 Due to its heavy use of higher mathematics, it may not be suitable for an introductory course.Lee Smolin argued that "General Relativity" bridges the gap between the presentation of the material in older textbooks and the literature.For example, while the early pioneers of the subject, including Einstein himself, employed coordinate-based methods, researchers since the mid-1960s have switched to coordinate-free formulations, of which Wald's text is entirely based.