[tarpherder]

You can. The problem is that there is a multi-frame (33ms * 2 or so) delay in getting any results back to the CPU. The GPU is set up for streaming, you compile command lists dynamically and feed it to it, this means it usually has at the very least 1 command list in execution and 1 being built on the CPU (the GPU is always kept busy, stalls are cycles going to waste). Hence the delay in getting results back.

And you will need some of those results on the CPU.

Physics for particles is not uncommon to be done on the GPU though. There is no feed-back to the CPU required so latency becomes a non-issue.