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E=mc² to the First Chain Reaction

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E=mc²: Mass Hides Energy

1905: Albert Einstein[^1] publishes his famous equation in his “miracle year”

E = mc²

What it means:

  • Energy (E) equals mass (m) times the speed of light (c) squared
  • A tiny amount of mass can become a HUGE amount of energy
  • Why? Because c = 299,792,458 m/s (and it’s squared)

The catch:

  • No one knew how to actually release this energy from atoms
  • Even Einstein was skeptical in the 1930s

The Problem: Cracking the Nucleus

After Rutherford discovered the nucleus (1911), scientists wanted to study it:

The challenge:

  • The nucleus is tiny, dense, and positively charged
  • How do you get inside to study it?

First attempt: Use electrons?

  • ❌ Electrons are too light (1/1836 the mass of a proton)
  • ❌ Like using a ping-pong ball to crack a boulder
  • ❌ Electric forces push them away

We needed something better…

Rutherford’s Prediction (1920)

Ernest Rutherford[^2] proposed a solution:

The “neutron” hypothesis:

  • A neutral particle in the nucleus
  • Same mass as a proton, but no electric charge
  • Would explain isotopes and extra nuclear mass

Why this would work:

  • Heavy enough to “crack” the nucleus
  • No charge means no electric repulsion
  • Could slip right past the atomic defenses

The problem: No one had found it yet

Ernest Rutherford

Discovery of the Neutron (1932)

James Chadwick[^3], Rutherford’s student, finally found it!

The neutron:

  • Mass: nearly identical to a proton
  • Charge: zero (neutral)
  • Location: inside the nucleus with protons

Why neutrons are perfect nuclear probes:

  • Heavy (good momentum to penetrate)
  • Neutral (no electric repulsion)
  • Can get deep inside nuclei

This opened the door to nuclear physics

James Chadwick

Fermi’s Discovery (1934)

Enrico Fermi[^4] experimented with neutrons:

Key findings:

  • Neutrons can induce radioactivity in atoms
  • Slow neutrons work even better than fast ones
  • Why? They spend more time near the nucleus

The setup:

  • Fire neutrons at various elements
  • Observe what happens
  • Record new radioactive isotopes

Fermi had no idea this would lead to…

Enrico Fermi

Nuclear Fission Discovered (1938-1939)

The team:

  • Otto Hahn & Fritz Strassmann[^5] (experiments in Berlin)
  • Lise Meitner & Otto Frisch[^6] (theoretical explanation)

What they found:

  • When neutrons hit uranium, the nucleus splits
  • Creates two medium-sized nuclei
  • Releases enormous energy
  • Also releases more neutrons!

The mass defect:

  • Products weigh ~0.1% less than the original
  • That “missing” mass → energy (E=mc²)
  • About 200 MeV per fission

Nuclear Fission Diagram

The Energy: How Much?

If 1 gram of uranium fully fissions:

  • Energy released: ~80 billion joules (80 GJ)
  • Electricity: ~22,000 kWh
  • TNT equivalent: ~19 tons
  • Power a 1 kW heater for: ~2.5 years
  • Power a 100W lightbulb for: ~25 years

The key insight:

  • Each fission releases more neutrons
  • Those neutrons can cause more fissions
  • Chain reaction possible!

The Skeptics Were Wrong

Ernest Rutherford (1933):

“Anyone who expects a source of power from the transformation of the atom is talking moonshine.”

Albert Einstein (early 1930s):

“There is not the slightest indication that nuclear energy will ever be obtainable.”

The problem:

  • They said this BEFORE fission was discovered
  • Nobody imagined a chain reaction was possible
  • The neutron changed everything

Albert Einstein

The First Chain Reaction (1942)

Chicago Pile-1 (CP-1)[^7]

  • Led by Enrico Fermi
  • December 2, 1942
  • Under the stands at University of Chicago

What it proved:

  • Controlled nuclear chain reaction is possible
  • One fission → more neutrons → more fissions
  • Can be sustained and controlled
  • Massive energy release is achievable

This moment changed the world

Chicago Pile-1

The Thread

1905: Einstein publishes E=mc² → Mass and energy are convertible

1911: Rutherford discovers the nucleus → Atoms have a tiny, dense core

1920: Rutherford predicts the neutron → Needed to explain isotopes and nuclear mass

1932: Chadwick discovers the neutron → Perfect tool for probing nuclei

1934: Fermi shows slow neutrons work best → Can induce radioactivity

1938-39: Fission discovered and explained → Nucleus can split, releasing energy

1942: First controlled chain reaction → Proves nuclear energy is practical

References