The ancient Greek and Indian philosophers who first conjectured that matter was made of tiny little bits of stuff would not believe that now, more than 25 centuries later, we can actually see them.
Granted, modern atoms are quite different from their old counterparts, given that they are actually not indivisible but, instead, made of electrons orbiting a positively-charged nucleus. Still, visualizing such tiny structures has remained a challenge since the idea took hold in modern times, during the turn of the 20th century.
In fact, many notable scientists at the time doubted that atoms actually existed — that is, that they were part of reality, since we couldn't see them. How can we tell if something exists if we can't have direct evidence of it? Good question, but one that certainly depends on the state of technology at the time. Assuming, of course, that the entity can be studied experimentally at least indirectly without violating any law of nature.
In 2013, a team of Dutch physicists led by Aneta Stodolna was able to visualize the elusive wave function of the electron in a hydrogen atom. Hydrogen, being the simplest chemical element, has only a single electron orbiting a single proton in the nucleus. The theory of quantum mechanics predicts that the electron should occupy "orbitals," sort of spherical shells around the proton where it can be found with a certain probability. The wave function is the mathematical object we use to compute this probability of finding the electron here or there when we measure its position.
As it turns out, the theory predicts that the orbitals have a beautiful and convoluted onion-like structure with gaps in between the shells, places where the electron can't be found. Stodolna's images, using a technique called photoionization microscopy, were able to construct a visual map of such orbitals that match the theory quite accurately. No one doubted that quantum physics was right, but seeing is believing, as they say.
Equally amazing, and at around the same time, a group of scientists at the University of California, Berkeley, visualized a chemical reaction atom-by-atom, showing not only how the atoms rearranged after the reaction but also the chemical bonds between them, the bridges that connect them together, a bit like Erector sets.
The visualization of such reactions allows scientists a much more hands-on control of the dynamics of chemical reactions, something crucial in applications where new molecules and materials are being designed.
But more than that, the techniques vindicate the power of the human imagination. Atoms and molecules have long been part of the physics and chemistry toolbox, and their reactions and level structure has been inferred theoretically and by means of ingenious experiments and spectroscopic techniques. (Studying the kinds of visual light and other types of electromagnetic radiation the atoms emit and absorb as they are cooled down or warmed up for example, or during reactions and collisions.)
During the first decades of the 20th century, a whole new kind of physics was developed, quantum physics, with only theoretical and indirect experimental evidence of the existence of such structures. We didn't doubt they were there, since the theory explained experiments so well, but we weren't quite sure of the details. Now we are. Students can turn to textbooks and look at those atomic orbitals and molecular bindings with the confidence of someone who knows that they are not just ideas or concepts but real entities that make us and every other kind of matter in the universe.
Marcelo Gleiser is a theoretical physicist and writer — and a professor of natural philosophy, physics and astronomy at Dartmouth College. He is the director of the Institute for Cross-Disciplinary Engagement at Dartmouth, co-founder of 13.7 and an active promoter of science to the general public. His latest book is The Simple Beauty of the Unexpected: A Natural Philosopher's Quest for Trout and the Meaning of Everything. You can keep up with Marcelo on Facebook and Twitter: @mgleiser