if photons in motion have mass and energy why don't they knockstuff over

wrote:

if photons in motion have mass and energy why don't they knock stuff
over
like little things like blades of grass or ants out of a tree

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

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


"Bjoern Feuerbacher" wrote in message
...
wrote:

if photons in motion have mass and energy why don't they knock stuff
over
like little things like blades of grass or ants out of a tree

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

In article , says...
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?

No - it's the higher momentum transfer to gas molecules from the warmer
dark sides of the vanes.

4B wrote:
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?

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

In message , Bjoern Feuerbacher
writes
4B wrote:
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?

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.

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

"4B" wrote in message ...
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


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

wrote:

like little things like blades of grass or ants out of a tree

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

Dear 4B:

"4B" wrote in message
...
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?

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

"4B" wrote in message ...
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?

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

Bjoern Feuerbacher wrote:

wrote:

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

like little things like blades of grass or ants out of a tree


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?

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
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net