# Thread: If Gravity is not a force, how do we stand on earth's surface?

1. I strongly believe in the Einstein's theory that gravity is a wave that it bends light and the object having mass warp the space-time giving it a curvature. But one thing I'm not sure about is why it is said gravity is not a force? Experimentally, the Galileo's trial is right that if a feather and a ball are dropped from a certain height in absence of any material medium then both will take equal time to make an impact on the surface towards which it is falling. But please clarify if gravity is not a force then why any object falls towards earth's surface? or maybe I'm wrong or misunderstood it. Please clarify.  2.

3. Gravity is a force, which is described by General relativity, and to a very good approximation by Newton's laws. Calling it not a force is simply word play.  4. Originally Posted by Chandrabhan But please clarify if gravity is not a force then why any object falls towards earth's surface? or maybe I'm wrong or misunderstood it. Please clarify.
OK - gravity _is_ a force.  5. Originally Posted by mathman Gravity is a force, which is described by General relativity, and to a very good approximation by Newton's laws. Calling it not a force is simply wordplay.
Ok, then why both the feather and the ball make an impact on the ground at the exact same time in a vacuum? It seems that mass has no significance with gravity (in this case)  6. Originally Posted by Chandrabhan  Originally Posted by mathman Gravity is a force, which is described by General relativity, and to a very good approximation by Newton's laws. Calling it not a force is simply wordplay.
Ok, then why both the feather and the ball make an impact on the ground at the exact same time in a vacuum? It seems that mass has no significance with gravity (in this case)
In Newtonian mechanics that's easy. The force, F, of gravity on an object is its weight. Its weight is proportional to its mass, m, right? So a 2kg mass has a weight double that of a 1kg mass.

But the acceleration, a, of a body is governed by F=ma. So your 2kg mass requires double the force required by your 1kg mass, in order to achieve the same rate of acceleration. Lo and behold this is exactly what the weight does. The 2kg mass experiences double the force, which is precisely what is needed to give it an identical rate of acceleration to that of the 1kg mass.

It all comes, in Newton's system, from the force of gravitation being proportional to mass, and the force needed to impart a certain acceleration also being proportional to mass.

(GR, famously, treats it all very differently, however.)  7. Originally Posted by Chandrabhan I strongly believe in the Einstein's theory that gravity is a wave that it bends light and the object having mass warp the space-time giving it a curvature. But one thing I'm not sure about is why it is said gravity is not a force? Experimentally, the Galileo's trial is right that if a feather and a ball are dropped from a certain height in absence of any material medium then both will take equal time to make an impact on the surface towards which it is falling. But please clarify if gravity is not a force then why any object falls towards earth's surface? or maybe I'm wrong or misunderstood it. Please clarify.
Gravity is indeed not a force - when you put an accelerometer into free fall, it will read exactly zero at all times. Hence, treating it as a force is not the right approach.
Objects fall towards other objects, because of a fundamental principle called the principle of extremal ageing - free-fall world lines in spacetime are such that the total proper time they trace out is maximised. This is a special case of the more general principle of least action. The falling down part of gravity is hence essentially due to gravitational time dilation, which comes down to the geometry of spacetime.  8. Originally Posted by Markus Hanke Gravity is indeed not a force - when you put an accelerometer into free fall, it will read exactly zero at all times. Hence, treating it as a force is not the right approach.
Objects fall towards other objects, because of a fundamental principle called the principle of extremal ageing - free-fall world lines in spacetime are such that the total proper time they trace out is maximised. This is a special case of the more general principle of least action. The “falling down part of gravity is hence essentially due to gravitational time dilation, which comes down to the geometry of spacetime.
Can you give any indication as to what it is that(if I am guessing correctly and in purely mathematical or geometric terms) actually causes flat spacetime to be curved in the presence of sources of mass, energy etc?

Again guessing is it the stress tensors?

Btw this principle of least action, is it an axiom that has been shown to accord with observation or is there actually an even more fundamental principle that lead to the formulation of the idea?

(I am immune to turtle analogies )

pps Do we know who first came up with the principle of least action?  9. Originally Posted by geordief pps Do we know who first came up with the principle of least action? Originally Posted by Wikipedia
Scholars often credit Pierre Louis Maupertuis for formulating the principle of least action because he wrote about it in 1744 and 1746. However, Leonhard Euler discussed the principle in 1744, and evidence shows that Gottfried Leibniz preceded both by 39 years.
In 1933, Paul Dirac discerned the quantum mechanical underpinning of the principle in the quantum interference of amplitudes.  10. [QUOTE=PhDemon;612016] Originally Posted by geordief  Originally Posted by Wikipedia
Scholars often credit Pierre Louis Maupertuis for formulating the principle of least action because he wrote about it in 1744 and 1746. However, Leonhard Euler discussed the principle in 1744, and evidence shows that Gottfried Leibniz preceded both by 39 years.
In 1933, Paul Dirac discerned the quantum mechanical underpinning of the principle in the quantum interference of amplitudes.
Thanks  11. Originally Posted by Markus Hanke  Originally Posted by Chandrabhan I strongly believe in the Einstein's theory that gravity is a wave that it bends light and the object having mass warp the space-time giving it a curvature. But one thing I'm not sure about is why it is said gravity is not a force? Experimentally, the Galileo's trial is right that if a feather and a ball are dropped from a certain height in absence of any material medium then both will take equal time to make an impact on the surface towards which it is falling. But please clarify if gravity is not a force then why any object falls towards earth's surface? or maybe I'm wrong or misunderstood it. Please clarify.
Gravity is indeed not a force - when you put an accelerometer into free fall, it will read exactly zero at all times. Hence, treating it as a “force” is not the right approach.
Objects fall towards other objects, because of a fundamental principle called the “principle of extremal ageing” - free-fall world lines in spacetime are such that the total proper time they trace out is maximised. This is a special case of the more general principle of least action. The “falling down” part of gravity is hence essentially due to gravitational time dilation, which comes down to the geometry of spacetime.
Your answer does not satisfy me. Assume me a new to science now will you please explain why are you saying that gravity is not a force? The above written answers were providing me a little confidence and you again confused me. Don't take any offence I'm just asking.  12. Assume me a new to science now will you please explain why are you saying that gravity is not a force?
If you drop an accelerometer from a high place, then the instrument will free-fall downwards. However, if you record what the accelerometer actually reads, you will find that it shows exactly zero at all points during the free fall. Since F=ma, with a=0, there is no force acting on the accelerometer, yet it still falls. This is true for all bodies in free fall. Hence, gravity cannot be consistently described as a force.

It is possible to come up with an observer-dependent description of gravity that uses the concept of forces (thats the old Newtonian mechanics you learn at school), but this works only as an approximation in special cases. It is not a complete and correct description of gravity.  13. Originally Posted by Markus Hanke Assume me a new to science now will you please explain why are you saying that gravity is not a force?
If you drop an accelerometer from a high place, then the instrument will free-fall downwards. However, if you record what the accelerometer actually reads, you will find that it shows exactly zero at all points during the free fall. Since F=ma, with a=0, there is no force acting on the accelerometer, yet it still falls. This is true for all bodies in free fall. Hence, gravity cannot be consistently described as a force.

It is possible to come up with an observer-dependent description of gravity that uses the concept of forces (that’s the old Newtonian mechanics you learn at school), but this works only as an approximation in special cases. It is not a complete and correct description of gravity.
Can the concept of force be applied to a system as a whole rather than ,as in this case a component of it ?

In this case the relationship between the Earth and the object (the accelerometer incidentally) changes .

A different kind of force perhaps but could it be referred to as "a force"? (Can the relationship be described as "being forced"?  14. Can you give any indication as to what it is that(if I am guessing correctly and in purely mathematical or geometric terms) actually causes flat spacetime to be curved in the presence of sources of mass, energy etc?
I do not have an answer to this, nor does anyone else at present. GR itself describes only the geometry of spacetime (which follows from the distribution of energy-momentum therein), but it has nothing to say as to the reasons why it works this way. This will be left for a more comprehensive model to answer.

Btw this principle of least action, is it an axiom that has been shown to accord with observation or is there actually an even more fundamental principle that lead to the formulation of the idea?
Historically, the principle followed from observations; I dont actually know if there is a more fundamental underpinning. If there is, then it would be found in the discipline of topology.

Do we know who first came up with the principle of least action?
It looks like it was Gottfried Leibniz.  15. In this case the relationship between the Earth and the object (the accelerometer incidentally) changes .
You can describe it as relative acceleration between the bodies, but that is a coordinate-dependent concept, and hence depends on the observer. The only thing everyone always agrees on is the physical reading of the accelerometer itself (proper acceleration, as opposed to coordinate acceleration), which is zero everywhere.

On a more fundamental note, it can be shown that vector fields (i.e. forces) are insufficient to capture all the degrees of freedom of gravity. You need a rank-2 tensor field for that, which is what GR uses.  16. Originally Posted by Markus Hanke In this case the relationship between the Earth and the object (the accelerometer incidentally) changes .
You can describe it as relative acceleration between the bodies, but that is a coordinate-dependent concept, and hence depends on the observer. The only thing everyone always agrees on is the physical reading of the accelerometer itself (proper acceleration, as opposed to coordinate acceleration), which is zero everywhere.

On a more fundamental note, it can be shown that vector fields (i.e. forces) are insufficient to capture all the degrees of freedom of gravity. You need a rank-2 tensor field for that, which is what GR uses.
Can it be (usefully) said that a system wide measurement is a contradiction in terms?  17. Now that I have studied too much but still lacking in building an image or thought concept that how the gravity works. Here I got a good video on youtube clearly depicting how that stuff actually works.  18. What do you want to discuss about the video??? ��  19. Originally Posted by Paleoichneum What do you want to discuss about the video??? ��
I do not want to discuss the video I have just posted it that it has cleared my doubt and I was unable to understand through statements.  20. Originally Posted by Chandrabhan Now that I have studied too much but still lacking in building an image or thought concept that how the gravity works. Here I got a good video on youtube clearly depicting how that stuff actually works.

Im glad that you understand it now. As I said already, objects under the influence of gravity do not experience acceleration ( a=0 => straight line! ), but because of gravitational time dilation, their worldlines are nonetheless curved. They are straight lines in a curved spacetime.

There is actually a lot more going on when it comes to gravity, but this gives the very basic idea. Just be careful not to confuse the visualisation with the actual model (i.e. General Relativity)!  21. What is the pentagon operator, dp and R.  Bookmarks
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