An interlude story, no monsters, no mystery to solve, no danger. Just The Doctor and Susan in melancholy mood, remembering the home they left behind so long ago. The temporal physics of the title, of course, comes from the experiment the boys have perfected, a temporal accelerator that allows them to grow trees in minutes. Remember the cúl nut The Doctor had in his pocket in Trouble in Paradise. Now he brings it out and the boys grow him a cúl nut tree in his granddaughter’s garden in Richmond-Upon-Thames, a little of Gallifrey growing on Earth. What that means to himself and to Susan only somebody who is a long way from home could possibly begin to know.

The house I had in my imagination when I invented the Lœngbǽrrow house on Southern Gallifrey doesn’t actually have Cúl nuts growing in the garden, but it is one I know very well. It is in South Dublin and is called The Hermitage or The Pearse Museum. One may wonder WHY Southern Gallifrey should have Georgian architecture, but not for long. The Doctor has long been described as an aristrocrat of his world. And this is his ancestral home. Well, it is now.

http://www.heritageireland.ie/en/HistoricSites/DublinArea/PearseMuseumDublin/