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

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

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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.

They don’t have mass but they have momentum.

It is possible because the mass was defined to be energy by einstein.

Solar sails are not hypothetical. IKAROS launched in 2010 used solar sales and practically demonstrated they work.

The answer is that photons have momentum.

The momentum is what is preserved in collisions. That is what Newton’s laws of motion are about.

In our everyday life momentum is mass * velocity. Mass in this context is the rest mass or invariant mass and it is the only thing we need to care about at a speed low compared to the speed of light.

At high-speed what is called relativistic mass is also relevant. A photon does have a relativistic mass.

The idea of mass–energy equivalence from relativity is to some degree known to most people that is what the E =mc^2 equation stands for.

That equation is for an object at rest for a moving object the formula i

E ^2 = ( m * c^2 )^2 + (p * c)^2

p is the momentum. So even if the rest mass m is zero the energy of the photons results in momentum this is called the Energy–momentum relation.

So rest mass is not required for momentum, mass is just what results in the momentum at the speed we are used to on earth.

We think that only things with mass can push when hitting something because we are used to everyday interactions with massive objects, including ourselves. Turns out photons can pack some punch too, because they have energy, and Einstein proved that energy is equivalent to mass in a certain sense which is what his famous equation (E = mc2) is all about. The more energetic a photon is (the shorter its wavelength) the more push it has to give.