DEAN
HARTLEY The Stories, Science, and Speculative Science in the Force-Fields Series |
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Keola |
Scalars, vectors, tensors, and spinors are all mathematical terms for rather straightforward ideas.
Think of a scalar as a thing with one slot that can be filled with a number. It can represent anything, but for example let's create a scalar for height. A scalar field is just a construct that contains a scalar for each point in some space. For example, we could have a scalar field for the height of the grass on a golf course. On the greens, the height will be nearly uniform and very short. On the fairways, the height will be fairly uniform and higher. In the rough, the height will be extremely variable and even higher.
A vector is a thing with two slots. For example, wind speed and compass direction. A weather map often shows a vector field of wind speeds and directions. In the figures below, two patches show uniform vectors within the patch but differences between the patches. In a real weather map, the vectors near a given point will not be uniform but will have similar directions and speeds, but the values will change smoothly as you change position.
A tensor is a generalization of a vector and usually has three or more slots, but there are also tensor fields. On example of a two-dimensional tensor field is the stress on the ground just prior to an earthquake. At each point, there are forces pushing on the point and if you look at each direction, the amount of forces is different. You might represent this on a map as different sizes and shapes of elipses, one elipse for each point.
Note that a scalar is a tensor of order 0, a vector is a tensor of order 1, and a general tensor has higher order. However, each of these can be applied to spaces of different dimensions. Thus, you can have an order-0 scalat field on a 2-dimensional plane or the 2-dimensional surface of a sphere, but you can also have an order-0 scalar field on the 3-dimensional volume of a sphere. Similarly, a tensor of any order might be applied to a space of any dimension. For nice situations, you can think of representing each slot in one of these things by a set of coordinate values, with the number of coordinates being equal to the dimension of the space. Therefore, the number of components of a vector field a three dimensional space would derive from three coordinates times three coordinates (for the two slots) = 9. In a 2-dimensional space, there would only by 2 times 2 or 4 components. But that stress tensor on a 2-dimensional surface would have 2 times 2 times 2 = 8 components. On a 3-dimensional space, it would have 27 components.
A spinor is another generalization in which the component values can be complex numbers or real numbers. This makes them much more complicated.
In these books, our heroes are modifying the component values of tensors. They use the short-hand and say they are modifying the component values. However, rather than meaning that they just change the numeric value of the tensor that is written on paper, they are modifying a property that causes the tensor to have that numeric value. In general, we don't know how to do this.
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