Just a quick note on the title of this piece.
What is meant by the words “the images of black holes are not photos” is that no everyday camera could take a photo of a black hole. Indeed not even the most sophisticated and expensive (single) camera could do so. That said, the eight telescopes specifically involved in the project of creating an image of the black hole in galaxy M87 are said to have “photographed” it multiple times.
In any case, back in 2019 we saw the first image of a black hole. That image became a major news item. Of course long before that image scientists and artists had created multiple paintings and drawings of black holes.
So what do we mean by the word “image” in the specific context of a black hole?
Firstly, a black hole isn’t something the naked eye could see. It’s not even something that can be seen through a single telescope either. And, finally, it’s not something that could be the subject of a single photograph.
Having said all that, the astronomer Mike Merrifield (of Sixty Symbols and the University of Nottingham) said that this image is the “best [proof] there has ever been for a super-massive black hole”.
So how should we interpret these images?
The Black Hole Itself
The black hole itself is located in a distant galaxy — Messier 87. More precisely, it’s 500 million trillion kilometres away. Thus the eight telescopes involved “saw” something from (our) distant past.
In terms of the black hole’s size: it’s 40 billion kilometres across. Or, to put that another way, it’s three million times the size of the Earth and has a mass 6.5 billion times that of our Sun. All in all, this black hole is larger that our entire Solar System.
So, in the context of what’s just been said, one can’t even imagine how a single person could see (or take a photo of) this black hole — or even use a single telescope to view it.
The final image of the black hole was “captured” by the Event Horizon Telescope (EHT). This itself was a network of eight telescopes which were linked together in various places throughout the world.
In terms of the image itself: what we can see is a bright “ring of fire” which surrounds a perfectly circular black hole. This bright ring of fire is itself caused by superheated gas falling into the hole. Indeed this ring is brighter than all the stars in the M87 galaxy put together. That’s partly why it could be “seen” through EHT’s eight telescopes.
In terms of the colours of the image. The white around the black hole is high-energy light; whereas the red is low-energy light.
Dr Katie Bouman (the scientist largely responsible for the image) emphasised the fact that what we “see” is actually what’s surrounding the black hole.
Bouman also uses the word “image”. She wrote on Facebook:
“Watching in disbelief as the first image I ever made of a black hole was in the process of being reconstructed.”
Previous to the publication of this image, Bouman said (during a TED talk) of another image of a black hole that it “isn’t a real photograph”: it’s “an interpretation of what a black hole might look like”. She also often used the word “picture” in this seminar.
Technical Stuff
No single telescope would have been powerful enough to capture this black hole. Bouman herself said (if before this particular image was created) that we’d “need a telescope the size of the earth to see a black hole”. Nonetheless, Bouman also said that a “computation” (or algorithmic) solution could be found to this problem. It was found. She found it. Hence the image.
So that’s why a network of eight telescopes was set up in locations ranging from Chile to Antarctica. (The project also involved over 200 scientists.) All eight telescopes together used a technique called interferometry. And then all the data from the eight telescopes was stored on hundreds of hard drives. That stored data, in turn, was processed in Bonn and Boston. (Interferometry is a group of techniques in which waves — often electromagnetic waves — are superimposed. That causes the phenomenon of (wave) interference; which, in turn, is used to extract information.)
Now for Bouman’s now well-known algorithms.
Much as been made of Bouman’s series of algorithms which made this image possible. So what, exactly, did these algorithms achieve?
Well, they converted the data from the eight telescopes into the image we can now see. More precisely, Bouman’s algorithms “recovered a photo” (Bouman herself repeatedly uses the word “image”) from that huge amount of data.
Interestingly enough, Bouman has previously said that “infinite images” can be taken to represent the same physical phenomenon. Thus, in that sense, Bouman and those involved in this project had their own analogue of the underdetermination of theory by data thesis. This is the thesis which states that the same physical phenomenon can be accounted for by numerous theories or by numerous models. And, because of that, Bouman has also accepted the possibility of “bias” (her word) when it comes to images of black holes. What she means in this case is that the images extracted from a huge pool of data may be those which coincide with the scientists previous exposure to the “simulations” (another of Bouman’s words), picture or paintings of other black holes.
Whatever the case may be, Bouman said that such images of black holes are “reconstructions” which are “pieced together” in the way that “puzzle pieces” are pieced together.
In technical terms, the colours on the image of a black hole (in this case, black, red, white, etc.) makeup what’s called a “false-colour image”. That is, the colours are only approximations of what human eyes would “interpret”. And that’s of course to forget that a human eye could never get close to this black hole in the first place. More basically, the black-hole image is a product of radio telescopes which can’t capture visible light (unlike an “ordinary” telescope). Hence the term pseudo (or false) colour.
Philosophical Questions
One may wonder what is meant by the word “image” (as in the phrase “image of a black hole”). This image (as stated) is the end result of the multiple recordings of eight different telescopes. They were somehow fused together in Bonn and Boston.
So this black hole isn’t what a human being would see — even through a telescope. A human being wouldn’t see this even if he or she could hypothetically get close enough to use a telescope to see a black hole. Indeed the scientists involved (or at least some of them) have happily admitted that the black hole is “unseeable” to the naked eye.
Thus how does that image of this black hole relate to what a human being could (or can) see? Does this even matter when it comes to this image? (Perhaps this means that, in a certain sense, the image of this black hole is not that unlike Niels Bohr’s model of the atom.)
Having said all that, despite the complexity of the overall project, the multiple images used, the algorithms required, etc., it doesn’t necessarily follow from all this that the image presented to the public isn’t a faithful representation (or image) of the black hole. Nonetheless, what would it mean for this image to be a faithful representation of this black hole? Indeed, to be pedantically philosophical, what do we mean by “image” or “representation” within the specific context of this black hole?
Let’s begin with the term “representation”.
This term is used in a multitude of different ways. However, unlike an image (or picture), representations are sometimes said to be (mere?) “symbols” or things which “take the place of something else”.
So the words “image” and “representation” are certainly not synonyms. That is, a representation needn’t also be an image. Similarly, an image needn’t be seen as a representation. (Though that would entirely depend on how the word “representation” is being used.)
Generally speaking, an image (unlike a representation) is said to “resemble a subject”. Thus it’s not surprising that an image is also said to “depict [replicate/mimic?] visual perception” — and this is clearly relevant to this image of the black hole. On some accounts, the category photograph is said to be a subset of the category image. Having said that, an image itself is also seen to be a subset of the category representation.
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