Nobel Prize in Physics · 1925
Gustav Hertz
His electron-collision experiment with James Franck proved that atoms take in energy only in fixed steps, a pillar of quantum physics.
The Nobel citation: “for their discovery of the laws governing the impact of an electron upon an atom”
- Born
- July 22, 1887, Hamburg, Germany
- Died
- October 30, 1975, Berlin, East Germany (now Germany)
- Shared with
- James Franck
- Affiliation at the time
- Halle University, Germany
Disputed inclusion. Sources disagree on whether this laureate meets our standard of at least one Jewish parent. The evidence is set out under Jewish background below. We count this laureate by default, and the methodology page shows how the results change without disputed cases.
Physics prize
1925
Shared with 1 other laureate.
Age that year
38years
Born in 1887.
Headline credited impact
$28.4–32billion in economic value
Cumulative world semiconductor sales, a proxy for the quantum-based electronics economy. How it was built
Sources cited
26
Fact-checked September 24, 2026.
- The Franck-Hertz experiment found that electrons hand energy to mercury atoms only in 4.9-electron-volt chunks, matching the energy of mercury's ultraviolet glow at 254 nm.
- His uncle Heinrich Hertz gave his name to the unit of frequency. Under the Nazis, some physicists proposed in 1939 renaming the unit the 'helmholtz'.
- He and his friend James Franck were awarded the 1925 physics prize, but received it a year late, in 1926.
- In 1932 he separated the isotopes of neon by passing the gas through a chain of diffusion stages, an approach later scaled up to more than a thousand stages to enrich uranium.
- A single Jewish-born grandfather, baptized at age seven in 1834, was enough for the Nazi ministry to move against his right to examine students in 1934.
The breakthrough
The Franck-Hertz experiment: electrons prove atoms have fixed energy levels (1914)
In 1913 the Danish physicist Niels Bohr proposed a strange rule: an atom can hold only certain fixed amounts of energy, nothing in between. In Berlin, Hertz and his friend James Franck tested it without meaning to. They filled a glass tube with thin mercury vapor, pushed electrons through it with an adjustable voltage, and measured how many reached a collecting plate. As the voltage rose, the current rose too, until 4.9 volts. Then it suddenly dropped, and it dropped again at each further multiple of 4.9 volts. The reason: a mercury atom ignores an electron carrying less than 4.9 electron-volts of energy, and the electron bounces off with its energy intact. Once an electron has at least that much, the atom can take exactly 4.9 electron-volts from it, no more and no less, like a toll booth that charges one fixed fare. The mercury then gives the energy back as ultraviolet light at 254 nanometers, whose light particles carry the same 4.9 electron-volts. Oddly, Franck and Hertz had not read Bohr's paper and at first thought they were knocking electrons out of the atoms. Bohr pointed out in 1915 that the results fit his theory, and after the war the two agreed. The Nobel committee later said their methods had turned Bohr's basic ideas from hypotheses into experimentally proved facts.[1],[3],[4],[5],[6],[7],[8]
“they have provided a direct experimental proof of the basic assumptions of Bohr’s theory of atoms”
What it meant for humanity
The Franck-Hertz experiment gave a direct experimental proof that atoms store energy in fixed steps. That idea is a foundation of quantum mechanics, and quantum science underpins lasers, transistors and the electronics of modern life. The Nobel committee also credited Franck and Hertz with opening a new chapter in physics: the study of what happens when electrons strike atoms and molecules. They turned an older method into a tool for probing the structure of atoms and molecules, and the experiment is still taught in university physics courses. Hertz's second big contribution came in 1932 in Berlin. To separate the isotopes of neon, atoms that are chemically identical and differ only slightly in weight, he sent the gas through a chain of stages, each enriching it a little and passing it on. The US government's 1945 report on the Manhattan Project, the Smyth Report, credited Hertz and others with using multi-stage recycling to separate neon's isotopes almost completely, and called such recycling necessary for nearly all methods of isotope separation. By 1938 the approach had been used to enrich heavy isotopes of carbon, nitrogen and oxygen ten- to twentyfold. Cascades became the standard way to enrich uranium: gaseous-diffusion plants needed more than a thousand stages, and the centrifuge plants that replaced them are also arranged in cascades. One French diffusion plant alone could fuel about ninety large reactors a year. After returning to East Germany in 1954, Hertz built up Leipzig University's physics institute, co-edited a three-volume nuclear physics textbook, and chaired the government's council for peaceful uses of atomic energy, where he argued for building nuclear reactors. Both of his sons became physicists.
- The Nobel committee said Franck and Hertz's methods turned Bohr's 1913 ideas about atomic energy states from hypotheses into experimentally proved facts, and opened a new chapter in physics: electron collisions with atoms.[4]
- Electrons lose 4.9 electron-volts per collision to mercury atoms, the same energy carried by mercury's 254 nm ultraviolet light; the experiment remains a classic of physics teaching.[6],[7]
- His 1932 cascade separated neon's isotopes. The Manhattan Project's official 1945 report (the Smyth Report) called such multi-stage recycling necessary for nearly all isotope-separation methods.[9],[20],[21]
- Uranium for reactors was enriched first in gaseous-diffusion cascades of about 1,400 stages, then in centrifuge cascades; a Halle University biography says his isotope work proved decisive for enriching uranium-235.[10],[22]
- From 1954 to 1961 he directed the Physics Institute at Leipzig, co-edited a three-volume nuclear physics textbook, and from 1955 chaired East Germany's council for peaceful uses of atomic energy.[2],[9],[10]
Impact in numbers
Hertz's legacy is mostly foundational. The Franck-Hertz experiment helped establish that atoms have fixed energy levels, so it sits beneath today's quantum-based electronics. We record a small ripple share of the semiconductor economy, split equally with James Franck, because the experiment confirmed a theory that others created and engineers turned into products. His 1932 neon cascade was an early working demonstration of the multi-stage method that became the basis of uranium enrichment, which has fueled nuclear power plants and also supplied nuclear weapons. We do not put a number on either side of that ledger, because the chain from a neon experiment to a power station or a bomb runs through many other people and decisions. We do record a token share of the deaths and injuries from First World War gas warfare, in which he served. We record no number for his work on the Soviet bomb, because Soviet nuclear weapons were never used in war.
Fundamental scienceTechnologyEconomyEnergyPeace
Each number is the laureate’s credited share of a real-world outcome, cumulative to 2025. The whole outcome, the share of credit, and the reasoning are shown so you can check the arithmetic. Outcomes shared with other laureates are counted once on the impact page.
- Low confidenceRippleModeledEconomy
Cumulative world semiconductor sales, a proxy for the quantum-based electronics economy
$28.4–32
billion in economic value, credited share
That is 0.2% of $14.2–16 trillion in economic value since 1955.
How this number was built
Summing WSTS annual worldwide semiconductor billings for 1986-2025 gives $10.28 trillion nominal; converting each year to 2024 dollars with the US CPI (Minneapolis Fed table) gives $14.2 trillion (low). High ($16T): $14.2T + rough, unsourced allowances of ~$0.5T for pre-1986 chip sales and ~$1T for lasers and other quantum devices outside WSTS = ~$15.7T, rounded up. Sales are only a proxy for value. NIST notes that quantum science underpins lasers and transistors. Share 0.002: the Nobel committee credited Franck and Hertz with turning Bohr's energy-level hypotheses into proved facts, but an experiment that confirms a theory earns less than the theory itself (0.01 each for founders such as Bohr and Born). We credit the experiment 0.004 in total, split equally with co-laureate James Franck. Credited: about $28-32 billion.[4],[23],[24],[25]
Sources: NobelPrize.org; National Institute of Standards and Technology (NIST), Taking Measure blog; World Semiconductor Trade Statistics (WSTS); Federal Reserve Bank of Minneapolis
- HarmLow confidenceDirectSourced totalPeace
Soldiers killed by chemical weapons in World War I
45–50
deaths caused, credited share
That is 0.05% of 90,000–100,000 deaths caused since 1915.
How this number was built
The OPCW states that more than 90,000 soldiers died from chemical agents in World War I; Prentiss's 1937 count, cited by Friedrich, is about 90,000. High allows about 10% for undercounting; this matches the range on the Fritz Haber profile. Share 0.0005: Hertz served as an officer in Haber's gas units for part of 1915, until he was severely wounded, and is named among the scientists of the gas brigades. Credited: about 45-50 deaths. He did not invent the weapon or decide to use it. Haber (0.35), the German high command and industry, and the Allied gas programs carry nearly all the responsibility, so this is a token share for months of direct participation.[2],[16],[17],[18]
Sources: Organisation for the Prohibition of Chemical Weapons (OPCW); Fritz Haber Institute of the Max Planck Society; Max Planck Society (PuRe repository); NobelPrize.org (from Nobel Lectures, Physics 1922-1941, Elsevier, 1965)
- HarmLow confidenceDirectSourced totalPeace
Soldiers injured by chemical weapons in World War I
450–600
people harmed, credited share
That is 0.05% of 900,000–1.2 million people harmed since 1915.
How this number was built
The OPCW says close to a million people left World War I battlefields blind, disfigured or with debilitating injuries from chemical weapons (low 0.9 million). Prentiss's 1937 tally of about 1.3 million gas casualties includes the roughly 90,000 deaths, so non-fatal injuries are at most about 1.2 million (high). This matches the range on the Fritz Haber profile. Share 0.0005, for the same reasons as the gas-death claim: a token share for an officer's months of service in the gas units, with responsibility lying overwhelmingly with Haber, military leaders and the gas programs on both sides. Credited: about 450-600 people injured.[16],[17],[18]
Sources: Organisation for the Prohibition of Chemical Weapons (OPCW); Fritz Haber Institute of the Max Planck Society; Max Planck Society (PuRe repository)
The double edge
Hertz's science was used for war as well as peace, and he took part himself. In 1915 he served in Fritz Haber's poison-gas troops. From 1945 to 1954 he worked for the Soviet atomic bomb project. He led an institute that developed isotope separation for enriching uranium, helped troubleshoot the first Soviet enrichment plant in 1948, and in 1951 shared a Stalin Prize for the theory of controlling enrichment cascades, awarded after the test of a Soviet bomb containing uranium-235. A 2000 study of German scientists in the Soviet project concluded that their help, though not essential, sped up the Soviet push for the bomb. Under the Nazis, he fought removal from his university partly by stressing his 'Aryan' ancestry. We record a token share of the harm from First World War gas warfare. We record no number for his Soviet work: Soviet nuclear weapons were never used in war, and the damage from nuclear testing and the arms race cannot be traced to one scientist's share.
- Moderate
Officer in Haber's poison-gas troops
In 1915 Hertz was among the scientists serving in the special gas units that Fritz Haber organized for Germany's chlorine-cloud attacks, which violated international law. A biography from a University of Flensburg physics-history project says he was an officer overseeing the release of gas at the front, and that he was badly hurt in Poland during a gas attack aimed at Russian troops.[2],[8],[16]
- Major
Uranium enrichment for Stalin's atomic bomb
From 1945 Hertz headed Institute G near Sukhumi, whose tasks included separating isotopes by diffusion and the theory of controlling diffusion cascades. In October 1948 he was among the experts called in to fix the failing first Soviet enrichment plant, D-1. In late 1951, after a Soviet test of a bomb with uranium-235 parts, he shared a second-degree Stalin Prize for cascade control theory. The Soviet physicist Isaak Kikoin said he had used Hertz as his model.[9],[10],[19]
- Minor
Stressing his 'Aryan' ancestry to keep his post
In 1938, when the education ministry sought to remove all 'quarter-Jews' from universities, Hertz argued that his 'Aryan' ancestors determined his essential hereditary traits. He was allowed to stay as an exception, helped by his university's backing and by officials' impression that he was sympathetic to Nazi concerns.[12]
Against the odds
Hertz faced partial persecution, not the full weight of Nazi policy, and his story should be told that way. His great-grandparents left Judaism in 1834, at a time when even a well-assimilated Jewish family was denied full civil rights. After baptism the family faced no legal limits and rose into Hamburg's elite. Hertz grew up a Lutheran, and the sources we found record no antisemitism in his early career. Nazi racial law changed that. Under the April 1933 civil service law, a single Jewish grandparent was enough to cost someone a state job. As a front-line veteran of the First World War, Hertz was at first exempt. But in 1934 the ministry moved to take away his right to examine students because of his Jewish ancestry, and on 30 June 1935 he left his professorship and state service to lead a research laboratory at Siemens. He kept an honorary professorship so he could still supervise doctorates, and in 1938, when the ministry tried to push all 'quarter-Jews' out of universities, he survived as an exception. People close to him fared worse. His friend and co-laureate James Franck resigned in protest in 1933 and left Germany. His cousin Mathilde Hertz, a scientist at a Berlin biology institute, was classed as 'non-Aryan' and emigrated to Cambridge in 1936. Most streets and institutions named after his uncle Heinrich Hertz were renamed.
1915
War
Mobilized in 1914, he was severely wounded in action in 1915 and returned to Berlin as a university lecturer only in 1917.[2],[8]
1934
Discrimination
The Reich education ministry moved to withdraw his right to examine students because of his Jewish ancestry, even though as a front-line veteran he had been exempt from the 1933 dismissals.[9],[12],[26]
1935
Dismissal
Facing these restrictions, he left his chair at the Technical University of Berlin and the state service on 30 June 1935, keeping only an honorary professorship, and moved to industry at Siemens.[2],[9],[12]
1938
Persecution
When the education ministry moved to remove all 'quarter-Jews' from universities, it pressed his university to get him to give up his honorary professorship voluntarily; he was eventually allowed to stay as an exception.[12]
—
Discrimination
In the Nazi era most streets and institutions named after his uncle Heinrich Hertz were renamed, and in 1939 some physicists proposed renaming the unit of frequency.[12]
Jewish background
Hertz was not Jewish by religion or upbringing. His paternal grandfather, a Hamburg lawyer who became a city senator, was born to a Jewish family and baptized a Lutheran in 1834 at age seven, with his parents and siblings. He married a Christian, and by the next generation Judaism had vanished from the family. Nazi law still classed Hertz as a 'quarter-Jew'. In 1938, fighting removal, he argued that his 'Aryan' ancestors defined his heredity. The Encyclopaedia Judaica and a Leipzig University page call his father Jewish, but the family genealogy does not support this.[11],[12],[13],[14],[15],[26]
Key dates
July 22, 1887
Born in Hamburg to the lawyer Gustav Hertz and his wife Auguste, née Arning; he is a nephew of the physicist Heinrich Hertz.[1],[2],[11]
1911
Earns his doctorate in Berlin under Heinrich Rubens, after studying at Göttingen, Munich and Berlin from 1906.[2],[10]
1914
With James Franck, his research partner since 1913, finds that electrons lose energy to mercury atoms only in fixed 4.9-electron-volt amounts: the Franck-Hertz experiment.[1],[2],[5],[6]
1915
Serves in Fritz Haber's poison-gas units and is severely wounded in action.[2],[8],[16]
1920
Joins the physics laboratory of the Philips lamp works in Eindhoven, the Netherlands, where he works on electrical discharges in gases.[2],[9]
1925
Is elected professor and director of the Physics Institute at the University of Halle, a post he takes up in 1926.[2],[10]
December 10, 1926
Receives the 1925 Nobel Prize in Physics, shared with James Franck, a year late.[1],[4]
1928
Becomes director of the Physics Institute at the Technical University in Berlin-Charlottenburg.[2],[9]
1932
Separates the isotopes of neon with a multi-stage diffusion cascade.[2],[9],[19]
June 30, 1935
After the Nazi ministry moves to strip his right to examine students over his Jewish ancestry, leaves his professorship and heads a Siemens research laboratory.[9],[12]
1945
Goes to the Soviet Union, as he and colleagues had agreed in 1944, and heads Institute G near Sukhumi, working on uranium isotope separation for the Soviet atomic project.[2],[9],[19]
1951
Shares a Stalin Prize for the theory of controlling uranium-enrichment cascades.[9],[19]
1954
Returns to East Germany as professor and director of the Physics Institute at Leipzig's Karl Marx University, retiring in 1961.[2],[9]
October 30, 1975
Dies in East Berlin, aged 88.[1]
Sources
- 1.Gustav Hertz – Facts · NobelPrize.org (Nobel Prize Outreach)
- 2.Gustav Hertz – Biographical · NobelPrize.org (from Nobel Lectures, Physics 1922-1941, Elsevier, 1965), 1965
- 3.Gustav Hertz – Nobel Lecture: The results of the electron-impact tests in the light of Bohr's theory of atoms (11 December 1926) · NobelPrize.org, 1926
- 4.The Nobel Prize in Physics 1925 – Presentation Speech by Professor C.W. Oseen · NobelPrize.org, 1926
- 5.James Franck – Biographical · NobelPrize.org (from Nobel Lectures, Physics 1922-1941, Elsevier, 1965), 1965
- 6.Franck-Hertz Experiment (HyperPhysics, by R. Nave) · Georgia State University, Department of Physics and Astronomy
- 7.The Franck-Hertz experiment supports Bohr's model (course lecture notes by Michael Richmond) · Rochester Institute of Technology
- 8.Biographie Gustav Hertz (1887-1975), StoryTelling project · Europa-Universität Flensburg, Department of Physics and its Didactics and History
- 9.Gustav Hertz (Catalogus Professorum) · Technische Universität Berlin, University Archive
- 10.Gustav Hertz (Catalogus Professorum Halensis) · Martin Luther University Halle-Wittenberg
- 11.Hertz, Heinrich Rudolf (Neue Deutsche Biographie 8, 1969, by Armin Hermann; genealogy section) · Deutsche Biographie (Historical Commission at the Bavarian Academy of Sciences), 1969
- 12.Juden wider Willen (Jews against their will), by Stefan L. Wolff · Jüdische Allgemeine, 2008
- 13.Hertz, Gustav (Encyclopaedia Judaica entry and other reference entries) · Encyclopedia.com
- 14.Jewish Nobel Prize Winners in Physics (note on laureates of one-quarter Jewish descent) · Jinfo.org
- 15.List of Jewish Nobel laureates · Wikipedia
- 16.From Berlin-Dahlem to the Fronts of World War I: The Role of Fritz Haber and His Kaiser Wilhelm Institute in German Chemical Warfare, by Bretislav Friedrich and Jeremiah James (in One Hundred Years of Chemical Warfare, Springer) · Max Planck Society (PuRe repository), 2017
- 17.A brief biography of Fritz Haber (1868-1934), by Bretislav Friedrich · Fritz Haber Institute of the Max Planck Society
- 18.History · Organisation for the Prohibition of Chemical Weapons (OPCW)
- 19.German Scientists in the Soviet Atomic Project, by Pavel V. Oleynikov (The Nonproliferation Review 7:2) · James Martin Center for Nonproliferation Studies, 2000
- 20.The Smyth Report (Atomic Energy for Military Purposes, the official 1945 US report on the Manhattan Project), Chapter IX: General Discussion of the Separation of Isotopes · US government report (1945), via the Nuclear Weapon Archive, 1945
- 21.Historical Note: Filtration Materials for Separating Isotopes, by Kevin J. Anderson (MRS Bulletin, December 1990) · MRS Bulletin (Cambridge University Press), 1990
- 22.Uranium Enrichment · World Nuclear Association
- 23.A Quantum Leap Forward: How Tiny Particles Can Bring Us Exciting New Tech, by Corey Stambaugh · National Institute of Standards and Technology (NIST), Taking Measure blog, 2025
- 24.Historical Billings Report (WSTS Blue Book monthly data, 1986 to date) · World Semiconductor Trade Statistics (WSTS)
- 25.Consumer Price Index, 1913- · Federal Reserve Bank of Minneapolis
- 26.Gustav Hertz (Leipzig University history pages) · Universität Leipzig
Fact-checked on September 24, 2026 by a separate AI fact-checking pass that re-opened the sources, with 10 corrections made. How we check
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