This evening I rediscovered the story of Nebulium. Nebulium doesn’t exist. But its “discovery” was a key step in both astronomy and quantum physics. It took sixty three years to realise what was really going on. What have we “discovered” recently that doesn’t mean what we think it means ? Dark energy maybe ?
I was looking for a book, but ran across an old favourite, and plucked it out instead, the way you do. The old favourite was an Edwardian book about the life of William Huggins, one of the founders of modern astrophysics – the man who turned the spectroscope to the stars. Huggins is a scientific hero, but its also a lovely wee book. Its one of “The People’s Books” – cheap pocket sized summaries of everything a person should know – from Inorganic Chemistry to The Life of Caesar through Kant’s Philosophy and Women’s Suffrage to The Stock Exchange and The Crusades. When the biography of Huggins was published in 1913, there were eighty four titles; by 1920 (the lastest volume I have) there were a hundred and thirty three. Whenever I drift by a second hand bookshop a magnet pulls me in and I check for People’s Books… I have twenty eight of them. Here is a picture of the frontispiece of the Huggins book.
Any other fans out there ? There seems to be little information on the Web …
Anyhoo … Huggins, working from his home in Tulse Hill in the suburbs of London, turned a spectroscope to the stars, and found them to contain Hydrogen, Sodium, Magnesium, Calcium, and Iron. Astronomy turned into Astrophysics in 1864. (You can read the real thing in Huggins and Miller 1864a). This was the start of the long confused path that led through Rutherfurd, Secchi, Pickering, Fleming and Cannon to the modern classification of stars. In 1866 Huggins took the first spectrum of a Nova, finding Hydrogen in emission, and in 1868, he took the first spectrum of a comet, finding, amongst other things, ethylene. These things alone would put him amongst the giants of astronomy; but his fame rests on the observations of nebulae.
On August 29th 1864, Huggins turned his spectroscope to the Cats Eye Nebula in Draco. He was amazed to find, not a continuous spectrum with dark lines, but a single bright emission line. (A few other lines were found later). He correctly deduced that nebulae were not aggregations of stars, but glowing gas. These observations and more were formally published in a series of papers starting with Huggins and Miller (1864b). However, my wee People’s Book contains a beautiful and moving informal account, written later by Huggins for the “Nineteenth Century Review” of 1897. Here is a link to a photo of the relevant pages.… I hope you can read it ok.
Now here is the problem. The “Chief Nebular Line” at 5007 Angstroms wavelength, had never been seen in a laboratory spectrum. It must be from a new substance, not found on Earth, which became known as “Nebulium”. (I am not sure when the word was first used.. ) This was a popular game in the 1860s. In 1868, Norman Lockyer found mystery lines in the Sun, which he decided were likewise due to a new element, christened Helium. Over the coming years, Helium lines were also found in nebulae, and nebulium lines found in novae, and all this spectroscopy was a major industry. By 1895, Helium had been found on Earth – William Ramsay managed to isolate it from the mineral Cleveite – but Nebulium was still confined to the Heavens, and was the subject of much speculation. By 1911, J.W.Nicholson had developed a full theory of the Nebulium atom, and calculated its size.
Finally in 1928 Ira Bowen solved the puzzle, in a classic paper. By this time, we knew about quantum mechanics, and how emission lines arise when atoms are excited, and then spontaneously decay to lower energy states. Some energy states are “metastable”; you have to wait a really long time before the spontaneous decay happens. In practice it never happens, as a collision with another atom always kicks it out of the excited state first. At least thats true on Earth … but in nebular gases the densities are so low these collisions happen very rarely .. and so the “forbidden” lines do occur after all. The Chief Nebular Line is not from Nebulium .. but from plain old Oxygen.
So all that time the wondrous nebulium did not exist. But the nebulium lines were a clue to the fundamental nature of the material world .. if only we had known.
So in modern times we measure the brightness and redshifts of distant supernovae, and find the universe, to our considerable surprise, to be accelerating. In the context of modern cosmology this requires a vacuum energy. Vacuum energy is proposed in some particle physics theories, but of a size many orders of magnitude different. This is something stranger, something new. The concordance cosmology has the universe made of ordinary (baryonic) matter, dark matter (non-bayonic particles), and “dark energy” .. a completely new substance … hmm..
Come back in sixty three years and maybe we will know what the hell this means.