Why Does Our Universe Have Three Dimensions?

Why does our universe look the way it does? In particular, why do we only experience three spatial dimensions in our universe, when superstring theory, for instance, claims that there are ten dimensions — nine spatial dimensions and a tenth dimension of time?

Japanese scientists think they may have an explanation for how a three-dimensional universe emerged from the original nine dimensions of space. They describe their new supercomputer calculations simulating the birth of our universe in a forthcoming paper in Physical Review Letters. Before we delve into the mind-bending specifics, it’s helpful to have a bit of background.

The Big Bang theory of how the universe was born has been bolsted by some pretty compelling observational evidence, including the measurement of the cosmic microwave background and the relative abundance of elements. But while cosmologists can gaze back in time to within a few seconds of the Big Bang, at the actual moment it came into existence, when the whole universe was just a tiny point — well, at that point, the physics we know and love breaks down. We need a new kind of theory, one that combines relativity with quantum mechanics, to make sense of that moment.

Over the course of the 20th century, physicists painstakingly cobbled together a reasonably efficient “standard model” of physics. The model they came up with almost works, without resorting to extra dimensions. It merges electromagnetism with the strong and weak nuclear forces (at almost impossibly high temperatures), despite the differences in their respective strengths, and provides a neat theoretical framework for the big, noisy “family” of subatomic particles. But there is a gaping hole. The standard model doesn’t include the gravitational force. That’s why Jove, the physicist in Jeanette Winterson’s novel, Gut Symmetries, calls the Standard Model the “Flying Tarpaulin” — it’s “big, ugly, useful, covers what you want and ignores gravity.” Superstring theory aims to plug that hole.

According to string theorists, there are the three full-sized spatial dimensions we experience every day, one dimension of time, and six extra dimensions crumpled up at the Planck scale like itty-bitty wads of paper. As tiny as these dimensions are, strings — the most fundamental unit in nature, vibrating down at the Planck scale — are even smaller.

The geometric shape of those extra dimensions helps determine the resonant patterns of string vibration. Those vibrating patterns in turn determine the kind of elementary particles that are formed, and generate the physical forces we observe around us, in much the same way that vibrating fields of electricity and magnetism give rise to the entire spectrum of light, or vibrating strings can produce different musical notes on a violin. All matter (and all forces) are composed of these vibrations — including gravity. And one of the ways in which strings can vibrate corresponds to a particle that mediates gravity.

Voila! General relativity has now been quantized. And that means string theory could be used to explore the infinitely tiny point of our universe’s birth (or, for that matter, the singularity that lies at the center of a black hole).

via Why Does Our Universe Have Three Dimensions?

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