Understanding Photon Collisions with Solar Sails

In summary: I am not proving this to you with algebra, or even declaring it to you as some kind of "authority". I just want to throw the idea your way so you can see if it works for you.In summary, solar sails are devices that use photons to generate thrust. The most efficient solar sail would be reflective, like a mirror. If it bounces the photon directly back then it captures about twice as much momentum as if it absorbs the photon.
  • #1
gonzo
277
0
I have some questions about solar sails. Basically, what happens to the photon?

I am under the impression that the "sail" is a giant mirror that reflects photons striking it. Since photons have momentum, they impart some of this momentum to this mirror. Is this correct?

But that must mean that each photon loses momentum/energy. Does this just mean that the reflected photons will always be of a lower frequency?

Does anyone know what determines during a photon collision with another particle how much momentum is transferred as opposed to other collision effects?

Thanks.
 
Astronomy news on Phys.org
  • #2
I don't think it's a mirror intended to reflect photons. It's a material that "captures" that momentum. The photon is absorbed & perhaps re-emited away as heat (lower frequency). (Sure, there's some reflection like anything else.)
 
  • #3
Phobos said:
I don't think it's a mirror intended to reflect photons. It's a material that "captures" that momentum. The photon is absorbed & perhaps re-emited away as heat (lower frequency). (Sure, there's some reflection like anything else.)

the most efficient solar sail would be reflective, like a mirror.
if it bounces the photon directly back then it captures about twice as much momentum as if it absorbs the photon
(this is just conservation of momentum)
 
  • #4
we can do a numerical example, assume we have a square meter of mirror-like mylar that is impacted by a kilowatt of sunlight. perpendicularly.

if it is perfectly reflective, the force in Newtons will be 2000 W/c

2000/3E8 Newtons = 666E-8 = 6.7 E-6 = 6.7 microNewtons.

but if it is perfectly absorbent (black) then the force will be 1000 W/c

3.3 microNewtons.
 
  • #5
gonzo said:
But that must mean that each photon loses momentum/energy. Does this just mean that the reflected photons will always be of a lower frequency?

the doppler effect means that the photon that is reflected has longer wavelength (from the standpoint of a stationary observer who sees the mirror moving and being accelerated) and so the light gives up energy


but if you position yourself on the mirror and say that the mirror is at rest
then the light just bounces off, in a direction depending on how the mirror is oriented, and momentum must be conserved.

I guess the picture is pretty much like a sail. One might tilt it 45 degrees to the sun and use it to gain orbital speed. then the analysis would be slightly more complicated than if the light is striking perpendicular
 
  • #6
I guess all there's left to do is find out which material has the best reflectiveness to density ratio.
 
  • #7
check said:
I guess all there's left to do is find out which material has the best reflectiveness to density ratio.
Mylar gift wrap!
:rofl:
 
  • #8
I've been thinking about this now, and I can't help thinking that it still seems like you are getting free energy if the photon doesn't lose energy.

It seems to me that if the photon is just reflected at the same frequency that you could theoretically construct a free energy machine that consisted of a "photon trap" with two mirrors just bouncing photons back and forth between them, and use the slight movement of the mirrors from the photons hitting them to gain energy.

If the photons never lost energy, they could in theory bounce back and forth forever.

What am I missing?
 
  • #9
gonzo said:
I've been thinking about this now, and I can't help thinking that it still seems like you are getting free energy if the photon doesn't lose energy...
...What am I missing?

marcus said:
the doppler effect means that the photon that is reflected has longer wavelength (from the standpoint of a stationary observer who sees the mirror moving and being accelerated) and so the light gives up energy
...

say you are the stationary observer and the light energy (as well as the mirror motion) is being measured on your intruments on your terms

then the energy books are going to balance because every little bit
of kinetic energy that is imparted to the mirror
is balanced by a doppler downshift in the energy of the light after it hits the mirror.

I am not proving this to you with algebra, or even declaring it to you as some kind of "authority". I just want to throw the idea your way so you can see if it works for you.

I think, for me, it works. I don't even have to check the algebra.
as the mirror moves back faster and faster it soaks up more and more energy from the light that is bouncing off it
and that fits the kinetic energy formula with its 1/2 m v2


of course in order to take energy the mirror has to be free to move
(otherwise it is like the wall you bounce tennis balls off of, they keep their energy almost because the wall is connected to the huge mass of the earth
so the wall barely moves when it is hit and the amount of energy it absorbs is negligible)

so it looks like you are considering sometimes a special case where the mirror is almost not moving----maybe because you just begin to accelerate it by shining the light on it. You ask how then can it absorb energy because there is almost no doppler shift?
Well as the mirror is just beginning to move it is also absorbing almost no kinetic energy.

when V is zero (at the very outset) then the Delta E is zero, because the kinetic energy has the V-square term.

I think you should consider a more typical case where the mirror is already moving some.
 
Last edited:
  • #10
thanks marcus
 
  • #11
Last edited:
  • #12
I thought the "sail" was powered by solar "wind" {particles} and not light

http://science.nasa.gov/ssl/pad/solar/sun_wind.htm [Broken]
 
Last edited by a moderator:

1. What is a solar sail?

A solar sail is a type of spacecraft propulsion technology that uses the momentum from photons (particles of light) to propel a spacecraft through space. It works by reflecting photons off a large, reflective sail, which creates a small but constant force that can gradually increase the velocity of the spacecraft.

2. How do photon collisions with solar sails work?

When photons from the sun hit the reflective sail, they transfer some of their momentum to the sail. This momentum transfer creates a small but continuous force that can gradually accelerate the spacecraft. This process is known as photon pressure or radiation pressure.

3. What are the advantages of using solar sails for space travel?

Solar sails have several advantages over traditional spacecraft propulsion methods. They do not require any fuel, so they can theoretically travel indefinitely as long as they are exposed to sunlight. They also have a much higher maximum velocity compared to traditional chemical rockets, making them ideal for long-distance space travel.

4. How do scientists calculate the trajectory of a solar sail?

To calculate the trajectory of a solar sail, scientists use equations that take into account the size and shape of the sail, the distance from the sun, and the solar radiation pressure. They also use computer simulations to model and predict the behavior of the sail in different conditions.

5. Are there any challenges or limitations to using solar sails?

One of the main challenges of using solar sails is the relatively low force of photon pressure, which means that it takes a long time for the spacecraft to reach high speeds. Additionally, solar sails can only be used in the vacuum of space and are not effective in environments with an atmosphere. Another limitation is the need for a constant source of sunlight for the sail to continue propelling the spacecraft.

Similar threads

Replies
2
Views
625
  • Sci-Fi Writing and World Building
Replies
9
Views
2K
  • Science Fiction and Fantasy Media
Replies
15
Views
2K
Replies
9
Views
1K
  • Astronomy and Astrophysics
Replies
6
Views
1K
  • Astronomy and Astrophysics
Replies
7
Views
3K
  • Astronomy and Astrophysics
Replies
14
Views
2K
Replies
8
Views
855
  • Astronomy and Astrophysics
Replies
13
Views
1K
Back
Top