A question about momentum

[rat156]

Hi All,

First post in the hard science side of the forums, take it easy on me please...

Anyway, I'm taking pretty pictures of RCW58 at the moment, some of the descriptions of the nebula mention that the shape is formed by light colliding with the gas from the expelled gas from earlier events. Now, I can see how this would energise the gas clouds and make them emit light, a fact that I'm now taking advantage of, but several websites describe this and other similar nebulae as "light blown", i.e. the pressure from the intense UV radiation is actually moving and shaping the gas clouds.

I was thinking about this on my drive to work this morning (where other more macro physics problems should have been entertained, but the traffic wasn't moving), how can something without mass transfer kinetic energy? How can "light pressure" actually move something?

Cheers
Stuart

[sjastro]

Photons carry energy E=hv where v is the frequency and h is Plancks constant.
When photons are scattered inelastically by particles some of the energy of the photon is lost. Since energy is conserved the lost energy is transferred to the particle in the form of kinetic energy, ionization energy and/or excitation energy.

Regards

Steven

[pdalek]

The amount of momentum carried by a photon is small.
e.g. The light pressure from the sun on the ISS solar panels (2500 sq m) gives a force about half that due to the weight of a 5c coin. Solar wind is a thousand times weaker.

W-R stars give off lots of light. Would give the ISS the same push as a small rocket engine (like a SEPR 844).
W-R stars also produce lots of stellar wind. The wind is material lifted from the star surface directly into space by the high photon pressure.
Both this wind and the light from the star push on other gas or dust which may be present.

The Encyclopedia of Astronomy and Astrophysics has a good basic section on photon pressure and stellar wind:
http://www.bartol.udel.edu/~owocki/preprints/encyc_hsw.pdf
More detail, from same author (about 100 pages):
http://www.bartol.udel.edu/~owocki/preprints/Oleron-review-Oct03.pdf

[Dave2042]

Not a dumb question at all.

On a general level, your question presupposes a distinction between particles (solid things like little cricket balls) and waves (insubstantial 'disturbances' in some field or background substance). Of course it's obvious the former can have a momentum and transfer it when they run into something, while the latter can't. And the latter can diffract around corners while the former can't.

This all makes perfect sense in the everyday world, however when you go into the tiny quantum world, it turns out the distinction breaks down. Things like electrons and photons are particles and waves at the same time, so photons have momentum (as per Steven's equation) and transfer it when they bump into things and electrons can diffract around corners.

Loopy, but real.

[sjastro]

Since energy and momentum are conserved, the "classical" equations for energy and momentum are interchangeable. (In relativity energy and momentum are linked into a single identity).

Instead of using the equation E=hv, one can use the photon momentum equation p=hv/c. In this case part of the momentum of the photon is transferred to the particle for inelastic collisions.

Also hard UV photons can indirectly impart momentum changes on matter. For example a photon can ionize or strip electrons from an atom. The resulting electrons carry momentum which is further increased by scattering photons. These electrons collide into matter imparting momentum.

Regards

Steven