What fills space?

Craig Hogan, director of the Center for Particle Astrophysics, wrote this column.

Craig Hogan

If you follow the news about physics, you might think that physicists don’t know what they are talking about when it comes to space.

I am not talking about the mysteries of outer space, or cataclysms like black holes. I mean ordinary space itself, the inner space between particles everywhere—what we used to call empty space or vacuum. What’s in it? Sometimes we hear that atoms are “mostly empty space.” Now we read in the papers that the newly discovered Higgs field “fills all of space” and “gives particles mass,” that it acts like a kind of space-filling “molasses,” or that it’s like a space-filling crowd of groupies hanging on as a celebrity’s posse.

On the other hand, astronomers tell us that space is expanding. Last year, the Nobel Prize in physics was awarded for the discovery that the cosmic expansion is speeding up. Scientists think that this acceleration is propelled by what they call “dark energy,” which fills and refills that ever-expanding void of intergalactic space. Cosmological space is said to be expanding in some places (between galaxies) and not expanding in others (such as Brooklyn, to choose Woody Allen’s example).

It gets even worse if you dig deeper. For example, the Higgs field is much weirder than the comparisons with molasses or crowds suggest, since it does not actually drag or impede particles, but still somehow shares its mass with them.

Stranger still, consider another space-filling field that also adds mass to everyday substances, in a way different from the Higgs field. The gluons of the strong nuclear force field create most of the mass of atoms through the energy of their incessant motion inside tiny bubbles of space that we call protons and neutrons. Since the mass-giving gluons are immune to the Higgs field, they have no mass themselves, but only add energy because of their motion. Moreover, they are held inside those bubbles by a gluon field that fills empty space everywhere between the bubbles…in just those places in space where the added mass isn’t.

Space is the first concept of physics we all learn as little kids, yet it is entangled with some of the deepest mysteries confronting physics. Confusing, koan-like paradoxes about space are not just pablum: They reflect a real and profound disparity of descriptions, at a deep level of mathematics, about what defines a vacuum, a position, a particle or a time.

It may be that all the space of the universe began, and may end, dominated by the energy of the vacuum, an expanding space devoid of particles. It may be that when examined over very short time intervals, space as we know it does not even exist, but dissolves into a cloud of quantum indeterminacy: It may never sit still, but constantly seethe in microscopic motion. It may be that space has many more than three dimensions on very small scales, while there may be only two truly independent dimensions on large scales. It may even be that all of these exotic possibilities actually apply in the real world.

At Fermilab, we are working on experiments including the Dark Energy Survey, the Holometer and the CMS experiment at the Large Hadron Collider that will probe these ideas in very different ways. If you want to find out more—watch this space!