The dumbest fucking arguments you've ever heard.

[TheRealJoeFish]

Light travels instantly in one direction.....I kid you not.

This one's actually interesting.

The version of this probably used is the "Anisotropic Synchrony Convention," as framed by fundie "scientist" Jason Lisle in his paper for the, ahem, Answers Research Journal.

It turns out "light travels the same speed in all directions" is a mathematical convention. Although there's absolutely no reason this shouldn't be the case, it cannot be proved, it seems.

Although it's way, way harder than doing physics with "all light travels at c in all directions," you can brute-force the math to also work with "light travels towards an observer at infinite speed and away from an observer at c/2." This is actually consistent with relativity. It's sort of analogous to brute-forcing the math of Newtonian physics to describe movement in relation to a non-obvious frame of reference (like, if we wanted to describe the movement of a car on earth from the frame of reference of an observer on Neptune, we could do it, but we'd have to add in terms for the rotation and revolution of the planets and it would be a mess).

The Anisotropic Synchrony Convention, though, is generally used not by physicists but by lunatics who somehow want to pretend it solves the Starlight Problem and suggests a young earth (it just so happens that Lisle is a physicist and a lunatic). And in that regard, almost certainly the context you heard it in, it is one of the dumbest fucking arguments anyone's ever heard.