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How Does Light From Distant Stars Remain Coherent?

-- Doug | July 27, 2022

Question:

How does light from stars that are billions and billions of miles away remain coherent? Wouldn’t the light diffuse as it travels that far and also encounters anything like dust/other particles/or even gravity?

-- Doug

Answer:

Since the light from a distant star is a point source for any measurement that we want to make, it is by definition spatially coherent when the light reaches us. Starlight is scattered by matter, such a dust, as it makes its way from the star to us, but the amount of scattering due to dust or other particles is wavelength-dependent. Shorter wavelengths are scattered more than longer wavelengths, so we can always minimize the amount of scattering that affects our measurements of a star by observing them at a longer wavelength. Gravity can also affect the path that photons take when traveling to us, but this is actually a bending of the space through which the photons travel to us, not something which directly affects the photons themselves.

-- Jeff Mangum

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