If you wanted to uncover the secrets of the Universe for yourself, all you’d have to do is interrogate the Universe until it revealed the answers in a way you could comprehend them. When any two quanta of energy interact — irrespective of their properties, including whether they’re particles or antiparticles, massive or massless, fermions or bosons, etc. — the result of that interaction has the potential to inform you about the underlying laws and rules that the system has to obey. If we knew all the possible outcomes of any interaction, including what their relative probabilities were, then and only then would we claim to have some understanding of what was going on. Being quantitative in precisely this fashion, asking not only “what happens” but also “by how much” and “how often,” is what makes physics the robust science that it is.

Quite surprisingly, everything that we know about the Universe can, in some way, be traced back to the most humble of all the entities we know of: an atom. An atom remains the smallest unit of matter we know of that still retains the unique characteristics and properties that apply to the macroscopic world, including the physical and chemical properties of matter. And yet, an atom is a fundamentally quantum entity, with its own energy levels, properties, and conservation laws. Moreover, even the humble atom couples to all four of the known fundamental forces. In a very real way, all of physics is on display, even inside a single atom. Here’s what they can tell us about the Universe.

scale of objects in the Universe

From macroscopic scales down to subatomic ones, the sizes of the fundamental particles play only a small role in determining the sizes of composite structures. Whether the building blocks are truly fundamental and/or point-like particles is still not known, but we do understand the Universe from large, cosmic scales down to tiny, subatomic ones. The scale of quarks and gluons is the limit to how far we’ve ever probed nature.
Credit: Magdalena Kowalska/CERN/ISOLDE team

Here on Earth, there are approximately ~90 elements that occur naturally: left over from the cosmic processes that created them. An element is fundamentally an atom, with an atomic nucleus made of protons and (possibly) neutrons and orbited by a number of electrons that’s equal to the number of protons. Each element has its own unique set of properties, including:

  • hardness,
  • color,
  • melting and boiling points,
  • density (how much mass occupied a given volume),
  • conductivity (how easily its electrons are transported when a voltage is applied),
  • electronegativity (how strongly its atomic nucleus holds onto electrons when bound to other atoms),
  • ionization energy (how much energy is required to kick an electron off),

and many others. What’s remarkable about atoms is that there’s only one property that defines what type of atom you have (and hence, what these properties are): the number of protons in the nucleus.

Every atom, with its unique number of protons in its nucleus, will form a unique set of bonds with other atoms, enabling a practically unlimited set of possibilities for the types of molecules, ions, salts, and larger structures that it can form. Primarily through the electromagnetic interaction, the subatomic particles that compose atoms will exert forces on one another, leading — given enough time — to the macroscopic structures we observe not only on Earth, but everywhere throughout the Universe.

At their very core, however, atoms all have the property of being massive in common with one another. The more protons and neutrons in the atomic nucleus, the more massive your atom is. Even though these are quantum entities, with an individual atom spanning no more than a single ångström in diameter, there’s no limit to the range of the gravitational force. Any object with energy — including the rest energy that gives particles their masses — will curve the fabric of spacetime according to Einstein’s theory of General Relativity. No matter how small the mass, or how small the distance scales are that we work with, the curvature of space induced by any number of atoms, whether ~1057 (like in a star), ~1028 (like in a human being), or just one (like in a helium atom), will occur exactly as the rules of General Relativity predict….