felix.speagel said:
You are right, but don't forget one thing: in the formula for the force of gravity there is also a mass. As a ball approaches the middle point the mass that's attracting the ball also drops to zero. In the middle point there is simply nothing that makes a gravity field.

Then the mass cancels itself out, because it ‘comes form all directions’?

taby said:
Actually, gravitation inside of planet is measured by multiplying by r, not dividing by r squared. https://profoundphysics.com/gravity-underground/​ It goes to zero because there is a balance in the forces.

So yes.
But then my hack is not needed. I can prevent explosions using correct physics:

		static bool testSimplePlanets = 0; ImGui::Checkbox("testSimplePlanets", &testSimplePlanets);
		if (testSimplePlanets)
		{
			constexpr int count = 10;
			struct Planet
			{
				sVec3 com = sVec3(0);
				sVec3 vel = sVec3(0);
				sVec3 force = sVec3(0);
				float mass = 1;
				float radius = 1;
			};
			
			static Planet planets[count];

			auto CalcPlanetRadius = [](const float mass, const float density = 1.f)
			{
				float vol = mass * density;
				float radius = pow (vol * 3.f / (4.f * float(PI)), 1.f/3.f);
				return radius;
			};

			// generate random planets (this code runs only once)
			static bool init = true;
			if (init || ImGui::Button("Reset Simulation"))
			{
				int k = 0;
				auto rnd = [&](float vMin, float vMax)
				{
					float f = PRN::randF(k++);
					return (f * vMax-vMin) + vMin;
				};

				for (int i=0; i<count; i++)
				{
					planets[i].com = sVec3(rnd(-1,1), rnd(-1,1), rnd(-1,1));
					planets[i].vel = sVec3(0);//sVec3(rnd(-1,1), rnd(-1,1), rnd(-1,1)) * 0.001f;//
					planets[i].force = sVec3(0);
					planets[i].mass = rnd(0.001f, 0.001f);
					planets[i].radius = CalcPlanetRadius(planets[i].mass);
				}

				init = false;
			}


			// function to update forces of a pair of planets
			auto GravityPair = [&](Planet &bodyI, Planet &bodyJ)
			{
				const float G = 1.f; // some number to relate your mass metric to gravity. i just assume we use a mass unit so G becoems 1.

				sVec3 diff = bodyJ.com - bodyI.com;

				float dist = diff.Length();
				
				if (dist > 0.f)
				{
					// simple model - too lazy to read up details from https://profoundphysics.com/gravity-underground/
					float massI = bodyI.mass;
					if (dist < bodyI.radius) massI *= dist / bodyI.radius;
					float massJ = bodyJ.mass;
					if (dist < bodyJ.radius) massJ *= dist / bodyJ.radius;


					float force = G * (massI * massJ) / (dist * dist);

					sVec3 direction = diff / dist;
					bodyI.force += direction * force;
					bodyJ.force -= direction * force;
				}
			};

			static float timestep = 0.001f; 
			ImGui::SliderFloat("timestep", &timestep, 0.0002f, 0.005f, "%.5f");


			// update (this code runs once every frame)

			// accumulate forces
			for (int i=0; i<count-1; i++)
			for (int j=i+1; j<count; j++)
				GravityPair(planets[i], planets[j]); 

			// integrate 
			for (int i=0; i<count; i++)
			{
				Planet &p = planets[i];

				sVec3 acc = p.force / p.mass;
				p.vel += acc * timestep;
				p.com += p.vel * timestep;

				p.force = sVec3(0); // reset accumulation
			}

			// render
			for (int i=0; i<count; i++)
			{
				const Planet &p = planets[i];
				RenderCircle (p.radius, p.com, sVec3(0), 1,1,1);
			}
		}

It works. Simulation is stable.

Thanks guys. My understanding of the universe has increased. : )

I think that it's important to reiterate that the shell theorem states that the net acceleration is zero inside of a shell.

The key word is net.

There is still gravitational time dilation!

Can someone go on to Physics Stack exchange and ask what the time dilation is inside of a shell. I am banned lol

I did read about that, and discovered the Schwarzschild radius, which also defines the event horizon of black holes.

It follows, you have to calculate the square root of a negative number to calculate time dilation below that.
Which means, there is an imaginary white space of increasing equilibrium, peace and harmony towards the center, where Jimi still rocks forever. <: )

Ha - he even just clamps the distance. What an amateur… knowing nothing about the universe. \:D/

I see you have borrowed his hacks.

That's fine, but now we need more particles and dimensions… ; )