I heard that the light has zero mass. But I searched in internet, some people say light has mass and some people said doesn't. I am not sure what is the right answer.
And also I heard that the reason we can't travel in light speed is because we have mass but light doesn't. Please tell me the right answer.
A photon (elementary particle that makes up light) does not have a (rest) mass, but it does have energy $E$ and momentum $p$, given by the relation $$E=pc$$ where $c$ is the speed of light.
(The full form of this equation for a general particle is $E^2=m^2c^4+p^2c^4$ - if you consider a stationary particle, which is not the case for a photon, then you end up with the famous $E=mc^2$ equation).
To accelerate towards the speed of light, a particle with mass would need infinite energy - this falls out of special relativity formulae. In addition to the above general formula I gave, another way of expressing the energy of a particle with mass is $$E=\gamma mc^2$$ where the "Lorentz factor" features here, which is: $$\gamma = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$$ which explodes to infinity as $v\rightarrow c$ and so if you want to get a massive particle with $v=c$, you would need infinite energy.
According to energy-momentum relation, energy possessed by a particle of mass $m_0$(rest mass) and having a momenta $p$ is $E^2=m_0^2c^4+p^2c^2$. Please note that the mass that goes into this equation is rest mass. Now the relativistic mass is given as $m=\frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}$. According to Einstein, rest mass of photon is $m_0=0$ and as it is moving at speed $c$, it's relativistic mass is indeterminant. The energy possessed by a photon is simply $E=pc$
