"Supposing he picked up your trail close to Terra?"
Jones smiled. "If he could detect us, we'd detect him," laughed Jones.
"Supposing he had a better detector."
"We're at the theoretical limit of sensitivity now," said Jones. "And we've been there for years. The noise level, thermal agitation in the set itself, and a horde of other things limit the ultimate sensitivity of any detector. And don't mention noise-eliminators. They aren't. You can't stop electrons from rubbing one another and that's that!"
"But—?"
"We—as he may—also use both pulse-type detectors and aperiodic receivers. People would have known that he was following them."
"Are you certain?"
Jones laughed. "Look, Lieutenant Jeffries, we're convoyed. There were two Solar Guard spacecraft that took off as we did, for convoy duty. Their job was to stick close by us all the way to Jupiter, right down to the landing on Callisto. Now, they'd follow anything that they saw suspicious. That's first. Secondly, we're at about three-quarters of the way to turnover now—and neither of the convoys are visible on the detector nor audible in the aperiodic receiver. If, Lieutenant Jeffries, two Guard ships, bearing the best in instrument and personnel, cannot stay within a million miles of us when they know our predicted course, how can you expect a pirate to barge in upon us when we're ramming space above two thousand miles per second? Detecting at these distances and at these velocities brings about a situation somewhat similar to Heisenberg's Uncertainty."
"Which is far above my policeman's mind," said Jeffries.
"You can detect where the spacecraft was when the transmitted pulse reached it and was echoed at X seconds ago. In order to know where it is, in truth, you must assume a velocity which you must get from the same gear. To assume the velocity, you must know exactly how far the ship traveled between pulses, which because of the fact that the pulses are transmitted different distances, is slightly difficult, especially when the doppler is changing."