The simulation is quite bad numerically. If I start with a proto disk and then put an non-moving OMFG particle immediately to the left of it, the whole system that is initially at rest will end a with a considerable momentum to the left, even though no or almost no particles carry any momentum to the right. That is not only not to physical scale, generating momentum out of nothing is completely unphysical.
It says it uses Euler integration, what do you expect? At least the really bad interactions are mostly handled by absorbing the planets into one another, even though not all of them are.
Nice observation. The inaccuracy is greatest when masses are near, so absorbing close masses limits how near they can get. When I wrote one of these simulators, I had masses zinging off everywhere when they "collided".
Question: does this inaccuracy systemically affect the development of a "solar system" from a "proto disk"? I think, by increasing randomness, it would actually tend to work against it somewhat.
Also, the odd effect of the proto disk expanding after the sun forms is caused (I think) by masses falling into the sun, and thus further away from the rim of the disk. This greater distances reduces the gravitational force on the rim, and so their velocity ("centrifical" force) can carry them away.
It looks right in my tests, the particles accelerate towards the OMFG with a large proportion hitting the OMFG in what looks like a perfect inelastic collision, that is they merge and the velocity and mass is added to OMFG. This creates a movement to the left of the OMFG particle.
Deleted my other comment as you replied sorry, it is 12 am and I am not explaining myself to my satisfaction.
As I said in my reply to your deleted comment, this is plain wrong. Momentum is conserved. Unlike kinetic energy (which gets transformed into heat), it is even conserved in inelastic collisions.
Take two masses A and B, place them at rest near each other. They should collide and be at rest at some point between them. However, based on the inaccuracy in the way the simulation works if one of those particles is really huge they sometimes gain a lot of energy and fly past each other.
It's hard to tell if momentum is being conserved or not with so many particle, but in the example you mention there is one particle moving to the right, which is the OMFG.
So to make the test simpler, let's start with a single particle. This particle is initially at rest, just as with the protodisk, and nothing happens if we wait for a while. Adding in another particle of the same mass, we see the two particles move towards each other and meet in the middle, implying momentum is conserved. So I am thinking momentum is probably conserved for all cases, to some degree of accuracy
No, momentum is conserved, that is one of the fundamental laws of physics, right up there with the conservation of energy. In the beginning, the OMFG particle is at rest, and the protodisk particles orbit symmetrically around the center of the protodisk, for a sum total momentum of zero.
