Question about light, mass, and the solar sail

[Fallingstar1971]

Light

To me, the most incredible thing in nature.

Is it a particle? Is it a wave? If it truly has zero mass, are we talking 100% mass/energy conversion?

Solar sails work by "bouncing" photons(light) off of the sail to propell it. But if photons have zero mass, why dont they pass right through it like a neutrino?

Could it be possible that the wave that light travels in has mass, but not the light itself? (mass less photon traveling in a wave that has mass) Is there a way to test that thought? Should there not be at least SOME mass to transfer the energy from the light to the sail upon impact?

Or do I have the mechanics of the solar sail wrong? The Sun DOES emit other forms of radiation then light. Perhaps one of those is whats truly propelling the sail?

Light.....one of the most plentiful substances in the universe, and one that is very hard to understand. (at least to me)

 

[drwayne]

Keep in mind that a photon does in fact have momentum, and it is conservation of momentum
that allows the solar sail to work - or allows a graduate student to impress his oral preliminary
committee that he knows how to do such a problem under duress.

Wayne

 

[Fallingstar1971]

After reading through the "What is light" thread I see that others have been thinking similar thoughts. Ill just throw one more into the mix.......

The solar sail works by transferring energy from the sun (or star) to the sail. The conductor for this energy is light. Without it, energy would not be able to travel to the sail or anywhere else.

So basically, (in electrical terms) The sun (or star) is the battery, the light is the wire or conductor, and the sail is the load.

Electricity is the movement of electrons through a conductive material like a copper wire. Electrons are forced to "hop" from one atom to another to another all the way down the line until the reach the load.

Could photons be "hopping" along a light beam until its reflected or refracted onto an object. (Is this "hop" the property that makes it look like a wave?)

Jeesh, its almost like its backwards. All mass, all objects absorbing or reflecting/refracting light with no conditions?

Electricity is lazy. Lazier than people. It always seems to take the path of least resistance.

Light has no conditions. It goes everywhere, regardless of resistance. Light does not appear to be lazy or take the shortest path.

But just how similar are electricity and light?

Could one "induct" light using coils? Like a "step up" or "step down" transformer in electricity? Take a small amount of potential and turn it into a larger one using magnetism?

Of course, in electricity there is a trade off. As you step up the pressure (voltage) you step down the current (amperage)

This is because of ohms law.

I wonder what would be lost in the induction of light?

 

[drwayne]

I think you might find these interesting:

http://www.thenakedscientists.com/forum ... _next=next

http://www.physlink.com/Education/askExperts/ae270.cfm

 

[Fallingstar1971]

Thank you wayne

So photons can transfer momentum. This force would be transferred to the sail upon impact. Now its starting to make some sense.

Thanks again

Star

 

[Fallingstar1971]

BTW thanks for those links. I am beginning to understand photon behavior a little bit better, but like all great things in the universe, the more you learn the more questions come up. But right now Im having trouble verbalizing my thoughts after reading those links. But thats where the fun is. Figuring it out. And light........gosh its like fuzzy logic, able to take on or adapt to whatever is needed (need a particle but no mass......light is for you. Does a particle totally mess up your equations, thats ok, Light can be a wave as well) Completely and totally amazing.

Thanks again

Star

 

[drwayne]

You are welcome. A fine gentleman who taught a number of astronomy courses that I took, Dr. Henry
Albers - first assigned me the clasic problem of radiation pressure about 30 years ago, and for some reason,
it stuck. Which did help me when a member of my Ph.D. committee, who was somewhat of a wise something-or-other,
tried to surprise me with it.

Interestingly, when he stated the problem, I had to tell him that he did not give me sufficient information
to do the problem. He somehwat snottily argued with me for a bit, and then asked what I *thought*
I needed to know. I responded calmly that he needed to tell me what the reflectivity of the object is.
You see, for a reflective surface, it is a hit and recoil momentum problem, where for an absorptive
surface it is a hit and stick problem.

After the oral exam, one of the committee members indicated that I passed the exam right at that moment.

Wayne

 

[arkady]

Light does not appear to be lazy or take the shortest path.

Light is indeed lazy as realized by Fermat while pondering the nature of prisms.

Fermat's Principle: The path taken between two points by a ray of light is the path that can be traversed in the least time.

 

[ramparts]

This is a pretty deep point - light, as with anything in a gravitational field under no forces, does indeed take the shortest path, even in curved space. These are called geodesics, and represent the equivalent of straight lines in curved space (where there can't actually be "straight" lines). What we see as gravity - as objects moving around in funny ways - is actually just them taking what they see as the shortest path

This extends even further - just about all modern physics theories, from general relativity to quantum field theory, are based on the universe's "laziness" is some way, taking whatever paths it can to minimize some quantity (called a Lagrangian, if you want to look it up!). Even the complicated laws Einstein discovered are just spacetime's effort to minimize the curvature of space, essentially, and there's a lovely bit of math you can do with this (it also allows you to modify general relativity in some fascinating ways, but that's a different topic!).

 

[mental_avenger]

You see, for a reflective surface, it is a hit and recoil momentum problem, where for an absorptivesurface it is a hit and stick problem.

Stated another way; For a reflective surface, it is an absorb and re-emit situation, whereas for a non-reflective surface it is merely an absorb situation.

 

[amaterasu]

Keep in mind that a photon does in fact have momentum, and it is conservation of momentum
that allows the solar sail to work - or allows a graduate student to impress his oral preliminary
committee that he knows how to do such a problem under duress.

Wayne

sorry for my amateuristic question but are you talking about photo-electric effect here? i mean that longer wavelength ~ lesser energy thing? and how about ionising radiation effects on the sail? i heard that a quarter of our sun's output is ionising.

intriguing discussion though. i wonder how realistic the future of sailing by the solar pressure torque can be?

 

[drwayne]

"sorry for my amateuristic question but are you talking about photo-electric effect here?"

Well, the math comes from the same place, since the momentum of a photon looks like:

P = h / lambda

Wayne

 

[amaterasu]

"sorry for my amateuristic question but are you talking about photo-electric effect here?"

Well, the math comes from the same place, since the momentum of a photon looks like:

P = h / lambda

Wayne

thanks.
so, it's really Compton effect which makes a solar sail work it seems?

E = hc / lambda

 

[ramparts]

Compton scattering is related, but it's not what causes the solar sail to work as it does. In principle, it's pretty simple: light particles have momentum, so even if they don't have mass, when they collide with objects, momentum is transferred, and in the case of massive objects, that translates to light "pushing" the object.

Just a matter of thinking about light in a different way

 

[amaterasu]

Compton scattering is related, but it's not what causes the solar sail to work as it does. In principle, it's pretty simple: light particles have momentum, so even if they don't have mass, when they collide with objects, momentum is transferred, and in the case of massive objects, that translates to light "pushing" the object.

Just a matter of thinking about light in a different way

i would reckon Compton scattering* is another way of explaining the kinetic energy/ momentum of the photon, at the quantum level, though, but yes, the solar pressure is far too small on the earth to be felt by us but it seems different in space due to much less air resistance there - combined that with ever hitting photons from our sun, solar sail crafts can supposedly run forever despite the force on it being weaker than that of the chemically propelled spacecrafts.

* http://prola.aps.org/pdf/PR/v21/i5/p483_1

 

[Vax]

Here are a few of a slew of questions that I get reading about this topic:

We know that this "craft" will be powered by concentrated photons collected (on the moon?) and sent colliding into the solar sail with enough momentum, it will be capable of reaching speeds of (insert answer here). Surely they do not expect they will be speeding about the galaxy at maximum warp.

Something you should consider about this idea is that it is in our means to utilize the power of the sun. However, to concentrate the amount of light/momentum it would take to actually move a craft through space at incredible speeds would be a very incredible feat in itself. Quite frankly it's out of our scientific league. You would have to first realize the size of the solar sail needed for it to be able to capture photons from great distances. It would have to be a thousand miles in diameter, not literally.

Then you must realize the science required to absorb energy from the sun, and concentrate it into to photon-beams for the sail to catch.

Here's some fun for all you kids at home!

Step #1: Take any ordinary flashlight from your house.
Step #2: Point it in the direction of your dads shiny red convertible, and switch it on.
Step #3: Watch in complete and utter amazement as the car moves in no which way.

Something we also have to remember is light becomes weaker over great distances. That might prove to be a big concern in the future.

What I'm trying to get at, is you would need first to construct a giant sail. Then you would have to construct small planetary outposts set up all of the solar system, perhaps reflecting concentrated beams from the sun, then from planet to planet, then to the solar sail. If that all runs smoothly, once you get out past the solar system you wouldn't have enough energy to make your way back to earth.

By all means though, I'm not bagging on NASA, I just think the idea is just a little far-fetched. But if we can somehow gather the resources to do this, go right on ahead. I'll be at a fundraiser with the "worm hole" people drinking beers and getting a good laugh.

...Or maybe we should all just stick to time-travel.

(This is a horrible post, but I think it has some good points. It's late for me I could be totally wrong so replies would be appreciated. If there is any way this stuff can work let me know what you got.)

 

[andrew_t1000]

I have always loved the idea of a light sail.
But the way I understand it, light or the photons from the sun is not what distorts the Earth's magnetic field.
Its the stream of charged particles.
Which brings up the idea of sailing a light sail down the south polar stream, out of the heliopause and out into the Galactic wind, logically we could assume if the Galactic Wind distorts the suns magnetic field as much as it does, it's going to make the solar wind seem like a gentle breeze.
Have either of the Voyager probes collected any data on how much Galactic Wind there is?
The idea of sending a light sail probe to Alpha Centauri in one persons lifetime is kinda appealing.

 

[Kpow]

I remember reading some sci-fi books in which a laser was mounted in the vessel and it projected a beam at the sail continuously until the halfway point then a separate laser was mounted in the front of the vessel to slow it down at the halfway point. Would this not work?

 

[MeteorWayne]

That's why it's sci Fi. If the laser's mounted on the vessel, you go nowhere.

 

[Kpow]

That's why it's sci Fi. If the laser's mounted on the vessel, you go nowhere.

I can see this if photons interatact with one another like molecules in a stream of water. Then the forward motion would be canceled by the backwards pressure of the stream but it seems like photons would be different? If we made the moon reflective and we fired a laser out of the back of a vessel in space would the vessel move assuming that the laser was sufficiently powerful?

 

[origin]

That's why it's sci Fi. If the laser's mounted on the vessel, you go nowhere.

I can see this if photons interatact with one another like molecules in a stream of water. Then the forward motion would be canceled by the backwards pressure of the stream but it seems like photons would be different? If we made the moon reflective and we fired a laser out of the back of a vessel in space would the vessel move assuming that the laser was sufficiently powerful?

Yes, but there is no need to aim the laser at a reflective moon. Just fire it out into space the momentum of the photons leaving the ship will impart an acceleration on the ship. Of course the accleration will be very very tiny....

 

[MeteorWayne]

What happened to the sail attached to the vehicle? I guess it blew away

 

[Kpow]

What happened to the sail attached to the vehicle? I guess it blew away

I was turning the question around to find out if the momentum of the photon was transferred to its source. I was trying to better understand why it would not work. While I understand that every action must have an equal and opposite reaction I had in my head the picture of a man on skateboard holding a fire hose and pointing it at a wall. When he turns on the water it will push back at him when it hits the wall. If he was on a sailboat and tried the same thing the forward momentum would be canceled by the backward momentum of the water on him. So I can see your point Wayne.

I was confused since it seems like the same principle but it is not really since the water is transferring force through itself backwards from the wall through the man and into the wheels of the skateboard whereas in space the momentum generated by the photon leaving and the momentum at the impact point of the photon are two separate events. The negative and positve momentums would cancel and give a net of zero, instead of having to wait for the impact force to travel back through the water it is imparted instantaneously at the source. It's more like shooting a machine gun at a wall that is on a skateboard while standing on the skateboard.

so I think I get it now amiright?

P.S. Thanks Origin you have shown me the light.

 

[MeteorWayne]

Closer, but still not yet. The wall isn't necessary, just as the moon isn't for photons.

"For every action, there is an equal and opposite reaction"

The mere act of squirting mass (water , ions, or photons) out in one direction, causes the squirter to move in the opposite direction, just as origin said. If you squirt a firehose into the air (the air is irrelevant) suddenly without being ready for it, it will still knock you on your can!

Take a garden hose with a nozzle attached. Hold the hose a foot or so from the end and turn the water on. Whether you aim it at a wall or not, it will still move backward.

It doesn't need anything to "bounce" off of.

MW

 

[drwayne]

I always found that the rocket problem was more constructively looked at as a conservation of
momentum problem.

A rocket is essentiallly a little guy throwing tennis balls out the back or a rocket.

Wayne