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> how the heck is this 100lb weight a 100lb weight? Because the scale that reads 100 lb is pushing up on the weight. The same would be true if the scale was in

by pdonis 3y ago

> how the heck is this 100lb weight a 100lb weight? Because the scale that reads 100 lb is pushing up on the weight. The same would be true if the scale was inside a rocket accelerating at 1 g in deep space, far from all gravitating bodies. (This is called the equivalence principle.) In other words, the 100 lb of force you are calling "weight" is not a "force of gravity"; there is no such thing. It's a non-gravitational force exerted by the scale on the weight.

lazide 3y ago
The force drawing the weight and the mass of the earth 'under' it together (with the scale in the middle) however clearly exists as much as the electromagnetic force though. Or stars, planets, etc. wouldn't (and couldn't) exist. As to the exact details of how we account for it is another matter. Calling gravity 'measured spacetime curvature' or 'gravitational force', or coming up with pseudo particles, or whatever, is an accounting method. Either way, it's still there. But it does clearly exist, as much as light (for example) does. All one needs to do to prove that is look out the window.
- pdonis 3y ago
  > The force drawing the weight and the mass of the earth 'under' it together There is no such force. Think of the accelerating rocket: there is no "force drawing the weight and the scale together". There is just the rocket pushing. The same is true standing on the Earth: there is just the ground pushing. One of the reasons we need "spacetime curvature" in GR is to explain how the ground can be pushing things in different directions in different parts of the Earth, while all those things still remain stationary relative to the Earth's center and each other. But the whole point is that, even with all that factored in, there is still no "force drawing the weight and the Earth together". There is just the ground pushing up, plus spacetime geometry to account for the global configuration. That's it.
  - lazide 3y ago
    Except that clearly isn't it, as the weight moves even if there is no 'ground' (say at altitude) in the same predictable fashion, until it collides with something - which merely attempts to arrest it's movement. In fact, in orbit the forces felt are still (quite clearly) there too. So if anything, gravity wise 'pushing' is not a property of gravity at all. Merely of matter which happens to produce the spacetime distortions which result in gravity. Unless dark matter exists, which would be neat. Or the moon wouldn't continue to be up there, instead of 'down here'. And clouds of hydrogen atoms will eventually collect together in a vacuum barring other outside gravitational influences. Which is notably why we even have a star. And the weight itself also has measurable (albeit extremely tiny) such effects on everything else too. So how do you model/name/account for that force which causes that to occur? Since it does clearly exist. Because a rocket doesn't just 'push' either. We can clearly model the chemical and physics behaviors going on there to generate that 'push', and all involve forces which we can account for. None of them meaningfully appear to be gravity. And gravity also involve forces (spacetime distortions causing very real effects!). Which we can also account for. So seriously, what are you even talking about?
    - pdonis 3y ago
      > the weight moves even if there is no 'ground' (say at altitude) in the same predictable fashion "Moves" is frame-dependent. In the weight's own rest frame, it is the Earth that moves. What is not frame-dependent is the fact that, if there is no ground and the weight (wrong word, as we'll see in a moment) is freely falling, there is no "weight"--the object feels no force and an accelerometer attached to it reads zero. This was the basic insight that started Einstein on the road to curved spacetime and General Relativity: if an object falls freely, it will not feel its own weight. > how do you model/name/account for that force which causes that to occur? Using the word "force" here is already wrong as far as General Relativity is concerned: in General Relativity gravity is not a force. > a rocket doesn't just 'push' either. We can clearly model the chemical and physics behaviors going on there to generate that 'push', and all involve forces which we can account for. None of them meaningfully appear to be gravity. Yes, exactly. Now apply the same principle to the Earth pushing up on the weight. We can clearly model all of the microscopic behaviors that lead to that push and the forces that account for them--and none of them are gravity. > gravity also involve forces (spacetime distortions causing very real effects!) The "spacetime distortions" you refer to, which do indeed cause real effects, are not "forces". That's the whole point. They are spacetime geometry. Spacetime geometry is not a force. That is what General Relativity says. > what are you even talking about? I am talking about standard General Relativity, as it has been understood and taught in textbooks for decades now. What are you talking about?
  - Scubabear68 3y ago
    Here is where an have a problem. The ground isn’t “pushing”. The ground is resisting my weight, and may even be compressed as a result.
    - SamBam 3y ago
      If you're just standing in an elevator, how can you tell the difference between the floor of the elevator "resisting" your weight due to gravity, vs. being out in space with silent rockets propelling the elevator at 9.8 m/s^2 and "pushing" you? Either way you'll feel like you and the elevator floor are being pushed together, and both you and the floor will experience some compression.
      - Scubabear68 3y ago
        Very true you cannot tell the difference. That was not the question, though. The earth isn’t pushing back.
        SamBam 3y ago
        I'm saying the distinction between "resisting" and "pushing" isn't real. When a rocket is accelerating and you are pressed against the back and being driven forward, is the rocket "pushing" you or "resisting" you? Either one is a fine way of describing it. The net result, though, is that you are squished into the back of the rocket in a way that's completely indistinguishable from gravity. You may say "but the ground can never push me away from the ground." Sure, but the back of the rocket can't push you away from the back of the rocket either, and yet it is clearly pushing you through space. So long as the rocket keeps accelerating, you are being pushed forward, yet you won't feel it as a push, you'll feel it as an attraction to the back of the rocket.
        deleted 3y ago
        [deleted]
        pdonis 3y ago
        > The earth isn’t pushing back. Yes, it is. The force you feel as weight is the ground pushing on you. By Newton's Third Law, the ground also feels a force of equal magnitude and opposite direction from you, which can, as you say, end up compressing the ground. But that isn't what you feel as weight.
silent_cal 3y ago
This is my first time hearing this. I don't study physics much but this doesn't sound right at all. You're telling me if I drop something there is no force pulling it down? That makes no sense.
- pdonis 3y ago
  > You're telling me if I drop something there is no force pulling it down? Yes. You and the ground are being pushed upwards by a force. You can feel that force--it's your weight. The dropped object falls relative to you precisely because it does not feel any force--it is weightless. This is exactly the same thing that would happen in a rocket whose engine was firing in deep space, far from all gravitating bodies. This is an example of the equivalence principle, which is a central experimental fact that Newtonian gravity just had to accept as being true for no apparent reason, but which GR accounts for naturally as an aspect of spacetime geometry.

by pdonis 3y ago

lazide 3y ago

The force drawing the weight and the mass of the earth 'under' it together (with the scale in the middle) however clearly exists as much as the electromagnetic force though. Or stars, planets, etc. wouldn't (and couldn't) exist. As to the exact details of how we account for it is another matter. Calling gravity 'measured spacetime curvature' or 'gravitational force', or coming up with pseudo particles, or whatever, is an accounting method. Either way, it's still there. But it does clearly exist, as much as light (for example) does. All one needs to do to prove that is look out the window.

pdonis 3y ago
> The force drawing the weight and the mass of the earth 'under' it together There is no such force. Think of the accelerating rocket: there is no "force drawing the weight and the scale together". There is just the rocket pushing. The same is true standing on the Earth: there is just the ground pushing. One of the reasons we need "spacetime curvature" in GR is to explain how the ground can be pushing things in different directions in different parts of the Earth, while all those things still remain stationary relative to the Earth's center and each other. But the whole point is that, even with all that factored in, there is still no "force drawing the weight and the Earth together". There is just the ground pushing up, plus spacetime geometry to account for the global configuration. That's it.
- lazide 3y ago
  Except that clearly isn't it, as the weight moves even if there is no 'ground' (say at altitude) in the same predictable fashion, until it collides with something - which merely attempts to arrest it's movement. In fact, in orbit the forces felt are still (quite clearly) there too. So if anything, gravity wise 'pushing' is not a property of gravity at all. Merely of matter which happens to produce the spacetime distortions which result in gravity. Unless dark matter exists, which would be neat. Or the moon wouldn't continue to be up there, instead of 'down here'. And clouds of hydrogen atoms will eventually collect together in a vacuum barring other outside gravitational influences. Which is notably why we even have a star. And the weight itself also has measurable (albeit extremely tiny) such effects on everything else too. So how do you model/name/account for that force which causes that to occur? Since it does clearly exist. Because a rocket doesn't just 'push' either. We can clearly model the chemical and physics behaviors going on there to generate that 'push', and all involve forces which we can account for. None of them meaningfully appear to be gravity. And gravity also involve forces (spacetime distortions causing very real effects!). Which we can also account for. So seriously, what are you even talking about?
  - pdonis 3y ago
    > the weight moves even if there is no 'ground' (say at altitude) in the same predictable fashion "Moves" is frame-dependent. In the weight's own rest frame, it is the Earth that moves. What is not frame-dependent is the fact that, if there is no ground and the weight (wrong word, as we'll see in a moment) is freely falling, there is no "weight"--the object feels no force and an accelerometer attached to it reads zero. This was the basic insight that started Einstein on the road to curved spacetime and General Relativity: if an object falls freely, it will not feel its own weight. > how do you model/name/account for that force which causes that to occur? Using the word "force" here is already wrong as far as General Relativity is concerned: in General Relativity gravity is not a force. > a rocket doesn't just 'push' either. We can clearly model the chemical and physics behaviors going on there to generate that 'push', and all involve forces which we can account for. None of them meaningfully appear to be gravity. Yes, exactly. Now apply the same principle to the Earth pushing up on the weight. We can clearly model all of the microscopic behaviors that lead to that push and the forces that account for them--and none of them are gravity. > gravity also involve forces (spacetime distortions causing very real effects!) The "spacetime distortions" you refer to, which do indeed cause real effects, are not "forces". That's the whole point. They are spacetime geometry. Spacetime geometry is not a force. That is what General Relativity says. > what are you even talking about? I am talking about standard General Relativity, as it has been understood and taught in textbooks for decades now. What are you talking about?
- Scubabear68 3y ago
  Here is where an have a problem. The ground isn’t “pushing”. The ground is resisting my weight, and may even be compressed as a result.
  - SamBam 3y ago
    If you're just standing in an elevator, how can you tell the difference between the floor of the elevator "resisting" your weight due to gravity, vs. being out in space with silent rockets propelling the elevator at 9.8 m/s^2 and "pushing" you? Either way you'll feel like you and the elevator floor are being pushed together, and both you and the floor will experience some compression.
    - Scubabear68 3y ago
      Very true you cannot tell the difference. That was not the question, though. The earth isn’t pushing back.
      - SamBam 3y ago
        I'm saying the distinction between "resisting" and "pushing" isn't real. When a rocket is accelerating and you are pressed against the back and being driven forward, is the rocket "pushing" you or "resisting" you? Either one is a fine way of describing it. The net result, though, is that you are squished into the back of the rocket in a way that's completely indistinguishable from gravity. You may say "but the ground can never push me away from the ground." Sure, but the back of the rocket can't push you away from the back of the rocket either, and yet it is clearly pushing you through space. So long as the rocket keeps accelerating, you are being pushed forward, yet you won't feel it as a push, you'll feel it as an attraction to the back of the rocket.
        deleted 3y ago
        [deleted]
      - pdonis 3y ago
        > The earth isn’t pushing back. Yes, it is. The force you feel as weight is the ground pushing on you. By Newton's Third Law, the ground also feels a force of equal magnitude and opposite direction from you, which can, as you say, end up compressing the ground. But that isn't what you feel as weight.

silent_cal 3y ago

This is my first time hearing this. I don't study physics much but this doesn't sound right at all. You're telling me if I drop something there is no force pulling it down? That makes no sense.

pdonis 3y ago
> You're telling me if I drop something there is no force pulling it down? Yes. You and the ground are being pushed upwards by a force. You can feel that force--it's your weight. The dropped object falls relative to you precisely because it does not feel any force--it is weightless. This is exactly the same thing that would happen in a rocket whose engine was firing in deep space, far from all gravitating bodies. This is an example of the equivalence principle, which is a central experimental fact that Newtonian gravity just had to accept as being true for no apparent reason, but which GR accounts for naturally as an aspect of spacetime geometry.