The situation you describe (for instance a horizontal linear polarizer followed by a 45° linear polarizer, which is in turn followed by a vertical linear polarizer) doesn’t have to be addressed at all by quantum mechanics and is perfectly described by using the wave nature of light. With all the light being horizontally polarized (after passing thru the first polarizer), it is incident on the 45° linear polarizer. The horizontally polarized light is made of components of 45° polarized light and 135° polarized light, thus the 45° polarized component passes thru the 45° linear polarizer. Similarly, 45° polarized light is made up of components of linearly polarized and vertically polarized light, thus the vertically polarized component passes through the laser filter to the user’s eye. There are several good references for Jones Calculus (which is used to calculate how polarization changes when passing through or reflection off optical components) on the internet.

Thanks for the explanation, that’s very interesting!

It is also possible to see the polarization of light with the human eye, but it’s difficult.

See: http://www.squidzone.ca/?p=204

Hi, I’m an author of the paper. I tried to answer a few other questions, but my posts seem not to appear. I will try once more….

The surface in this case is basically a surface in optical depth; a surface beyond which photons will have passed a certain depth of gas. When light of any kind travels through a gas the photons travel – on average – until the probability of their interaction becomes 1. Or very loosely, until they have crossed an optical depth of 1. What determines this optical depth, and the interaction probability at a given position, is just the density of the medium. That’s your surface – it’s a pretty deep one, but a ‘surface’ nonetheless.

Then, in the scenario where photons are produced centrally and travel outward radially, in every direction, at some point they traverse a high enough depth of gas that on average they reach the point where their interaction probability has become ~1. Here they scatter. Now, the critical point here is that for Ly-alpha and the hydrogen atom, the polarization fraction is highest when photons scatter at an angle of about 90 degrees. It’s analogous to Rayleigh scattering, and the reason why the blue sky is polarized in a ring around the sun. Thus photons that travel initially in the plane of the observations – the x,y plane only – that are observed after scattering, can *only* have scettered through ~90 degrees; otherwise they would not be observable. Photons that travel in our direction initially and scatter through 90 deg in any direction can never be seen. Similarly photons that star off coming in our direction, scatter, and are subsequently observed can not have scattered through 90 degrees, and will exhibit low polarization fractions. So in the case where photons are produced centrally and scatter at large radii, the prediction is an increase in the fraction of polarization with radius.

The gas cloud is being lit up by galaxies inside it. Since the galaxies are young, they’re probably undergoing furious bouts of star formation, which means they’re pouring out Lyman-α. It’s this that’s getting scattered and polarized by the gas cloud.

So can we say that these young, distant galaxies are all wearing one huge LAB coat then?

Make that Hanny’s Voorweep Voorwerp instead.
(No wonder it took me a while to find online images & info.)

See :

http://en.wikipedia.org/wiki/Hanny%27s_Voorwerp

&

http://apod.nasa.gov/apod/ap080625.html

&

http://blogs.discovermagazine.com/badastronomy/2011/01/11/voorwerp/

for comparison.

BTW. Wikipedia is featuring :

Astronomers announce that TrES-2b [aka Kepler-1b - ed.] has the lowest known albedo of any planet, reflecting less than 1% of the starlight falling upon it.

on its main front news page currently in case folks are interested.

I think I remember space-dot-com having a news item on that Hot Jupiter recently too.

Remarkable discovery – & great write-up BA thanks – reminds me of Hanny’s Voorweep a bit in appearance.

It’s fairly easy to see that light bouncing from an interface, such as the surface of water, or the hot air/cold air boundary above a road surface, would have an orientation based on that surface – oriented elsewise, it’s going to go elsewhere. But supposedly these galaxies are located within the LAB, and the edges of the blob are going to be too gradual to really make a good interface to bounce off of – and for that matter, the edges are pretty random…

Other polarized phenomena in space, I can understand – masers depend on one photon exciting the emission of the next, so it’s easy to see why their polarization would be aligned, and synchrotron radiation has the plane in which the radiation is being curved to provide alignment. Pulsars have their magnetic field – likewise sunspots and solar flares…

So – how do the photons communicate to know which way they’re supposed to line up in this LAB?

What I don’t get is this: how can you distinguish between a distant gas blob emitting ultraviolet, red-shifted to appear green, and a nearby gas blob emitting green?

I’ve often wondered that myself. My understanding now is that they look at the object’s entire spectrum, not just particular wavelengths. It’s not just “Lyman-α has been shifted into the green”, but rather “if we shift the entire spectrum back X, everything lines up with known emissions”, heavily implying that everything was shifted by X in the first place.

The trick is looking at a detailed spectrum. Various elements in the gas absorb very specific frequencies of light in distinct patterns, and looking at a spectogram will show lots of “drop outs” where frequencies have been completely or nearly completely absorbed. You can then line these drop-outs up, and figure out how much the lines have been shifted, telling you the distance of the intervening gas (and its composition: oxygen has different lines in a different pattern than, say, sulfur or iron or nitrogen).

Most often the intervening gas is in the envelope of the star. However, for distant galaxies, sometimes several sets of aborption lines can be found, corresponding to clouds of gas between the source and us, each taking out the same set of frequencies, each red-shifted by a different amount.

Another fun game is to go into a store, find some sunglasses that claim to be polarized, and hold them up together perpendicular and see that they aren’t nearly opaque as they should be, and then inform the clerk on duty that these glasses are not actually polarized and that’s false advertising. The game is to see if they care. Hint: Probably not!

For even more mindblowing polarization fun, you’ll need 3 pairs of sun glasses (or at least 3 lenses). Hold the two glasses perpendicular to each other and look through them. Theoretically all the light should be gone. Now put the third pair in between the first two at about 45 degrees relative to the others. The light is back! The fun of quantum mechanics.

I suspect the answer lies in the known emission wavelengths of hydrogen (and other elements). So, what if it was nearby and emitting green? What would it be then? Or at any of the intermediate distances, emitting something between green and ultraviolet? How are all of those infinite possibilities ruled out?

On another note. Did anyone see the ABC show on Psychics last night. I missed the last 10 mins, but I think they did a good job from a reality standpoint. I hope Phil gives his opinion about it.

numbing sex is for young galaxies.