So, I've been doing research on how no mass could pass the speed of light. So, I've always had a question in mind. What would happen if someone was going at 99.99% the speed of light and they decided to accelerate. What would happen?

  • $\begingroup$ Theoretically, if you were able to strictly accelerate something to faster than the speed of light, they would actually go back in time. However, as it is impossible to go faster than the speed of light, the point is kinda moot. $\endgroup$
    – Phiteros
    Commented Oct 25, 2016 at 3:47
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    $\begingroup$ @Phiteros that isn't really how it works... Although it can be interpreted as going "back in time" you would theoretically be traveling faster than the information about you could travel, so you would actually just be moving through space while existing as a time-independent being. See Doug's answer, and then for a fictional non-realistic example watch Dark Matter... $\endgroup$
    – LaserYeti
    Commented Oct 25, 2016 at 4:43
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    $\begingroup$ We need an autobot to close any question with the phrase "faster than light" $\endgroup$ Commented Oct 25, 2016 at 12:00
  • $\begingroup$ I'm voting to close this question as off-topic because this is a physics and space exploration question, not an astronomy one. $\endgroup$
    – called2voyage
    Commented Oct 25, 2016 at 14:43

1 Answer 1


If someone was going speed $v = \beta c$ where $0 \leq \beta < 1$ and they decided to accelerate, their velocity would increase and approach $c$ but never pass it. This is due to the fact that it would require an infinite amount of energy to accelerate a massive particle to the speed of light in the first place. You can see this from the energy in special relativity:

$$E = \frac{mc^2}{\sqrt{1-\beta^2}}$$

Where $\beta = v/c$ from above. If $\beta = 1$, or $v = c$, $E \to \infty$. There is a signularity in this equation when $v=c$ or $\beta = 1$, which is interpreted as taking infinite energy just to get to the speed of light. This also restricts massive objects from going the speed of light ever, which means for a massive object $ v < c $ strictly.

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    $\begingroup$ In layman's terms, you would accelerate from 99.99% to 99.993% or even 99.996% but never $\geq$ 100%. You would simply be adding additional significant figures. $\endgroup$
    – LaserYeti
    Commented Oct 25, 2016 at 4:46

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