This episode is devoted to the topic of time travel; time travel in a conventional sense, which relies on time machines to travel either into the future or the past. (I do not discuss forms of time travel, which may be permissible by the laws of physics.) It is advisable to separate these two different scenarios. Time travel into the future - as in H. G. Wells's novel The Time Machine - leads to odd situations, giving rise to the opportunity of 'harvesting' future knowledge for its use in the present. For instance, you could observe the results of a lottery draw tomorrow, return to today and tick the winning numbers.

However, time travel into the past leads to straightforward contradictions. It is agreed that the past is fixed and cannot be changed. But time travel into the past seems to imply that you can go back and change the past. Lots of possibilities seem to offer themselves. You could change world history, kill your younger self or simply change your appearance. Then the Second World War happened and did not happen! These contradictions seem to rule out time travel into the past. However, if you are willing to contemplate parallel universes, these logical inconsistencies can be avoided. You can travel from your own to another universe and do there, as I illustrate, what you are prevented from doing in your familiar universe.

Literature:

One of the most comprehensive books on time travel is P. J. Nahin, Time Machines (1993). The material for this episode is drawn from Friedel Weinert, The March of Time: Evolving Conceptions of Time in the Light of Scientific Discoveries (2013).