According to Einstein E²=(mc²)²+(pc)². In the case of photons m would be equal to zero since photons are massless, so the equation simplifies to E²=(pc)². p represents momentum but because p=mv and mass is still zero, p would also be zero. Therefore E should also be equal to zero, but this doesn’t make sense since light must have energy (eg. solar energy). I hope I’ve explained my thoughts clearly and thanks to anyone who can help me understand.