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11.1 Classical tests of general relativity

3 min readLast Updated on August 7, 2024

General relativity predicts mind-bending effects like Mercury's wonky orbit and light bending near the Sun. These wild ideas were put to the test, with scientists racing to prove Einstein right or wrong.

Experiments like watching stars during eclipses and bouncing signals off planets confirmed Einstein's crazy predictions. From Mercury's dance to warped starlight, general relativity keeps acing its exams.

Perihelion Precession and Gravitational Deflection

Mercury's Orbit and Gravitational Lensing

Top images from around the web for Mercury's Orbit and Gravitational Lensing
Top images from around the web for Mercury's Orbit and Gravitational Lensing
  • Perihelion precession of Mercury occurs when the planet's elliptical orbit rotates gradually over time
    • Newtonian mechanics could not fully explain this precession
    • General relativity accurately predicts the observed precession rate of 43 arcseconds per century
  • Gravitational deflection of light happens when light rays bend as they pass near massive objects like the Sun
    • This effect is due to the curvature of spacetime caused by the object's mass
    • Deflection angle is proportional to the mass of the object and inversely proportional to the distance from it
  • Einstein ring forms when a distant light source, a massive object, and the observer are perfectly aligned
    • The massive object acts as a gravitational lens, bending the light from the source into a ring shape
    • Radius of the ring depends on the mass of the lensing object and the distances between the source, lens, and observer

Experimental Confirmation of General Relativity

  • Eddington expedition in 1919 aimed to observe the gravitational deflection of starlight during a total solar eclipse
    • Measured deflection angles matched the predictions of general relativity
    • This observation provided the first experimental confirmation of Einstein's theory
    • Results were widely publicized, making Einstein and his theory famous worldwide
  • Further observations of gravitational lensing effects have consistently supported general relativity
    • Hubble Space Telescope has captured numerous images of gravitational lenses and Einstein rings
    • Gravitational lensing is now a powerful tool in astronomy for studying distant galaxies and dark matter distribution

Gravitational Redshift and Time Dilation

Effects on Light and Time

  • Gravitational redshift is the stretching of light wavelengths as photons climb out of a gravitational well
    • Photons lose energy and their frequency decreases, shifting toward the red end of the spectrum
    • Magnitude of the redshift depends on the strength of the gravitational field
    • Observed in the spectra of stars and in experiments on Earth (Pound-Rebka)
  • Shapiro time delay occurs when light signals pass near massive objects, experiencing a longer path due to spacetime curvature
    • Round-trip travel time of the signal is slightly longer than it would be in flat spacetime
    • Delay depends on the mass of the object and the closest approach distance of the light signal
    • Measured using radar signals bounced off planets and spacecraft

Experimental Tests of Gravitational Redshift

  • Pound-Rebka experiment in 1959 measured the gravitational redshift of gamma-ray photons in a laboratory setting
    • Used the Mössbauer effect to detect the small frequency shift of photons moving vertically in Earth's gravitational field
    • Measured redshift agreed with the predictions of general relativity to within 10%
    • Later refined experiments improved the precision to better than 1%
  • Gravitational redshift has also been measured in the spectra of white dwarf stars and neutron stars
    • Intense gravitational fields of these compact objects lead to significant redshifts
    • Observations match the predictions of general relativity, confirming the theory in strong-field regimes
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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