The Biggest Ideas in the Universe summary

You’re probably aware of the concept of space. It’s that “thing” where everything takes place. Where “objects” are located. But have you ever really thought about what space actually is? To start unpacking the concept, it might help to wind the clock back some 300 years.

In the early 1700s, there were two schools of thought. One posited that space was a substance and had an existence of its own – it was the “container” for everything else. The other considered space to be nothing at all. There was much discussion at the time, especially in a series of letters between Samuel Clarke in England – who agreed with Isaac Newton that space was indeed a substance – and the German Gottfried Wilhelm Leibniz, an adversary of Newton who believed space was relational. Leibniz’s death brought the exchange to an abrupt end in 1716, but by then, they’d talked about everything from space to free will to God.

Most physicists today agree with Newton: space is indeed a “thing.” Why do they believe that? Well, first, space isn’t empty. There are fields of one type or another – gravity, for instance – operating within it. And second, space (as part of spacetime) can change on its own. 

At the moment, the whole picture still isn’t completely understood. We do know some basics, though. Take space’s dimensionality, for instance. Grab a couple of pencils, and tie them together at right angles. Then pick up a third, and tie it at the same intersection so it’s at right angles to the other two. Now pick up a fourth and tie it at the same intersection at right angles to the other three. That’s right! It’s not possible. You’ve just effectively demonstrated that space is three-dimensional.

Occasionally, physicists treat some things as having only one dimension – a stream of electrons moving rapidly along the length of a long wire, for example. Or having two – a thin film or the surface of a three-dimensional object. But although they can model systems as if they have just one or two dimensions, they in fact have all three. And when it comes to string theory or some other models, physicists envision even more than three dimensions. Not quite as simple as it first seems, huh? Well, you’ll be pleased to know that’s all beyond the scope of this Blink!

Having three-dimensional space, though, provides us with a neat explanation as to why the gravitational force between two objects is inversely proportional to the square of the distance between them. Imagine lines radiating out from the center of the sun – these represent lines of gravitational force. Now imagine another bigger sphere with the sun at its center. All the lines radiating from the sun pass through this sphere. Finally, imagine another even bigger sphere even further out. All the lines also pass through the radius of this megasphere. But due to its size, fewer lines pass through any given area compared with the first. The area of a sphere is proportional to its radius squared. So it follows that the gravitational force is inversely proportional to the square of the distance between two objects.

OK, enough about space for now. Let’s turn our attention to the next big idea in the universe: time.