At the highest level of abstraction ( = the lowest level of magnification), most experiments look like this:
Set up the physical system to be studied.
Set up and calibrate the instrumentation
Run the experimental system and record the outputs of the instrumentation.
Change experimental and instrumental setup
Go to 3.
Publish paper -> tenure -> fame -> etc.....
The important part of step 6) is that the published paper includes enough detail about the experimental setup and how it was run so that other labs with access to the same equipment can recreate the experiment and test the repeatability of the results. This is hardly ever done (or even possible) in the context of experiments run in computers, and the crucial process of independent verification via replication of results is almost unheard of in computer simulation. One goal of Swarm is to bring simulation writing up to a higher level of expression, writing applications with reference to a standard set of simulation tools.
First, let's look at what happens when we port the above stages into the world of a computer. In a computer, you don't just drag the pieces of your experiment in from the outside world and hook them up. You have to create a world with space and time, a bunch of objects in that world (stuff to study and stuff to look at it with), schedules of events over those objects, all sorts of computer widgetry to interact with that artificial world and to manage multiple experimental runs and the data that they generate, and so forth. In other words, in a computer, one usually has to first *create* from scratch all of the bits and pieces of the experimental setup - the virtual equivalent of beakers, bunsen burners, microscopes etc.
Perhaps the most important difference between an experiment in the "real" world and an experiment inside of a computer is the nature of time. In the real world, everything in one's experimental setup is moved forward in time via a very concurrency courtesy of the laws of physics. In a computer experiment, however, the experimenter has to explicitly move every object in his/her artificial universe forward in time, making sure that everything remains within some well-understood state of synchronization. Many fundamental problems in computer science have arisen in the course of trying to understand how to control and use concurrency. Furthermore, most people who implement computer simulations aren't even aware of the subtle, but quite-possibly dominating, impacts of assumptions that they aren't even aware that they are making about concurrency in their model when they code it up and run it.
Therefore, a very important aspect of setting up an experiment in a computer is how one weaves the multiple threads of time that must be woven together coherently in order to produce reliable, repeatable results. Much of our work on Swarm has been devoted to not only making the task of managing concurrency manageable, but towards mechanisms to make people aware that they are always making implicit assumptions about how multiple threads of time are interacting with one another in their experimental setups. Swarm forces experimenters to make their concurrency assumptions explicit, so that others can reproduce their results by implementing the same assumptions about the flow of time.