They further developed torsion balances in the style of Eötvös and used them to study the possible deviation of the gravitational force from Newton's
Heckel and his colleagues also used the torsion balance to test Lorentz invariance with polarized electrons and to look for new spin-dependent forces.
In the 1980s his research dealt with experimental atomic physics, including searches for violations of parity and violation of time reversal invariance, by means of measuring upper limits for the electric dipole moment of atoms such as 199Hg.
[4] Heckel and his colleagues also measured the coupling constants of the weak interaction of neutrons with nucleons.
His team used beams of cold polarized neutrons from the National Institute of Standards and Technology (NIST) reactor to bombard a liquid helium target, measuring the parity-violating spin of the beam polarization.
[5] In 2021 he was awarded, jointly with Eric Adelberger and Jens H. Gundlach, the Breakthrough Prize in Fundamental Physics for “precision fundamental measurements that test our understanding of gravity, probe the nature of dark energy, and establish limits on couplings to dark matter.