Sometime, we don't need to consider both space and time in a model.

So for instance, you may be just interested in global quantity

like how many individual of a given species I have in an area.

So, it's basically counting the number of individual in a population.

I'm not really interested in where they are, especially.

We can also be interested in the total amount of CO2 in the atmosphere.

So in some model, you can remove the spacial dimension and

sometime you can remove the temporal dimension if time doesn't evolve.

The process is steady.

Maybe you can get rid of the time and

just consider the evolution in space.

So I give an example, for instance, the temperature in the room,

you maybe interested whether it's cold near the window or warm in the middle.

But if you have a good heating system,

probably doesn't change over the day and it's something which is steady.

So if we focus now on the time dimension and

would like to see how we can describe it in a model and

we'll see that there are several solutions for us.

First, we know from physics that time is a continuous variable.

So time can be as detailed as you want.

You can have microsecond, nanosecond.

So every real value is potentially possible, although

some physicists claim that maybe at some scale we should stop and fill the gaps.

We'll assume that time is a continuous variable and

this is very difficult to describe in the computer.

So, only mathematical model like a differential equation that can deal with

the continuous time really with a real variable.

So most of the time when you do a model and it goes to the computer,

you have to change this continuous time.

For instance, by discretidizing it, so you basically take your time

into value you wanna describe and you split it in several time step and

then you look at your system at every of this time step.

So for instance, if I call delta t the time stamp can be one second,

one millisecond, one hour depending on the model or

even one year depending on the time scale we wanna capture.

Basically, you look at your system at time zero,

at time delta, and so on until you reach the end of your simulation.