'39 was written about a man who leaves the Earth at a speed near that of light. At such speeds, time would appear to go more slowly for the traveller than for those remaining on Earth. This effect is predicted by Einstein's theory of special relativity. If he goes round in a loop to get back again, Einstein's general relativity has to be used and it's beyond my capabilities to calculate it exactly, but the upshot is that, at certain very high speeds, a year could elapse for the traveller, but a hundred years for his relatives on Earth. So he sees his daughter (or Granddaughter?!) on his return and imagines he sees his wife for a moment. That's the concrete level. But on another level it's about emotional journeys, which is harder to explain, and rather sadder.