eli5 – if photons have no mass, how can they be used to push hypothetical “solar sails”?

Question

eli5 – if photons have no mass, how can they be used to push hypothetical “solar sails”?

In: 20

Answer 1

Momentum (aka oomph) was originally defined as mass X velocity. The assumption being that you have more “oomph” is you are faster or heavier.

Modern physics finds that it is more sensible to define “oomph” with a different equation. One reason for this is the fact that photons (which have no mass) clearly have oomph; they are able to push stuff (albeit only a little). So, momentum isn’t actually defined as mass times velocity anymore.