How do people really know that atoms exist even though they can't see them?
No one has ever really seen an atom. Humans like to see something before they believe in it. I am sure there are some people who object to that since there have been claims that electron microscopes have imaged atoms. I believe that illuminating an object with electrons, capturing those electrons and recreating an image is also indirect evidence. Just imagine a primitive human. We'll call him Og. Just having a fifth grade education would make you a genius compared to Og. But Og, being human and kind of hairy, has one thing really powerful in his favor. He has an overpowering curiosity. Og wonders about almost everything. Og is bothered by everything he knows he doesn't know. Og does know that the world is made up of lots of different things and sometimes those things can change. For instance if you burn wood it turns to ash and ash is different from wood. Eventually Og, or perhaps one of his descendants, learns to make a stone knife and learns to cut things. Og's teenage son is playing with Og's stone knife trying to see how small he can cut an object.
This was probably repeated thousands of times before we also learned to write and share information. Then around 400 BC a greek dude named Democritus came up with the idea that something could be cut into it's smallest piece and it would still be the same object. He was also the first person to write the word atom down. For these reasons Democritus was given credit for the idea and the name. Those ancient Greeks became really good at sitting around thinking, but they weren't all that good at building instruments to prove their thoughts and ideas. It took a few thousand years, until just recently when we got good enough at making machines that we could prove Democritus' atom theory. Can you imagine that? He came up with an idea, but it took 2,400 years before anyone figured out that he was right!
Now I said no one has seen an atom, but we have seen so much evidence of their existence that most of us believe in them. Most of the experiments, like those here at Jefferson Lab, work by bouncing something off an atom, like an electron, and watching where the electron goes after it bounces out of the atom. Of course, an electron is smaller than an atom, so it would seem to get trickier. An electron is actually pretty easy to see since it leaves tracks in many things. Using those tracks, we can build a pretty nice picture of an atom. The story is typical in science and many human endeavors. Someone comes up with an idea and then tries to prove it. One could argue that Columbus' trip was an experiment to prove or disprove his theory that India could be reached by sailing West from Europe.
The end of our primitive story is that Og's son accidentally cut himself with the knife. Og caught him playing with the knife and grounded him for a week.
Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)