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So, I just asked a question (FTL drives and Quantum Mechanics experiments) about the intersection of quantum mechanics, FTL drives, and some form of relativity (not sure if it's general or special), and it got put on hold for "no questions about non-mainstream physics".

But relativity, FTL drives like the Alcubierre drive or wormholes, and quantum mechanics are all mainstream physics, so I'm not sure why it was closed. Could someone please explain?

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    $\begingroup$ Well, FTL drives are not mainstream physics, so... Sure, some people might like to presume that they are, but, well, they aren't. $\endgroup$ – Jon Custer Dec 6 '18 at 23:07
  • $\begingroup$ en.wikipedia.org/wiki/Alcubierre_drive It's a solution of Einstein's field equations in general relativity. That sounds like mainstream physics to me. $\endgroup$ – nick012000 Dec 6 '18 at 23:10
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    $\begingroup$ I've heard a great phrase, "Physics doesn't describe what happens when the laws of physics are broken." It seems relevant for a bulk of FTL questions. $\endgroup$ – user191954 Dec 7 '18 at 0:54
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    $\begingroup$ I started to write an answer, but it had enough overlap with this recent answer about close reasons that I've decided just to link to it instead. $\endgroup$ – rob Dec 7 '18 at 5:03
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First of all, thanks for asking about it here. We always appreciate it when people take the time to learn more about why the community made a decision (rather than, say, ranting about how we are all closed-minded fools :p).

After a quick look at your question, it looks like this is at the core of people's objections:

building an FTL drive (assuming that constructing such a device is possible)

The thing is, we don't know if such a device is possible, and if it is possible, we don't know how it would work. There are many details about FTL travel that are not known which could potentially affect the answer to your question. For that reason, we consider your question to fall under "fictional physics", which includes pretty much anything about the behavior of a system that may not necessarily have a self-consistent set of rules. That's a subcategory of non-mainstream physics, as far as our hold reasons go.

One way to improve your question would be to narrow it down to a specific physical system that is (real or) theoretically well-understood. In this meta post and comments you keep referencing the Alcubierre drive, which is one option for how an FTL drive might be implemented, so if you were to edit your question to ask about an Alcubierre drive specifically (not a generic FTL drive), that might be enough to get the hold removed. Or it might not; I'm not particularly familiar with that area of study, but people who are might say that you have to be even more specific than that, e.g. if there are several variants of Alcubierre drives, you would have to be precise about which one(s) you want to ask about.

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To me, it would appear that the sum of the parts is non-mainstream - probably due to a very large number of misconceptions you have about mainstream physics. An Alcubierre drive is "main-stream physics" in the isolated context of analyzing a hypothetical solution to Einstein's Field Equations (it does break some energy conditions though so most physicists take its validity with a grain of salt), and many worlds interpretation is main-stream physics in the isolated context of discussing interpretations of standard quantum mechanics (but as an interpretation of QM and not a physical theory, it's true physical content is debatable). If you sprinkle on top of that a bunch of truly vague statements (which I can't make any heads or tails of) like

So, my understanding is that information about a location's time propagates along its light-cone, right?

And

If the Many-Worlds interpretation of quantum mechanics is correct, wouldn't that mean information about the collapse of alternate timelines also propagate along its light cone?

I would say your question is now firmly in the realm of non main-stream physics.

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  • $\begingroup$ "truly vague statements" I'm probably not using the correct terminology, because I'm not a subject matter expert, but my understanding is that information about time propagates at the speed of light, which is why FTL implies time travel, right? It's why diagrams like these ones work: physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel Am I wrong here? $\endgroup$ – nick012000 Dec 7 '18 at 1:07
  • $\begingroup$ A bit more looking around and it looks like they're called Minkowsky diagrams. en.wikipedia.org/wiki/Minkowski_diagram $\endgroup$ – nick012000 Dec 7 '18 at 1:24
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    $\begingroup$ "Information of time" is not a thing. Nor does a location have a time. So "information about a location's time" makes no sense. Minkowski diagrams show the causal cones in SR - regions of space-time which can be causally connected (can have a cause and effect relationship with) to an event or which an event can causally connect to. $\endgroup$ – enumaris Dec 7 '18 at 16:55
  • $\begingroup$ The comments highlighted here, plus the comment highlighted by @DavidZ, make it clear to me this question is primarily speculative and not mainstream. $\endgroup$ – ZeroTheHero Dec 8 '18 at 4:33
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You asked why your question was closed. I can't speak for the other voters, but I voted to close because you referred to a "location's time", the meaning of which I cannot fathom, and then ignored multiple requests to clarify.

In particular, Carnegie Hall is a location, and I asked you what you mean by the time associated with that location. You ignored the invitation to clarify your meaning, which very much creates the impression that you never had any meaning in mind in the first place. Unwillingness to engage with a simple clarifying question is pretty much always ample reason to close, in my opinion.

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  • $\begingroup$ I thought you were making some sort of snide reference to some sort of physicist in-joke. The "time of Carnegie Hall" is exactly that: the information associated with the time at Carnegie Hall. If you draw a Minkowsky diagram of it, Carnegie Hall would be the zero position, just like a diagram of the time of Alpha Centauri would have Alpha Centauri at the zero position. I think the technical term might involve "reference frame"? I'm not sure. $\endgroup$ – nick012000 Dec 7 '18 at 7:18
  • $\begingroup$ Here, I drew a quick diagram with MS Paint, does this help? imgur.com/a/SaoBTgB $\endgroup$ – nick012000 Dec 7 '18 at 7:30

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