Why do economists disagree so much? I'm coming at this from a scientific point of view, having had no formal training in economics - most of what I know about it was from self-studying an undergraduate economics textbook.

My question is, why do economists disagree so much? This doesn't happen in the sciences. For example, for the question "will a driver in a car with these safeguards survive a collision at 100km/h", most scientists will agree on an answer. It might be conditional on various things (e.g. age of the driver), but most scientists will come to the same conclusion.

However in economics, it's different. Faced with a question such as "should we provide more fiscal stimulus for the economy", it's possible that one big group of economists says "yes", and then another big group says "no". To some extent this might be because of personal interpretation of what is desirable. However I would expect that economists should still be able to say something like "if you provide fiscal stimulus, this will happen, and it's up to you to judge whether the consequences are desirable", and yet it seems there's no consensus on what will actually happen. It doesn't help that when I see economics debated in the media, both sides advance what looks like reasonable arguments.

Does economics have predictive power? If so, why can't economists simply tell policymakers what to do? If not, what's the point of economics (it might as well be astrology)?
Some areas of economics have more consensus and predictive power than others. Most economists would agree on the effects of trade barriers, could fairly accurately forecast the effects of a price change given a good demand estimate, would come to the same conclusion about the effects of allowing a merger between two large competing firms, know how asset prices will react to a change in interest rates, etc.

The effect of a fiscal stimulus is one of the most complicated questions in economics because you are asking about the effect of stimulating a system with millions of moving parts (people and firms) and many dimensions (consumption/saving, employment/work, trade, investment, innovation, ...). This is a long way from the simple, closed systems for which natural science is able to give sharp predictions. In fact, when you look at parts of the natural sciences that deal with similar levels of systemic complexity, the overall (lack of) predictive power looks similar to that in economics:


although principles of ecology are well understood, it is almost impossible to accurately predict what the final effect of, say, introducing a new species into an ecosystem will be
the theory of evolution is probably one of the greatest scientific theories of all time, but can only be used to make the vaguest predictions about the future
medics cannot accurately predict the onset of a wide range of diseases—only diagnose them ex post
models of future global temperature increases have wide error bars
weather forecasts more than one or two days into the future have wide error bars


The other thing to note is that fiscal stimulus is a highly politicised topic and economists often do a (frustratingly) poor job of keeping politics out of the debate. If you read the economic literature, you will find a story similar to that for your car crash: any outcome is possible but some look more likely, depending on the exact circumstances. Given this ambiguity, politicians of different persuasions have no difficulty finding an economist willing to ignore this nuance and take a politically expedient position (just as conservative politicians can always find scientists willing to play down anthropogenic global warming). But that is less a failing with economics than with economists.