these are basically the only classes of experts that can be trusted to be both reasonably accurate in the analysis and honest about it. the only ones that i can think of anyway.
every other field of expertise is either too complicated for anyone to be able to reliably come up with accurate analysis (psychoanalysis comes to mind) or too influenced by money for any expert's stated opinion to be trusted to be their honest opinion (the unfailingly rosy predictions of california real estate professionals over the past 18 months as prices have continuously and rapidly dropped come to mind as a topical example).
so, for me to trust a field of experts, i have to believe that they are likely to have a solid understanding of what they're talking about and not have money pushing them one way or the other.
physicists qualify because everyone in the field is in it long term and being wildly wrong could cost them their careers and there just isn't that much money in the field.
poetry critics qualify because there is no such thing as right or wrong, but they will always say what they actually think because there is no economic incentive whatsoever to say anything else.