I just encountered the wikipedia article on phonons, which says that these quasiparticles represent vibrations through matter. Wikipedia says that Phonons have negative mass and negative gravity.
But then the article says that "In 2019, researchers were able to isolate individual phonons without destroying them for the first time." The citation is:
Enamul Haque and M. Anwar Hossain. (2018). prediction of phonon-mediated superconductivity in XBC (X= Mg, Ca, Sr, Ba) Arviv.org, Retrieved November 27, 2018
So do these things actually exist? Or, rather, do mainstream physicists think that they exist? They clearly exist in PDF files.