Mankind uses nuclear weapons to finally breach the force field and discover that a signal was activated to aliens. Our ability to breach the force field told aliens we were advanced enough to come back check on! I read this in college and can't find it again!

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    And when would "college" have been? :) more generally: could you take a look at these guidelines, see if that triggers any more memories you could edit in? – Jenayah Dec 22 '18 at 0:32

"The Sentinel" aka "Sentinel of Eternity", the short story by Arthur C. Clarke on which the 1968 movie 2001: A Space Odyssey was based, and the beginning of Clarke's Space Odyssey series; first published in 10 Story Fantasy, Spring 1951, available at the Internet Archive; reprinted in New Worlds Science Fiction #22, April 1954, also available at the Internet Archive.

"The Sentinel" came up before as the answer to the question Story about astronauts who set off an alien warning beacon on the moon (or Mars?).

Wikipedia summary:

The story deals with the discovery of an artefact on Earth's Moon left behind eons ago by ancient aliens. The object is made of a polished mineral, is tetrahedral in shape, and is surrounded by a spherical forcefield. The narrator speculates at one point that the mysterious aliens who left this structure on the Moon may have used mechanisms belonging "to a technology that lies far beyond our horizon, perhaps to the technology of paraphysical forces."

The narrator speculates that for millions of years (evidenced by dust buildup around its forcefield) the artifact has been transmitting signals into deep space, but it ceases to transmit when, sometime later, it is destroyed "with the savage might of atomic power". The narrator hypothesizes that this "sentinel" was left on the moon as a "warning beacon" for possible intelligent and spacefaring species that might develop on Earth.

In 2001: A Space Odyssey, the operation of the sentinel is activated when sunlight touches it for the first time after it is dug up.


Then I went back toward that circle in the dust. I picked up a fragment of splintered rock and tossed it gently toward the shining enigma. If the pebble had vanished at that invisible barrier I should not have been surprised, but it seemed to hit a smooth, hemispherical surface and slide gently to the ground.

I knew then that I was looking at nothing that could be matched in the antiquity of my own race. This was not a building, but a machine, protecting itself with forces that had challenged Eternity. Those forces, whatever they might be, were still operating, and perhaps I had already come too close.

[. . . .]

The mystery haunts us all the more now that the other planets have been reached and we know that only Earth has ever been the home of intelligent life in our Universe. Nor could any lost civilization of our own world have built that machine, for the thickness of the meteoric dust on the plateau has enabled us to measure its age. It was set there upon its mountain before life had emerged from the seas of Earth.

When our world was half its present age, something from the stars swept through the Solar System, left this token of its passage, and went again upon its way. Until we destroyed it, that machine was still fulfilling the purpose of its builders, and as to that purpose, here is my guess.

[. . . .]

Those wanderers must have looked on Earth, circling safely in the narrow zone between fire and ice, and must have guessed that it was the favorite of the Sun's children. Here, in the distant future, would be intelligence; but there were countless stars before them still, and they might never come this way again.

So they left a sentinel, one of millions they have scattered throughout the Universe, watching over all worlds with the promise of life. It was a beacon that down the ages has been patiently signaling the fact that no one had discovered it.

[. . . .]

I can never look now at the Milky Way without wondering from which of those banked clouds of stars the emissaries are coming. If you will pardon so commonplace a simile, we have set off the fire alarm and have nothing to do but to wait.

I do not think we will have to wait for long.

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