Tests of General Relativity
General relativity replaced Newton's view of gravity as a force with a new picture: massive objects curve spacetime, and everything (planets, light, even time itself) responds to that curvature. For most situations, Newton's equations work fine. But in extreme conditions, only general relativity gives the right answers. The tests covered here show exactly where and how Einstein's predictions were confirmed.
Mercury's Orbit and Spacetime Curvature
Mercury's orbit provided one of the earliest confirmations of general relativity, and it didn't even require a new experiment. The data already existed; it just needed the right theory to explain it.
Precession is the gradual rotation of an orbit's orientation over time. Think of it as the entire ellipse slowly pivoting around the Sun. Newtonian gravity, accounting for the gravitational tugs of all the other planets, predicts Mercury should precess at about 5,557 arcseconds per century. (An arcsecond is 1/3600 of a degree, so these are tiny angles.) But the observed rate is 5,600 arcseconds per century.
That leaves a gap of 43 arcseconds per century that Newtonian gravity simply can't account for. Before Einstein, astronomers even proposed a hidden planet ("Vulcan") near the Sun to explain it.
General relativity solves this cleanly. Because Mercury orbits so close to the Sun, it moves through a region where spacetime is noticeably curved. That curvature causes the orbit's closest approach point (the perihelion) to shift a tiny bit extra with each revolution. When you add it up, the extra shift matches the 43-arcsecond discrepancy exactly.
Light Bending Near Massive Objects
General relativity predicts that light follows the curvature of spacetime. When light passes near a massive object, its path bends. The more massive the object, the stronger the curvature and the greater the deflection.
The 1919 Solar Eclipse
This prediction was tested during the total solar eclipse of 1919, led by astronomer Arthur Eddington. During totality, the Moon blocked the Sun's glare, making it possible to photograph stars whose light passed close to the Sun on its way to Earth.
- The stars appeared slightly shifted from their normal positions.
- The measured deflection matched general relativity's prediction.
- Crucially, the observed bending was twice the amount that Newtonian gravity would predict. Newton's theory does predict some light deflection (treating light as particles affected by gravity), but only half of what Einstein's equations give. This factor-of-two difference made the eclipse test a decisive win for general relativity.
Gravitational Lensing
The same light-bending effect operates on a much larger scale with galaxies and galaxy clusters. A massive foreground object can act as a gravitational lens, bending and focusing light from a more distant source behind it.
Depending on the alignment and mass distribution of the lens, this can produce:
- Multiple images of the same background object (like a quasar appearing in several places at once)
- Arcs of stretched, distorted light
- Einstein rings, where the source, lens, and observer align so perfectly that the background light forms a complete ring
These lensing effects match general relativity's predictions and are now used as tools to map the distribution of mass (including dark matter) in the universe.
Newtonian Gravity vs. General Relativity
For weak gravitational fields, both theories give nearly identical results. The motion of most planets, moons, and spacecraft can be calculated with Newtonian gravity and you'll get the right answer. The differences only show up in strong gravitational fields.
| Newtonian Gravity | General Relativity | |
|---|---|---|
| Gravity described as | A force between masses | Curvature of spacetime caused by mass and energy |
| Works well for | Everyday situations, most of the solar system | All situations, including extreme gravity |
| Black holes | Not predicted | Predicted (event horizon, singularity) |
| Light bending by Sun | Predicts half the observed value | Predicts the correct observed value |
| Time dilation | Not predicted | Predicted and confirmed |
Gravitational Time Dilation and Redshift
General relativity predicts two related effects in strong gravitational fields:
- Gravitational time dilation: Time passes more slowly closer to a massive object. Clocks at lower elevations tick slightly slower than clocks at higher elevations. This has been confirmed with atomic clocks at different altitudes on Earth.
- Gravitational redshift: Light climbing out of a strong gravitational field loses energy, shifting to longer (redder) wavelengths. This has been observed in light from dense objects like white dwarfs and neutron stars.
These effects are tiny near Earth's surface but become dramatic near compact objects like neutron stars and black holes.
Additional Tests and Concepts
- The equivalence principle is the foundation of general relativity. It states that gravitational acceleration and inertial acceleration are physically indistinguishable. A person in a sealed elevator can't tell whether they're standing on Earth's surface or accelerating through space at the same rate.
- The Einstein field equations are the mathematical core of the theory, describing how the distribution of matter and energy determines the curvature of spacetime.
- Gravitational waves are ripples in spacetime produced by accelerating masses, especially violent events like merging black holes or neutron stars. General relativity predicted them in 1916, and LIGO made the first direct detection in 2015.
- The Shapiro delay is a subtler test: light signals passing near a massive object take slightly longer to arrive than they would in flat spacetime. Radar signals bounced off planets when they're on the far side of the Sun show this extra delay, matching general relativity's predictions.