Eventually, we are going to reach our limits in particle physics, or maybe find the theory of everything. Particle physics isn't easy by any means. It requires at the minimum years of graduate school and more to master. The main motivation of learning is to come up with original research later. But if there is no more research to do in the future, hardly anyone would be motivated to learn particle physics given its intrinsic difficulty. What would happen then? Would knowledge of particle physics disappear because no one would be around to remember it? Or would it become a scholastic tradition passed down dogmatically from generation to generation.
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There are huge challenges left in particle physics. There are number of free parameters which are put by hand to make proper predictions. Physicists will continue to struggle to find a theory which will predict those parameters. There are other compelling reasons to consider that there must be some post standard model physics. There is still the problem of a completely satisfactory quantum theory of gravity.
String theory is one answer which is an work in progress. It has not predicted those parameters from the theory till now but who knows what surprises may come in this journey.
Huge fields of study in particle physics lay before the physicists and the investigation is by no means over.
As someone says "Confusion and disorientation reign…. Very likely we are still confused beginners with very wrong mental pictues, and ultimate reality remains far beyond our grasp… The more we discover, the less we seem to know. That’s physics in a nutshell...".
A view of history helps:
I will repeat the trite that knowledge is a circle with an expanding radius. Within the circle is the known, without is the unknown. The more the known is enlarged the greater the contact with unknowns.
Now specifically for particle physics, maybe it will go the way of thermodynamics, or electromagnetism :) . These disciplines are still being studied by physicists because new frontiers also open new questions in old disciplines.
Particle physics is no harder than other physics branches when one tries to describe and learn as much as possible. Now suppose that strings do turn up to be the theory of everything. Have you looked at what this everything is? at least six more dimension and God knows what this means as far as our reality goes. Rather future generations of physicists will try to see what this means.
In addition the tools available to physicists for calculations and experiments could not even be dreamed about during Lord Kelvin's time. Why should we be able to dream successfully about how physics and experiments will be 100 years hence?