Rules for Time Travelers

We can't travel through time, but we do know enough to say that if time travel were possible, certain rules would have to be obeyed.

Cosmic Variance
By Sean Carroll
May 14, 2009 4:55 PMApr 26, 2020 10:59 PM
Bulb Exploding Physics - Shutterstock
(Credit: gualtiero boffi/Shutterstock)

Newsletter

Sign up for our email newsletter for the latest science news
 

With the new Star Trek out, it's long past time (as it were) that we laid out the rules for would-be fictional time-travelers. (Spoiler: Spock travels to the past and gets a sex change and becomes Kirk's grandfather — no — lover.*) Not that we expect these rules to be obeyed; the dramatic demands of a work of fiction will always trump the desire to get things scientifically accurate, and Star Trek all by itself has foisted half a dozen mutually-inconsistent theories of time travel on us.

But time travel isn't magic; it may or may not be allowed by the laws of physics — we don't know them well enough to be sure — but we do know enough to say that if time travel were possible, certain rules would have to be obeyed. And sometimes it's more interesting to play by the rules. So if you wanted to create a fictional world involving travel through time, here are 10+1 rules by which you should try to play.

0. There are no paradoxes.

This is the overarching rule, to which all other rules are subservient. It's not a statement about physics; it's simply a statement about logic. In the actual world, true paradoxes — events requiring decidable propositions to be simultaneously true and false — do not occur. Anything that looks like it would be a paradox if it happened indicates either that it won't happen, or our understanding of the laws of nature is incomplete. Whatever laws of nature the builder of fictional worlds decides to abide by, they must not allow for true paradoxes.

1. Traveling into the future is easy.

0 free articles left
Want More? Get unlimited access for as low as $1.99/month

Already a subscriber?

Register or Log In

0 free articlesSubscribe
Discover Magazine Logo
Want more?

Keep reading for as low as $1.99!

Subscribe

Already a subscriber?

Register or Log In

Stay Curious

Sign up for our weekly newsletter and unlock one more article for free.

 

View our Privacy Policy


Want more?
Keep reading for as low as $1.99!


Log In or Register

Already a subscriber?
Find my Subscription

More From Discover
Stay Curious
Join
Our List

Sign up for our weekly science updates.

 
Subscribe
To The Magazine

Save up to 40% off the cover price when you subscribe to Discover magazine.

Copyright © 2025 LabX Media Group