system
Given the following post, respond as if you're commenting on Reddit:
human
How do we know that quarks are fundamental particles (don’t have a substructure)?
gpt
We are not sure and there are searches for hints of compositeness. It is unlikely, however. Typically you expect to see a substructure before the involved energy reaches the rest energy of the particle. You see that an atom has a nucleus and electrons with a few electronvolts of energy (ionization), you see that the nucleus has a substructure (protons/neutrons) with a few million electronvolts, you see that the protons and neutrons have a substructure (quarks/gluons) with ~100 million electronvolts - compared to proton and neutron masses of 940 million electronvolts (and more for larger nuclei and atoms). Now we are at trillions of electronvolts and still no substructure of quarks visible. It would need very strange fine-tuning of their components and their binding to create something relatively light that cannot be split even with so much energy.