# How to infer that the black hole in M87 is rotating clockwise?

The spokesperson mentioned that the rotation of M87 supermassive blackhole is rotating in the clockwise direction by looking at the bright and dim region of the image taken, is it the resolution of the images or looking at the direction of the astrophysical jet from the blurry images? Exactly how did they infer the rotation does any one know?

The ring of light is produced by plasma that is swirling around the black hole. Light (microwave emission at 1.3 mm wavelengths) is bent by the extreme gravitational field so that many light rays are bent in our direction by passing close to what is called the "photon sphere". i.e. The ring of light is not the accretion disk, it is a magnified halo of light caused by gravitational lensing of the general illumination surrounding the black hole.

However the black hole is likely to be spinning and its spin-axis is likely to be aligned with the large scale "jet" that is seen to emanate from the core of M87. The direction of that jet is more-or-less E-W as you look at the picture, but the jet is also coming towards us, with an angle of just 17 degrees between the jet axis and our line of sight.

The black hole is therefore likely to be spinning either almost clockwise or anti-clockwise as we view it from Earth.

The spin of the black hole means that plasma falling into it will be dragged in the direction of rotation at relativistic speeds. As it does so, any light emitted will be Doppler boosted in the direction that the material is moving.

That 17 degree tilt then becomes important, because there is a component of the velocity of the material circulating around the black hole that is either towards or away from us. Evidently, from the picture, we see brighter light to the south, which means that material circulating around the black hole is coming towards us from south of the black hole (and hence there is more light from here), but away from us to the north of the black hole. To arrange this, the spin-axis of the black hole must be aligned with the jet (i.e. it's projected spin-axis is roughly E-W on the picture) but pointing in to the picture, with most of the spin then being characterised as clockwise as we view it, but with a component rotating into the picture at the top and out of the picture at the bottom.

Our article "Measurement of the spin of the M87 black hole from its observed twisted light", available in preprint form from https://arxiv.org/abs/1904.07923, might shed some light on this (no pun intended). Be warned that it may be hard to read and digest for non-experts.

To explain what we did: We managed to extract the intercepted 1.3 mm radio wavefront and its phase from the brightness temperature images made public by the EHT team 13-24 April 2019. From these reconstructed profiles, we calculated the photon orbital angular momentum (OAM) spectrum of the radiation. By analysing and comparing the asymmetry of the OAM spectra from two two-day datasets we were able to determine not only the sense of rotation (clockwise) of the M87 black hole (M87*), but also the M87* rotation parameter (~0.9) and its inclination (17 degrees). This allowed us to estimate the M87* rotational energy (10^64 erg). This grotesquely high energy is comparable to the energy that would be emitted during one billion years from the strongest quasar that we know.

• Hi Bo, I downvoted this because it's not an answer to the question. Moreover, considering that most readers here are non-experts, the link provided is not very helpful. If you can summarize the arguments of your paper in an answer here, it would be great. I think it's okay if the answer is for experts, but it should be answer here, not just a link to you paper. As it stands not, it mostly looks like self-promotion. – pela May 6 at 9:48
• Our article explains explicitly how we used the EHT data from M87 to show that the black hole rotates clockwise, precisely what the question is all about. That we were also were able to measure, from the same data, the rotational speed and the inclination of this black hole cannot possibly be a reason to downvote my comment! – Bo Thidé May 6 at 9:56
• I didn't downvote because you were able to measure the rotational speed and the inclination, I downvoted because rather than answering the question, you merely link to a paper. Answers on SE are expected to be "self-contained". Why not extract the parts of your paper relevant to the OP's question and summarize it here? I'll be happy to upvote! I think a non-expert version is to be preferred since >99% of the readers here are non-experts, and it will deffo have more readers, but that may just be my opinion. – pela May 6 at 11:20
• OK. I have now added a detailed description how we did it. – Bo Thidé May 6 at 14:42
• I'm sorry, but while this might be a great abstract for your talk at the BHI Conference in 2 weeks, with all respect I really don't think it is very helpful on a site like this. You basically say that you calculate, analyse, and estimate. It might just be me who don't get it, so I'll remove my downvote. – pela May 6 at 19:47