Oscar Berg has written Will the real expert please stand up? on his experience in a job interview and a familiar take on expertise / experts.
Being an expert is not about knowing everything within a certain domain. It is about being able to ask the right questions, and having the skills and network to find the answers.
Experts know how to ask good questions, and they usually know that there isn't a single "right" answer in any situation. That said, every expert has their own experiences and training and thinking models that guide what questions they ask and what evidence they look for in a situation. Chemists and chemical engineers tend to look at the same problem from slightly different perspectives.
I also like that Oscar highlights not only having the skills but also the network to find the answers. In today's world of many moving parts and many options, it is almost impossible to expect one person to be able to all the answer questions in a given field. They must have access to a network of people and communities where they can bounce ideas and ask questions. Sometimes that network is fellow colleagues, but it is even more valuable when that network extends beyond organizational boundaries.
[Photo: "Ask Here" by Madison Guy]