# Would time reverse if we were to go faster than the speed of light? [duplicate]

This question already has an answer here:

Einstein states that the fastest thing in the universe is light. He also states that velocity interacts with time (faster you go the slower time is relative to you). The speed limit of the universe is light, so if you were to go at the speed of light time would stop... If something were to go faster than the speed of light then it makes sense for time to go backwards. This would technically not be possible because if time was to go backwards then the speed of light would return to its "fastest" state hence making it not possible to go faster....Is this true fact? Or am I completely off....

## marked as duplicate by WillO, M. Enns, Chris♦, Emilio Pisanty, Kyle KanosMar 4 '18 at 19:00

• Are you saying that if you could exceed the speed of light, then time would run backwards, taking you back to when you were going slower than the speed of light? – D. Halsey Mar 4 '18 at 1:52
• Yes, in return this would make speed of light the limit.. – user537166 Mar 4 '18 at 1:56

The speed limit of the universe is light, so if you were to go at the speed of light time would stop... If something were to go faster than the speed of light then it makes sense for time to go backwards.

It's at this point that I think your logic starts to fall apart.

Fundamentally, a physical theory can't answer questions about what happens when you violate the theory. Special relativity says that material objects travel at velocities less than $c$, so it can't answer questions that assume, to the contrary, that a material object travels at a velocity greater than or equal to $c$.

Another, less fundamental way of getting at this is to look at the equation for the time dilation factor

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

For $v>c$, it doesn't give a negative result, it gives an imaginary number.

The prior answer: your proper time does not compute, is correct. Here is what other observed would see:

Consider just the end points of your journey. Suppose you traveled from here, now (event A) to the Moon in 100 milliseconds (event B). This is not possible b/c the Moon is 1.5 light-seconds away--but we're suspending that problem. Note that B is outside the forward light-cone of A, so there are reference frames in which B is before A. There are reference frames in which A and B are simultaneous (your speed must have been infinite), and there are frames in which B is after A. As far as causality goes, it is a mess.