Theoretical physicists are quite faithful to the image that one is generally the scientific kind. If you imagine the distracted, bald, bizarre, scratching his chin, deeply immersed in their thoughts ... well, in general you will not be far from reality.
Their task is to try to understand how nature: how is it that the stars shine, the water is flowing and the sky is blue; what are we made and why "it" was there a weight, why is it expanding universe, or actually ARE the energy and matter ...
One of the most amazing facts of life is that scientists can actually answer some of these questions, and that the quality of responses is gradually improving (that is to say they agree to more and more with observation). The fact that we humans enjoy poetry and music may be, if you think about it, the only other Equally surprising.
Two repeated questions: "What is the difference between theoretical physicists and experimenters and why is there a difference? ". The answer to the second question is simple: the two "species" have activities rather different and often highly specialized. It is increasingly difficult to find people such as Leonardo da Vinci, who know everything and are active in all areas. The experimenter asks directly the Nature, the passively observing - as do astronomers - or actively, such particle experimentalists who "play" with the smallest constituents of nature directly to understand their behavior.
The relationship between theorists and experimentalists often healthy competition in the quest for the truth, but also less healthy competition for glory. Here's a riddle illustrating this fact:
What are the similarities and differences between these two groups?
The first is a farmer, his pig and truffles:
The second includes the theorist, the experimentalist and the big discoveries:
(You will notice, in my drawing the second set, I am not an experimenter).
The answer to the riddle:
The farmer takes his pig in the woods. The pig sniffing around looking for a truffle. Once the pig in a landmark and is about to eat the farmer strike his head with his stick and grabs the nose. Here the similarities: a theorist would also claim a discovery, made by an experimenter (if it has any relationship with his theories), even if he is not the author!
The difference is that the farmer takes his pig in the woods where there are indeed truffles, while in most cases, the suggestions made by theorists led experimenters to "woods" no "truffles" (in suggesting experiments that lead to interesting discoveries).
Not to be unfair to the theorists, we must add, however, that there are notable exceptions to these rules, as progress is made through trial and error, and that the advice theorists sometimes show the right direction! And more often, so they are looking for "truffles" theorists, experimenters discover "gold": something unexpected but even more interesting! (Nature tends to be more creative than us).
The main specialty of theoretical physicists at CERN is trying to understand the "elementary particles", which are the fundamental constituents of the Universe and the agents of basic forces of nature, such as gravity. It turns out that our growing knowledge of these small "elementary" things is the basis of our understanding of the universe as a whole!
A short visit to the CERN Theoretical Physics Group might lead you to believe that you are in a zoo. This is not entirely accurate. It is true that you will find men and women of dozens of nationalities, cultures, languages and others ... some of them even having some resemblance to ET. But their work is what defines our species as a whole question, and sometimes respond to some of the deepest questions. The zoo is rather a circus magicians, where artists - so to speak - are mainly concerned to make you their number ... and whose entry is free!