# Why are separable spaces called "separable"?

1. Feb 20, 2015

### pellman

What is getting separated from what? I presume there is some historical founding case that involved separating something. Like how the original vector spaces were mental arrows in R^3.

2. Feb 21, 2015

### Svein

Definition: A metric space X is called separable if it has a subset D which has a countable number of points and which is dense in X, that is, for which the closure of D in X is equal to X.

3. Feb 21, 2015

### pellman

Thanks, Svein. I know the definition. But what is the significance of the label "separable"? For example, once you see that compactness is a generalization of closed and bounded, you understand why they chose the term "compact". But why "separable"? What is the fundamental metaphor at work here? I feel that if I can understand that, I will understand the concept much better than simply being able to repeat the strict definition.

4. Feb 21, 2015

### Svein

For example: ℚ is dense in ℝ. This is important for some proofs.

5. Feb 21, 2015

### micromass

So the idea is to approximate certain elements by other certain elements. Separable means that any element of the set can be approximated by a limited number of elements.
For example, with $\mathbb{R}$, we can approximate an arbitrary element (for example $e$) by their decimal representation. So we can approximate $e$ better and better by
$$2. ~2.7, ~ 2.71, ~ 2.718, ......$$
We can do exactly the same with any real number. We can do this in two dimensions too. For example, the couple $(e,\pi)$ can be approximated by
$$(2,3),~(2.7,3.1),~(2.71, 314),......$$
So separability actually is a very far-reaching generalization of the decimal representation. So basically, we have a countable number of "basis" elements, and then we succeed in approximating any element by the basis elements.

In the theory of Hilbert spaces, the analogy becomes even better with the existence of countably orthonormal bases. But I can only tell this if you know Hilbert spaces.

In a very general sense, you should see separable spaces and spaces which are "not too large" and where countable many terms suffice in many cases. In the same way that rational numbers can be used to describe real numbers.

It turns out that a very large portion of all spaces encountered "in nature" are separable. This is especially true in physics. This is good because it allows us to describe a physical system by countably many terms (like position, momentum, etc.) instead of dealing with uncountabilities.

6. Feb 21, 2015

### pellman

This is very helpful, micromass. Thank you.

7. Feb 21, 2015

### Staff: Mentor

8. Feb 21, 2015

### WWGD

How do you use the approximation analogy in non-metrizable separable spaces?

9. Feb 21, 2015

### micromass

With nets

10. Feb 21, 2015

### WWGD

Well sure, but I don't see how the analogy is helpful in non-metrizable cases.

11. Feb 21, 2015

### micromass

To be honest, I don't think many people care about separability in a non-metrizable (or similar) context.