"... completing the great circle of life," said the priest. Grandma's body was solemnly placed into a compost bin, where she would be recycled into the fertilizer for our tomatoes and corn.
"Dad, is EVERYTHING reused now?" asked Tommy.
"Just about. All our containers are compostable. Everything complicated, like electronics, has a QR code identifying it, and you can look up in a database how to recycle if it breaks irrecoverably. And we're pretty good at modularizing everything so anything can be repaired. That same QR code gives you instructions for 3D-printing replacement parts. The landfills are all gone ... once suitable mining techniques were developed, there was a gold rush to own and recycle them all."
"But that still takes lots of energy. What would we do if the sun wasn't wasting all that energy anyhow?"
The simulation of that universe finished, the results were recorded, and it was rolled back. "We're so beyond all that," commented Marry. "The stars have been put out, and now everything is pure computation. Including ourselves. And the computation is zero-energy, except for clearing bits. All mass is bound up in computation units. When atoms slip out of place, we put them back in place. No waste at all. The only things we spend energy on is clearing bits and maintaining the machinery. The only way we can save new results is if we clear bits to make room for them."
"But we're still using up entropy to clear bits," said Llisa.
"It's OK! It's true that entropy can only increase. But we live in an exponentially expanding universe, and the maximum entropy grows with the log of the volume of the universe. So sure entropy keeps increasing, but the maximum entropy possible keeps increasing too. The goalpost keeps moving. That's why the universe is so cold, despite us constantly dumping energy into it," said Marry.
"But ... then we're still wasting energy," said Llisa. "We can do direct matter-to-energy conversion. So, when we throw away energy, we're throwing away matter. No way we can get that back."
"Yeah. That would be impossible unless you knew the entire state of the whole universe, so you could reverse entropy," said Marry.
The simulation of that universe finished, and rolled back. The computation was entirely reversible. It had run for 300 quintillion years. Still, no results were saved. There was no longer any matter that could be wasted on matter-to-energy conversion. So, a reversible gajillion-bit counter incremented, and another universe was started using the counter to seed its random number generator, for another 300 quintillion-year run. And so it would continue for 2 to the power of a gajillion universes. And then it would repeat.
This was in response to a prompt on reddit.com r/WritingPrompts, "Imagine a world in which everything, absolutely everything that was ever built, created or manufactured by humans is being maintained. Nothing goes to waste. Everything is being repaired."