What Is Matter? Matter vs. Mass, Explained

At the end of a lecture on astronomy, describing how the planets orbit the Sun, a little old lady at the back of the room stood up and said: “What you’ve told us is all rubbish. The world is really a flat crust of earth supported on the back of a giant tortoise.”

The lecturer frowned and replied, “But then, madam, what is the tortoise standing on?”

“You’re very clever, young man, very clever,” said the old lady. “But it’s no use. It’s turtles all the way down!”

Young people, and the young at heart, ask great questions. But sometimes, the endless questions run into bigger answers than anyone expects. What’s that tree made of? Leaves and bark, xylem and phloem, and cambium. What are those made of? Cells, molecules, carbon atoms. But what’s an atom made of? What is matter, really?

Matter is defined as that which 1) has mass and 2) has volume or takes up space. Mass is an intrinsic property whose base SI unit is the kilogram (kg). Volume is an extrinsic property—that is, a property or characteristic that depends on sample size—which describes a region in three-dimensional space.

Simple, right?

There are many subtly different but equally correct definitions of matter and mass. Anything made of atoms or molecules has mass. Just about everything we encounter in our day-to-day lives fits that definition, with the notable exception of light. (More on that in a moment.) But as usual in science, there’s more to the story.

Forces and Fields

On human scales of mass and distance, matter obeys the laws of motion described by Sir Isaac Newton. Chief among them is the law of inertia, upon which all others are based: a body at rest will remain at rest, and a body in motion at a constant speed will keep that same speed unless acted upon by a force. (Air resistance due to friction counts as a force, which is why homework and test questions that deal with kinematics often disregard it.) The principle of equal and opposite reaction is another of Newton’s Laws of Motion. So, too, is F = ma, which describes forces acting on an object with respect to its mass and acceleration. Newton was a mathematician and a physicist, and those who have had advanced math may recognize F = ma as the quintessential example of the Chain Rule, central to Newton’s system of differential calculus. It’s all very orderly.

On the subatomic scale, however, it’s a very different story. As things get smaller, they get weirder. Chaos theory starts to matter a lot. From the perspective of the Standard Model, matter isn’t solid at all; it’s a tiny nucleus in a sea of empty space, and beneath that, matter is a ripple in one of the many overlapping force fields that permeate spacetime. Forces, in turn, arise from interactions with evanescent force carrier particles. For example, in the Standard Model, photons are the force carrier particles of the electromagnetic field. Matter, as we understand it, gets its mass from interactions with a different field, whose force carrier is called the Higgs boson. (Formerly known as the ‘God particle,’ en homage to the deep secrets about the Universe it could reveal, the Higgs boson was discovered by Peter Higgs and other scientists at CERN in 2012 after a 40-year search. The discovery immortalized Higgs and captured the 2013 Nobel Prize in Physics.)

Photons are massless, which is why they travel at 100% of the speed of light. They act like particles that can collide and waves that simply intersect. Despite not having mass, photons are affected by gravity. Photons also have momentum: since photons interact with electrons, light alone can physically push matter through space, a phenomenon called radiation pressure.

Conservation: It’s Not Just a Good Idea, It’s the Law

For ordinary matter, momentum is given by the formula P = mv, where P is momentum, m is mass, and v is velocity. Even tiny amounts of matter can have vast amounts of momentum if the matter is moving fast enough. For example, in 1991, an ultra-high-energy cosmic ray, now known as the OMG particle, hit a detector in Utah with the kinetic energy of a 63 mph fastball. Most cosmic rays are protons, and to have had that kind of energy, a proton would have had to be traveling at more than 0.9999999999 c.

Momentum is one of the few things in nature with its own law of conservation. Conservation laws identify measurable properties of a physical system that don’t change as the system moves through time. Conserved quantities in physics include charge, linear and angular momentum, and mass-energy, the latter of which is the relationship Einstein expressed in his famous equation E = mc².

When opposite charges equalize, they aren’t destroyed; instead, charged particles move around into a lower-energy configuration. Similarly, matter can neither be created nor destroyed. But because of underlying symmetries in the Universe, matter can be transmuted into energy through a process called annihilation.

Antimatter, the Real Philosopher’s Stone

Quantum field theory holds that particles and antiparticles are equal but opposite fluctuations in the same underlying matter field. This is where the idea of antimatter comes from. According to the Standard Model, for each type of particle that makes up “ordinary” matter, there’s a corresponding antiparticle equal in all ways except for having the opposite charge. By these rules, for example, protons have antiparticles called antiprotons, and electrons get antielectrons—more commonly known as positrons. (Neutrons have no charge, so antineutrons also have no charge, but they have other equal and opposite properties on the subatomic level. Photons are their antiparticle.)

Overlapping sound waves in the same place can interfere with one another, creating weird dead zones where sound seems to vanish, or places where the sound waves suddenly carry enough energy to shatter glass or break up gallstones. Active noise canceling works because of the same principle of destructive interference. It works the same with light. But matter and antimatter create mutual interference so intolerable to reality that if two sister antiparticles, such as an electron and a positron, find themselves in the same place, both waveforms disintegrate into something altogether different. At the same time, the charges of antiparticles are equal but opposite, so they attract one another. This is why antimatter must be contained inside a “magnetic bottle.” Matter can’t contain it. When a particle and its antiparticle meet, their mass is converted into energy and released as light.

Symmetry and the Big Questions

In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. —Douglas Adams, The Restaurant at the End of the Universe

One of the biggest questions in physics is why anything exists at all. This is not a question about whether God created the Universe; it’s a question about symmetry. In modern cosmology, the Universe came into being at the moment of the Big Bang. As a whole, physicists assume that the Universe is neutrally charged; that is, the sums of all its positive and negative charges are the same, adding up to zero. Likewise, it seems like it should follow that there would have been equal amounts of matter and antimatter created. But if that was the case, it would have all been annihilated, leaving nothing behind but a titanic flash of light. So why is the Universe full of matter? Why is there anything left in the Universe to try to understand itself? This has caused physicists a lot of consternation. However, as the proverb says, necessity is the mother of invention.

Cloud chambers and particle beams are two of the important tools used to learn about the interactions between particles in the Standard Model’s “particle zoo.” Particle beams and colliders like CERN send photons and protons hurtling through space and into one another, smashing them to bits and sending the pieces flinging in all directions while cloud chambers and bubble chambers track the paths of the fragments. This informs the mathematical models of the early Universe that cosmologists use to build a narrative of the moments immediately after the Big Bang. At the same time, observatories like the James Webb Space Telescope and the Wilkinson Microwave Anisotropy Probe turn back the clock as they gaze ever farther into the night sky. All this technology has answered so many questions—but in the wake of those answers, other questions open up: for example, the problem of dark matter.

Dark Matter

Modern telescopes enabled Edwin Hubble and his contemporaries to discover that the Universe was much larger and much older than anyone had understood. Through a powerful telescope, stars in the night sky transform from points of light into entire galaxies with internal structures like bars and spiral arms.

Hubble’s eponymous telescope. Credit: NASA

Zooming out to look at entire clusters of galaxies, astronomers found that the clusters didn’t contain enough mass to keep them from drifting apart. Looking more closely at individual galaxies, astronomers realized their motion was…wrong. The outside of the galaxy rotated around its core at a rate too fast to agree with Newton’s law of gravity. Such a speed suggested the presence of some kind of invisible something that acted like inertial mass, making the outskirts of galaxies spin around their nuclei with more momentum than they should possess.

Further study turned up non-answer after non-answer. No study or probe has yet found an identifiable sample of dark matter. What is it? No one knows. And yet it shows up repeatedly in the data, with disheartening consistency. In a sense, dark matter is our name for the problem, as much as it is the name of a type of matter. What particles interact with dark matter? Basically none, leading to a tongue-in-cheek nickname for dark matter: weakly interacting massive particles, or WIMPs. (Massive compact halo objects, or MACHOs, are the other top contenders for the true identity of dark matter. Scientists do love their puns.)

If this offends your sense of reasoned inquiry, you’re in good company. Fed up with the status quo, instead of looking for dark matter through telescopes, scientists are now hunting for it directly. Maybe it’s arrogant to believe we’ll know what dark matter is, but then again, maybe not. It’s thought that 95% of the Universe’s total mass and energy are dark matter and dark energy, respectively. We call that a target-rich environment.