When do we feel gravity

[alpha2cen]

Which step we obtain gravity?

[elfmotat]

I don't understand the question. Gravity is present whenever there's energy/momentum.

[JohnStu]

the origin of gravity or the cause of gravity probably can never be explained. I mean, even if we attempt to explain it, the explaination often ends up being the preset justification for its existance, which is not really an explaination.

[J.C.MacSwell]

We feel gravity when we feel the first tugs of our upcoming spaghettification as we approach a black hole. On Earth we really don't feel it.

[doG]

On Earth we really don't feel it.

Huh? I feel it every time I stand up, climb the stairs, climb a mountain, etc.. Where is the weightlessness you speak of on Earth?

[J.C.MacSwell]

Huh? I feel it every time I stand up, climb the stairs, climb a mountain, etc.. Where is the weightlessness you speak of on Earth?

 

Where did I mention weightlessness?

 

You feel the force of your resistance to gravity, but that is your resistance not gravity. When you stop standing up, climbing etc, gravity has not gone away. If you jump from a cliff you do not feel gravity any more or less, just the recognition that you have stopped resisting...and when you land that's not gravity either that's a collision.

 

Compare a free fall on the moon or on an Earth size planet with no atmosphere...you cannot tell the difference in the two even though the gravity is significantly stronger on the Earth Planet. You cannot feel it.

[Tres Juicy]

 

Where did I mention weightlessness?

 

You feel the force of your resistance to gravity, but that is your resistance not gravity. When you stop standing up, climbing etc, gravity has not gone away. If you jump from a cliff you do not feel gravity any more or less, just the recognition that you have stopped resisting...and when you land that's not gravity either that's a collision.

 

Compare a free fall on the moon or on an Earth size planet with no atmosphere...you cannot tell the difference in the two even though the gravity is significantly stronger on the Earth Planet. You cannot feel it.

?! Any significant difference in gravity will be noticable.

 

I would wager that I could do more press ups on the moon than I can here on earth.

[zapatos]

 

 

You feel the force of your resistance to gravity, but that is your resistance not gravity.

So when I get stabbed, I don't actually feel the knife entering my body, but the resistance of my body to the knife?

[StringJunky]

I think what J C Macswell is saying is: in the absence of resistance you can't be directly aware of gravity. Freefalling towards a massive object in a vacuum, you would feel that you were stationary and absent of any feeling of weight.

Edited December 10, 2012 by StringJunky

[zapatos]

I understand the point, but that sounds to me to be splitting hairs. Sort of, 'in the absence of a sense to feel gravity, you will not feel gravity'. Or, 'in the absence of eyes, you will not see light'. I mean, I don't actually see light, I experience nerve cell stimulation to light.

 

It may be true, but is it meaningful?

[elfmotat]

I understand the point, but that sounds to me to be splitting hairs. Sort of, 'in the absence of a sense to feel gravity, you will not feel gravity'. Or, 'in the absence of eyes, you will not see light'. I mean, I don't actually see light, I experience nerve cell stimulation to light.

 

It may be true, but is it meaningful?

 

They're not the same thing at all. The point is that if you were inside a box with no windows floating in space, you wouldn't be able to tell if you were free-falling towards a massive object or just floating around in the absence of anything. It's called the equivalence principle, and it's what helped Einstein come up with general relativity. It's what allows gravity to be represented as a consequence of geometry. Einstein himself called it his "happiest thought."

[zapatos]

 

They're not the same thing at all. The point is that if you were inside a box with no windows floating in space, you wouldn't be able to tell if you were free-falling towards a massive object or just floating around in the absence of anything. It's called the equivalence principle, and it's what helped Einstein come up with general relativity. It's what allows gravity to be represented as a consequence of geometry. Einstein himself called it his "happiest thought."

But why aren't they the same?

 

If I'm in a box with no windows I cannot tell if I'm free falling or floating. That is, I cannot tell if gravity is acting on me or not.

If I cannot see anything, I cannot tell if my vision is blocked, or if there actually is no light. That is, I cannot tell if light is acting on me or not.

 

Why is my butt finally hitting the ground NOT an experience of gravity, but suddenly seeing a neon sign in front of me IS an experience of light?

[elfmotat]

But why aren't they the same?

 

If I'm in a box with no windows I cannot tell if I'm free falling or floating. That is, I cannot tell if gravity is acting on me or not.

If I cannot see anything, I cannot tell if my vision is blocked, or if there actually is no light. That is, I cannot tell if light is acting on me or not.

 

Why is my butt finally hitting the ground NOT an experience of gravity, but suddenly seeing a neon sign in front of me IS an experience of light?

 

They're not the same because one of them has useful theoretical implications, whereas the other does not. That was your original question: "It may be true, but is it meaningful?"

[zapatos]

 

They're not the same because one of them has useful theoretical implications, whereas the other does not. That was your original question: "It may be true, but is it meaningful?"

I don't mean to appear dense, but what is the useful theoretical implication of saying that "You feel the force of your resistance to gravity, but that is your resistance not gravity"? How has the equivalence principle been compromised by my suggesting that feeling the pressure on your feet is indeed a gravitational experience?

 

Does the equivalence principle make a distinction between sensing gravity via spahgettification and falling on my butt?

[StringJunky]

 

They're not the same because one of them has useful theoretical implications, whereas the other does not. That was your original question: "It may be true, but is it meaningful?"

 

The two scenarios zapatos put forward are not equivalent anyway because, using our senses, we can't feel gravity in the absence of some resistance to our continued motion but we can see photons directly.

[elfmotat]

I don't mean to appear dense, but what is the useful theoretical implication of saying that "You feel the force of your resistance to gravity, but that is your resistance not gravity"? How has the equivalence principle been compromised by my suggesting that feeling the pressure on your feet is indeed a gravitational experience?

 

Does the equivalence principle make a distinction between sensing gravity via spahgettification and falling on my butt?

 

You're focusing far too much on the words being used, and you're asking meaningless philosophical questions that have no bearing on the physics. The only reason I replied to you was because you implied the equivalence principle was meaningless. What you do or do not decide to classify as "feeling gravity" is not something I'm interested in discussing.

[zapatos]

 

The two scenarios zapatos put forward are not equivalent anyway because, using our senses, we can't feel gravity in the absence of some resistance to our continued motion but we can see photons directly.

I guess I was trying to say that sensing resistance via nerves in our feet and sensing photons via nerves in our eyes seemed equivalent.

 

 

You're focusing far too much on the words being used, and you're asking meaningless philosophical questions that have no bearing on the physics. The only reason I replied to you was because you implied the equivalence principle was meaningless. What you do or do not decide to classify as "feeling gravity" is not something I'm interested in discussing.

I never suggested the equivalence principle was meaningless. That was your interpretation of what I said.

 

J.C.MacSwell is the one who made what sounded like a physics statement (as opposed to a philosophical statement) that you cannot feel gravity on earth. Not me. I would just like to know why.

 

If that is something you don't wish to discuss, please don't respond any more.

[J.C.MacSwell]

I don't mean to appear dense, but what is the useful theoretical implication of saying that "You feel the force of your resistance to gravity, but that is your resistance not gravity"? How has the equivalence principle been compromised by my suggesting that feeling the pressure on your feet is indeed a gravitational experience?

 

Does the equivalence principle make a distinction between sensing gravity via spahgettification and falling on my butt?

It does. Spaghettification is due to significant gradient in a gravitational field. You would feel it even in a box, free falling toward a sufficiently large mass.

 

?! Any significant difference in gravity will be noticable.

 

I would wager that I could do more press ups on the moon than I can here on earth.

You would lose the bet if being accelerated sufficiently upward while on the moon, pinned to the upward accelerating gym floor.

[zapatos]

It does. Spaghettification is due to significant gradient in a gravitational field. You would feel it even in a box, free falling toward a sufficiently large mass.

Ah, thank you. So presumably the gradient exists on earth as well, but the gradient is so small here that we couldn't possibly physically detect it?

Edited December 10, 2012 by zapatos

[D H]

So presumably the gradient exists on earth as well, but the gradient is so small here that we couldn't possibly physically detect it?

 

Gravity gradient is what drive the tides.

 

And yes, it is physically possible to detect gravity gradient. It's what a gravity gradiometer does. Note that while accelerometers cannot measure gravity (no local experiment can measure gravitation; J.C. Macswell is correct), a pair of rigidly connected accelerometers can measure the gradient in gravity between the pair. The European Space Agency's GOCE satellite uses three pairs of accelerometers to measure the gravity gradient tensor.

[Tres Juicy]

...You would lose the bet if being accelerated sufficiently upward while on the moon, pinned to the upward accelerating gym floor.

 

Yes, but that's not due to gravity is it? That's due to accelleration which is not the same thing.

[D H]

 

Yes, but that's not due to gravity is it? That's due to accelleration which is not the same thing.

 

It's exactly the same thing. Suppose you wake up in a strange room. You sit up and try to figure out where you are. The last thing you remember is going out to a bar with some friends on December 21 to celebrate the end of the world (or maybe not the end of the world). How can you tell, just based on feeling your butt on the floor whether

• It was the end of the world. The evil aliens who brought about the end of the world kidnapped you, brought you up to their rotating space station, and are about to do unthinkable things to you. The space station's rotation gives the illusion of normal Earth gravity.
   
• It was the end of the world. The benevolent aliens who foresaw the end of the world rescued you and brought you up to their rocket, which is currently accelerating at 1g toward planet X.

It wasn't the end of the world. You had too much too drink last night and your friends wouldn't let you drive home. One of them took you to their place, where you promptly crashed on the floor.

Aside: This new interface bites. Completely.

Edited December 11, 2012 by D H

[Tres Juicy]

 

 

It's exactly the same thing. Suppose you wake up in a strange room. You sit up and try to figure out where you are. The last thing you remember is going out to a bar with some friends on December 21 to celebrate the end of the world (or maybe not the end of the world). How can you tell, just based on feeling your butt on the floor whether

 

 

 

Aside: This new interface bites. Completely.

 

• • It was the end of the world. The evil aliens who brought about the end of the world kidnapped you, brought you up to their rotating space station, and are about to do unthinkable things to you. The space station's rotation gives the illusion of normal Earth gravity.

     

     
  • It was the end of the world. The benevolent aliens who foresaw the end of the world rescued you and brought you up to their rocket, which is currently accelerating at 1g toward planet X.
  • It wasn't the end of the world. You had too much too drink last night and your friends wouldn't let you drive home. One of them took you to their place, where you promptly crashed on the floor.

While the 2 effects are indisdinguishable from each other the causes are not the same

[D H]

While the 2 effects are indisdinguishable from each other the causes are not the same

The causes of what you feel are exactly the same in all three scenarios. With one exception, you do not feel gravitation. (That one exception: You would feel extreme tidal forces; these would cause you to undergo spaghettification. You'd feel yourself being pulled apart.)

 

Local experiments (e.g., what you feel) cannot sense gravity. You feel forces other than gravity. You feel the floor pushing up on the parts of your body in contact with the floor. That's the normal force. You feel this when you push against a wall or pick up a book. This normal force induces stresses and strains on your body as that force propagates through your body. You feel those stresses and strains.

 

Your inner ear contains the biological equivalent of an accelerometer. It works just like an accelerometer does. Some parts are (somewhat) free to follow an inertial path, other parts are more or less rigidly attached to your skeletal structure, and yet other parts sense the stresses that result when the accelerometer case (or your skeleton) is forced to follow a non-inertial path. So what's an inertial path? This is one of the key places where Newtonian physics and general relativity differ. In Newtonian mechanics, Newton's falling apple does not follow an inertial path. It does in general relativity. Einstein described his realization of this key distinction as his happiest thought.

[Tres Juicy]

The causes of what you feel are exactly the same in all three scenarios. With one exception, you do not feel gravitation. (That one exception: You would feel extreme tidal forces; these would cause you to undergo spaghettification. You'd feel yourself being pulled apart.)

 

Local experiments (e.g., what you feel) cannot sense gravity. You feel forces other than gravity. You feel the floor pushing up on the parts of your body in contact with the floor. That's the normal force. You feel this when you push against a wall or pick up a book. This normal force induces stresses and strains on your body as that force propagates through your body. You feel those stresses and strains.

 

Your inner ear contains the biological equivalent of an accelerometer. It works just like an accelerometer does. Some parts are (somewhat) free to follow an inertial path, other parts are more or less rigidly attached to your skeletal structure, and yet other parts sense the stresses that result when the accelerometer case (or your skeleton) is forced to follow a non-inertial path. So what's an inertial path? This is one of the key places where Newtonian physics and general relativity differ. In Newtonian mechanics, Newton's falling apple does not follow an inertial path. It does in general relativity. Einstein described his realization of this key distinction as his happiest thought.

I'm going to have to disagree I'm afraid. While accelleration causes the same effect it is not the same thing as gravity. Gravity is a force caused by the distortion of space time generated by mass, accelleration is a ficticious force

 

http://en.wikipedia.org/wiki/Fictitious_force