Okay, sure, let's discuss telepathy. "Three planets behind the moon." Why would the telepathic alien communicate the thought "three planets behind the moon" when realistically, his mind would translate the conveyed thought into a system of measurement that he would understand, like miles or even lightyears? It could even be some sort of celestial coordinate system (assuming his grandfather is knowledgable about celestial coordinates). Do you think that his grandfather measures long distances by "planets" or by miles/kilometers?
In any case, we've already determined that no planet that close to the Earth could ever orbit the Earth, as it would just end up orbiting the sun, which would mean we'd end up seeing it. But sure, let's assume that his grandfather for some reason measures distance by "planets". Let's also assume the laws of physics refuse to work for this particular moment in history. These planets orbit the Earth while they are lined up perfectly behind the moon such we never see them. Let's even say that the light of the Sun magically can't illuminate any of these three planets, so our probes would never see them. The way to the alternate dimension by which these aliens arrived must be three planets behind the moon, right? Why wouldn't they telepathically communicate to his grandfather that "there is a gateway to an alternate dimension located three planets behind the moon and that's where we are headed"? Or are you saying that the grandfather and his brother happened to just exist in an alternate dimension where they were looking down upon Earth with aliens who telepathically couldn't communicate the idea that, "you are in an alternate dimension all of a sudden and we measure everything by planets here for some reason and we are headed to a place that is three planets behind the moon"?
The laws of our universe can work differently in other universes, I will acknowledge that. But literally everything else does not check out.
Honestly, I was fixating on it because I was annoyed about something earlier, and yeah that's my fault. However, to be fair, I have seen some pretty far-fetched posts on this subreddit, all of which I try to give the benefit of the doubt, though they are clearly just... untrue... based on, you know, common sense. I take no issue with the expression of creativity through wild stories –– frankly, the absolute BS stories that are often on here are sorta interesting to X-ray for flaws. This, however, was the most bizarrely done story I've read thus far. "Three planets from behind the moon." Lol.
My apology really is sincere, though –– I wouldn't have commented if I could go back. It was just provocative for no reason. The dude's probably just a kid trying to believe in something fantastical.