3 ms·

> Your acceleration is zero--you're in free fall. So if I jump out of an aeroplane, I don't accelerate towards the ground? Apparently my understanding of the n

> Your acceleration is zero--you're in free fall. So if I jump out of an aeroplane, I don't accelerate towards the ground? Apparently my understanding of the natural world needs some enhancements.

ninkendo 3y ago
Nope, you're in an inertial reference frame when in free-fall. Ignoring air resistance, if you were to take an apple out of your pocket and place it next to you in mid-air, it would be motionless relative to yourself. Put a box around you with no windows and you'll have no idea whether you're floating through space in the absence of gravity, or if you're free-falling inside a gravity well. This is the main difference between general relativity and the newtonian notion of gravity: GR says gravity is just spacetime curvature, and by following an inertial (geodesic) path through spacetime, it appears to be acceleration to certain observers, but is in fact inertial. Newtonian mechanics says instead that gravity is a force and you're indeed accelerating, but it's a view that breaks down at high mass/speed, and GR makes correct predictions where Newtonian mechanics does not, so we generally prefer the GR interpretation.
- smallnamespace 3y ago
  > you're in an inertial reference frame when in free-fall. But the falling person's frame isn't the only one available. Someone on the ground would rightfully describe the falling person as accelerating towards the ground (or them) (at least until they reach terminal velocity). The most important lesson of relativity is that no reference frame is privileged. That means both descriptions are valid, provided you attach the reference frame to the description.
  - pdonis 3y ago
    > The most important lesson of relativity is that no reference frame is privileged. No, the most important lesson of relativity is that only invariants are physically meaningful. IIRC Einstein once commented that he wished his theory had been called the "theory of invariants" since that would have been a better description of its most important feature. The second most important lesson of relativity is that yes, you can use whatever frame you wish, because invariants are the same no matter what frame you use to calculate them. So you will come up with the same physically meaningful quantities no matter what frame you choose.
    - 867-5309 3y ago
      all this talk of GR and invariants, not to confuse a mathematical singularity with a gravitational one, at which point GR itself breaks down per the need for a quantum mechanical description. the truth is we don't know what happens within a black hole, and probably never will
      - pdonis 3y ago
        > GR itself breaks down per the need for a quantum mechanical description This is the current predominant belief among physicists, yes, but that doesn't mean it's established fact. > the truth is we don't know what happens within a black hole, and probably never will This is way too pessimistic. The predictions of GR for this regime are precise and unequivocal. Even if you believe GR breaks down when spacetime curvatures reach the Planck scale (which is the current predominant belief, as I said above), that still leaves most of the black hole's interior within GR's domain of validity.
        867-5309 3y ago
        but if most of the interior is concentrated into an infinite mass, i.e. impossible, that kinda invalidates its validity
        denton-scratch 3y ago
        > that still leaves most of the black hole's interior within GR's domain of validity. Thanks, that's what I (as a layman) supposed: that inside the event horizon isn't some space that you can't think about; it's qualitatively not distinguishable from outide. Upthread, someone mentioned that the singularity wasn't a place in space, it was a location in time. That made me think; the mass of a BH isn't the mass of a volume of space, it's the mass of the "singularity", which (presumably?) is located at the center of the space described by the event horizon. So there must be a place where the singularity is located, and there must be a direction towards it. Someone else suggested there are two things: a mathematical singularity and a gravitational singularity, and that they aren't the same. That hadn't crossed my mind.
- hans0l074 3y ago
  So is it correct to say that Newtonian mechanics is "just" an approximation (based on our observations) for all practical purposes? (EDIT: Spelling)
akomtu 3y ago
Falling in a black hole is a different experience. It must be more like being squeezed in a narrow tube with a glowing light that was once the universe on one end of the tube and something indescribable on the other end. Perception of time will change too, as all the particles that let us perceive the time will be messed up in strange ways. Perhaps time will seem multidimensional or there will be many timelines. In other words, your perception of a fall from an aeroplane isn't true or absolute, it's mediated by particles that define, for you, space and time, and if those particles get messed up, your reality will change completely.
- pdonis 3y ago
  None of what you say is correct as a description of what General Relativity says will happen to someone who falls into a black hole.
- denton-scratch 3y ago
  I think the "tube" business concerns what you see a you approach the speed of light. All the light/future events are in front of you, and you see nothing if you stand on the stern of your ship. I'm not sure how that relates to falling into a BH, bit I don't know; there's a lot of this that I'm struggling to understand. I'm not a physicist, and not clever, just very interested.
  - pdonis 3y ago
    > I'm not sure how that relates to falling into a BH The special relativistic effect you describe has nothing whatever to do with falling into a BH.
pdonis 3y ago
> if I jump out of an aeroplane, I don't accelerate towards the ground? Not in your rest frame. In your rest frame, the ground accelerates towards you. However, the acceleration described in both frames (yours and the Earth's) is coordinate acceleration, which is frame-dependent, and in relativity frame-dependent quantities don't have any direct physical meaning. The quantity that has physical meaning in terms of acceleration is proper acceleration--what an accelerometer strapped to you would read--and in free fall that is zero.
AdamH12113 3y ago
> So if I jump out of an aeroplane, I don't accelerate towards the ground? In a very real sense (although not a useful one for a skydiver), the answer is yes. This is a key insight of the Equivalence Principle and the curved-spacetime view of gravity. You (in free-fall) are on an inertial "straight line" path through spacetime towards the center of the Earth. When you reach the ground, it pushes on you, messily accelerating you off of that path.
rocqua 3y ago
In some sense, the ground is accelerating towards you. By jumping out of the plane, you've stopped the plane floor from accelrating you away from the ground at the same rate the ground is accelerating towards you. This is barely useful for humans since we are very used to 'standing still' actually involving the force of your weight, so we consider that distinct from acceleration. But physics gets much easier if you don't make that distinction.

> Your acceleration is zero--you're in free fall. So if I jump out of an aeroplane, I don't accelerate towards the ground? Apparently my understanding of the natural world needs some enhancements.

ninkendo 3y ago

Nope, you're in an inertial reference frame when in free-fall. Ignoring air resistance, if you were to take an apple out of your pocket and place it next to you in mid-air, it would be motionless relative to yourself. Put a box around you with no windows and you'll have no idea whether you're floating through space in the absence of gravity, or if you're free-falling inside a gravity well. This is the main difference between general relativity and the newtonian notion of gravity: GR says gravity is just spacetime curvature, and by following an inertial (geodesic) path through spacetime, it appears to be acceleration to certain observers, but is in fact inertial. Newtonian mechanics says instead that gravity is a force and you're indeed accelerating, but it's a view that breaks down at high mass/speed, and GR makes correct predictions where Newtonian mechanics does not, so we generally prefer the GR interpretation.

smallnamespace 3y ago
> you're in an inertial reference frame when in free-fall. But the falling person's frame isn't the only one available. Someone on the ground would rightfully describe the falling person as accelerating towards the ground (or them) (at least until they reach terminal velocity). The most important lesson of relativity is that no reference frame is privileged. That means both descriptions are valid, provided you attach the reference frame to the description.
- pdonis 3y ago
  > The most important lesson of relativity is that no reference frame is privileged. No, the most important lesson of relativity is that only invariants are physically meaningful. IIRC Einstein once commented that he wished his theory had been called the "theory of invariants" since that would have been a better description of its most important feature. The second most important lesson of relativity is that yes, you can use whatever frame you wish, because invariants are the same no matter what frame you use to calculate them. So you will come up with the same physically meaningful quantities no matter what frame you choose.
  - 867-5309 3y ago
    all this talk of GR and invariants, not to confuse a mathematical singularity with a gravitational one, at which point GR itself breaks down per the need for a quantum mechanical description. the truth is we don't know what happens within a black hole, and probably never will
    - pdonis 3y ago
      > GR itself breaks down per the need for a quantum mechanical description This is the current predominant belief among physicists, yes, but that doesn't mean it's established fact. > the truth is we don't know what happens within a black hole, and probably never will This is way too pessimistic. The predictions of GR for this regime are precise and unequivocal. Even if you believe GR breaks down when spacetime curvatures reach the Planck scale (which is the current predominant belief, as I said above), that still leaves most of the black hole's interior within GR's domain of validity.
      - 867-5309 3y ago
        but if most of the interior is concentrated into an infinite mass, i.e. impossible, that kinda invalidates its validity
      - denton-scratch 3y ago
        > that still leaves most of the black hole's interior within GR's domain of validity. Thanks, that's what I (as a layman) supposed: that inside the event horizon isn't some space that you can't think about; it's qualitatively not distinguishable from outide. Upthread, someone mentioned that the singularity wasn't a place in space, it was a location in time. That made me think; the mass of a BH isn't the mass of a volume of space, it's the mass of the "singularity", which (presumably?) is located at the center of the space described by the event horizon. So there must be a place where the singularity is located, and there must be a direction towards it. Someone else suggested there are two things: a mathematical singularity and a gravitational singularity, and that they aren't the same. That hadn't crossed my mind.
hans0l074 3y ago
So is it correct to say that Newtonian mechanics is "just" an approximation (based on our observations) for all practical purposes? (EDIT: Spelling)

akomtu 3y ago

Falling in a black hole is a different experience. It must be more like being squeezed in a narrow tube with a glowing light that was once the universe on one end of the tube and something indescribable on the other end. Perception of time will change too, as all the particles that let us perceive the time will be messed up in strange ways. Perhaps time will seem multidimensional or there will be many timelines. In other words, your perception of a fall from an aeroplane isn't true or absolute, it's mediated by particles that define, for you, space and time, and if those particles get messed up, your reality will change completely.

pdonis 3y ago
None of what you say is correct as a description of what General Relativity says will happen to someone who falls into a black hole.
denton-scratch 3y ago
I think the "tube" business concerns what you see a you approach the speed of light. All the light/future events are in front of you, and you see nothing if you stand on the stern of your ship. I'm not sure how that relates to falling into a BH, bit I don't know; there's a lot of this that I'm struggling to understand. I'm not a physicist, and not clever, just very interested.
- pdonis 3y ago
  > I'm not sure how that relates to falling into a BH The special relativistic effect you describe has nothing whatever to do with falling into a BH.

pdonis 3y ago

> if I jump out of an aeroplane, I don't accelerate towards the ground? Not in your rest frame. In your rest frame, the ground accelerates towards you. However, the acceleration described in both frames (yours and the Earth's) is coordinate acceleration, which is frame-dependent, and in relativity frame-dependent quantities don't have any direct physical meaning. The quantity that has physical meaning in terms of acceleration is proper acceleration--what an accelerometer strapped to you would read--and in free fall that is zero.

AdamH12113 3y ago

> So if I jump out of an aeroplane, I don't accelerate towards the ground? In a very real sense (although not a useful one for a skydiver), the answer is yes. This is a key insight of the Equivalence Principle and the curved-spacetime view of gravity. You (in free-fall) are on an inertial "straight line" path through spacetime towards the center of the Earth. When you reach the ground, it pushes on you, messily accelerating you off of that path.

rocqua 3y ago

In some sense, the ground is accelerating towards you. By jumping out of the plane, you've stopped the plane floor from accelrating you away from the ground at the same rate the ground is accelerating towards you. This is barely useful for humans since we are very used to 'standing still' actually involving the force of your weight, so we consider that distinct from acceleration. But physics gets much easier if you don't make that distinction.