There is a physics engine underneath, but the physics interactions are simplified for accelerated training. I.e. blocks that the agents carry can pass through o

There is a physics engine underneath, but the physics interactions are simplified for accelerated training. I.e. blocks that the agents carry can pass through other blocks, but the agents can't pass through blocks that are already placed. Such simplification allows the high throughput. There is a section in the article that explains the motivation behind this: there are a lot of questions in embodied AI that can be answered with such simplified simulators, i.e. you need something more complex than Atari, but you don't need the full complexity of the real world.