Einstein’s theory of general relativity predicts that a clock at a higher gravitational potential will tick faster than an otherwise identical clock at a lower potential, an effect known as the gravitational redshift. Here we perform a laboratorybased, blinded test of the gravitational redshift using differential clock comparisons within an evenly spaced array of 5 atomic ensembles spanning a height difference of 1 cm. We measure a fractional frequency gradient of [ − 12.4 ± 0. 7_{(stat)} ± 2. 5_{(sys)}] × 10^{−19}/cm, consistent with the expected redshift gradient of − 10.9 × 10^{−19}/cm. Our results can also be viewed as relativistic gravitational potential difference measurements with sensitivity to mm scale changes in height on the surface of the Earth. These results highlight the potential of localoscillatorindependent differential clock comparisons for emerging applications of optical atomic clocks including geodesy, searches for new physics, gravitational wave detection, and explorations of the interplay between quantum mechanics and gravity.
This content will become publicly available on September 28, 2024
Einstein’s general theory of relativity from 1915^{1}remains the most successful description of gravitation. From the 1919 solar eclipse^{2}to the observation of gravitational waves^{3}, the theory has passed many crucial experimental tests. However, the evolving concepts of dark matter and dark energy illustrate that there is much to be learned about the gravitating content of the universe. Singularities in the general theory of relativity and the lack of a quantum theory of gravity suggest that our picture is incomplete. It is thus prudent to explore gravity in exotic physical systems. Antimatter was unknown to Einstein in 1915. Dirac’s theory^{4}appeared in 1928; the positron was observed^{5}in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted^{6}by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter^{7–10}. In the general theory of relativity, the weak equivalence principle (WEP) requires that all masses react identically to gravity, independent of their internal structure. Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHAg apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive ‘antigravity’ is ruled out in this case. This experiment paves the way for precision studies of the magnitude of the gravitational acceleration between antiatoms and the Earth to test the WEP.
more » « less Award ID(s):
 1806305
 NSFPAR ID:
 10466773
 Author(s) / Creator(s):
 ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
 Publisher / Repository:
 Nature
 Date Published:
 Journal Name:
 Nature
 Volume:
 621
 Issue:
 7980
 ISSN:
 00280836
 Page Range / eLocation ID:
 716 to 722
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
More Like this

Abstract 
null (Ed.)We propose a method by which one could use modified antimatter gravity experiments in order to perform a highprecision test of antimatter charge neutrality. The proposal is based on the application of a strong, external, vertically oriented electric field during an antimatter freefall gravity experiment in the gravitational field of the Earth. The proposed experimental setup has the potential to drastically improve the limits on the chargeasymmetry parameter ϵ¯q of antimatter. On the theoretical side, we analyze possibilities to describe a putative chargeasymmetry of matter and antimatter, proportional to the parameters ϵq and ϵ¯q, by Lagrangian methods. We found that such an asymmetry could be described by fourdimensional Lorentzinvariant operators that break CPT without destroying the locality of the field theory. The mechanism involves an interaction Lagrangian with field operators decomposed into particle or antiparticle field contributions. Our Lagrangian is otherwise Lorentz, as well as PT invariant. Constraints to be derived on the parameter ϵ¯q do not depend on the assumed theoretical model.more » « less

ABSTRACT In this paper, we study the gravitationalwave (GW) radiation and radiative behaviour of relativistic compact binary systems in the scaleindependent energy–momentum squared gravity (EMSG). The field equations of this theory are solved approximately. The gravitational potential of a gravitational source is then obtained by considering two matter Lagrangian densities that both describe a perfect fluid in general relativity (GR). We derive the GW signals emitted from a compact binary system. The results are different from those obtained in GR. It is shown that the relevant nonGR corrections modify the wave amplitude and leave the GW polarizations unchanged. Interestingly, this modification depends on the choice of the matter Lagrangian density. This means that for different Lagrangian densities, this theory presents different predictions for the GW radiation. In this case, the system loses energy to modified GWs. This leads to a change in the secular variation of the Keplerian parameters of the binary system. In this work, we investigate the nonGR effects on the radiative parameter, that is, the first time derivative of the orbital period. Next, applying these results together with GW observations from the relativistic binary systems, we constrain/test the scaleindependent EMSG theory in the strongfield regime. After assuming that GR is the valid gravity theory, as a priori expectation, we find that the free parameter of the theory is of the order 10−5 from the direct GW observation, the GW events GW190425 and GW170817, as well as the indirect GW observation, the double pulsar PSR J0737−3039A/B experiment.

null (Ed.)The application of the CPT (chargeconjugation, parity, and time reversal) theorem to an apple falling on Earth leads to the description of an antiapple falling on anti–Earth (not on Earth). On the microscopic level, the Dirac equation in curved spacetime simultaneously describes spin1/2 particles and their antiparticles coupled to the same curved spacetime metric (e.g., the metric describing the gravitational field of the Earth). On the macroscopic level, the electromagnetically and gravitationally coupled Dirac equation therefore describes apples and antiapples, falling on Earth, simultaneously. A particletoantiparticle transformation of the gravitationally coupled Dirac equation therefore yields information on the behavior of “antiapples on Earth”. However, the problem is exacerbated by the fact that the operation of charge conjugation is much more complicated in curved, as opposed to flat, spacetime. Our treatment is based on secondquantized field operators and uses the Lagrangian formalism. As an additional helpful result, prerequisite to our calculations, we establish the general form of the Dirac adjoint in curved spacetime. On the basis of a theorem, we refute the existence of tiny, but potentially important, particleantiparticle symmetry breaking terms in which possible existence has been investigated in the literature. Consequences for antimatter gravity experiments are discussed.more » « less

The huge amounts of undetected and exotic dark matter and dark energy needed to make general relativity work on large scales argue that we should investigate modifications of gravity. The only stable, metricbased and invariant alternative to general relativity is f(R) models. These models can explain primordial inflation, but they cannot dispense with either dark matter or dark energy. I advocate nonlocal modifications of gravity, not as new fundamental theories but rather as the gravitational vacuum polarization engendered by infrared quanta produced during primordial inflation. I also discuss some of the many objections which have been raised to this idea.more » « less