light time vs length

light time and length both are nouns.

Word	Noun	Adjective	Verb	Adverb
light time	Yes	No	No	No
length	Yes	No	No	No

As nouns, length is a hypernym of light time; that is, length is a word with a broader meaning than light time:

light time: distance measured in terms of the speed of light (or radio waves)
length: size of the gap between two places

Other hypernyms of light time include distance.

light time (noun)	length (noun)
distance measured in terms of the speed of light (or radio waves)	a section of something that is long and narrow
	the linear extent in space from one end to the other; the longest dimension of something that is fixed in place
	continuance in time
	the property of being the extent of something from beginning to end
	size of the gap between two places