High School Gravity: Force or Distortion of Spacetime?

[gnnmartin]

I come late to these discussions, and the discussion has moved on from Lunct's original question. However, I can't resist replying.We do, like Humpty Dumpty in ‘Alice Through the Looking Glass' make words mean what we want them to mean. When you look at the sky at night you see lots of points of light (well perhaps not in London where I see Lunct is situated). Some people will call these stars, others will protest that the easiest ones to see are not stars, they are planets. In the same way, we might talk about the force of gravity even though gravity is not a force. The “force of gravity” is a metaphor, and like a well chosen metaphor it allows us to simplify descriptions. A lot of phenomena are hard to describe without that metaphor.That aside, I argue most strongly that gravity is not in fact a force. We must start by saying what we mean by a force. Newton said an object remains still or moves in a straight line with constant velocity unless it is acted upon by a force, and I take that as a starting point. Once you realize that space and time are linked, and that space/time is not necessarily flat, then you need to refine Newton's definition. It is always possible to describe local smooth space/time as flat space with time the same everywhere, and if an object is not acted on by a force when space/time is so described, then it is moving along a timelike geodesic, hence we can define force as that which causes a body to deviate from a timelike geodesic. With this definition, gravity is not a force.We can describe the space/time of the solar system very accurately as flat space with time running slightly slower as you approach a planet or a moon. We can then describe with very great accuracy (though not perfect accuracy) how a body will move in such a space by assuming that a force acts on the body in the direction opposite to the rate of change in space of the rate of time. That imagined force is the force of gravity.There is another pitfall when talking about force. Force acts at a point, not over a distance. Thus when a body falls towards a star it suffers tidal ‘forces’ which squeeze and elongate the body. The tidal ‘forces’ do not make a body deviate from a timelike geodesic, so they are not truly forces: again, again, to describe them as forces is to use metaphor.

[Post edited by moderator]

 

[Arkalius]

I experience the force when I am stationary with respect to the gravitating source due to something preventing me from accelerating, like the floor.

The floor isn't "preventing" you from accelerating. It's accelerating you upwards, away from the spacetime geodesic that gravity would have you move along otherwise. In general relativity, a net force results in proper acceleration, and proper acceleration is that which is measurable by an accelerometer within the accelerating frame without referencing any other frame.

In a Newtonian model, an astronaut in orbit in a spaceship with no windows is being constantly accelerated by the gravitational force to remain in orbit. In GR, there is no way the astronaut could measure any proper acceleration from within this spaceship. He is experiencing no net forces. On the ground, you are experiencing a proper acceleration upward, and thus an upward force by the ground. Thus, what you're feeling isn't the force of gravity, but pressure from the ground pushing you upward. This set of circumstances exists because of gravity, so in a sense you could say you're "feeling" gravity here, but that's only possible because there are external cues that let you see that there is enough mass-energy around to curve spacetime in such a way for gravity to influence you in this way. In a small room with no windows, those clues would be gone and you couldn't say for sure if gravity was responsible for the force you're feeling holding you to the floor (though it would be a safe assumption-- most humans don't randomly find themselves on spacecraft in deep space).

 

[Buzz Bloom]

The tidal ‘forces’ do not make a body deviate from a timelike geodesic, so they are not truly forces: again, again, to describe them as forces is to use metaphor.

Hi gnnmartin:

I have no problem with accepting that some uses of the word "force" are metaphors. However, I now need a new word to refer to what a tidal force is, or a generalization that includes "force" and "tidal force". Although a "tidal force" does not cause movement from a timelike deviation, it does act at each point of a space a to cause a change in what occupies that space, like e.g. pressure does. Perhaps you might invent a useful word for this. What I have come up with is "distorter", but I don't think that will attract much usage. However, for the purpose of this post we have:

Force is a kind of distorter which causes a body to deviate from a timelike geodesic.
Pressure is a kind of distorter which causes a body to compress.
"Tidal force" (new word needed) is a kind of distorter which causes a body to be pulled apart in one direction while compressing in the other two directions.​

BTW: I very much enjoyed reading through the two articles. I don't think I fully digested them as I read them, so I intend to re-read them after a while.

Regards,
Buzz

 

[Buzz Bloom]

The floor isn't "preventing" you from accelerating. It's accelerating you upwards, away from the spacetime geodesic that gravity would have you move along otherwise.

Hi Arkalius:

I do not disagree with you, but my description is intended to describe a direct real world experience. Your description is correct regarding physical models and math, but does not describe the experience.

I am not trying to be philosophical, but just commenting that scientific language is not readily intuitive, and that pedagogically it might be useful to connect what is intuitive to what the math and models say, or vice versa.

Remembering Feinmann's quote ("No one understands quantum mechanics".), my attempts to understand QM have taught me that an intuitive complete understanding of what the math says is impossible. But it might be possible for relativity.

Regards,
Buzz

 

[PeterDonis]

I experience the force when I am stationary with respect to the gravitating source due to something preventing me from accelerating, like the floor.

The force you experience in this case is not gravity. It's the non-gravitational force of the floor pushing on you. You never experience the "force" of gravity. That's a key reason why the GR model of gravity as curved spacetime works.

 

[pervect]

Hi pervect:

The point I was hinting at was that there is a perception of a gravitational force on your feet which is directly experiential in the real world in which we live. It does not depend on which model we use to understand the properties of this "force". In that sense, there is an undeniable gravitational "force" in the real world.

I'm sure you're convinced of that. And in Newtonian gravity it's even correct, gravity is a force in Newtonian theory. Where you would be going wrong would be if you were to say that this implies that gravity is a force in General Relativity. It's not. We're trying to explain to you why it is not.

If you aren't making any claims about general relativity, I"m not understanding what you're trying to say, and you might need to rephrase it. My main concern here would be that if you're not familiar with GR, and you are also not curious about it and don't want to learn about it, this discussion can't possibly go anywhere, and is probably posted to the wrong forum.

But before we can go onto the issues related at all to understanding of what GR has to say, we need to clear up some points regarding Newtonian theory and what it has to say.

Suppose you have an accelerating elevator, something I've been calling Einstein's elevator, and you want to analyze the force on an object in said elevator. The first set of observations is this:

1) We want to use Newton's laws, so we will use an inertial frame of reference to describe this problem.

2) In this inertial frame of reference there is only one force on the object (a person standing on the floor, in your example). This is the force that the floor exerts on the person to accelerate them. This is the only "real" force in this situation.

There is more to say, I've only made half of my point, and perhaps the relevance isn't yet clear. But I don't think I can proceed much further until I have some sign that we have some level of agreement thus far. Would you agree with point 2, above, or not?

If we're agreed thus far, we can address such issues as "what the person feels", and "what are psuedo forces". But if we're not agreed so far, we need to straighten that out first before proceeding further.

 

[Buzz Bloom]

Would you agree with point 2, above, or not?

Hi pervect:

I agree. What I seem to be unable to explain clearly is that what is experienced by my feet is the same whether the elevator is on the ground or being accelerated in empty space. What bothers me is not the use of language in discussing the physics. It is what seems to me to be a dogma that this is the only correct way to use the language and fails to recognize that there is a different natural use of language which is to be discouraged. Earlier in this thread this natural use was referred to as metaphorical. Is seems to me that from a linguistic point of view the natural use reflects natural experience, and the scientific use is metaphorical.

Regards,
Buzz

 

[Buzz Bloom]

The force you experience in this case is not gravity.

Hi Peter:

I agree with you. In the context of my post responding to pervect I would concede that the use of the term "gravity" may be metaphorical when referring to what my feet feel, but the use of "force" is not.

Regards,
Buzz

 

[Herman Trivilino]

What I seem to be unable to explain clearly is that what is experienced by my feet is the same whether the elevator is on the ground or being accelerated in empty space.

Well, you did call only one of them the force of gravity. That means you've made a distinction between two things that you're now saying are the same.

What bothers me is not the use of language in discussing the physics. It is what seems to me to be a dogma that this is the only correct way to use the language and fails to recognize that there is a different natural use of language which is to be discouraged.

The floor exerts a force on your feet that is electromagnetic. When you say it's gravitational you're getting the physics wrong. That force arises because the matter that makes up both the floor and your feet is held together by electromagnetic interactions, not by gravitational interactions. This is not dogma. It's physics.

 

[Buzz Bloom]

The floor exerts a force on your feet that is electromagnetic.

Hi Mister T:

I agree that the immediate cause of the experience is electromagnetic, but I think it is reasonable to identify the root cause when the elevator is on the Earth's surface to be the influence of the Earth's gravitational field. Does this make sense to you? If so, then it is not also reasonable to describe the experience as a force caused by gravity? I agree that technically with respect to the Einstein model for gravity that the gravity is not the force experienced, but it is the root cause of the force.

With respect to the question posed by the OP, is it not reasonable to answer that the choice of words and phrases used to describe something depends on the context? If so, then in the context of the science that preceded the 20th century, gravity is (was?) a force.

Regards,
Buzz

 

[Drakkith]

Thread locked for moderation.

 

[Drakkith]

This thread has attracted multiple crackpots and other problematic posts. Since the original question seems to have been answered, this thread will remain locked.