Context: Time travel is the concept of moving backwards or forwards to different points in time, in a manner analogous to moving through space, and different from the normal "flow" of time to an earthbound observer. In this view, all points in time (including future times) "persist" in some way. Time travel has been a plot device in fiction since the 19th century. Traveling backwards in time has never been verified, presents many theoretic problems, and may be an impossibility. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine.

Question: How long has time travel been a topic in science fiction?
Answer: since the 19th century

Question: Traveling which direction in time has never been verified and presents many problems?
Answer: backwards

Question: A time machine is known as any technological devise that is used for what purpose?
Answer: to achieve time travel

Question: How long has space been a plot device in fiction?
Answer: Unanswerable

Question: What does the idea of traveling in space present?
Answer: Unanswerable

Question: What was space travel thought to be in the 19th century?
Answer: Unanswerable

Question: What did authors go beyond in the 19th century when writing fiction?
Answer: Unanswerable

Question: What are some devices thought to be when in the process of invention?
Answer: Unanswerable

Question: How long has time travel been a topic in theory?
Answer: Unanswerable

Question: Traveling which direction in time has never been verified and presents earthbound problems?
Answer: Unanswerable

Question: A theoretic problem is known as any technological devise that is used for what purpose?
Answer: Unanswerable

Question: What was a plot device in the 18th century?
Answer: Unanswerable

Question: What does not persist in some way?
Answer: Unanswerable