Hughes–Drever experiment
Encyclopedia
Hughes–Drever experiments (also clock comparison-, clock anisotropy-, mass isotropy-, or energy isotropy experiments) are testing the isotropy
of mass
and space
. As in Michelson–Morley experiments, the existence of a preferred frame
of reference, or deviations from Lorentz invariance
can be tested, which also affects the validity of the equivalence principle
. Thus these experiments concern fundamental aspects of both special
and general relativity
. Contrary to Michelson–Morley, Hughes–Drever experiments test the isotropy of the interactions of matter itself, that is, of proton
s, neutron
s, and electron
s. The accuracy achieved makes this kind of experiment one of the most accurate confirmations of relativity (see also Tests of special relativity).
and Edwin Ernest Salpeter
(1958) theorized, that inertia
depends on the surrounding masses according to Mach's principle
. Anisotropic distribution of matter thus would lead to anisotropy
of inertia in different directions. This might be observed by investigating the Zeeman effect
in atomic nuclei
.
Hughes et al. (1960) and Ronald Drever
(1961), independently conducted experiments of that kind. They observed the nucleus of lithium
-7, whose ground state
possesses a spin
of . By that, four magnetic energy level
s exist when measured in a magnetic field in accordance with its allowed magnetic quantum number
. If mass isotropy is satisfied, no frequency shift in the energy levels arises and only one resonance line should exist, if not, a triplet or broadened resonance line should exist. In fact, no frequency shift of the energy levels was observed, and due to the experiment's high precision, the maximum anisotropy only amounted 0,04 Hz
= 10-25 GeV.
and thus special relativity
. This is because anisotropy effects also occur in the presence of a preferred
and Lorentz-violating frame of reference – usually identified with the CMBR
-rest frame as some sort of luminiferous aether
. Therefore, the negative results of the Hughes-Drever experiments (like the Michelson–Morley experiments) rule out the existence of such a frame. In addition, one fundamental statement of the equivalence principle
of general relativity
says that Lorentz invariance locally holds in freely moving reference frames = local Lorentz invariance (LLI). This means that the results of this experiment concern both special and general relativity.
Or, as it was put by Clifford Will, it's about the question of whether the limiting velocity of matter is identical with the speed of light, as predicted by relativity. It they are not the same, the properties and frequencies of matter interactions will change and that's exactly what can be measured by Hughes-Drever experiments. Due to the fact that different frequencies are compared, those experiments are also denoted as clock-comparison experiments.
are considered, especially in the course of the continuing development of quantum gravity
models. Therefore, a series of modern variants of Hughes-Drever experiments have been conducted. Those measurements are related to neutron
s and proton
s, and by application of spin-polarized
systems and co-magnetometers (to suppress magnetic influences), it was possible to considerably increase the accuracy. In addition, by using spin-polarized torsion balances the electron
sector was also tested.
All of those experiments gave negative results, so there is still no sign of the existence of a preferred frame or any other form of Lorentz violation. The values of the following table are related to the coefficients given by the Standard-Model Extension
(see also Test theories of special relativity
). From that, any deviation of Lorentz invariance can be connected with specific coefficients. Since a series of coefficients are tested in those experiments, only the value of maximal sensitivity is given (for precise data, see the individual articles):
Isotropy
Isotropy is uniformity in all orientations; it is derived from the Greek iso and tropos . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary...
of mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
and space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...
. As in Michelson–Morley experiments, the existence of a preferred frame
Preferred frame
In theoretical physics, a preferred or privileged frame is usually a special hypothetical frame of reference in which the laws of physics might appear to be identifiably different from those in other frames....
of reference, or deviations from Lorentz invariance
Lorentz covariance
In standard physics, Lorentz symmetry is "the feature of nature that says experimental results are independent of the orientation or the boost velocity of the laboratory through space"...
can be tested, which also affects the validity of the equivalence principle
Equivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
. Thus these experiments concern fundamental aspects of both special
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
and general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
. Contrary to Michelson–Morley, Hughes–Drever experiments test the isotropy of the interactions of matter itself, that is, of proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s, neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s, and electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s. The accuracy achieved makes this kind of experiment one of the most accurate confirmations of relativity (see also Tests of special relativity).
Early experiments
Giuseppe CocconiGiuseppe Cocconi
Giuseppe Cocconi was a former director of the Proton Synchrotron at CERN in Geneva. He is known for is work in particle physics and for his involvement with SETI.-Life:...
and Edwin Ernest Salpeter
Edwin Ernest Salpeter
Edwin Ernest Salpeter FRS was an Austrian-Australian-American astrophysicist. Born to a Jewish family, he emigrated from Austria to Australia while in his teens to escape the Nazis. He attended Sydney University, where he obtained his bachelor's degree in 1944 and his master's degree in 1945...
(1958) theorized, that inertia
Inertia
Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...
depends on the surrounding masses according to Mach's principle
Mach's principle
In theoretical physics, particularly in discussions of gravitation theories, Mach's principle is the name given by Einstein to an imprecise hypothesis often credited to the physicist and philosopher Ernst Mach....
. Anisotropic distribution of matter thus would lead to anisotropy
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
of inertia in different directions. This might be observed by investigating the Zeeman effect
Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
in atomic nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
.
Hughes et al. (1960) and Ronald Drever
Ronald Drever
Ron Drever is a Scottish physicist. He is currently a Professor Emeritus at the California Institute of Technology.Among other accomplishments he co-founded the LIGO project, and was a co-inventor of the Pound-Drever-Hall technique for laser stabilisation....
(1961), independently conducted experiments of that kind. They observed the nucleus of lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
-7, whose ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...
possesses a spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
of . By that, four magnetic energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
s exist when measured in a magnetic field in accordance with its allowed magnetic quantum number
Magnetic quantum number
In atomic physics, the magnetic quantum number is the third of a set of quantum numbers which describe the unique quantum state of an electron and is designated by the letter m...
. If mass isotropy is satisfied, no frequency shift in the energy levels arises and only one resonance line should exist, if not, a triplet or broadened resonance line should exist. In fact, no frequency shift of the energy levels was observed, and due to the experiment's high precision, the maximum anisotropy only amounted 0,04 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
= 10-25 GeV.
Modern interpretation
While this experiment was initially only related to Mach's principle, it is also considered as an important test of Lorentz invarianceLorentz covariance
In standard physics, Lorentz symmetry is "the feature of nature that says experimental results are independent of the orientation or the boost velocity of the laboratory through space"...
and thus special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
. This is because anisotropy effects also occur in the presence of a preferred
Preferred frame
In theoretical physics, a preferred or privileged frame is usually a special hypothetical frame of reference in which the laws of physics might appear to be identifiably different from those in other frames....
and Lorentz-violating frame of reference – usually identified with the CMBR
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
-rest frame as some sort of luminiferous aether
Luminiferous aether
In the late 19th century, luminiferous aether or ether, meaning light-bearing aether, was the term used to describe a medium for the propagation of light....
. Therefore, the negative results of the Hughes-Drever experiments (like the Michelson–Morley experiments) rule out the existence of such a frame. In addition, one fundamental statement of the equivalence principle
Equivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...
of general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
says that Lorentz invariance locally holds in freely moving reference frames = local Lorentz invariance (LLI). This means that the results of this experiment concern both special and general relativity.
Or, as it was put by Clifford Will, it's about the question of whether the limiting velocity of matter is identical with the speed of light, as predicted by relativity. It they are not the same, the properties and frequencies of matter interactions will change and that's exactly what can be measured by Hughes-Drever experiments. Due to the fact that different frequencies are compared, those experiments are also denoted as clock-comparison experiments.
Modern experiments
Besides Lorentz violations due to a preferred frame or influences based on Mach's principle, also spontaneous violations of Lorentz invariance and CPT symmetryCPT symmetry
CPT symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity, and time simultaneously.-History:...
are considered, especially in the course of the continuing development of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...
models. Therefore, a series of modern variants of Hughes-Drever experiments have been conducted. Those measurements are related to neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s and proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s, and by application of spin-polarized
Spin polarization
Spin polarization is the degree to which the spin, i.e., the intrinsic angular momentum of elementary particles, is aligned with a given direction. This property may pertain to the spin, hence to the magnetic moment, of conduction electrons in ferromagnetic metals, such as iron, giving rise to...
systems and co-magnetometers (to suppress magnetic influences), it was possible to considerably increase the accuracy. In addition, by using spin-polarized torsion balances the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
sector was also tested.
All of those experiments gave negative results, so there is still no sign of the existence of a preferred frame or any other form of Lorentz violation. The values of the following table are related to the coefficients given by the Standard-Model Extension
Standard-Model Extension
Standard-Model Extension is an effective field theory that contains the Standard Model, General Relativity, and all possible operators that break Lorentz symmetry.Violations of this fundamental symmetry can be studied within this general framework...
(see also Test theories of special relativity
Test theories of special relativity
Test theories of special relativity give a mathematical framework for analyzing results of experiments to verify special relativity.An experiment to test the theory of relativity cannot assume the theory is true, and therefore needs some other framework of assumptions that are wider than those of...
). From that, any deviation of Lorentz invariance can be connected with specific coefficients. Since a series of coefficients are tested in those experiments, only the value of maximal sensitivity is given (for precise data, see the individual articles):
| Author | Year | max. anisotropy in GeV | ||
|---|---|---|---|---|
| Proton | Neutron | Electron | ||
| Prestage et al. | 1985 | 10−27 | ||
| Phillips | 1987 | 10−27 | ||
| Lamoreaux et al. | 1989 | 10−29 | ||
| Chupp et al. | 1989 | 10−27 | ||
| Wineland David J. Wineland David J. Wineland is an American physicist at the National Institute of Standards and Technology physics laboratory in Boulder... et al. |
1991 | 10−25 | ||
| Wang et al. | 1993 | 10−27 | ||
| Berglund et al. | 1995 | 10−27 | 10−30 | 10−27 |
| Bear et al. | 2000 | 10−31 | ||
| Walsworth et al. | 2000 | 10−27 | 10−31 | |
| Phillips et al. | 2000 | 10−27 | ||
| Humphrey et al. | 2003 | 10−27 | 10−27 | |
| Hou et al. | 2003 | 10−29 | ||
| Canè et al. | 2004 | 10−32 | ||
| Wolf et al. | 2006 | 10−25 | ||
| Heckel et al. | 2006 | 10−30 | ||
| Heckel et al. | 2008 | 10−31 | ||
| Altarev et al. | 2009 | 10−20 | ||
| Brown et al. | 2010 | 10−32 | 10−33 | |
| Gemmel et al. | 2010 | 10−32 |
External links
- T. Roberts (2007) & S. Schleif, Relativity FAQ, What is the experimental basis of Special Relativity?