Possibly they thought we had developed a faster-than-light drive and were waiting for them.

Time travel can be a vehicle, like a faster-than-light drive.

"But I thought no humans could be citizens until we invented a faster-than-light drive."

I also know that you have almost perfected a faster-than-light drive.

"A faster-than-light drive needs a tremendous amount of energy," he said to the three reporters.

"Why not use the faster-than-light drive for the big ships?"

The crew can then use the ship's faster-than-light drive to get home.

Later it might give men the stars, or, if not a faster-than-light drive, then the longevity to reach them.

"How about putting that brain of yours to work on a faster-than-light drive?"

We will trade you our faster-than-light drive for your incredible probes.