Photons

Yes, But doesn't the photon need a constant "push" to keep moving? Who will provide that push?

@Sharmilakg Absolutely not. Newton's First Law of Motion states that "In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force." In other words, ANY body to which an initial force has been applied will continue to move at a constant velocity until it meets 'an obstacle' - that is, until some other net force is applied to stop it, accelerate it or change its direction. The energy that any particle has to keep it moving at a constant velocity is called 'momentum'. Photons are something of a special case, though, because they are not something which you can stop, slow down or pick up and hold. In fact, they are really only a metaphor for one of the ways that light behaves. What that means is that "IF you consider light as a stream of 'particles' - and we call those particles 'photons' - then it can be said to obey the First Law of Motion inasmuch as it will continue forever at the same velocity unless acted upon by a net force. 'Photons', in other words, would have zero mass if they were at rest (which they can never be) but have momentum at the speed of light, as can be shown by the simple device known as a 'Light Mill' (Crooke's Radiometer), where the momentum of light pushes the sails of the mill around: https://youtu.be/MtSUGYcEWAY

@owlwings thank you..that's a beautiful explanation..it clarified my doubt.