You can do a lot. 'Digital sundials' come to mind: https://en.wikipedia.org/wiki/Digital_sundial https://en.wikipedia.org/wiki/Digital_sundial https://gwern.net

You can do a lot. 'Digital sundials' come to mind: https://en.wikipedia.org/wiki/Digital_sundial https://en.wikipedia.org/wiki/Digital_sundial https://gwern.net/doc/math/1991-stewart.pdf https://gwern.net/doc/math/1991-stewart.pdf But yes, you are restricted to linear things and you can't make a good photonic cat detector out of that easily. So all the photonic neural networks you may have heard of like https://arxiv.org/abs/2106.11747 https://arxiv.org/abs/2106.11747 wind up sticking some mechanical or electrical nonlinearity somewhere.