Are inertial forces "real"

[pmb]

This thread is not about what an inertial force is, what Einstein thought of it, what you think GR has to say on it etc. This thread is whether you think that inertial forces can be thought of as "real" or not. And. of course, that will be a function of your own personal opinion on how the term "real? should be interpreted, at least in this scenario/case.

The definition of inertial force is well known and never debated. For those who don’t know the definition please see

http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm

In that page I've described various viewpoints from some well-known physics textbooks on mechanics general relativity and cosmology.

Before any of you claim to know how it's viewed in the physics community please see the quotes at the end which expresses one side of the viewpoint.

My question for you is - Do you believe that the gravitational force cannot be thought of as a "real" force and must therefore be called, at best, a pseudo force? Or to phrase it another way - How many of you believe that if a particle is accelerating under the action of a field for which the 4-acceleration on the particle is zero that any attempt to define a "force" on the particle must imply that it should be thought of/defined as a pseudo-force?

If you're able to vote either yes, nor or other then please do so. or example; "Other" might mean that you don't know what "real" means or that its a bad word to use in physics.

Note - No. This is not a trick question.

 

[Pete]

- Randall Munroe, xkcd: Centrifugal Force

 

[Pete]

I vote yes, but with some reservation. There is good reason to call inertial forces pseudo forces in at least some situations... but that doesn't stop them from crushing the life out of you.

 

[Maxila]

I vote yes, but with some reservation. There is good reason to call inertial forces pseudo forces in at least some situations... but that doesn't stop them from crushing the life out of you.

I agree with Pete and vote yes. As he said the fact they will change the dynamics, and/or nature, of energy and matter subject to them is undeniably as real as it gets.

 

[Billy T]

I agree with Pete and vote yes. As he said the fact they will change the dynamics, and/or nature, of energy and matter subject to them is undeniably as real as it gets.

I vote no, but agree theses ACCLERATIONS, can kill you as can other large ones, even linear accelerations like sitting on a steel plate when a 100 pounds of TNT under the plate explodes. That is no reason to call an acceleration a force.

Also if you do call it a force, then you invalidate Newton's third law (the one about every force having an equal and opposite force). If you can't tell me what is the "equal and opposite force" to the one you feel when on a merry-go-round and may not know enough physics to know that feeling is caused by an acceleration, not a force.

 

[RJBeery]

In physics, a force is any influence that causes an object to undergo a certain change, either concerning its movement, direction, or geometrical construction. - Wiki

The question comes down to "certain change" relative to what? In flat Minkowski space gravity is an actual force, not a pseudo one. In a rotating system described by the Bond villain even a "stationary object" would be under constant acceleration.

Anyway, since it can't really be answered objectively I'm voting yes only because I think most would agree that objects undergo certain changes when subjected to inertial forces by our common sense definitions of "certain changes".

 

[Aqueous Id]

My question for you is - Do you believe that the gravitational force cannot be thought of as a "real" force and must therefore be called, at best, a pseudo force? Or to phrase it another way - How many of you believe that if a particle is accelerating under the action of a field for which the 4-acceleration on the particle is zero that any attempt to define a "force" on the particle must imply that it should be thought of/defined as a pseudo-force?

If you're able to vote either yes, nor or other then please do so. or example; "Other" might mean that you don't know what "real" means or that its a bad word to use in physics.

I voted "other" because, although in the context of your article (excellent BTW) the best answer might be "yes", there are several layers of mystery that surround the actual meaning of a particle in free fall as we lean toward interpreting the gravitational force as a pseudoforce. It left me a little puzzled about what real vs pseudo force might even mean in this context, other than as a simple distinction between reference frames undergoing linear translation and those in rotation. Then comes the mystery of what rotation actually means in GR, how it manifests other than to say space and time curl and uncurl infinitesimally in the gravity well, which isn't even done justice by a wireframe sketch such as the ones used to illustrate the findings of Gravity Probe B. And I mention this because it's hard to connect the more intuitive wireframe to the implied rotation of the particle as is traverses the curved path that imparts the acceleration that would lead to the likely result that gravitational force is a pseudoforce. I'm not at all a skeptic (at least I think I'm not) but I'm a little agnostic (or maybe confused is a better word) over the several counterintuitive layers of reasoning that connect these ideas. The same is true to an extent in managing the counterintuitive notion in SR that there is a spatio-temporal rotation that accompanies motion, but only in a relative sense. What does it really mean, physically? That remains shrouded in the mystery that's associated with the questions "what is space" and "what is time".

One thing I liked about your article was the particular way you wove those excellent cites together to present this idea which I find difficult to articulate. I liked thinking that there is a way to treat SR and GR as duals, which allows for more than just reconciling the reason they both involve Lorentz rotation. There is something much deeper to this (deep for my shallow noggin) which has something to do with that uncurling of space and time, for which the duality is a correspondence between relative velocity and -- well, not just mass -- but the traversal of the particle through the gradient of the well produced by mass. (My language is a little klunky for lack of a vocabulary to express this, but I will forge ahead anyway.)

I'm a little tempted to say that two phenoma -- the cutting space by time (as we might think of dx/dt) by relative motion in SR, and the static gravitational case in GR in which the nonmoving object is simultaneously in free fall (as if occupying two worlds simultaneously) and is therefore evidently cutting space by time in a virtual way, and perhaps merely by its existence -- give me the uncanny feeling that they run counter to the natural process of inflation, which (not sure if this word is appropriate here ) uncurls (?) space and time at the inflationary pace, and these other contrary actions, since they seem to run counter to that natural pace of expansion, would undergo a warp that has more to do with being out of sync with that pace than it does with being "real" or "pseudo". The natural consequence of the counterposed action would seem to be a curl in the reverse sense, hence the observed rotation described in the Lorentz transformation. (I probably massacred what I was really trying to say, so bear with me.) But before we can refer one frame to another it seems we have to refer all of space and all of time to the inflation that creates them. I think that gets me closer to what I wanted to say. The distinction between real and pseudo force seems to lie in the way the underlying action proceeds "toward" or "against" the inflation.

I feel like I still haven't gotten to the bottom of my reservations, and I'd like to think that at some time in the future I'll work through this in my mind and hopefully see the light. And most of all, I hope to fill some of the glaring holes in my understanding of the science. But I'll leave it at that, having touched on the flavor of my reason for voting "other". I also hope I didn't get too far off in the weeds for the more practical and better grounded folks here. Not to mention the ones who actually know what they're talking about.

 

[brucep]

This thread is not about what an inertial force is, what Einstein thought of it, what you think GR has to say on it etc. This thread is whether you think that inertial forces can be thought of as "real" or not. And. of course, that will be a function of your own personal opinion on how the term "real? should be interpreted, at least in this scenario/case.

The definition of inertial force is well known and never debated. For those who don’t know the definition please see

http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm

In that page I've described various viewpoints from some well-known physics textbooks on mechanics general relativity and cosmology.

Before any of you claim to know how it's viewed in the physics community please see the quotes at the end which expresses one side of the viewpoint.

My question for you is - Do you believe that the gravitational force cannot be thought of as a "real" force and must therefore be called, at best, a pseudo force? Or to phrase it another way - How many of you believe that if a particle is accelerating under the action of a field for which the 4-acceleration on the particle is zero that any attempt to define a "force" on the particle must imply that it should be thought of/defined as a pseudo-force?

If you're able to vote either yes, nor or other then please do so. or example; "Other" might mean that you don't know what "real" means or that its a bad word to use in physics.

Note - No. This is not a trick question.

Hi Pete

I'm not going to vote since I don't have an opinion. The reasoning for me is 'what we call it' won't change physics associated with the description of natural phenomena. I think folks argue about this stuff on occasion. Tell the Star, recorded by the HST being torn apart as it falls towards the black hole, the tidal accelerations/forces are fictitious. LOL.

 

[paddoboy]

I'm a yes voter, as I believe anything we can see, smell, touch, taste, or feel is real enough.

 

[wellwisher]

This thread is not about what an inertial force is, what Einstein thought of it, what you think GR has to say on it etc. This thread is whether you think that inertial forces can be thought of as "real" or not. And. of course, that will be a function of your own personal opinion on how the term "real? should be interpreted, at least in this scenario/case.

Say we have a train and car heading toward each other in a head on collision. If we ignore inertial forces we can say the velocity is relative since either reference can give us the velocity. But if we allow them to collide, we would find that these relative references are not equal, due to inertial forces.

One can do this experiment conceptually. If the car is moving at V and it hits the stopped train, head-on, the car stops into a pancake. If the train is moving at v and the car is stopped, after the train punts the car, the train continues to move. If one was on each reference, when inertial forces manifest, in it is not relative anymore. I would prefer be on the relative train references since you won't feel the car punted.

 

[pmb]

Also if you do call it a force, then you invalidate Newton's third law (the one about every force having an equaland opposite force).

It's well known that Newton's 3rd law doesn't always hold. In fact it's violated when two charges in relative motion interact.

If you can't tell me what is the "equal and opposite force" to the one you feel when on a merry-go-round and may not know enough physics to know that feeling is caused by an acceleration, not a force.

The source of inertial forces is the matter of the universe. This force is a result of what's called inertial induction. This is explained in the link I gave which quote's Peacock's text

http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm

A hint at an answer to this question comes by returning to the expression for the inertial forces. The most satisfactory outcome would be to dispose of the notion of inertial frames altogether, and to find a direct physical mechanism for generating ‘fictitious’ forces. Following this route in fact leads us to conclude that Newtonian gravitation cannot be correct, and that inertial forces can be affectively attributed to gravitational radiation. Since we cannot at this stage give a relativistic argument, consider the analogy with electromagnetism … It is highly plausible that something similar goes on in the generation of inertial forces via gravity, and we can guess the magnitude by letting e/(4pe0) ® Gm. This argument was proposed by Dennis Sciama, and is known as inertial induction. …. Thus, it does seem qualitatively valid to think of inertial forces as arising from gravitational radiation. Apart from being a startling different view of what is going on in non-inertial frames , this argument also sheds light on Mach’s principle: for a symmetric universe, inertial forces clearly vanish in the average rest frame of the matter distribution. Frames in constant relative motion are allowed because (in this analogy) a uniformly moving charge does not radiate.

 

[pmb]

I...although in the context of your article (excellent BTW)

That's very gracious of you. Thank you so much!

 

[pmb]

Hi Pete

I'm not going to vote since I don't have an opinion. The reasoning for me is 'what we call it' won't change physics associated with the description of natural phenomena. I think folks argue about this stuff on occasion. Tell the Star, recorded by the HST being torn apart as it falls towards the black hole, the tidal accelerations/forces are fictitious. LOL.

Thanks for the input.

The reason most GRists interpret gravity as a curvature in spacetime is that they don’t perceive gravitational forces as being “real” and that’s because they can be transformed away whereas tidal forces (a second order effect) cannot be transformed away.

I’ve see people make mistakes because of this interpretation. There’s an article in the American Journal of Physics where the author assumes that because a uniform gravitational field is a gravitational field that it must have a curved spacetime. Yet a uniform gravitational field is defined as having zero spacetime curvature. Thus his thinking got mixed up because of his views and interpretation on what “real” is. That’s one of the reasons this topic is of interest to me.

 

[BdS]

Haven't voted yet, still trying to figure out what inertial force is. If you compare it to a gravity well with negative curvature the inertial force would be the opposite, a hill with positive curvature. Gravity puts denser matter on the inside (center) and the less dense matter outward in a density decreasing array from the center. The inertial force does the opposite puts the densest matter on the outside and with the lower density matter moving inward creating an opposite array. In gravity the boundary is in the inner (center) and the inertial is on the outer. The potential energy the matter contains in the two would also be opposite, in gravity its the distance from center and in inertial the distance from the outer boundary toward the center with the center being the highest potential for inertia.

Just taking some notes... Is it ok to imagine the inertial force like this?

 

[wellwisher]

In the head on collision example, inertial force appears upon impact, as the car and the train try to occupy the same space. Before they collide what we have is kinetic energy. The conservation of energy does not allow for relative references unless both reference are exactly equal. The collision test is the best way to test relative references to demonstrate each reference will have different sum of energy and inertial forces upon impact.

Inertial is tangible, space-time is all in the mind. You can't prove space-time without math and conceptual constraints. Inertial is different in that inertial can hit you on the side of the head, with a wet sponge and be demonstrated. You don't need math or abstractions, when the pie hits you in the face. Once mass tries to occupy the same space as other mass, forces appear. Relative reference examples are never allowed to collide references, so they do not have to deal with energy balances and inertial considerations.

 

[brucep]

Thanks for the input.

The reason most GRists interpret gravity as a curvature in spacetime is that they don’t perceive gravitational forces as being “real” and that’s because they can be transformed away whereas tidal forces (a second order effect) cannot be transformed away.

I’ve see people make mistakes because of this interpretation. There’s an article in the American Journal of Physics where the author assumes that because a uniform gravitational field is a gravitational field that it must have a curved spacetime. Yet a uniform gravitational field is defined as having zero spacetime. Thus his thinking got mixed up because of his views and interpretation on what “real” is. That’s one of the reasons this topic is of interest to me.

I'm not sure what you mean by 0 spacetime. I started out with GR so I haven't paid much attention to stuff that is a consequence of using Newtons theory but doesn't exist in GR. Like frame dependent forces. A basic uniform gravitational field would be g_shell over r_shell expressed in the Schwarzschild coordinates. The metric reduces to SR in the uniform gravitational field, a special case where the effects of gravity can generally be ignored in a local laboratory frame. Spacetime curvature over this uniform field is a constant that has no requirement to be 0. Just uniform over the entire path through the field.

 

[Tach]

The metric reduces to SR in the uniform gravitational field, a special case where the effects of gravity can generally be ignored in a local laboratory frame.

No, it doesn't. It reduces to the Minkowski metric in ZERO gravitational field, not in uniform gravitational field.

Spacetime curvature over this uniform field is a constant that has no requirement to be 0. Just uniform over the entire path through the field.

This is false, I suggest that you consult a book on the subject, the stuff that you are making up is patently wrong.

 

[Tach]

I’ve see people make mistakes because of this interpretation. There’s an article in the American Journal of Physics where the author assumes that because a uniform gravitational field is a gravitational field that it must have a curved spacetime. Yet a uniform gravitational field is defined as having zero spacetime. Thus his thinking got mixed up because of his views and interpretation on what “real” is. That’s one of the reasons this topic is of interest to me.

"Zero spacetime"? This is nonsense. You must be referring to "zero intrinsic curvature".

 

[Undefined]

Hi pmb.

Thanks for the input.

The reason most GRists interpret gravity as a curvature in spacetime is that they don’t perceive gravitational forces as being “real” and that’s because they can be transformed away whereas tidal forces (a second order effect) cannot be transformed away.

I’ve see people make mistakes because of this interpretation. There’s an article in the American Journal of Physics where the author assumes that because a uniform gravitational field is a gravitational field that it must have a curved spacetime. Yet a uniform gravitational field is defined as having zero spacetime. Thus his thinking got mixed up because of his views and interpretation on what “real” is. That’s one of the reasons this topic is of interest to me.

If I read you correctly IN CONTEXT, did you inadvertently make a 'typo' by omitting the word "curvature" after "spacetime" there, mate?

If so, no biggie; I read you in context ok despite that typo. Cheers.


PS: pmb, everyone, I haven't voted; but I will of course be reading this poll thread/discussion with interest. Thanks for the interesting discussion, pmb, everyone.

 

[Trapped]

The source of inertial forces is the matter of the universe. This force is a result of what's called inertial induction. This is explained in the link I gave which quote's Peacock's text.

We don't actually know if the sum of masses in the universe contributes the properties of inertia on moving or stationary objects. The idea that inertia had something to do with all the matter in the universe was Machian theory of gravity one which Einstein never fully resolved.

Inertia as far as we know, is a purely intrinsic property of all matter. The idea that inertia is resultant from all the mass in the universe, a Machian proposal which was rooted in an idea carried on from the birth Newtonian physics, the proposal/hypothesis/assumption that all matter gravitational influences all other matter in the universe, no matter what their distance. This obviously cannot be tested.