Question: What do you think will happen when the world ends? How does it make you feel that all the world's discoveries will be forgotten and all inventions gone forever?

Laurence Perreault Levasseur answered on 24 Jun 2014:

There are many things you might mean by ‘world’.
Planet Earth will be destroyed by the sun for sure in 5.4 billion years, when the sun will have run out of hydrogen to burn and progressively become a red giant. By that time, the outer layers of the sun will have expanded so much that chances are it will become bigger than the orbit of the Earth. So Earth will literally be ‘eaten’ by the sun by that time. But chances are life on Earth won’t make it that far, because way before that (in about 2.8 to 3 billion years), the temperature on our planet will become so high (about 150C) that even unicellular life will be impossible.

However, even 1 billion year is an insane amount of time. I mean, modern human (Homo Sapiens) appeared about 200 000 years ago, and the first permanent settlements appeared with the invention of agriculture about 8000 to 5000 B.C. So if you think about how far we’ve got since then, and the amount of time left before the end of life on Earth happens, a LOT of things can happen.

So I think we shouldn’t worry too much about the end of the Earth, by that time chances are we’ll have colonized many planets in the solar system, and many moons of the giant gas planets. Maybe we’ll even have colonized the planetary of other stars. So I don’t think all of humanity’s knowledge will disappear at that point.

If by ‘world’ you mean the Universe, on an even longer timescale, however, things don’t look as good. I’ll outline the current predicted events and doing so I’ll assume that the current best estimates for things like the amount of dark energy. In 100 billion years all galaxies beyond the local group will have disappeared beyond the cosmic light horizon (that’s because of the expansion of the Universe). That’s only about 47 galaxies! (Recall that the Universe right now is only 14 billion years old, and it’s already pretty old!! So all these numbers are INSANELY big, many times the current age of the full Universe!!!)

In about 450 billion years, all those galaxies (in particular, our Milky Way and Andromeda) will have collided into one gigantic galaxy: Milkomeda. Between 10^12 and 10^14 years from now (that’s one trillion to 100 trillion years), there won’t be enough hydrogen left in the galaxy to ensure normal star formation, so no new stars will be formed and remaining stars will slowly burn their fuel and die.

In about 110 to 120 trillion years, all remaining stars will have exhausted their fuel, and the only remaining objects will be star remnants (white dwarfs, neutron stars and black holes) and brown dwarves. Sometimes collision between those things will make new stars, but we can estimate that about 100 stars will be shining in the galaxy. So nights are going to be pretty dark.

In 1 quadrillion years, the sun will have cooled down to 5 degrees above absolute zero.

By 10^30 years from now (there is no name for such a large number), all stars remnants will either have been ejected from the galaxy or collapsed into the central black hole of the galaxy. Between 10^36 to 10^43 years, the protons of all atoms in the Universe will (probably, but we don’t know if protons are stable) decay, and the only remaining objects will be black holes. This is the Black Hole Era.

Black holes will eventually evaporate through Hawking radiation, and by 1.7×10^106 years from now, they will have disappeared. This is the heat death of the Universe.

By this time, things look pretty dreadful. The first time I read about that honestly I was pretty depressed. But the next thing that will happen is great, because by 10^{10^{56}} (that is an INSANE number, by the way) random quantum fluctuations will have created a new big bang, and so everything will start all over again!!

The thing to realize here is that these estimates are based on what we currently understand of physics (and a lot of theories that have not yet been proven!), and so with such big extrapolations, chances are they are completely wrong (the further in the future, the higher the chances it’s wrong!). Recall that only a bit more than 100 years ago, we thought the total life span of the sun was about 5800 years. Pretty far off.

I think that the bottom line is that we shouldn’t worry too much about it (yet): we literally have many times the current age of the Universe to figure it out! (but it’s important to do research to understand those things better!!) And if we, humans, eventually die off and all our knowledge, discoveries and technologies disappear with us, chances are that other intelligent life forms, somewhere else in the Universe, will figure it out.