Primary types of time paradoxes
A situation where an event or object exists without a clear origin because it is passed back in time and becomes its own source.
Example: You travel back and hand a young scientist a notebook of equations. The scientist publishes them. Years later you copy those same equations from the published paper and carry them back to give to the scientist. The notebook has no original author.
2. Consistency Paradox (Grandfather Paradox)
An action in the past prevents the cause of that action, producing a logical contradiction in the causal chain.
Example: You go back in time and kill your grandfather before your parent is conceived. If your parent is never born you could not exist to travel back and commit the act.
3. Ontological Paradox
A subtype of bootstrap paradox focused on objects, knowledge, or beings that appear to exist without an originating creation event.
Example: A composer receives a score from their future self, performs it, and years later goes back to hand the same score to their younger self. The composition has no point of creation.
4. Predestination Paradox
Attempts to change the past end up causing the same events that were intended to be prevented; the past is self-consistent and enforces its own outcome.
Example: You learn of a fire and rush back to stop it; your frantic actions accidentally knock over a lamp, which starts the same fire you tried to prevent.
5. Polchinski’s Paradox
A physics thought experiment (often using closed timelike curves) where a physical trajectory into the past interacts with itself to cause inconsistency—for example, collisions that prevent the original motion.
Example: A billiard ball travels through a wormhole to the past and strikes its earlier self so that the earlier ball never enters the wormhole—creating an impossible trajectory.
6. Information Paradox (Temporal Information Problem)
Sending data or knowledge into the past that undermines the causal chain of computation, discovery, or decision-making—information appears without origin or breaks protocols.
Example: A programmer receives a compiled program from their future self, runs it, then later extracts the source from the compiled binary and sends it back—who actually wrote the code?
7. Observer Paradox (Temporal Reference Inversion)
Observers who travel through time alter frames of simultaneity or measurements, producing contradictions in what different observers record as the order or occurrence of events.
Example: Two observers traveling different paths through spacetime disagree on whether event A happened before event B; one observer's actions that depend on that order create a paradox in shared records.
8. Dual Timeline / Multiverse Paradox (Branching Resolution)
Instead of creating a logical contradiction, changes to the past spawn a new branching timeline or universe; identity continuity and causation split across branches.
Example: You go back and prevent an accident; your original timeline still exists, but a new branch forms where the accident never occurred—your original past remains different from the new branch.