Spinning Black Holes Twist Light

[astroron (Ron)]

A way to Identify Black holes in galaxy centers
http://www.bbc.co.uk/news/science-environment-12434007
Cheers

[mswhin63 (Malcolm)]

Sort of wondering if this looks similar to gravitation lensing?
If so could the so called dark matter we see be isolated black holes travelling in an empty part of the universe. Difficult to accept as it should be sucking in some matter around producing a small amount of light around the event horizon.

Just a silly question

[CraigS]

Firstly, the Detection method:
The light is detected and passed through interferometers. The arms are then rotated with respect to one another and another stage of the interferometer sorts photons with even and odd orbital momentum into separate exit ports. The odd valued ones are sent into a hologram designed to increase their orbital momentum. They manipulate the prisms involved and separate out photons with discrete whole numbered momemtum values. These values are then a strong indicator of the orbital momentum of the photons.

Originally Posted by mswhin63

Sort of wondering if this looks similar to gravitation lensing?

Lensing is observed for light coming from behind the suspected BH, (say from another galaxy). The resulting light received by us on the line of sight is a distorted image of the galaxy which originally emitted the light.

If the intervening BH was spinning, then I'd say that the light from the distorted galaxy, would return the predicted results from the orbital angular momentum detector, (outlined above).

Originally Posted by mswhin63

If so could the so called dark matter we see be isolated black holes travelling in an empty part of the universe. Difficult to accept as it should be sucking in some matter around producing a small amount of light around the event horizon.

Just a silly question

As mentioned above, in this case, the light would probably have to come from some external background source (eg a galaxy). If the BH passes between us and a background light source along our line of sight, then we'd receive the 'twisted' light.

SMBHs would probably generate the same phenomenon courtesy of their accretion disks.

I wonder whether Quasar and Pulsar light exhibits the same phenomenon ?
(If so, their orbital angular momentum measured by this gizmo would probably return a different value when compared with a rotating BH).

Numerous surveys have found that rogue BHs, could constitute only a tiny percentage of the total amount of Dark Matter (and are referred to as MACHOs). Their detection in those measurements involved the use of lensing. Can't see any reason why that 'lensed' light couldn't be passed into the interferometer, to see whether it was rotating or not (and to quantify the mass of the rotating object).

Cheers

[CraigS]

Interesting .. Physorg's article provides more detail on how they're doing the detection. It is here.

It appears the information in my previous post, which was sourced from the BBC referenced AstroPhys published paper, dated 2003, was the original paper which documented the hypothesised method.

The Physorg article cites a revised mehtod of detection, by using modelled phase distributions of the light …

In the latest work, Fabrizio Tamburini of the University of Padova in Italy and colleagues instead show how to detect the rotation by measuring changes to the light from a distant star or from the disk of accreted material surrounding a black hole.

They point out that a wavefront travelling in a plane perpendicular to the black hole’s axis of spin will get twisted as it passes close to the black hole, since half of the wave front will be moving in the direction of advancing space-time and the other half in the direction of receding space-time. In other words, the phase of the radiation emanating from close to a rotating black hole should have a distinctive distribution in space.
..
They say the way to measure it is to point an array of radio telescopes at the centre of the galaxy, using different telescopes to observe different segments of the approaching wave front, and then superimpose these segments to calculate their relative phase. This procedure would be repeated, each time the telescopes pointing to a different section of the tiny patch of sky surrounding the black hole.

The principle behind the two descriptions is the same but interestingly, the more recent work describes the principle using the 'twisted wavefront' analogy (not the photon spin model as per the original concept paper) …

As a result of thinking about it all from the wavefront analogy, they developed a way of doing the observation using a phased array of radio telescopes .. far easier than using interferometry …

Fascinating .. always worthwhile thinking about other ways to 'skin a cat' (apologies to the cat lovers .. no offence intended).


Cheers

[bojan]

OK, let's first define/clarify what is "twisted light" ?
Is it circular polarisation? And what else it could be.. spin?
How spin is manifested in terms of RF signal and what is it's distribution? Does it mean "twisted light" has preferred spin of it's photons?

[CraigS]

Originally Posted by bojan

OK, let's first define/clarify what is "twisted light" ?
Is it circular polarisation? And what else it could be.. spin?
How spin is manifested in terms of RF signal and what is it's distribution? Does it mean "twisted light" has preferred spin of it's photons?

G'Day Bojan;

Welcome back to the Science Forum !


From the first paper:

Photons are endowed with spin angular momentum along their direction of propagation.
Beams of photons all carrying the same spin are circularly polarized.
Less well known is that photons can also carry orbital angular momentum (OAM), 'l', quantized in units 'h'. Curtis, Koss, & Grier (2002) have produced beams of photons each with OAM as high as l = 200h.

They differentiate between the two (spin, and the spin angular momentum along the direction of propagation). The latter is quantised and in the model, one of its discrete values can be applied to light having been effected by a rotating BH (as distinct from other values, which can be used to infer other effects).

Cheers

[bojan]

Originally Posted by CraigS

G'Day Bojan;

Welcome back to the Science Forum !


From the first paper:


They differentiate between the two (spin, and the spin angular momentum along the direction of propagation). The latter is quantised and in the model, one of its discrete values can be applied to light having been effected by a rotating BH (as distinct from other values, which can be used to infer other effects).

Cheers

So, in a nutshell, we are talking about circular polarisation of the incoming radio signal.
The receiving of such signals is nothing really new.. Helical and some patch antennas are circularly polarised.

They are suggesting that the radio signal, passing near BH will be circularly polarised as a consequence of passing near rotating BH ?

[Robh (Rob)]

Weren't they making the point that it is not due to spin angular momentum (polarisation) but orbital angular momentum, a result of a relativistic effect i.e. frame-dragging.

Here's another article on the effect ...
http://www.universetoday.com/83247/m...emos-in-light/

Regards, Rob

[CraigS]

Originally Posted by CraigS

They differentiate between the two (spin, and the spin angular momentum along the direction of propagation).

Oops .. I accidentally left the word 'spin' in the above sentence (typo).
Apologies for any confusion.

They flip around in their terminology usage in the paper as well. Sometimes they refer to simply Orbital Angular Momentum (OAM) and Photon Orbital Angular Momentum (POAM), which makes it a little confusing as well. Both of these terms refer to the same property, ie: that induced by the rotating BH.

I yield to Rob's article .. much clearer.

Cheers

[bojan]

Hmmm..
So how is this supposed to be manifested in received signal?

[CraigS]

Originally Posted by bojan

Hmmm..
So how is this supposed to be manifested in received signal?

See the quote in Post #4, and check out the original paper (Section 3 page 1267 called: ASTRONOMICAL INSTRUMENTATION TO MEASURE POAM, 3.1: Dove-Prism Mach-Zehnder Interferometers).

Cheers

[sheeny (Al)]

Originally Posted by bojan

They are suggesting that the radio signal, passing near BH will be circularly polarised as a consequence of passing near rotating BH ?

Not necessarily, as I understand it. I've just been reading about it today from Nature's article on the same. As I understand it, if you measure the polarisation of the radio waves passing near the black hole, with multiple radiotelescopes simultaneously, the frame dragging will be evidenced by a change in polarisation of the light between the radiotelescopes at any given time.

So the polarisation of the light is changed by the frame dragging.

Al.

[Robh (Rob)]

Here's some more on twisted light ...
http://www.popularscience.co.uk/features/feat8.htm

The link (click on this page) gives some more technical stuff with some nice diagrams.
Note the statement ...
"It is important to understand that the light is not following a helical path. The phase of the light is changing in such a way that it describes a helix."

Regards, Rob

[CraigS]

Thanks for that article, Rob.

I think its an extremely cool way to find evidence in support of BHs.

I really hope they can apply it to a 'suspected' BH asap.

Cheers

[bojan]

Very interesting...
I wonder how wide antennas (or receptors) must be spaced to detect the phase shift.
Comparable to the size of BH? or smaller...

[bojan]

Actually, what I have just written in the previous post was a nonsense.
What will be detected is the phase shift of (narrow band) signal around the image of the potential BH.

[CraigS]

It would seem that they would need the array to get sufficient resolution to make sure the light wasn't coming from anything other than the accretion disk of the object in question. (Ie: using the resolving power to improve pointing accuracy). The extra gain would also be handy.

Looks to me, like they're creating interference patterns to measure the phase differences in different parts of the image. This holographic plate must enable them to establish the interference pattern (a bit like a diffraction grating ... in 3D)

Interesting, none-the-less.

I wonder how they differentiate between different objects having different grav. fields ? I mean, it would also work for other objects like a neutron star wouldn't it ? I guess the more the intense the phase shift pattern is, the more intense the grav field of the object. I also guess the calibration of the model involves a lot of theoretical calculations ..

Cheers