Editing Relativistic Doppler effect (section)

=== Direct measurement of transverse Doppler effect ===
The advent of [[particle accelerator]] technology has made possible the production of particle beams of considerably higher energy than was available to Ives and Stilwell. This has enabled the design of tests of the transverse Doppler effect directly along the lines of how Einstein originally envisioned them, i.e. by directly viewing a particle beam at a 90° angle. For example, Hasselkamp et al. (1979) observed the ''H''α line emitted by hydrogen atoms moving at speeds ranging from 2.53×10<sup>8</sup>&nbsp;cm/s to 9.28×10<sup>8</sup>&nbsp;cm/s, finding the coefficient of the second order term in the relativistic approximation to be 0.52±0.03, in excellent agreement with the theoretical value of 1/2.<ref group=p>{{cite journal |last1=Hasselkamp |first1=D. |last2=Mondry |first2=E. |last3=Sharmann |first3=A. |title=Direct observation of the transversal Doppler-shift |journal=Zeitschrift für Physik A |date=1979 |volume=289 |issue=2 |pages=151–155|doi=10.1007/BF01435932 |bibcode=1979ZPhyA.289..151H |s2cid=120963034 }}</ref>

Other direct tests of the TDE on rotating platforms were made possible by the discovery of the [[Mössbauer effect]], which enables the production of exceedingly narrow resonance lines for nuclear gamma ray emission and absorption.<ref>{{cite book |author1=Saburo Nasu |editor1-last=Yoshida |editor1-first=Yutaka |editor2-last=Langouche |editor2-first=Guido |title=Mössbauer Spectroscopy: Tutorial Book |url=https://archive.org/details/mssbauerspectros00nasu |url-access=limited |publisher=Springer |isbn=978-3642322198 |pages=[https://archive.org/details/mssbauerspectros00nasu/page/n7 1]–22 |year=2013 |chapter=General Introduction to Mössbauer Spectroscopy}}</ref> Mössbauer effect experiments have proven themselves easily capable of detecting TDE using emitter-absorber relative velocities on the order of 2×10<sup>4</sup>&nbsp;cm/s. These experiments include ones performed by Hay ''et al.'' (1960),<ref group=p>{{cite journal|author1=Hay, H. J. |author2=Schiffer, J. P. |author3=Cranshaw, T. E. |author4=Egelstaff, P. A. |date=1960|title=Measurement of the Red Shift in an Accelerated System Using the Mössbauer Effect in <sup>57</sup>Fe|journal=Physical Review Letters|volume=4|issue=4|pages=165–166|doi=10.1103/PhysRevLett.4.165|bibcode=1960PhRvL...4..165H}}</ref> Champeney ''et al.'' (1965),<ref group=p>{{cite journal|author1=Champeney, D. C. |author2=Isaak, G. R. |author3=Khan, A. M. |date=1965|title=A time dilatation experiment based on the Mössbauer effect|journal=Proceedings of the Physical Society|volume=85|issue=3|pages=583–593|doi=10.1088/0370-1328/85/3/317|bibcode = 1965PPS....85..583C }}</ref> and Kündig (1963).<ref group=p name=Kundig/>