A Black Hole’s Shadow and your Cloak of Invisibility

The buzz around the lunch table and my group’s D&D table is about the image of the black hole shadow in the galaxy M87 (M87*). People are talking, discussing, conjecturing and even imagining the future of space or time travel. Moments like this are great for both the scientific community and the public as it creates intrigue, wonder, and gets both communities talking with each other.

The shadow is really quite amazing, though it should not be confused with the event horizon as some lunchtime conversations have. The shadow of M87* is something separate from the event horizon and perhaps the best way to describe the shadow is to chat D&D, or Harry Potter if you prefer. Either way, we need to talk cloaks of invisibility.

Black holes are already invisible by their own right. They pull in light that comes towards them and trap it forever. This makes photographing them directly a pipe dream because there is never any light leaving them for us to see. The only way to see the presence of black holes is either when they have an accretion disk or strong jets, like M87*, or through their gravitational influence, like Sagittarius A* at the centre of our galaxy zipping stars around it at breakneck speeds.

A perfect cloak of invisibility might not absorb light like a black hole but instead works by bending the light around the person and focusing it on the other side again, giving the appearance of no one being in the way of the light. This type of cloak will also prevent you from ever being photographed because you too are not sending out any light to be captured by a camera. Unfortunately, not every treasure chest in your dungeon will have one, nor do most average witches and wizards have access to a personal Dumbledore. Us regular dungeon crawlers and novice wizards and witches are more likely to get a cloak of partial invisibility (or a cloak of un-invisibility, which is mostly good for a half decent ghost costume at your next themed party).

A perfect cloak of invisibility (left) and a black hole (right).

If we are lucky enough to get a partial cloak of invisibility it will bend light around you, but it may have some tells. The fringes might shimmer, objects might be blurry, or your feet are clearly visible because it is too short and there goes that lovely bonus on your sneak attack rolls (and forget that practical joke of levitating your friend’s lunch with wingardium leviosa unseen).

Black holes can act as a partial cloak of invisibility when lensing distant objects behind them. We still cannot see the black hole directly, but we see the multiple images of the object behind it being projected in a ring around the black hole. In this way, it gives itself away without revealing any interesting details about itself.

A cloak of partial invisibility (left) and a black hole lensing a distant object (right).

Like a lensing black hole, your cloak of partial invisibility is not working out very well and you’ve been spotted. If you can’t stay out of sight, maybe try being seen but not recognized. For this trick, you’ll need a hula hoop of light, available in lower level dungeons or your local Weasleys’ Wizard Wheezes.

By keeping the hula hoop of light spinning around you, you might give yourself away, but people will probably be too distracted by the really neat shadow you’re creating between you and the hula hoop.

Most of the light from the part of the hoop that is behind you is absorbed by your faulty cloak of invisibility. Some of the light, however, is bent around you and focussed in front of you ahead of where the absorbed light should have been focused. The region where the absorbed light should have appeared is dark because it has no light being bent into it – in effect a shadow.

Ah, but what of the light from the ring in front of you that is cast backwards you say? Sharp eye young adventurer (wizard or witch), but just like the light from behind, the light going from the ring towards you gets absorbed by the faulty cloak or bent around behind you – no reflected light reaches an observer. With this disguise, you might not go unnoticed, but you could try for the Guinness Book of World Records as the first shadow to ever hula hoop.

Cloak and hula hoop of light (left) compared with a black hole and photon ring (right).

In the case of M87* its hula hoop of light is the photon ring that surrounds it. Just like our hoop, most of the light from behind the black hole is pulled in, while some of it is bent around and refocused in front ahead of where the missing light would appear under perfect conditions – just like with our cloak.

This leaves a dark region in front of the black hole between it and the distorted image of the photon ring (The ring is larger on the bottom because it is rotating towards us at that point. The Doppler shift makes it brighter – that’s another article.). The shadow exists in front of the event horizon but behind the ring and this is why it is interesting. The shadow’s presence is sort of a rough outline of the event horizon, but unlike the event horizon, the light that goes into the shadow has a chance of escaping – albeit slim. This is also what makes the image so amazing, by seeing the shadow we are effectively looking at a black hole. We have finally taken a picture of the one thing we’ve not been able to take a picture of directly. By seeing a shadow we have an outline of the event horizon! I would call that rolling a natural 20 or a performing a perfect Patronus as far as photos go.


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