# Why does string theory need 10 dimensions?

In our unique human experiences, we appear to inhabit in a three-dimensional cosmos. Some physics theories, particularly string theory, involve the concept of extra spatial dimensions. According to such ideas, there may be a fixed number of spatial dimensions, such as ten. The extra dimensions would be smaller than an atom and might be large enough to create other particles besides just quarks and electrons.

The idea of additional dimensions was first proposed by mathematician Nicholas Copernicus in 1543. He suggested that Earth is not the center of the universe but rather one of many planets orbiting around the Sun. His idea was based on mathematical evidence suggesting that Earth could be replaced with another planet without affecting the behavior of planetary orbits. This idea was later developed by Johannes Kepler who proved in 1609 that any number of planets can orbit around the Sun provided they each have their own elliptical orbit. In other words, all solar systems can be described using three dimensions (the length of the major axis of an ellipse) times two angles (the phases of the moon or the positions of the planets).

It was also proposed by Kepler that there are exactly eight planets in the Solar System including Earth. He derived this number by considering how many points there are in a circle when you divide it into seven parts. He assumed that each part occupied an angle of 90 degrees which is incorrect because angles greater than 180 degrees do not exist.

## How many dimensions are known?

The world as we know it has three spatial dimensions (length, breadth, and depth) and one temporal dimension. However, there is the mind-boggling potential that there are many more dimensions out there. According to string theory, one of the most influential physics theories of the last half-century, the universe has ten dimensions. If this is true, then it follows that there could be universes with anything from 3-10+ dimensions.

Our knowledge of dimensionalities is limited because all we can see are lengths scales below about 10-35 m. The other dimensions might be smaller than this or they might not exist at all. It's also possible that higher-dimensional spaces are compactified by gravity into lower-dimensional spaces that we experience as our universe. This idea was first proposed by Kaluza and Klein in the 1920s, but it wasn't until the 1980s that it was developed further by Randall and Sundrum. In their model, our four-dimensional space-time is a thin shell located at the edge of a five-dimensional manifold called an "anti-de Sitter" space. There are other models that describe our universe as being embedded in a higher-dimensional space. These models often involve extra dimensions that are small compared to those considered by string theory but may still be larger than what we can see.

## How many dimensions does string theory predict?

Current forms of string theory need a total of ten dimensions, but M-theory, an even more speculative uber-string theory, requires eleven. When we gaze around the cosmos, however, we only perceive the standard three spatial dimensions plus the dimension of time. So how many dimensions are there really out there? The answer is: at least five and possibly as many as eleven.

The number of dimensions in our universe is called its "dimensionality". Physicists have only been able to study physics on a scale small enough to see just three spatial dimensions, so they can't say for sure how many others might exist. But based on what we know now, here's a list of the most likely numbers of additional dimensions:

• Three space dimensions plus one more dimension that we can move in (but not touch). This makes four total dimensions. • Three space dimensions plus two more dimensions that we can move in and touch (six total). • Three space dimensions plus three more dimensions that we can move in and touch (nine total). • Three space dimensions plus four more dimensions that we can move in and touch (13 total). • And finally, three space dimensions plus five more dimensions that we can move in and touch (18 total).

So which version of increased dimensionality is right? It depends on how you look at it.

## What dimension is space?

The universe is four-dimensional, with three dimensions for space and one for time. There are nine, 10, or eleven dimensions in the cosmos. It's only a matter of rephrasing the words. For all we know, space is three dimensions and spacetime is four dimensions, but if string theory is correct, space is nine dimensions and spacetime is ten dimensions. The idea that space is three-dimensional and time is one-dimensional has been very useful for understanding certain phenomena, but it's not the whole story.

## How do multiple dimensions work?

Several Dimensions When we hear the phrase "different dimensions," we usually think of parallel universes—alternate realities that exist alongside our own but in which things operate or happen differently. Scientists think that there may be many more dimensions outside these three apparent dimensions. Addressing this idea directly, physicist Andrei Linde wrote: "People believe that physics is about our world, but actually it's about another world — a multidimensional world."

In mathematics and physics, a dimension is a measure of size. In other words, it's a way to describe how wide or tall something is. There are three known physical dimensions of length, width, and height. However, physicists also say that there are at least two more dimensions of space that we don't know anything about. They're called spatial dimensions and they're measured in nanometers (nm) or microns (mm).

At a distance of 1 millimeter (mm), the head of a pin is too small to see with the naked eye. Yet an electron microscope shows us that the head of the pin has three dimensions: length, breadth, and depth. So, even though you can't see them, particles such as atoms and molecules are not one-dimensional objects. An atom is made up of electrons in orbit around a nucleus composed of protons and neutrons. The electrons are in quantum mechanical states called orbits, which have different sizes depending on their energy level.

##### Susan Hernandez

Susan Hernandez loves to teach people about science. She has a background in chemistry, and she's been interested in teaching people about science ever since she was a child.

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