Willem F Esterhuyse Posted February 21, 2023 Posted February 21, 2023 The Earth is a ball. How can one side and the opposite side accelerate upwards (their local "up").
Genady Posted February 21, 2023 Posted February 21, 2023 If you are talking about the Earth spinning around its axis, then the points on the surface everywhere accelerate downwards rather than upwards. (except the poles)
Lorentz Jr Posted February 21, 2023 Posted February 21, 2023 It sounds like a reference to GR, saying the falling object isn't accelerating downward.
Willem F Esterhuyse Posted February 21, 2023 Author Posted February 21, 2023 If space accelerates in towards the center of Earth, where does it go?
Genady Posted February 21, 2023 Posted February 21, 2023 2 minutes ago, Willem F Esterhuyse said: If space accelerates in towards the center of Earth, where does it go? No, space does not accelerate. The matter on the Earth surface accelerates downwards.
mistermack Posted February 21, 2023 Posted February 21, 2023 6 minutes ago, Willem F Esterhuyse said: If space accelerates in towards the center of Earth, where does it go? There was a previous thread on this subject some time ago that got a lot of posts, so it might be worth a look. https://www.scienceforums.net/topic/110805-gravity-please-knock-this-down/#comments
Genady Posted February 21, 2023 Posted February 21, 2023 9 minutes ago, mistermack said: There was a previous thread on this subject some time ago that got a lot of posts, so it might be worth a look. https://www.scienceforums.net/topic/110805-gravity-please-knock-this-down/#comments I think that OP rather is about a centripetal acceleration.
MigL Posted February 21, 2023 Posted February 21, 2023 You ask, or make the most inane statements. Realize that in Physics we deal with models. And if the 'reality' of the situation can best be represented bysomething accelerating, or a made-up particle which might not really exist, or any number of other assumptions which are unsupported, we explore them, and see if they lead to consequences we can observe and measure. 2
studiot Posted February 21, 2023 Posted February 21, 2023 12 minutes ago, MigL said: You ask, or make the most inane statements. Realize that in Physics we deal with models. And if the 'reality' of the situation can best be represented bysomething accelerating, or a made-up particle which might not really exist, or any number of other assumptions which are unsupported, we explore them, and see if they lead to consequences we can observe and measure. +1 But we do not deal in unsupported assumptions. We (like to think) we have sound reasons for these assumptions 52 minutes ago, Willem F Esterhuyse said: If space accelerates in towards the center of Earth, where does it go? It does and it compresses. The compression of rocks at depth is enormous.
Phi for All Posted February 21, 2023 Posted February 21, 2023 27 minutes ago, MigL said: You ask, or make the most inane statements. They all follow the same format. "You can't pet a hairless cat because 'petting' is defined as stroking fur! I'm going to insist on using this single, rigid definition no matter how you reply, even though I alone find it interesting and meaningful!" And the questions in the OP are always incredulity-based, which suggests they aren't being asked in good faith. I don't think it's a successful format for learning anything, and certainly hobbles any attempt at a decent discussion.
mistermack Posted February 21, 2023 Posted February 21, 2023 1 hour ago, Genady said: I think that OP rather is about a centripetal acceleration. No, I posted it, I was picturing something akin the the 'river model' which is tied to general relativity. 1 hour ago, studiot said: It does and it compresses. The compression of rocks at depth is enormous. Yes, but that's not the compression of space.
Genady Posted February 21, 2023 Posted February 21, 2023 1 minute ago, mistermack said: No, I posted it, I was picturing something akin the the 'river model' which is tied to general relativity. Sorry, when I said, "I think that OP rather is about a centripetal acceleration", I meant OP in this thread we are here now, namely, Willem, https://www.scienceforums.net/topic/128865-the-earth-is-not-accelerating-upwards/
studiot Posted February 21, 2023 Posted February 21, 2023 26 minutes ago, mistermack said: Yes, but that's not the compression of space. It must be because the thing you and I call space does not accelerate, or even move.
mistermack Posted February 21, 2023 Posted February 21, 2023 It does in the river model. In fact, if you say that space-time is curved, than that is equvalent to saying that space moves. If it didn't, space-time could never be curved. 32 minutes ago, Genady said: when I said, "I think that OP rather is about a centripetal acceleration", Oh yes, I see that could be read both ways. 🙂
studiot Posted February 21, 2023 Posted February 21, 2023 7 minutes ago, mistermack said: It does in the river model. In fact, if you say that space-time is curved, than that is equvalent to saying that space moves. If it didn't, space-time could never be curved. I don't think so. That would be true if and only if 'space' was embedded in a higher dimensional manifold. Current theory is that this is not the case and that the physical manifold we call space is not embedded in anything. That is possible because of Gauss famous little theorem - The theorema egregium - which distinguishes between intrinsic and extrinsic curvature.
mistermack Posted February 21, 2023 Posted February 21, 2023 1 minute ago, studiot said: I don't think so. You lost me there, (not your fault, mine) My reasoning is that if space is always stationary over time, how can you say that space-time can be curved? If the shape of anything is different after an elapsed time, then something has to move.
Genady Posted February 21, 2023 Posted February 21, 2023 (edited) 24 minutes ago, mistermack said: if space is always stationary over time, how can you say that space-time can be curved? An example could be the spacetime around non-rotating spherically symmetrical massive body, aka Schwarzschild metric. The space there is stationary, and the spacetime is curved, isn't it? Edited February 21, 2023 by Genady
mistermack Posted February 21, 2023 Posted February 21, 2023 1 minute ago, Genady said: Example could be the spacetime around non-rotating spherically symmetrical massive body, aka Schwarzschild metric. The space there is stationary, and the spacetime is curved, isn't it? If the space there is stationary, how can a stationary object begin to move towards the massive body, without accelerating?
Genady Posted February 21, 2023 Posted February 21, 2023 5 minutes ago, mistermack said: If the space there is stationary, how can a stationary object begin to move towards the massive body, without accelerating? Without accelerating? It will accelerate. How is it related to the space being stationary? Perhaps, we mean different things.
MigL Posted February 21, 2023 Posted February 21, 2023 51 minutes ago, studiot said: I don't think so. This is a common procedure in Black Hole dynamics.. The use of Rindler frame/co-ordinates transforms the infalling observer to a stationary one, where the Event Horizon accelerates outward. See Here Rindler coordinates - Wikipedia
mistermack Posted February 21, 2023 Posted February 21, 2023 14 minutes ago, Genady said: Without accelerating? It will accelerate. How is it related to the space being stationary? Perhaps, we mean different things. In general relativity, the object is not considered to accelerate, but to simply float free in curved spact-time.
Genady Posted February 21, 2023 Posted February 21, 2023 3 minutes ago, mistermack said: In general relativity, the object is not considered to accelerate, but to simply float free in curved spact-time. In GR, free falling object considered to have a locally inertial reference frame. The things float freely inside that frame, i.e., relative to the free falling object. Relative to the outside observer who is stationary relative to the central mass, the free falling object accelerates.
mistermack Posted February 21, 2023 Posted February 21, 2023 1 hour ago, Genady said: In GR, free falling object considered to have a locally inertial reference frame. The things float freely inside that frame, i.e., relative to the free falling object. Relative to the outside observer who is stationary relative to the central mass, the free falling object accelerates. And therefore, relative to the same outside observer, the locally inertial reference frame accelerates.
Genady Posted February 21, 2023 Posted February 21, 2023 37 minutes ago, mistermack said: And therefore, relative to the same outside observer, the locally inertial reference frame accelerates. Yes.
mistermack Posted February 21, 2023 Posted February 21, 2023 3 hours ago, Genady said: Yes. Well, either the observer accelerates, or a free falling object accelerates. But to decide which is happening, I'm looking at which is experiencing a force. In general relativity, gravity is not a force. But the observer IS experiencing a force through his feet. So from a GR point of view, the observer is the one that is accelerating, and hence the surface that he's standing on is likewise accelerating. So the OP is pointing out that the surface of the Earth is accelerating upwards, away from the centre, all over the world. So here in the UK, the surface is accelerating upwards, and on the opposite side, in New Zealand it's also accelerating upwards but in the opposite direction because the Earth is a ball. In the thread that I linked, I was trying to argue the notion that this happens because space (or the element of space that determines the position and motion of inertial frames) is constantly falling into massive bodies, whether they are a black hole, or a planet like the Earth.
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