Spinning Light: How Does it Work With No Mass?

[MNC007]

Imagine a glass sphere container. You took all the air inside so you have vacuum. Inside you have a very thin metal sheet that is mounted on an equator axis, so it is allowed to rotate. If you expose the mecanism to ligth it will spin...What´´s happening? howcome it spins if the photon doesn´t have mass?

 

[nitsuj]

I think it is heat energy that causes the thing to spin.

It may be that one side has to be colored darker then the other to cause it to spin.

Not sure if we are thinking of the same apparatus.

 

[tiny-tim]

Welcome to PF!

Hi MNC007! Welcome to PF!

howcome it spins if the photon doesn´t have mass?

The photon has momentum, which is all that is needed …

in principle, you could sail a spacecraft away from the Sun relying on "light pressure".

(btw, any such device probably still has enough air inside for there to be currents, or even for pressure from air on the hot side to be enough to turn it)

 

[MNC007]

I think it is heat energy that causes the thing to spin.

It may be that one side has to be colored darker then the other to cause it to spin.

Not sure if we are thinking of the same apparatus.

i think so... but in order to be a thermal mechanism it could only be convection and for that you would need air.

 

[nitsuj]

cool, I never thought of heat like that. heat is a "state" not something on it's own.

I can see now that in a true vaccum, heat would have nothing to do with it.

I guess mass and momentum aren't directly related in the sense I thought.

I thought momentum was a consiquence of mass & velocity. Perhaps it is just a consiquence of velocity.

 

[tiny-tim]

welcome to pf!

hi nitsuj! welcome to pf!

… I thought momentum was a consiquence of mass & velocity. Perhaps it is just a consiquence of velocity.

momentum isn't a consequence of anything, momentum just is …

momentum is 3 of the 4 coordinates of the energy-momentum of a body

photons are created with energy and momentum already installed and running

 

[MNC007]

Hi tiny-tim, Thanks!
so... a massless particle can have momentum. i thougth the sailing example only apllied to sun wind (ie sun particles) and not light... i think i´ve got it but i´m still kinda confused. my phisics are pretty basic..

 

[ZapperZ]

Hi tiny-tim, Thanks!
so... a massless particle can have momentum. i thougth the sailing example only apllied to sun wind (ie sun particles) and not light... i think i´ve got it but i´m still kinda confused. my phisics are pretty basic..

Please start by reading the FAQ thread in the General Physics forum.

Zz.

 

[Drakkith]

Hi tiny-tim, Thanks!
so... a massless particle can have momentum. i thougth the sailing example only apllied to sun wind (ie sun particles) and not light... i think i´ve got it but i´m still kinda confused. my phisics are pretty basic..

Yep, we actually have a spacecraft that has already used this method of propulsion, the IKAROS spacecraft .
See here for more info on the Solar Sail. http://en.wikipedia.org/wiki/Solar_sail

 

[rcgldr]

My guess is that you're referring to Crookes radiometers. The internal movement is due to the molecules in a particle (not full) vacuum) flowing from the cooler brighter sides of the vanes to the hotter darker sides.

http://en.wikipedia.org/wiki/Crookes_radiometer

 

[MNC007]

My guess is that you're referring to Crookes radiometers. The internal movement is due to the molecules in a particle (not full) vacuum) flowing from the cooler brighter sides of the vanes to the hotter darker sides.

http://en.wikipedia.org/wiki/Crookes_radiometer

That´s it! So the reason it moves is because it is not in a perfect vaccuum... It still has enougth air for the convection effect. Probably the same experience with perfect vacuum would not work... unless the photons had enough momentum to move it.