I want to know if this is possible and any limitations it has.

Time Dilation is an arse to say the least. It is one the things that ought to stop humankind from ascending to sci-fi badassness and I am not ok with it, BUT, I have been told by a very reputable scientist (

6:55-9:09 mins in) that there are two types of time dilation, so to speak.. The infamous one is the one where time runs slower for a moving clock than a stationary one (mechanical, biological, etc.) but the slightly less known one is the one I am interested in. I will get to that later. If you go at any speed under the speed of light, time will run slower for you as seen by a stationary observer. Say you went at the speed of light for 10 minutes, you would experience no travel time and would instantly teleport to your location but the world around you would be 10 minutes in the future. Once you exceed lightspeed, external time would run backwards instead of forwards. . So, say you have your hypothetical super fast magical ship that can somehow go at speeds up to 1000 times the speed of light. If that ship goes at several times the speed of light until external time, is at, say, 5 years in the past, would the ship be able to escape the clawing grasp of time dilation by lowering its speed to just below lightspeed so that external time runs forward again until it is normal, at which point the ship would go faster than light and so on until it reaches its destination, free of T.D because of playing jump rope with the laws of the universe.

(this is kind of a weird question, but I am going to write a sci-fi book series about the end of the universe in around a decade and I want it to be as scientifically accurate as possible and there is a depressing void of this sort of info on the internet so thought I would ask you guys. plz don't get salty about this question)

Limits of google

  • 3
    $\begingroup$ this feels like a WorldBuilding question $\endgroup$
    – Rory Alsop
    Commented Oct 10, 2017 at 8:54
  • $\begingroup$ Are you asking if you can get no net time dilation despite net FTL? $\endgroup$ Commented Oct 10, 2017 at 9:14
  • $\begingroup$ You can't exceed speed of light - To reach speed of light you need infinite energy. As you accelerate towards speed of light, the closer you get the more of the energy you spend towards acceleration goes towards increasing your mass as opposed to speed. $\endgroup$
    – SF.
    Commented Oct 10, 2017 at 12:46
  • $\begingroup$ The good news is due to dilation, for the crew of such craft the travel time would be as if they were traveling 2x the speed of light (speed near speed of light, but distance is made shorter - Lorenz Contraction). The bad news is for everyone else they'll be crawling just below 1c. And the worst news is the best propulsion means we have developed so far fall woefully short of any considerable fraction of speed of light. Regardless, all "if we moved faster than light" falls apart at the impossibility of crossing the light speed barrier. $\endgroup$
    – SF.
    Commented Oct 10, 2017 at 12:49
  • $\begingroup$ Exceeding the speed of light puts this in the fictional category. Voting to close. $\endgroup$ Commented Oct 10, 2017 at 12:56

1 Answer 1


Once you exceed lightspeed, external time would run backwards instead of forwards.

This is where it all falls down. You're applying the laws of relativity to FTL travel, but the laws of relativity, specifically the Lorentz factors, don't make sense for velocities above the speed of light, $c$.

Just look at the mathematics - the Lorentz Factor is given by

$ \gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} $

If $v > c$, then you have the square root of a negative number which doesn't make sense.


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