General relativity

General relativity, also called the general theory of relativity and Einstein's theory of gravity, is the idea about how gravity works. It was published by Albert Einstein in May 1916. It is the main way we describe how big objects pull each other in modern physics. This theory builds on special relativity and improves Isaac Newton's law of universal gravitation. It shows that gravity is a property of space and time, or four-dimensional spacetime.

One important idea of general relativity is that the curvature of spacetime depends on the energy, momentum, and stress of what is there, like matter and radiation. This idea is described by the Einstein field equations. General relativity makes predictions that go beyond Newton's ideas. These include gravitational time dilation, gravitational lensing, gravitational redshift of light, and the existence of black holes.

Einstein's theory has changed how we understand the universe. It helped create the modern study of cosmology. This led to the discovery of the Big Bang and cosmic microwave background radiation. General relativity also predicts gravitational waves. These waves have been observed directly by places like LIGO. Even though this theory works very well, mixing it with the rules of quantum physics is still hard. We do not yet have a full theory of quantum gravity.

History

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

Before Albert Einstein created his theory of general relativity, scientists like Henri Poincaré were thinking about how gravity might connect to the ideas of relativity. Poincaré thought that gravity might move at the same speed as light and that our measurements could change how we see gravity.

Einstein started working on his theory of gravity in 1907. After many years of thinking and testing, he shared his main ideas in 1915. These ideas showed how the shape of space and time changes because of matter and energy around us. Later, scientists used Einstein's ideas to learn about objects like black holes and to understand how the universe changes over time.

From classical mechanics to general relativity

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

Classical physics already gives clues with ideas like free-fall paths being straight in space and time. General relativity builds on this, adding ideas from special relativity to fully describe gravity. 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 helps us understand space, time, and gravity in new ways. One important idea is that gravity can change how time works. Clocks near big, heavy objects like Earth tick a little slower than clocks far away. We know this because of experiments with atomic clocks and tools like GPS satellites, which need to make small time adjustments to work right.

Another effect is that light bends when it goes close to a heavy object. This was first seen when we watched how starlight bends near the Sun. General relativity also tells us that space-time can have waves, called gravitational waves. These are like ripples that move at the speed of light. We first measured these waves in 2016 from two black holes coming together. These findings show how Einstein's ideas about gravity change how we see 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 and create special effects in space. When a big object, like a galaxy, is between us and a faraway star, it can make the star look like it has many images or even a ring. This is called gravitational lensing. Scientists use this to study distant galaxies and to find dark matter, which we can’t see but which has gravity.

Gravitational-wave astronomy

Main articles: Gravitational wave and Gravitational-wave astronomy

Scientists are working to find gravitational waves, which are ripples in space caused by big events like black holes hitting each other. They have special tools on Earth to catch these waves. These waves help us learn 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 places in space where gravity is so strong that even light can’t escape. They form when very 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 strong streams of energy. Scientists look for 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 began with a Big Bang and how it has changed since then. We know the universe is still getting bigger, and there is something called dark energy that seems to be making it expand even faster. Scientists are still learning more about these mysterious parts of our universe.

Advanced concepts

In special relativity, scientists study how space and time work together using something called the Poincaré group. This group includes movements and rotations of space and time.

In general relativity, scientists look at space and time from far away, where gravity is weak. They found that space and time have more complex patterns than expected. This shows that general relativity is different from special relativity, even when gravity is weak.

In general relativity, light always travels faster than anything else. This helps us understand how events in the universe can affect each other. Scientists use special diagrams to show these relationships.

General relativity also tells us about black holes. These are places where gravity is so strong that nothing, not even light, can escape. Black holes follow special rules, similar to the rules that govern heat and energy. The theory also suggests that space and time can have singularities. These are points where the usual rules do not work, such as inside black holes or at the very beginning of the universe.

Relationship with quantum theory

General relativity and quantum theory are two big ideas in modern physics. General relativity explains how gravity works, while quantum theory helps us understand very small particles. But we do not yet know how to make these two ideas work together perfectly.

Some scientists study how quantum theory behaves in places where gravity is very strong, using ideas from general relativity. Others try to create a new 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 still do not have a complete theory that works for all situations, and we need more experiments to test these ideas.

Current status

General relativity is a successful theory that explains how gravity works. It matches what scientists see in the universe. But there are still big questions, like how to combine it with quantum physics. Scientists also study mysteries like dark energy and dark matter.

Even so, general relativity is still widely used today. Researchers study strange points in space called singularities and run computer simulations of events like black holes merging. In 2016, scientists made an important discovery by detecting gravitational waves. These are ripples in space caused by violent events, and this confirmed one of Einstein's predictions.