I was really curious what would be this article, based on the title. The answer was, oddly, that there was not much there. I really don't understand what the point of the article was. But it brings up an odd question in my mind: what does the phrase "artificial gravity" really mean?
I think both phenomena discussed in the article are situations where momentum or stress exerts non-negligible gravitational influence, instead of mass, but that's just what general relativity predicts, so should that really be considered "artificial" gravity? What about the apparent gravity experienced inside, say, a rotating space station? Arguably that's not gravity at all.
I guess I'd just never before appreciated how odd the notion of "artificial gravity" is. I think if you look at how it's often used, "artificial gravity" probably usually means any phenomenon where people or objects experience having weight which is not due to the presence of mass nearby (which means it can include things that are not artificial and things that are not gravity).
I think SF writers and TV producers have done us a disservice in using "artificial gravity" everywhere (I get why they had to but it's still annoying) in that people now have this idea that "artificial gravity" is ever going to be A Thing.
Really, there's just Acceleration,
...which you'll either feel because you're accelerating/spinning or because you're near some massive body and somehow *not* accelerating when you're "supposed" to (due to ST curvature), -- say, because a planetary surface is getting in your way --
and that is all. Equivalence Principle says there's no difference.
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Even if you want to say something like "it's not real gravity unless there's a tide", that doesn't work either; even with a uniformly spaced fleet of accelerating ships, the ones in the back have to be accelerating harder to keep up, so it's NOT really a uniform field the way you'd expect.
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@wrog As I was acknowledging, I think the semantics are a bit unclear, but I guess I'd consider it to be gravity if spacetime has non-zero intrinsic curvature (not true of the set of uniformly accelerated observers), so, yes, that relates to tides. Of course, the entire point of the equivalence principle is that for an infinitesimal little lab you can't tell the difference, but assuming you can look out and see things further way then you can distinguish.
@johncarlosbaez I guess my question is would they classify it as artificial gravity any time they accelerate quickly in their car and feel pressed back into their seat? I would guess not. But what you're saying is along the lines of the definition I gave above, experiencing weight due to something other than the presence of mass.
It's just funny that people speak of "artificial gravity" as if it's a well-defined concept, but it's not really clear we agree on exactly what we mean by either term in this phrase. @wrog
@johncarlosbaez Oh, I was mostly thinking of when it's mentioned in a sci-fi context. I agree that I'd generally steer away from people using the term in a scientific context as probable cranks, but the term was used in the BBC article I linked to in the OP, which was what sent me down the path of thinking, "hey, what does that phrase really mean exactly?" @wrog
sorry I think your OP got disconnected (why I might not have been replying to the Right Things...); where is this BBC article?
(n.b. Even though they've got the Authoritative East-Anglian British Accents and they do tend to be better on average than US outlets, that's still kind of a low bar, and the BBC -- esp. more recent BBC after Tory funding cuts -- tends to suck more than Americans would expect on matters of scientific/historical accuracy.)
@wrog Oh, I thought your responses were reasonable in any case. Sorry the BBC post was missing, I think that must be an issue with how the quote posting function on my instance interacts with other instances. Here's the post in question:
ok, this article is disappointingly short. On the other hand it's better than most in that it doesn't try to do too much, and what it does do it seems to get right (...assuming the 2016 Füzfa result -- something I hadn't heard of before but I'll give them the benefit of the doubt -- is legit...)
I think the fundamental problem is when we teach things like centrifugal force or the Coriolis force being somehow "fictitious" forces. They are plenty real in that if you're being accelerated in any manner whatsoever then you are going to feel it (and be crushed by it if it's of sufficiently large magnitude).
I would say the definition of what makes a force "fictitious" is circular (*) They're the forces you feel when you're not in an inertial frame. What's an inertial frame? A frame in which you're not (to first order) experiencing fictitious forces.
(*) Update: well okay, I should be more careful. Clearly there's a way to make it work and Einstein depends on it, but it's still kind of circular...)
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And then SF writers further muddy the water by adding lots of imaginary concepts that not only will not ever be realized but arguably don't even make any sense in actual physics:
. "artificial gravity"
. "inertial dampeners"
(two sides of the same coin)
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@internic - it may make sense to say there's "gravity" iff spacetime is curved. But ordinary folks say there's gravity when they feel the ground pushing up on their feet, so such folks might say there's "artificial gravity" when they're in an elevator accelerating upwards or a space station spinning around. In other words, when they're tracing out a non-geodesic path through spacetime. If that's how they want to talk, I would not bother fighting them!
@wrog