6 ms·

I’d like to see a map of the known universe visualizing atomic density on a log scale. I’d never seen “a hundred atoms per cubic meter”, but it’s always been m

by dfee 3y ago

I’d like to see a map of the known universe visualizing atomic density on a log scale. I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light. And on the other hand, I’ve seen claims that “space is really big” as you mentioned; but that claim has always seemed dubious.

wongarsu 3y ago
100 atoms per cubic meter is on the lower end of typical densities in the interstellar medium. It can get many orders of magnitude denser than that. Outside galaxies you have better chances of surviving high speeds. The intergalactic medium is only 1-10 particles per cubic meter in the web of gas we call the warm–hot intergalactic medium, and possibly less outside of that.
- micw 3y ago
  So would probably better to get out of the galaxy disc, travel -fast- to the other end and dive back into rather that go straight through the galaxy?
  - wordpad25 3y ago
    Except we can't get to edge of the galaxy quickly. I guess Hitchhikers guide to the galaxy building space highways destroying everything in the way could be more than a gag after all.
  - ben_w 3y ago
    Go up and over, that's only a few hundred light years. To the edge of the disk is 30,000.
eru 3y ago
> I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light. It's not that bad. With currently known science, your fuel would most likely be hydrogen so you can run a fusion reactor. The rocket equation tells you that most of your starship by mass would be fuel, if you want to go fast. Of all the stuff on your ship that's not fuel, you'd probably need quite a bit of water for survival needs. So you would make your spaceship relatively long and thin (to maximize internal volume for a given frontal area), and you would store your fuel (and water) in front of you to serve as exactly that shield.
- tsimionescu 3y ago
  Wouldn't your water and food then become highly radioactive over time? Even if no, it would mean your shield will be gradually consumed. Not sure this is the best idea.
  - eru 3y ago
    Your fuel is the outermost layer of your shields (modulo whatever is necessary to keep the fuel in place. But you might use magnetic fields perhaps). That's basically free shielding: you have to carry the fuel around anyway, so you might as well put it to good use. If you run a nuclear fusion reactor, you won't really lose much of the mass of your fuel, unless you want to. Eg you could use the helium you produce as the reaction mass for your ion drive. (I haven't done the numbers to see how the required mass per second for your ion drive compares to the helium mass per second a nuclear fusion reactor would spit out.) Because it's a free shield, you don't really get to complain about your shield being gradually consumed. Of course, you can have some extra shielding further inside. You would keep your water forward of your people, but behind your fuel. So your water would not bear the brunt. Hydrogen doesn't really get all that radioactive: you can use chemical means to remove any helium or so you might accidentally produce; and hydrogen's isotopes are both pretty short lived and relatively easy to separate. (At least much easier than eg enriching uranium.) Your water and food is also only a very small fraction of the overall mass of your rocket: as always, the vast majority is made up of fuel.
    - tsimionescu 3y ago
      Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel. And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually impossible to isolate tritium out of water. So if any tritium forms (which is an extremely common by-product of any fusion reactions which might happen, and the most common decay product of heavier hydrogen isotopes), it will render your water quite poisonous for human consumption, virtually irrevocably.
sega_sai 3y ago
One nice illustration of "space is really big" is a fact that if you take the cube with the side equal to the distance from the Sun to the nearest star and fill it with water, then the mass of this cube will be roughly equal to the mass of the whole visible Universe.
- wincy 3y ago
  Another good one for me is that a cubic light year or butter would immediately collapse into a black hole with a Schwarzchild radius larger than the observable universe.
  - jemfinch 3y ago
    It's impossible for your fact and the fact you're replying to both be true. Water is denser than butter, and the nearest star to the sun is about 4.3ly away; if your fact were true, the universe would be a black hole. A cubic lightyear is about 8.468e+50 liters, and butter weighs 911 g/L, giving the mass of a cubic lightyear of butter to be 7.714348e+50, whose Schwarzchild radius is about 121,103,293 lightyears, about 100x smaller than the radius of the known universe.
    - heads 3y ago
      Wasn’t one of the great puzzles of the 20th century exactly this — will the universe collapse back into a singularity or not?
    - lloeki 3y ago
      > if your fact were true, the universe would be a black hole ... maybe it is? Hear my pet theory out. Extrapolating backwards from the expansion of our universe, the Big Bang model posits a hyperdense state that exceeds black hole levels originating from a singularity, yet it's thought that somehow it did not collapse back, handwaving it as "physics as we know it did not apply". But maybe physics as we know it does apply. Notably physics as we know it does not imply a specific direction for the arrow of time. So our universe might very well be a black hole, but we have time backwards compared to the usual way we think of black holes: what we think of as the origin of time and space is what we think of as the irremediable end of time and space in a black hole.
sliken 3y ago
From quora: The vacuum in solar space is around 10^7 atoms per cubic metre; the vacuum of interstellar space is around 10^6 atoms per cubic metre; and the vacuum of intergalactic space is around 1 atom per cubic metre.

by dfee 3y ago

wongarsu 3y ago

100 atoms per cubic meter is on the lower end of typical densities in the interstellar medium. It can get many orders of magnitude denser than that. Outside galaxies you have better chances of surviving high speeds. The intergalactic medium is only 1-10 particles per cubic meter in the web of gas we call the warm–hot intergalactic medium, and possibly less outside of that.

micw 3y ago
So would probably better to get out of the galaxy disc, travel -fast- to the other end and dive back into rather that go straight through the galaxy?
- wordpad25 3y ago
  Except we can't get to edge of the galaxy quickly. I guess Hitchhikers guide to the galaxy building space highways destroying everything in the way could be more than a gag after all.
- ben_w 3y ago
  Go up and over, that's only a few hundred light years. To the edge of the disk is 30,000.

eru 3y ago

> I’d never seen “a hundred atoms per cubic meter”, but it’s always been my intuition that, without some quite interesting shielding, you couldn’t make it anywhere near the speed of light. It's not that bad. With currently known science, your fuel would most likely be hydrogen so you can run a fusion reactor. The rocket equation tells you that most of your starship by mass would be fuel, if you want to go fast. Of all the stuff on your ship that's not fuel, you'd probably need quite a bit of water for survival needs. So you would make your spaceship relatively long and thin (to maximize internal volume for a given frontal area), and you would store your fuel (and water) in front of you to serve as exactly that shield.

tsimionescu 3y ago
Wouldn't your water and food then become highly radioactive over time? Even if no, it would mean your shield will be gradually consumed. Not sure this is the best idea.
- eru 3y ago
  Your fuel is the outermost layer of your shields (modulo whatever is necessary to keep the fuel in place. But you might use magnetic fields perhaps). That's basically free shielding: you have to carry the fuel around anyway, so you might as well put it to good use. If you run a nuclear fusion reactor, you won't really lose much of the mass of your fuel, unless you want to. Eg you could use the helium you produce as the reaction mass for your ion drive. (I haven't done the numbers to see how the required mass per second for your ion drive compares to the helium mass per second a nuclear fusion reactor would spit out.) Because it's a free shield, you don't really get to complain about your shield being gradually consumed. Of course, you can have some extra shielding further inside. You would keep your water forward of your people, but behind your fuel. So your water would not bear the brunt. Hydrogen doesn't really get all that radioactive: you can use chemical means to remove any helium or so you might accidentally produce; and hydrogen's isotopes are both pretty short lived and relatively easy to separate. (At least much easier than eg enriching uranium.) Your water and food is also only a very small fraction of the overall mass of your rocket: as always, the vast majority is made up of fuel.
  - tsimionescu 3y ago
    Your fuel still gets consumed, so you still can't rely on your fuel as the main form of shielding. Towards the end of your journey, your rocket is approximately 0% fuel. And at the point of highest speed, before you start decelerating, it is roughly 50% fuel. And you are entirely wrong about the isotopes of hydrogen. Tritium is highly radioactive, with a half life of ~12 years. And it is not just hard, but virtually impossible to isolate tritium out of water. So if any tritium forms (which is an extremely common by-product of any fusion reactions which might happen, and the most common decay product of heavier hydrogen isotopes), it will render your water quite poisonous for human consumption, virtually irrevocably.

sega_sai 3y ago

One nice illustration of "space is really big" is a fact that if you take the cube with the side equal to the distance from the Sun to the nearest star and fill it with water, then the mass of this cube will be roughly equal to the mass of the whole visible Universe.

wincy 3y ago
Another good one for me is that a cubic light year or butter would immediately collapse into a black hole with a Schwarzchild radius larger than the observable universe.
- jemfinch 3y ago
  It's impossible for your fact and the fact you're replying to both be true. Water is denser than butter, and the nearest star to the sun is about 4.3ly away; if your fact were true, the universe would be a black hole. A cubic lightyear is about 8.468e+50 liters, and butter weighs 911 g/L, giving the mass of a cubic lightyear of butter to be 7.714348e+50, whose Schwarzchild radius is about 121,103,293 lightyears, about 100x smaller than the radius of the known universe.
  - heads 3y ago
    Wasn’t one of the great puzzles of the 20th century exactly this — will the universe collapse back into a singularity or not?
  - lloeki 3y ago
    > if your fact were true, the universe would be a black hole ... maybe it is? Hear my pet theory out. Extrapolating backwards from the expansion of our universe, the Big Bang model posits a hyperdense state that exceeds black hole levels originating from a singularity, yet it's thought that somehow it did not collapse back, handwaving it as "physics as we know it did not apply". But maybe physics as we know it does apply. Notably physics as we know it does not imply a specific direction for the arrow of time. So our universe might very well be a black hole, but we have time backwards compared to the usual way we think of black holes: what we think of as the origin of time and space is what we think of as the irremediable end of time and space in a black hole.

sliken 3y ago

From quora: The vacuum in solar space is around 10^7 atoms per cubic metre; the vacuum of interstellar space is around 10^6 atoms per cubic metre; and the vacuum of intergalactic space is around 1 atom per cubic metre.