Comment on ‘Ultradense protium p(0) and deuterium D(0)and their relation to ordinary Rydberg matter: a review’ (2019 Physica Scripta 94 075005)

The article by Holmlid and Zeiner-Gundersen (2019 Physica Scripta 94 075 005) contains a number of claims on the production and properties of a new state of matter, referred to as ultra-dense hydrogen (UDH). These claims are very far reaching as they include the observation of processes violating the conservation of baryon number (not observed before in spite of decades of dedicated efforts); furthermore the structure assigned to UDH is not consistent with quantum mechanics, in particular with the concept of kinetic energy operators and the Heisenberg indeterminacy relations. No independent experimental confirmation on production and properties of UDH has been published, whilst possible explanations in terms of instrumental artefacts have been put forward.