Maitland removed his three-day beard with an effective depilatory cream he discovered in the bathroom, and settled down to wait. When Swarts arrived, the engineer said quietly, "Sit down, please. I have to talk with you."
Swarts gave him the look of a man with a piece of equipment that just won't function right, and remained standing. "What is it now?"
"Look," Maitland said, "Ingrid has told me that men never reached the planets. You ought to know how I feel about space flight. It's my whole life. Knowing that my work on rockets is going to pay off only in the delivery of bombs, I don't want to go back to the 20th Century. I want to stay here."
Swarts said slowly, "That's impossible."
"Now, look, if you want me to cooperate...."
The big man made an impatient gesture. "Not impossible because of me. Physically impossible. Impossible because of the way time travel works."
Maitland stared at him suspiciously.
"To displace a mass from its proper time takes energy," Swarts explained, "and it's one of the oldest general physical principles that higher energy states are unstable with respect to lower ones. Are you familiar with elementary quantum theory? As an analogy, you might regard yourself, displaced from your proper time, as an atom in an excited state. The system is bound to drop back to ground state. In the atomic case, the time which elapses before that transition occurs is a matter of probabilities. In the case of time travel, it just depends on the amount of mass and the number of years the mass is displaced.
"In short, the laws of nature will insist on your returning to 1950 in just a few days."