Atom Annoying
August 24, 2011 12:26 AM
Is the number of atoms in the universe static (or as near dammit)?
A question from a colleague has stumped me. While comparing the number of orders a deck of cards can be shuffled into against the number of atoms in the universe (we looked it up rather than counting by hand) it turns out the sometimes-quoted fact that the former is greater than the latter is wrong.
My colleague then asked, "will the number of atoms stay the same?" and I had to admit defeat. I know that fusion will reduce the number of atoms, and fission increase them, but I have no idea which occurs most often and add in stuff like antimatter and I'm baffled before I can even start thinking about it. Any ideas?
A question from a colleague has stumped me. While comparing the number of orders a deck of cards can be shuffled into against the number of atoms in the universe (we looked it up rather than counting by hand) it turns out the sometimes-quoted fact that the former is greater than the latter is wrong.
My colleague then asked, "will the number of atoms stay the same?" and I had to admit defeat. I know that fusion will reduce the number of atoms, and fission increase them, but I have no idea which occurs most often and add in stuff like antimatter and I'm baffled before I can even start thinking about it. Any ideas?
I think the key to answering this is to look at the life cycle of a single star: for up to billions of years, it's mostly converting hydrogen to helium (i.e. fission, reducing the number of atoms).
A star the sun's size or less (and the sun is way above average for stars) will eventually become a red giant, fusing helium to create carbon and oxygen.
A much bigger star will supernova, creating higher-atomic-weight elements, also by fusion, and spreading them round the galactic neighborhood. We're living on a chunk made of such by-products.
Some of those heavier elements will then decay by fission. But the radioactive elements are a tiny fraction of the planet, and the heavier elements in general are a small fraction of the total mass of the solar system.
So unless I'm missing something, fusion runs way, way ahead of fission.
posted by zompist at 1:11 AM on August 24, 2011
A star the sun's size or less (and the sun is way above average for stars) will eventually become a red giant, fusing helium to create carbon and oxygen.
A much bigger star will supernova, creating higher-atomic-weight elements, also by fusion, and spreading them round the galactic neighborhood. We're living on a chunk made of such by-products.
Some of those heavier elements will then decay by fission. But the radioactive elements are a tiny fraction of the planet, and the heavier elements in general are a small fraction of the total mass of the solar system.
So unless I'm missing something, fusion runs way, way ahead of fission.
posted by zompist at 1:11 AM on August 24, 2011
Dang similar-looking words. 'Fission' in the first paragraph was supposed to be fusion.
posted by zompist at 1:12 AM on August 24, 2011
posted by zompist at 1:12 AM on August 24, 2011
It's probably too early in our understanding of the universe for us to answer this question definitively. Our cosmological models and understanding of the basic structure of the universe are very immature; relatively recently, astronomer's have come to believe that perhaps 83% of the mass of the universe may be in dark matter, a form of matter about which we have no real understanding. Thus, only about 17% of the mass of the observed universe is in normal matter.
So, as soon as we get any real understanding of the nature of dark matter, we'll get back to you on the rate of decrease or increase in the number of atoms in the universe.
posted by paulsc at 1:29 AM on August 24, 2011
So, as soon as we get any real understanding of the nature of dark matter, we'll get back to you on the rate of decrease or increase in the number of atoms in the universe.
posted by paulsc at 1:29 AM on August 24, 2011
You may wish to consult the Wikipedia article Future of an expanding universe. The half-life of the proton may be >1x10^34 years, if proton decay does occur. For comparison, the current age of the universe is estimated at 1.37×10^10 years.
posted by benzenedream at 2:20 AM on August 24, 2011
posted by benzenedream at 2:20 AM on August 24, 2011
What zompist said. Heat death is irrelevant, and while there's a lot of stuff we really don't understand, things relating to the number of atoms are pretty solid with the exception of the mentioned proton decay (which is on extreme timescales). There's not much you need to worry about in terms of small details of the physics - baryon number, which is broadly the number of protons and neutrons, is pretty well conserved so you don't need to worry about antimatter or anything else. To make antimatter you have to make matter to go with it, so the total baryon number stays the same (this clearly isn't always true as it must have gone wrong early in the universe to leave us with so much matter, but it's pretty much not going to go significantly off now).
On a historical note, there was at one point a suggestion by those liking steady-state theories (where the universe pretty much always looks the same at all times) that there was a mechanism by which atoms would regularly pop into existence at a low rate, but there's no reason to think that now that the big bang is basically accepted as fact.
posted by edd at 3:00 AM on August 24, 2011
On a historical note, there was at one point a suggestion by those liking steady-state theories (where the universe pretty much always looks the same at all times) that there was a mechanism by which atoms would regularly pop into existence at a low rate, but there's no reason to think that now that the big bang is basically accepted as fact.
posted by edd at 3:00 AM on August 24, 2011
Way over my head, but can't you have a constant number of baryons and a changing number of atoms? Four hydrogen atoms fusing into one helium atom is still 4 baryons but only 2 atoms at the end of the process.
posted by Quietgal at 8:47 AM on August 24, 2011
posted by Quietgal at 8:47 AM on August 24, 2011
Quietgal: exactly, which is why the important thing is what zompist said. The baryon number stuff just means there's no other way you can mess around with the atom count except by fusion and fission.
posted by edd at 9:06 AM on August 24, 2011
posted by edd at 9:06 AM on August 24, 2011
The baryon number stuff just means there's no other way you can mess around with the atom count except by fusion and fission.
What about when small amounts of mass are 'destroyed' during fission, creating the huge amounts of energy? By converting mass into energy (there's no reverse process, is there?) are we 'losing' atoms?
posted by liquidindian at 9:25 AM on August 24, 2011
What about when small amounts of mass are 'destroyed' during fission, creating the huge amounts of energy? By converting mass into energy (there's no reverse process, is there?) are we 'losing' atoms?
posted by liquidindian at 9:25 AM on August 24, 2011
There are reverse processes that can turn energy into mass, they are just not common as it takes a lot of energy to get mass (plus, we're usually interested in getting energy out of things, not cramming energy back into things, for economic reasons). It does happen in particle colliders though, or in cosmic ray interactions in the atmosphere.
When mass is converted to energy in a fission process, you actually GAIN atoms, while losing energy. For example, uranium-235 (that's the fun "let's build a bomb" isotope), undergoes natural alpha decay, turning into thorium-231 and an alpha particle (which is just a helium-4 nucleus): so 1 atom turns into 2. In fission, you throw a free neutron at U-235, and it will break apart, releasing two lighter nuclei, at least two free neutrons and some gamma rays. The neutrons (if they don't continue to induce fission), would eventually decay into protons (and electrons+antineutrinos), leaving at least 4 new atoms (the heavy elements generated this way will themselves decay as well). So either way, you end up with more atoms than you started with. The numbers of baryons (protons+neutrons) stays the same in all cases though, because (ignoring some theoretical and known-to-be-EXTREMELY rare interactions) baryon number is an absolutely conserved quantum number in the Universe. (by the way, beta decay is a totally different process, that will not change the number of atoms)
If the baryon number is conserved, where does the energy of fission come from? This is due to the "binding energy" of nuclei, basically the energy that is required to hold the positively charged protons in a small volume. As positive charges repel each other, so the protons would naturally fly apart. They are held together by the residual strong force, which has a limited range. Elements that fission (or decay via alpha processes) are so large they are at the limit of their residual strong interaction. So, by fissioning into two smaller nuclei, the two resulting products are more strongly bound. Binding energy counts as "negative," so what's going on is that we move from a slightly negative "barely-bound" nuclei to two more negative "strongly-bound" states. The resulting energy change is where we get the energy from a fission reaction.
Fusion goes the other way. The nuclei involved are all small, so adding more nucleons gets to more negative binding energy (because you've added a new source of residual strong interaction, and while you added some new protons as well, the strong interaction still wins over the electric repulsion). The turning point is iron: this is the element that is too heavy to fuse, but too light to fission (fize?). Stars fuse light elements until the core is pure iron, then run out of available fuel, and collapse. Anything heavier than iron was created in a supernova, when a collapsing star gets even hotter and the resulting available energy is enough to force the nuclei to bind together into configurations that would normally not be energetically favorable. Some of these configurations are long-lasting, and those are the heavy elements that make it out of the supernova remnant, float around for awhile, and then get reprocessed into a new solar system like our own (so, another example when energy -> mass, not mass -> energy).
To get back to your question, as zompist said, since there are more stars fusing things than fissionable atoms, the number of atoms will tend to decrease over time, not increase. However, since most of the atoms in the Universe are not in stars (they just float around in very cold intergalactic space), the percentage decrease in the number of atoms is going to be very negligible.
posted by physicsmatt at 10:35 AM on August 24, 2011
When mass is converted to energy in a fission process, you actually GAIN atoms, while losing energy. For example, uranium-235 (that's the fun "let's build a bomb" isotope), undergoes natural alpha decay, turning into thorium-231 and an alpha particle (which is just a helium-4 nucleus): so 1 atom turns into 2. In fission, you throw a free neutron at U-235, and it will break apart, releasing two lighter nuclei, at least two free neutrons and some gamma rays. The neutrons (if they don't continue to induce fission), would eventually decay into protons (and electrons+antineutrinos), leaving at least 4 new atoms (the heavy elements generated this way will themselves decay as well). So either way, you end up with more atoms than you started with. The numbers of baryons (protons+neutrons) stays the same in all cases though, because (ignoring some theoretical and known-to-be-EXTREMELY rare interactions) baryon number is an absolutely conserved quantum number in the Universe. (by the way, beta decay is a totally different process, that will not change the number of atoms)
If the baryon number is conserved, where does the energy of fission come from? This is due to the "binding energy" of nuclei, basically the energy that is required to hold the positively charged protons in a small volume. As positive charges repel each other, so the protons would naturally fly apart. They are held together by the residual strong force, which has a limited range. Elements that fission (or decay via alpha processes) are so large they are at the limit of their residual strong interaction. So, by fissioning into two smaller nuclei, the two resulting products are more strongly bound. Binding energy counts as "negative," so what's going on is that we move from a slightly negative "barely-bound" nuclei to two more negative "strongly-bound" states. The resulting energy change is where we get the energy from a fission reaction.
Fusion goes the other way. The nuclei involved are all small, so adding more nucleons gets to more negative binding energy (because you've added a new source of residual strong interaction, and while you added some new protons as well, the strong interaction still wins over the electric repulsion). The turning point is iron: this is the element that is too heavy to fuse, but too light to fission (fize?). Stars fuse light elements until the core is pure iron, then run out of available fuel, and collapse. Anything heavier than iron was created in a supernova, when a collapsing star gets even hotter and the resulting available energy is enough to force the nuclei to bind together into configurations that would normally not be energetically favorable. Some of these configurations are long-lasting, and those are the heavy elements that make it out of the supernova remnant, float around for awhile, and then get reprocessed into a new solar system like our own (so, another example when energy -> mass, not mass -> energy).
To get back to your question, as zompist said, since there are more stars fusing things than fissionable atoms, the number of atoms will tend to decrease over time, not increase. However, since most of the atoms in the Universe are not in stars (they just float around in very cold intergalactic space), the percentage decrease in the number of atoms is going to be very negligible.
posted by physicsmatt at 10:35 AM on August 24, 2011
« Older Birth Control Filter: I think the pill might be... | Single-click Android app for time tracking sought.... Newer »
This thread is closed to new comments.
posted by michaelh at 12:34 AM on August 24, 2011