There is an electrostatic repulsive force.
And if you were just to look at electrostatic repulsive force,
there would be sort of hyperbola going up to infinity particles at zero size.
And most particles cannot come close enough for
the nuclear forces that can grab them in and then they're purely attractive.
So once you get past that barrier, it's fine.
So if classical physics were working all the way down
to the nuclear world, this will never happen.
But there is a phenomenon of quantum tunneling that this is
intrinsic to quantum physics.
That sometimes particle can go through some barrier that normally wouldn't go,
that can tunnel through walls, so to speak.
This is really a tiny wall, and
three is probability it can be computed, how to do this.
And it's highest for the highest energy particles.
And so this very specifically quantum effect is what enables occasional
proton to actually pass through this electrostatic barrier and
fuse with whatever's inside the nucleus.