Gravity: Force or Distortion of Spacetime?

[maria_phys]

Is that like gravitational waves?

No gravitational waves are something else.

 

[PeterDonis]

other forces may be occur in such events. If you drop an object inside the black hole it will feel the huge tidal forces from the black hole

Tidal gravity is spacetime curvature in GR. It is not a force. An object that is affected by tidal gravity might have internal forces/stresses inside it, yes, but those are not due to tidal gravity; they are due to the non-gravitational forces between the object's parts (electromagnetic forces between the atoms) that are resisting the effects of tidal gravity.

The effect of the tidal forces wouldn't be the same like the system Earth moon. I will stressed out or shrink in one direction.

You will be stretched in one direction, and squeezed in the other two orthogonal directions.

A force has a direction.

In GR, a force is something that causes proper acceleration (weight). Tidal gravity does not do that. So tidal gravity in GR is not a force. It is, as I said above, spacetime curvature.

Newtonian dynamics works really good for objects with low masses.

The difference between Newtonian gravity and GR is not in which domain each one "works really good". GR also "works really good" for objects with low masses, in fact it works even better than Newtonian gravity does--it has to, because Newtonian gravity is just an approximation to GR, and leaves out effects that GR includes. Those effects are very, very small for objects with low masses, but they are not zero.

Take for example the gravitational force between Earth and Moon. The space-time curvature is so negligible that has no meaning.

You are incorrect. Tidal gravity is spacetime curvature, and the tidal gravity of the Earth and Moon on each other is easily observable.

 

[maria_phys]

Thank you for the clarification. So, the only forces in GR are those between atoms (electromagnetism, weak and strong force)?

they are due to the non-gravitational forces between the object's parts (electromagnetic forces between the atoms) that are resisting the effects of tidal gravity.

Can the strong tidal forces from a black hole change the structure of an atom?

 

[Mister T]

Newtonian dynamics works really good for objects with low masses. Take for example the gravitational force between Earth and Moon. The space-time curvature is so negligible that has no meaning.

The tides are explained by spacetime curvature and it's not negligible. Just ask anyone who lives near the sea. They are also explained by Newton's model. What's negligible is the quantitative differences between their predictions. But qualitatively the two models are very different.

When tidal gravity is large the two models make very different quantitative predictions and its the spacetime curvature model that makes the more accurate predictions.

My point is that the Newtonian model doesn't have its own separate realm of validity. Einstein's model is perfectly valid everywhere that Newton's model is valid.

 

[PeterDonis]

the only forces in GR are those between atoms (electromagnetism, weak and strong force)?

Those forces aren't just between atoms. But yes, anything that produces proper acceleration, and therefore counts as a "force" in GR, will be due to one of those three interactions.

Can the strong tidal forces from a black hole change the structure of an atom?

Tidal gravity is not a force in GR, as I said before. Strong enough tidal gravity could in principle break atoms apart, yes--because the force between the electrons and the nucleus is not strong enough to hold the atom together against the effects of sufficiently strong tidal gravity (meaning, the effects of sufficiently strong spacetime curvature).

 

[Orodruin]

Exactly. We assume that gravity is a force for systems with low masses (like our Earth and moon) .

No, not exactly. I did not say this. In Newtonian gravity, gravity is a force. In GR it is an effect of curved spacetime. Whether the masses are small or not does not matter.

The space-time curvature is so negligible that has no meaning.

This is not true, it keeps the Moon orbiting the Earth.

But I've tried to clarify when GR has a meaning.

GR is applicable in all situations where Newtonian gravitation is applicable and then some. It works perfectly well for weak fields.

 

[maria_phys]

Tidal gravity is not a force in GR, as I said before. Strong enough tidal gravity could in principle break atoms apart, yes--because the force between the electrons and the nucleus is not strong enough to hold the atom together against the effects of sufficiently strong tidal gravity (meaning, the effects of sufficiently strong spacetime curvature).

Thank you very much all for your replies. According to your post can we have quarks moving freely, Inside the black hole ? Could the strong tidal gravity overcome the strong force between the quarks?

 

[PeterDonis]

Could the strong tidal gravity overcome the strong force between the quarks?

When the tidal gravity is strong enough, yes.

 

[maria_phys]

When the tidal gravity is strong enough, yes.

Interesting. So, assuming that quarks are moving freely in an environment with very strong tidal forces and gravity, can the shape (and properties) of them be changed?

 

[PeterDonis]

assuming that quarks are moving freely in an environment with very strong tidal forces and gravity, can the shape (and properties) of them be changed?

I'm not sure what you mean by the "shape" of a quark, or "properties". Tidal gravity can't change things like the quark's mass or color charge.

 

[maria_phys]

I'm not sure what you mean by the "shape" of a quark, or "properties". Tidal gravity can't change things like the quark's mass or color charge.

That exactly what I meant. Thank you.

(By the word "shape") A particle could be transform into a string, inside a back hole because of the huge tidal forces?

 

[PeterDonis]

A particle could be transform into a string, inside a back hole because of the huge tidal forces?

What do you mean by a "string"?

 

[maria_phys]

What do you mean by a "string"?

I mean, if the tidal force can stressed out the quark, transform it into a string..

 

[PeterDonis]

if the tidal force can stressed out the quark, transform it into a string..

Again, what do you mean by a "string"? A quark doesn't have a shape to begin with; it's not a little sphere.

 

[maria_phys]

Yes, I thought about that after I posted my question.

Forget about the string. Having a quark moving freely inside the black hole, could tidal forces stretch it out?

 

[PeterDonis]

Having a quark moving freely inside the black hole, could tidal forces stretch it out?

Stretch what? Once again, a quark is not a little sphere. It doesn't have a shape. I have said that several times now. Please read my posts again.

 

[maria_phys]

The "shape" has no meaning assuming that quarks has no inner structure. I'm talking about the case of quarks consists of strings.

 

[PeterDonis]

I'm talking about the case of quarks consists of strings.

Do you mean strings as in string theory?

 

[maria_phys]

Do you mean strings as in string theory?

Exactly.

 

[PeterDonis]

Exactly.

Ok, then what does "stretching" mean in string theory terms? (Hint: this is not a "B" level question and the obvious "B" level answer is not correct.)

The short answer is that the simple conceptual model you are using is not valid. A longer answer would be an "A" level discussion, for which it does not appear that you have the background. But briefly, in string theory, spacetime itself is an emergent phenomenon, so tidal gravity is also an emergent phenomenon. In other words, spacetime, and consequently tidal gravity, are "made of" strings in string theory; they're not things that can do things to strings. So your question is not even well-defined in string theory terms.

 

[Buzz Bloom]

I experience gravitational "force" with an additional different meaning than what has been discussed here so far. I directly feel an upward force on the bottom of my feet when I stand on a floor.

 

[pervect]

I experience gravitational "force" with an additional different meaning than what has been discussed here so far. I directly feel an upward force on the bottom of my feet when I stand on a floor.

You can feel that same "force" on your feet in an accelerating elevator (usually called Einstein's elevator) as well as by gravity. Sometimes the analogy of the "force" you feel when you're in a car and the car accelerates is also used to illustrate this point. The physical experience (the pressure you feel on your feet) is the same when you're standing on a floor in a gravitational field, or when you're standing on the floor in Einstein's elevator. In Newtonian physics at an introductory level, what you feel in Einstein's accelerating elevator is typically regarded as an pseudo force. People are generally cautioned about the difference about pseudo forces and real forces, but rather than repeat or attempt to explain these cautions I'm going to take a different route.

Rather than repeating cautions, or going into the underlying mathematics (which is great, if one has the necessary background) I'll focus on some simpler physical experiments that one can do on Einstein's elevator that illustrate why the model of the "gravity" in the elevator being "a force" is at best incomplete. Suppose there are two clocks on such an elevator, one on the floor, the other on the ceiling. The two clocks can exchange light signals, and if the elevator accelerates at a uniform rate, the propagation delay of the exchange of these light signals is constant. This constant propagation delay allows the rates of the clocks on the floor and ceiling to be compared.

Famously, when one compares the rates of the two clocks on Einstein's elevator in this manner, the rates of the two clocks are found to be different. This was noticed by Einstein (as far as I know he was the first to notice this), and rightly seen as an obstacle to describing a model of gravity that was compatible with Special relativity. The "force" model of gravity doesn't lead us to the expectation that the clocks will run at different rates due to their elevations - but that's what the analysis shows.

Going into the details of these calculations is beyond the scope of what I want to write for this short post, but I hope it helps illuminate some of the concepts involved. I'll also add that the effect, which has been termed "gravitational time dilation" has been confirmed for clocks on the Earth's surface in an actual gravitational field, for instance the famous Harvard tower experiment. As far as I know a comparison of clocks on an accelerating elevator is just a "thought experiment" rather than something that's been directly tested. While the direct test of this nature hasn't been done, many other tests of the principles involved have been carried out. See the usual FAQ references on experimental tests of the principle of equivalence for more.

 

[Buzz Bloom]

You can feel that same "force" on your feet in an accelerating elevator (usually called Einstein's elevator) as well as by gravity.

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.

By the way, in the thread

https://www.physicsforums.com/searc...e+beam+interruption.&o=date&c[user][0]=547865​

there is a discussion of how a person in a ten meter elevator can distinguish whether the elevator is on the Earth's surface, or it is in space accelerating at 1 g. The method involves measuring tidal force with an apparatus that could be constructed with current technology, but might take a very long time to accumulate sufficient statistics.

Regards,
Buzz

 

[Mister T]

I experience gravitational "force" with an additional different meaning than what has been discussed here so far. I directly feel an upward force on the bottom of my feet when I stand on a floor.

Even in the Newtonian approximation that's not the force of gravity that you feel. Or measure. It's a contact force exerted on you by the floor. And you exert an equal but opposite force on the floor ala Newton's Third Law.

If the floor (and you) are in free fall the gravitational interaction between you and planet Earth is still present, but you can't feel or measure it locally, by for example, standing on a bathroom scale.

 

[Buzz Bloom]

If the floor (and you) are in free fall the gravitational interaction between you and planet Earth is still present, but you can't feel or measure it locally, by for example, standing on a bathroom scale.

Hi Mister T:

I agree. When I am in free fall, I do not directly experience gravitational force, nor do I experience any acceleration from the gravitation force. I experience the force when I am stationary with respect to the gravitating source due to something preventing me from accelerating, like the floor.

Regards,
Buzz

 

[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

 

[Mister T]

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