Why is it bad to pick basis for a vector space?

Reading `This Week's Finds', http://math.ucr.edu/home/baez/week247.html, I'm informed that one should avoid picking coordinate systems and I'm unsure why that is the case. Any help on the matter is appreciated.

Linear algebra is all about vector spaces and linear maps. One of the lessons that gets drummed into you when you study this subject is that it's good to avoid picking bases for your vector spaces until you need them. It's good to keep the freedom to do coordinate transformations... and not just keep it in reserve, but keep it manifest!

Why?

Is there a simple example of when a choice of coordinates makes a difference in a result? ---thereby giving reason to avoid basis.

As Hermann Weyl wrote, "The introduction of a coordinate system to geometry is an act of violence".

Why?

Some googling has informed me that this issue is part of the larger notion of categorical naturality...

• In the opposite direction, what do you get for choosing an explicit basis? – anomaly Mar 18 '16 at 21:30
• It can be very good for computation to pick a suitable basis, such as a basis of eigenvectors. – André Nicolas Mar 18 '16 at 21:59
• @AndréNicolas Yes, but then we are not working in the category of vector spaces anymore. We are then dealing with vector spaces equipped with a fixed linear map. Or, in other words, linear representations of the monoid $(\mathbb N,0,+)$. That is why some bases are better than others. – Gejza Jenča Mar 25 '16 at 15:04

You have a mission to find out how strong the wind blows on the pick of the mountain. So you climb to the top, put an aparatus and you get a result that it blows 200km/h in one direction. Then you turn it by 90 degree clockwise and get record 80km/h. You end up with such picture:

You know that tangent space to the surface of our planet is 2 dimentional vector space. So you choose coordinates and write your results in your coordinates [200,80]. You are sure that it is all data you can collect and you go back to give an report. In the base you say that wind blows 200km/h in one direction and 80km/h in other. Report receivers asks you additionally how were you oriented while taking first record. But you no longer remember. Hence your mission fails.

Conclusion

While modeling "real world" situations with vector spaces (in particular $\mathbb{R}^2$ or $\mathbb{R}^3$) you must be aware that in reality there is no canonical/natural choice of coordinates. In our situation by choosing coordinates, you assumed that tangent space has canonical coordinates. Obviously it does not. Just stand on the ground, look around and see if there are any.

Assume you define a concept about vector spaces by using a basis. Then what tell you that this concept remains relevant with another basis ? Generally when a mathematician uses this method, he must prove that his concept doesn't depend of the chosen basis, which can be not difficult but nonetheless annoying.

Therefore, one generally prefers to have inherent properties which do not depend on the basis or the local coordinates and which are directly defined thanks to the manifold/vector space.

• Any chance you would be able to give an example for when something might hold in one basis but not another? – Aran Mar 19 '16 at 9:20
• your answer makes it seem that a choice of a basis is merely a hindrance and not a problem that merits discussion. You would do well to present an example, as Aran suggests. – Musa Al-hassy Jun 7 '16 at 3:17