Greetings. 

Those solar sails proposed for space probes collect the impact of photons to provide propulsion.  I think.   The photons stop moving when reach/hit the 'solar sail' or always bounce and keep going at the speed of light ?  Is the collision to those sail surfaces the one that creates a force as reaction from their action ?   How different is from a boat sail and wind ?

The photons from the sun reaching earth provide some push ?  How many photons hit the earth per unit of time; is there a tiny shift of the orbit due to reaction to photons ? -Explain as simplistic as possible, please-

Photons do not stop or accelerate, they are absorbed or reflected.
Photons do not have mass, but they do have momentum, which is directly proportional to its frequency, or inversely proportional to its wavelength.
So    p = hv   where v is the frequency, or   p = h/l    where l is the wavelength.This momentum is always conserved, in both reflection and absorption, and accounts for the induced momentum on the solar sail.

I'm sure the same effect applies to radiation reaching the Earth, but I'm not sure of its magnitude or its effect on orbital dynamics.

 

I had done these calculations before for another post awhile back. However don't particularly have time atm. However this site performs the same relevant calculations

https://www.forbes.com/sites/chadorzel/2016/04/12/how-hard-does-the-sun-push-on-the-earth/

The numbers are roughly in the same orders of magnitude that I recall when I did them.

Edited July 8, 20232 yr by Mordred

1 hour ago, Externet said:

Greetings. 

Those solar sails proposed for space probes collect the impact of photons to provide propulsion.  I think.   The photons stop moving when reach/hit the 'solar sail' or always bounce and keep going at the speed of light ?  Is the collision to those sail surfaces the one that creates a force as reaction from their action ?   How different is from a boat sail and wind ?

The photons from the sun reaching earth provide some push ?  How many photons hit the earth per unit of time; is there a tiny shift of the orbit due to reaction to photons ? -Explain as simplistic as possible, please-

It's not mass but momentum, p. For a photon, E=pc. The change of momentum when a photon is reflected at normal incidence will be 2p. A boat sail uses Bernoulli's principle, i.e. that of an aerofoil, so that is very different.

 

The momentum is E/c, meaning the force is P/c (P being the power). Twice that if reflected, as exchemist notes.

Randall Monroe did a calculation (which I confirmed) that levitating a 1 kg squirrel on a perfectly reflecting mirror requires about a “Back to the Future”-esque 1.21 GW. So not a big effect for light with a more pedestrian power level. Solar sails have to be big for any reasonable payload 

It does affect the earth by a tiny amount. Somewhat more dramatically, it can have an effect on small satellites, including causing non-uniformly-shaped ones (or ones with a non-uniform albedo) to tumble, after enough time. It contributes to the YORP effect.

https://en.m.wikipedia.org/wiki/YORP_effect#:~:text=The Yarkovsky–O'Keefe–,of its own thermal radiation.

Thanks.

1 hour ago, MigL said:

Photons do not have mass, but they do have momentum ... ... p = hv 

OK.  We are then not talking momentum p = mv = mass times velocity.    It is then momentum p = hv  =  h times frequency  =    p = h/lambda   

What is h ?   What is the formula E energy = hv ?

Or, it is not momentum; it is impetus, or impact, or some other wording  ?  Is it wrong terminology ?  Calling two things the same name ?

Just now, Externet said:

Thanks.

OK.  We are then not talking momentum p = mv = mass times velocity.    It is then momentum p = hv  =  h times frequency  =    p = h/lambda   

What is h ?   What is the formula E energy = hv ?

Or, it is not momentum; it is impetus, or impact, or some other wording  ?  Is it wrong terminology ?  Calling two things the same name ?

h is Planck’s constant

It is very definitely momentum. You can use light to accelerate atoms from the recoil of an absorption and subsequent emission of photons. Using that to cool atoms down won the 1997 Nobel prize.

12 minutes ago, Externet said:

momentum p = hv  =  h times frequency

Rather,

p = hv/c  =  h times frequency / speed of light

Thanks.  Then  impulse, impact, impetus... are not to be used.  Momentum it is.

Now; if momentum is hv   and   Energy is hv ---> momentum is energy ? 

Where am I peeing out of the can ?

Just now, Externet said:

Thanks.  Then  impulse, impact, impetus... are not to be used.  Momentum it is.

Now; if momentum is hv   and   Energy is hv ---> momentum is energy ? 

Where am I peeing out of the can ?

 

8 minutes ago, Genady said:

Rather,

p = hv/c  =  h times frequency / speed of light

 

Isn't speed of light / frequency = wavelenght ?  and speed of light / wavelenght = frequency ?

Instead of frequency / speed of light ???

17 minutes ago, Genady said:

p = hv/c  =  h times frequency / speed of light

 

3 minutes ago, Externet said:

Isn't speed of light / frequency = wavelenght ?  and speed of light / wavelenght = frequency ?

Instead of frequency / speed of light ???

 

frequency times wavelength = speed of light

p = E/c = hf/c = h/l

Bear in mind that when you go out into the sunshine, you don't feel the force of the light being exerted on you, so it is evidently quite small in magnitude.