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 May 1916. It is the accepted description of the gravitation of macroscopic objects in modern physics. This theory generalizes special relativity and refines Isaac Newton's law of universal gravitation, showing that gravity is a geometric property of space and time, or four-dimensional spacetime.

One of the key ideas of general relativity is that the curvature of spacetime is directly related to the energy, momentum, and stress of whatever is present, including matter and radiation. This relation is specified by the Einstein field equations. General relativity makes several predictions that go beyond Newton's law of gravity, such as gravitational time dilation, gravitational lensing, gravitational redshift of light, and the existence of black holes.

Einstein's theory has had profound implications for our understanding of the universe. It provided the modern framework for cosmology, leading to the discovery of the Big Bang and cosmic microwave background radiation. General relativity also predicts gravitational waves, which have been observed directly by observatories like LIGO. Despite its success, reconciling general relativity with the laws of quantum physics remains a challenge, as no complete theory of quantum gravity has yet been found.

History

Main articles: History of general relativity and Classical theories of gravitation

Before Albert Einstein developed his theory of general relativity, scientists like Henri Poincaré were already thinking about how gravity might relate to the ideas of relativity. Poincaré suggested that gravity might travel at the same speed as light and that our measurements might affect how we see gravity.

Einstein began working on his theory of gravity in 1907. After many years of thinking and testing ideas, he finally presented his main ideas in 1915. These ideas showed how the shape of space and time changes because of matter and energy around us. Later, other scientists used Einstein's ideas to describe objects such as black holes and to understand how the universe itself changes over time.

From classical mechanics to general relativity

General relativity explains gravity by showing how it shapes space and time. Unlike older ideas, it pictures gravity as the bending of a four-part space called spacetime, caused by mass and energy.

Classical physics already hints at this with ideas like free-fall paths being straight in space and time. General relativity builds on this, mixing in ideas from special relativity to describe gravity fully. This creates a new way to understand how objects move when pulled by gravity.

Definition and basic applications

See also: Mathematics of general relativity and Physical theories modified by general relativity

General relativity is a way to understand gravity. Instead of thinking of gravity as a force that pulls objects, it describes gravity as the shape of space and time. When space and time are curved, objects move along the curves, which looks like they are being pulled by gravity.

This theory helps us understand things like the orbits of planets and the paths of spacecraft. It works well even when gravity is very weak or things are moving slowly, much like Newton’s old ideas about gravity. Scientists use computers to solve the complex math of general relativity, which helps them study black holes and the expansion of the universe.

Consequences of Einstein's theory

General relativity has many important effects on how we understand space, time, and gravity. One key idea is that gravity affects time itself. Clocks closer to heavy objects, like Earth, tick slightly slower than clocks far away from such objects. This has been shown through experiments with atomic clocks and tools like GPS satellites, which need to account for these tiny time differences to work properly.

Another important effect is that light bends when it passes near a heavy object. This was first shown by watching how starlight bends as it passes close to the Sun. General relativity also predicts that there are waves in space-time called gravitational waves. These are like ripples that travel at the speed of light and were first directly detected in 2016 from two black holes merging. These discoveries show how Einstein's ideas about gravity change our understanding of the universe.

Main article: Gravitational time dilation Main articles: Schwarzschild geodesics, Kepler problem in general relativity, Gravitational lens, and Shapiro delay Main article: Gravitational wave Main article: Two-body problem in general relativity Main article: Apsidal precession Main articles: Geodetic precession and Frame dragging

Astrophysical applications

Gravitational lensing

Main article: Gravitational lensing

Gravity can bend light, creating interesting effects in space. When a big object, like a galaxy, is between us and a distant star, it can make the star look like it has multiple images or even a ring. This is called gravitational lensing. Scientists use this to study faraway galaxies and even to find dark matter, which is invisible but has gravity.

Gravitational-wave astronomy

Main articles: Gravitational wave and Gravitational-wave astronomy

Scientists are working hard to detect gravitational waves, which are ripples in space caused by big events like black holes crashing into each other. They have special tools on Earth to catch these waves. These waves can tell us more about black holes, neutron stars, and what happened right after the Big Bang.

Black holes and other compact objects

Main article: Black hole

Black holes are areas in space where gravity is so strong that even light can’t escape. They form when big stars die. There is usually one huge black hole at the center of each galaxy. These black holes can pull in gas and dust, making very bright lights and powerful streams of energy. Scientists look for these black holes by watching for these bright lights and also by trying to catch the gravitational waves they make when they crash together.

Cosmology

Main article: Physical cosmology

Our understanding of the universe comes from Einstein’s ideas about gravity. These ideas help us explain how the universe started with a Big Bang and how it has been changing ever since. We know the universe is still growing bigger, and there is something called dark energy that seems to be pushing it to expand even faster. There might also have been a very fast growth phase called inflation very early on. Scientists are still trying to learn more about these mysterious parts of our universe.

Advanced concepts

The spacetime symmetry group for special relativity is the Poincaré group, which includes movements and rotations of space and time. In general relativity, scientists study symmetries by looking at spacetime from far away, where gravity is weak. In 1962, researchers found that instead of the simple Poincaré group, a more complex, infinite group of symmetries exists. This means that general relativity does not simply become special relativity in weak fields.

In general relativity, light always travels faster than anything else. This helps us understand the causal structure of spacetime, showing how events can influence each other. Scientists use special diagrams to map out these relationships. General relativity also predicts the existence of black holes, regions where gravity is so strong that nothing, not even light, can escape. These objects follow specific laws similar to thermodynamics. The theory also suggests that spacetime can have singularities, points where the usual rules break down, such as inside black holes or at the beginning of the universe.

Relationship with quantum theory

General relativity and quantum theory are two main ideas in modern physics. General relativity explains gravity, while quantum theory helps us understand tiny particles. But we still don’t know how to make these two ideas work together perfectly.

Some scientists study how quantum theory behaves in spaces where gravity is strong, using ideas from general relativity. Others try to create a full theory called quantum gravity, where gravity is described using quantum physics. There are many ideas for this, like string theory, which imagines tiny vibrating strings instead of points, and loop quantum gravity, which pictures space as a network. But we don’t yet have a complete theory that works for all situations, and we need more experiments to test these ideas.

Current status

General relativity is a very successful theory that explains how gravity works. It matches what scientists observe in the universe, but there are still some big questions, like how to combine it with quantum physics. Scientists are also studying mysteries like dark energy and dark matter, which might mean we need to update or change the theory.

Even so, general relativity is still widely used and studied today. Researchers work on understanding strange points in space called singularities and run computer simulations of events like black holes merging. In 2016, scientists made a big discovery by detecting gravitational waves, a type of ripple in space caused by violent events, confirming one of Einstein's predictions.