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Gravity Feilds vs Gravity Wells

Ian

Unregistered
I
I know that there are a lot of people who believe that the Earth and everything that has mass in the universe has a gravity field and there are others who think that everything in the universe has a gravity well. I found a contradiction. I heard in a book about Einstein's theroy of gravity when he said that he either saw someone fall off a building or he heard about someone falling off a building and that person didn't feel his or her own weight. Now if there is a gravity feild pulling down on everybody, then that person would have felt heavier as he or she approached the ground. How could this happen if Einstein clamied that that person didn't feel his/her own weight? There's a contradiction here to the gravity feild theory. That's something which I would like to know which theroy is right?
 
Could you provide a reference of some kind so that we could investigate this ?

As I was taught at university, masses distort the spacetime continuum. This distortion enables masses to interact.

Someone in freefall may or may not be aware of their own weight. In freefall you are "weightless" in that there is a force acting upon you ("gravity") but apart from wind resistance there is no counteracting force you experience. This contrasts with the equal and opposite forces you are currently experiencing in which the "downward" gravitational force is equal to the upward force from your chair. You only feel "weight" if there is something preventing your accelleration towards the Earth's centre of mass.

Incidentally, you have an equal and opposite attractional force pulling the earth towards you, it's just that the Earth is very massive and so the accelleration is tiny.
 
There's no difference between the gravity field and gravity well ideas, it's just a matter of perspective. The gravity field is how we percieve gravity in 3-D Euclidean geometry, whilst the gravity well is the perception from the 4-D space-time perspective.

As for someone not feeling their weight when falling, this is due to the fact that there is no resistance to their motion*, they are in "free-fall", which according to Einstein's equivalence principle feels exactly the same as not being in a gravity well.



*They would of course feel some air resistance, but we'll ignore that for now!
 
Also I believe, the difference between gravitational attraction at sea level and the top of any building on the planet is going to be negligible relative to the mass of the earth. I don't think anyone would be able to notice an increase in feeling heavier over so short a distance.
 
Gravity Field vs Gravity Well

I think that the refrence frame would be the person falling off of the building because how can the person not feel his/her own weight if he/she is pushing agianst whatever makes up the gravity field. If that happens, then that person would feel the force of the gravity field against them and this doesn't make sense because Einstein said that that person wouldn't feel his/her own weight. If the person fell and there was a gravity field, then the gravity field would bend around the person. Would gravity be different 5 feet away from the person who is falling reletive to the gravity field/well?
 
Re: Gravity Field vs Gravity Well

Ian said:
I think that the refrence frame would be the person falling off of the building because how can the person not feel his/her own weight if he/she is pushing agianst whatever makes up the gravity field.
They aren't pushing against the gravity field. Not sure where you got that idea from. Gravity pulls. That's all it does, it's a one way field. The Earth pulls on us and we pull on it.
If that happens, then that person would feel the force of the gravity field against them and this doesn't make sense because Einstein said that that person wouldn't feel his/her own weight.
What you feel as your weight is the action/reaction force pairing from physical contact with the surface of the Earth. The Earths gravity is pulling you down, but the Earths surface is pushing you up, the point of contact being the soles of your feet. Your ankles are pushing down on your feet, which are pushing up on your ankles. Your lower legs are pushing down on your ankles, which are pushing up on your lower legs. And so on, all the way up your body. Without the resistance of the Earths surface there are no action/reaction pairs from which to measure the force, so your senses cannot discern any push or pull in any direction. And here's where the equivalence principle comes in - with no discernable forces you can't tell whether you are in free-fall or there's no gravitational field.
If the person fell and there was a gravity field, then the gravity field would bend around the person.
No, gravity fields don't bend, they permeate.
Would gravity be different 5 feet away from the person who is falling reletive to the gravity field/well?
5 feet laterally - no difference at all. 5 feet straight up or down and there would be an infinitessimal difference.
 
The Don about a refrence frame

I think that the refrence frame to test this would be the falling object relative to the building and the ground or relative to a tall object in a room and the floor.
 
Re: The Don about a refrence frame

Ian said:
I think that the refrence frame to test this would be the falling object relative to the building and the ground or relative to a tall object in a room and the floor.
There's no need for a reference frame as this is non-relativistic.

P.S. I think The Don meant a reference as in "where did you read this bit of information?"
 
Gravitational Field Vs Gravitational Well

I think that I just heard some people call it a field and then I heard people call it a gravitational well and I wonder what the gravitational well is made of?
 
As you're falling down from the rooftop (assuming in a vacuum--hold your breath) and you take off your wooden leg and drop it, it will appear to float beside you (assuming you don't see the windows rushing past you). Perhaps this adds to the "not sensing" gravity. Of course, it's falling just as fast as you are.
 
Re: Gravitational Field Vs Gravitational Well

Ian said:
I think that I just heard some people call it a field and then I heard people call it a gravitational well and I wonder what the gravitational well is made of?
I think the "well" comes from the 3-D representation of the 4-D space-time interpretation of how gravity works in relativity. (that sentence does make sense, although you may have to read it 2 or 3 times, sorry.)

If you think of 4-D space time as a 2-D sheet of rubber pulled tight, then gravitational masses can be thought of as indentations in the surface. Small masses make small indentations, large masses make big indentations. A very small mass (ball) rolled across the surface will fall into any indentation it comes across. The indentations are sort of like "wells" in the surface of space-time, a bit like this image.
 
Ian said:
I know that there are a lot of people who believe that the Earth and everything that has mass in the universe has a gravity field and there are others who think that everything in the universe has a gravity well. I found a contradiction. I heard in a book about Einstein's theroy of gravity when he said that he either saw someone fall off a building or he heard about someone falling off a building and that person didn't feel his or her own weight. Now if there is a gravity feild pulling down on everybody, then that person would have felt heavier as he or she approached the ground. How could this happen if Einstein clamied that that person didn't feel his/her own weight? There's a contradiction here to the gravity feild theory. That's something which I would like to know which theroy is right?

"Field" just means that it's a field theory, which means that for every point in space there are some numbers. "Well" is just a word for what the field is like in certain areas.

As for your falling person, taking Newton's theory, one could say that the person doesn't feel anything because all of the parts of the person are being pulled by roughly the same amount (except for a slight tidal variation).

Einstein had a completely different view of gravity, that it isn't a force, that a person falling off a building is moving according to inertia, but you need to treat spacetime as a single thing to make sense of it. It's still a field theory, though.
 
I know that there are a lot of people who believe that the Earth and everything that has mass in the universe has a gravity field and there are others who think that everything in the universe has a gravity well.

No, these aren't two different things. A "well" is just a description of the energy diagram that is associated with a gravitational field. The surface of the earth and the moon are both low points, in the sense that it takes energy from either one to get to a point in the middle, and that an object displaced to either side of the high point will fall, of its own accord, to the surface. So we describe both those surfaces as being in gravity wells.

I found a contradiction.

I think what you found is something you are confused about. Einstein's description of gravity and how it is perceived can't differ substantially from Newton's. Newton's description works very very well, and relativity only adds tiny corrections except in cases of extreme gravity, such as neutron stars or black holes.

I heard in a book about Einstein's theroy of gravity when he said that he either saw someone fall off a building or he heard about someone falling off a building and that person didn't feel his or her own weight.

It would be useful if you could find the original quote. But the fact is that while you are falling you feel "weightless". This is well known. It's how astronauts train. It's how the movie "Apollo 13" was filmed. While in freefall, you do not feel a sensation of weight. That feeling comes from having the ground to press against.

Now if there is a gravity feild pulling down on everybody, then that person would have felt heavier as he or she approached the ground.

Not true. You wouldn't feel heavier as you approached the ground. When you're in free fall, you can't tell if you're on the space shuttle or on an elevator right off the surface of earth. Have you really never been in freefall, never felt that stomach-dropping-out sensation?

How could this happen if Einstein clamied that that person didn't feel his/her own weight?

It wouldn't. There's no such sensation of feeling heavier as you get closer to a massive object. Not in freefall.

There's a contradiction here to the gravity feild theory.

You haven't said what you think the "gravity field theory" is, but the term "gravitational" field simply means that there is a force associated with every point in space. This is what Newton expressed when he wrote F = G M m/r^2, which tells you the
force between to masses M and m, at a distance r.

We believe this to be a pretty accurate description for most of the universe.

Perhaps what's confusing you is the fact that you EXPERIENCE a force, which pulls you more and more as you approach an object, yet you don't FEEL the force. Both of those are true. There's no contradiction. You feel a force when a car is accelerating you, because the back of the car is pushing against you in order to accelerate you. But you don't feel any force when you and your elevator, or your aircraft, are in free fall under the influence of gravity. That's because the walls or the floor aren't pushing you. Everything is falling together. No solid object has to push on you for gravity to accelerate you.
 
Re: Gravitational Field Vs Gravitational Well

Ian said:
I think that I just heard some people call it a field and then I heard people call it a gravitational well and I wonder what the gravitational well is made of?

It's the same thing, looked at differently. There is no difference, and it's space itself that has the "dips" in it, of course not in any dimension you can see.
 
The falling person would feel an acceleration if the field changed rapidly as they fell. This doesn't happen to any appreciable degree on the surface of the earth.

When the free-falling object is very large, or when the gravitational field changes very rapidly, a force is felt.

An example is the Earth, free-falling in the gravitational field of the moon; the difference between the field at the near and far side of the earth is enough to stretch the hydrosphere of the earth by several feet, causing the daily tides.

It is hypothesized that objects free-falling into a black hole would experience this force in a more dramatic way.
 

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