When describing an impersonal character performing something, one has several options. Which option is selected depends largely on personal preference or the preference expressed in any style-guides the author has to follow.
You can use the second person: if you keep travelling in a certain direction, etc. This is quite informal and tends to appear as a try to include the reader personally into the story. It is very often used in informal oral conversation:
Well, if you ever open a hard disk drive, make sure that you clean the platters with a cloth before closing it again.
The advantage of you is that it can be used to introduce the character, as well as to refer to it later on.
Another option is the use of he or she. To introduce this, you'd need to use one, someone or something similar.
If a traveller keeps travelling in a certain direction, he will not encounter..., he will come back where he started.
The main issue with this option is that some people get upset if an author assumes only male characters are relevant (or vice versa, if one uses she every time, that men don;t play a role). In some cases, he and she are used seemingly at random in different scenarios. Although the idea behind that is nicely politically correct, for some, it works as a distraction in the text. I have noticed that, for instance, Microsoft, has the tendency to switch a lot:
When a user encounters this error, she can take this action. When the developer so wishes, he is free to implement this solution. If a DBA needs to speed up the process, she can use indexes.
Then there is the gender-neutral they. It also needs a separate introduction of the character:
If a traveller keeps travelling in a certain direction, they will not encounter..., they will come back where they started.
Although this use of they is anything but new, there are still plenty of people who stumble over it when reading a text. Some keep interpreting it as a plural, others feel it is an artificial trick to introduce gender-neutrality.
And then of course, there is the good old one. The problem with one is that it seems to always have been less common in English than in some other languages (on in French, men/man in Dutch/German) and it "feels" old-fashioned to some. It does have the advantage, though, of being both singular and gender neutral, and the old-fashioned feeling will not matter much in a (semi-)formal setting. Stephen Hawking's usual audience is no stranger to formal language, and it is hardly surprising that he (or his editor) would opt for this linguistic gem - in a semi-formal explanation of the entire universe, they may well expect one will forgive the small inconvenience of a slightly dated expression. By opting for one, they also avoid the potential unrest that can be caused by every single other option.