Where Some See Strings, She Sees a Space-Time Made of Fractals

Astrid Eichhorn spends her days thinking about how the laws of physics change at the tiniest scales.

Imagine zooming in closer and closer to the device on which you’re reading this article. Its apparently smooth screen quickly dissolves into a jiggling lattice of molecules, which in turn resolve into clouds of electrons buzzing around atomic nuclei. You dive into a nucleus, and atoms disappear as you enter the domain of quarks. It is here, where protons loom as large as solar systems, that Eichhorn’s explorations begin.

Past this point, the fundamental forces themselves shift. Electromagnetism and the weak interaction intensify, while the strong force slackens. The changes happen in a fairly regular way, so physicists have a good sense of how they work … until they don’t.

When an atom appears as large as the observable universe, the established laws of physics can no longer tell you what happens between particles separated by an atom’s width. Gravity, a force that’s too weak to notice at the scale of atoms, grows strong in an erratic way. You’ve just crossed over into the “Planck” realm.

The apparent breakdown of particle physics at this scale has inspired some dramatic theories. Some physicists argue that this failure point in our understanding tells us that the universe is fundamentally composed not of particles, but of vibrating strings and membranes. Others argue that at these smallest scales, space and time themselves must dissolve into structures such as loops.

Eichhorn and her colleagues are pursuing a different possibility. In 1976, Steven Weinberg, a theorist who would eventually earn a Nobel Prize, pointed out that if you zoomed in far enough, you might reach a place where the rules of physics would stop changing. New realms would stop appearing; the intensities of the forces would stabilize; and gravity would turn out to make perfect sense after all.

Eichhorn, a physicist at Heidelberg University in Germany, has over the last decade become a leading theorist investigating this idea, called asymptotic safety. In particular, Eichhorn has emphasized the importance of taking into account the ways in which matter affects space-time, and vice versa. “She is the expert of gravity-matter systems in asymptotic safety,” said Alessia Platania, a physicist at the University of Copenhagen who has worked with Eichhorn.