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Do Light Waves Get Stretched to Straight Lines By Moving Objects?

-- | May 27, 2018

Question: Does light wave disintegrates(i.e. its crest and troughs vanishes leaving a straight line) when it travels unhindered and stretched too far due to expansion of universe? Though it can’t be observed, is it theoretically possible? — Vinod

Answer: The apparent stretching of light waves caused by the expansion of the universe is in fact due to two potential affects: (1) the stretching of space due to gravity (or gravitational redshift) and/or (2) the wavelength shift of light emitted by an object that is moving (which would be a “redshift” for an object moving away from us). For gravitational redshift, it is the longer path that light has to travel that gives the appearance of light being “stretched”. For the second case of a moving object, the lower frequency is measured by the observer because each successive crest of the light wave emitted by the moving object is emitted from a position further away from the observer than the previous wave. This appears to the observer as a lowering of the frequency of the light coming from the moving object. The light wave itself is, in fact, not changed or stretched. [Editor’s note: This answer was updated to include the gravitational redshift contribution, which was left-out of the original answer to this question.]

Jeff Mangum

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