ⓘ Equivalence principle. In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein ..


ⓘ Equivalence principle

In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einsteins observation that the gravitational "force" as experienced locally while standing on a massive body is the same as the pseudo-force experienced by an observer in a non-inertial frame of reference.


1. Einsteins statement of the equality of inertial and gravitational mass

A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body. For Newtons equation of motion in a gravitational field, written out in full, it is:

Inertial mass ⋅ {\displaystyle \cdot } Acceleration = {\displaystyle =} Intensity of the gravitational field ⋅ {\displaystyle \cdot } Gravitational mass.

It is only when there is numerical equality between the inertial and gravitational mass that the acceleration is independent of the nature of the body.


2. Development of gravitational theory

Something like the equivalence principle emerged in the early 17th century, when Galileo expressed experimentally that the acceleration of a test mass due to gravitation is independent of the amount of mass being accelerated.

Kepler, using Galileos discoveries, showed knowledge of the equivalence principle by accurately describing what would occur if the moon were stopped in its orbit and dropped towards Earth. This can be deduced without knowing if or in what manner gravity decreases with distance, but requires assuming the equivalency between gravity and inertia.

If two stones were placed in any part of the world near each other, and beyond the sphere of influence of a third cognate body, these stones, like two magnetic needles, would come together in the intermediate point, each approaching the other by a space proportional to the comparative mass of the other. If the moon and earth were not retained in their orbits by their animal force or some other equivalent, the earth would mount to the moon by a fifty-fourth part of their distance, and the moon fall towards the earth through the other fifty-three parts, and they would there meet, assuming, however, that the substance of both is of the same density.

The 1/54 ratio is Keplers estimate of the Moon–Earth mass ratio, based on their diameters. The accuracy of his statement can be deduced by using Newtons inertia law F=ma and Galileos gravitational observation that distance D = 1 / 2 a t 2 {\displaystyle D=1/2at^{2}}. Setting these accelerations equal for a mass is the equivalence principle. Noting the time to collision for each mass is the same gives Keplers statement that D moon /D Earth =M Earth /M moon, without knowing the time to collision or how or if the acceleration force from gravity is a function of distance.

Newtons gravitational theory simplified and formalized Galileos and Keplers ideas by recognizing Keplers "animal force or some other equivalent" beyond gravity and inertia were not needed, deducing from Keplers planetary laws how gravity reduces with distance.

The equivalence principle was properly introduced by Albert Einstein in 1907, when he observed that the acceleration of bodies towards the center of the Earth at a rate of 1 g = 9.81 m/s 2 being a standard reference of gravitational acceleration at the Earths surface is equivalent to the acceleration of an inertially moving body that would be observed on a rocket in free space being accelerated at a rate of 1 g. Einstein stated it thus:

we. assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.

That is, being on the surface of the Earth is equivalent to being inside a spaceship far from any sources of gravity that is being accelerated by its engines. The direction or vector of acceleration equivalence on the surface of the earth is "up" or directly opposite the center of the planet while the vector of acceleration in a spaceship is directly opposite from the mass ejected by its thrusters. From this principle, Einstein deduced that free-fall is inertial motion. Objects in free-fall do not experience being accelerated downward e.g. toward the earth or other massive body but rather weightlessness and no acceleration. In an inertial frame of reference bodies and photons, or light obey Newtons first law, moving at constant velocity in straight lines. Analogously, in a curved spacetime the world line of an inertial particle or pulse of light is as straight as possible in space and time. Such a world line is called a geodesic and from the point of view of the inertial frame is a straight line. This is why an accelerometer in free-fall doesnt register any acceleration; there isnt any between the internal test mass and the accelerometers body.

As an example: an inertial body moving along a geodesic through space can be trapped into an orbit around a large gravitational mass without ever experiencing acceleration. This is possible because spacetime is radically curved in close vicinity to a large gravitational mass. In such a situation the geodesic lines bend inward around the center of the mass and a free-floating weightless inertial body will simply follow those curved geodesics into an elliptical orbit. An accelerometer on-board would never record any acceleration.

By contrast, in Newtonian mechanics, gravity is assumed to be a force. This force draws objects having mass towards the center of any massive body. At the Earths surface, the force of gravity is counteracted by the mechanical physical resistance of the Earths surface. So in Newtonian physics, a person at rest on the surface of a non-rotating massive object is in an inertial frame of reference. These considerations suggest the following corollary to the equivalence principle, which Einstein formulated precisely in 1911:

Whenever an observer detects the local presence of a force that acts on all objects in direct proportion to the inertial mass of each object, that observer is in an accelerated frame of reference.

Einstein also referred to two reference frames, K and K. K is a uniform gravitational field, whereas K has no gravitational field but is uniformly accelerated such that objects in the two frames experience identical forces:

We arrive at a very satisfactory interpretation of this law of experience, if we assume that the systems K and K are physically exactly equivalent, that is, if we assume that we may just as well regard the system K as being in a space free from gravitational fields, if we then regard K as uniformly accelerated. This assumption of exact physical equivalence makes it impossible for us to speak of the absolute acceleration of the system of reference, just as the usual theory of relativity forbids us to talk of the absolute velocity of a system; and it makes the equal falling of all bodies in a gravitational field seem a matter of course.

This observation was the start of a process that culminated in general relativity. Einstein suggested that it should be elevated to the status of a general principle, which he called the "principle of equivalence" when constructing his theory of relativity:

As long as we restrict ourselves to purely mechanical processes in the realm where Newtons mechanics holds sway, we are certain of the equivalence of the systems K and K. But this view of ours will not have any deeper significance unless the systems K and K are equivalent with respect to all physical processes, that is, unless the laws of nature with respect to K are in entire agreement with those with respect to K. By assuming this to be so, we arrive at a principle which, if it is really true, has great heuristic importance. For by theoretical consideration of processes which take place relatively to a system of reference with uniform acceleration, we obtain information as to the career of processes in a homogeneous gravitational field.

Einstein combined postulated the equivalence principle with special relativity to predict that clocks run at different rates in a gravitational potential, and light rays bend in a gravitational field, even before he developed the concept of curved spacetime.

So the original equivalence principle, as described by Einstein, concluded that free-fall and inertial motion were physically equivalent. This form of the equivalence principle can be stated as follows. An observer in a windowless room cannot distinguish between being on the surface of the Earth, and being in a spaceship in deep space accelerating at 1g. This is not strictly true, because massive bodies give rise to tidal effects caused by variations in the strength and direction of the gravitational field which are absent from an accelerating spaceship in deep space. The room, therefore, should be small enough that tidal effects can be neglected.

Although the equivalence principle guided the development of general relativity, it is not a founding principle of relativity but rather a simple consequence of the geometrical nature of the theory. In general relativity, objects in free-fall follow geodesics of spacetime, and what we perceive as the force of gravity is instead a result of our being unable to follow those geodesics of spacetime, because the mechanical resistance of matter prevents us from doing so.

Since Einstein developed general relativity, there was a need to develop a framework to test the theory against other possible theories of gravity compatible with special relativity. This was developed by Robert Dicke as part of his program to test general relativity. Two new principles were suggested, the so-called Einstein equivalence principle and the strong equivalence principle, each of which assumes the weak equivalence principle as a starting point. They only differ in whether or not they apply to gravitational experiments.

Another clarification needed is that the equivalence principle assumes a constant acceleration of 1g without considering the mechanics of generating 1g. If we do consider the mechanics of it, then we must assume the aforementioned windowless room has a fixed mass. Accelerating it at 1g means there is a constant force being applied, which = m*g where m is the mass of the windowless room along with its contents including the observer. Now, if the observer jumps inside the room, an object lying freely on the floor will decrease in weight momentarily because the acceleration is going to decrease momentarily due to the observer pushing back against the floor in order to jump. The object will then gain weight while the observer is in the air and the resulting decreased mass of the windowless room allows greater acceleration; it will lose weight again when the observer lands and pushes once more against the floor; and it will finally return to its initial weight afterwards. To make all these effects equal those we would measure on a planet producing 1g, the windowless room must be assumed to have the same mass as that planet. Additionally, the windowless room must not cause its own gravity, otherwise the scenario changes even further. These are technicalities, clearly, but practical ones if we wish the experiment to demonstrate more or less precisely the equivalence of 1g gravity and 1g acceleration.


3.1. Modern usage The weak equivalence principle

The weak equivalence principle, also known as the universality of free fall or the Galilean equivalence principle can be stated in many ways. The strong EP includes astronomic bodies with gravitational binding energy. The weak EP assumes falling bodies are bound by non-gravitational forces only. Either way:

The trajectory of a point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition and structure. All test particles at the alike spacetime point, in a given gravitational field, will undergo the same acceleration, independent of their properties, including their rest mass. All local centers of mass free-fall in vacuum along identical parallel-displaced, same speed minimum action trajectories independent of all observable properties. The vacuum world-line of a body immersed in a gravitational field is independent of all observable properties. The local effects of motion in a curved spacetime gravitation are indistinguishable from those of an accelerated observer in flat spacetime, without exception. Mass measured with a balance and weight measured with a scale are locally in identical ratio for all bodies the opening page to Newtons Philosophiæ Naturalis Principia Mathematica, 1687.

Locality eliminates measurable tidal forces originating from a radial divergent gravitational field e.g., the Earth upon finite sized physical bodies. The "falling" equivalence principle embraces Galileos, Newtons, and Einsteins conceptualization. The equivalence principle does not deny the existence of measurable effects caused by a rotating gravitating mass frame dragging, or bear on the measurements of light deflection and gravitational time delay made by non-local observers.


3.2. Modern usage Active, passive, and inertial masses

By definition of active and passive gravitational mass, the force on M 1 {\displaystyle M_{1}} due to the gravitational field of M 0 {\displaystyle M_{0}} is:

F 1 = M 0 a c t M 1 p a s r 2 {\displaystyle F_{1}={\frac {M_{0}^{\mathrm {act} }M_{1}^{\mathrm {pass} }}{r^{2}}}}

Likewise the force on a second object of arbitrary mass 2 due to the gravitational field of mass 0 is:

F 2 = M 0 a c t M 2 p a s r 2 {\displaystyle F_{2}={\frac {M_{0}^{\mathrm {act} }M_{2}^{\mathrm {pass} }}{r^{2}}}}

By definition of inertial mass:

F = m i n e r t a {\displaystyle F=m^{\mathrm {inert} }a}

If m 1 {\displaystyle m_{1}} and m 2 {\displaystyle m_{2}} are the same distance r {\displaystyle r} from m 0 {\displaystyle m_{0}} then, by the weak equivalence principle, they fall at the same rate i.e. their accelerations are the same

a 1 = F 1 m 1 i n e r t = a 2 = F 2 m 2 i n e r t {\displaystyle a_{1}={\frac {F_{1}}{m_{1}^{\mathrm {inert} }}}=a_{2}={\frac {F_{2}}{m_{2}^{\mathrm {inert} }}}}


M 0 a c t M 1 p a s r 2 m 1 i n e r t = M 0 a c t M 2 p a s r 2 m 2 i n e r t {\displaystyle {\frac {M_{0}^{\mathrm {act} }M_{1}^{\mathrm {pass} }}{r^{2}m_{1}^{\mathrm {inert} }}}={\frac {M_{0}^{\mathrm {act} }M_{2}^{\mathrm {pass} }}{r^{2}m_{2}^{\mathrm {inert} }}}}


M 1 p a s m 1 i n e r t = M 2 p a s m 2 i n e r t {\displaystyle {\frac {M_{1}^{\mathrm {pass} }}{m_{1}^{\mathrm {inert} }}}={\frac {M_{2}^{\mathrm {pass} }}{m_{2}^{\mathrm {inert} }}}}

In other words, passive gravitational mass must be proportional to inertial mass for all objects.

Furthermore, by Newtons third law of motion:

F 1 = M 0 a c t M 1 p a s r 2 {\displaystyle F_{1}={\frac {M_{0}^{\mathrm {act} }M_{1}^{\mathrm {pass} }}{r^{2}}}}

must be equal and opposite to

F 0 = M 1 a c t M 0 p a s r 2 {\displaystyle F_{0}={\frac {M_{1}^{\mathrm {act} }M_{0}^{\mathrm {pass} }}{r^{2}}}}

It follows that:

M 0 a c t M 0 p a s = M 1 a c t M 1 p a s {\displaystyle {\frac {M_{0}^{\mathrm {act} }}{M_{0}^{\mathrm {pass} }}}={\frac {M_{1}^{\mathrm {act} }}{M_{1}^{\mathrm {pass} }}}}

In other words, passive gravitational mass must be proportional to active gravitational mass for all objects.

The dimensionless Eotvos-parameter η A, B {\displaystyle \eta A,B} is the difference of the ratios of gravitational and inertial masses divided by their average for the two sets of test masses "A" and "B."

η A, B = 2 m g m i A − m g m i B m g m i A + m g m i B {\displaystyle \eta A,B=2{\frac {\left{\frac {m_{g}}{m_{i}}}\right_{A}-\left{\frac {m_{g}}{m_{i}}}\right_{B}}{\left{\frac {m_{g}}{m_{i}}}\right_{A}+\left{\frac {m_{g}}{m_{i}}}\right_{B}}}}

3.3. Modern usage Tests of the weak equivalence principle

Tests of the weak equivalence principle are those that verify the equivalence of gravitational mass and inertial mass. An obvious test is dropping different objects, ideally in a vacuum environment, e.g., inside the Fallturm Bremen drop tower.


Experiments are still being performed at the University of Washington which have placed limits on the differential acceleration of objects towards the Earth, the Sun and towards dark matter in the galactic center. Future satellite experiments – STEP Satellite Test of the Equivalence Principle, Galileo Galilei, and MICROSCOPE MICROSatellite à trainee Compensee pour lObservation du Principe dEquivalence – will test the weak equivalence principle in space, to much higher accuracy.

With the first successful production of antimatter, in particular anti-hydrogen, a new approach to test the weak equivalence principle has been proposed. Experiments to compare the gravitational behavior of matter and antimatter are currently being developed.

Proposals that may lead to a quantum theory of gravity such as string theory and loop quantum gravity predict violations of the weak equivalence principle because they contain many light scalar fields with long Compton wavelengths, which should generate fifth forces and variation of the fundamental constants. Heuristic arguments suggest that the magnitude of these equivalence principle violations could be in the 10 −13 to 10 −18 range. Currently envisioned tests of the weak equivalence principle are approaching a degree of sensitivity such that non-discovery of a violation would be just as profound a result as discovery of a violation. Non-discovery of equivalence principle violation in this range would suggest that gravity is so fundamentally different from other forces as to require a major reevaluation of current attempts to unify gravity with the other forces of nature. A positive detection, on the other hand, would provide a major guidepost towards unification.


3.4. Modern usage The Einstein equivalence principle

What is now called the "Einstein equivalence principle" states that the weak equivalence principle holds, and that:

The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.

Here "local" has a very special meaning: not only must the experiment not look outside the laboratory, but it must also be small compared to variations in the gravitational field, tidal forces, so that the entire laboratory is freely falling. It also implies the absence of interactions with "external" fields other than the gravitational field.

The principle of relativity implies that the outcome of local experiments must be independent of the velocity of the apparatus, so the most important consequence of this principle is the Copernican idea that dimensionless physical values such as the fine-structure constant and electron-to-proton mass ratio must not depend on where in space or time we measure them. Many physicists believe that any Lorentz invariant theory that satisfies the weak equivalence principle also satisfies the Einstein equivalence principle.

Schiffs conjecture suggests that the weak equivalence principle implies the Einstein equivalence principle, but it has not been proven. Nonetheless, the two principles are tested with very different kinds of experiments. The Einstein equivalence principle has been criticized as imprecise, because there is no universally accepted way to distinguish gravitational from non-gravitational experiments see for instance Hadley and Durand.


3.5. Modern usage Tests of the Einstein equivalence principle

In addition to the tests of the weak equivalence principle, the Einstein equivalence principle can be tested by searching for variation of dimensionless constants and mass ratios. The present best limits on the variation of the fundamental constants have mainly been set by studying the naturally occurring Oklo natural nuclear fission reactor, where nuclear reactions similar to ones we observe today have been shown to have occurred underground approximately two billion years ago. These reactions are extremely sensitive to the values of the fundamental constants.

There have been a number of controversial attempts to constrain the variation of the strong interaction constant. There have been several suggestions that "constants" do vary on cosmological scales. The best known is the reported detection of variation at the 10 −5 level of the fine-structure constant from measurements of distant quasars, see Webb et al. Other researchers dispute these findings. Other tests of the Einstein equivalence principle are gravitational redshift experiments, such as the Pound–Rebka experiment which test the position independence of experiments.


3.6. Modern usage The strong equivalence principle

The strong equivalence principle suggests the laws of gravitation are independent of velocity and location. In particular,

The gravitational motion of a small test body depends only on its initial position in spacetime and velocity, and not on its constitution.


The outcome of any local experiment gravitational or not in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.

The first part is a version of the weak equivalence principle that applies to objects that exert a gravitational force on themselves, such as stars, planets, black holes or Cavendish experiments. The second part is the Einstein equivalence principle with the same definition of "local", restated to allow gravitational experiments and self-gravitating bodies. The freely-falling object or laboratory, however, must still be small, so that tidal forces may be neglected hence "local experiment".

This is the only form of the equivalence principle that applies to self-gravitating objects such as stars, which have substantial internal gravitational interactions. It requires that the gravitational constant be the same everywhere in the universe and is incompatible with a fifth force. It is much more restrictive than the Einstein equivalence principle.

The strong equivalence principle suggests that gravity is entirely geometrical by nature that is, the metric alone determines the effect of gravity and does not have any extra fields associated with it. If an observer measures a patch of space to be flat, then the strong equivalence principle suggests that it is absolutely equivalent to any other patch of flat space elsewhere in the universe. Einsteins theory of general relativity including the cosmological constant is thought to be the only theory of gravity that satisfies the strong equivalence principle. A number of alternative theories, such as Brans–Dicke theory, satisfy only the Einstein equivalence principle.


3.7. Modern usage Tests of the strong equivalence principle

The strong equivalence principle can be tested by searching for a variation of Newtons gravitational constant G over the life of the universe, or equivalently, variation in the masses of the fundamental particles. A number of independent constraints, from orbits in the solar system and studies of Big Bang nucleosynthesis have shown that G cannot have varied by more than 10%.

Thus, the strong equivalence principle can be tested by searching for fifth forces deviations from the gravitational force-law predicted by general relativity. These experiments typically look for failures of the inverse-square law specifically Yukawa forces or failures of Birkhoffs theorem behavior of gravity in the laboratory. The most accurate tests over short distances have been performed by the Eot–Wash group. A future satellite experiment, SEE Satellite Energy Exchange, will search for fifth forces in space and should be able to further constrain violations of the strong equivalence principle. Other limits, looking for much longer-range forces, have been placed by searching for the Nordtvedt effect, a "polarization" of solar system orbits that would be caused by gravitational self-energy accelerating at a different rate from normal matter. This effect has been sensitively tested by the Lunar Laser Ranging Experiment. Other tests include studying the deflection of radiation from distant radio sources by the sun, which can be accurately measured by very long baseline interferometry. Another sensitive test comes from measurements of the frequency shift of signals to and from the Cassini spacecraft. Together, these measurements have put tight limits on Brans–Dicke theory and other alternative theories of gravity.

In 2014, astronomers discovered a stellar triple system including a millisecond pulsar PSR J0337+1715 and two white dwarfs orbiting it. The system provided them a chance to test the strong equivalence principle in a strong gravitational field with high accuracy.


4. Challenges

One challenge to the equivalence principle is the Brans–Dicke theory. Self-creation cosmology is a modification of the Brans–Dicke theory. The Fredkin Finite Nature Hypothesis is an even more radical challenge to the equivalence principle and has even fewer supporters.

In August 2010, researchers from the University of New South Wales, Swinburne University of Technology, and Cambridge University published a paper titled "Evidence for spatial variation of the fine structure constant", whose tentative conclusion is that, "qualitatively, results suggest a violation of the Einstein Equivalence Principle, and could infer a very large or infinite universe, within which our local Hubble volume represents a tiny fraction."


5. Explanations

Dutch physicist and string theorist Erik Verlinde has generated a self-contained, logical derivation of the equivalence principle based on the starting assumption of a holographic universe. Given this situation, gravity would not be a true fundamental force as is currently thought but instead an "emergent property" related to entropy. Verlindes entropic gravity theory apparently leads naturally to the correct observed strength of dark energy; previous failures to explain its incredibly small magnitude have been called by such people as cosmologist Michael Turner who is credited as having coined the term "dark energy" as "the greatest embarrassment in the history of theoretical physics". These ideas are far from settled and still very controversial.


6. Experiments

  • Satellite Test of the Equivalence Principle STEP
  • University of Washington
  • Satellite Energy Exchange SEE
  • ".Physicists in Germany have used an atomic interferometer to perform the most accurate ever test of the equivalence principle at the level of atoms."
  • Lunar Laser Ranging
  • Galileo-Galilei satellite experiment

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