# Are there examples of sub-light propulsion that are not subject to time dilation?

A common theme in space sci-fi are means of moving faster than the speed of light without experiencing the effects of time dilation (to a substantial enough degree, anyways). Such methods often use extra-dimensional means, such as hyperspace, subspace, folding space, etc.

But travelling at less-than-light speeds can still result in significant dilation.

In the Star Trek universe, impulse drive can propel a starship upwards of 80% the speed of light. At those speeds, a single second inside the vessel would be hours if not days outside of it. And to do anything interesting in space, you have to travel at significant speeds even if such speeds are slower than light.

So, are there any examples of sub-light drives that operate in a manner whereby occupants are unaffected by time dilation?

• I'm not sure what you're asking here, since most scifi travel ignores time dilation (there's no actual time dilation with impulse drive, for example) Commented Dec 31, 2011 at 22:30
• @thedaian I guess the real question here then is why? Commented Dec 31, 2011 at 22:41
• Time dilation at sub-light speeds appears in "The Sparrow" by Mary Doria Russell (a really amazing book, very under-appreciated in scifi).
– user4050
Commented Jan 2, 2012 at 17:39
• 80% of the speed of light makes time run at 60% speed, when compared to an Earth observer, so it's not quite as extreme as the OP makes it at those speeds. Commented Nov 17, 2014 at 15:14

There are several drive types in various games and shows that avoid dilation by not actually generating velocity.

The 2300AD game's stutterwarp involves macro-scale quantum tunneling effect to generate both high sublight through several hundred C.

Star Trek makes use of "Heisenberg Compensators" that negate dilation effects at both high sub-C and at warp. Further, at warp it "moves a bubble of space" rather than the ship itself through space, which theoretically could avoid some or all dilation effects as well, and there are implications in some novels that warp and impulse are related effects. (Some of those may be unintentionally making use of Amarillo Design Bureau's Star Fleet Universe, a spin-off setting from TOS/TAS for their game lines, originally a sublicense from Franz Joseph. in the SFU, impulse drives can generate warp fields and do low warp speeds.)

The Spelljammer setting for AD&D (and its derived stories) simply redefines the universe to ignore quantum physics.

The Novels (and the related GURPS sourcebook) for the Lensmen setting make use of a drive which disconnects the ship from accumulating inertia. Doc Smith seems to have believed that Inertia and Dilation were in fact closely linked, and that the Inertialess ships would be limited in speed only by friction. While most consider the Inertialess Drive to be FTL, it operates sublight as well.

Many harder settings also ignore relativity - some by predating common awareness of it, a few by predating it (Verne's works).

The general pattern is to simply keep sublight speeds under 0.8C where it isn't really noticed.

As a corollary, almost any good means of evading dilation also enables FTL, since the inability to attain C is a function of dilation.

At 80% of the speed of light, the time dilation is only 5:3 - one second on the ship is equal to 1.67 outside. To get a ratio of 'hours' (7071:1, not quite two hours per second), you need to be going 0.99999999c. To get a ratio of 'days' (223606:1, not quite three days per second), you need to be going 0.99999999999c. That speed is 6 miles per year slower than light.

With any sort of sub-light propulsion, you can usually ignore time dilation unless you're very close to light speed.

• Although if you're on a trip decades long those seconds would add up eventually. Commented Jan 1, 2012 at 0:23
• 5:3 is pretty darn big if you're trying to do something like talk or deal with a daily schedule with someone who isn't moving. Commented Jan 1, 2012 at 2:12
• A rule of thumb often used by physicists is that relativistic effects (like time dilation) become significant once you reach 10% of the speed of light. Of course in practice it depends entirely on your definition of "significant." Commented Jan 1, 2012 at 8:33
• 5:3 is pretty big over a 40 light-year trip to another star system - at 80% the speed of light, the trip would take 48 years, and everyone back at Earth would have experienced 12 years more than you did Commented Jan 2, 2012 at 0:23
• But even a few ms can make a difference. GPS calculations take time dilation into account, and those satellites are only moving at a small fraction of c. Commented Jan 2, 2012 at 23:26

There are many forms of space-drive that are discussed in the real world as well as science-fiction that do not approach the speeds that time-dilation becomes an issue. Many authors have used drives that are either have very low acceleration and/or that drift for a majority of their journey.

One example is a light-sail drive - where the acceleration would drop the further away from the sun or launching light source was based. In The Mote in God's Eye by Niven and Pournelle, the light-sail powered Crazy Eddie Probe takes 150 years to arrive in New Caledonia from the Mote, a relatively small distance between systems. This journey is unlikely to have encountered a very significant time-dilation effect.