Are there warm places in space?

[Pbartender]

It's called a radiometer.

The black side absorbing light (being "pulled" toward the light)

That's the conceptual problem you're having... Collisions don't pull, they push.

If you treat photons as particles, you can treat the problem as two different types of collision:

On the white side, the photons are "ricocheting" off of the paddle. Presuming an elastic collision in which kinetic energy is conserved, the massless reflected photon will retain practically all its energy and the paddle will gain virtually none.

On the black side, the photons are being absorbed into the paddle. Using the same inelastic model, the combined paddle plus absorbed photon have all the kinetic energy of the incoming photon. While this doesn't translate to much momentum, since the energy of a single photon is small and the mass of the paddle is comparatively large, the paddle now has kinetic energy equal to that of the absorbed photon, and it the same direction as the photon was traveling (away from the source of light).

So, you've effectively got nothing pushing on one side, and oodles of teeny tiny pushes on the other side. Given the quantity of photons hitting the black side, it adds up to be enough to make the paddles move.

 

[Bullgrit]

Thanks for the explanation. I understand completely, now. I guess I just misheard or misremembered the demonstration from my school days, and for all these years since, I've assumed physics matched what I thought. Until seeing the demonstration again and learning that physics really doesn't care much about what I think.

Edit: What makes this long-time, basic, misunderstanding funnier (to me) is that I love physics (except for the numbers) -- A Brief History of Time is one of my favorite books (have read it several times). And I read xkcd each week.

Bullgrit

 

[freyar]

It's called a radiometer.



That's the conceptual problem you're having... Collisions don't pull, they push.

If you treat photons as particles, you can treat the problem as two different types of collision:

On the white side, the photons are "ricocheting" off of the paddle. Presuming an elastic collision in which kinetic energy is conserved, the massless reflected photon will retain practically all its energy and the paddle will gain virtually none.

On the black side, the photons are being absorbed into the paddle. Using the same inelastic model, the combined paddle plus absorbed photon have all the kinetic energy of the incoming photon. While this doesn't translate to much momentum, since the energy of a single photon is small and the mass of the paddle is comparatively large, the paddle now has kinetic energy equal to that of the absorbed photon, and it the same direction as the photon was traveling (away from the source of light).

So, you've effectively got nothing pushing on one side, and oodles of teeny tiny pushes on the other side. Given the quantity of photons hitting the black side, it adds up to be enough to make the paddles move.

Actually, that's not right. (Here's the description on wikipedia, btw.) If the motion were due to the impacts of the photons, it would spin in the opposite direction (from white to black) due to conservation of momentum: when light is absorbed by the dark side, it imparts its original momentum; when the light bounces off the white side, it gains some negative momentum and therefore must impart more than its original momentum to the spinner. The momentum gained by the spinner is greater on the white side, making white move away from the light.

The real reason for the motion has to do with how the different colored sides heat the minute amount of gas remaining in the bulb. Wikipedia gives two explanations about that, but this site favors one of those specifically (the author of it is usually pretty good with getting things right).

 

[Pbartender]

Actually, that's not right. (Here's the description on wikipedia, btw.) If the motion were due to the impacts of the photons, it would spin in the opposite direction (from white to black) due to conservation of momentum: when light is absorbed by the dark side, it imparts its original momentum; when the light bounces off the white side, it gains some negative momentum and therefore must impart more than its original momentum to the spinner. The momentum gained by the spinner is greater on the white side, making white move away from the light.

The real reason for the motion has to do with how the different colored sides heat the minute amount of gas remaining in the bulb. Wikipedia gives two explanations about that, but this site favors one of those specifically (the author of it is usually pretty good with getting things right).

Fair enough.

I was stuck on correcting the "pulled by absorbing light" idea (both sides are being pushed, just one is being pushed harder), and remembered the original, and since disproved, theory, which muddled me up thoroughly.

I stand corrected and mildly embarrassed.

 

[Bullgrit]

The momentum gained by the spinner is greater on the white side, making white move away from the light.

Wait, what? So the spinner should move: white away from the light, black towards the light? That's what I had thought, and the demo was doing the opposite. Huh? Now I'm back to confused.

Bullgrit

 

[Umbran]

Wait, what? So the spinner should move: white away from the light, black towards the light?

"When exposed to sunlight, artificial light, or infrared radiation (even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing."

Or, another way of saying it - each vane move light side forward when the device is heated.

 

[Pbartender]

Wait, what? So the spinner should move: white away from the light, black towards the light? That's what I had thought, and the demo was doing the opposite. Huh? Now I'm back to confused.

Bullgrit

It should move as if the light was "pushing" on the black sides.

But the explanation I gave for why it does that is wrong.

Shortly: The vacuum inside the glass bulb is not perfect. Under light, the black side heats up the trace amounts of air nearby. This sets up minute currents that push on the black side and pull on the white side.

 

[Theo R Cwithin]

Wait, what? So the spinner should move: white away from the light, black towards the light? That's what I had thought, and the demo was doing the opposite. Huh? Now I'm back to confused.

One thing it helps to keep in mind: when you heat air, that means the particles that make up the air move faster.

Faster particles hit things harder than slower particles. Frex, think about a baseball: a professional pitcher's 100mph fastball hits a lot harder than a little kid's toss of the same ball.

Back to the radiometer:

When light hits the vane, the black side gets hotter than the white side.

So the air particles next to the black side heat up and move faster than those on the white side-- and therefore the air particles on the black side hit the black surface harder than the air particles on the white side hit the white surface.

Net result: there's more "push" on the black side than on the white side.

.
Is that clearer, or did I just make it even worse?? If so, apologies
.

 

[Bullgrit]

Ok, ok, I think I got it now.

Why couldn't we make a space ship move (away from the sun) by painting it's rear (facing the sun) black?

Or maybe make a space ship sort of out of a flashlight mounted behind a black panel. The light from the flashlight pushes the black panel which brings the flashlight along with it.

(By the way, I have no delusion that I'm asking brilliant or original questions. I'm just asking what comes to mind.)

Bullgrit

 

[Mustrum_Ridcully]

Ok, ok, I think I got it now.

Why couldn't we make a space ship move (away from the sun) by painting it's rear (facing the sun) black?

We can do this. It's not very fast, but it provides some propulsion. And the further away you get from the sun, the weaker the effect.

IIRC, probes like from the Voyager series that reached the edge of our solar system (as far as one can define such an edge) have experienced noticeable course changes due to the solar radiation. (But gravity from planets and solar winds in form of ionized particles also play a role.)

Or maybe make a space ship sort of out of a flashlight mounted behind a black panel. The light from the flashlight pushes the black panel which brings the flashlight along with it.

Actio = Reactio, as Newton said, IIRC. The flashlight does create a propelling force on the black panel, but also a propelling force on itself in the opposite direction. If you do not couple flashlight and black panel, it would work. As the distance grows, less light arrives at the black panel itself, making the effect weaker and weaker.
Better to use some kind of Laser, of course, as the beam will stay coherent for longer. But as long as you don't create a perfect laser beam, it will always widen a little over distance and you will lose efficiency.

Depending on how powerful you can get your laser, this might be a decent way. You could build a giant construction powering the laser and have a small probe "ride" on the beam. Of course, changing course is limited now.