Lise Meitner (1878–1968) was an Austrian-Swedish physicist whose life was tragically marked by both brilliant discovery and historical injustice. She was a crucial member of the Berlin research team—alongside Otto Hahn and Fritz Strassmann (Article 162)—that discovered nuclear fission. While Hahn received the Nobel Prize for the chemical identification, it was Meitner who provided the essential theoretical physics explanation for the startling experimental results, calculating the immense energy released from the splitting of the atom using Einstein's famous mass-energy equivalence equation ($E=mc^2$).

For years, Meitner, Hahn, and Strassmann had been bombarding uranium nuclei with neutrons, expecting to create slightly heavier transuranic elements. By 1938, however, the results became baffling: the resulting radioactive products contained lighter elements, particularly Barium (atomic number 56).

• Hahn and Strassmann's Role: After Meitner was forced to flee Germany due to Nazi persecution in mid-1938, Hahn and Strassmann performed the critical chemical analysis that proved the Barium was definitely present (Article 162). They were certain of the chemistry but could not explain the physics—how could a heavy uranium nucleus ($Z=92$) break apart to form a medium-sized barium nucleus ($Z=56$)?

Meitner, corresponding with Hahn from her exile in Sweden, quickly recognized the true significance of the findings with the help of her nephew, physicist Otto Robert Frisch.

• The Model: They applied the Liquid-Drop Model (a nuclear model) to visualize the process. They realized that the uranium nucleus, upon capturing a neutron, became unstable, vibrating so violently that it stretched and split into two smaller, roughly equal-sized "drops." Frisch later coined the term "fission" for this process, borrowing the term from biology (cell division).

• The Energy Calculation: Meitner's greatest insight was calculating the energy released. She realized that the combined mass of the fission products (Barium, Krypton, and residual neutrons) was slightly less than the mass of the original uranium nucleus plus the initial neutron.

Meitner correctly concluded that the missing mass ($\Delta m$) had been converted directly into kinetic energy ($E$), exactly as predicted by Albert Einstein's equation:

$$E = \Delta m c^2$$

Meitner calculated that a single fission event released approximately 200 million electron volts ($\text{MeV}$) of energy—a colossal amount compared to chemical reactions. This confirmed that fission was a physical process (not a strange chemical artifact) and was the key to unlocking atomic power.

Meitner's explanation was published in Nature in early 1939, immediately alerting the scientific community (especially in the U.S.) to the immense possibilities of a nuclear chain reaction (Article 147).

Despite her pivotal role in guiding the experiment and providing the theoretical framework, Lise Meitner was tragically overlooked by the Nobel Committee. Otto Hahn received the 1944 Nobel Prize in Chemistry alone for the discovery of fission. Though Meitner was nominated multiple times, she was never awarded the Nobel, a historical omission widely regarded as unjust and compounded by the political and gender bias of the time.

In Conclusion: Lise Meitner provided the crucial theoretical physics explanation for nuclear fission. Recognizing that the massive uranium nucleus had split upon neutron capture, she calculated the enormous energy release by demonstrating that the process involved a conversion of mass into energy, precisely according to Einstein's mass-energy equivalence ($E=mc^2$). Her work was the essential conceptual catalyst that launched the nuclear age.

Lise Meitner (1878–1968) was an Austrian-Swedish physicist whose life was tragically marked by both brilliant discovery and historical injustice. She was a crucial member of the Berlin research team—alongside Otto Hahn and Fritz Strassmann (Article 162)—that discovered nuclear fission. While Hahn received the Nobel Prize for the chemical identification, it was Meitner who provided the essential theoretical physics explanation for the startling experimental results, calculating the immense energy released from the splitting of the atom using Einstein's famous mass-energy equivalence equation ($E=mc^2$).

For years, Meitner, Hahn, and Strassmann had been bombarding uranium nuclei with neutrons, expecting to create slightly heavier transuranic elements. By 1938, however, the results became baffling: the resulting radioactive products contained lighter elements, particularly Barium (atomic number 56).

• Hahn and Strassmann's Role: After Meitner was forced to flee Germany due to Nazi persecution in mid-1938, Hahn and Strassmann performed the critical chemical analysis that proved the Barium was definitely present (Article 162). They were certain of the chemistry but could not explain the physics—how could a heavy uranium nucleus ($Z=92$) break apart to form a medium-sized barium nucleus ($Z=56$)?

Meitner, corresponding with Hahn from her exile in Sweden, quickly recognized the true significance of the findings with the help of her nephew, physicist Otto Robert Frisch.

• The Model: They applied the Liquid-Drop Model (a nuclear model) to visualize the process. They realized that the uranium nucleus, upon capturing a neutron, became unstable, vibrating so violently that it stretched and split into two smaller, roughly equal-sized "drops." Frisch later coined the term "fission" for this process, borrowing the term from biology (cell division).

• The Energy Calculation: Meitner's greatest insight was calculating the energy released. She realized that the combined mass of the fission products (Barium, Krypton, and residual neutrons) was slightly less than the mass of the original uranium nucleus plus the initial neutron.

Meitner correctly concluded that the missing mass ($\Delta m$) had been converted directly into kinetic energy ($E$), exactly as predicted by Albert Einstein's equation:

$$E = \Delta m c^2$$

Meitner calculated that a single fission event released approximately 200 million electron volts ($\text{MeV}$) of energy—a colossal amount compared to chemical reactions. This confirmed that fission was a physical process (not a strange chemical artifact) and was the key to unlocking atomic power.

Meitner's explanation was published in Nature in early 1939, immediately alerting the scientific community (especially in the U.S.) to the immense possibilities of a nuclear chain reaction (Article 147).

Despite her pivotal role in guiding the experiment and providing the theoretical framework, Lise Meitner was tragically overlooked by the Nobel Committee. Otto Hahn received the 1944 Nobel Prize in Chemistry alone for the discovery of fission. Though Meitner was nominated multiple times, she was never awarded the Nobel, a historical omission widely regarded as unjust and compounded by the political and gender bias of the time.

In Conclusion: Lise Meitner provided the crucial theoretical physics explanation for nuclear fission. Recognizing that the massive uranium nucleus had split upon neutron capture, she calculated the enormous energy release by demonstrating that the process involved a conversion of mass into energy, precisely according to Einstein's mass-energy equivalence ($E=mc^2$). Her work was the essential conceptual catalyst that launched the nuclear age.