Miracle by Ray Cummings - A summons from yesterday, a promise from tomorrow–they had commanded Alan Dane to tear apart the pages of history–to save his unborn son!
'But how can you possibly know that time traveling has never been done?' the chemist protested. 'Someone from our future may have gone into the past many times.'
'I should think they'd have created quite a commotion,' the lawyer observed. 'Wouldn't we have heard of it from our historical records?'
'Of course.' The chemist was smiling now. 'We probably have. History tells of many important occasions on which a 'vision' appeared. A miraculous presence, such as Joan of Arc, for instance, or the Angel of Mons.'
'Or the appearance of the Sun God to the Aztecs. I get your point,' one of the other men interjected. 'You think that there might have been a time traveler who materialized just long enough to take a look—and the superstitious natives took him for a god. Why not? That's probably just what would happen.'
Young Alan Dane sat in a corner of his grandfather's laboratory, listening to the argument of the group of men. He was well over six feet in height, a sun-bronzed, crisply blond young Viking. Beside him sat Ruth Vincent, his fiancée, a slim girl of twenty. Alan's heart was pounding. Somehow it seemed as though this bantering talk of time traveling were something momentous to him, something requiring a great and irrevocable decision.
Then abruptly old Professor Dane held up his hand and, quite casually, said, 'What you do not know, gentlemen, is that for half my life I have been working to discover the secret of time travel.'
His audience was suddenly tense. Professor Dane was loved and respected by each of them, and his word in his chosen field of physics was final. If he said a thing could be done there was no mistake.
