# if photons in motion have mass and energy why don't they knock stuff over

Discussion in 'Digital Photography' started by Old Man, Apr 8, 2004.

1. ### Old ManGuest

Quantum or Classical, photon or field, p = E / c (sans h).
Sans mass, waves and photons push, and they push twice
as hard when reflected. Momentum is conserved. Observe
Compton. [Old Man]

Old Man, Apr 8, 2004

2. ### Bjoern FeuerbacherGuest

Do the math. Visible light has a wavelength of 5 * 10^(-9) m. Momentum
of a photon is p = h/lambda. Thus p is approx. 1 * 10^(-25) kg m/s. Even
for gamma rays, whose wavelengths are about 10^(-6) smaller, the
momentum of a photon is only about 1 * 10^(-19) kg m/s. I don't think
that could knock even an ant or a blade of grass over - do you?

Bye,
Bjoern

Bjoern Feuerbacher, Apr 8, 2004

3. ### 4BGuest

What about those small propels that "mysteriously" spin when they are put
close to a source of light. Is not the momentum of the photons that make
them spin?

4B

4B, Apr 8, 2004
4. ### CeeBeeGuest

If morons like you have an Internet connection, why are they obsessed
with trolling and crossposting?

Go visit the dark side of the moon, and report back if light knocks you
over there.

CeeBee, Apr 8, 2004
5. ### Bjoern FeuerbacherGuest

1) Don't top-post, please.
2) *Lots* of photons are acting there together.
3) You need an apparat with very few friction forces for this to work.
4) It is constructed especially to maximize the effect, by making one
side black and the opposite one light-reflecting.

Bye,
Bjoern

Bjoern Feuerbacher, Apr 8, 2004
6. ### Martin BrownGuest

And the cheap ones sold commercially spin the wrong way since the vacuum
in them is relatively poor. Warm gas molecules recoiling on the black
side of the vane dominate the forces acting.

When you pump them down to much lower pressures the recoil of photons
off the mirror side eventually dominates. It is quite a fun demo.

Regards,

Martin Brown, Apr 8, 2004
7. ### Paul CardinaleGuest

No. The vanes are black on one side, white on the other. This
results in a temperature differential which causes a pressure
differential.

Paul Cardinale

Paul Cardinale, Apr 8, 2004
8. ### Robert J. KolkerGuest

How about making holes in a steel plate. A sufficiently powerful laser
can put a hole in a 1/2 inch steel plate. If the light is coherent it
can pack a punch.

Bob Kolker

Robert J. Kolker, Apr 8, 2004
9. ### Don StaufferGuest

A number of deep space probes have had solar reflector attitude control
panels. These use the photons to help stabilize or move the craft to a
new attitude using the photon momentum. There have been many proposals
over the years for solar sail propulsion for space probes.

You don't ordinarily see the effect because the pressures are very small
compared to pressures from even the most gentle breeze or air current.
But they ARE there.

Don Stauffer, Apr 8, 2004
10. ### N:dlzc D:aol T:com \(dlzc\)Guest

Dear 4B:

I'll let you do your own search. Those devices are called "radiometers".
If the envelope they are contained in contains any gas, then the black side
moves away from the light source. If there is no gas, the white side moves
away from the light source. I have never seen a radiometer with the white
side moving away from the light, so I have never seen a completely
evacuated one.

I wonder if any of the space missions ever took a non-enclosed radiometer
up with them?

David A. Smith

N:dlzc D:aol T:com \(dlzc\), Apr 8, 2004
11. ### puppet_sockGuest

The usual setup for such a critter is a small "turbine" with,
most often, four blades. Each blade is black on one side and
silvery on the other. The "glib" explanation is that the silvery
side reflects light, and so gets double the push from each photon
that hits it.

The problem with the glib explanation is, the typical device of
this type that you can buy in a novelty store or such, turns the
wrong way. They spin such that there would appear to be more
push on the dark sides of the turbine than the light. Then you
find out that the bulb this thing is built in does not contain
a vacuum, but only a very thin gas.

See, what's really happening is, at least in the "novelty store"
version of this thing, the black sides are warmer. And warmer
means the gas next to them is warmer. And that means the gas
molecules move faster. So it's a little gas turbine, powered by
the energy of the light involved.

Let's think about the approximate force that would be involved
from light pressure. Say you've got bright sunlight, which is
roughly 1 killowatt/m^2. (Depends on lattitude, clouds, etc.,
but we are just doing an estimate here.) Suppose the fins were
2 mm across, giving an area of 4E-6 m^2. So you've got a power
of 4E-3 Watts. The force is power/c or a bit more than
1E-11 Newtons. What does the turbine in one of these gizmos mass?
Say it's .01 grams, or 100,000 of them to the kilo. Probably
masses more than that, but let's play. So you've got F/m to
give you the approx acceleration. You'd need to work out the
torque and know the shape of the silly thing to get it correct.
This will probably get us within a factor of 3 or so. So F/m
gives 1E-11/1E-5 or 1E-6m/s^2. So to get the thing going to
1 cm/s would take about 1E4 seconds, or about 3 hours. And that
is neglecting any friction. And my experience with these gadgets
is that they speed up very quickly in bright sun, and slow down
again fairly quickly when you put them in less light.

So it's probably not light momentum turning the fins.
Socks

puppet_sock, Apr 8, 2004
12. ### puppet_sockGuest

Um? A number of craft have had solar power panels. Which craft
have had solar reflector attitude control?
Proposals yes. Examples built, none.
Socks

puppet_sock, Apr 8, 2004
13. ### John NavasGuest

[POSTED TO rec.photo.digital - REPLY ON USENET PLEASE]

Mariner IV (the first Mars probe, which I worked on back in 1964) had solar
pressure vanes, mounted on the ends of each of the four solar panels, that
adjusted automatically when the attitude jets fired. After they found the
right balance, the need for attitude gas was so greatly reduced that the
spacecraft used only a fraction of the attitude gas that would have otherwise
been required. <http://nssdc.gsfc.nasa.gov/nmc/tmp/1964-077A.html>

The earlier Mariner II (the first Venus probe) had a solar sail, but it was
less effective than the four solar pressure vanes on Mariner IV.
<http://nssdc.gsfc.nasa.gov/nmc/tmp/1962-041A.html>

John Navas, Apr 8, 2004
14. ### William C. KeelGuest

The solar panels on Mariner 10 were used for attitude control
via radiation pressure, by tilting them appropriately between
Mercury encounters. This conserved attitude-control gas and
helped give the third flyby. See
http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1973-085A
http://www.fukuoka-edu.ac.jp/~kanamitu/study/solar/solar/marin10.htm

This was possible because the solar panels had to be large enough to
deliver power not only at launch but during a Venus swingby, so
there was more tolerance for off-normal pointing. In fact, the
status report at
indicates that the controllers had to tilt the panels on some direction
anyway for thermal control after the first Mercury encounter.

Bill Keel

William C. Keel, Apr 8, 2004
15. ### Rene TschaggelarGuest

Lets take a mol of these photons then.
You can indeed lift glass spheres with a laser.
It needn't be especially powerfull actually.
A few mW are sufficient.

Rene

Rene Tschaggelar, Apr 8, 2004
16. ### Rudy GarciaGuest

Indeed!

And if the radiometer (the small propeller thingie) has a vacuum inside
the glass bulb, the propeller spins in one direction, but, if the there
is air inside, it spins in the other direction.

Rudy Garcia, Apr 9, 2004
17. ### Old ManGuest

In sci.physics, regardless of the OP's expectations, the point
is always physics, and there's plenty of that in this thread.
Jonathan is requested to peddle his cracked pottery elsewhere.

[Old Man]

[snip diarrhea]

Old Man, Apr 9, 2004
18. ### Old ManGuest

At the time of writing, PoincarĂ© probably knew little about
relativistic invariance. He is forgiven. Jonathan isn't. Even
Newton knew that acceleration and rotation rate weren't
relative. Both are absolute and self-referential. Overcast
skys that hide the "fixed stars" aren't an obstacle to their
measurement. Local, self-referential, measurements are
guarantied to reveal Mother Nature. Global concoctions
are the fairy tales of metaphysics. The local speed of light
is absolute. E(dot)B and E^2 - B^2 are local relativistic
invarients. For electromagnetic plane waves, they are both
absolutely zero, no matter the observer's relative velocity.
[Old Man]

Old Man, Apr 9, 2004
19. ### Don StaufferGuest

What is the one leaving solar system? Voyager, I believe it is. The
attitude control paddles are on the ends of the solar array panels.
These paddles are not the entire attitude control. If I remember right,
there are CMGs for primary, gas jets to desaturate wheel, and the solar
paddles. The paddles take some of the load off the CMGs, so they do not
approach saturation as often. There is a limited supply of gas, and
using it too often to unload CMGs would exhaust gas sooner.

Now, my memory is not as good as it should be, so maybe it was not
Voyager, but one of the later probes that went out to Jupiter and Saturn
(Mariner?). I know it was a deep space probe series, but it has been a
lot of years since I worked with it, so I am having a hard time
remembering its name. Anyway, it is one with four solar arrays, with a
paddle at end of each array. Clearly visible in pictures.

Don Stauffer, Apr 9, 2004
20. ### Richard SchumacherGuest

Well, that's a common misconception: actually, photons are knocking things over
all the time.

Richard Schumacher, Apr 9, 2004