4 ms·

It would be interesting to hear your answer to mekkkkkk‘s question

maxekman 6y ago
It was answered, as far as I can tell: > However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed.
- thaumasiotes 6y ago
  OK, we have a meter stick. It's made of pure iron and, for whatever physical reasons, it is 1 meter long exactly. After some time, the scale factor of the universe grows, so that 1 new meter is 1.05 old meters. But, by hypothesis, as this happens, our meter stick contracts, so that it ends up being just 0.952 new meters (= 1 old meter) long. Why? Did the physical laws governing iron atoms change so that they pack more densely than before? And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule?
  - ddevault 6y ago
    >And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule? The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.
    - thaumasiotes 6y ago
      But the scale of quarks increases just as much as every other scale, doesn't it? Why did the iron rod shrink while space grew? Why are the iron atoms closer together now than they were before? Why are the protons in each iron atom closer together now than they were before?
      - ddevault 6y ago
        The iron rod didn't shrink, it's more like it just held together. If I have a rubber sheet with a ball on it, and I pull at the ends of the rubber sheet, it's not going to pull the ball apart. The binding force of the ball's constituent atoms is sufficient to overcome the pulling force. And in the case of dark energy, the pulling force is absoultely miniscule at those scales.
        thaumasiotes 6y ago
        The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?
        ddevault 6y ago
        >The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink? In the sense that it did not become 0.952 meters long. It didn't "become" anything - it did not change at all. The universe grew around it, or perhaps "underneath" it. More space appeared for it to exist in. It's like putting an iron rod in a 10 meter square house, then moving the walls out another meter. Edit: let me phrase this another way. Let's imagine that you and I were pulling on the bar at either end. We could pull it forever and ever and it would never come apart (unless it rusted and became brittle or something, but let's imagine it doesn't). That's because we aren't putting in enough force to overcome the forces which are holding it together. Well, the expansion of the universe is putting like 10^-20 times less force into it than our arms would be.
        thaumasiotes 6y ago
        > More space appeared for it to exist in. This is a very different idea from "it's the same amount of space, but bigger", which is what I understand by the claim that a scale factor is increasing.
        ddevault 6y ago
ddevault 6y ago
I didn't really understand mekkkkkk's question, but I wrote up a longer reply here at your prompting: https://news.ycombinator.com/item?id=24424626 https://news.ycombinator.com/item?id=24424626
- joshspankit 6y ago
  Thank you. If I understand what you’ve written already, the essence of the answer in terms of mekkkkkk’s question (for reference: “ Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would seem that way. In your balloon example, the dots would get bigger.”), I think the answer would be: Anything held together (for example by gravity, or the force that keeps out atoms together) is not growing along with the universe simply because those forces are (for now) stronger than the expansion. This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us?
  - ddevault 6y ago
    >This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us? Yep.
    - joshspankit 6y ago
      (thinking emoji) (thinking emoji) (thinking emoji) (mindblown emoji)
noizejoy 6y ago
I thought it was explained. Local gravity is moving things together much faster than general expansion is moving them apart. At super super long distances, gravity is weaker than expansion, so something very far is disappearing while something nearby may be collapsing. However, that questions a different future I’ve seen in popular science talks: That the future becomes dark because of expansion. That makes less sense to me now, since it would seem that a merged set of galaxies would keep plenty of local stars in the skies. Assuming that stars are born as fast as they die (but I don’t recall seeing an answer to the ratio of star births vs star deaths at a local level).
- ddevault 6y ago
  The rate of expansion is speeding up. If it continues to increase, eventually it'll have observable effects at interstellar scales, and even interplanetary scales, and ultimately tear everything apart in the "big rip". Note that I'm omitting a lot of context here, there's a lot of other factors which make the big rip scenario much less likely. Also, the age of star formation will eventually end. Stars will stop forming somewhere between 1 trillion and 100 trillion years in the future.

It was answered, as far as I can tell: > However, local forces like gravity and the strong and even the weak nuclear force are more than sufficient to hold everything together despite this, so the expansion cannot be locally observed.

thaumasiotes 6y ago
OK, we have a meter stick. It's made of pure iron and, for whatever physical reasons, it is 1 meter long exactly. After some time, the scale factor of the universe grows, so that 1 new meter is 1.05 old meters. But, by hypothesis, as this happens, our meter stick contracts, so that it ends up being just 0.952 new meters (= 1 old meter) long. Why? Did the physical laws governing iron atoms change so that they pack more densely than before? And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule?
- ddevault 6y ago
  >And if so, why do all particles pack at exactly the same new density? Shouldn't it differ from molecule to molecule? The dominant factor on this scale is the strong nuclear force, which governs interactions at the scale of quarks - much smaller than molecules.
  - thaumasiotes 6y ago
    But the scale of quarks increases just as much as every other scale, doesn't it? Why did the iron rod shrink while space grew? Why are the iron atoms closer together now than they were before? Why are the protons in each iron atom closer together now than they were before?
    - ddevault 6y ago
      The iron rod didn't shrink, it's more like it just held together. If I have a rubber sheet with a ball on it, and I pull at the ends of the rubber sheet, it's not going to pull the ball apart. The binding force of the ball's constituent atoms is sufficient to overcome the pulling force. And in the case of dark energy, the pulling force is absoultely miniscule at those scales.
      - thaumasiotes 6y ago
        The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink?
        ddevault 6y ago
        >The iron rod started out being 1 meter long, and now it's 0.952 meters long. In what sense did it not shrink? In the sense that it did not become 0.952 meters long. It didn't "become" anything - it did not change at all. The universe grew around it, or perhaps "underneath" it. More space appeared for it to exist in. It's like putting an iron rod in a 10 meter square house, then moving the walls out another meter. Edit: let me phrase this another way. Let's imagine that you and I were pulling on the bar at either end. We could pull it forever and ever and it would never come apart (unless it rusted and became brittle or something, but let's imagine it doesn't). That's because we aren't putting in enough force to overcome the forces which are holding it together. Well, the expansion of the universe is putting like 10^-20 times less force into it than our arms would be.
        thaumasiotes 6y ago
        > More space appeared for it to exist in. This is a very different idea from "it's the same amount of space, but bigger", which is what I understand by the claim that a scale factor is increasing.
        ddevault 6y ago

ddevault 6y ago

I didn't really understand mekkkkkk's question, but I wrote up a longer reply here at your prompting: https://news.ycombinator.com/item?id=24424626 https://news.ycombinator.com/item?id=24424626

joshspankit 6y ago
Thank you. If I understand what you’ve written already, the essence of the answer in terms of mekkkkkk’s question (for reference: “ Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would seem that way. In your balloon example, the dots would get bigger.”), I think the answer would be: Anything held together (for example by gravity, or the force that keeps out atoms together) is not growing along with the universe simply because those forces are (for now) stronger than the expansion. This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us?
- ddevault 6y ago
  >This does produce an interesting side question though: aren’t we (humans) shrinking in relation to the universe? Or, without the click-baity phrasing: isn’t the universe growing without us? Yep.
  - joshspankit 6y ago
    (thinking emoji) (thinking emoji) (thinking emoji) (mindblown emoji)

noizejoy 6y ago

I thought it was explained. Local gravity is moving things together much faster than general expansion is moving them apart. At super super long distances, gravity is weaker than expansion, so something very far is disappearing while something nearby may be collapsing. However, that questions a different future I’ve seen in popular science talks: That the future becomes dark because of expansion. That makes less sense to me now, since it would seem that a merged set of galaxies would keep plenty of local stars in the skies. Assuming that stars are born as fast as they die (but I don’t recall seeing an answer to the ratio of star births vs star deaths at a local level).

ddevault 6y ago
The rate of expansion is speeding up. If it continues to increase, eventually it'll have observable effects at interstellar scales, and even interplanetary scales, and ultimately tear everything apart in the "big rip". Note that I'm omitting a lot of context here, there's a lot of other factors which make the big rip scenario much less likely. Also, the age of star formation will eventually end. Stars will stop forming somewhere between 1 trillion and 100 trillion years in the future.