Physicists take seriously questions about the physical possibility of time travel. This is a matter of consistency of time travel with the laws of nature. This website primarily concentrates on philosophical matters that are even more basic than the issue of physical possibility. There are arguments that seem to show that time travel doesn’t even make sense, that it is illogical, that it is contradictory. Proponents of these arguments think that time travel is impossible no matter what the laws of nature are.

When apparently sound reasoning leads to a contradictory conclusion, a paradox is the result. Many paradoxes arise in the consideration of time travel. Below are some of the paradoxical topics to be explored:

The Grandfather Paradox: Can a person who has traveled to the past kill his or her own grandfather?

The Self-Visitation Paradox: Can a person visit himself or herself? How can there be two of one person at once?

The Nowhere Argument: If only the present moment exists, how could we travel to the past or the future?

The Double-Occupancy Problem: Can a person time travel backwards in time without colliding with him- or herself?

There is no Loop in Looper: How should we distinguish a true causal loop from a merely apparent causal loop in time-travel plots?

Causal Loops : What is a causal loop? Are causal loops impossible?

If you would like some exposure to the physics of time travel, see:

Relativity and Time Travel: What is it about spacetime that opens the door to time travel to the future and time travel to the past?

For something a little bit different, consider what time travel might be like if our universe included multiple timelines:

Multi-Dimensional Time: Find out what time would have to be like for you to go back in time to prevent Lincoln’s assassination.

We don’t provide a detailed manual on how to build your own time machine, but we think you will enjoy learning about the metaphysics of time travel.

Why have we created and maintained a website about time travel that focuses on philosophical and—more specifically—metaphysical issues about time travel? In his book, Time Travel in Einstein’s Universe, Princeton astrophysicist, J. Richard Gott says,

Why are physicists like me interested in time travel? It’s not because we are hoping to patent a time machine in the near future. Rather, it is because we want to test the boundaries of the laws of physics. The paradoxes associated with time travel pose a challenge. Such paradoxes are often a clue that some interesting physics is waiting to be discovered (p. 29).

What Professor Gott says about physicists applies equally well to philosophers. Philosophers like to test the boundaries of our logic and our concepts, testing philosophical theories of free will, identity, time, modality and causation against unfamiliar situations. Thinking about time travel often pushes the boundaries of these theories in ways that deepen our understanding of more day-to-day aspects of reality.

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