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Is this new image (below) of polarized light surrounding Sagittarius A, showing actual frame dragging being captured by the magnetic field? The image is from this article If not, how would a photo showing frame dragging in the magnetic field look any different?

Sagittarius A

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    $\begingroup$ Does the source of the image make such a claim ? Without knowing a great deal about the very complex image processing that must have gone into this image, I doubt that anyone can say with any certainty just how it should be interpreted. $\endgroup$
    – gandalf61
    Commented Mar 28 at 17:01
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    $\begingroup$ @gandalf61 All the article says is "twisting and turning like a spiral," but I would not expect an NBC article to talk about something as GR specific as frame dragging. $\endgroup$
    – foolishmuse
    Commented Mar 28 at 17:11
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    $\begingroup$ So the NBC article links to this EHT press release eventhorizontelescope.org/blog/… but there is no mention of frame dragging. $\endgroup$
    – gandalf61
    Commented Mar 28 at 17:31
  • $\begingroup$ @foolishmuse The thin lines on the image are fake. They were not photographed, but added on top of the image by those who claim that the image implies more than it actually shows. The evident resolution of this image is 3 by 3 pixels, so about 10 pixels total (just pixels, not megapixels). If you take a 3 by 3 photo of your yacht, no one would be able to tell it from a black hole ;) Plus they used a specific imaging technique that showed “a donut”. Well, when my camera is out of focus, any light looks like a donut. So there is exactly zero confidence in this “Event Horizon Telescope” project. $\endgroup$
    – safesphere
    Commented Mar 30 at 4:42
  • $\begingroup$ @safesphere I did not realize there was so much imagination involved in the photo. Thanks $\endgroup$
    – foolishmuse
    Commented Mar 30 at 17:03

1 Answer 1

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It is not frame dragging, it is a visualisation of the linear polarisation present in 230 GHz light emerging from the vicinity of Sgr A* at the centre of the Milky Way. But getting an answer took some detective work...

The press release links to a paper by the Event Horizon Telescope Collaboration. This paper present images of the region around the (very likely) black hole at the centre of M87 in circularly polarised (CP) light. The CP signature is v "both weak and sensitive to calibration errors". The CP signature is a couple of vague blobs with only very crude spatial structure at a resolution roughly similar to the Schwarzschild radius, with positive and negative CP either side of the centre. This paper does not contain the image in your press release and is nothing to do with Sgr A*, the black hole at the centre of the Milky Way.

So, after that wild-goose chase, I tracked down the correct paper, which is also from the EHT collaboration and is a study of the linear polarisation (LP) and CP present in the emission from the region immediately surrounding Sgr A*. In addition to the problems inherent to analysing the M87 polarisation data, the Sgr A* dataset is also afflicted by short timescale (20s) variability and interstellar scattering. However, they do detect a very strong LP signal and it is from this that the visualisation has been created.

It makes more sense to see the "raw" data (though note that the image has been reconstructed from interferometric data) and the visualisation together (as they are in the paper). Here it is in the top plot: They grey scale shows the total intensity of the 230GHz radiation, the tick marks show the direction of the electric field vector polarisation angle (for the linearly polarised component) and the colour scale indicates what fraction of the light is linearly polarised. The white dotted contours indicate the strength of the linearly polarised component.

The bottom plot is the image in the press release and has been created by overlaying dark "streamlines" of linear polarisation over the top of an orange-tinted grey-scale image of the total intensity, in a loosely defined way. The electric polarisation vector ticks in the upper plot have been given a length and opacity that is proportional to the square of the polarised intensity.

EHT resconstructed image

Thus the answer to your question is that this has nothing to do with frame-dragging. It is a visualisation of what the linear polarisation field looks like (the dark lines aren't "real").

Another point to mention is that the image is not a direct image of the accretion flow around the black hole. The ring-like structure is a consequence of light rays passing from material near the black hole, approaching the photon sphere at $1.5 r_s$ and being bent around into our line of sight. This ring would be seen from whichever direction we view the black hole and for any orientation of the accretion flow.

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    $\begingroup$ +1 It might be worth noting that "streamline" plots (or "streamplots") are a pretty standard tool for visualizing vector fields. E.g., pyplot has a function to do this sort of thing. $\endgroup$
    – Mike
    Commented Mar 28 at 19:19
  • $\begingroup$ Would an image showing frame dragging in the magnetic field look any different? Would there be more lines or less? A tighter twist? Anything else. $\endgroup$
    – foolishmuse
    Commented Mar 28 at 19:54
  • $\begingroup$ @foolishmuse - not sure I understand. It is supposed to be a spinning black hole, so whatever frame dragging there is, is there... $\endgroup$
    – ProfRob
    Commented Mar 28 at 19:59
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    $\begingroup$ @foolishmuse I think the key point here is that there is no evidence at all of frame dragging. Frame dragging results from spin of the central black hole, and EHT don't mention any measurement of that. (Other recent work varies wildly from $\chi <0.1$ to $\chi>0.9$ out of [0,1].) On the other hand, there is surely ordinary angular momentum of the material flowing into the BH, which is a lot more important to the spiral structure that you see in this picture. (Besides, all those pictures of frame dragging are not really showing frame dragging anyway.) $\endgroup$
    – Mike
    Commented Mar 29 at 14:14
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    $\begingroup$ @Mike thanks for pointing out that issue of ordinary angular momentum. That clarifies it significantly. $\endgroup$
    – foolishmuse
    Commented Mar 30 at 17:04

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