4 ms·

Let's say the universe is turn-based, and consists of a single dimension. At the end of each turn the distance between two points in this universe doubles (beca

• stavros 5y ago
  Hmm, I see, thank you. So it's space itself that expands, not just the objects in it that accelerate apart from one another. But then doesn't that mean that some of that space is expanding inside the objects? Do we know what happens there? How do objects just get more space inside them?
  • cygx 5y ago
    Thinking of spacetime as some sort of expanding fabric permeating every-day objects is not a good analogy in my opinion. First, like any other gravitational effect, locally, spatial expansion would manifest as pseudo-forces that can be counter-acted by all the other forces that are far more relevant at human scale. Second, the local effect of spatial expansion should be an indirect one: Friedmann cosmology - which is how we describe an isotropic, expanding universe - is a large-scale approximation. More realistic would be 'swiss-cheese' models, where spacetime in our neighbourhood can look vastly different from the Friedmann one, except that the spacetime patches need to properly fit together to yield the correct large-scale behaviour.
    • stavros 5y ago
      Ah I see, so this all happens at a quantum level that doesn't translate well to our intuition? That makes a certain amount of sense, thank you.
      • cygx 5y ago
        This is still relativity, not quantum mechanics. Point is, illustrative models and analogies are limited. Ideas like 'space itself expanding' or 'space flowing like a river and falling into a black hole like a waterfall' might help visualize some things, but can also lead to wrong ideas if taken too seriously.
        • stavros 5y ago
          > This is still relativity, not quantum mechanics. Oh hmm, I see. > but can also lead to wrong ideas if taken too seriously. Yeah, and it's hard to know how far to take the analogy unless you're already familiar with the concept it's trying to describe, unfortunately.
  • JZumun 5y ago
    (Assuming space expands evenly) we don't notice the expansion of space inside objects because the electromagnetism and gravity are vastly more powerful on the scales we care about and nullifies the effect. That's why atoms and galaxies in the Local Group stay together, but everything farther away is flying further away. If expansion never stops accelerating, one day in the far future it will overcome all the other fundamental forces and everything will be torn apart in a Big Rip.
    • grkvlt 5y ago
      wait - does this mean that some super capable civilisation could link the various galaxies in their inter-galactic empire with a space-railway (i.e. using actual physical tracks between them in space) assuming they have the staggeringly vast amount of matter and/or energy needed, then this would gravitationally bind them together and space between them could not expand?
      • lalaithion 5y ago
        No, "gravitationally binding" is not a thing that you can do by connecting things via railways. Over the long distances, the railways would be torn apart by expansion. If you could drastically increase the mass of the local group, then you could increase the range at which gravitational attraction to it was dominating, but it would require seriously increasing the mass of the local group.
        • grkvlt 5y ago
          > over the long distances, the railways would be torn apart by expansion i'm not sure i get it. as i understood it, it was the fact that forces over a small scale dominate the effects of space expanding that prevents e.g. atoms getting bigger. so why would my space-railway tracks (made of continuous welded steel space-rails) not stay the same size (2m wide by thousands of light years long) as well?
          • Thiez 5y ago
• toss1 5y ago
  So, Zeno finally gets his paradox to work!