This post is a part of an experiment for me to improve the distinctions I use in my world. I have daily conversations with interesting and well-educated people who tell me how a plane flies or how digital cookies are used in a login process. If I then try to share what I've learned with others, I'm unable to reproduce what I've learned in an eloquent and clear way. And so I decided to see if writing things down will help me refine my own stories.
Disclaimer: Don't read these posts as if I know what I'm talking about. I'm a lay person in the fields I'm writing about and you should approach these posts as such. In fact, if you know more on what I'm talking about and have something to add or correct, please do so.
So, what is a surface?
(from the perspective of math and, in particular, Algebraic Geometry)
You might be wondering why I'm writing a post on what a surface is. Well, my fiancé is a mathematician who studies surfaces, and even though I have spoken to him many times about his work, I realized recently that I do not have a good lay-person distinction for the objects which he studies.
A couple weeks ago, I was speaking with a non-mathematical friend, Guillermo Wechsler, about what a surface was. He claimed that a surface was two-dimensional. I, having heard my fiancé, a mathematician in Algebraic Geometry, talk about the hundreds of different variables he was working with to describe the surface he was studying, had a different idea about what a surface was. I said...”A surface can be multi-dimensional – imagine a sphere on which you have a point with three coordinates. As there are three variables involved in describing that point, it is a three dimensional surface.” Right?
A week or so ago, I recapped this conversation to my fiancé. And...I learned quickly that I didn't know what I was talking about. What I had been calling a surface was based on a common sense, non-rigorous definition. What I learned from the conversation is the following:
- A surface is a two-dimensional object. Plain and simple. If something is three-dimensional, it is called a three-fold. If something is four-dimensional, it is called a four-fold. My first mistake was a tautological one.
- A sphere is a surface. It is not a solid object but instead a hollow casing of a ball. But even with this made explicit to me, I had a hard time seeing a sphere as a surface. I imagined a sphere with radius 1 centered around the origin (0,0,0) of the x, y, and z-axes. In order to describe the point that intersects the y-axis, one would write (0,1,0)...three variables and so three dimensions. Tony said, “Nope. You can describe that same point on a sphere simply with longitude and latitude." The dimensions of any particular object are those that the object can be reduced to. And so a sphere is two-dimensional.
- A surface can be produced at the intersection of multiple multi-dimensional objects. So what about the hundreds of variables Tony has to work with in order to describe his two-dimensional Del Pezzo surfaces of Degree 1? Well, a surface can live in a three-dimensional, or four-dimensional, or n-dimensional space. Imagine a one-dimensional line. It can live on a plane or in a three-dimensional world.