The story was about the value of pi changing throughout the history of civilisation, suggesting not that technology was improving but that the universe itself was changing.
"The New Reality", a novelette by Charles L. Harness, also my (unaccepted) answer to this old question; first published in Thrilling Wonder Stories, December 1950, available at the Internet Archive. Here's the part about the changing value of π:
"Memo to Astronomy. Were erroneous Ptolemaic measurements due to lack of precision instruments?"
Soon he had his reply:
To Director: Source of Ptolemy’s errors in solar measurement not clearly understood. Used astrolabe precise to 10 seconds and clepsydra water clock incorporating Hero's improvements. With same instruments, and using modern value of pi, Ptolemy measured moon radius (0.29 earth radius vs. 0.273 actual) and distance (59 earth radii vs. 60 1/3 actual). Hence instruments reasonably precise. And note that Copernicus, using quasi-modern instruments and technique, "confirmed" Ptolemaic figure of sun's distance at 1200 earth radii. No explanation known for glaring error.
Unless, suggested something within Prentiss' mind, the sun were closer and much different before the 17th century, when Newton was telling the world where and how big the sun ought to be. But that solution was too absurd for further consideration. He would sooner assume his complete insanity.
Puzzled, the ontologist gnawed his lower lip and stared at the message in the scriptor.
In his abstraction he found himself peering at the symbol "pi" in the scriptor message. There, at least, was something that had always been the same, and would endure for all time. He reached over to knock out his pipe in the big circular ash tray by the scriptor and paused in the middle of the second tap. From his desk he fished a tape measure and stretched it across the tray. Ten inches. And then around the circumference. Thirty-one and a half inches. Good enough, considering. It was a result any curious schoolboy could get.
He turned to the scriptor again. "Memo to Math Section. Rush paragraph history on value of pi. Prentiss." He didn’t have to wait long.
To Director. Re history "pi." Babylonians used value of 3.00. Aristotle made fairly accurate physical and theoretical evaluations. Archimedes first to arrive at modern value, using theory of limits. . . .
There was more, but it was lost on Prentiss. It was inconceivable, of course, that pi had grown during the two millennia that separated the Babylonians from Archimedes. And yet, it was exasperating. Why hadn’t they done any better than 3.00? Any child with a piece of string could have demonstrated their error. Countless generations of wise, careful Chaldean astronomers, measuring time and star positions with such incredible accuracy, all eoming to grief with a piece of string and pi. It didn't make sense. And certainly pi hadn't grown, any more than the Babylonian 360-day year had grown into the modern 365-day year. It had always been the same, he told himself. The primitives hadn’t measured accurately, that was all. That had to be the explanation. He hoped.
A comment by J King says "I've got the story, the finale involves a scientist building a system which sets up a single atom for the wave-particle 'choice' and ....... But as of now, I can't find it. Some mention of a new Adam and Eve, I think." That's the same story. Here's the part about the experiment to force a single photon to make a choice:
"Your apparatus," said Prentiss, "is going to provide just such a photon. And I think it will be a highly confused little photon, just as your experimental rat was, that night not so long ago. I think it was Schroedinger who said that these physical particles were startlingly human in many of their aspects. Yes, your photon will be given a choice of equal probability. Shall he reflect? Shall he refract? The chances are 50 percent for either choice. He will have no reason for selecting one in preference to the other. There will have been no swarm of preceding photons to set up a traffic guide for him. He'll be puzzled; and trying to meet a situation for which he has no proper response, he'll slow down. And when he does, he'll cease to be a photon, which must travel at the speed of light or cease to exist. Like your rat, like many human beings, he solves the unsolvable by disintegrating."
Luce said : "And when it disintegrates, there disappears one of the lambdas that hold together the Einstein space-time continuum. And when that goes, what's left can be only final reality untainted by theory or imagination. Do you see any flaw in my plan?"
And the new Adam and Eve:
Meta-universe, by whatever name you called it, was beautiful, like a gorgeous garden. What a pity he must live and die here alone, with nothing but a lot of animals for company. He’d willingly give an arm, or at least a rib, if — "Adam Prentiss! Adam!"
He whirled and stared toward the orchard in elated disbelief.
She'd got through!
The whole world, and just the two of them!
His heart was pounding ecstatically as he began to run lithely upwind.
And they'd keep it this way, simple and sweet, forever, and their children after them. To hell with science and progress! (Well, within practical limits, of course.)
As he ran, there rippled about his quivering nostrils the seductive scent of apple blossoms.