Fundamental physics appears to be entering a new phase. And this new phase has many physicists worried.
Human curiosity about natural phenomena has always exceeded what was merely necessary for survival. On one hand, this curiosity has led to the generation of elaborate mythologies and religions. On the other, it brought about the ascent of science.
The desire to explain the universe, and to make definitive predictions about cosmic phenomena on all scales, has been one of the key drivers of science since the pioneering thinking of Galileo, Descartes, and Newton. The achievements have been truly astounding.
In fact, we now have a verifiable "story" for the evolution of the universe from the time it was no more than a minute old to the present.
One of the main pillars on which scientists have always constructed their theories (the so-called "scientific method") was falsifiability—theories had to make clear predictions that could be directly tested by experiments or observations.
The hope has been, that we can find a theory that would explain not only all the forces that we observe in nature (gravity, electromagnetism, and two nuclear forces), but also all the values of all the "constants of nature" (such as, the relative strength of the forces, the ratio of the masses of the elementary sub-atomic particles, and so on).
In the past two decades, however, ideas started to emerge, that maybe some of the constants of nature are not truly "fundamental", but rather "accidental." In other words, instead of one universe, these theories propose, there is really a huge ensemble of universes (a "multiverse").
Those "accidental" constants take different random values in different members of the ensemble, and the value we happen to observe in our universe is simply one that is consistent with the fact that complexity and life has evolved. This means that there is no true physical explanation to the value of some constants.
The details of these models are unimportant. What is important is that all of these parallel universes may be directly unobservable. And this is what keeps many physicists up at night. A huge number of unobservable universes seems to depart from the scientific method into the realm pure metaphysics.
To some scientists, admitting that some parts of our universe do not have an explanation from first principles marks the end of fundamental science. Well, does it? I don't think so.
We cannot directly observe free quarks (the constituents of protons), and yet all physicists believe in their existence. Why? Because the theory of quarks (the "standard model") makes enough predictions that can be directly verified, so we accept all of its predictions.
For the idea of the multiverse and accidental constants to be accepted, it would also have to make a sufficient number of verifiable predictions in the observable universe. If it will not, it will remain as a mere speculation, not a theory. There is also still a chance that physicists will eventually succeed in formulating a theory that does not require the concept of accidental variables.
This will clearly be fantastic, and in the spirit of the historical development of science. But we do have to keep an open mind to the possibility that our dream of a "theory of everything" was based on a misunderstanding of what is truly fundamental. In that case, the multiverse may simply mark the beginning of a new and exciting era of scientific thinking, rather than the end of science.