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

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

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

In article ,
Bjoern Feuerbacher wrote:

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



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

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

Richard Schumacher wrote:

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

The rhodopsin molecules in the cones take quite a beating.

Bob Kolker

"Richard Schumacher" wrote in message
...
| Well, that's a common misconception: actually, photons are knocking things
over
| all the time.

Yep, all you have to do is push on something unstable. Over it goes. With
nothing but photons between your hand and the object.

FrediFizzx