# Can a gravitational field be represented as an acceleration? (2nd try)

I think he used the term "equivalence" strictly to mean that all measurements would be the same in the two cases. For example, the lengths and widths of the two groups of yard sticks (those along the circumference, and those along the diameter) would be the same in both scenarios. I think he definitely considered the two scenarios to be different: in one case, you have acceleration with no gravitational field, and in the other case, you have a gravitational field, and no acceleration.

This sounds like the opposite of what Q-reeus just said. He said the rotating and gravitational cases produce different measurements. I don't know which is correct, but it seems like Einstein had the opportunity to say the measurements would be the same in both cases, but I don't recall him doing so specifically.

I also believe that he saw the value in the equivalence principle to be in what special relativity can tell us about general relativity, not the reverse.

Yes, that is what I think as well. SR is built up from its own postulates, in flat spacetime, and should therefore stand on its own in flat spacetime. GR is built up from SR, for the cases where there is not flat spacetime. GR should therefore reduce to SR in the case of flat spacetime.

Thank you, very interesting. I was wondering whether the gravitational non-Euclidean effects were similarly-analogous or oppositely-analogous to the rotating case. And you have answered it clearly, (assuming you are correct, which I assume you are).

Intuitively, to me, it makes sense that it would be opposite because the creatures living on the rotating disk have a type of gravity which tends to make objects fall away from the center, whereas the creatures living on the non-rotating mass have a type of gravity which tends to make objects fall toward the center. Do you suppose that is one of the reasons why it is opposite, or is that more of a coincidence?
The Equivalence Principle only strictly applies locally i.e. 'at a point'. Owing to the very different origins of 'g-forces' in the self-gravitating vs rotating disk, you can't even rely on integrating proper 'g-forces' over a radial path, to determine the global non-Euclidean geometry. If you did, the conclusion would be that in both cases it's only the radial metric that differs from the 'zero g-forces' limit. The global metric needs to be evaluated from either a non-rotating inertial frame (for rotating disk), or 'at infinity' (for gravitating disk).
Keep in mind too that even if the vastly larger centripetal or gravitationally induced stresses & strains could be exactly cancelled, the disk shape itself would not reflect the global metric distortions. The overall shape will adjust to produce the least net elastic strain energy possible (D'Alembert's principle I think covers it).

have a type of gravity which tends to make objects fall away from the center,
Is that not known as Centrifugal force?

It was in his popular book "Relativity", originally published in 1916, and revised in 1952, and now published by Crown in paperback. I always recommend that little book as the best starting place for anyone wanting to learn relativity theory. It's out of favor now by a lot of people who consider Einstein to have been naive and unsophisticated by modern standards. But I don't agree ... what he had that no one else had was uncommon intuition, and the ability to question things that "everyone knows is true".

I inadvertently left out one VERY important trait that Einstein had: although he could think as abstractly as anyone (and often more so than most), he also kept himself grounded in the practical. In particular, from the beginning, before he talked about time and space, he insisted on being clear about how he was prepared to measure those quantities.

Q-reeus:

Maybe it would be a good idea for you to dial back the passive aggressiveness in 2019? What do you think?

That thread being yet still in Physics & Maths speaks of an upstairs SciForums illness.
You may have noticed that we've just had Christmas and the New Year period. Moderators are not on call 24 hours a day, and do not read every thread.

I note that you took no steps at all to draw moderator attention to either of the two threads you are concerned about. Nevertheless, you wasted no opportunity to criticise the administration.

If you have some kind of personal issue with me or another moderator, perhaps we can discuss it by private messaging. That might be more productive than your regular whines.

Q-reeus:

Maybe it would be a good idea for you to dial back the passive aggressiveness in 2019? What do you think?

You may have noticed that we've just had Christmas and the New Year period. Moderators are not on call 24 hours a day, and do not read every thread.

I note that you took no steps at all to draw moderator attention to either of the two threads you are concerned about. Nevertheless, you wasted no opportunity to criticise the administration.

If you have some kind of personal issue with me or another moderator, perhaps we can discuss it by private messaging. That might be more productive than your regular whines.
Looks like my so-called 'whining' - what I would term apt if unflattering observations - have finally had an effect re the two threads formerly parked in Physics & Math. One could hope that's a good sign for SF starting into 2019.

As for dealing with any personal issues via 'conversations' (what used to be laughably labelled 'private' messages), such has from past experiences never been worth the effort. Always win-win for admin/mod, always lose-lose for yours truly. Forum rules and guidelines can say one thing, but we both know interpretation and 'discretionary application' is what really matters.

Looks like my so-called 'whining' - what I would term apt if unflattering observations - have finally had an effect re the two threads formerly parked in Physics & Math.
No. I moved the threads because I happened to stumble across them when reviewing my unread threads list. Nothing to do with you.

No. I moved the threads because I happened to stumble across them when reviewing my unread threads list. Nothing to do with you.
At face value imo an amazing admission. Given just how modest is the typical posting rate, aggregated across the entire SF site, let alone in a Science sub-forum. Where presumably you alone among the current staff would have any notional competency and/or interest in monitoring/evaluating . On auto-pilot here for the most part it seems.

At face value imo an amazing admission. Given just how modest is the typical posting rate, aggregated across the entire SF site, let alone in a Science sub-forum. Where presumably you alone among the current staff would have any notional competency and/or interest in monitoring/evaluating . On auto-pilot here for the most part it seems.
It sounds like you imagine I occupy a paid position here, wherein one of my responsibilities is to jump whenever you call. Both notions are mistaken.

Is a curvature the same as acceleration? Or is acceleration a product of curvature?

Is a curvature the same as acceleration? Or is acceleration a product of curvature?

Neither is true. I think it is a BIG mistake to think of them as the same thing. They are distinct concepts. When there are no gravitational fields present, special relativity is ALL that is needed. The usefulness of the equivalence principle is to allow us to sometimes use the solution to a special relativity scenario to tell us some things about what general relativity must be like, or to give us the solution to a general relativity scenario, without having to use the more cumbersome general relativity machinery.

Is a curvature the same as acceleration? Or is acceleration a product of curvature?
Neither is true.
Why do you say that?
Write4U's second question was correct, what we sense as gravitational acceleration is due to the curvature of space.

Why do you say that?
Write4U's second question was correct, what we sense as gravitational acceleration is due to the curvature of space.

Acceleration in the presence of a gravitational field (and no other forces) is indeed caused by curvature of space (or spacetime) ... that's general relativity. What I was objecting to is that some people view acceleration in the absence of any significant masses as itself producing a gravitational field, and I think that's wrong-headed.

What I was objecting to is that some people view acceleration in the absence of any significant masses as itself producing a gravitational field, and I think that's wrong-headed.
I agree.

Acceleration in the presence of a gravitational field (and no other forces) is indeed caused by curvature of space (or spacetime) ... that's general relativity. What I was objecting to is that some people view acceleration in the absence of any significant masses as itself producing a gravitational field, and I think that's wrong-headed.
Thanks for that additional info. I was not aware of that factoid.......

But IMO, gravity and acceleration are very closely related.

I am thinking of Einstein's man-in-the-box thought experiment.
What is Einstein's principle of equivalence?
In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.
https://en.wikipedia.org/wiki/Equivalence_principle
How did Einstein explain relativity?
Einstein then spent 10 years trying to include acceleration in the theory and published his theory of general
relativity in 1915. In it, he determined that massive objects cause a distortion in space-time, which is felt as gravity.Nov 7, 2017
Inside Einstein's Mind | Gravity Is Acceleration
Watch a visualization of the thought experiment that Albert Einstein used to conclude that gravity and acceleration are the same phenomenon, in this video from NOVA: Inside Einstein’s Mind. To work out a complex idea that would later feature his theory of general relativity, Einstein carried out an experiment in his mind. He envisioned a man in a box. Einstein realized that there was no way this man could tell whether he was sitting in a gravitational field or being accelerated. Because of this, these two situations were equal. By extension, Einstein concluded that gravity and acceleration are the same thing.
https://idahoptv.pbslearningmedia.o...nside-einsteins-mind-gravity-is-acceleration/

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