GRAVITY: Force or Farce?

[DEFinning]

Let me first stress that I do not have a physical science background. This does not prevent me, however, from being at least competent at logical thinking. I was impressed w/ how willing the people here w/ advanced understanding were to explain things to lay-people. Hence, my question.

As I understand it, Einstein explains the puzzle of why things of different mass fall to the ground at the same speed (Galileo's experiment) being because they are following the same space-time curvature created by the planet (let's use Earth, in this example). In that case, would not, "gravity," only be our perception of that curvature, not an actual force?  

Alternately, if it is a force that is generated by the degree of curvature, doesn't that bring us back to the original question of why it has an identical effect on bodies of vastly different mass?

[studiot]

Good morning,

Well I will take you at face value about being able to think logically, despite the aattitude expressed in the title by the use of the word 'farce'.

Before you can understand the answer, you have to know and understand that we employ (consider) two kinds of 'force' in Science and Technology.

Real forces such as those which hold the atom together,

and imaginary forces which appear to be acting because of circumstances.

The commonest example of an imaginary force is 'centrifugal force', which is 'felt' by someone on a roundabout, but not by an observer standing by.

So your question boils down to is the force of gravity real or imaginary and what are the implications of each.

 

[DEFinning]

Okay, I'm ready & w/ you. The alliteration was too much of a temptation-- & now I'm rhyming-- in the title. My orig. title was very close to what you say is my actual question (Is Gravity REAL?) but I thought I'd get more notices if I added a little, "sizzle."

[SergUpstart]

1 hour ago, DEFinning said:

Alternately, if it is a force that is generated by the degree of curvature, doesn't that bring us back to the original question of why it has an identical effect on bodies of vastly different mass?

By the same influence, we mean the same acceleration. In this case, the force acting on the body is proportional to its mass, and the acceleration of the body is inversely proportional to its mass (Newton's second law). Thus, the mass of the body is reduced and it turns out that the acceleration of the body does not depend on its mass.

[DEFinning]

10 minutes ago, SergUpstart said:

By the same influence, we mean the same acceleration. In this case, the force acting on the body is proportional to its mass, and the acceleration of the body is inversely proportional to its mass (Newton's second law). Thus, the mass of the body is reduced and it turns out that the acceleration of the body does not depend on its mass.

So then, with his, "curvature," theory, Einstein didn't explain anything, initially, that wasn't already known.

53 minutes ago, studiot said:

Good morning,

Well I will take you at face value about being able to think logically, despite the aattitude expressed in the title by the use of the word 'farce'.

Before you can understand the answer, you have to know and understand that we employ (consider) two kinds of 'force' in Science and Technology.

Real forces such as those which hold the atom together,

and imaginary forces which appear to be acting because of circumstances.

The commonest example of an imaginary force is 'centrifugal force', which is 'felt' by someone on a roundabout, but not by an observer standing by.

So your question boils down to is the force of gravity real or imaginary and what are the implications of each.

Okay.

Okay.I'm ready & w/ you. The alliteration was too much of a temptation-- & now I'm rhyming-- in the title. My orig. title was very close to what you say is my actual question (Is Gravity REAL?) but I thought I'd get more notices if I added a little, "sizzle."

Edited September 25, 2020 by DEFinning
I think I mistakenly addended the person's post I was trying to answer, with my answer; I think my phone is not interfacing all that well w/ this feature of your site. The protocol for responding is not as obvious as I would have hoped.

[SergUpstart]

17 minutes ago, DEFinning said:

So then, with his, "curvature," theory, Einstein didn't explain anything, initially, that wasn't already known.

The deflection of light rays by the mass of the sun was twice as large as predicted by Newton's theory. Einstein's theory explained this (although an alternative explanation has recently been found for this effect within the framework of a small analogy of Newton's theory). In addition, Einstein's theory explained the precession of mercury's orbit, and here no alternative explanations have yet been found.

[studiot]

Understanding GR curvature requires quite a lot of deep Maths and Physics.

Better to start with how Science approaches its description and analysis of what we observe around us using models and in doing so understand how far models can go in prediction.

[DEFinning]

1 minute ago, SergUpstart said:

The deflection of light rays by the mass of the sun was twice as large as predicted by Newton's theory. Einstein's theory explained this (although an alternative explanation has recently been found for this effect within the framework of a small analogy of Newton's theory). In addition, Einstein's theory explained the precession of mercury's orbit, and here no alternative explanations have yet been found.

All right, that's very helpful to learn. I knew that Einstein, "proved" his theory through an astronomer's seeing light from a star going around some other body, but I didn't know that was the problem he was trying to solve.

I had, also, wondered if there could be some possible explanation for this, "proof," based on some as yet unknown quality of light, that made it act similarly to the way it bends when a person views it through water to take the shortest route (the effect that makes a pole under water in a pool look off kilter from what you'd expect from the part that's above water-- do you know what I'm talking about?). But you say there now IS an alternate proposed explanation! Do you think that either it is, or I am, too dense for you to explain it?

[Eise]

20 minutes ago, DEFinning said:

So then, with his, "curvature," theory, Einstein didn't explain anything, initially, that wasn't already known.

That is true. That objects with different mass accelerate exactly the same in a gravity field was already discovered by Galileo, and mathematically underpinned by Newton. Where Einstein went further is to wonder why mass has such totally different effects (inertia against forces, and the same attraction in a gravitational field).

He realised (at least) two things (which are in fact just mirror phenomena, but it helps to spell them out):

• In a windowless laboratory, it is impossible to experimentally find out if you are put in a (homogeneous) gravitational field or suddenly accelerating
• if the same laboratory is in free fall in a (homogeneous) gravitational field, you can do no single experiment that shows you are accelerating

From this he concluded that 'inertial mass' and 'gravitational mass' are exactly the same thing, and so are gravitation and acceleration. However, normally homogeneous gravity fields do not exist (only on small scales gravity fields can be approximated by homogeneous fields). To get the mathematics sound, Einstein had to use (difficult, even for him) techniques from differential geometry. And so he came to the conclusion that gravity can be described as spacetime curvature.

In this way Einstein could explain the orbit of Mercury, which showed deviations from Newtonian gravity, and the bending of light in gravitational fields. And there followed many more discoveries and predictions which only make sense when using General Relativity. Phenomena that would not exist at all is Newtons theory would be correct.

[DEFinning]

22 minutes ago, studiot said:

Understanding GR curvature requires quite a lot of deep Maths and Physics.

Better to start with how Science approaches its description and analysis of what we observe around us using models and in doing so understand how far models can go in prediction.

That seems like taking a very long running start. Initially, I'm just asking, in terms of thinking about an object being, "affected by gravity," in pre-GR terms (though I bet, in practice, physicists still say this) how is there a difference in thinking of it as actually being affected by the curvature of space, in practical terms?

[Eise]

5 minutes ago, DEFinning said:

All right, that's very helpful to learn. I knew that Einstein, "proved" his theory through an astronomer's seeing light from a star going around some other body, but I didn't know that was the problem he was trying to solve.

AFAIK, the real problem he wanted to solve is that Newtonian gravity does not fit to Special Relativity, especially that nothing can go faster than light. In Newtonian gravity, gravity is instantaneous. To say it a bit more technically: Newtonian gravity is not Lorentz invariant.

@DEFinning: Just to tell you: on the day that somebody joins the forum, he can only post 5 times. So you reached the limit for today. From tomorrow on you can post again, and as much as you want. 

[joigus]

16 minutes ago, DEFinning said:

Okay, I'm ready & w/ you. The alliteration was too much of a temptation-- & now I'm rhyming-- in the title. My orig. title was very close to what you say is my actual question (Is Gravity REAL?) but I thought I'd get more notices if I added a little, "sizzle."

Point taken.

I see other knowledgeable users are addressing the more traditional/geometric point of view. Just to try to be complete, and because I have a feeling that your idea that gravity may not "be real" might come from a certain trend in theoretical physics that started in the late 90's, let me tackle the "information" perspective:

In the last decades some messages have been filtering out from the community of theoretical physics in the way of "gravity is an illusion", "gravity is not a real force", "gravity is entropic", etc.

https://www.scientificamerican.com/article/the-illusion-of-gravity-2007-04/

Sometimes physicists try to develop an intuitive picture of what really is much more subtle and complicated, and involves a high level of mathematical sophistication to express precisely. Strong suggestions have cropped up that the best language to describe what gravity consists of is the language of information. That gravity is made up of information.

That may turn out not to be the case, but it's the direction the research seems to be going. Now, it is for you to decide whether information is real or not. I'll take the view that an action is real if it can destroy something, crash it to smithereens, modify it, change its course of motion.

Do you think information can modify, change the course of events, even destroy things? If you do, and it is true that gravity is made up of information, then gravity is real. 

Gravity can bend your spine, and it will, if you live long enough. So can information. Information can bend your spine, make you turn your head, nod in agreement, etc.

[studiot]

1 hour ago, Eise said:

AFAIK, the real problem he wanted to solve is that Newtonian gravity does not fit to Special Relativity, especially that nothing can go faster than light. In Newtonian gravity, gravity is instantaneous. To say it a bit more technically: Newtonian gravity is not Lorentz invariant.

@DEFinning: Just to tell you: on the day that somebody joins the forum, he can only post 5 times. So you reached the limit for today. From tomorrow on you can post again, and as much as you want. 

It's nice to see someone else offering the warning about the 5 first day posts rule.    +1

 

Also you have hit the nail on the head with your  comment I underlined, about the real reason Einstein's two relativity theories have been so well received.

Newton formally introduced the concept of Force so I will start there.
It was known from Newton's time onwards that there was a unresolved difficulty hidden in the Laws of Mechanics (as they were then known).
The difficulty is this and is called 'Action at a Distance'.
If I walk up to you and push you, then it is easy to say and understand 'I exert a force on you'.
But what if I drag a piece of iron along the ground by pulling it along with a separated magnet ?
How does the magnet exert a pulling force on the piece of iron when there is nothing between them?
Some secondary questions arise.
What happens as the distance between the bodies increases?
Does it take any time for the effect to happen, so if my magnet is suddenly knocked off course how long before the dragged piece of iron also changes course?

All these questions were known to Newton and (some of) his contemporaries.

It is these questions the Einstein's General theory address with great (but not total) observational success.

1 hour ago, DEFinning said:

That seems like taking a very long running start. Initially, I'm just asking, in terms of thinking about an object being, "affected by gravity," in pre-GR terms (though I bet, in practice, physicists still say this) how is there a difference in thinking of it as actually being affected by the curvature of space, in practical terms?

 

Faint heart ne'er won fair Lady.

Since you seem to be genuinely asking a thoughtful questions I thought I would offer you something very important and much easier than the Maths route, which is very tough,
yet still vitally important and of use in other (if not all) areas of Science and Technology, unlike the rather specialist Maths and Physics needed for Relativity.
 

[Markus Hanke]

3 hours ago, DEFinning said:

Initially, I'm just asking, in terms of thinking about an object being, "affected by gravity," in pre-GR terms (though I bet, in practice, physicists still say this) how is there a difference in thinking of it as actually being affected by the curvature of space, in practical terms?

I’m actually having difficulty understanding what exactly it is you are asking. Can you rephrase or reformulate the question? 
A test particle in free fall cannot not follow (if that makes sense) the geometry of spacetime, just as you cannot not follow the curvature of Earth’s surface as you move about on it. You can of course compensate for these effects by equipping yourself with suitable thrusters - but then you are no longer in free fall. Furthermore, there are scenarios where you cannot counteract gravity at all, regardless of how much you fire your thrusters; so it is evidently quite a real thing, in the sense that it has real consequences for the motion of bodies.

[MigL]

4 hours ago, joigus said:

In the last decades some messages have been filtering out from the community of theoretical physics in the way of "gravity is an illusion", "gravity is not a real force", "gravity is entropic", etc.

https://www.scientificamerican.com/article/the-illusion-of-gravity-2007-04/

In the good old days, theoretical Physicists used to add dimensions to make problems more tractable; Kaluza-Klein and SString theory come to mind.
Now the Holographic Principle is going the other way, and subtracting dimensions.

But is this an indication of 'reality', or just a mathematical 'trick' to facilitate modelling of that 'reality' ?

[joigus]

34 minutes ago, MigL said:

In the good old days, theoretical Physicists used to add dimensions to make problems more tractable; Kaluza-Klein and SString theory come to mind.
Now the Holographic Principle is going the other way, and subtracting dimensions.

But is this an indication of 'reality', or just a mathematical 'trick' to facilitate modelling of that 'reality' ?

I think that's a very sharp observation. I'm in two minds about that, to be honest, and ultimately I can't give you an answer. But that shift seems to be going on.

For one thing I rather like geometrical theories. They are mathematically complicated, but very beautiful and simple to formulate in principle.

But for the other, principles of negative content, the "don't go there" kind or principles, like thermodynamics' laws; IOW, the "impossibility to tell this from that", kind of principles, tend to be very stable, very robust. They may be signaling that new variables, or that a new creative splitting of the significant variables must be proposed. Maybe the distinction between source and interaction field cannot be assumed to be always valid. Or maybe, as Nima Arkani-Hamed says, "space-time is doomed."

I wish I knew the answer to that.

Edit: I'm not even sure I'm addressing your question properly.  

Edited September 25, 2020 by joigus