The following is a paper by H. Aspden published in Hadronic Journal, v. 10, pp. 167-172 (1987).
Abstract: Technetium (Z=43) and promethium (Z=61) are by far the least abundant of all atoms below the radioactive elements (Z=84 onwards). Their scarcity confirms theoretical predictions emerging from a theory of the photon derived from synchronous lattice electrodynamics. This theory has given precise theoretical values for the fine-structure constant α and the constant of gravitation G and is now shown in this paper to indicate resonant interactions between the vacuum lattice oscillations and technetium and promethium. In the case of promethium there is strong reason for believing that this atom can assume supergravitational or antigravitational properties, accounting for its scarcity. This paper not only adds support to the earlier theoretical work on the photon and gravitation, but suggests a research route that might lead to new technology based on controlled interactions with gravity fields.
Commentary: This paper was written because the author was asked if his theory of gravitation might have any practical implications. Although the author had, for nearly 30 years prior to this request, had a clear understanding of the electrodynamic nature of gravity, he had always believed that the frequencies needed to promote controlled interaction with the gravity field put that possibility out of reach.
In the early years of the author's theory thought had been given to electron activity in the rare earth atoms, particularly erbium, which has a Z value close to half of α-1=137. However, the thought of suggesting a practical research route seemed too difficult and it was too remote from the circumstances facing the author, namely the need first to find acceptance for his theory. Confronting the question anew and with a fresh mind at the time of the above request, the author saw that until then he had never considered how electrons in certain atoms that could possibly be caused to resonate with the gravitational field might be affected by collision with the vacuum aether lattice.
Once this thought registered, attention converged on the significance of this with regard to promethium and the author woke up to the fact that this element plus the element technetium were the only elements in the periodic table below the upper radioactive range that simply were missing as having any natural abundance.
One day, when the author's theory has become the accepted theory of gravitation, it will be deemed right to deploy enough scientific resource into devising and building a solid-state anti-gravity machine. This paper might suggest one route to that objective.
The full text of this paper may also be seen in PDF format as Paper No. 4 in the author's 1996 book Aether Science Papers which is of record in the author's website http://www.haroldaspden.com.