Reply to: The Great Global Warming Swindle

[TimberWolf]

Momentum of the photons moving at 45 degrees remains as before (unless the photon changes energy

Actually, a couple of corrections to my post, this is a physics forum after all

Momentum is a vector quantity, so of course the photon momentum is changed by being bent by the lens in being refracted, I should have said the energy of the photon is unchanged. This isn't precisely true either though, since in order that both energy and momentum are conserved, either a new low energy photon is created (which I find unlikely) or energy is lost by the photon (its wavelength increases) to account for the lens being pushed away from the Sun and gaining some momentum. The lens won't actually get pushed radially away from the Sun, it would tend to be pushed into an elliptical orbit, though by choosing the right (more sunward) L1 position forces could presumably be balanced without needing to supply extra external energy/forces. I do hope we don't need quantum theory to get this solar reflector working... Anyway I imagine the astrophysicist who came up with the idea does this kind of calculation before breakfast.

[NickFitz]

I appreciate that crumpling could result as a reaction to photons being refracted in a direction perpendicular to the ecliptic, but I think momentum changes in a radial direction still need to be accounted for.

To clarify let's me try to explain what I mean in 2 dimensions, where say the Sun-Earth axis is called X and photons passing through the lens on this axis are refracted by some perpendicular angle, in a Y direction.

The net force on the lens in the Y direction is zero since photons are homogeneously refracted through the lens. This could potentially crumple the lens if it is weak or not spun though.

In the X direction photons come in on some axis parallel to the X-axis, and are refracted by some amount into the Y-axis. For example lets say they enter parallel to the X-axis and emerge 45 degrees to the X-axis. Momentum of the photons moving at 45 degrees remains as before (unless the photon changes energy?), but along X (and Y) this is now srqt(0.5) respectively. Momentum in the Y direction has been explained without needing to balance any books. To balance momentum lost in the X direction, the lens must experiences a force (sqrt0.5) away from the Sun

If that's how it works in two dimensions, I hate to think what the explanation in three dimensions is going to be like

[TimberWolf]

I don't think the transmitted light would have any effect in moving the lens relative to the Sun. Any light absorbed by the lens and re-emitted at longer wavelengths, as heat in effect, would presumably radiate equally in all directions.

But as the transmitted light is slightly divergent, its reaction would be a very slight tendency to "crumple" the lens towards its centre. and over time this would need counteracting by thrusters (or light reflected from the steering lenses I mentioned) to pull the lens back out in its own plane.

I appreciate that crumpling could result as a reaction to photons being refracted in a direction perpendicular to the ecliptic, but I think momentum changes in a radial direction still need to be accounted for.

To clarify let's me try to explain what I mean in 2 dimensions, where say the Sun-Earth axis is called X and photons passing through the lens on this axis are refracted by some perpendicular angle, in a Y direction.

The net force on the lens in the Y direction is zero since photons are homogeneously refracted through the lens. This could potentially crumple the lens if it is weak or not spun though.

In the X direction photons come in on some axis parallel to the X-axis, and are refracted by some amount into the Y-axis. For example lets say they enter parallel to the X-axis and emerge 45 degrees to the X-axis. Momentum of the photons moving at 45 degrees remains as before (unless the photon changes energy?), but along X (and Y) this is now srqt(0.5) respectively. Momentum in the Y direction has been explained without needing to balance any books. To balance momentum lost in the X direction, the lens must experiences a force (sqrt0.5) away from the Sun

[OwlHoot]

.. In addition to radiative pressure on the lens from partial reflection, wouldn't the divergent lens be subject to a second force, again radially directed away from the Sun, as it gains momentum lost by light rays bent off the radial (Sun-Earth) axis? ..

I don't think the transmitted light would have any effect in moving the lens relative to the Sun. Any light absorbed by the lens and re-emitted at longer wavelengths, as heat in effect, would presumably radiate equally in all directions.

But as the transmitted light is slightly divergent, its reaction would be a very slight tendency to "crumple" the lens towards its centre. and over time this would need counteracting by thrusters (or light reflected from the steering lenses I mentioned) to pull the lens back out in its own plane.

[TimberWolf]

A couple of Earth sunshade proposals are discussed here.

Arthur C Clarke, and more recently Gregory Benford, proposed a Fresnel mirror hovering at the L1 Lagrange Point between the Earth and the Sun.

It would be a large spinning circular disk of thin plastic, several hundred miles across (or lots of smaller ones) with an etched pattern of concentric circles that cause it to act like a concave lens and slightly diverge sunlight in a small range of wavelengths. But as you say, the proportion of sunlight it reflected back would cause a reaction tending to push it in the opposite direction, away from the Sun.

Maybe that could be counteracted by positioning it slightly closer to the Sun so a small net gravitational attraction toward the Sun would counteract the radiation pressure pushing it away. But being closer to the Sun might (?) give it a slight tendency to run ahead of the Earth's orbit. Also, the Moon would be slightly pulling it this way and that all the time. So you'd need thrusters to adjust its position.

Maybe rather than thrusters it would be simpler and cheaper to have several extra "steering mirrors" orbiting closer to the Earth and controlling the main Fresnel lens position by reflecting light onto reverse-facing mirrors round its circumference.

Hmm, interesting, thanks. I see that it looks possible in theory and also that some serious people have looked at this. I'd heard of something similar but thought it was just some half-baked idea. In addition to radiative pressure on the lens from partial reflection, wouldn't the divergent lens be subject to a second force, again radially directed away from the Sun, as it gains momentum lost by light rays bent off the radial (Sun-Earth) axis? This may only be a small amount though, especially since the angles involved are so small. It's hard to get an intuitive grip on the forces and directions in an optical/semi-transparent system - all of which would change significantly if the plastic lens should become bent or opaque

Perhaps a new L1 just has to be calculated to take into account the (in affect) weakened force from the Sun, that would be closer to the Sun than would be the case, if not for the extra forces it is subjected to from sunlight.

[NickFitz]

See linky for an honest discussion of the science.

I think you posted the wrong link - that site isn't an "honest discussion of the science", it's just some bozo's blog where he slags off anything that doesn't fit in with his political views, and praises anything that does. No discussion, no intellectual honesty, and definitely no science.

It's a bit like describing this as an honest discussion of modern English literary theory.

[Addanc]

You might think its some left wing conspiracy - but im a Tory voter, ...

Tim Nice but dim?

Man made globe warming is a load of bollocks; it is a device used by the political class to justify taxes by frighten people out of money. This technique is used over and over again e.g. look at the nasty terrorist threatening your existence, gives us honourable politicians more powers and we'll sort'em. Knobs like you just fall for it hook, line, sinker, fishing rod, angler, river bank ...

See linky for an honest discussion of the science.

[OwlHoot]

Presumably positioned at a langrange point? I've not seen the proposal, what's to stop sunlight pushing it out of position, very quickly?

A couple of Earth sunshade proposals are discussed here.

Arthur C Clarke, and more recently Gregory Benford, proposed a Fresnel mirror hovering at the L1 Lagrange Point between the Earth and the Sun.

It would be a large spinning circular disk of thin plastic, several hundred miles across (or lots of smaller ones) with an etched pattern of concentric circles that cause it to act like a concave lens and slightly diverge sunlight in a small range of wavelengths. But as you say, the proportion of sunlight it reflected back would cause a reaction tending to push it in the opposite direction, away from the Sun.

Maybe that could be counteracted by positioning it slightly closer to the Sun so a small net gravitational attraction toward the Sun would counteract the radiation pressure pushing it away. But being closer to the Sun might (?) give it a slight tendency to run ahead of the Earth's orbit. Also, the Moon would be slightly pulling it this way and that all the time. So you'd need thrusters to adjust its position.

Maybe rather than thrusters it would be simpler and cheaper to have several extra "steering mirrors" orbiting closer to the Earth and controlling the main Fresnel lens position by reflecting light onto reverse-facing mirrors round its circumference.

[TimberWolf]

It wouldn't have to reflect anything, only fan it out so more goes past Earth without hitting it.

Presumably positioned at a langrange point? I've not seen the proposal, what's to stop sunlight pushing it out of position, very quickly?

[TimberWolf]

Just remember that Arhenius modelled the greenhouse effect (the extent to which the earth with CO2 and H2O was warmer than it would be with only O2/N2) to a fair degree of accuracy in the 19th century. The basic mechanism does not require immense simulation models.

It's far more complex than that. and gets worse the harder you look. For instance a whole new carbon cycle involving continental subduction and volcanoes has been discovered since then which I believe is not well understood. They can only model what they understand, and can get any result they want.

[Marina]

...well a giant mirror put into space has been proposed to reflect the sunlight back, but I sort of wonder whether this may cause a positive feedback loop that might blow the sun up.

It wouldn't have to reflect anything, only fan it out so more goes past Earth without hitting it.

[xoggoth]

Just remember that Arhenius modelled the greenhouse effect (the extent to which the earth with CO2 and H2O was warmer than it would be with only O2/N2) to a fair degree of accuracy in the 19th century. The basic mechanism does not require immense simulation models.

While there does seem to be a lot of vested interest in promoting it which should make us sceptical of some of the claims and some data that does not seem to fit with the idea that it is the MAIN warming mechanism (specifically the profile of temperature with altitude is not right) do not forget that there is a great deal of uninformed and biased bulltulipe on the other side too eg the recent survey of a a lot of people with science degrees but no specific knowledge of the subject that does not prove a thing or the irrelevant and wrong claim that volcanos produce more CO2 emissions than mankind.

I would be interested to know how many of you more dogmatic lot have any knowledge whatever of radiant heat transfer or the other basic physical mechanisms involved. Come on then threaded, what's your qualification?

[lambrini_socialist]

Or possibly under artificial light for certain, er, varieties.

yeah, you can't beat the taste of a fresh, hydroponically grown tomato. and they smoke just great.

p.s. climate change deniers, deluded, obligatory Hitler comparison, yadda yadda, etc. i really can't be arsed today.

[sasguru]

i'm with you brother zathras! let's ressurect the dead threads!

I'm with you brothers. Lets send down the BB thread!
Viva La Revolucion, compadres.

[DS23]

i'm with you brother zathras! let's ressurect the dead threads!