More than Meets the Eye: An Evolving Perspective on the Universe

Much of human evolution (perhaps all of it) has been a game of chance. We might have been subterranean beings; strangers to an ocean breeze or the warmth of the sun or pollen allergies. We might never have developed any senses at all, save for touch — the world would be coarse, and lumpy, and scalding, and slippery, and a million other things, but never a billion more. Could these alternative humanities ever comprehend the sheer vastness of sensations that we behold without a second’s thought? It is unlikely. Depending on your choice of alternate-human-conversation-partner (how you are conversing being none of my business), you may be greeted with awe at best, a scoff at worst, and fear somewhere in the middle.

But now consider a third, and final, hypothetical: what if we had evolved to be able to see more than we can?

Ah! (So says the conscientious reader) I know precisely where this path proceeds, you conniving rascal! You will talk about the electromagnetic spectrum, and how despairingly limited the human perspective is on it. You will remind me that every room I have ever occupied has been awash with a silent radiance beyond my reckoning, and you will induce me to despair over the fact that reality itself will forever be richer than my experience of it.

So I will, conscientious reader. Perhaps more optimistically.

The collection of wavelengths that our eyes have evolved to notice (visible light) lies on a measly range of 380 to 780 nm, and accounts for 42% to 43% of sunlight reaching the earth. It is not difficult to imagine why getting such a significant bite would help a species flourish (1). But different species see different worlds: the bee sees ultraviolet, the snake in infrared, and deep sea creatures have adjusted their eyesight for absurdly dim conditions. The world is not one thing seen by many animals. There are, rather, many worlds being sampled from the same physical reality.

In January 1610, Galileo Galilei aimed his most powerful telescope (20x magnification) at Jupiter, and over an extended period of observation noted that four tiny points of light were dancing around it, not erratically, but in a shape suspiciously reminiscent of revolution (2). A catastrophe: an object revolving around something that was not the earth? We knew already, from Aristotle’s respectable (and very fashionable) philosophy, that the earth was immovably the centre of the universe, while the Sun, moon, planets and stars all travelled around it in their celestial spheres. Now a middle-aged Italian polymath had taken to chucking crockery at this tidiness, for Jupiter’s moon begged the awkward question: why were their centres of orbit not the earth?

The answer, of course, is that Jupiter did not give a hoot about either Aristotle or Galileo, and while 20x magnification might not seem all too impressive to the modern reader, it was sufficient for the heavens to descend into disarray as the moon acquired scars and Venus began to wax and wane in a scandalously heliocentric fashion (2). Humanity’s vision had grown, and with it, its universe.

But these extended organs had yet space for improvement — and improve they would! The reader will recognise the human eye as a wondrously impatient instrument, in that it receives what happens now and moves on posthaste. A faint object sending only a handful of photons towards the retina must be observed in the moment, and the moment cannot be negotiated with. How would one deign to see such an object then?

Louis Daguerre’s daguerreotype, unveiled in 1839, worked by exposing a highly polished, iodine-sensitised silver-plated copper sheet to light inside a camera and then developing it using mercury vapour to create a single, highly detailed mirror image directly on the metal (3). In March 1840, John William Draper produced a rather crude photograph of the moon from New York using this method. His result, however unattractive, was the first instance of the celestial image being made persistent rather than momentary (3). In this, one may devise an answer to our old question: to see the faint object, let the detector towards which the photons are aimed remain open for longer and so they accumulate. The photograph had graduated from preserving what an astronomer had seen to recording what would have been elsewise inaccessible. 

Now, telescopes could magnify, and photographic plates could circumvent dimness. But visible light remained visible light, and the human eye remained the final arbiter of what counted as ‘seeing’. It was not long before nature came knocking, to inform Karl Jansky, American physicist, that she had been sending more letters than we could have ever dreamed of, only in another alphabet.

In 1932, Jansky, hard at work at Bell Telephone Laboratories, was engaged in a rather prosaic investigation on the source of a radio interference troubling transatlantic telephone communication. But as his antenna rotated with the Earth, a persistent radio signal was found that would make his inquiry far more interesting — its position seemed to change with the stars. Jansky eventually traced the source to the direction of the centre of the Milky Way, in Sagittarius (4). Radio astronomy went on to be quite the extraordinary field. Pulsars, quasars and the cosmic microwave background would all be feathers in the cap for an area with nothing to do with the narrow strip of electromagnetic radiation for which our eyes happen to be equipped (5).

And now for another awkward question: What does an X-Ray photograph look like? Strictly speaking, there is no natural human answer. This is also where colour enters the story (6).

It is easy for the word ‘colourised’ to give the impression of someone with the artistic enthusiasm of a bored child adding swatches of purple to an otherwise acceptable photo. Astronomical imaging, conscientious reader, has more to it than this. A colour photograph can be thought of as three measurements: red, green and blue. Each is a map of intensity. Put another way, every pixel can be represented by a vector (R, G, B). A pixel with the value (255, 0, 0) is assigned maximum red and no green or blue. A pixel with (255, 255, 255) is white, while one with (0, 0, 0) is black (7). Now point a telescope such as your personal best friend Hubble at a nebula and observe it three times, each time through a different filter. The first filter permits one range of wavelengths to reach the detector; the second permits another; the third permits another again. Each exposure therefore produces a monochrome image: a map of how much signal arrived through that filter. Combine them and lo and behold! It is your colour photograph.

Except now ‘red’ has acquired a slightly suspicious meaning. Astronomical imaging sidesteps the implicit promise of your ordinary photograph (‘if you were here to see this, you would have called it red’), in favour of ‘this filter produced the following measurements and we have elected to call it red’. Colour, as experienced by the human observer, is a relationship between electromagnetic radiation and a particular visual system. Our eyes translate a wavelength range to the sensation of red, and similarly, the person preparing the astronomical image performs another.

There are reasons for these assignments. Consider the emission lines of a nebula. Emission lines, highly summarised, are chemical signatures that indicate the presence of specific elements due to the emission of photons of certain wavelengths as the electrons jump around states. A planetary nebula might be imaged through filters targeting H-alpha (whose peak wavelength lies at approximately 656.3 nm), [O III] (peak at 500.7 nm) and [N II] (peak at 658.3 nm) (8). The measurements could then be assigned to green, blue and red respectively. H-alpha, in physical terms, is a particular transition of hydrogen, and its peak wavelength corresponding to green in visible light makes the colourising convention easier. The same applied to the other two filters, but switching the colours around would simply change the legend of the map; the features they are meant to encode persist.

Then what colour is a nebula? This is three questions in a trenchcoat:

1. What wavelengths does it emit?

2. What would a human observer see if sufficiently favourably placed, sufficiently dark-adapted, and possessed of sufficient visual sensitivity?

3. What assigned colours could communicate the measurements most effectively?

Sometimes the useful question is not “What would I have seen?” but “Where is the oxygen?” Or “Where is the ionised hydrogen?” Or “Which parts of this object are hottest?” Or “How does the infrared structure differ from the visible structure?” Now the colour may become more ambitious. Modern astronomical images can contain considerably more information than three channels permit. Hubble’s image of the galaxy cluster MACS J1206.2-0847, for instance, was constructed from ten different filters across Hubble’s ACS and WFC3 instruments, with groups of filters assigned to the blue, green and red channels. Ten observations enter, and three colours leave! What happened in the middle? Interpretation. Colour, in such an image, has become a coding system. It can represent wavelength, certainly, but it can also encode temperature, ionisation state, chemical composition, density, or some other measured property. A red patch means merely that some particular physical quantity is greater there than elsewhere.

After all, science is as much an enterprise of communication as it is one of measurement. Evolution gave us eyes, but eyes were never enough. Galileo gave them a telescope; photography taught them patience; detectors turned invisible light into numbers; colour returned those numbers to the eye. We may not have learnt to fast-track evolution, but we have learnt to see beyond it.

References

1. Yu J, Zhang L, Wang L, Zhu B. S-scheme Heterojunction Photocatalysts. Elsevier; 2023.

2. Drake S. Galileo’s first telescopic observations. Journal for the History of Astronomy. 1976;7(3):153—68.

3. Draper JW. XXXII. On the process of Daguerreotype, and its application to taking portraits from the life. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 1840 Sept;17(109):217—25.

4. M JC. The beginnings of radio astronomy. American Scientist [Internet]. 1957;45(1):5—12. Available from: https://www.jstor.org/stable/29773690

5. Norris RP. Extragalactic radio continuum surveys and the transformation of radio astronomy. Nature Astronomy [Internet]. 2017;1(10):671—8. Available from: https://www.nature.com/articles/s41550-017-0233-y

6. Lesser M. A summary of charge-coupled devices for astronomy. Publications of the Astronomical Society of the Pacific. 2015 Nov;127(957):1097—104.

7. Lupton R, Blanton MR, Fekete G, Hogg DW, O’Mullane W, Szalay A, et al. Preparing red‐green‐blue images from CCD data. Publications of the Astronomical Society of the Pacific [Internet]. 2004;116(816):133—7. Available from: https://www.astro.princeton.edu/~rhl/Papers/truecolor.pdf

8. Anandarao BG, Banerjee DPK, Desai JN, Jain SK, Mallik DCV. Morphology of the bipolar planetary nebula NGC 2346 from emission line profile studies. Monthly Notices of the Royal Astronomical Society [Internet]. 1988];235(1):221—8. Available from: https://academic.oup.com/mnras/article/235/1/221/991889