To complete the analogy, we need something that normally travels in a straight line (as light does).
1	To	to	PART	TO	_	2	mark	2:mark	Discourse=purpose-goal:141->142:1
2	complete	complete	VERB	VB	VerbForm=Inf	7	advcl	7:advcl:to	_
3	the	the	DET	DT	Definite=Def|PronType=Art	4	det	4:det	Entity=(179-abstract-new-cf3-2-sgl
4	analogy	analogy	NOUN	NN	Number=Sing	2	obj	2:obj	Entity=179)|SpaceAfter=No
5	,	,	PUNCT	,	_	2	punct	2:punct	_
6	we	we	PRON	PRP	Case=Nom|Number=Plur|Person=1|PronType=Prs	7	nsubj	7:nsubj	Discourse=topic-solutionhood:142->145:2|Entity=(53-person-giv:act-cf1*-1-ana)
7	need	need	VERB	VBP	Mood=Ind|Number=Plur|Person=1|Tense=Pres|VerbForm=Fin	0	root	0:root	_
8	something	something	NOUN	NN	Number=Sing	7	obj	7:obj|11:nsubj	Entity=(180-object-new-cf4-1-sgl
9	that	that	PRON	WDT	PronType=Rel	11	nsubj	8:ref	Discourse=elaboration-attribute:143->142:0
10	normally	normally	ADV	RB	Degree=Pos	11	advmod	11:advmod	_
11	travels	travel	VERB	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	8	acl:relcl	8:acl:relcl	_
12	in	in	ADP	IN	_	15	case	15:case	_
13	a	a	DET	DT	Definite=Ind|PronType=Art	15	det	15:det	Entity=(181-abstract-new-cf5-3-sgl
14	straight	straight	ADJ	JJ	Degree=Pos	15	amod	15:amod	_
15	line	line	NOUN	NN	Number=Sing	11	obl	11:obl:in	Entity=181)
16	(	(	PUNCT	-LRB-	_	19	punct	19:punct	Discourse=mode-manner:144->143:0|SpaceAfter=No
17	as	as	SCONJ	IN	_	19	mark	19:mark	_
18	light	light	NOUN	NN	Number=Sing	19	nsubj	19:nsubj	Entity=(182-abstract-new-cf2-1-sgl)
19	does	do	VERB	VBZ	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	11	advcl	11:advcl:as	SpaceAfter=No
20	)	)	PUNCT	-RRB-	_	19	punct	19:punct	Entity=180)|SpaceAfter=No
21	.	.	PUNCT	.	_	7	punct	7:punct	_
