Einstein's Weak Equivalence Research Articles

Upper limits on Einstein’s weak equivalence principle placed by uncertainties of dispersion measures of fast radio bursts

Fast radio bursts (FRBs) are astronomical transients with millisecond timescales occurring at cosmological distances. The observed time lag between different energies of each FRB is well described by the inverse-square law of the observed frequency, i.e., dispersion measure. Therefore, FRBs provide one of the ideal laboratories to test Einstein's weak equivalence principle (WEP): the hypothetical time lag between photons with different energies under a gravitational potential. If WEP is violated, such evidence should be exposed within the observational uncertainties of dispersion measures, unless the WEP violation also depends on the inverse-square of the observed frequency. In this work, we constrain the difference of gamma parameters ($\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}$) between photons with different energies using the observational uncertainties of FRB dispersion measures, where $\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}=0$ for Einstein's general relativity. Adopting the averaged `Shapiro time delay' for cosmological sources, FRB 121002 at $z=1.6\ifmmode\pm\else\textpm\fi{}0.3$ and FRB $180817.\mathrm{J}1533+42$ at $z=1.0\ifmmode\pm\else\textpm\fi{}0.2$ place the most stringent constraints of $\mathrm{log}\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}<\ensuremath{-}20.8\ifmmode\pm\else\textpm\fi{}0.1$ and $\mathrm{log}(\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}/{r}_{E})<\phantom{\rule{0ex}{0ex}}\ensuremath{-}20.9\ifmmode\pm\else\textpm\fi{}0.2$, respectively, where ${r}_{E}$ is the energy ratio between the photons. The former is about three orders of magnitude lower than those of other astrophysical sources in previous works under the same formalization of the Shapiro time delay while the latter is comparable to the tightest constraint so far.

Weak equivalence principle, swampland and H0 tension with fast single radio bursts FRB 180924 and FRB 190523

Two new fast single radio bursts FRB 180924 and FRB 190523 well localized to massive galaxies have opened a new window to probe and characterize how cosmic baryons are allocated between galaxies, their surroundings and intergalactic medium. We are motivated by testing Einstein's weak equivalence principle with these two cosmic transients which have accurate redshifts. Using photons with different energies emitted by FRB 180924, we obtain, so far, the most stringent bound $\Delta\gamma<2.16\times10^{-10}$ for non-repeating FRBs with accurate redshifts when only considering the gravitational potential of the Milk Way. If using the gravitational potential of the Laniakea supercluster instead of the Milk Way one, we also obtain the strictest bound $\Delta\gamma<1.06\times10^{-14}$ to date. In light of rapid progress of FRB cosmology, towards the next two decades, we give an universal limitation $\Delta\gamma<8.24\times10^{-22}$ from photons with different energies emitted by single FRBs with accurate redshifts. Moreover, we analyze detailedly the effects of various astrophysical parameters on the precision of weak equivalence principle. We also estimate the abilities of single FRBs with known redshifts to test the validity of swampland criterion, and to distinguish which value of $H_0$ is preferred.

Multimessenger tests of Einstein's weak equivalence principle and Lorentz invariance with a high-energy neutrino from a flaring blazar

The detection of the high-energy (∼290 TeV) neutrino coincident with the flaring blazar TXS 0506+056, the first and only 3σ neutrino-source association to date, provides new, multimessenger tests of the weak equivalence principle (WEP) and Lorentz invariance. Assuming that the flight time difference between the TeV neutrino and gamma-ray photons from the blazar flare is mainly caused by the gravitational potential of the Laniakea supercluster of galaxies, we show that the deviation from the WEP for neutrinos and photons is conservatively constrained to have an accuracy of 10−6–10−7, which is 3–4 orders of magnitude better than previous results placed by MeV neutrinos from supernova 1987A. In addition, we demonstrate that the association of the TeV neutrino with the blazar flare sets limits on the energy scales of quantum gravity for both linear and quadratic violations of Lorentz invariance (LIV) to EQG,1>3.2×1015–3.7×1016 GeV and EQG,2>4.0×1010–1.4×1011 GeV. These improve previous limits on both linear and quadratic LIV energy scales in neutrino propagation by 5–7 orders of magnitude.

A New Method to Test the Einstein’s Weak Equivalence Principle

Abstract The Einstein’s weak equivalence principle (WEP) is one of the foundational assumptions of general relativity and some other gravity theories. In the theory of parametrized post-Newtonian (PPN), the difference between the PPN parameters γ of different particles or the same type of particle with different energies, Δγ, represents the violation of WEP. Current constraints on Δγ are derived from the observed time delay between correlated particles of astronomical sources. However, the observed time delay is contaminated by other effects, such as the time delays due to different particle emission times, the potential Lorentz invariance violation, and none-zero photon rest mass. Therefore, current constraints are only upper limits. Here, we propose a new method to test WEP based on the fact that the gravitational time delay is direction-dependent while others are not. This is the first method that can naturally correct other time-delay effects. Using the time-delay measurements of BASTE gamma-ray burst sample and the gravitational potential of local super galaxy cluster Laniakea, we find that the constraint on Δγ of different energy photons can be as low as 10−14. In the future, if more gravitational wave events and fast radio bursts with much more precise time-delay measurements are observed, this method can give a reliable and tight constraint on WEP.

GW170817 falsifies dark matter emulators

On August 17, 2017 the LIGO interferometers detected the gravitational wave (GW) signal (GW170817) from the coalescence of binary neutron stars. This signal was also simultaneously seen throughout the electromagnetic (EM) spectrum from radio waves to gamma-rays. We point out that this simultaneous detection of GW and EM signals rules out a class of modified gravity theories, termed ``dark matter emulators,'' which dispense with the need for dark matter by making ordinary matter couple to a different metric from that of GW. We discuss other kinds of modified gravity theories which dispense with the need for dark matter and are still viable. This simultaneous observation also provides the first observational test of Einstein's Weak Equivalence Principle (WEP) between gravitons and photons. We estimate the Shapiro time delay due to the gravitational potential of the total dark matter distribution along the line of sight (complementary to the calculation in arXiv:1710.05834) to be about 400 days. Using this estimate for the Shapiro delay and from the time difference of 1.7 seconds between the GW signal and gamma-rays, we can constrain violations of WEP using the parameterized post-Newtonian (PPN) parameter $\gamma$, and is given by $|\gamma_{\rm {GW}}-\gamma_{\rm{EM}}|<9.8 \times 10^{-8}$.

New test of weak equivalence principle using polarized light from astrophysical events

Einstein's weak equivalence principle (WEP) states that any freely falling, uncharged test particle follows the same identical trajectory independent of its internal structure and composition. Since the polarization of a photon is considered to be part of its internal structure, we propose that polarized photons from astrophysical transients, such as gamma-ray bursts (GRBs) and fast radio bursts (FRBs), can be used to constrain the accuracy of the WEP through the Shapiro time delay effect. Assuming that the arrival time delays of photons with different polarizations are mainly attributed to the gravitational potential of the Laniakea supercluster of galaxies, we show that a strict upper limit on the differences of the parametrized post-Newtonian parameter $\gamma$ value for the polarized optical emission of GRB 120308A is $\Delta\gamma<1.2\times10^{-10}$, for the polarized gamma-ray emission of GRB 100826A is $\Delta\gamma<1.2\times10^{-10}$, and for the polarized radio emission of FRB 150807 is $\Delta\gamma<2.2\times10^{-16}$. These are the first direct verifications of the WEP for multiband photons with different polarizations. In particular, the result from FRB 150807 provides the most stringent limit to date on a deviation from the WEP, improving by one order of magnitude the previous best result based on Crab pulsar photons with different energies.

Testing Einstein’s weak equivalence principle with a 0.4-nanosecond giant pulse of the Crab pulsar

Einstein's weak equivalence principle (EEP) can be tested through the arrival time delay between photons with different frequencies. Assuming that the arrival time delay is solely caused by the gravitational potential of the Milky Way, we show that a ``nano-shot'' giant pulse with a time delay between energies corrected for all known effects, e.g., $\mathrm{\ensuremath{\Delta}}t&lt;0.4\text{ }\text{ }\mathrm{ns}$, from the Crab pulsar poses a new upper limit on the deviation from the EEP, i.e., $\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}&lt;(0.6--1.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$. This result provides the hitherto most stringent constraint on the EEP, improving by at least 2 to 3 orders of magnitude from the previous results based on fast radio bursts.

Limits on the neutrino velocity, Lorentz invariance, and the weak equivalence principle with TeV neutrinos from gamma-ray bursts

Five TeV neutrino events weakly correlated with five gamma-ray bursts (GRBs) were detected recently by IceCube. This work is an attempt to show that if the GRB identifications are verified, the observed time delays between the TeV neutrinos and gamma-ray photons from GRBs provide attractive candidates for testing fundamental physics with high accuracy. Based on the assumed associations between the TeV neutrinos and GRBs, we find that the limiting velocity of the neutrinos is equal to that of photons to an accuracy of ∼ 1.9 × 10−15 – 2.5 × 10−18, which is about 104 – 107 times better than the constraint obtained with the neutrino possibly from a blazar flare. In addition, we set the most stringent limits up to date on the energy scale of quantum gravity for both the linear and quadratic violations of Lorentz invariance, namely EQG, 1 > 6.3 × 1018 – 1.5 × 1021 GeV and EQG, 2 > 2.0 × 1011 – 4.2 × 1012 GeV, which are essentially as good as or are an improvement of one order of magnitude over the results previously obtained by the GeV photons of GRB 090510 and the PeV neutrino from a blazar flare. Assuming that the Shapiro time delay is caused by the gravitational potential of the Laniakea supercluster of galaxies, we also place the tightest limits to date on Einstein's weak equivalence principle through the relative differential variations of the parameterized post-Newtonian parameter γ values for two different species of particles (i.e., neutrinos and photons), yielding Δγ ∼ 10−11 – 10−13. However, it should be emphasized again that these limits here obtained are at best forecast of what could be achieved if the GRB/neutrino correlations would be finally confirmed.

The WEAX experiment

Einstein's weak equivalence principle (WEP) states that gravitationalmass is identical to inertial mass. This hypothesis has withstoodexperimental tests to an impressive accuracy of one part in 1011.Various hypotheses based on theory and observations with matter suggestviolations of WEP for antimatter may exist anywhere from the one part in106 to the 200% level. An observed violation at any level would havea profound impact, e.g. it would offer an explanation as to why matterand antimatter are so distinctly separated in the cosmos. We propose aprecise test of WEP for antiprotons in microgravity. We expect to testWEP for antimatter to about one part in 106, and foresee thatadditional advancements of several orders of magnitude in precision couldfollow with further technical developments.

Precision tests of Einstein's Weak Equivalence Principle for antimatter

Einstein's Weak Equivalence Principle (WEP) states that gravitational mass is identical to inertial mass. This hypothesis has withstood experimental tests to an impressive accuracy of one part in one hundred billion. Various hypotheses based on theory and observations with matter suggest violations of WEP for antimatter may exist anywhere from the one part per million to the 200% level. An observed violation at any level would have a profound impact, e.g. it would offer an explanation of why matter and antimatter are so distinctly separated in the Cosmos. We propose a precise test of WEP for antiprotons in microgravity. We expect to test WEP for antimatter to about one part per million, and foresee that additional advancements of several orders of magnitude in precision could follow with further technical developments.