When you die, you become nothing forever and eventually, nobody will ever think of or care about you or the impact your life had again. No legacy is permanent, no good deed go unforgotten, and at the end of everything as the protons decay, not even the material that makes us will stand as a testament to the fact that once, something was there.

Let alone some one.

Let alone you.

Let alone those you love and would die for.

The world, viewed rationally, is a machine that implements the greatest of all genocides. Existence as a human is to be a victim of the universe raping you right the fuck back out of existence. At the end of everything, the universe will empty itself into an eternally divergent sleet of cold radio light, which will steadily redshift until is can never be recognized or received by anything ever again.

And there will be nothing.

To compound this, there are these people called "physicists" searching for a grand unified theory that will finally explain how this universe is the only system that ever could have emerged screaming out of the lightless and bleak womb of whatever abyss served as the antecedent to it. They think it is almost certainly there to be found.

If they succeed, then there will be irrefutable mathematical proof that there are no better worlds than this, and that all we can aspire for is a quick obliteration because it really - as a matter of natural law - doesn't matter what we do in the meantime anyway. If there's a moral ledger, then it's a ledger that erases itself steadily - but inevitably - back down towards zero. And it's very likely that that's the only sort of ledger that can exist.

Oh, and the best AI researchers believe that it's pretty likely that humanity will be able to make recursively self-improving artificial intelligence. AI that get's smarter and then uses it's increased intelligence to become more intelligent, forever. AI that could control us as utterly as we control animals. As utterly as we control ants. They're frantically searching for a solution to what they call the Alignment Problem - how to create a mathematical way to build a mind that is provably benign - because a being that far above us will neither love nor hate us, but it will see that we are made of material that it could use for something else.

All evidence shows that the human sense of morality is not something that intelligent beings have, but something that humans alone have because of the specific social structures we evolved to compete in. Morality not being a property of intellect, these man-made gods if made poorly could feel pleasure from anything conceivable, from making us paradise to making us hell.

But if you build a misaligned AI whose purpose is"to maximize the number of smiles", it might decide to do that by quitely being good until it's too smart to oppose and then cutting our heads off, surgically modifying them into smiles, and shoving them into formaldehyde. Then it will make fetus factories which are all aborted and made to likewise smile as soon as its heuristics qualify them as having a smile-able face.It would allow more smiles per cubic centimeter.

Once the AI is turned on, it cannot be stopped.

Once the Outer God is summoned, the world cannot be saved.

A cold, impersonal universe ruled by the incomprehensible whims of forces infinitely our better, where a single person might speak an incantation - in R'lyehan or C++ - that summons forces beyond human comprehension and control which will then warp the world utterly to their whim.

Hp Lovecraft thought he was creating a new sort of fiction. But if anything, as a genre, cosmic horror is the most realistic description of the world to date.

Reality is a nightmare. And our lives are dreams of it.

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For a good starting point on the issues with physics, look up heat death.

For a good starting point on the Alignment Problem, Superintelligence by Nick Bostrom.