Traditionally, dot products or something that's
introduced really early on in a linear algebra course typically right at the start. So it might seem strange that I push them
back this far in the series. I did this because there's a standard way
to introduce the topic which requires nothing more than a basic understanding
of vectors, but a fuller understanding of the role the
dot products play in math, can only really be found under the light of
linear transformations. Before that, though, let me just briefly cover the standard way that products are introduced. Which I'm assuming is at least partially review
for a number of viewers. Numerically, if you have two vectors of the
same dimension; to list of numbers with the same length, taking their dot product, means, pairing up all of the coordinates, multiplying those pairs together, and adding the result. So the vector [1, 2] dotted with [3, 4], would be 1 x 3 + 2 x 4. The vector [6, 2, 8, 3] dotted with [1, 8,
5, 3] would be: 6 x 1 + 2 x 8 + 8 x 5 + 3 x 3. Luckily, this computation has a really nice
geometric interpretation. To think about the dot product between two
vectors v and w, imagine projecting w onto the line that passes
through the origin and the tip of v. Multiplying the length of this projection
by the length of v, you have the dot product v・w. Except when this projection of w is pointing
in the opposite direction from v, that dot product will actually be negative. So when two vectors are generally pointing
in the same direction, their dot product is positive. When they're perpendicular, meaning, the projection of one onto the other is the
0 vector, the dot product is 0. And if they're pointing generally the opposite
direction, their dot product is negative. Now, this interpretation is weirdly asymmetric, it treats the two vectors very differently, so when I first learned this, I was surprised
that order doesn't matter. You could instead project v onto w; multiply the length of the projected v by
the length of w and get the same result. I mean, doesn't that feel like a really different
process? Here's the intuition for why order doesn't
matter: if v and w happened to have the same length, we could leverage some symmetry. Since projecting w onto v then multiplying the length of that projection
by the length of v, is a complete mirror image of projecting v
onto w then multiplying the length of that projection by the length of w. Now, if you “scale” one of them, say v
by some constant like 2, so that they don't have equal length, the symmetry is broken. But let's think through how to interpret the
dot product between this new vector 2v and w. If you think of w is getting projected onto
v then the dot product 2v・w will be exactly twice the dot product v・w. This is because when you “scale” v by
2, it doesn't change the length of the projection
of w but it doubles the length of the vector that
you're projecting onto. But, on the other hand, let's say you're thinking
about v getting projected onto w. Well, in that case, the length of the projection
is the thing to get “scaled” when we multiply v by 2. The length of the vector that you're projecting
onto stays constant. So the overall effect is still to just double
the dot product. So, even though symmetry is broken in this
case, the effect that this “scaling” has on
the value of the dot product, is the same under both interpretations. There's also one other big question that confused
me when I first learned this stuff: Why on earth does this numerical process of
matching coordinates, multiplying pairs and adding them together, have anything to do with projection? Well, to give a satisfactory answer, and also to do full justice to the significance
of the dot product, we need to unearth something a little bit
deeper going on here which often goes by the name "duality". But, before getting into that, I need to spend some time talking about linear
transformations from multiple dimensions to one dimension which is just the number line. These are functions that take in a 2D vector
and spit out some number. But linear transformations are, of course, much more restricted than your run-of-the-mill
function with a 2D input and a 1D output. As with transformations in higher dimensions, like the ones I talked about in chapter 3, there are some formal properties that make
these functions linear. But I'm going to purposely ignore those here
so as to not distract from our end goal, and instead focus on a certain visual property
that's equivalent to all the formal stuff. If you take a line of evenly spaced dots and apply a transformation, a linear transformation will keep those dots
evenly spaced, once they land in the output space, which
is the number line. Otherwise, if there's some line of dots that
gets unevenly spaced then your transformation is not linear. As with the cases we've seen before, one of these linear transformations is completely determined by where it takes
i-hat and j-hat but this time, each one of those basis vectors
just lands on a number. So when we record where they land as the columns
of a matrix each of those columns just has a single number. This is a 1 x 2 matrix. Let's walk through an example of what it means
to apply one of these transformations to a vector. Let's say you have a linear transformation
that takes i-hat to 1 and j-hat to -2. To follow where a vector with coordinates,
say, [4, 3] ends up, think of breaking up this vector as 4 times
i-hat + 3 times j-hat. A consequence of linearity, is that after
the transformation the vector will be: 4 times the place where
i-hat lands, 1, plus 3 times the place where j-hat lands,
-2. which in this case implies that it lands on
-2. When you do this calculation purely numerically,
it’s a matrix-vector multiplication. Now, this numerical operation of multiplying
a 1 by 2 matrix by a vector, feels just like taking the dot product of
two vectors. Doesn't that 1 x 2 matrix just look like a
vector that we tipped on its side? In fact, we could say right now that there's
a nice association between 1 x 2 matrices and 2D vectors, defined by tilting the numerical representation
of a vector on its side to get the associated matrix, or to tip the matrix back up to get the associated
vector. Since we're just looking at numerical expressions
right now, going back and forth between vectors and 1
x 2 matrices might feel like a silly thing to do. But this suggests something that's truly awesome
from the geometric view: there's some kind of connection between linear
transformations that take vectors to numbers and vectors themselves. Let me show an example that clarifies the
significance and which just so happens to also answer the
dot product puzzle from earlier. Unlearn what you have learned and imagine that you don't already know that
the dot product relates to projection. What I'm going to do here is take a copy of
the number line and place it diagonally and space somehow
with the number 0 sitting at the origin. Now think of the two-dimensional unit vector, whose tips sit where the number 1 on the number
line is. I want to give that guy a name u-hat. This little guy plays an important role in
what's about to happen, so just keep them in the back of your mind. If we project 2D vectors straight onto this
diagonal number line, in effect, we've just defined a function that
takes 2D vectors to numbers. What's more, this function is actually linear since it passes our visual test that any line of evenly spaced dots remains
evenly spaced once it lands on the number line. Just to be clear, even though I've embedded the number line
in 2D space like this, the output of the function are numbers, not
2D vectors. You should think of a function that takes
into coordinates and outputs a single coordinate. But that vector u-hat is a two-dimensional
vector living in the input space. It's just situated in such a way that overlaps
with the embedding of the number line. With this projection, we just defined a linear
transformation from 2D vectors to numbers, so we're going to be able to find some kind
of 1 x 2 matrix that describes that transformation. To find that 1 x 2 matrix, let's zoom in on
this diagonal number line setup and think about where i-hat and j-hat each
land, since those landing spots are going to be
the columns of the matrix. This part's super cool, we can reason through
it with a really elegant piece of symmetry: since i-hat and u-hat are both unit vectors, projecting i-hat onto the line passing through
u-hat looks totally symmetric to protecting u-hat
onto the x-axis. So when we asked what number does i-hat land
on when it gets projected the answer is going to be the same as whatever
u-hat lands on when its projected onto the x-axis but projecting u-hat onto the x-axis just means taking the x-coordinate of u-hat. So, by symmetry, the number where i-hat lands
when it’s projected onto that diagonal number line is going to be the x coordinate of u-hat. Isn't that cool? The reasoning is almost identical for the
j-hat case. Think about it for a moment. For all the same reasons, the y-coordinate
of u-hat gives us the number where j-hat lands when
it’s projected onto the number line copy. Pause and ponder that for a moment; I just
think that's really cool. So the entries of the 1 x 2 matrix describing
the projection transformation are going to be the coordinates of u-hat. And computing this projection transformation
for arbitrary vectors in space, which requires multiplying that matrix by
those vectors, is computationally identical to taking a dot
product with u-hat. This is why taking the dot product with a
unit vector, can be interpreted as projecting a vector
onto the span of that unit vector and taking the length. So what about non-unit vectors? For example, let's say we take that unit vector u-hat, but we “scale” it up by a factor of 3. Numerically, each of its components gets multiplied
by 3, So looking at the matrix associated with that
vector, it takes i-hat and j-hat to 3 times the values
where they landed before. Since this is all linear, it implies more generally, that the new matrix can be interpreted as
projecting any vector onto the number line copy and multiplying where it lands by 3. This is why the dot product with a non-unit
vector can be interpreted as first projecting onto
that vector then scaling up the length of that projection
by the length of the vector. Take a moment to think about what happened
here. We had a linear transformation from 2D space
to the number line, which was not defined in terms of numerical
vectors or numerical dot products. It was just defined by projecting space onto
a diagonal copy of the number line. But because the transformation is linear, it was necessarily described by some 1 x 2
matrix, and since multiplying a 1 x 2 matrix by a
2D vector is the same as turning that matrix on its
side and taking a dot product, this transformation was, inescapably, related
to some 2D vector. The lesson here, is that anytime you have
one of these linear transformations whose output space is the number line, no matter how it was defined there's going
to be some unique vector v corresponding to that transformation, in the sense that applying the transformation
is the same thing as taking a dot product with that vector. To me, this is utterly beautiful. It's an example of something in math called
“duality”. “Duality” shows up in many different ways
and forms throughout math and it's super tricky to actually define. Loosely speaking, it refers to situations
where you have a natural but surprising correspondence between two types of mathematical thing. For the linear algebra case that you just
learned about, you'd say that the “dual” of a vector
is the linear transformation that it encodes. And the dual of a linear transformation from
space to one dimension, is a certain vector in that space. So, to sum up, on the surface, the dot product
is a very useful geometric tool for understanding projections and for testing whether or not vectors tend
to point in the same direction. And that's probably the most important thing
for you to remember about the dot product, but at deeper level, dotting two vectors together is a way to translate one of them into the
world of transformations: again, numerically, this might feel like a
silly point to emphasize, it's just two computations that happen to
look similar. But the reason I find this so important, is that throughout math, when you're dealing
with a vector, once you really get to know its personality sometimes you realize that it's easier to
understand it, not as an arrow in space, but as the physical embodiment of a linear
transformation. It's as if the vector is really just a conceptual
shorthand for certain transformation, since it's easier for us to think about arrows
and space rather than moving all of that space to the
number line. In the next video, you'll see another really
cool example of this "duality" in action as I talk about the cross product.
