There is a human settlement, and there seems to have been an alien invasion, but it's long over.

I've read this in a German anthology, but am sure it's British or American.

It begins with a man at home, a circular door pops up in his room. On the other side he finds strange, giant buildings and a giant empty pit. He finds some screen-device and makes it play back some imagery. This shows some alien lifeform in the pit. The images are so horrifying, that he almost goes mad and only after some time he barely recovers. He leaves the compound and finds a human settlement, with very healthy but somehow primitive people. They seem to worship him. He returns home by the teleporter door, picks up some books and returns to stay as their leader. I read this about 30 years ago, but I´m quite sure the story is from the fifties or sixties. Unique detail at the end: He grabs all the books about agriculture and leadership he can get his hands on and jumps back through the circular opening to the other end. He then learns, that a part of his book stack was cut in half by touching the rim of the circle, one of the ruined ones being Hitler´s Mein Kampf, which he then applauds as all for the better being ruined. I remember this especially, because that book is forbidden for sale in Germany, you can only get it legally for scholarly reasons at university or such places, and by this story I first learned that drivel seems to have been received elsewhere in the world as well and seemingly even recognized as a legitimate work of political literature, which I found appalling at the time but also fascinating. So maybe the story was from the fifties, if that book was still in circulation there at that time...

  • Hi, welcome to SF&F. When did you read this? Check out the other suggestions to see if they help you remember any more details you can edit into your question. – DavidW Oct 15 '20 at 17:03

This is "By His Bootstraps" by Robert Heinlein.

It is a portal into the future that opens up while the main character is writing his doctor's thesis.

The story is an example of a time loop.

The main character Bob Wilson travels through the portal into the future at the invitation of an older man.

Before going, he picks up some things that the older man requested: books on science and technology, and a mechanical phonograph and some records of specific pieces of music.

In the future, humans have been "pacified" by some extraterrestrial visitors. Besides changing the character of humanity, they left behind some enormous buildings.

In one of those buildings is the time machine that took Bob into the future.

Over time, Bob grows old in the future. He uses the books and the records to try to rekindle human society.

He also uses the time machine to view earlier times, and eventually manages to get a look at the aliens. What he sees frightens him so badly that his hair turns white.

Eventually, he views himself back in his appartment working on his thesis and the whole cycle starts again.

  • Thank you very much!! That must be it. I Shall try to get my hands on a copy. And try and get everything from Heinlein, since everything seems to be the best o f the best. Recently finished the YA novel Time For The Stars, which I think to be perfect in every way. – Spoeken Oct 15 '20 at 18:20
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    @Spoeken best to avoid anything written after 1967. Most of the YA books are excellent. – Organic Marble Oct 15 '20 at 19:58

I think the story is By His Bootstraps by Robert Heinlein, but you have misremembered it as the story is far more complicated than you describe. It is a 20,000 word novella rather than a short story and explores some of the paradoxes inherent in time travel. It starts as you describe with a portal opening in the protagonists living room:

Bob Wilson did not see the circle grow.

Nor, for that matter, did he see the stranger who stepped out of the circle and stood staring at the back of Wilson’s neck‌—‌stared, and breathed heavily, as if laboring under strong and unusual emotion.

Wilson had no reason to suspect that anyone else was in his room; he had every reason to expect the contrary. He had locked himself in his room for the purpose of completing his thesis in one sustained drive. He had to‌—‌tomorrow was the last day for submission, yesterday the thesis had been no more than a title: “An Investigation Into Certain Mathematical Aspects of a Rigor of Metaphysics.”

And it has the scene where Mein Kampf is cut in half by the portal:

The Hall of the Gate was empty, to his great relief. What a break, he told himself thankfully. Just five minutes, that’s all I ask. Five uninterrupted minutes. He set the suitcases down near the Gate to be ready for a quick departure. As he did so he noticed that a large chunk was missing from a corner of one case. Half a book showed through the opening, sheared as neatly as with a printer’s trimmer. He identified it as “Mein Kampf.”

And the giant buildings on the other side of the portal:

The architecture of the place confused him. Not only was it strange to his experience‌—‌he had expected that‌—‌but the place, with minor exceptions, seemed totally unadapted to the uses of human beings. Great halls large enough to hold ten thousand people at once‌—‌had there been floors for them to stand on. For there frequently were no floors in the accepted meaning of a level or reasonably level platform. In following a passageway he came suddenly to one of the great mysterious openings in the structure and almost fell in before he realized that his path had terminated. He crawled gingerly forward and looked over the edge. The mouth of the passage debouched high up on a wall of the place; below him the wall was cut back so that there was not even a vertical surface for the eye to follow. Far below him, the wall curved back and met its mate of the opposite side‌—‌not decently, in a horizontal plane, but at an acute angle.

But it isn't a beast in a pit that nearly drives him mad. It's when he attempts to research the High Ones who built the palace:

When he pulled himself together he was halfway down the passageway leading away from the hall. He realized that he had been screaming. He still had an attack of the shakes.

Somewhat later he forced himself to return to the hall, and, with eyes averted, enter the control booth and return the spheres to zero. He backed out hastily and left the hall for his apartment. He did not touch the controls or enter the hall for more than two years.

It had not been fear of physical menace that had shaken his reason, nor the appearance of the creature‌—‌he could recall nothing of how it looked. It had been a feeling of sadness infinitely compounded which had flooded through him at the instant, a sense of tragedy, of grief insupportable and unescapable, of infinite weariness. He had been flicked with emotions many times too strong for his spiritual fiber and which he was no more fitted to experience than an oyster is to play a violin

  • Curse it. Beaten to the punch by two minutes! :-) – John Rennie Oct 15 '20 at 18:17
  • Thank you!! This here must be the best place on the Internet :-) – Spoeken Oct 15 '20 at 18:30

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