Abstract
A fundamental question in physics that has yet to be addressed experimentally is whether particles of antimatter, such as the antiproton or positron, obey the weak equivalence principle (WEP). Several theoretical arguments have been put forward arguing limits for possible violations of WEP. No direct `classical' gravitational experiment, the measurement of the free fall of an antiparticle, has been performed to date to determine if a particle of antimatter would experience a force in the gravitational potential of a normal matter body that is different from normal gravity. 30 years ago we proposed a free fall experiment using protons and antiprotons, modeled after the experiment to measure the gravitational acceleration of a free electron. At that time we gave consideration to yet another possible observation of gravitational differences between matter and antimatter based on the gravitational red shift of clocks. I will recall the original arguments and make a number of comments pertaining to the technical problems and other issues that prevented the execution of the antiproton free fall measurement. Note that a different gravitational force on antimatter in the gravitational field of matter would not constitute a violation of CPT, as this is only concerned with the gravitational acceleration of antimatter in the gravitational field of an antimatter body.
Highlights
A fundamental question in physics that has recently attracted the interest of several experimental collaborations at CERN1−3 and elsewhere[4] is whether particles of antimatter, such as the antiproton, the positron, or an antihydrogen atom obey the weak equivalence principle
From the above argument, if the proton or electron respectively respect the weak equivalence principle, and we assume that any violation of equivalence for the antiproton occurs through an anomalous coupling strength of gravity to its energy, the antiproton’s cyclotron frequency will red shift by an amount different from the proton when they are lowered to the same height in a gravitational field from “infinity”, resulting in a measurable frequency difference
In 1986 we proposed measuring the acceleration of antiprotons in the Earth’s gravitational field by launching antiprotons from a thermal distribution at 4 K in groups of approximately 100 particles upwards against the force of gravity and measuring their time-of-flight (TOF) for a 1 m flight path
Summary
A fundamental question in physics that has recently attracted the interest of several experimental collaborations at CERN1−3 and elsewhere[4] is whether particles of antimatter, such as the antiproton, the positron, or an antihydrogen atom obey the weak equivalence principle. During the time of development of the PS200 apparatus, highprecision experimental results on the equality of the inertial masses (or more precisely the equality of the charge–to-mass ratios) of protons and antiprotons had become available from an experiment comparing the cyclotron frequencies of the particles in the same magnetic field.[11] Since these measurements have been improved by nearly 3 orders of magnitude.[12] These results are usually regarded as very sensitive tests of CPT symmetry.[13] assuming exact CPT symmetry, they can provide tests of the weak equivalence principle for a gravitational coupling to the energy of positrons and antiprotons, using certain assumptions for the coupling of gravity to energy and mass. This possibility arises because the frequencies in question constitute local ”clocks” and as such are subject to a gravitational red shift, which may be formulated as a test of weak equivalence for their energy content
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