"Alain" <ele### [at] netscapenet> wrote in message 
news:47274dcc@news.povray.org...
> ... Gamma radiation is light. The kynetic energy mass gets transformed 
> into light. And remember: light can push objects. It's just that this push 
> is normaly to small for you to notice, but it can easily be demonstrated 
> with a very simple experiment.
> Take an empty globe of glass, place a needle holding a light rotor made 
> from a glass axis and holding 3 or 4 blades, white on one side, black on 
> the other. Place the rotor on the needle. Remove all air from the globe. 
> Have ANY light shining on that rotor, even a candle light, and the rotor 
> will spinn. That experiment is over 100 years old!

I saw that experiment in a high-school physics lab three decades ago.  I was 
mightily impressed -- in fact it was one of my first "WOW" moments in 
science.  It completely turned my mind to suddenly realise that:

a.  Light actually has a mass that exerts force on "solid" matter.

b.  Even at typical levels, light's force on macro-scale objects is strong 
enough that I could stand in an average lab in an average country-town 
high-school and watch a finely-balanced vane turn in a vacuum, impelled by 
nothing but light.

Until then, I'd thought that the effects of things down at quantum 
dimensions could not be observed so simply at our size.

-- 
Brian

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Brian Elliott wrote:
> "Alain" <ele### [at] netscapenet> wrote in message 
> news:47274dcc@news.povray.org...
>> ... Gamma radiation is light. The kynetic energy mass gets transformed 
>> into light. And remember: light can push objects. It's just that this 
>> push is normaly to small for you to notice, but it can easily be 
>> demonstrated with a very simple experiment.
>> Take an empty globe of glass, place a needle holding a light rotor 
>> made from a glass axis and holding 3 or 4 blades, white on one side, 
>> black on the other. Place the rotor on the needle. Remove all air from 
>> the globe. Have ANY light shining on that rotor, even a candle light, 
>> and the rotor will spinn. That experiment is over 100 years old!
> 
> I saw that experiment in a high-school physics lab three decades ago.  I 
> was mightily impressed -- in fact it was one of my first "WOW" moments 
> in science.  It completely turned my mind to suddenly realise that:
> 
> a.  Light actually has a mass that exerts force on "solid" matter.
> 
> b.  Even at typical levels, light's force on macro-scale objects is 
> strong enough that I could stand in an average lab in an average 
> country-town high-school and watch a finely-balanced vane turn in a 
> vacuum, impelled by nothing but light.
> 
> Until then, I'd thought that the effects of things down at quantum 
> dimensions could not be observed so simply at our size.
> 

It is impressive but the understanding that the rotation is caused by 
light pressure in the way you recall is incorrect.

The commonly known instrument is a Crookes Radiometer 
(http://en.wikipedia.org/wiki/Crookes_radiometer) and the rotation is 
due to a thermal effect in the thin gas near the vanes.  It is 
definitely not a vacuum inside.  In fact if the gas pressure is reduced 
to approach a vacuum the effect disappears.

Light pressure would not produce a strong enough effect to show up here.

Also you'll find that the white/silver sides rotate towards the light 
rather than the reverse that you would expect if the cause was momentum 
being transferred by light bouncing off the more reflective side of the 
vanes.

The proper explanation can be found in the Wikipedia article.

A true light pressure effect was measured using a Nichols radiometer 
(http://en.wikipedia.org/wiki/Nichols_radiometer).  Not nearly as 
spectacular or easy to produce in the typical classrooms.

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Arttu Voutilainen wrote:
> Bill Pragnell wrote:
>> Darren New wrote:
>>> Bill Pragnell wrote:
>>>> Warp was trying to clarify his understanding of momentum (which can
>>>> be a tricky concept to separate from energy if you've not learned
>>>> physics to quite a high level) and this isn't likely to help him. :-)
>>> I plead guilty to drifting off-topic on p.off-topic!  :-)
>> Ha, that's nothing, they're discussing rendering algorithms further
>> down. Now that's definitely off-topic for this group... ;-)
> 
> But.. if it's off-topic, then doesn't it just belong here? ;)

Checkmate. You win again, gravity! :-)

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Alain <ele### [at] netscapenet> wrote:
> Take an empty globe of glass, place a needle holding a light rotor made from a 
> glass axis and holding 3 or 4 blades, white on one side, black on the other. 
> Place the rotor on the needle. Remove all air from the globe. Have ANY light 
> shining on that rotor, even a candle light, and the rotor will spinn. That 
> experiment is over 100 years old!

  AFAIK that's the same kind of flawed experiment as the "measure the weight
of an empty balloon, then fill it with air and then weight it again" which
supposedly demonstrates the weight of air. (In actuality that's completely
silly. It's like trying to demonstrate the weight of water underwater by
weighting an empty balloon and a balloon filled with water: You won't get
any difference. What this air balloon experiment actually demonstrates is
the compressibility of air and that compressed air inside the balloon has
a higher density.)

  AFAIK that "experiment" does *not* work in complete vacuum and, moreover,
makes the blades rotate in the wrong direction. The real reason for the
rotation is that the black sides get heated while the white sides don't,
and the heated black sides heat up the air close to them, making the air
push that side of the blade.

  AFAIK photon pressure would not be even near enough to cause the rotation
of such small blades with such (relatively) huge mass.

-- 
                                                          - Warp

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And lo on Tue, 30 Oct 2007 14:54:20 -0000, Jim Henderson  
<nos### [at] nospamcom> did spake, saying:

> On Mon, 29 Oct 2007 15:17:18 -0500, Warp wrote:
>
>>   Then wikipedia is horribly wrong, I suppose.
>
> Wikipedia isn't always right....

Blasphemer! May a thousand sysops poop on your head :-)

-- 
Phil Cook

--
I once tried to be apathetic, but I just couldn't be bothered
http://flipc.blogspot.com

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Brian Elliott wrote:
> Until then, I'd thought that the effects of things down at quantum 
> dimensions could not be observed so simply at our size.

Here's another one:

Go into a place selling polarized sunglasses. Hold up two pairs a few 
inches apart at right angles. No light comes thru, obviously.

Now, without moving those glasses, put a third one in between at a 
45-degree angle. Suddenly, light comes thru.

When I see a ray-tracer do *that*, I'll believe in "using physical 
equations". ;-)

-- 
   Darren New / San Diego, CA, USA (PST)
     Remember the good old days, when we
     used to complain about cryptography
     being export-restricted?

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Darren New a écrit :
> Brian Elliott wrote:
>> Until then, I'd thought that the effects of things down at quantum 
>> dimensions could not be observed so simply at our size.
> 
> Here's another one:
> 
> Go into a place selling polarized sunglasses. Hold up two pairs a few 
> inches apart at right angles. No light comes thru, obviously.
> 
> Now, without moving those glasses, put a third one in between at a 
> 45-degree angle. Suddenly, light comes thru.
> 
> When I see a ray-tracer do *that*, I'll believe in "using physical 
> equations". ;-)
> 

If you are really into that kind of things, there is at least a paper 
about that here:
http://www.cg.tuwien.ac.at/research/publications/2001/wilkie-2001-crp/

So it has been done, just not in a widely known renderer... Perhaps 
because the effect is not so ubiquitous in everyday life yet ;-)

-- 
Vincent

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On Wed, 31 Oct 2007 16:56:23 +0000, Phil Cook wrote:

> And lo on Tue, 30 Oct 2007 14:54:20 -0000, Jim Henderson
> <nos### [at] nospamcom> did spake, saying:
> 
>> On Mon, 29 Oct 2007 15:17:18 -0500, Warp wrote:
>>
>>>   Then wikipedia is horribly wrong, I suppose.
>>
>> Wikipedia isn't always right....
> 
> Blasphemer! May a thousand sysops poop on your head :-)

Ah, well, that's already happened.  (I was a SysOp on CompuServe years 
ago <g>)

Jim

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Jim Henderson escribió:
> On Mon, 29 Oct 2007 15:17:18 -0500, Warp wrote:
> 
>>   Then wikipedia is horribly wrong, I suppose.
> 
> Wikipedia isn't always right....

Then edit it.

I hate people saying "don't use wikipedia because it's wrong". Make it 
right instead of whining!

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>> Friction against what?  Newton's 3rd law...
>
>  Against the ground

Equal and opposite torque on the person and the Earth ... angular momentum 
is still conserved.

> and the air.

Same for the air - assuming you take the air as being part of the "Earth". 
If not, then well you've got an external force on the system and angular 
momentum will not be conserved (but at some point the air will have some 
other effect on the Earth that will cancel out, the wind can't keep getting 
faster and faster for ever).

>  Friction causes heat, and the energy for that heat must come from 
> somewhere.
> I cannot be produced from nothing.

The heat is produced from the energy you need to maintain the force over 
time (ie work done), eg from your food that you use to push your legs, or 
the electricity in some electric motor.  Without that there would be no 
force against the frictional force and the spinning would eventually stop.

Momentum (both angular and linear) is ALWAYS conserved in a system with no 
external forces, always always always.  It's simply impossible to change the 
momentum of a system without applying some external force (or torque).

Energy is also always conserved, but you have to consider ALL the energy 
going into and out of the system, including heat, kinetic, any potentials in 
gravitational/magnetic fields, chemical etc.  Look up the laws of 
thermodynamics for further info.

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