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Comment by mg | Hacker News Reader

That everything is so empty is the aspect of the universe that I find most surprising.

This also holds for small things like atoms. They are mostly empty space, too. And for bigger things like galaxies.

Most other aspects seem to be "good" choices. Like limiting the speed of things. The way it is limited (as described by special relativity) is even really elegant. The uncertainty as described by quantum theory and how it is coupled to the observer is downright cool. I often think "Yes, if I made a universe from scratch, this seems like a nice choice to go with".

But that everything is so empty? I would not have made that choice, I think.

You?

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It may look empty to human eyes but there’s actually a lot going on in “empty space”. Aside from physical objects that are just too small to detect from Earth (small asteroids (some gravitationally bound to the sun, others just passing through), dust, fragments of ancient collisions, gas, isolated molecules, and hydrogen and helium leftover from the formation of the solar system), there is also the ever-blowing solar wind - particles from the sun heading outwards - and a hailstorm of photons emitted from far off sources (other stars and galaxies, nebulae, pulsars, the extreme red-shifted radiation of distant energetic events… and much much more), passing through allowing us to see the rest of the universe. On top of these natural sources of EM radiation, our solar system at least tingles with human-created radio waves. Empty space also vibrates with gravitational waves, which we are only just starting to be able to detect. We theorise that any remaining emptiness might actually be boiling over with the continual creation and destruction of virtual particles. And who knows what’s going on with dark matter?

I think the emptiness of space is an illusion—really, there is a lot going on, in terms of gravitational forces, etc.

Well, if I want my universe to be manageable (don't ask me why), introducing the hierarchy of scales from the beginning is a natural solution.

  > I often think "Yes, if I made a universe from scratch, this seems like
  > a nice choice to go with".

One interpretation of that thought: All the universes that were not so elegant, collapsed quickly or did not develop life, or did not develop life as complex as us. We are here, living and complex, in _this_ universe because _this_ universe is so elegant.

> This also holds for small things like atoms. They are mostly empty space, too

Not quite so simple, see: https://physics.stackexchange.com/questions/126512/why-doesn...

Too much stuff you just end up with a big black hole, perhaps?

Mass is energy, localised in space-time, thats why space mostly empty, but mot completly emty, not 0 Kelvin.

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