Last modified on January 6, 2019, at 19:52

Cockcroft and Walton Experiment

This experiment by John Cockcroft and Ernest Walton is famous for being the first nuclear transmutation by artificially accelerated particles, as opposed to mere observations of natural radioactive decay.[1] Such particle accelerators were called "atom smashers" at the time.

Conducted in April 1932 at the University of Cambridge's Cavendish Laboratory in England, the physicists Cockroft and Walton successfully split lithium nuclei by colliding them with artificially accelerated protons. Cockroft and Walton were honored with the Nobel Prize in 1951 for this.[2]

Cockcroft and Walton "did not see their experiments as a "test" or "proof" of Einstein's mass-energy relationship; that was not the goal of the experiment. 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 analyzed for nuclear decays, more than 20 years earlier. The scope of this experiment was too narrow to purport to verify the equation in the general case. See Experimental and Observational Evidence Confirming Relativity for a discussion of this point.

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 discover 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

Nevertheless, this experiment is often championed by physicists as being confirmation of Albert Einstein's famous formula E=mc². It is not a good "proof" of the formula; it was too specific to make such a sweeping claim. It was simply consistent with the formula, and that consistency was noted at the time and continues to be noted.[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