Equivalence principle for quantum objects and antimatter

5 problems

• Does antihydrogen fall with g to within 1 percent?

  Open

  In 2023 the first direct experiment showed that antihydrogen falls down, not up, but only to about 20% precision. A 1% measurement would test whether antimatter feels gravity exactly like matter.

• Atom-interferometric test of universality of free fall at $10^{-15}$

  Open

  Two kinds of atoms dropped together should fall at exactly the same rate. Atom interferometers have checked this to about one part in $10^{12}$, while a satellite with solid masses reached one part in $10^{15}$.

• Gravitational time dilation within a single-atom spatial superposition

  Open

  A clock held higher runs faster, so an atom whose internal ticking is split between two heights at once should carry two different ages that later interfere. This quantum version of gravitational time dilation has been proposed but not measured.

• Gravitational free fall of muonium or positronium

  Open

  Muonium (an antimuon with an electron) and positronium (an electron with its antiparticle) contain no protons or neutrons. Measuring how they fall tests gravity on matter made purely of leptons, partly of antimatter.

• Do atoms in internal-energy superpositions obey the equivalence principle?

  Open

  Since energy has mass, an atom in a quantum superposition of two energy levels has no single definite mass. Whether it still falls exactly like a normal atom is a question that classical physics cannot pose.