zotero-db/storage/P8QB9UUW/.zotero-ft-cache

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/281514516
Direct Terrestrial Test of Lorentz Symmetry in Electrodynamics to 10$^{-18}$
Article in Nature Communications · September 2015
DOI: 10.1038/ncomms9174 · Source: PubMed

CITATIONS
94
8 authors, including: Stephen R. Parker University of Western Australia 40 PUBLICATIONS 368 CITATIONS
SEE PROFILE
John Gideon Hartnett University of Adelaide 262 PUBLICATIONS 3,233 CITATIONS
SEE PROFILE

READS
223
Paul Stanwix University of Western Australia 102 PUBLICATIONS 4,118 CITATIONS
SEE PROFILE
Eugene N Ivanov University of Western Australia 80 PUBLICATIONS 1,625 CITATIONS
SEE PROFILE

All content following this page was uploaded by Michael E. Tobar on 29 October 2015.
The user has requested enhancement of the downloaded file.

ARTICLE

Received 17 Jan 2015 | Accepted 25 Jul 2015 | Published 1 Sep 2015

DOI: 10.1038/ncomms9174

OPEN

Direct terrestrial test of Lorentz symmetry in electrodynamics to 10 À 18

Moritz Nagel1,*, Stephen R. Parker2,*, Evgeny V. Kovalchuk1, Paul L. Stanwix2, John G. Hartnett2,3, Eugene N. Ivanov2, Achim Peters1 & Michael E. Tobar2

Lorentz symmetry is a foundational property of modern physics, underlying the standard model of particles and general relativity. It is anticipated that these two theories are low-energy approximations of a single theory that is unified and consistent at the Planck scale. Many unifying proposals allow Lorentz symmetry to be broken, with observable effects appearing at Planck-suppressed levels; thus, precision tests of Lorentz invariance are needed to assess and guide theoretical efforts. Here we use ultrastable oscillator frequency sources to perform a modern Michelson–Morley experiment and make the most precise direct terrestrial test to date of Lorentz symmetry for the photon, constraining Lorentz violating orientation-dependent relative frequency changes Dn/n to 9.2±10.7 Â 10 À 19 (95% confidence interval). This order of magnitude improvement over previous Michelson–Morley experiments allows us to set comprehensive simultaneous bounds on nine boost and rotation anisotropies of the speed of light, finding no significant violations of Lorentz symmetry.

1 Institut fu¨r Physik, Humboldt-Universita¨t zu Berlin, Newtonstrasse 15, 12489 Berlin, Germany. 2 School of Physics, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia. 3 Institute for Photonics and Advanced Sensing, School of Physical Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia. * These authors contributed equally to this work. Correspondence and requests for materials should be
addressed to S.R.P. (email: stephen.parker@uwa.edu.au).

NATURE COMMUNICATIONS | 6:8174 | DOI: 10.1038/ncomms9174 | www.nature.com/naturecommunications

1

& 2015 Macmillan Publishers Limited. All rights reserved.

ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9174

A significant consequence of Lorentz symmetry is the isotropic nature of the speed of light, which remains invariant under rotation and boost transformations. Measuring the isotropy of the speed of light has played an important role in physics, starting with the seminal Michelson and Morley interferometer experiment in the late nineteenth century1. What began as a hunt for a luminiferous ether soon evolved into tests of the revolutionary theory of special relativity. Current motivation arises from the search for hints of new physics2–4, to provide direction in the quest for a unified theory of quantum mechanics and general relativity5.
Despite the success of unifying the weak force and electrodynamics6, unification of the standard model with gravity remains elusive. Many approaches invoking string theory7, quantum gravity8–10 and noncommutative field theories11 either explicitly require or naturally permit Lorentz symmetry to be broken5. Efforts to test these theories experimentally focus on searches for violations of Lorentz and charge–parity–time (CPT) symmetry, which are expected to occur at the Planck scale (5.4 Â 10 À 44 s, 1.6 Â 10 À 35 m and 1.2 Â 1019 GeV), with suppressed effects manifesting in regimes experimentally accessible via precision measurement12. Many tests of Lorentz and CPT symmetry have been performed for a variety of fields, particles and forces, with no violations reported to date13.
Just as the theories motivating Michelson–Morley style experiments have changed, so too has the technology used. The transition from conventional optical interferometer searches to resonator-stabilized frequency source tests14–19 has enabled a rapid increase in experimental sensitivity; an overview is provided in Fig. 1. The exceptional frequency stability performance of these modern sources makes tests of Lorentz symmetry of the photon one of the most powerful experimental tools in the search for clues towards unification frameworks such as quantum gravity.
Here we present the results of the most sensitive Michelson– Morley style frequency comparison experiment performed to date. We use 1 year of data to set new bounds on the nine possible rotational and boost isotropies of the speed of light, with our results expressed as constraints on coefficients of the standard model extension (SME)20. We find no evidence of any statistically significant violation of Lorentz symmetry of the photon.

Results
Experiment design. A schematic of our Michelson–Morley oscillator experiment, located in Berlin (latitude 52.4°), is presented in Fig. 2. Two cylindrical copper cavities were each loaded with a nominally identical cylindrical sapphire dielectric crystal. Whispering gallery modes were excited within the crystals with a resonance frequency of 12.97 GHz. Pound control electronics are used to build two loop oscillator circuits, with each oscillator locked to the resonance frequency of a cavity21. The cavity crystal axes were aligned perpendicular to each other such that the Poynting vectors and thus path of light propagation for the resonant modes were orthogonal to each other (see insert of Fig. 2). A fractional change in the speed of light would induce a proportional fractional change in the beat note frequency of the two oscillators. Thus, to be highly sensitive to the signals of Lorentz invariance violation (LIV) one needs to employ extremely low-noise frequency sources. Of course, systematic noise sources also lead to frequency changes; thus, we require both low-noise sources and a well-controlled setup. At cryogenic temperatures, the frequency stability of the loop oscillator circuit was optimized, which ultimately dictated the sensitivity of the experiment (see Fig. 3).
We analysed the experiment relative to a sun-centred inertial reference frame22. As such, passive rotation transformations were provided courtesy of Earth’s daily and annual cycles. In addition, the apparatus was actively rotated in the laboratory on a tiltstabilized high-precision air-bearing turntable that was run with a period of 100 s. This corresponds to the optimal performance of the experiment, which is a trade-off between oscillator frequency stability, rate of data acquisition and the influence of systematic

Electronics

To data acquisition

Δv/v or Δc /c Liquid helium Cryogenic dewar

10–9

10–10

10–11 10–12 10–13

Legend Interferometer tests Cavity tests

10–14

10–15

10–16

10–17 10–18 10–119880

1900

1920

1940 1960 Year

1980

This work 2000 2020

Figure 1 | Historical overview of Michelson–Morley style tests of Lorentz invariance in electrodynamics. Presented are interferometer tests (squares) that measure fractional shifts in the speed of light, Dc/c, and cavity-based tests (circles) such as this work that measure a fractional change in frequency, Dn/n. Bounds are taken directly from the results reported in original publications. A full list is provided in the Supplementary References, with numerical values given in Supplementary Table 1.

Tilt sensor
Rotation platform
Tilt control
Figure 2 | Schematical overview of the experimental setup. Two sapphire cylinders were loaded in a dual-cavity mount with their crystal axes (black arrows) aligned orthogonally. This was housed within two vacuum cans and cooled to 4 K in a liquid helium dewar. A whispering gallery mode resonance was excited in each sapphire, the Poynting vector is shown by the dashed magenta arrow. Microwave and DC electronics were used to create two loop oscillators, each locked to the resonance of a sapphire. The two oscillators were beat against each other and the difference frequency was recorded. The apparatus was continuously rotated with a 100 s period on a tilt-controlled air-bearing turntable. Comparison with the original Michelson–Morley arrangement is presented in the top right to demonstrate experimental concept.

2

NATURE COMMUNICATIONS | 6:8174 | DOI: 10.1038/ncomms9174 | www.nature.com/naturecommunications

& 2015 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9174

a 20

Relative beat frequency (mHz)

15

10

5

0

–5

–10

–15

–20 0

50

100

150

200

250

300

350

400

Time (days)

b

c

10–14

Rotating Stationary

10–3

Rotating Stationary

10–15

10– 4

ARTICLE

Amplitude spectral density (Hz Hz–1/2 )

Allan deviation of fractional beat frequency

10–16100

101

102

Integration time (s)

103

10–

5
10–3

10–2

10–1

100

Fourier frequency (Hz)

Figure 3 | Beat frequency of the two cryogenic oscillators. (a) Sampled beat frequency recorded over the course of the experiment from June 2012 to June 2013, with linear drift (B86 mHz per day) and a 347 kHz offset subtracted for display purposes. The gap in data was because of a short period of downtime during the 2012/2013 transition. (b) Allan deviation of the fractional beat frequency, demonstrating the fundamental frequency stability of the oscillators when they are stationary in the laboratory (black trace) compared with the stability when actively rotated (red trace). Rotated stability represents typical performance and was calculated from a subset of the data taken during April 2012. (c) Comparison of amplitude spectral density of the oscillators beat frequency when stationary (black trace) and rotated (red trace) in the laboratory, computed from the same data as in b. Peaks corresponding to the fundamental turntable rotation frequency and higher-order harmonics can be resolved clearly, whereas the background noise level has not increased. Signals of interest are located at sidereal and annual sideband frequencies around the second turntable harmonic (2oR ¼ 0.02 Hz), where the noise is not increased by rotation.

noise. A 200 l liquid helium reservoir allowed for 3 weeks of continuous operation before refilling was required.
Owing to the symmetry of the setup with respect to the suncentered inertial reference frame, the signal of interest occurs at twice the turntable rotation frequency, 2oR, with additional sideband modulations arising from Earth’s sidereal rotation, o", and orbit, O". This has the added benefit of suppressing the influence of any rotation-induced sources of systematic noise that manifest at the fundamental turntable rotation frequency (see Fig. 3). The experimental setup is first-order sensitive to LIVs of rotational transformations, with a suppressed sensitivity to symmetry breaking of boost transformations. The suppression is of order 10 À 4, which is the ratio of Earth’s orbital velocity to the speed of light.

Analysis. Data were collected over the course of a year; the beat

frequency between the oscillators for the duration of the experi-

ment is displayed in Fig. 3. The beat frequency data were analysed

for periodic signals of variation corresponding to modulation

frequencies of interest. Least-squares regression was used to

perform a fit to the fundamental turntable rotation frequency and

the first harmonic,

Dnbeat nbeat

¼

A

þ

Bt

X þ Cn
n¼1;2

cosðnoR

t

þ

fnÞ

þ

Sn

sinðnoRt þ fnÞ:

ð1Þ

Error-weighted least-squares regression was then used to fit the
amplitudes Cn and Sn from equation (1) to the daily variations (o", 2o") and finally to the annual frequencies (O", 2O"). A detailed description of the data analysis process can be found in previous work19,23,24 and in the Methods section.
Taking error-weighted averages of relevant amplitudes from equation (1) we found a 2oR amplitude of À 98±6 nHz. This value of interest, 2oR, is only statistically significant, owing to the influence of systematic noise sources (see Fig. 3), the most dominant of which is the dependency of oscillator resonance
frequency on external magnetic fields, arising from the presence of impurities in the sapphire crystal25 and ferrite-based
microwave components. The frequency variations induced by moving the oscillators through the quasi-static magnetic field of
the Earth in the laboratory are indistinguishable from a Lorentz violating signal. However, the leakage into sidereal and annual
sidebands is negligible. Calculating the weighted average of quadrature amplitudes for
daily and twice daily variations (o", 2o"; see Supplementary Figs 1–8), we found a frequency variation of 12±14 nHz
(95% confidence interval), leading to our reported bound on the overall sensitivity of the experiment, Dn/nr9.2±10.7 Â 10 À 19 (95% confidence interval). The presence of a statistically significant LIV signal in any solitary amplitude would propagate
through to the final value.

NATURE COMMUNICATIONS | 6:8174 | DOI: 10.1038/ncomms9174 | www.nature.com/naturecommunications

3

& 2015 Macmillan Publishers Limited. All rights reserved.

ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9174

Discussion We use our data to place limits on coefficients of the SME20,

which is an effective field theory framework containing the

standard model and general relativity along with any possible

Lorentz and CPT symmetry violating coefficients that could arise

from the various fields. It is important to note that the SME is a

purely phenomenological framework, designed to enable

comprehensive searches for Lorentz and CPT symmetry

violations and facilitate cataloguing and comparisons between

experiments. It is conventional to express results in a single

inertial reference frame; in this case, the widely adopted sun-centred celestial equatorial frame is used22.

It is important to point out that technically, at a fundamental

level, a change in the beat frequency of the experiment is

proportional to not just a change in the speed of light (owing to

Lorentz violation of the photon) but also to changes in the

propagating medium, owing to Lorentz violation of the electron26–28. What we are actually measuring and constraining

is a difference or combination of these potential effects. Owing to

the choice of coordinates, the proton sector remains Lorentz

invariant and does not contribute to any changes in the propagating medium22.

We restrict our attention to the photon sector of the minimal SME22, which only contains operators of renormalizable

dimension in flat spacetime. The resulting possible violations of

Lorentz symmetry can be divided into polarization-dependent or -independent effects; astrophysical constraints29 have limited

polarization-dependent violations far beyond the reach of this

work and are thus ignored. What remains are nine coefficients

constraining different effects—rotation violations, described by the five coefficients of the 3 Â 3 symmetric traceless matrix k~jekÀ , boost violations, described by the three coefficients of the 3 Â 3 antisymmetric matrix k~jokþ and an overall isotropic deviation of c, described by the scalar k~tr (ref. 24). Experimental sensitivities to

these coefficients are provided in Supplementary Table 2.

For sapphire with the c axis parallel to the crystal axis, as is the

case in this work, the sensitivity to Lorentz violation of electrons

is reduced27,28. The effect is such that the orientation coefficients,

kk~~jejekkÀÀ

, and the À 2:25cjk

isotropic deviation, k~tr, should be replaced with and k~tr þ 1:5ce00, respectively. In presenting the

results of this work, we adhere to established convention and

assume that the electron sector remains Lorentz invariant, as it

has been for all modern cavity-based Michelson–Morley

experiments.

Bounds for coefficients of the minimal SME are presented in

Table 1. It is important to note that k~Ze ÀZ is constrained solely by the amplitude of the cosine variation at twice the turntable

Table 1 | Bounds on non-birefringent photon-sector coefficients of the minimal SME.

Coefficient
k~Xe YÀ k~Xe ZÀ k~Ye ZÀ k~XeXÀ À k~YeYÀ k~ZeZÀ k~XoYþ k~XoZþ k~YoZþ
k~tr

Bound (Error) À 0.7 (1.6) À 5.5 (4.0) À 1.9 (3.2) À 1.5 (3.4) À 286 (279) À 3.0 (3.4) 0.2 (1.7) À 2.0 (1.6) À 6.0 (4.0)

SME, standard model extension. Errors are standard 1s of statistical origin. Values for k~e À are given in 10 À 18, k~o þ in 10 À 14 and k~tr in 10 À 10.

frequency, 2oR, which is dominated by known systematic noise processes (magnetic field). Therefore, the presented bound is obtained by taking the statistical mean of the time-dependent C2 amplitudes from equation (1), with the error given by the s.d. of the amplitudes. All coefficients are statistically insignificant; we report no evidence for violations of Lorentz symmetry in electrodynamics.
For the rotation symmetry breaking coefficients, k~e À , our bounds are comparable to recent results from a trapped ion experiment30 that put constraints on four sets of linear combinations of electron and photon coefficients. In contrast, our reported bounds on the five k~e À coefficients were set under the assumption that Lorentz invariance of the electron is conserved (that is, cjk ¼ 0); ultimately, this works derives bounds from nine linear combinations of photon and electron coefficients (see Supplementary Table 2), which in principle offers better opportunities for disentangling coefficients. Compared with bounds derived from previous modern Michelson–Morley cavity tests18,19, where it was also assumed that the electron remained Lorentz invariant, we improve by up to a factor of B4.
The trapped ion experiment30 was not sensitive to the isotropic deviation, k~tr, or the boost symmetry breaking coefficients, k~o þ . For k~o þ , we improve on the current state-of-the-art13,18,19 by up to a factor of B5. We also improve on our existing bounds24 for the isotropic shift, k~tr, by a factor of B20, as recent work31 demonstrated that double-pass asymmetric resonators constructed out of a single material are only sensitive to higherorder coefficients (d44) of the SME.
Despite thoughtful advances in the implementation and design of alternative experiments, cavity-based Michelson–Morley tests such as the one presented in this work still remain the most powerful and comprehensive way to search for non-birefringent LIV effects in electrodynamics.
Scheduled upgrades to the experiment to improve the fundamental stability of the microwave oscillators32 will soon allow for even more sensitive tests. The installation of superior magnetic shielding around the oscillators will reduce the influence of systematic noise sources. The addition of a separate optical cavity system in the same cryogenic environment will also open up opportunities for exploring additional higher-order coefficients31,33 and matter sector coefficients34, allowing a more complete disentangling of electron and photon sector coefficients.
A modern Michelson–Morley experiment has been performed with two orthogonally aligned stable microwave oscillators. Using data of the beat note frequency between the two oscillators recorded over the course of a year, we are able to constrain LIV-induced Dn/n to be o10 À 18, the most precise measurement ever made for electromagnetic cavity experiments. No violations of Lorentz symmetry were observed.
Methods Experiment. Two nominally identical cylinders (51 mm diameter and 30 mm
height) of HEMEX grade ultra-pure sapphire (GT Advanced Technologies) were machined from the same boule. The crystals were cleaned in a solution of 70 % nitric acid containing several drops of hydrofluoric acid and then mounted in copper cavities and sealed in a stainless steel vacuum can, evacuated to sub-10 À 6 mbar. Whispering gallery resonant mode WGE16,0,0 was used for both cavities. This mode features a dominant radial electric field with 32 variations around the circumference of the sapphire crystal. The majority of the electromagnetic fields are contained within the sapphire dielectric, with minimal evanescent field leaking out to the copper walls of the cavity structure. The first cavity had a resonant frequency of 12.9688 GHz and a loaded quality factor of 109 at 4.4 K, while the second cavity had a resonant frequency of 12.9685 GHz and a loaded quality factor of 1.5 Â 109 at 4.4 K.
Small concentrations of impurities within the sapphire give rise to a temperature/frequency turning point21; for the 347 kHz beat frequency between the two resonators, this turning point occurred at 5.5 K. A temperature controller (model 340, Lake Shore Cryotronics, Inc.) was used in conjunction with a resistive heater and a carbon-glass temperature sensor to operate the resonators at the

4

NATURE COMMUNICATIONS | 6:8174 | DOI: 10.1038/ncomms9174 | www.nature.com/naturecommunications

& 2015 Macmillan Publishers Limited. All rights reserved.

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9174

ARTICLE

turning point. Custom microwave circuits and control electronics are used to create a loop oscillator out of each resonator. Pound locking is employed, whereby modulation sidebands reflected back from the resonator are demodulated with a lock-in amplifier (model SR830, Standford Research Systems) and from this a correction signal is applied to a voltage-controlled phase shifter to align the frequency of the oscillator with that of the resonator. Power incident on the cavity is monitored with a detector (model DT8016, Herotek, Inc.), the signal is compared against a user-defined set-point and a correction voltage is applied to a voltagecontrolled attenuator placed in situ with the loop oscillator. The oscillator comparison beat frequency was logged on a frequency counter (model 53142A, Agilent Technologies, Inc.) referenced to a 10 MHz rubidium standard.
The oscillators were rotated on a high-precision air-bearing rotation table (Kugler GmbH); an 18,000 point optical encoder was used to track the angular position of the table and maintain a constant rotation velocity. The table sat on three aluminium legs, with each one in turn placed on a force sensor; these were used to align the centre of mass with the axis of rotation. A biaxial high-gain tilt sensor (model 755, Applied Geomechanics) sits at the centre of the experiment. Variations in tilt were compensated for by heating or cooling two of the three aluminium legs. All three legs were heated above ambient temperature to improve performance of the tilt-control system.

Data analysis. Original time tags are converted into time in seconds since the
Vernal Equinox before the start of data collection (20 March 2012, 05:14 UTC þ 0). This format assists with calculating the relevant phase offsets required to analyse the data in the context of the SME22. The rotation turntable features an optical encoder with 18,000 points and a trigger mark to indicate that a full rotation has occurred; the data are scanned and incomplete rotations are discarded. The rotation points are converted into a modular angle value in radians. The data are broken up into subsets containing ten full turntable rotations (corresponding to B1,000 s) and an ordinary least-squares regression is used to fit the subset to the following model:

nbeat ¼A þ Bt þ CoR cosðoRt þ fRÞ þ SoR sinðoRt þ fRÞ

ð2Þ

þ C2oR cosð2oRt þ 2fRÞ þ S2oR sinð2oRt þ 2fRÞ:

The value of oRt comes straight from the modular angular position of the turntable recorded in the data. The phase offset, fR, is the angular difference between the

start of data collection and the alignment of the crystal axis of the top cavity with

geographical East. The start of data collection is triggered at the same point for

each experimental run. The phase offset is once again only needed to aid with the

reporting of coefficients in the SME framework. The amplitudes of equation (2)

and the mean time of each subset are stored to file, producing 19,597 entries. A

histogram of the magnitude of errors for the fits to C2oR and S2oR is produced (Supplementary Fig. 9); all subsets with an error further than 3s from the mean are

discarded, resulting in 299 entries being removed (1.5% of the full data set). These

points correspond to data with excessive additional noise present that does not fit

our model or expected signals and would otherwise corrupt the quality of the

subsequent analysis.

The demodulated data set is then broken up into subsets containing 100 entries

each (B1.2 days) and fit to the following model via a weighted least-squares

regression, using the square of the s.e. of the fits from equation (2) as the weights.

À

Á

À

Á

C2oR ðtÞ ¼C0 þ CCoÈ coÀs o È t þ f È Áþ CSoÈ sin oÀ È t þ f È Á ð3Þ

þ CC2oÈ cos 2o È t þ 2f È þ CS2oÈ sin 2o È t þ 2f È

À

Á

À

Á

S2oR ðtÞ ¼S0 þ SCoÈ coÀs o È t þ f È Áþ SSoÈ sin oÀÈ t þ f È

Á ð4Þ

þ SC2oÈ cos 2o È t þ 2f È þ SS2oÈ sin 2o È t þ 2f È

The phase offset f" is the difference between the 2012 Vernal Equinox and the alignment of geographical East with the Y axis of the Sun Centred frame used for determination of the SME coefficients22. The value of o" used was 7.3 Â 10 À 5 rad s À 1 and t is the relevant mean time calculated during the previous demodulation (equation (2)). Once again the amplitudes, s.e. and mean time for each subset is stored to file. The fitted amplitudes and s.e. from equations (3) and (4) are shown in Supplementary Figs 1–8. The eight amplitudes from equations (3) and (4) representing the first two harmonics of daily variations are used to bound the overall sensitivity of the experiment to LIVs. We take the s.e.-weighted average of all the amplitudes, which is equivalent to weighting by the variance. Noting the distribution of the histogram of all the amplitudes (Supplementary Fig. 10), whereby 95% of the points lie within 2 s.d. of the mean, we multiply the associated s.e. by 2, to determine the 95% confidence interval for our bound, Dn/nr9.2±10.7 Â 10 À 19.
The final stage of the data analysis is used to set bounds on coefficients of the SME. Each amplitude and s.e. from equations (3) and (4) is used to perform a weighted least-squares regression fit to an offset and variations at harmonics of Earth’s orbital frequency O" and 2O". Supplementary Table 2 summarizes the relevant amplitudes, their sensitivity to different coefficients of the SME and the corresponding numerical weights.
For SME coefficients where more than one bound is derived from Supplementary Table 2, we report the error-weighted average of all contributions. The coefficient k~Ze ÀZ is only accessible via the second harmonic of turntable rotation

(amplitude C2oR in equation (2), or offset C0 in equation (3)), where systematic noise sources are present. The bound for this coefficient is set by taking the statistical mean and s.d. of all the values obtained for C0, noting that the bound should be consistent with a null result for systematic noise with a random phase, while a substantial Lorentz violating signal with constant phase would be statistically significant.
References
1. Michelson, A. A. & Morley, E. W. On the relative motion of the earth and the luminiferous ether. Am. J. Sci. 34, 333–345 (1887).
2. Bluhm, R. in Special Relativity. (eds Ehlers, J. & La¨mmerzahl, C.) 702 of Lecture Notes in Physics 191–226 (Springer, Berlin, 2006).
3. Liberati, S. & Mattingly, D. in Proceedings of the 2009 SIGRAV Graduate School in Contemporary Relativity and Gravitionational Physics (Villa Olmo, Como, Italy, 2009).
4. Tasson, J. D. What do we know about lorentz invariance? Rep Prog Phys 77, 062901 (2014).
5. Weinberg, S. in Proceedings of the sixth International Workshop on Chiral Dynamics, no. UTTG-09-09, Proceedings of Science 1–15 (University of Bern, Bern, Switzerland, 2009).
6. Weinberg, S. A model of leptons. Phys. Rev. Lett. 19, 1264–1266 (1967). 7. Kostelecky´, V. A. & Samuel, S. Spontaneous breaking of lorentz symmetry in
string theory. Phys. Rev. D 39, 683–685 (1989). 8. Myers, R. C. & Pospelov, M. Ultraviolet modifications of dispersion relations in
effective field theory. Phys. Rev. Lett. 90, 211601 (2003). 9. Hoˇrava, P. Quantum gravity at a lifshitz point. Phys. Rev. D 79, 084008 (2009). 10. Sotiriou, T. P., Visser, M. & Weinfurtner, S. Phenomenologically viable
lorentz-violating quantum gravity. Phys. Rev. Lett. 102, 251601 (2009). 11. Carroll, S. M., Harvey, J. A., Kostelecky´, V. A., Lane, C. D. & Okamoto, T.
Noncommutative field theory and lorentz violation. Phys. Rev. Lett. 87, 141601 (2001). 12. Kostelecky´, V. A. & Potting, R. CPT , strings, and meson factories. Phys. Rev. D 51, 3923–3935 (1995). 13. Kostelecky´, V. A. & Russell, N. Data tables for lorentz and cpt violation. Rev. Mod. Phys. 83, 11–31 (2011). 14. Essen, L. A new aether-drift experiment. Nature 17, 793–794 (1955). 15. Brillet, A. & Hall, J. L. Improved laser test of the isotropy of space. Phys. Rev. Lett. 42, 549–552 (1979). 16. Lipa, J. A., Nissen, J. A., Wang, S., Stricker, D. A. & Avaloff, D. New limit on signals of lorentz violation in electrodynamics. Phys. Rev. Lett. 90, 060403 (2003). 17. Stanwix, P. L. et al. Test of lorentz invariance in electrodynamics using rotating cryogenic sapphire microwave oscillators. Phys. Rev. Lett. 95, 040404 (2005). 18. Eisele, C., Nevsky, A. Y. & Schiller, S. Laboratory test of the isotropy of light propagation at the 10 À 17 level. Phys. Rev. Lett. 103, 090401 (2009). 19. Herrmann, S. et al. Rotating optical cavity experiment testing lorentz invariance at the 10 À 17 level. Phys. Rev. D 80, 105011 (2009). 20. Colladay, D. & Kostelecky´, V. A. Lorentz-violating extension of the standard model. Phys. Rev. D 58, 116002 (1998). 21. Locke, C. R., Ivanov, E. N., Hartnett, J. G., Stanwix, P. L. & Tobar, M. E. Invited article: design techniques and noise properties of ultrastable cryogenically cooled sapphire-dielectric resonator oscillators. Rev. Sci. Instrum. 79, 051301 (2008). 22. Kostelecky´, V. A. & Mewes, M. Signals for lorentz violation in electrodynamics. Phys. Rev. D 66, 056005 (2002). 23. Stanwix, P. L., Tobar, M. E., Wolf, P., Locke, C. R. & Ivanov, E. N. Improved test of lorentz invariance in electrodynamics using rotating cryogenic sapphire oscillators. Phys. Rev. D 74, 081101 (2006). 24. Hohensee, M. A. et al. Improved constraints on isotropic shift and anisotropies of the speed of light using rotating cryogenic sapphire oscillators. Phys. Rev. D 82, 076001 (2010). 25. Benmessai, K. et al. Hybrid electron spin resonance and whispering gallery mode resonance spectroscopy of fe3 þ in sapphire. Phys. Rev. B 87, 094412 (2013). 26. Mu¨ller, H., Herrmann, S., Saenz, A., Peters, A. & La¨mmerzahl, C. Optical cavity tests of lorentz invariance for the electron. Phys. Rev. D 68, 116006 (2003). 27. Mu¨ller, H., Braxmaier, C., Herrmann, S., Peters, A. & La¨mmerzahl, C. Electromagnetic cavities and lorentz invariance violation. Phys. Rev. D 67, 056006 (2003). 28. Mu¨ller, H. Testing lorentz invariance by the use of vacuum and matter filled cavity resonators. Phys. Rev. D 71, 045004 (2005). 29. Kostelecky´, V. A. & Mewes, M. Sensitive polarimetric search for relativity violations in gamma-ray bursts. Phys. Rev. Lett. 97, 140401 (2006). 30. Pruttivarasin, T. et al. Michelson-Morley analogue for electrons using trapped ions to test Lorentz symmetry. Nature 517, 592–595 (2015). 31. Mewes, M. Optical-cavity tests of higher-order lorentz violation. Phys. Rev. D 85, 116012 (2012). 32. Nand, N. R. et al. Resonator power to frequency conversion in a cryogenic sapphire oscillator. Appl Phys Lett 103, 043502 (2013).

NATURE COMMUNICATIONS | 6:8174 | DOI: 10.1038/ncomms9174 | www.nature.com/naturecommunications

5

& 2015 Macmillan Publishers Limited. All rights reserved.

ARTICLE

NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9174

33. Parker, S. R., Mewes, M., Stanwix, P. L. & Tobar, M. E. Cavity bounds on higher-order lorentz-violating coefficients. Phys. Rev. Lett. 106, 180401 (2011).
34. Mu¨ller, H. et al. Tests of relativity by complementary rotating michelson-morley experiments. Phys. Rev. Lett. 99, 050401 (2007).
Acknowledgements
This work was supported by Australian Research Council Discovery Project DP130100205 and by the Go8-DAAD Australia-Germany Joint Research Co-operation Scheme.
Author contributions
M.T. and A.P. conceived and instigated the experiment. S.P., P.S., M.N., J.H. and E.I. built the cryogenic oscillators and relevant control systems. M.N., E.K., S.P. and P.S. commissioned and optimized the experimental setup. M.N. and E.K. collected data and operated the experiment. S.P., M.N., E.K., A.P. and M.T. analysed the data. S.P. and M.N. wrote the manuscript and Supplementary Materials. All authors contributed to interpretations and discussions of results, and to the content of the manuscript.

Additional information
Supplementary Information accompanies this paper at http://www.nature.com/ naturecommunications
Competing financial interests: The authors declare no competing financial interests.
Reprints and permission information is available online at http://npg.nature.com/ reprintsandpermissions/
How to cite this article: Nagel, M. et al. Direct terrestrial test of Lorentz symmetry in electrodynamics to 10 À 18. Nat. Commun. 6:8174 doi: 10.1038/ncomms9174 (2015).
This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

6
View publication stats

NATURE COMMUNICATIONS | 6:8174 | DOI: 10.1038/ncomms9174 | www.nature.com/naturecommunications & 2015 Macmillan Publishers Limited. All rights reserved.