Pound–Rebka experiment

In Pound Rebka Experiment Robert Pound had in 1960 together with his assistant Glen Rebka the gravitational redshift of gamma - radiation in the gravitational field of the earth after. Previously, both had in 1959 proposed the experiment. The experiment used the Mössbauer effect, which allows precise frequency measurement, and was conducted at Harvard University in Jefferson Tower. See also tests of general relativity.

Physical Basics

Gravity

Einstein showed in 1911 that follows from energy conservation already for classical considerations that photons are affected in the gravitational field as well as massive particles. His thought experiment describes a particle that has gained kinetic energy in free fall and sends to the ground by annihilation radiation. This particle has before the fall only rest energy and then a total energy of rest energy and kinetic energy. The hypothetical produced in the annihilation photon would have this energy and could now be sent to the exit point of the falling particle. When the photon would not be affected by gravity, it would have at the top end of the fall distance is still the full particle and could be used for re- production of a covered particle. The excess energy of would be free. Only when the photon on the way up, loses energy, the energy conservation is guaranteed.

Therefore applies to the photon energy in the units of the generating particle and. In the units of the photon from the following or in frequency.

A height difference of 22.56 meters results in this Newtonian approximation, an expected frequency shift.

Measurement and structure

The change in energy of the photon on its way through the gravitational field is reflected in a change in frequency. The gamma radiation used has a very small line width, whereby the frequency variation clearly. To measure the change between the source and the absorber, and Pound Rebka selected resonant absorption of the radiation, which has the same mechanism as by the emission of the same line width. Thus, the absorber is used, depending on the atomic nucleus sensitive only to its own very narrow frequency range. In the case of Mößbauereffekts, so the recoilless emission and absorption, these areas are the same for the emitter and absorber. So it follows that if the frequency changes of the radiation on the way, shows up in a system of mutually at rest emitter and absorber with cores whose line positions are the same, no absorption. However, since the atoms move due to their thermal energy emitting and receiving atoms are not to each other in peace. This effect of the thermal Doppler broadening is compensated for by strong cooling at the Mössbauer also occurs. If now the source or the absorber moves at a predetermined speed relative to the other can be measured by the Doppler effect on the absorption at different frequencies. In the case of the pound- Rebka experiment, the source was mounted on a hydraulic plate, and placed so exactly in position. Between the hydraulic source and the different electro-acoustic transducers were used to place the source in sinusoidal up - and - down movement during the experiment. Of the time within said cycle of movement and thus the current speed of the source, in which the absorption occurs can infer how much has changed, the frequency of the photon.

Source and absorber are mounted in this experiment at a vertical distance of 74 feet, or approximately 22.56 m. During the experiment, the positions were repeatedly exchanged in order to demonstrate the influence of gravity with the difference of the frequency shift for the flight of the photon upwards or downwards. In the space, there was a foil bag, was pumped through the helium to reduce the scattering of gamma-rays in comparison to air.

The largest systematic effect on the frequency shift, however, had the temperature difference between source and absorber. This resulted in a four-fold higher than the gravitational effect and had to be determined with high precision.

Implementation and Results

Up to the first publication of the results on April 1 they had measured on 10 days since the start of the measurements on 22 February 1960. In the first two days, the source was constructed on the ground and the measured frequency change during the flight of the photon was in the 6 measurements made after consideration of the temperature difference in the means. Subsequently was measured in 2 days to the source at the top of the structure and the frequency change in the eight measurements made averaged. When flying upwards as previously described, energy loss, so expect a red shift, and down a blue shift. In the measurement results, plug a gravitational component and various solid-state physics effects. Applies in the case of the red shift and blue shift. The difference of the displacements for the two different directions thus giving the double effect, which would be expected for the easy route. The measurement result corresponds in size and accuracy of a pure Rotverschiebungsexperiment with 45 m riser height or a blue-shift experiment with 45 m head. For the first 4 of the 10 days of measurement is a quantitative difference was found in accordance with the prediction of. Subsequently, the accuracy could be improved by the larger sample and the published results Once the measurement was ( according to the sign convention for the difference used by the authors ), thus confirming the prediction with an accuracy of 10 %.