The first great popularising of the idea of a time machine was undoubtedly H.G. Wells' novel of the same name, but lesser known is the fact that scientists from the same time period forward to today have been speculating in earnest about the factual possibilities of time travel and time machines. These kinds of speculations do not take the form of machines that look like go-carts with umbrellas on top (such as the films portray), but they are nonetheless fascinating. Once upon a time, the idea that human beings would send ships to the moon and other planets seemed like the stuff of fanciful science fiction; time machines and time travel still has that veneer, but as recently as a few years ago, physicist Paul Davies was able to state with all seriousness that there is no theoretical problem with building a time machine.
Jenny Randles has put together an intriguing text looking at the history of time machine and time travel speculation and research. This includes a good dose of science fiction, but more interestingly, a strong selection of science fact. Scientists with well-known names such as Einstein, Fermi, Hawking, and Penrose are joined with lesser-known figures such as Kaku and Chernobrov, the latter of whom has claimed to have built a time machine of sorts already.
Of course, this flies in the face of the law of chronological protection - a speculation advanced by Hawking (among others) that there is an as-yet undiscovered law of nature that enforces the cause-preceding-effect sequence of events. Just because it hasn't been discovered yet doesn't mean it's not there, and for good measure, the idea was advanced that civilisations with time-travel capabilities would have already made their presence known (if not destroyed us entirely) if such capabilities were ever found in fact. Others hold for a less rigid law of restrictive behaviour - you cannot go back and prevent your own birth, for example. However, where the boundary exists between chronology protection and flexible but restrictive boundaries is impossible to tell.
Randles discusses in general terms experiments, theoretical physical and mathematical models, and concepts that deal with dimensional analysis and speculation. How many dimensions are there, really? Even scientists such as Einstein could not come up with a single answer over the course of his life. Do we live in a universe or a 'multi-verse'? Just what is a multi-verse, anyway? These are some of the questions discussed. Randles does not get into equations and technical details, but sticks with general narrative discussion; thus, the level of science in this text never advanced much further than popular levels. However, there are some references listed in the back that can lead the interested reader to further texts. This part could be expanded to be more helpful for those who are technically inclined.
This is an interesting text, a quick read, full of personality and intrigue as well as scientific (and science fiction) ideas.