by very special experiments at LHC completed in 2012 showing existence of the Higgs boson after a 40 year long fruitless search, as a little jump on an energy graph.
But if the experiment does not agree with theoretical prediction, what to do? Throw the theory away because it fails on one experiment? Maybe quite reasonable if the experiment is relevant. The alternative is to modify the theory by e g some new parameter to agree with the experiment, but then the prediction aspect is missing. Of course it is also possible to modify the experiment until agreement with theory, but again without true prediction.
To fully confirm a general theory by experiments is impossible. To confirm a general theory by very special experiments, as seems to be the current standard, is not very convincing to me.
Newton's theory of gravitation is a general theory, which is supported by a generality of experiments/observations and contradicted by none, and can be understood from conservation principles. It can serve as the role model for all of theoretical physics.
If the element of understanding is missing, then theoretical science seems to reduce to fitting theory to experiment or vice versa. Is this the reason for the current crisis of fundamental physics?
What then about GR? Again, we have a theory which is very difficult to understand. Einstein said he could not understand it, but how is it possible to formulate a theory without understanding it? Of course a modern physicists would proudly say that there are two main theories of modern physics, quantum mechanics and GR (which happen to be contradictory/incompatible), and then act as if he/she understand these theories quite well, if not in full detail, while showing no willingness to go into a discussion about specifics of the theories and referring instead to the very rich literature explaining GR written by people who really understand GR.
It means that a modern theoretical physicist will have to struggle with general theories, which cannot be understood nor confirmed by experiments, since only very special experiments are available, if any at all.
At the same time the modern theoretical physicist must give the impression of understanding and presence of confirmation.
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