Physics noise in simulation

Does Unity’s inbuilt Physics system have some form of random noise used in the simulation? If so, how do you disable it? I want the physics system to behave exactly the same each time the scene is loaded, but there appears to be minor differences that are causing problems.

@Casiell is basically right but it’s not exactly spot on.

The floating point issue is only one side of the problem. You can read a bit about machine accuracy if you have further intrest in this topic. Exact timing is another crutial part of this problem. Even with a certain given machine (in)accuracy, even unity would basically always get the almost same result for a given phyics simulation.

But here we are with another factor: Update rate and start conditions.
Unity tackles the Update rate issue by using the Fixed Update. This way it tries to assure that you will always have the same amount of phyics updates in the same timespan. For a numerical problem this is crutial. If you do more or less calculations you will without doubt end up at a different endpoint. But this timing also is never spot on the same. There are inaccuracies given by the system itself. (e.g. when your operating system decides to provide processor time to your application) which already leads to “noise” every update. This might also be amplified if you have actions in your FixedUpdate that sometimes are called and sometimes are not. → different code execution time needed.

But for the most part the most important thing is most likely the staring condition. Here you could think of it a bit like the Butterfly-effect. (If you have never heard of this then this is a good point to google it :)) → this means a small difference in your starting conditions will have the most impact on the result itself.

Best thing is probably to see the issue for yourself.
Check out this website with a double pendulum animation where you can give a certain animationspeed and start conditions. Even a slight difference in this mechanic system with only 2! degrees of freedom make a huuuge difference on the actual outcome.
Doublependulum

SO answers in short: Yes there is a “builtin noise”. No you cannot disable it. You can only try to slim down your system and programm to be more efficient. But in the end you can then only say it works on your system. Other computers may behave different.