I was reading an article online from the Express when I read the following: “Black holes are often found at the hearts of galaxies and up until April this year have been purely theoretical.“ The statement threw me for a bit of a loop, mostly because black holes have been an active part of astrophysics since the discovery of Cygnus X-1 in 1971. Using the day to day word theoretical misleads the general public into thinking that black holes were not confirmed in science until this past April with the discovery of M87*’s shadow. This is a dangerous statement to make.
In day to day language, theory is used to mean a hunch or an educated guess. However, in the sphere of science, a theory is an explanation of what something is or how it works. Theories are often large bodies of work and research and are quite detailed.
Take gravity for example. Newton’s law of universal gravitation does not explain what gravity is, it only shows mathematically how two bodies affect each other through gravitational attraction. It is the theory of general relativity that explains how gravity works and what it is (a result of mass curving space-time). Black holes are part of the general theory of relativity. To start they were a mathematical quirk of Einstein’s field equations, but the discovery of Cygnus X-1 showed that black holes were not a mathematical quirk.
Further observations about black holes have been made over time adding to the theory and supporting the existence of these massive objects. Accretion disks, relativistic jets, active galactic nuclei, gravitational waves and other observations were all documented well before the Event Horizon Telescope document M87*’s shadow.
None of these observations, including the observation of a shadow, have moved black holes out of the realm of scientific theory. Instead, they help keep black holes as active parts of the general theory of relativity.
Science writers and communicators need to take extra caution when using terms like theory, theoretical, law, fact, hypothesis or conjecture. They have different meanings inside and outside of the sphere of science and using them inappropriately can build a false impression of what science does; that in turn can cause misunderstandings and mistrust of science as a whole.
I am not without fault and am often rereading and refamiliarizing myself with the different terms. It is part and parcel of the job as a science communicator. One of my go-to’s is this article from liveScience.com. Writer or reader, it is probably worth a second look and remember to give critical consideration when you see those terms in an article.
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).
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.
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.
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.
On April 10th The Event Horizon Telescope Collaborative released an image so exciting that I, like my parents with the Moon landing, will remember where I was and what I was doing when I saw it – which happened to be on my partner’s stationary bike in our garage watching the YouTube broadcast on my smartphone.
The image that popped up on the screen before me was a ring of orange hues, weighted and thicker towards the bottom left. A dark, gaping, empty, expanse of black sat inside the ring. I was looking at a black hole and the shadow its event horizon. The orange hues were ionized gasses dizzyingly swirling around it at speeds a fraction of the speed of light; sending out their blazingly hot swansong before crossing a frontier into an area of space so unknown we can only conjecture at what is behind the veil of the event horizon.
The light from that gas travelled incredible distances of time and space before reaching not our eyes, but a group of radio telescopes spanning the globe, interconnected through an ambitious and creative collaborate effort. The end result of which is nothing short of breathtaking.
Being so enthralled in the image, I missed a good portion of what the researchers announced about their findings so far. To get an idea, I turned to the five articles that were published in The Astrophysical Journal Letters and thumbed through them. Between the formulas, diagrams and interpretations, I quickly saw the incredible amount of collaboration and work that went into capturing and processing the images taken between April 4 and 11, 2017. Numerous radio telescopes across the Earth all had to simultaneously have good weather, the petabytes of data that had to be transferred, standardized, aligned and consolidated. New algorithms were created, faster data processing were invented and countless hours spent to produce an image of a dark region in space, the shadow of the black hole, at the centre of M87 that spans 19 to 38 microarcseconds!
If you are like me, you want to know how much that is in light years not arcseconds and you’re not worried about the margins of error. Let us have a little fun and work that out for ourselves. We’ll need a few things: the small angle formula, the distance to M87 and a calculator.
The small angle formula (SAF) is: arcseconds = 206,265(diameter of object/ distance to object)
The distance to M87 is about 53.5 million Light Years
Let us take the upper end of the measurement because who really wants a small shadow? 38 microarcseconds become … 3.8 x 10^-5 arcseconds.
We want the diameter of the shadow, that means we rewrite the SAF to become diameter of object = (distance to object x arcseconds)/206,265 then plug in the numbers.
diameter = (53,500,000 x 0.000,038)/206,265
We get around 0.01 light years which we can convert into km by multiplying by 9.5 x 10^12 … and voila! 9.5 x 10^10 km or 95 billion km! Not bad for a shadow.