Gravity Probe A (GP-A) was a space-based experiment to test the equivalence principle, a feature of Einstein's theory of relativity. It was performed jointly by the Smithsonian Astrophysical Observatory and the National Aeronautics and Space Administration. The experiment sent a

hydrogen maser A hydrogen maser, also known as hydrogen frequency standard, is a specific type of maser that uses the intrinsic properties of the hydrogen atom to serve as a precision frequency reference. Both the proton and electron of a hydrogen atom have s ...

a highly accurate

frequency standard A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to p ...

into space to measure with high precision the rate at which time passes in a weaker gravitational field. Masses cause distortions in

spacetime In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differ ...

, which leads to the effects of

length contraction Length contraction is the phenomenon that a moving object's length is measured to be shorter than its proper length, which is the length as measured in the object's own rest frame. It is also known as Lorentz contraction or Lorentz–FitzGerald ...

and

time dilation In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them ( special relativistic "kinetic" time dilation) or to a difference in gravitational ...

, both predicted results of Albert Einstein's theory of

general relativity General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

. Because of the bending of spacetime, an observer on Earth (in a lower gravitational potential) should measure a slower rate at which time passes than an observer that is higher in altitude (at higher gravitational potential). This effect is known as

gravitational time dilation Gravitational time dilation is a form of time dilation, an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass. The lower the gravitational potential (the closer ...

. The experiment was a test of a major consequence of Einstein's general relativity, the equivalence principle. The equivalence principle states that a reference frame in a uniform gravitational field is indistinguishable from a reference frame that is under uniform acceleration. Further, the equivalence principle predicts that phenomenon of different time flow rates, present in a uniformly accelerating reference frame, will also be present in a stationary reference frame that is in a uniform gravitational field. The probe was launched on June 18, 1976 from the NASA-Wallops Flight Center in Wallops Island, Virginia. The probe was carried via a

Scout rocket The Scout family of rockets were American launch vehicles designed to place small satellites into orbit around the Earth. The Scout multistage rocket was the first orbital launch vehicle to be entirely composed of solid fuel stages. It was also t ...

, and attained a height of , while remaining in space for 1 hour and 55 minutes, as intended. It returned to Earth by splashing down into the Atlantic Ocean.

Background

The objective of the Gravity Probe A experiment was to test the validity of the equivalence principle. The equivalence principle is a key component of Albert Einstein's theory of

, and states that the laws of physics are the same in an accelerating reference frame as they are in a reference frame that is acted upon by a uniform gravitational field.

Equivalence principle

The equivalence principle can be understood by comparing a rocket ship in two scenarios. First, imagine a rocket ship that is at rest on the Earth's surface; objects dropped within the rocket ship will fall towards the floor with an acceleration of . Now, imagine a distant rocket ship that has escaped Earth's gravitational field and is accelerating at a constant due to thrust from its rockets; objects in the rocket ship that are unconstrained will move towards the floor with an acceleration of . This example shows one way that a uniformly accelerating reference frame is indistinguishable from a gravitational reference frame. Furthermore, the equivalence principle postulates that phenomena that are caused by inertial effects will also be present due to gravitational effects. Consider a beam of light that is shined horizontally across a rocket ship, which is accelerating. According to a non-accelerating observer outside the rocket ship, the floor of the rocket ship accelerates towards the light beam. Therefore, the light beam does not seem to travel on a horizontal path according to the inside observer, rather the light ray appears to bend toward the floor. This is an example of an inertial effect that causes light to bend. The equivalence principle states that this inertial phenomenon will occur in a gravitational reference frame as well. Indeed, the phenomenon of

gravitational lensing A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer that is capable of bending the light from the source as the light travels toward the observer. This effect is known ...

states that matter can bend light, and this phenomenon has been observed by the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

, and other experiments.

Time dilation

Time dilation refers to the expansion or contraction in the rate at which time passes, and was the subject of the Gravity Probe A experiment. Under Einstein's theory of general relativity, matter distorts the surrounding

. This distortion causes time to pass more slowly in the vicinity of a massive object, compared to the rate experienced by a distant observer. The Schwarzschild metric, surrounding a spherically symmetric gravitating body, has a smaller coefficient at

dt^2

closer to the body, which means slower rate of time flow there. There is a similar idea of time dilation occurrence in Einstein's theory of

special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws ...

(which involves neither gravity nor the idea of curved spacetime). Such time dilation appears in the

Rindler coordinates In relativistic physics, the coordinates of a ''hyperbolically accelerated reference frame'' constitute an important and useful coordinate chart representing part of flat Minkowski spacetime. In special relativity, a uniformly accelerating particle ...

, attached to a uniformly accelerating particle in a flat spacetime. Such a particle would observe time passing faster on the side it is accelerating towards and more slowly on the opposite side. From this apparent variance in time, Einstein inferred that change in velocity affects the relativity of simultaneity for the particle. Einstein's equivalence principle generalizes this analogy, stating that an accelerating reference frame is locally indistinguishable from an inertial reference frame with a gravity force acting upon it. In this way, the Gravity Probe A was a test of the equivalence principle, matching the observations in the inertial reference frame (of special relativity) of the Earth's surface affected by gravity, with the predictions of special relativity for the same frame treated as being accelerating upwards with respect to free fall reference, which can thought of being inertial and gravity-less.

Experimental setup

The Gravity Probe A spacecraft housed an atomic

system that operated throughout the mission. Maser is an acronym for microwave amplification by stimulated emission of radiation, and is similar to a laser, as it produces coherent electromagnetic waves in the microwave region of the electromagnetic spectrum. A hydrogen maser produces a very accurate signal (1.42 billion cycles per second), which is highly stableto one part in a quadrillion (). This is equivalent to a clock that drifts by less than two seconds every 100 million years. The probe was launched nearly vertically upward to cause a large change in the gravitational potential, reaching a height of . At this height, general relativity predicted a clock should run 4.5 parts in faster than one on the Earth, or about one second every 73 years. The maser oscillations represented the ticks of a clock, and by measuring the frequency of the maser as it changed elevation, the effects of gravitational time dilation were detected.

Doppler shift

Along with the hydrogen maser, a microwave repeater was also included in the probe in order to measure the Doppler shift of the maser signal. A Doppler shift occurs when a source is moving relative to the observer of that source, and results in a shift in the frequency that corresponds to the direction and magnitude of the relative motion. The maser's signal was Doppler shifted because it launched vertically at a high speed relative to the ground station on the Earth.

Results

The goal of the experiment was to measure the rate at which time passes in a higher gravitational potential, so to test this the maser in the probe was compared to a similar maser that remained on Earth. Before the two clock rates could be compared, the Doppler shift was subtracted from the clock rate measured by the maser that was sent into space, to correct for the relative motion between the observers on Earth and the motion of the probe. The two clock rates were then compared and further compared against the theoretical predictions of how the two clock rates should differ. The stability of the maser permitted measurement of changes in the rate of the maser of 1 part in for a 100-second measurement. The experiment was thus able to test the equivalence principle. Gravity Probe A confirmed the prediction that deeper in the gravity well, the flow of time is slower, and the observed effects matched the predicted effects to an accuracy of about 70 parts per million.

References

External links

Gravity Probe A Collection, The University of Alabama in Huntsville Archives and Special Collections
Physics experiments Tests of general relativity 1976 in science 1976 in spaceflight