Solar panels - effect of photovoltaics on photonic momentum

[Aladdin123]

Solar panels exploit photonic momentum to propel objects. If instead of a plain "sail" we place a solar panel (that harvests light for electric power) would that reduce the efficiency of the propelling action? If so by how much?

[swansont]

Solar panels exploit photonic momentum to propel objects. If instead of a plain "sail" we place a solar panel (that harvests light for electric power) would that reduce the efficiency of the propelling action? If so by how much?

Yes, by as much as a factor of 2. A reflected photon imparts twice its momentum to the target (2E/c), while an absorbed photon can only impart its momentum (E/c).

[Sensei]

How about momentum vector.. ?

[swansont]

How about momentum vector.. ?

Momentum being a vector is why reflection imparts 2E/c of momentum to the solar sail. The photon changes direction.

[Sensei]

So suppose so we have spaceship with mirror on rear.

And laser on the Earth, or "launcher" on the orbit,

it's emitting photons toward spaceship,

spaceship is accelerated each time photon is reflected (not just absorbed) (like you said by p=2E/c),

then that photon returns back to the "launcher",

then suppose so, there is mirror (again),

which reflects photons back to spaceship,

and spaceship again receives the same photon but twice reflected,

then it's reflected by spaceship mirror,

and again it's reflected by launcher mirror..

Repeat millions or billions repetitions.,

 

[J.C.MacSwell]

So suppose so we have spaceship with mirror on rear.

And laser on the Earth, or "launcher" on the orbit,

it's emitting photons toward spaceship,

spaceship is accelerated each time photon is reflected (not just absorbed) (like you said by p=2E/c),

then that photon returns back to the "launcher",

then suppose so, there is mirror (again),

which reflects photons back to spaceship,

and spaceship again receives the same photon but twice reflected,

then it's reflected by spaceship mirror,

and again it's reflected by launcher mirror..

Repeat millions or billions repetitions.,

 

 

To quote myself from a recent thread...

 

 

Unbalanced equations are the greatest source of renewable energy...

 

There ought to be a name for perpetual motion machines based on this concept as it comes up "perpetually"...ignore a small change in energy as insignificant and then use it over and over to perpetually drive the machine.

 

We also see it's cousin that thrusts itself forward in space by creating it's own momentum...all by claiming as insignificant what is used over and over in some sort of circuit.

Edited September 29, 2016 by J.C.MacSwell

[wayne_m]

Momentum being a vector is why reflection imparts 2E/c of momentum to the solar sail. The photon changes direction.

 

That's something I have a hard time understanding. If the reflected photon carries away the energy it brought into the collision, where does the equation get the energy that goes into changing the momentum of the sail?

Do all of the photons reflect, or do some get absorbed? Do the ones reflected just lose energy to a drop in frequency?

[Strange]

Do the ones reflected just lose energy to a drop in frequency?

 

 

Yes.

But also, not every photon will be reflected because the mirror will not be perfect.

[swansont]

So suppose so we have spaceship with mirror on rear.

And laser on the Earth, or "launcher" on the orbit,

it's emitting photons toward spaceship,

spaceship is accelerated each time photon is reflected (not just absorbed) (like you said by p=2E/c),

then that photon returns back to the "launcher",

then suppose so, there is mirror (again),

which reflects photons back to spaceship,

and spaceship again receives the same photon but twice reflected,

then it's reflected by spaceship mirror,

and again it's reflected by launcher mirror..

Repeat millions or billions repetitions.,

 

 

 

You are describing an optical buildup cavity. These are really hard to align when the length is on order of a meter.

 

That's something I have a hard time understanding. If the reflected photon carries away the energy it brought into the collision, where does the equation get the energy that goes into changing the momentum of the sail?

Do all of the photons reflect, or do some get absorbed? Do the ones reflected just lose energy to a drop in frequency?

 

 

There is a small drop in frequency. But E/c is a really small number (maybe 3 x 10^-18J for a visible or UV photon, so the momentum change is 10^-24 kg-m/s) and it's hitting a massive object so the change in speed of the target is tiny. The energy loss is insignificant for purposes of this calculation.