B Is Gravity an illusion of a force but really a curvature of spacetime?

[ESponge2000]

TL;DR Summary

And does that mean when jumping off a plane without a parachute (or regardless of one in a world without air resistance), I’m not really accelerating in a universal way and would actually be at rest ?

And right now while we are at rest relative to earth we are not universally at rest because the floor not allowing us to fall deeper into the earth is a real force acting acceleration upon us?

I heard this from one of Sabine’s YouTube videos and it is inconsistent with “Gravity is a Natural Force’ or how we define ‘force’?

 

[PeroK]

TL;DR Summary: And does that mean when jumping off a plane without a parachute (or regardless of one in a world without air resistance), I’m not really accelerating in a universal way and would actually be at rest ?

And right now while we are at rest relative to earth we are not universally at rest because the floor not allowing us to fall deeper into the earth is a real force acting acceleration upon us?

I heard this from one of Sabine’s YouTube videos and it is inconsistent with “Gravity is a Natural Force’ or how we define ‘force’?

If you jump out of a plane, you are essentially in freefall - following a natural force-free trajectory through spacetime. Motion is relative. In the reference frame of the Earth, you are accelerating towards the surface.

By contrast, someone standing on the surface of the Earth is subject to an upward force from the ground and is not following a natural trajectory. They are at rest relative to the Earth and accelerating upwards relative to you.

There is no such thing as a state of absolute rest. That idea goes back to Newton and Galileo.

 

[PeroK]

PS if you thing gravity is an illusion, then that delusion will be shattered when you hit the ground.

 

[Ibix]

TL;DR Summary: And does that mean when jumping off a plane without a parachute (or regardless of one in a world without air resistance), I’m not really accelerating in a universal way and would actually be at rest ?

And right now while we are at rest relative to earth we are not universally at rest because the floor not allowing us to fall deeper into the earth is a real force acting acceleration upon us?

There's no such thing as "universally at rest". There is such a thing as "moving inertially" or "accelerating". In a force model of gravity if you stand on the floor you are inertial because the net force (gravity plus normal reaction from the floor) is zero. In a spacetime curvature model you are accelerating because there is an unbalanced reaction force from the floor pushing you out of your free-fall path.

Currently our best theory of gravity is a spacetime curvature based one, but there's nothing wrong with switching perspectives and using a force model when it is precise enough for your purposes.

I heard this from one of Sabine’s YouTube videos and it is inconsistent with “Gravity is a Natural Force’ or how we define ‘force’?

Be wary of Sabine Hossenfelder's videos on relativity. At least one she's posted, attempting to explain the twin paradox, went down the "acceleration causes time dilation" route, which is inconsistent with relativity.

 

[ESponge2000]

If you jump out of a plane, you are essentially in freefall - following a natural force-free trajectory through spacetime. Motion is relative. In the reference frame of the Earth, you are accelerating towards the surface.

By contrast, someone standing on the surface of the Earth is subject to an upward force from the ground and is not following a natural trajectory. They are at rest relative to the Earth and accelerating upwards relative to you.

There is no such thing as a state of absolute rest. That idea goes back to Newton and Galileo.

But a “change” in velocity is an absolute because as we know it is what allows for twin paradox and for differences in spacetime travel to have absolute meaning such as where 2 observers are reunited who took different paths through space-time

 

[PeroK]

But a “change” in velocity is an absolute

Even in Newtonian mechanics, there is the concept of an accelerating reference frame. This is the problem with trying to learn advanced physics without ever having studied the basics. A change in velocity may be nothing more than being measured in an non-inertial reference frame.

because as we know it is what allows for twin paradox and for differences in spacetime travel to have absolute meaning such as where 2 observers are reunited who took different paths through space-time

The twin paradox is fundamentally about flat spacetime. Gravity takes you into the realm of General relativity. Imagine two satellites in almost the same orbit, but orbitting in opposite directions. Each is in constant relative motion and acceleration relative to the other. By symmetry, however, each time their paths cross their clocks must remain synchonized.

 

[Ibix]

But a “change” in velocity is an absolute

No. You can always choose a frame in which either twin has zero velocity. Proper acceleration is absolute in the sense of being a direct measurable, but whether or not it leads to a change in velocity depends on your choice of coordinate system.

 

[ESponge2000]

Even in Newtonian mechanics, there is the concept of an accelerating reference frame. This is the problem with trying to learn advanced physics without ever having studied the basics. A change in velocity may be nothing more than being measured in an non-inertial reference frame.

The twin paradox is fundamentally about flat spacetime. Gravity takes you into the realm of General relativity. Imagine two satellites in almost the same orbit, but orbitting in opposite directions. Each is in constant relative motion and acceleration relative to the other. By symmetry, however, each time their paths cross their clocks must remain

The change in velocity has to be fundamentally different from being at rest since it enacts a change in inertia “under acceleration in a car for example you have a seatbelt for a reason

 

[PeroK]

The change in velocity has to be fundamentally different from being at rest since it enacts a change in inertia “under acceleration in a car for example you have a seatbelt for a reason

Did you come here to learn physics or try to persuade the rest of us that we are wrong?

 

[ESponge2000]

If there is a moving platform underneath a car and the car is in neutral, and the car engine is off and not operating at all … it will appear relative to the moving floor that the car is in motion … now if the moving floor is what’s being powered by an electric current , not the car

And then the moving floor is suddenly shut off with a rapid de acceleration, and I’m inside the car , is it true or not true that i would see the illusion of the car coming to a rapid stop, the wheels of the car forced to stop spinning round and round , but I would not need a seatbelt because my inertia was and remains at rest with the earth and it’s the moving platform beneath the car that changed inertial frames ?

My understanding is yes in this situation there would not be a change in inertia inside the car but yet if i am staring at the moving floor as if its a rest frame , at constant velocity I will get the illusion the car could be in operation but once the platform turns off i won’t detect this change in inertia which is very important as it illustrates the point that while velocity is relative , help me understand how a change in inertia isn’t an absolute ? In this exercise you can distinguish it by the need for a seatbelt vs not

 

[PeroK]

If there is a moving platform underneath a car and the car is in neutral, and the car engine is off and not operating at all … it will appear relative to the moving floor that the car is in motion … now if the moving floor is what’s being powered by an electric current , not the car

And then the moving floor is suddenly shut off with a rapid de acceleration, and I’m inside the car , is it true or not true that i would see the illusion of the car coming to a rapid stop, the wheels of the car forced to stop spinning round and round , but I would not need a seatbelt because my inertia was and remains at rest with the earth and it’s the moving platform beneath the car that changed inertial frames ?

My understanding is yes in this situation there would not be a change in inertia inside the car but yet if i am staring at the moving floor as if its a rest frame , at constant velocity I will get the illusion the car could be in operation but once the platform turns off i won’t detect this change in inertia which is very important as it illustrates the point that while velocity is relative , help me understand how a change in inertia isn’t an absolute ? In this exercise you can distinguish it by the need for a seatbelt vs not

The relevant concept is that of a non-inertial reference frame. In Newtonian physics, Newton's laws hold in any inertial reference frame. But, in a non-inertial frame, Newton's laws do not hold - unless you include one or more fictitious forces.

In an inertial frame, an object only accelerates if acted on by an unbalanced (real) force. In a non-inertial frame, an object may accelerate when acted on by no real forces. I.e. Newton's first law does not hold.

When analysing motion you must know, therefore, whether you are using an inertial or non-inertial reference frame.

This also holds in special relativity, although dealing with non-inertial frames is trickier than it is in Newtonian physics.

In GR, there are no global inertial reference frames. You can never simplify things to the same extent that you can in Newtonian physics or Special Relativity.

For example, in GR, in the reference frame of the Sun, the Earth moves in a circular orbit (which implies kinematic, centripetal acceleration). However, there is no real centripetal force acting on the Earth. This shows that the Sun's reference frame is non-inertial.

There is no way to explain this using Newtonian physics - which would require a force to keep the Earth in orbit. There is no point, therefore, in starting to study GR with Newton's laws (or even SR) as your guiding principle. GR is a different ball-game altogether.

 

[Dale]

PS if you thing gravity is an illusion, then that delusion will be shattered when you hit the ground.

When you hit the ground you are subject to very large non-gravitational forces. A better example of non-illusory gravity would be tidal forces.

 

[PeroK]

When you hit the ground you are subject to very large non-gravitational forces. A better example of non-illusory gravity would be tidal forces.

In my capacity as an amateur mountaineer, tidal forces concern me much less than gravity as manifested by a relatively large free-fall acceleration towards the Earth.

 

[Dale]

The change in velocity has to be fundamentally different from being at rest since it enacts a change in inertia “under acceleration in a car for example you have a seatbelt for a reason

That is a proper acceleration, not a change in velocity. While I sit here at my desk my phone's accelerometer registers that I have an acceleration of 9.8 m/s^2 upwards. However, in the reference frame of the ground my velocity is not changing. And in a free-fall reference frame my velocity is changing.

Change in velocity is not the same thing as acceleration. Specifically proper acceleration, which is the physically important concept of acceleration.

But a “change” in velocity is an absolute

Proper acceleration is an absolute. Change in velocity is not.

 

[Dale]

In my capacity as an amateur mountaineer, tidal forces concern me much less than gravity as manifested by a relatively large free-fall acceleration towards the Earth.

That may be so, but it is not the gravity that hurts you when you collide with the earth. It is the real contact force (electromagnetic) that causes the damage.

 

[PeroK]

That may be so, but it is not the gravity that hurts you when you collide with the earth. It is the real contact force (electromagnetic) that causes the damage.

The speed with which you hit the ground needs an explanation, IMO. It's all very well to blame the ground, but if you just floated gently down to Earth the ground would be little to fear. When you're walking, your feet are colliding with the ground all the time. The ground doesn't change, so there must be some difference between a step and a fall. There must be another factor beyond the hardness of the ground.

 

[ESponge2000]

Now I’m understanding the terminology better. What we call the “force” of gravity means what we call an “acceleration” operating in a “non-inertial frame” …. The part of earth beneath our feet represents a non-inertial frame.

The natural flow of spacetime itself is not a linear path and that’s true for light as well, which is why these statements would be seemingly contradictory

1) the floor under Honolulu Hawaii is pushing up, enacting the real acceleration that jn any “inertial frame” explains why there’s a sense of free fall when jumping off a building in Honolulu

2) if one were to drill a hole straight through the earth from Honolulu they would come out the other side of the planet in Botswana African continent . The floor under Botswana is pushing up, enacting the real acceleration that in any “inertial frame” explains why there’s a sense of free fall when jumping off a building in Botswana.

3) these up-forces to us are in diametrically opposite directions in 3D space since the earth is a sphere . But somehow these 2 upward accelerations do not result in Botswana and Honolulu moving away from each other nor earth inflating like a balloon. Is that because curvature of spacetime leads to closed loops or explain that ?

 

[Dale]

The speed with which you hit the ground needs an explanation, IMO.

Agreed. But that explanation isn't always gravity. That explanation can also be that the ground is accelerating upwards under the action of real contact forces pushing upwards.

The explanation of the injury may either be due to the real contact force alone, or due to the real contact force and gravity. There is no explanation of the injury that can be made due to gravity alone. So, IMO, it isn't a good example of the reality of gravity.

Furthermore, all of the devices and procedures for safety involve reducing the contact force, none reduce gravity even where gravity is part of the explanation.

I agree that gravity can be demonstrated to be "real". Your example has the benefit of being familiar to everyone. But the disadvantage of involving more than just gravity.

 

[jbriggs444]

I would not need a seatbelt because my inertia was and remains at rest with the earth and it’s the moving platform beneath the car that changed inertial frames ?

The platform beneath the car did not change inertial frames. It is still present in all of them. And changed its state of motion relative to every one of them.

Your lack of a need of a seat belt is not because you remain at rest relative to the Earth but because both you and the car remained in the same state of motion relative to every inertial frame. Or, perhaps more directly relevant, you and the car remained in the same state of motion relative to one another -- so you did not subsequently strike the windshield with your cranium.

Edit: When I say "inertia", I normally mean "inertial mass". When you say "inertia", you apparently mean "momentum". So when you say that your "inertia remains at rest with the Earth", you apparently mean that your momentum remains zero relative to an inertial frame in which the Earth is at rest.

My understanding is yes in this situation there would not be a change in inertia inside the car but yet if i am staring at the moving floor as if its a rest frame , at constant velocity I will get the illusion the car could be in operation but once the platform turns off i won’t detect this change in inertia which is very important as it illustrates the point that while velocity is relative , help me understand how a change in inertia isn’t an absolute ?

Proper acceleration is an "absolute" in that it can be measured without need for a reference frame. The tool that does such a measurement is called an accelerometer.

In this exercise you can distinguish it by the need for a seatbelt vs not

Technically, the seat belt is there because it can constrain your motion relative to the car more gently than the windshield, dashboard, hard top or side windows. And because ejections are quite dangerous.

In a more ordinary situation, you can also detect the presence of proper acceleration by the force of the seat underneath your buttocks. Or by the fact that an accelerometer on your lap usually records 1g of proper acceleration in the upward direction.

 

[ESponge2000]

“Or, perhaps more directly relevant, you and the car remained in the same state of motion relative to one another --”

no that is always the case when in a car. Regardless, if the driver slams the brake it results in a force on all objects in the car , and you’re never not at rest with the car

 

[Dale]

But somehow these 2 upward accelerations do not result in Botswana and Honolulu moving away from each other nor earth inflating like a balloon. Is that because curvature of spacetime leads to closed loops or explain that ?

No closed loops are needed, this is simply a feature of curvature even without loops.

Consider the simplest curved surface, a sphere. While it does have loops, we are only going to look at a small section. Suppose that you have two people that are both walking, one a little north of the equator and one a little south of the equator. If they both start by going due east and walk in a straight line, their paths will gradually get closer. Instead, if they want to keep the same distance they will need to follow their respective latitude lines rather than a straight line. Latitude lines are curved, so to do that they will need to continually turn with their latitude line. The one to the north will need to turn left and the one to the south will need to turn right. So, to keep the distance constant between them, on a curved surface, requires that they continually turn in opposite directions.

In spacetime, proper acceleration is how much an object's path turns. So two objects on opposite sides of the earth must accelerate in opposite directions in order to keep their distance constant. Just as two paths on opposite sides of the equator must turn in opposite directions to keep their distance constant.

 

[jbriggs444]

no that is always the case when in a car. Regardless, if the driver slams the brake it results in a force on all objects in the car , and you’re never not at rest with the car

Nonsense.

Have you ever watched a crash test video? The dummies move violently relative to the car.

As a child, I saw the windshield of my neighbor's car after an occupant had been nearly ejected through the windshield. It was a pretty impressive deformation. And how I learned about laminated safety glass. It was non-fatal, thankfully.

 

[PeroK]

But somehow these 2 upward accelerations do not result in Botswana and Honolulu moving away from each other nor earth inflating like a balloon. Is that because curvature of spacetime leads to closed loops or explain that ?

Even simpler, consider two people on opposite sides on a merry-go-round. They are both continuously accelerating (centripetally) towards the centre - hence towards each other. There are real forces on each in that direction. Yet, they never get any closer together.

 

[A.T.]

But somehow these 2 upward accelerations do not result in Botswana and Honolulu moving away from each other nor earth inflating like a balloon.

This is where the intrinsic curvature of space-time comes in.

First, try to understand how gravity works in GR locally, which is visualized in the animation below. Note that the conical-space-time in the animation below doesn't have intrinsic curvature, because you can unroll a cone mantle onto a flat plane without distortion.

As you see, the apple hanging on the branch must continuously turn away from the locally straight (geodesic) path in space-time, just to stay at constant height. That turn away from the geodesic path is what an accelerometer measures as upwards proper acceleration.



Then, when you zoom out to see both sides of the planet, the space-time geometry looks like in the image below. As you see, the local cone is just an approximation of a thin slice, of that geometry. And on the other side the cone point the other way.

To keep constant distance, and avoid drifting towards each other by moving locally straight ahead in space-time (on geodesics), objects on opposite sides of the Earth must continuously turn in space-time in opposite directions, away from geodesic paths (red line), and thus experience proper acceleration in opposite directions.

Check out chapter 10 of this book for more explanation:
https://archive.org/details/L.EpsteinRelativityVisualizedelemTxt1994Insight/page/n157/mode/2up

 

[Dale]

@ESponge2000 note that the loops in the @A.T. digrams are just for convenience in graphing. The loops do not represent closed timelike curves in spacetime

 

[A.T.]

@ESponge2000 note that the loops in the @A.T. digrams are just for convenience in graphing. The loops do not represent closed timelike curves in spacetime

@ESponge2000 Yes, imagine the diagram is multi layered, like a roll of foil. So after each round around that bulged cylinder you arrive on a new layer.

The red path is a geodesic for an object falling through the planet without resistance. Like a non-spinning planet without atmosphere, and a tunnel trough the center to the other side.

 

[PeterDonis]

universally at rest

There is no such thing. You have been told that repeatedly in previous threads. The other misconceptions in your posts in this thread have also been repeatedly corrected before. Enough is enough.

This thread is now closed. @ESponge2000 if you continue to make posts based on misconceptions that have already been repeatedly corrected, you will start receiving warnings.