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A type of time paradox which involves information or matter that that is never created or destroyed.

Example 1:
You meet yourself from the future, who gives you a notebook about time travel. Using the information, you build the first time machine and at some point go back in time to give yourself a notebook about time travel.

Where did the information in the note book come from? You didn't compile it since you got it from your future self.

Example 2: (See also: Star Trek IV: The Voyage Home)
You find equipment somewhere that you use during your time travels. At some point on a trip to the past, you lose it. (You will find it "later".)

When was it manufactured? When will it break?

Example 2a: (From the 1984 novel Them Bones by Howard Waldrop)
You find a hurt animal in your basement and tend it back to health. You then use the animal in your time travel experiments and accidentally injure it before sending it back in time to your basement.

When is the animal born? When will it die?

Example 3: (From the journal Analysis, 1979 by Jonathan Harrison)
A woman finds a freezer containing a young man who has a book describing how to make a time machine and how to make a freezer to preserve a person. They marry (or not) and have a child. The child learns from the book and builds a time machine. The boy and his father get into the time machine with the book and travel back in time. They run low on supplies and the boy kills and eats his own father. In the past, the boy destroys the time machine and builds a freezer. He freezes himself and is thawed out by the woman...

This is interesting because there is no issue of having an immortal creature or matter that is immune to entropy but Where did the genetic information for the boy/man come from? Also, where did the time travel information come from?

Some causal loops are more complicated but ultimately involve an event causing itself.
Causal loops are often referred to as closed temporal loops. This is usually done when an object makes a "round trip".

Special thanks to: Paul Nahin and his book, Time Machines, 2nd Ed
tdent points out that these are known as closed timelike curves to the hard-core.

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