What will happen if a person with say weight of 100 kilograms, starts to travel with,

a) equal to speed of light?

b) greater than speed of light?

  • 1
    $\begingroup$ the problem with this question is that there is no real way to get up to that speed for the matter with rest mass. As you approach the speed of light you actually are traveling through time, and there is a fundamental limit to how quick an outside observer can see you going This outside observer is the only person who can actually tell you how fast you are going because in relativity there is no absolute reference frame. $\endgroup$
    – Skyler
    Nov 10, 2014 at 10:38
  • $\begingroup$ It's an okay, but somewhat misguided question, but I'd suggest you pull it down before half the community bullies this question down. $\endgroup$
    – Skyler
    Nov 10, 2014 at 10:42
  • $\begingroup$ See also the recent question physics.stackexchange.com/q/143615/8851 $\endgroup$
    – b_jonas
    Nov 10, 2014 at 10:45
  • 1
  • $\begingroup$ @Skyler, one should not accept bullying (nor accept its subtle variation of a bad but given unavoidable reality). And if this is an ok question (as your comment hints) i would expect to stand up to it, instead of using these kinds of subtle variations (which the comment is assumed to disagree) $\endgroup$
    – Nikos M.
    Nov 11, 2014 at 17:53

1 Answer 1


The energy required to accelerate a massive object from below the speed of light to the speed of light (or beyond it) would be infinite, so it's not thought to be possible at all. This can be derived from the fact that the momentum $p$ of an object with nonzero rest mass $m$ and velocity $v$ is given by $p = mv / \sqrt{1 - v^2/c^2}$, which approaches infinity as $v$ approaches the speed of light $c$, and the total energy of an object, including both the energy due to rest mass and the kinetic energy, is given by $E^2 = m^2 c^4 + p^2 c^2$.

However, it would not be inconsistent with relativity to have hypothetical particles labeled "tachyons" that always travel faster than light; in order to get physically meaningful predictions about them, though, we would have to assume their rest mass was an imaginary number, as discussed on this page. Also, if tachyons existed and relativity wasn't violated, it would either have to be impossible to use them to transmit information faster than light (which could be a natural consequence of analyzing them using quantum field theory, as discussed in the link above), or if they could be used to transmit information FTL, this would imply they could also be used to transmit information into one's own past, violating causality (see my answer here to a question about why tachyons would violate causality). Although tachyons wouldn't violate relativity, there is no evidence that they actually exist.

  • $\begingroup$ The issue of "infinite" mass/energy is a red herring here. You cannot attain the speed of light by accelerating because of the velocity addition formula: en.wikipedia.org/wiki/Velocity-addition_formula - it is a hard limit build into spacetime itself. $\endgroup$
    – m4r35n357
    Nov 10, 2014 at 16:01
  • $\begingroup$ @m4r35n357 - The velocity addition formula can also be used to derive the impossibility of reaching the speed of light, but only if you include a premise that your proper acceleration can't approach infinity at some finite proper time (or coordinate time); as it turns out this premise would be false in Newtonian physics, so it doesn't necessarily seem any better as a starting premise than energy considerations. $\endgroup$
    – Hypnosifl
    Nov 10, 2014 at 16:26
  • $\begingroup$ Surely infinite proper acceleration means infinite boost(rapidity) which would mean v = 1. I would say that is covered already. I stand by the statement that it is unnecessary to multiply the velocity four vector by m as the speed limit is there even if you don't. It is just an extra layer over the fundamentals. $\endgroup$
    – m4r35n357
    Nov 10, 2014 at 17:13
  • $\begingroup$ @m4r35n357 - Sure, by definition they imply one another, but again what is the physical argument for saying we should expect infinite proper acceleration/infinite boost to be impossible, if you don't want to get into energy considerations? Light is in fact defined to have infinite rapidity, so obviously the mere fact that some quantity would be infinite under some circumstances does not automatically imply we should say those circumstances are physically impossible. $\endgroup$
    – Hypnosifl
    Nov 10, 2014 at 18:50

Not the answer you're looking for? Browse other questions tagged or ask your own question.