I've enjoyed writing answers to a few questions from children over the past few weeks, including "How strong is space?" And "Can atoms touch?The best questions for kids are formulated in a way that seems to have an obvious answer, but upon closer inspection leads to some deeper problems that turn out to be a bit subtle. I got another one this week, as a preliminary query for a "Skype a scientist' which I'll do later this week, but it's fun so I'll type it out now.
The question is the title of this article: How were atoms created? This is a question with a long history, dating back nearly fourteen billion years.
The seemingly obvious answer here is that a tremendous amount of energy was released during the Big Bang, and because matter and energy are different forms of the same thing, some of that energy manifested as fundamental particles. Initially, this was a huge mass of quarks and electrons and photons, and in the very first moment the universe was so hot and dense that they could not combine into atoms: each time three quarks tried to combine into one proton or neutron, another Particle would rush in at high speed, disrupting the would-be nucleon. And in that case, forget about electrons sticking to a nucleus...
Eventually, as the universe began to expand and cool, things expanded and slowed to the point where quarks could combine into nucleons without immediately breaking apart. This begins a time when protons, neutrons, and electrons are running around, colliding too frequently and too hard for stable atoms to form. After a while things cool down to the point where protons and neutrons can stick together to form nuclei without being broken apart immediately, and then quite a while after that things cool down to the point where nuclei Capturing electrons and mostly forming atoms can hydrogen without being immediately blown apart by a passing photon.
The stages here vary in length of time—a few seconds to get to protons and neutrons, a few minutes to get to nuclei, and a few hundred thousand years to get to atoms. This final step is the first thing we can see in the universe: These famous images of the cosmic microwave background capture the light that rattled through the universe at the point where nuclei and electrons combined into atoms and stopped absorbing the whole light before it could travel very far.
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That's a nice simple story - Big Bang makes stuff, stuff cools, stuff sticks together, atoms! – which hides a lot of subtleties. There are two things I want to mention in particular, one that is reasonably well understood and one that is a very deep mystery.
The reasonably understandable point is this: You can't make a hydrogen atom out of just one electron and one proton. This may sound strange, because after all, a hydrogen atom is just an electron attached to a proton, but it's absolutely true: if you only have a proton and an electron and nothing else, there's no way you can put them together into one atom.
This happens because there are two important quantities to consider when thinking about the interaction of these particles: energy and momentum. These are similar in that they both depend on the mass and velocity of the particle in question, but momentum is just mass times velocity (but can be either positive or negative depending on which direction it's moving), while energy is (roughly speaking) is ) mass times velocitychecked(and therefore always positive). If you double the speed, you double the momentum, but quadruple the energy.
Both quantities must also be conserved, which means that if you add up the total energy and momentum after some particles have interacted with each other, they must equal the total energy and momentum before they started interacting. And it turns out that because of the differential dependence on the particles' speed and direction, there's no way to start with just two particles and glue them together while preserving both. If you get the momentum part to work, the energy will end up being lower than it was at the beginning, and if you just worry about the energy, the momentum will come out all wrong.
So how do we get atoms? You need a third particle: another electron, another proton or a photon. If you have three bodies, two of them can stick together, lowering their overall energy, and the third carries off the extra energy.
This is a simple idea, but it has many implications. Because of this, we can date the formation of atoms to a very specific moment in the history of the universe: the temperature and density of the particles must be just right: high enough for three particles to come together for electrons to stick to atoms, but not like that so high that the newly formed atoms are thrown apart again. This is also the reason why experiments are so popularCATCHAndALPHAhave such a hard time making atoms out of antihydrogen: they have to find the same kind of balance.
Mentioning these two experiments is also a good place to move on to the deep conundrum here, which is that all atoms formed a few hundred thousand years after the Big BangObjectand not antimatter. Under normal circumstances, the conversion of energy into mass produces equal amounts of matter and antimatter: one positron for each electron and one antiquark for each quark. However, if you have equal amounts of matter and antimatter, they should quickly find each other and annihilate back into pure energy.
The fact that everything we see in the observable universe is made of matter and not antimatter suggests that contrary to our normal physical experience, when the energy released in the Big Bang was converted to mass, there was slightly more matter than antimatter was -object. Most of what was created was destroyed, but the tiny excess of ordinary matter remained. To steal a jokeEric Cornel, we are all descended from the uncool loser particles of matter that failed to bond with antimatter in the minutes after the Big Bang.
So why did this happen? Here's the secret: no one really knows. The fact that some excess matter was created in the moments after the Big Bang implies that there must be a violation of the fundamental symmetry between matter and antimatter, but while we have seen this in some processes, the violations that we know are aren't big enough to support the existence of, well,everything. This is why ATRAP and ALPHA are working to study antihydrogen in detail, and this is why other experiments are looking for tiny clues about differences between particles and antiparticles.
As I said in the beginning, this is a great kids question as it includes both a simple story as a basic answer and some subtleties that reward deeper diving. A closer look at how atoms were formed leads to all sorts of interesting places.