Cockcroft and Walton Experiment

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This experiment by John Cockcroft and Ernest Walton is championed by physicists as being confirmation of Albert Einstein's famous formula E=mc²,[1] even though no such claim was made by the experimenters at the time and experimental error was too large for it confirm the formula. Moreover, an isolated experiment such as this could not demonstrate the sweeping claim made by the formula.

Conducted in April 1932 at the University of Cambridge's Cavendish Laboratory in England, the physicists Cockroft and Walton successfully split lithium atom nuclei by colliding them with artificially accelerated protons. This experiment is general hailed as being the first transmutation of an element using artificially accelerated particles, for which they were honored with the Nobel Prize in 1951.[2]

Cockcroft and Walton "did not see their experiments as a test of Einstein's mass-energy relationship; rather, they simply used that relationship in their analysis, assuming it to be valid "[3] The E=mc² equation had in fact been proposed, and considered for nuclear decays, more than 20 years earlier. The experimental error was too large -- and the scope of the experiment to narrow -- to purport to verify the equation for all artificially accelerated particles. As explained by Stanford Encyclopedia of Philosophy:[4]

As Stuewer (1993) has suggested, Cockcroft and Walton use mass-energy equivalence to confirm their hypothesis about what happens when 7Li is bombarded by protons. Hence, it does not seem we ought to regard this experiment as a confirmation of E=mc². However, if we take some of the other evidence that Cockcroft and Walton provide concerning the identification of the products in reaction p + 7Li → α + α as sufficient to establish that the products are indeed α-particles, then we can interpret this experiment as a confirmation of mass-energy equivalence, which is how this experiment is often reported in the physics literature.

Others have asserted that Cockcroft and Walton did discovery some kind of confirmation of the E=mc² equation, such as physicist Kenneth Bainbridge in 1933:

The gain in energy in the reaction is 16.97×106 e-volts, an energy equivalent to 0.0182 mass units on the O16 scale if ΔE =C²Δm. Taking Aston's values for the mass of helium and hydrogen and the author's value, 7.0146±0.0006 for Li7, the mass change is 0.0181 ± 0.0006 in the reaction which may be represented as Li7 + p → 2α. Within the probable error of the measurements the equivalence of mass and energy is satisfied. --Kenneth Bainbridge

Neither the Nobel Prize committee nor the prize recipients made the claim that their experiment verified of Albert Einstein's famous E=mc² formula.[5][6]


  1. Mike Poole Cockcroft's subatomic legacy: splitting the atom, Cern Courier, Nov 20, 2007
  2. The Nobel Prize in Physics 1951,, 23 Jan 2013
  3. Roger H. Stuewer: Mass-Energy in Einstein in Context, Cambridge University Press
  4. Fernflores, Francisco, "The Equivalence of Mass and Energy", The Stanford Encyclopedia of Philosophy (Spring 2012 Edition), Edward N. Zalta (ed.)
  5. The Center for History of Physics: Einstein: Image and Impact, © 1996-2004 - American Institute of Physics
  6. Louise Boylan: Cockroft and Walton Experiment: Converting Mass into Energy