4 ms·

How is it not a force though? Regardless of curvature, a ball starts moving if you let it go without applying any force. Curvature alone can't account for tha

How is it not a force though? Regardless of curvature, a ball starts moving if you let it go without applying any force. Curvature alone can't account for that could it?

doomrobo 6y ago
When you are at rest on the surface of the earth, you’re actually accelerating. I barely understand it but veritasium explains it in the newest vid https://youtu.be/XRr1kaXKBsU https://youtu.be/XRr1kaXKBsU
saagarjha 6y ago
In general, it is convenient to assume that forces exist. However, the model presented here shows that you can instead dispense of the need for there to be a force and show that the apparent acceleration is just the object moving along its curved path in spacetime. You could consider it as if the curvature itself making it look like there are forces.
tim_hutton 6y ago
An object at rest is still travelling through time. In fact it continues to travel at the same speed through spacetime when you let it go. It's just that spacetime is distorted such that the straight line takes it across space too, as seen in the inertial frame on the right.
gilgoomesh 6y ago
The falling object doesn't accelerate. You, standing on the ground are the one that's accelerating. You see the object as accelerating but that's an illusion due to frames of reference. As evidence: which object feels a force on it? You can feel the force the ground continually pushes up at you. The ground is accelerating you up. The falling object is completely idle in its inertial frame and feels nothing.
- colordrops 6y ago
  Ok so that makes sense, but if we are accelerating by standing on the ground, isn't that implying that we are continuously increasing in energy? Couldn't that be harvested for a perpetual motion machine of some sort?
  - drdeca 6y ago
    First, "it all adds up to normality". Second, hm, I'm not sure. Well, the glass on the table next to me is "accelerating", but in my frame of reference (which is the same as that of the glass) it is also stationary...? If I drop a ball above the glass, and we consider things from the frame of reference where the ball is stationary, which (neglecting air resistance) is an inertial reference frame. In this frame, the glass is accelerating ball-wards , because of the table pushing on it, and, this force is applied over a distance as the glass approaches the ball. So, yes, it seems that the KE of the glass should be increasing in the reference frame of the ball. Hm, I'm confused. I suppose if we consider a geodesic representing a periodic orbit around the earth, that the positions and velocities of various objects on earth will be in a cyclic pattern?
    - colordrops 6y ago
      The layman descriptions are probably incomplete or rough abstractions missing nuance. Reminds me of the idea of a rubber membrane with balls on it representing the curvature of spacetime explaining how gravity works - I've always disliked that example because you still need plain old gravity to pull the balls into the membrane.
  - gilgoomesh 6y ago
    Energy is Force times Distance. You're not moving (even though you're accelerating) so there's no energy expended. See Veritasium's video at 10m17s: https://youtu.be/XRr1kaXKBsU?t=617 https://youtu.be/XRr1kaXKBsU?t=617 He addresses that basic science makes this confusing because there's a curvature term that is usually left out of acceleration equations which balances out when you're accelerating along your curvature (e.g. against gravity), instead of along your spatial coordinates.
mikelevins 6y ago
It acts like an acceleration, rather than a force. Imagine a bowling ball and a marble at rest from your perspective, and an apparatus with a pair of pneumatic guns that eject pistons with the same precisely-calibrated amount of force. You arrange the guns so that they will hit the marble and the bowling ball at the same moment, and you trigger them together. The marble and the bowling ball are hit at the same time by the same amount of force. The marble, being much lighter takes off much faster than the bowling ball. Now take the same marble and bowling ball to a place a mile above the moon (so that there is no confounding atmosphere to complicate things) and release them next to one another at the same moment. If gravity is a force, then we should expect that the marble will fall much faster than the bowling ball, because the same force is acting on two different masses; the lighter mass should be accelerated more, just as it was when the source of the force was the pneumatic gun. F = ma, after all. If the force is the same and the mass is less, then the acceleration must be more. That's not what happens, though. The marble and the bowling ball fall together at the same accelerating rate. Gravity acts like an acceleration, not a force. Lo these many years ago when I was an undergraduate physics student, my advisor told me that we should say "the force due to gravity", not "the force of gravity". In every day colloquial speech it doesn't matter, of course.

How is it not a force though? Regardless of curvature, a ball starts moving if you let it go without applying any force. Curvature alone can't account for that could it?

doomrobo 6y ago

When you are at rest on the surface of the earth, you’re actually accelerating. I barely understand it but veritasium explains it in the newest vid https://youtu.be/XRr1kaXKBsU https://youtu.be/XRr1kaXKBsU

saagarjha 6y ago

In general, it is convenient to assume that forces exist. However, the model presented here shows that you can instead dispense of the need for there to be a force and show that the apparent acceleration is just the object moving along its curved path in spacetime. You could consider it as if the curvature itself making it look like there are forces.

tim_hutton 6y ago

An object at rest is still travelling through time. In fact it continues to travel at the same speed through spacetime when you let it go. It's just that spacetime is distorted such that the straight line takes it across space too, as seen in the inertial frame on the right.

gilgoomesh 6y ago

The falling object doesn't accelerate. You, standing on the ground are the one that's accelerating. You see the object as accelerating but that's an illusion due to frames of reference. As evidence: which object feels a force on it? You can feel the force the ground continually pushes up at you. The ground is accelerating you up. The falling object is completely idle in its inertial frame and feels nothing.

colordrops 6y ago
Ok so that makes sense, but if we are accelerating by standing on the ground, isn't that implying that we are continuously increasing in energy? Couldn't that be harvested for a perpetual motion machine of some sort?
- drdeca 6y ago
  First, "it all adds up to normality". Second, hm, I'm not sure. Well, the glass on the table next to me is "accelerating", but in my frame of reference (which is the same as that of the glass) it is also stationary...? If I drop a ball above the glass, and we consider things from the frame of reference where the ball is stationary, which (neglecting air resistance) is an inertial reference frame. In this frame, the glass is accelerating ball-wards , because of the table pushing on it, and, this force is applied over a distance as the glass approaches the ball. So, yes, it seems that the KE of the glass should be increasing in the reference frame of the ball. Hm, I'm confused. I suppose if we consider a geodesic representing a periodic orbit around the earth, that the positions and velocities of various objects on earth will be in a cyclic pattern?
  - colordrops 6y ago
    The layman descriptions are probably incomplete or rough abstractions missing nuance. Reminds me of the idea of a rubber membrane with balls on it representing the curvature of spacetime explaining how gravity works - I've always disliked that example because you still need plain old gravity to pull the balls into the membrane.
- gilgoomesh 6y ago
  Energy is Force times Distance. You're not moving (even though you're accelerating) so there's no energy expended. See Veritasium's video at 10m17s: https://youtu.be/XRr1kaXKBsU?t=617 https://youtu.be/XRr1kaXKBsU?t=617 He addresses that basic science makes this confusing because there's a curvature term that is usually left out of acceleration equations which balances out when you're accelerating along your curvature (e.g. against gravity), instead of along your spatial coordinates.

mikelevins 6y ago

It acts like an acceleration, rather than a force. Imagine a bowling ball and a marble at rest from your perspective, and an apparatus with a pair of pneumatic guns that eject pistons with the same precisely-calibrated amount of force. You arrange the guns so that they will hit the marble and the bowling ball at the same moment, and you trigger them together. The marble and the bowling ball are hit at the same time by the same amount of force. The marble, being much lighter takes off much faster than the bowling ball. Now take the same marble and bowling ball to a place a mile above the moon (so that there is no confounding atmosphere to complicate things) and release them next to one another at the same moment. If gravity is a force, then we should expect that the marble will fall much faster than the bowling ball, because the same force is acting on two different masses; the lighter mass should be accelerated more, just as it was when the source of the force was the pneumatic gun. F = ma, after all. If the force is the same and the mass is less, then the acceleration must be more. That's not what happens, though. The marble and the bowling ball fall together at the same accelerating rate. Gravity acts like an acceleration, not a force. Lo these many years ago when I was an undergraduate physics student, my advisor told me that we should say "the force due to gravity", not "the force of gravity". In every day colloquial speech it doesn't matter, of course.