Light Can Twist as Well as Spin

NASA -- Astronomy Picture of the Day ^ | 11-19-03 | apod

[Boot Hill]

Explanation:   Light is more complicated than we thought. When astronomers measure light, they are usually concerned with its direction, energy, and spin polarization (sometimes). Recently, however, it has been more broadly realized that photons can also have orbital angular momentum (OAM), an attribute classically analogous to the Earth orbiting the Sun as well as spinning on its axis. Pictured above, the wave-front of a photon with OAM is shown to be twisted, in contrast to the flat plane of zero OAM light. Light with OAM might be used to increase the information content of communication or to discern specific types of astronomical sources. Passing through a common lens, light without OAM focuses to a point, whereas light with OAM focuses to a ring. Most light bouncing around the cosmos, however, is expected to have so little (or zero) OAM that the created ring is too small to measure. Even given other promising methods for measurement, exploiting OAM for astronomical discovery might be as much an issue of observational practicality as theoretical possibility.

[richtig_faust]

You are making that assumption based upon the particle nature which is indeed related to plank's constant (but you forgot the divisor of 2*pi). BUT doppler shift is a frequency shift measurement, they are NOT measuring momentum, they measure FREQUENCY.


A good example of equipment that indirectly measures angular momentum is a Magnetic Resonance Imaging machine which catches frequency shift based upon the change in angular precession.

[edwin hubble]

Orbital Angular Momentum of Photons

MEASURING THE ORBITAL ANGULAR MOMENTUM OF SINGLE PHOTONS
http://www.physics.gla.ac.uk/Optics/projects/singlePhotonOAM/

and this one from Phys Rev Letters:
http://optics.org/articles/news/8/6/14/1

Single photons show momentum state
18 June 2002

An optical technique reveals the orbital angular momentum state of single photons for the first time.

A new technique that measures the orbital - rather than the spin - angular momentum of single photons could lead to the development of super-efficient quantum communication systems. Previously, physicists had only been able to measure this quantity for many photons in a beam, or detect a certain value of it for single photons. But the set-up devised by Johannes Courtial of the University of Glasgow and colleagues should be able to reveal any orbital angular momentum state of a single photon (Phys. Rev. Lett. 88 257901).

[RadioAstronomer]

Thanks for the links :-)

[ValerieUSA]

Light penetrates the darkness like a drill?

[ValerieUSA]

The helix is so dynamic, it may be God's signature on His creation.....

[Boot Hill]

richtig_faust says:   "you forgot the divisor of 2*pi."

I forgot no such thing. Energy equals h*c, not h-bar*c. The equation properly uses Planck's constant (h), not Dirac's constant (h-bar). Planck's constant is the correct choice in the above equations.

richtig_faust says:   "BUT doppler shift is a frequency shift measurement, they are NOT measuring momentum, they measure FREQUENCY."

Of course they measure frequency, nobody has suggested otherwise. However the result of a decrease in momentum of a photon is a commensurate decrease in the observed frequency.

--Boot Hill

[Boot Hill]

Interesting way of viewing it!

--Boot

[Consort]

The old Chubby Checker Effect.

[MaeWest]

Cute. Took a minute.

[richtig_faust]

However the result of a decrease in momentum of a photon is a commensurate decrease in the observed frequency.


But not angular momentum

[richtig_faust]

Well, i'm not a physicist, i'm an engineer, so as far as my knowledge goes I would have to say no. I could be wrong though.

[PatrickHenry]

Are there any forces in interstellar space that would cause a decrease in the momentum and hence frequency of photons, leading to an observed red shift caused by something other than recessional velocity?

There's a gravitational redshift. Light leaving a very massive body will have predictably stretched wavelengths, and thus it will appear to be shifted into the red end of the spectrum. It's a well-known phenomenon, and taken into consideration by astronomers. I don't think this affects momentum, however.

If I'm wrong, I invite RA to clean up behind me.

[richtig_faust]

I don't think this affects momentum, however.


I was thinking of the exact scenario you mentioned, I believe the phase velocity would have to change in order for this to occur - and according to general relativity I do believe this is the case.

[mikegi]

Off topic here but I wonder if you'd take a look at a page I put on the web and give me some comments? The page contains a condensed version of John N. Dodd's Semiclassical Compton Effect theory. Semiclassical means Maxwell plus Special Relativity. I run through the equations including where I think Dodd makes an error (where I write "Dodd assumes that all this energy is added to the ZM electron’s total energy").

What I like about Dodd's theory is that it gives a physical explanation for the quantized interaction of light and a charged particle. The quantization is not due to energy coming in bundles but instead is due to the electron transitioning between steady states. The first state in the Compton Effect is rest. The second is when the electron has been accelerated by a circularly-polarized electromagnetic wave to a constant speed in the wave's direction by the magnetic force. Superimposed on this constant downstream speed is an orbital motion due to the rotating electric field of the incident em wave.

Dodd's theory covers the first half of Compton (absorption of the incident photon and scattered frequency) but has no explanation of the apparent concentration of scattered energy in the photon direction or the scattered electron direction.

Anyway, if you're interested, here's a link: Dodd's Semiclassical Compton Effect.

[Boot Hill]

richtig_faust says:   "But not angular momentum."

LOL, OK, I surrender, have it your way, there are no consequences for a loss of momentum!

(frames 4 & 5)

--Boot Hill

[Boot Hill]

Buckhead asks:   "Are there any forces in interstellar space that would cause a decrease in the momentum and hence frequency of photons...?"

Yes! Here is a list...

1. Since I first posted my speculation to you about the possibilities of so-called twisted light (light with "orbital angular momentum"), I have found that it has already been demonstrated that twisted light does indeed have a "red shift" which is a consequence of a loss in the linear momentum of the photon caused by a transfer of linear momentum to (orbiting) angular momentum.

   CITATION:
   "MECHANICAL EFFECTS OF SPIN AND ORBITAL ANGULAR MOMENTUM: SPINNING AND ORBITING and ROTATIONAL DOPPLER EFFECT"

2. Light colliding with interstellar gas, resulting in a loss of momentum and thereby a loss in frequency has been shown to cause a red shift.

3. As "PatrickHenry" correctly pointed out in #53, there is also a gravitational caused red shift. Again, as with all the others causes noted, the red shift is due to a loss of momentum.

4. And of course there's our old perennial favorite, the Doppler or velocity induced red shift.

One final word:   When I speak of a "loss" of momentum, I don't want to mislead you. Momentum must be conserved. So to be precise, it is not a actually a "loss", but rather a transfer of momentum from the linear motion of the photon to other things. Sort of like taxes!

--Boot Hill

[PatrickHenry]

I donno. The mass of the photon (whatever that is) won't change, nor it's speed (in vacuuo). So I don't see any changes in momentum. Only changes in wavelength seem likely (and spin and twist -- whatever that amounts to). Anyway, thanks for a truly momentous post.

[richtig_faust]

You have to admit... I may not be right, but i'm damn persistent... he he