My trade as an astronomer , involves travelling to the very darkest spots on Earth, peering up to see what is there, and returning to report to you, my fellow humans. You poor devils can’t see much of the splendour because you have blinded yourselves with artificial lighting.
These expeditions into the dark started in the Arizona desert, then on to the Warumbungles in the Australian bush, to the island peaks of Hawaii and La Palma, and finally to La Silla up in the Chilean Andes where I measured the darkness in between the stars at 22.5 Blue magnitudes per square arc second. In my ignorance I was impressed though I was disappointed to see little more from up up there with the naked eye than I could espy from a moonless beach in Wales. ‘Why not?’ I wondered.
It turns out that the rod cells in the eye, which enable us to see in the dark — if we treat them properly, are critically dependent on Oxygen. Climb a mountain, where of course all our telescopes are constructed, and you’ve lost it. I discovered that by accident when I woke up in the bottom of my little sailing boat up a remote creek in West Wales to find a colossal glow several moon-widths across, peering down at me from on high. Yes I’d had a few pints but….surely…. It took minutes to realize that I was being watched by the Andromeda Nebula, our fellow Spiral galaxy, which I’d never seen before, even from those remote mountain peaks. [Try it for yourself on a moonless night in summer. Get as far away as you can from city lights, and go to sleep in the open (that ‘dark-adapts’ ones eyes), then wake and look up — and with any luck you might see a marvel you will never forget.]
Why hadn’t the big telescopes I’d been using make much difference? First because their fields of view are far too small. Second because the light you want to see is accompanied by much more background sky-light that you don’t. The big mirror amplifies both, only weakly improving the contrast. And that is what one needs to discern the dimmest structures in the universe — more contrast!
When I joined the Hubble Space Telescope Team it was natural to suppose that seen from up there in Space the sky would be really dark. But no. In between the planets drift tiny motes of dust which reflect sunlight back down into our dark — the so called Zodiacal Light — which you can actually see , if you know where to look. So disappointment once again.
Perhaps, if we could escape from the Solar System? But no again; there will still be faint starlight out there from the Milky Way and scattered starlight too. What a disappointment. Is there nowhere in the cosmos from where we could see the Universe as it really is? What about out there between the galaxies out in Intergalactic Space? You and I will never be able to go out there, but perhaps our distant descendants?
If I couldn’t go at least I could calculate how dark it ought to be out in that farthest, remotest, darkest immensity, an unimaginable distance away from any luminous star. It wouldn’t be absolutely black of course because some light would still be leaking from the nearest galaxies several million light years away.
What a shock I was to get from my calculation when I finally made it in my eighties. Out there the sky would have a brightnesss, or rather a dimness of 31.5 Blue magnitudes per square arc second, 9 magnitudes darker than the darkest site on Earth (Remember? 22.5 of the same magnitudes in the Andes). Now 5 magnitudes is one hundred by definition, so 9 magnitudes is one hundred times forty, or four thousand. Turn the calculation around and you can see why I was stunned. We live on a planet where the darkest night sky we will ever see is four thousand times brighter than it has to be if we are ever to see the Universe properly . We’re dazzled, blinded, blind. It is far more likely than not that we are blinded to most of the structures out there, and that all we will ever see, for all our technology, is a tiny fraction of the true Universe. Think on that. Knowing what we cannot know is sometimes more informative than knowing what we can.
If you want to see how the calculation was done go to:
https://mjdisney.org/wp-content/uploads/2020/11/hgreview7sept-1.pdf
Tags: galaxies, hidden galaxies, my books
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