> scientific progress has really been linear. We didn't "get" physics 50 times better in 1990 compared to 1940

It's very hard to quantify scientific progress. The main reason is that we first pick the "low hanging fruits" and advancements get progressively harder. Just think of particle physics: While the first steps could be done with basic lab bench equipment, today we need to build gigantic accelerators to push the boundaries even a little bit. The same is true in other areas: The first steps are comparatively easy, but the more you know the harder it gets to solve the remaining problems, because you already got the easy ones.

So even if perceived progress is linear, considering the increasing difficulty we may have exponential scientific progress.