I've got a question here about the validity, though. I asked my older brother and he agreed too: When you jump, you and the earth push each other (though the earth doesn't move very far). As soon as you reach the height of your jump, the earth and you pull each other back (though, again, the earth not very far). So, by hitting the ground again, you reverse all the movement and energy spent pushing the earth. Imagine the common example in high school science textbooks: two people on rollerblades. Expecting 600,000,000 million people to move the earth's orbit is like saying that if I push another rollerblader, he'll move, and I won't. We'll both move in proportion to our mass. If we pull each other back, we will end up on the same spot on the road. I have another idea, but I'll see what everybody's opinion to this is first.