"Greg Goss" <email@example.com> wrote in message news:buqcijF4m7fU1@mid.individual.net > "Michael J. Strickland" <firstname.lastname@example.org> wrote: > > > "Greg Goss" <email@example.com> wrote > > > > I love conspiracy theories, though I'm rational enough to set them > > > aside when I'm done with them. > > > > > > My favourite at the moment is that "chemtrails" is a project to > > > disperse high altitude sulfates to cancel out some global warming. > > > After all, we started departing from the model predictions about > > > 2000. > > > I'm not so sure because the soot would probably just have absorbed > > incoming and outgoing heat and shed it to the rest of the air. > > > When something is perfectly reflected (like light into a mirror), it > > delivers twice the momentum (and 4 times the energy, E = p^2/2m) as > > when it is absorbed. Any attempt to "screen" out radiation with air > > chemicals will just double the energy deposited in the air after > > the reflector molecules are accelerated and shed their energy to > > neighboring molecules. Absorbing chemicals will only add half as > > much energy to the air. > > > > How is this "chemtrail" cancelation supposed to work. If its trying > > to increase screening, I don't have much hope for it. If its trying > > to react the chemicals with existing greenhouse molecules to > > precipitate them out, it might work. > > Soot and sulfate work differently. Soot absorbs the energy and > re-radiates it. ("black body")
With loss, or else the soot would not heat up.
> Sulfate reflects it. When something is perfectly reflected it absorbs > double the energy? Momentum maybe, but the energy departs back into > space. Energy is a scalar amount, not a vector.
No, an amount of energy equal to twice the incoming energy of the photon is deposited in the reflector as recoil kinetic energy.
The total energy of the system (photon and reflector molecule) must remain the same before and after reflection since no external energy is added.
The initial energy of the incoming photon is E_i = pc.
After reflection, the photon's energy is E_f = -pc since its momentum (p = hf/c) has been reversed (c -> -c).
In order for system energy to be conserved, the reflector must acquire an energy E_r = 2pc.
E_i = E_p + E_r or pc = -pc + 2pc
This balances the energy equation.
Therefore, the reflector molecule must acquire energy, E_r = 2pc.
This energy shows up as recoil energy for an isolated atom. For a lattice of bound atoms it shows up as macroscopic kinetic energy because the lattice atoms are bound to their neighboring atoms by much stronger forces than the incoming photon delivers.
Alternative explanation: You could say the photon is red-shifted to a lower frequency (and energy) upon reflection due to the Doppler imparted by the reflector which starts moving away in the opposite direction. This seems kind of non-causal to me though, since the reflector is originally stationary when the photon hits it.
This is how a solar sail accelerates (gains kinetic energy).
Think about it. How can the reflector acquire linear momentum (mv) without acquiring translational kinetic energy (1/2 mv^2)? It can't. If v increases, both energy and momentum increase.
Btw, I believe maybe energy should be expressed as vector, but that's another story.
> If you shine a bathroom heat lamp onto a mirror, are you saying that > the back of the mirror gets HOTTER than the guy standing next to the > heat lamp?
Heat is random direction energy (e.g. vibration). A solar sail acquires energy but little heat (random motion) because its molecules are tightly bound in a lattice so they can't move independently of the atoms they are bound to like a liquid or gas molecule. The energy is given to the crystal lattice as a whole, so the whole lattice experiences a force away from the light source. Not so for individual gaseous reflector molecules. They are not tightly coupled in a lattice and absorb the energy individually when they "reflect". They recoil and transfer energy to neighboring atoms which shows up as heat (random motion).
For the heat lamp, the mirror gains energy and is in fact pressed against the wall and both heat up some due to their electrons clouds being pushed together. Again, since its solid, its electrons and atoms are tightly bound in a lattice by much stronger forces than the incoming EM and so incoming light tends to move the entire lattice of the mirror away from the light.
The guy standing next to the heat lamp is a better absorber than the mirror molecules. His water molecules are more mobile (unbound) and absorb (vibrate)well in infra-red which he feels as heat.