Why is Our Universe Just Three Dimensions?

March 21, 2016 | Joanne Kennell

3D puzzle piece containing grass
Photo credit: Chriss Potter/flickr (CC BY 2.0). Stockmonkeys.com

It is one of the most baffling questions in the field of physics.

We live in a three-dimensional world plus one dimension of time. How do we know this? Just look around! Everything in the known universe, including us, everything we interact with — our pets, the clothes we wear, and the food we eat — and even giant supernovas in space have a width, a height and a depth, and they all move along with time.

But why does the universe look this way? This exact question has perplexed physicists for years, especially since superstring theory states there could be as many as ten dimensions — nine spatial dimensions and a tenth dimension of time. Not to mention, general relativity breaks down the moment the universe came into existence as a tiny point, right before the Big Bang.

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The simple answer is that the universe evolved in three dimensions, so that is the way we perceive it, but that stirs the question: Why did the universe evolve in three dimensions in the first place?

Back in 2012, a team of three Japanese scientists developed a computer model of the universe’s birth based on superstring theory. They found that at the moment of the Big Bang, the universe had ten dimensions, but only three of these spatial dimensions expanded. In short, the three-dimensional space that we live in could have formed from ten dimensions, just like superstring theory predicts.

However, this result is only a small piece of the puzzle. Physicists are still trying to make sense of our universe. In fact, there are a lot of questions that still need to be answered about why our universe evolved three-dimensionally — what makes three dimensions so special?

Taking a look at gravity, its strength depends on the distance squared between massive objects. However, in two dimensions, the strength depends on the distance, whereas in four dimensions it depends on the distance cubed.

What this means is that, in a two-dimensional world, gravity would be too strong for solar systems to form, and in four dimensions, gravity would be too weak, according to Explanimator, who posted the YouTube video you can watch below.

So did we just get really lucky, or is something deeper at play? Maybe space is not the most fundamental thing in the universe and it actually emerged from something even more rudimentary. For now, it remains a mystery, but I believe one day, hopefully in this lifetime, we will have the answers to one of physics’ most mystifying questions.

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