Why can't we travel faster than light? And is the theory of relativity absolutely flawless?

  • 3
    $\begingroup$ Special relativity is thoroughly tested and incorporated in our understanding of the world, and one of its implications is that moving faster than light would be the equivalent of moving backwards in time; that is you could violate causality in some frames of reference. So the reason you can't travel faster than light is in some sense the same reason that you can't go backwards in time. Check physics stackexchange for more thorough answers. $\endgroup$
    – antlersoft
    Nov 6, 2018 at 17:10
  • 4
    $\begingroup$ I'm voting to close this question as off-topic because it's basic Physics $\endgroup$ Nov 7, 2018 at 20:12
  • $\begingroup$ It would cause logical contradiction in the special relativity. Google for "tachyonic anti-telephone". Some trickery on the general relativity makes it theoretically possible, but all of them require matter with negative mass. Read this. If it will be ever possible, it will be very different what we call physics today. Note also, the experimental proof behind SR and GR is very strong. Thus, the answer on the best knowledge of the today is unfortunately no. $\endgroup$
    – peterh
    Nov 8, 2018 at 1:01
  • $\begingroup$ That's two questions. The answer to the second one is no because no theory explains everything. Why not just stick to the first question? If you want to ask about things not explained about relativity that would be a different question (but maybe too broad) $\endgroup$
    – user1569
    Nov 8, 2018 at 11:12

2 Answers 2


No scientist really considers a theory flawless, but as theories put together by mankind go, it is one of the best ones put out there. One of the things that makes for a great theory is not only being able to describe what's already happened, but to make predictions about new stuff, and general relativity did that with predictions of things like gravitational waves that took 80 years for technology to get good enough to see.

General relativity also passes an aesthetics test, where some people feel that the most fundamental laws are beautifully simple and elegant, and though as far as we know, there is no scientific reason that says the Universe is simple and elegant, most of the best theories turn out to be that way, including general relativity.

So if you acknowledge that relativity handles all the many many tests that have been thrown at it, the answer to your first question has to do with the Lorentz factor, which goes as $ \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} $, so you see that as your velocity $v$ gets closer and closer to the speed of light $c$, the bottom of the fraction gets closer to zero and the Lorentz factor blows up. Since the kinetic energy of something moving really fast involves the Lorentz factor, that means as you approach the speed of light, our energy blows up, and to actually travel at the speed of light, you'd need an infinite amount of energy.


One of the predictions of Special Relativity is that the relativistic mass of an object increases toward infinity as the object approaches the speed of light. The formula is given by:

mr = m0 /sqrt(1 - v2/c2)

where mr = relativistic mass, m0 is rest or invariant mass, v = velocity, and c = the speed of light.

Since the mass of an object approaches infinity near the speed of light, it takes more and more energy to cause acceleration, and at the speed of light, it would take infinite energy to cause any further acceleration, since the object has infinite mass. Of course, you could never get to the speed of light because there isn't enough energy represented as mass in the universe to make that happen.


Not the answer you're looking for? Browse other questions tagged .