Because the three parts of the Einstein equivalence principle discussed above are so very different in their empirical consequences, it is tempting to regard them as independent theoretical principles. On the other hand, any complete and self-consistent gravitation theory must possess sufficient mathematical machinery to make predictions for the outcomes of experiments that test each principle, and because there are limits to the number of ways that gravitation can be meshed with the special relativistic laws of physics, one might not be surprised if there were theoretical connections between the three sub-principles. For instance, the same mathematical formalism that produces equations describing the free fall of a hydrogen atom must also produce equations that determine the energy levels of hydrogen in a gravitational field, and thereby the ticking rate of a hydrogen maser clock. Hence a violation of EEP in the fundamental machinery of a theory that manifests itself as a violation of WEP might also be expected to show up as a violation of local position invariance. Around 1960, Schiff conjectured that this kind of connection was a necessary feature of any self-consistent theory of gravity. More precisely, Schiff’s conjecture states that any complete, self-consistent theory of gravity that embodies WEP necessarily embodies EEP. In other words, the validity of WEP alone guarantees the validity of local Lorentz and position invariance, and thereby of EEP.
If Schiff’s conjecture is correct, then Eötvös experiments may be seen as the direct empirical foundation for EEP, hence for the interpretation of gravity as a curved-spacetime phenomenon. Of course, a rigorous proof of such a conjecture is impossible (indeed, some special counter-examples are known [204, 194, 62]), yet a number of powerful “plausibility” arguments can be formulated.
The most general and elegant of these arguments is based upon the assumption of energy conservation. This assumption allows one to perform very simple cyclic gedanken experiments in which the energy at the end of the cycle must equal that at the beginning of the cycle. This approach was pioneered by Dicke, Nordtvedt, and Haugan (see, e.g., ). A system in a quantum state A decays to state B, emitting a quantum of frequency . The quantum falls a height H in an external gravitational field and is shifted to frequency , while the system in state B falls with acceleration gB. At the bottom, state A is rebuilt out of state B, the quantum of frequency , and the kinetic energy m B gB H that state B has gained during its fall. The energy left over must be exactly enough, mA gA H, to raise state A to its original location. (Here an assumption of local Lorentz invariance permits the inertial masses mA and mB to be identified with the total energies of the bodies.) If gA and gB depend on that portion of the internal energy of the states that was involved in the quantum transition from A to B according to (TEGP 2.5 ).
Box 1. The formalism
where and subscript “0” refers to a chosen point in space. If EEP is satisfied, .
The first successful attempt to prove Schiff’s conjecture more formally was made by Lightman and Lee . They developed a framework called the formalism that encompasses all metric theories of gravity and many non-metric theories (see Box 1). It restricts attention to the behavior of charged particles (electromagnetic interactions only) in an external static spherically symmetric (SSS) gravitational field, described by a potential . It characterizes the motion of the charged particles in the external potential by two arbitrary functions and , and characterizes the response of electromagnetic fields to the external potential (gravitationally modified Maxwell equations) by two functions and . The forms of , , , and vary from theory to theory, but every metric theory satisfies
Lightman and Lee then calculated explicitly the rate of fall of a “test” body made up of interacting charged particles, and found that the rate was independent of the internal electromagnetic structure of the body (WEP) if and only if Equation (9) was satisfied. In other words, WEP EEP and Schiff’s conjecture was verified, at least within the restrictions built into the formalism.
Certain combinations of the functions , , , and reflect different aspects of EEP. For instance, position or -dependence of either of the combinations and signals violations of LPI, the first combination playing the role of the locally measured electric charge or fine structure constant. The “non-metric parameters” and (see Box 1) are measures of such violations of EEP. Similarly, if the parameter is non-zero anywhere, then violations of LLI will occur. This parameter is related to the difference between the speed of light , and the limiting speed of material test particles , given by
The rate of fall of a composite spherical test body of electromagnetically interacting particles then has the form).
The formalism also yields a gravitationally modified Dirac equation that can be used to determine the gravitational redshift experienced by a variety of atomic clocks. For the redshift parameter (see Equation (6)), the results are (TEGP 2.6 (c) ):
The redshift is the standard one , independently of the nature of the clock if and only if . Thus the Vessot–Levine rocket redshift experiment sets a limit on the parameter combination (see Figure 3); the null-redshift experiment comparing hydrogen-maser and SCSO clocks sets a limit on . Alvarez and Mann [7, 6, 8, 9, 10] extended the formalism to permit analysis of such effects as the Lamb shift, anomalous magnetic moments and non-baryonic effects, and placed interesting bounds on EEP violations.
The formalism can also be applied to tests of local Lorentz invariance, but in this context it can be simplified. Since most such tests do not concern themselves with the spatial variation of the functions , , , and , but rather with observations made in moving frames, we can treat them as spatial constants. Then by rescaling the time and space coordinates, the charges and the electromagnetic fields, we can put the action in Box 1 into the form (TEGP 2.6 (a) )
The electrodynamical equations which follow from Equation (17) yield the behavior of rods and clocks, just as in the full formalism. For example, the length of a rod which moves with velocity relative to the rest frame in a direction parallel to its length will be observed by a rest observer to be contracted relative to an identical rod perpendicular to the motion by a factor . Notice that does not appear in this expression, because only electrostatic interactions are involved, and appears only in the magnetic sector of the action (17). The energy and momentum of an electromagnetically bound body moving with velocity relative to the rest frame are given by
The electrodynamics given by Equation (17) can also be quantized, so that we may treat the interaction of photons with atoms via perturbation theory. The energy of a photon is times its frequency , while its momentum is . Using this approach, one finds that the difference in round trip travel times of light along the two arms of the interferometer in the Michelson–Morley experiment is given by . The experimental null result then leads to the bound on shown on Figure 2. Similarly the anisotropy in energy levels is clearly illustrated by the tensorial terms in Equations (18, 20); by evaluating for each nucleus in the various Hughes–Drever-type experiments and comparing with the experimental limits on energy differences, one obtains the extremely tight bounds also shown on Figure 2.
The behavior of moving atomic clocks can also be analyzed in detail, and bounds on can be placed using results from tests of time dilation and of the propagation of light. In some cases, it is advantageous to combine the framework with a “kinematical” viewpoint that treats a general class of boost transformations between moving frames. Such kinematical approaches have been discussed by Robertson, Mansouri and Sexl, and Will (see ).
For example, in the “JPL” experiment, in which the phases of two hydrogen masers connected by a fiberoptic link were compared as a function of the Earth’s orientation, the predicted phase difference as a function of direction is, to first order in , the velocity of the Earth through the cosmic background,L = 21 km is the baseline, and where and are unit vectors along the direction of propagation of the light at a given time and at the initial time of the experiment, respectively. The observed limit on a diurnal variation in the relative phase resulted in the bound . Tighter bounds were obtained from a “two-photon absorption” (TPA) experiment, and a 1960s series of “Mössbauer-rotor” experiments, which tested the isotropy of time dilation between a gamma ray emitter on the rim of a rotating disk and an absorber placed at the center .
Kostelecký and collaborators developed a useful and elegant framework for discussing violations of Lorentz symmetry in the context of the standard model of particle physics [63, 64, 155]. Called the Standard Model Extension (SME), it takes the standard field theory of particle physics, and modifies the terms in the action by inserting a variety of tensorial quantities in the quark, lepton, Higgs, and gauge boson sectors that could explicitly violate LLI. SME extends the earlier classical and frameworks, and the framework of Ni  to quantum field theory and particle physics. The modified terms split naturally into those that are odd under CPT (i.e. that violate CPT) and terms that are even under CPT. The result is a rich and complex framework, with many parameters to be analyzed and tested by experiment. Such details are beyond the scope of this review; for a review of SME and other frameworks, the reader is referred to the Living Review by Mattingly .
Here we confine our attention to the electromagnetic sector, in order to link the SME with the framework discussed above. In the SME, the Lagrangian for a scalar particle with charge interacting with electrodynamics takes the form
The tensor can be decomposed into “electric”, “magnetic”, and “odd-parity” components, by defining
In the rest frame of the universe, these tensors have some form that is established by the global nature of the solutions of the overarching theory being used. In a frame that is moving relative to the universe, the tensors will have components that depend on the velocity of the frame, and on the orientation of the frame relative to that velocity.
In the case where the theory is rotationally symmetric in the preferred frame, the tensors and can be expressed in the form
© Max Planck Society and the author(s)