Where does fission collide?

The splitting of a heavy atomic nucleus, such as uranium, into two lighter nuclei when it absorbs a neutron, releasing a large amount of energy and further neutrons that can sustain a chain reaction. Meitner and Frisch's 1939 Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction interpreted the experiments of Hahn and Strassmann and named the process fission, borrowing the word from the way a living cell divides. The referent is a physical event inside a single atom's core, counted in released energy and neutrons, not a whole cell splitting in two or a human group breaking apart.[1]

Unit

One heavy nucleus splitting into lighter fragments, with energy and neutrons released

Recovery means

Not applicable, because the split fragments are new, lighter nuclei and do not reassemble into the original atom

Measured by

Detected by the energy and neutrons released and by identifying the lighter fragment nuclei with radiochemistry and particle detectors

The asexual reproduction in which a single parent cell divides into two roughly equal daughter cells, the ordinary way most bacteria multiply: the cell copies its genetic material, grows, and splits across the middle. Angert's 2005 Alternatives to Binary Fission in Bacteria reviewed this standard mode of division alongside less common patterns, such as multiple fission, that depart from it. The referent is the reproduction of a whole living cell, counted by generations and doubling time, not a nucleus breaking inside an atom or a social group dividing.[2]

Unit

One parent cell dividing into two daughter cells

Recovery means

Not applicable, because each division yields two independent cells and the parent does not persist as a separate thing

Measured by

Counted by cell number, generation time, and doubling rate in culture, and observed directly under the microscope

The splitting of a social group, lineage, or political unit into two or more separate parts, as when a band, community, or faction divides. Aureli and colleagues' 2008 Fission-Fusion Dynamics set out a framework for the way animal and human groups repeatedly break into smaller parties and merge again, changing in size and membership over time. The referent is the division of a group of individuals, described by who leaves with whom and how membership shifts, not a cell reproducing or an atom's nucleus breaking apart.[3]

Unit

One social group dividing into two or more subgroups

Recovery means

Often reversible, because split parties may later fuse again, which is the fusion half of fission-fusion dynamics

Measured by

Tracked by group membership, party size, and the changing composition of subgroups over time