Supergravity facts for kids

Supergravity is a big idea in theoretical physics that brings together two important concepts: supersymmetry and general relativity. Think of it as a way to describe how gravity works, but with an extra twist!

General relativity is Albert Einstein's famous theory about gravity, which explains how massive objects like planets and stars bend the fabric of space and time. Supersymmetry is a theory that suggests every known particle in the universe has a "superpartner" particle. For example, a particle called a "fermion" would have a superpartner called a "boson," and vice versa.

Supergravity tries to combine these two ideas. It suggests that the force of gravity itself might have a superpartner.

Gravitons and Gravitinos

In any theory about gravity, there's a special particle called a graviton. This particle carries the force of gravity, much like a photon carries light.

In supergravity, because of the idea of supersymmetry, the graviton must have a superpartner. This superpartner is called a gravitino. The graviton is a spin-2 particle, while the gravitino is a spin-3/2 particle. The number of gravitino particles depends on how many "supersymmetries" are in the theory.

History of Supergravity

Scientists have been working on supergravity for a long time.

Early Ideas

The first idea for a local supersymmetry theory, which is a key part of supergravity, was suggested in 1975 by Dick Arnowitt and Pran Nath. They called it gauge supersymmetry.

The first full model of supergravity in four dimensions (the dimensions we experience: up, down, left, right, and time) was created in 1973 by Dmitri Vasilievich Volkov and Vyacheslav A. Soroka. They thought it was important for supersymmetry to "break" spontaneously to make a realistic model.

The Breakthrough

The most well-known version of 4-dimensional supergravity was built in 1976 by Dan Freedman, Sergio Ferrara, and Peter van Nieuwenhuizen. In 2019, these three scientists received a special Breakthrough Prize in Fundamental Physics for their discovery. Around the same time, Deser and Zumino also independently proposed a similar model.

Scientists quickly expanded these ideas to create many different supergravity theories in various numbers of dimensions. Some of these theories, called "extended supergravity" (SUEGRA), involve more than one type of supersymmetry.

The 11-Dimensional Theory

One supergravity theory, which exists in 11 dimensions, got a lot of attention. Scientists thought it might be the first step towards a "theory of everything" – a single theory that explains all the forces and particles in the universe.

Here's why it was so exciting:

• Werner Nahm showed that 11 dimensions was the largest number of dimensions that could have a single graviton without creating particles with spins greater than 2.
• In 1981, Ed Witten suggested that 11 dimensions was the smallest number of dimensions large enough to include the forces of the Standard Model (like the strong and electroweak forces).
• In 1978, Eugène Cremmer, Bernard Julia, and Joël Scherk (CJS) found the basic equations for an 11-dimensional supergravity theory. This is still the only known 11-dimensional theory with local supersymmetry and no particles with spin higher than two.
• In 1980, Peter Freund and M. A. Rubin showed that if 11 dimensions were "compactified" (meaning some dimensions are curled up very small), it could result in our universe having only 4 or 7 large, observable dimensions.

These findings seemed to point to 11 dimensions as special, and the theory appeared to explain why our universe looks 4-dimensional.

Challenges and New Directions

However, the excitement about 11-dimensional supergravity eventually faded. Scientists found several problems:

• The way dimensions were "curled up" (compactified) didn't seem to work with supersymmetry or allow for particles like quarks or leptons.
• Supergravity models often predicted a very large amount of energy in empty space (called the cosmological constant), which didn't match observations. This is still a puzzle today.
• When trying to combine the theory with quantum mechanics, it ran into issues called "anomalies" that made it inconsistent.

Because of these problems, scientists started looking at theories in 10 dimensions, especially those involving superstrings. While moving to 10 dimensions meant losing the "uniqueness" of the 11-dimensional theory, it opened new doors.

A big breakthrough in 10-dimensional theories, known as the first superstring revolution, showed that only a few supergravity models in 10 dimensions could work without inconsistencies.

The Second Superstring Revolution

Interest in 10-dimensional theories also slowed down by the late 1980s. But in the 1990s, new tools were developed. Scientists realized that different superstring theories were connected by "dualities," meaning they were different ways of looking at the same underlying physics.

Then came the second superstring revolution. Joseph Polchinski discovered that strange objects in string theory, called D-branes, were related to "p-branes" known in supergravity. This connection helped scientists understand these objects much better.

With this new understanding, Edward Witten and others showed that all the different string theories were actually parts of a single, larger theory called M-theory. He argued that when you look at M-theory at very long wavelengths (like looking at something from far away), it looks just like the 11-dimensional supergravity theory that had been set aside earlier.

So, supergravity came back into focus. It became a key part of understanding string theories, M-theory, and how they relate to our 4-dimensional universe.

Supergravity and Superstrings

Some 10-dimensional supergravity theories are called "low energy limits" of string theories. This means they describe what string theories look like when you're not looking at very tiny, high-energy details.

The idea is that the proposed 11-dimensional M-theory also has 11-dimensional supergravity as its "low energy limit." However, this doesn't mean that string theory or M-theory are the *only* ways to complete supergravity. Research in supergravity is important on its own, even without these connections.

N=8 Supergravity in 4 Dimensions

N=8 Supergravity is a very special theory. It's the most "symmetric" quantum field theory that includes gravity and a limited number of particles. It can be created by taking the 11-dimensional supergravity theory and making 7 of its dimensions incredibly small, effectively reducing it to 4 dimensions.

This theory has 8 supersymmetries, which is the maximum a gravity theory can have without introducing particles with spins higher than 2. (Remember, the graviton has a spin of 2). If there were more supersymmetries, the particles would need superpartners with spins greater than 2, which are usually only found in theories with an infinite number of particles, like string theory.

Stephen Hawking once thought that N=8 Supergravity might be the "Theory of Everything". However, later on, string theory became more popular. But in the 21st century, there's been new interest in N=8 Supergravity, with some scientists thinking it might be a "finite" theory, meaning it doesn't have the problematic infinities that often appear in quantum theories.

Higher-Dimensional Supergravity

Higher-dimensional supergravity is simply the idea of supergravity existing in more than four dimensions. Supergravity can be described in any number of dimensions up to eleven.

The number of "supercharges" (a way to count supersymmetry) depends on the number of dimensions and how space-time is structured. Some theoretical examples of higher-dimensional supergravity include:

• 12-dimensional theories (with two time dimensions)
• 11-dimensional maximal supergravity
• Various 10-dimensional supergravity theories (like Type IIA, Type IIB, and Type I)
• 9-dimensional supergravity theories

Scientists are most interested in supergravity theories that don't have particles with spins higher than two. However, the study of theories with higher-spin particles is a very active area of research today!

See also

In Spanish: Supergravedad para niños