Yes, they are. I always imagine what it would be like if we tried to colonize a planet with twice the gravity. In most science fiction I have read the gravity is always less, except in "The Moon is a Harsh Mistress" where the point of reference is the moon. I am sure there would be a lot of stresses on our body as we didn't evolve in those conditions, but if you could survive what would the people look like? Would obesity be cut in half? Would everyone have plantar fasciitis? Would all our imported plants grow short and stubby? How you it affect the way water behaves? Would it be scary to jump off a four-foot ledge? I think it would be a pretty humorous story.