"[The] Doomsday Device", a short story by John Gribbin, published in Analog Science Fiction/Science Fact, February 1985, apparently never reprinted. The title is given as "The Doomsday Device" in the table of contents, but just "Doomsday Device" in the heading of the story.
A major research facility (like the Large Hadron Collider) is being commissioned. Unfortunately the project appears to be jinxed, because the device always breaks down before it can be successfully turned on. Every time the researchers try to run it, some random failure occurs.
"OK. This is the biggest particle accelerator in creation, right? The next best thing to the Big Bang itself. We smash the beams together out there," waving vaguely at the desert view outside the windows, "and use the energy to make particles that haven't existed since the moment of creation. Only it doesn't work. Up to ten or a hundred trillion GeV, everything is hunky dory. But when we push the energy above a trillion [sic] GeV, nothing. There's no reason why it shouldn't work at higher energies, and the low energy runs check out everything in the theory. So I reckon the answer's simple. It is working, but we can't see it."
[. . . .]
"Look at what actually happens. We test all the components separately, and they check out 100 per cent. We stick it all together, we press the button, and nothing happens. So we take it all to bits, and find some trivial fault. Not once; not twice. Every time we try to make the Beast perform as specified."
Somehow the researchers come to the conclusion that every time the experiment runs, it destroys the Earth. For example, if it is a collider, it may catalyze a strangelet soup.
The theory in the story is a lower energy vacuum state:
"There could be lots of minima, but let's just consider two. It isn't like a U shaped valley, but an uneven W, with one side dipping much lower than the other. What I'm suggesting is that when the Universe cooled after the Big Bang, the vacuum settled down into the state corresponding to the higher valley—the higher local minimum. There is at least one even lower energy state, but we are separated from it by a little energy hill. It's in the valley next door."
He turned back to his seat, put the pen away and sat down. "When we put enough energy into our Beast in the Desert, we are pushing the particles we make up over that hill and down into the next valley. In fact, they tunnel through they get to the top, because of quantum uncertainty. But it comes to the same thing."
As a result, the only time it's possible for the researchers to observe the state of the experiment is if it fails to run. Effectively, the device is pruning all the world lines in which it actually runs.
"What I believe is, every time we run that machine it does destroy the Universe. It destroys a whole slew of universes. The only universes that continue to exist afterwards are the ones in which, for some reason, the machine didn't work. So we observe a plague of little faults that stop the machine working as planned."
If I'm not conflating this with another story, the researchers eventually hook the experiment up to a detector that will trigger the device to run if an atmospheric nuclear detonation is detected, with the idea that they will "prevent" a nuclear war by pruning any world-lines in which one occurs.
"Peace prize, Jim? How come?"
"Like this. Now we know what we are doing, we can redesign the Beast so that there are no moving parts to go wrong—everything working at the quantum level. Then, if anyone presses the button it really will be the end of the Universe. I bet you can see what will happen. Anyone who tries to do the job will have a silly accident, or just change his mind. Nobody will ever bring himself to try it, in our Universe, because in all the universes where it is tried, we won't be around any more to notice. The universe in which we are around to see what is going on just has to be one of the universes in which nothing happens. So you have to automate the Beast fully, seal it up and leave it to trigger under one condition, and one only—if it detects an upsurge in radiation consistent with the start of a nuclear war."
