Why does light have no mass but still carry momentum?
Quick Answer
Photons are massless particles, but they still carry energy and momentum because of relativity. That is why light can push on matter, as in solar sails.
The Full Story
In classical physics, momentum is associated with moving mass, but light does not fit that picture. In Einstein’s relativity, energy and momentum are linked, and photons carry both even though their rest mass is zero. When light is absorbed or reflected, it transfers momentum to the object it hits, producing a tiny radiation pressure. This effect is extremely small in everyday life, but it becomes useful in space engineering and precision experiments. It is one reason sunlight can very slowly push dust and spacecraft.
Key Facts
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