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The '''Compton effect''' is the increase in wavelength of X-rays or gamma rays when they are scattered by electrons. Discovered by [[Arthur Holly Compton]] and published in 1923, it is conventionally regarded as the decisive evidence that light carries ''momentum'' in discrete amounts, and therefore as the strongest of the three classic arguments for the [[Photon|photon]].

==The standard account==

Compton directed molybdenum K-alpha X-rays at a graphite target and measured the wavelength of the scattered radiation as a function of scattering angle. Classical Thomson scattering predicts that the scattered wave has the same frequency as the incident one. Compton found instead a shifted component, with

: Δλ = (''h''/''m''e''c'') (1 − cos θ)

where θ is the scattering angle. The constant ''h''/''m''e''c'' is the '''Compton wavelength''' of the electron, 2.426 × 10−12 m. The shift depends only on angle — not on wavelength, not on the target material, not on intensity. Compton derived it by treating the collision as an elastic two-body encounter between a particle of energy ''hf'' and momentum ''hf''/''c'' and a free electron at rest, and applying conservation of energy and momentum. He shared the 1927 Nobel Prize in Physics with C. T. R. Wilson.

Two points of precision are worth keeping. First, the shift is an ''additive'' constant in wavelength, not a multiplicative one: for visible light it is utterly negligible, for hard X-rays it dominates. Second, the same kinematic formula can be obtained by treating the process as a double Doppler shift from a recoiling electron, a wave-mechanical derivation given by Schrödinger in 1927; what the photon picture adds is not the shift but the ''coincidence'' between individual scattered quanta and individual recoil electrons, which Bothe and Geiger demonstrated in 1925 and which is much harder to reproduce without quantization.

==On this wiki==

Two distinct interests run through the Compton material catalogued here.

===Compton scattering as a redshift mechanism===

The larger group treats the Compton shift as a candidate mechanism for cosmological [[Redshift|redshift]] — a [[Tired Light|tired-light]] proposal in which light loses energy to scattering on its way to us rather than to the recession of its source. [[John W Kierein]] is the principal advocate here, in "[[Implications of the Compton Effect Interpretation of Red Shift]]", "[[The Compton Effect Interpretation of Solar Red Shift]]" and "[[Gravitation as a Compton Effect Redshift of Long Wavelength Background Radiation]]", the last extending the idea to gravitation itself. Related papers include "[[Cosmological Redshift, Compton Effect and Age of the Stars]]" and [[Biagio Buonaura]]'s "[[Electromagnetic Waves, Inertial Transformations and Compton Effect]]".

The standard objection should be stated, because it is a real one and readers deserve it: Compton scattering redirects the photon as well as reddening it, so a scattering-based redshift would blur distant images and broaden spectral lines, and the sharpness of quasar images and of absorption lines at high redshift constrains such mechanisms severely. The wavelength dependence is also wrong — the Compton shift is additive in λ, whereas cosmological redshift is proportional to λ. Proponents here respond by invoking variant or forward-scattering mechanisms rather than ordinary Compton scattering; whether that rescue works is the substance of the dispute, and it is not settled by assertion on either side. See also the wider [[Tired Light]] and [[Redshift]] articles.

===Compton scattering without the photon===

The smaller group disputes the quantum reading of the effect itself.

* [[Johann Marinsek]], "[[Compton Effect is a Raman Wave Effect]]" — treats the shift as an inelastic wave-scattering phenomenon of the Raman type rather than a particle collision.
* [[Qing Zeng]], "[[Compton Effect Obeys Newton's Law]]".
* [[Joe Alexander Nahhas]], "[[Compton's "Apparent" Wave-Length Contraction Effect]]".
* [[William C Daywitt]], "[[Origin of the Compton and de Broglie Relations]]" — derives the Compton and [[Louis de Broglie|de Broglie]] relations from a vacuum-structure model rather than postulating them.
* [[Jenner Barretto Bastos-Filho]], "[[Correspondence and Commensurability in Modern Physics (a Study of the Compton Effect)]]" — a philosophical study of what the effect does and does not establish.

The Compton effect belongs with the [[Photoelectric Effect]] and [[Blackbody Radiation]] as the standard trio of quantization arguments, and this wiki holds classical alternatives to all three; the Compton case is the hardest of the three for a classical account, because of the Bothe–Geiger coincidences rather than because of the wavelength shift.

==See also==

* [[Photon]]
* [[Photoelectric Effect]]
* [[Blackbody Radiation]]
* [[Redshift]]
* [[Tired Light]]
* [[Arthur Holly Compton]]

[[Category:Light]]
[[Category:Quantum Theory]]
[[Category:Redshift]]

Compton Effect - Revision history

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