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

In article , (Tom Kirke) writes:
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

They do. A recent ( last year? ) NOVA had a segment about a guy
in New Mexico who blasted aluminum disks about 20 meters straight
up using a laser DOE had left over from fusion research. He could
keep them up as long as the laser was on and the disk was in the
laser beam.

If he was doing that in air, he wasn't using photon pressure.

Assume photon pressure is responsible.

Assume a 1kg aluminum disk. That's 1 kg * 9.8 m/sec of momentum
per second. Divide by 2 because that aluminum better be a damned
good reflector. E = pc. So you have 1 kg * 9.8 m/sec / 2 * 3 * 10^8
meters /sec ~= 7*10^8 joules every second = 700 megawatts of light power
hitting an aluminum disk massing 1 kg.

Assume 99.99 reflectivity. That's 70 kilowatts of heating power.
Divide by 4.2 to get calories/sec, multiply by 10 because the
specific heat of aluminum must be pathetic, divide by 1000 because
it's one kilogram and you have around 150 degrees Celsius per
second temperature rise, back-of-the-envelope.

Note that disk mass factors out of the result. The bigger the
disk, the bigger the mass, the more power you need and dividing
out the thermal mass leaves you back where you started.

Poof goes the aluminum disk, inside of ten seconds.

I imagine you could pull the trick off by ablating aluminum. That'd
cut your power requirements quite a bit and take care of the cooling
concerns at the same time.

Because of the inefficencies of the process it takes a lot more
than a few mW laser to do this.

Also simple glass-bulb radiometers demonstrate photon momentum.

Actually, they demonstrate the opposite. They demonstrate that
whatever photon pressure there might be is inadequate to offset
the temperature and pressure effects induced by those same photons.
They spin the wrong way.

John Briggs

In article ,
wrote:

In article ,
(Tom Kirke) writes:
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

They do. A recent ( last year? ) NOVA had a segment about a guy
in New Mexico who blasted aluminum disks about 20 meters straight
up using a laser DOE had left over from fusion research. He could
keep them up as long as the laser was on and the disk was in the
laser beam.

If he was doing that in air, he wasn't using photon pressure.

Assume photon pressure is responsible.

Assume a 1kg aluminum disk.

Much too heavy, this was a highly polished Al disk about
20cm x 1mm:

10cm^2 x 0.1cm x Pi x 2.7 gm/cc ~ 84 gm = 0.084 kg.

It was in fact a whopping BIG laser, DOE had used it
in fusion research. Also since this is a reflection
effect you multiply by 2, not divide.

...pressure effects induced by those same photons.

What pressure effects, they're in a vacuum. If you
mean the pressure of the photons, then you agree that
they have mass.

tom

--
We have discovered a therapy ( NOT a cure )
for the common cold. Play tuba for an hour.

In article , (Tom Kirke) writes:
In article ,
wrote:

In article ,
(Tom Kirke) writes:
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

They do. A recent ( last year? ) NOVA had a segment about a guy
in New Mexico who blasted aluminum disks about 20 meters straight
up using a laser DOE had left over from fusion research. He could
keep them up as long as the laser was on and the disk was in the
laser beam.

If he was doing that in air, he wasn't using photon pressure.

Assume photon pressure is responsible.

Assume a 1kg aluminum disk.

Much too heavy, this was a highly polished Al disk about
20cm x 1mm:

10cm^2 x 0.1cm x Pi x 2.7 gm/cc ~ 84 gm = 0.084 kg.

Still way too much. On total reflection it takes 150 MW per Newton.
So, for the disk above you would need 12.6 MW. That's CW power.
Ain't gone get it.

It was in fact a whopping BIG laser, DOE had used it
in fusion research.

The lasers DOE used in Fusion research were pulsed, not CW.

Also since this is a reflection
effect you multiply by 2, not divide.

...pressure effects induced by those same photons.

What pressure effects, they're in a vacuum.

Eh?

If you mean the pressure of the photons, then you agree that
they have mass.

Nope. They have momentum. They don't have mass.

Mati Meron | "When you argue with a fool,
| chances are he is doing just the same"

skrev

In article ,
(Tom Kirke) writes:

They do. A recent ( last year? ) NOVA had a segment about a guy
in New Mexico who blasted aluminum disks about 20 meters straight
up using a laser DOE had left over from fusion research. He could
keep them up as long as the laser was on and the disk was in the
laser beam.

If he was doing that in air, he wasn't using photon pressure.


They used an infrared laser to heat air:

http://members.nova.org/~sol/station/planetar.htm

(Fourth-Generation RLV : beam-assisted launch vehicles.)



--
Øystein Olsen, , http://folk.uio.no/oeysteio
Institute of Theoretical Astrophysics, http://www.astro.uio.no
University of Oslo, Norway

Tom Kirke 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


They do. A recent ( last year? ) NOVA had a segment about a guy
in New Mexico who blasted aluminum disks about 20 meters straight
up using a laser DOE had left over from fusion research. He could
keep them up as long as the laser was on and the disk was in the
laser beam.

Because of the inefficencies of the process it takes a lot more
than a few mW laser to do this.

The glass spheres lift with few mW because they have only
10um diameter in this case.

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