Andy Updegrove writes: "Has it ever occurred to you that while you've read articles theorizing about how gravity relates to other forces, you've never read even one that pretends to explain what gravity actually "is," or why it exists? That's because no one has a clue — we can observe and describe its effects, but we don't know why it affects things at all. As we begin to create virtual worlds, we have the opportunity to create our own virtual physics, which our un-real creations have no choice but to obey, no matter how arbitrarily we set the rules. As these virtual worlds become more real, it can leave an uncomfortable feeling that perhaps the rules that control our world are only artificial constraints as well. Maybe we can't explain gravity because there is no explanation to be found, now or ever — its just an arbitrarily defined standard imposed by...who?"