## Abstract

This paper contains a tutorial introduction to the ideas of geometric algebra, concentrating on its physical applications. We show how the definition of a “geometric product” of vectors in 2-and 3-dimensional space provides precise geometrical interpretations of the imaginary numbers often used in conventional methods. Reflections and rotations are analyzed in terms of bilinear spinor transformations, and are then related to the theory of analytic functions and their natural extension in more than two dimensions (monogenics), Physics is greatly facilitated by the use of Hestenes' spacetime algebra, which automatically incorporates the geometric structure of spacetime. This is demonstrated by examples from electromagnetism. In the course of this purely classical exposition many surprising results are obtained—results which are usually thought to belong to the preserve of quantum theory. We conclude that geometric algebra is the most powerful and general language available for the development of mathematical physics.

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## Additional information

The title of this paper is inspired by David Hestenes, who is known to have a fondness for deliberate ambiguity.^{(1)}

Supported by a SERC studentship.

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### Cite this article

Gull, S., Lasenby, A. & Doran, C. Imaginary numbers are not real—The geometric algebra of spacetime.
*Found Phys* **23, **1175–1201 (1993). https://doi.org/10.1007/BF01883676

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### Keywords

- Reflection
- Analytic Function
- Mathematical Physic
- Conventional Method
- Physical Application