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FIRAS compared sky radiation with an adjustable internal blackbody using an interferometer and cold bolometers. An external blackbody could cover the sky entrance for measurement calibration. The team adjusted the internal temperature to minimize the difference signal.^[ref]

Comparisons across detectors and mirror strokes tested instrumental errors; sky mapping separated Milky Way emission. John C. Mather's lecture reports a final root-mean-square spectral departure of 50 parts per million of peak brightness. This is neither the preliminary 1990 precision nor a relative error in every spectral bin. Calibration drift and nonideal reference emission required correction.^[ref]

The advantage of a null measurement is that a small difference can be resolved without assigning the same absolute precision to a large detector output. Nevertheless, matching an internal reference is meaningful only after its relation to an independently calibrated blackbody has been tested. A bound on departures from a Planck spectrum constrains energy redistribution after thermal equilibrium; it does not require the cosmic microwave background to have exactly the same temperature in every direction.

Jocelyn Bell Burnell helped build Antony Hewish's radio telescope for finding compact sources through interplanetary scintillation in the solar wind. Operating it and inspecting its chart recorder output in 1967, she recognized an unusual recurring trace, checked earlier charts, and obtained fast recordings resolving pulses in late November. Her observation, recognition and persistent analysis were indispensable.^[ref]

The 1968 paper by Antony Hewish, Jocelyn Bell Burnell (bylined S. J. Bell), J. D. H. Pilkington, P. F. Scott and R. A. Collins reported $1.337\mathrm{s}$ spacing and proposed compact-star oscillations. Its radio telescope had 2,048 full-wave dipole antennas in sixteen rows. Four receivers selected declinations; two array halves formed an east-west interferometer. The $81.5\mathrm{MHz}$ receivers passed $1\mathrm{MHz}$. Observations used $0.05\mathrm{s}$ receiver time constant and Rapidigraph chart recorder with $0.03\mathrm{s}$ time constant; radio second pips calibrated timing.^[ref]

Antony Hewish checked sidereal time recurrence. Jocelyn Bell Burnell recalls that P. F. Scott and R. A. Collins reproduced the pulses with another telescope and receivers, while J. D. H. Pilkington measured pulsar dispersion delay: independent checks of instrument independence and intervening plasma.^[ref]

A useful numerical comparison uses the approximate sidereal time day of $1436.07\min$. Relative to a $1440\min$ schedule, the same sky direction transits about $3.93\min$ earlier each day, or $39.3\min$ after ten days. This compares successive passages through the receiving beam; it is a different clock from the roughly $1.3\mathrm{s}$ pulse spacing within one passage. Sky recurrence alone cannot rule out every transmitter or reflection, and pulsar dispersion delay alone does not uniquely establish an emission mechanism. Their strength lies in independent constraints agreeing.

The discovery lesson is that an unexpected signal must survive changes of readout, instrument and observing date. Increasing chart recorder speed exposed structure hidden in a compressed trace; it did not make the source begin pulsing. The pulsar interpretation now separates the rotating clock, beam geometry, pulsar magnetosphere emission and pulsar dispersion delay. The initial observation established periodic arrival, not a direct measurement of the neutron star's surface motion or radius.

An interferometer combines waves from two or more paths to measure their interference.

An interferometer compares round-trip light travel times along perpendicular arms as its orientation changes. The absence of the classical stationary-aether fringe shift at the expected scale constrains orientation dependence of the round-trip speed of light. The result is consistent with special relativity, but alone does not select it uniquely from every alternative theory.^[ref]

A mirror redirects light according to the law of reflection. Its orientation determines the reflected beam direction in an interferometer.

The original report by Albert Abraham Michelson and Edward W. Morley describes an interferometer experiment constraining the orientation dependence of light propagation. Scanned report: history.aip.org/exhibits/gap/PDF/michelson.pdf

Experimental evidence. Tests of special relativity check linked predictions about light propagation, clocks and four-momentum. Several independent kinds of experiment support those predictions.

The observed partial-drag coefficient agrees with this first-order prediction rather than simple Galilean addition of $u$. It tests a different consequence of the Lorentz transformation from a particle-lifetime measurement.^[ref]

Atmospheric muons and the two frame descriptions. Atmospheric cosmic rays create unstable particles whose decay chains produce muons. A stopped-muon experiment measures the time between a stopping signal and the subsequent decay electron, obtaining the proper mean lifetime ${\tau }_0\simeq 2.2\mu \mathrm{s}$. The associated light-travel distance is only $c{\tau }_0\simeq 660\mathrm{m}$, yet substantial numbers of energetic atmospheric muons reach detectors after travelling several kilometres.

For an ideal selected cohort produced height $h$ above a detector and moving downward at constant speed $v$, the flight time in the terrestrial frame is $h/v$. Time dilation predicts a laboratory mean lifetime $\gamma {\tau }_0$, hence survival fractionWithout time dilation, the predicted exponent would be $-h/(v{\tau }_0)$ and the surviving population much smaller. Altitude comparisons constitute a cosmic-ray muon time-dilation experiment: select comparable momenta or energies and account for detector acceptance, atmospheric energy loss and backgrounds before interpreting the count ratio.

For illustration, not as a claimed measurement, take $h=10\mathrm{km}$ and $v=0.998c$. Then $\gamma =15.82$, the laboratory flight time is $33.4\mu \mathrm{s}$, and the predicted surviving fraction is about $0.38$. Omitting time dilation would give only about $2.5\times 10^{-7}$ on the same idealized constant-speed model.

In the muon rest frame, its decay still follows $e^{-\Delta \tau /{\tau }_0}$. The atmosphere and ground move upward. Their separation, measured simultaneously in that frame, is shortened by length contraction:The same survival fraction follows. The terrestrial description uses a longer-lived moving particle; the muon-frame description uses a shorter atmospheric flight path. They describe the same production and detection events. This consistency illustrates both time dilation and length contraction, while the survival measurement is not two independent experimental tests merely because it can be described in two frames.

Energy-momentum invariant. For a particle with rest mass $m$,Since $E=m{c}^2+K$, substitution and cancellation of $m^2{c}^4$ giveThis relativistic energy-momentum relation reduces to $K\simeq p^2/(2m)$ when $K\ll m{c}^2$.

Laboratory beam energies. If both final muons are at rest in the laboratory, their total momentum is zero and their total energy is $2m_{\mu }{c}^2$. Four-momentum conservation therefore requires equal and opposite incoming momenta. Because the Electron and Positron have equal rest masses, they then have equal total energies:Subtract each particle's own rest energy:Using the usual handbook rest energies gives approximately $105.1\mathrm{MeV}$ for each beam. These are the ideal kinematic threshold values. Just above threshold, the pair has small, equal and opposite momenta in this laboratory frame.

Positron momentum. Apply the relativistic energy-momentum relation to $E_{+}=m_{\mu }{c}^2$ and use its stipulated negative-$x$ direction:The Electron has the opposite momentum, $p_{e,x}=+p_0$ with $p_0=c\sqrt{m_{\mu }^2-m_e^2}$.

Boost to the electron rest frame. The electron's velocity is $v=p_{e,x}{c}^2/E_{-}=p_0/m_{\mu }>0$. Thus $S^{\prime }$ moves in the positive-$x$ direction, and its Lorentz factor satisfiesThe positron has the same laboratory total energy as the electron, so $v{E}_{+}/c^2=v{m}_{\mu }=p_0$. The supplied Lorentz transformation then givesAs a sign check, the same boost gives $p_{e,x}^{\prime }=\gamma (p_0-v{E}_{-}/c^2)=0$, as required for the Electron rest frame.

A physicist working on interferometers.^[ref]