Is Poincaré's Space Dilemma the Key to Understanding Gravity?

[PeterDonis]

that is what gravity does to you too

No, it isn't. If you are moving solely under the influence of gravity, you feel no acceleration; you do not notice any inertia acting on you.

 

[yoron]

Well, if we use that spaceship at one constant gravity the same will hold true, will it not? But yes, inertia may be what defines a change in acceleration, whereas gravity is what we have when without change.

 

[PeterDonis]

if we use that spaceship at one constant gravity the same will hold true, will it not?

What do you mean by "spaceship at one constant gravity"? Do you mean a spaceship that is firing its engine to produce a 1 g proper acceleration? Such a ship is not moving solely under the influence of gravity.

More generally, an observer who feels a nonzero acceleration (like one at rest in the spaceship firing its engine) is not equivalent to an observer who feels zero acceleration (like one who is freely falling in a gravitational field). If you are trying to think of them as equivalent, you are doing it wrong.

 

[yoron]

I was thinking of the equivalence principle Peter, between a constantly accelerating, uniformly moving spaceship, at one gravity versus a planetary gravity at one G. Ignoring spin.

 

[PeterDonis]

I was thinking of the equivalence principle Peter, between a constantly accelerating, uniformly moving spaceship, at one gravity versus a planetary gravity at one G.

Here is what you originally said:

the point to notice here is that a acceleration always must be 'first person', meaning that you will notice when inertia acts on you, and you accelerate. And that is what gravity does to you too as I read it

And I replied that no, that is not what gravity does to you. If you are moving solely under the influence of gravity, you feel no acceleration, you notice no inertia acting on you, etc.

If you are in a spaceship with its engine firing, or standing on the surface of a planet, then yes, you feel acceleration, and you "notice inertia acting on you"--but the reason you feel those things is not gravity; it's the spaceship's engine firing, or the planet's surface pushing on you. In short, something other than gravity is acting on you, preventing you from being in free fall. So feeling acceleration, noticing inertia acting on you, is not what gravity does to you; it's what something other than gravity does to you.

 

[yoron]

Yes, I know :)

But if you let a moment of doubt into it you will see that I write "And that is what gravity does to you too as I read it"
What I do there is to connect it to the idea of Higgs particles. I've had, and still have, a lot of doubt in the view that 'gravity' is a result of a 'Higgs field'. Somewhere I read a rather good explanation to why it, to some, can be seen as equivalent though. I should probably have been clearer on that subject. Never the less, if we ignore a Higgs field, the equivalence principle presume that this 'spaceship' and 'Earth' both are equivalent as I understands it.
==

And, just to not clutter this thread.. Wes, what you write about 'global' relative 'local' interpretations is one of my big dilemmas for the moment. I like the local interpretation myself, and are not sure what a global should be. Although Einstein defined one. Where I differ is in what makes a universe. I presume it to need a logic, to become 'whole', that meaning explainable without needing to refer to magic. But, logic is a mindset.

 

[PeterDonis]

What I do there is to connect it to the idea of Higgs particles. I've had, and still have, a lot of doubt in the view that 'gravity' is a result of a 'Higgs field'

First, this is completely unrelated to the equivalence principle, which is what you had previously said you were talking about.

Second, gravity is not a result of a Higgs field. Nonzero rest mass is, according to the Standard Model of particle physics. But nonzero rest mass is not the same as gravity; it's not even the same as being a source of gravity. The source of gravity is the stress-energy tensor, and objects with zero rest mass (like photons) still have a nonzero stress-energy tensor.

if we ignore a Higgs field, the equivalence principle presume that this 'spaceship' and 'Earth' both are equivalent as I understands it.

The EP says that proper acceleration due to being in a spaceship with its engine firing, and proper acceleration of the same magnitude due to standing on the surface of a gravitating mass, are equivalent, yes. And it says this regardless of whether a Higgs field is present or not. See above.

 

[yoron]

"Second, gravity is not a result of a Higgs field. Nonzero rest mass is, according to the Standard Model of particle physics. But nonzero rest mass is not the same as gravity; it's not even the same as being a source of gravity. The source of gravity is the stress-energy tensor, and objects with zero rest mass (like photons) still have a nonzero stress-energy tensor. "

Nicely put Peter, and that goes back to how one want to define a universe, to me. I don't remember where the link is, to how you connect a Higgs field to gravity, but it was that one I was thinking of when commenting. Maybe someone else have a link to it? Otherwise I'll try to find it again, been some time since I read about it.
=

And no, I'm not speaking about inertia in this case, although that is the view I have myself when it comes to a Higgs field influence. Typical, isn't it :) then again, was another laptop I had it on. This is actually a minefield I'm traversing, thinking of it again. Einstein had this to say about it.

"Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the Earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation."

And if now 'inertia' is 'gravity'?

Ouch. And I think that was it Peter, even if I don't find a specific article discussing it. It doesn't really explain how one 'type' of gravity can be so different from another, well, in my eyes then. Inertia do exist everywhere, as soon as you accelerate, but?
=

Also, if I imagine a perfectly spherical restmass, non rotating, evenly distributed, in a presumed empty space. Then 'gravity' points inwards, doesn't it? If I now somehow shrink this restmass into a 'black hole' then the direction still will be the same, right? (This last question actually has a relevance to me wondering about 'locality', which is how I view what really (reality) is. The 'Global' interpretation is not what I will observe, other than theoretically. And this saddles meta physics too I'm afraid)

 

[PeterDonis]

I don't remember where the link is, to how you connect a Higgs field to gravity

That's because there isn't a link, as I said. There's only a link between the Higgs field and nonzero rest mass.

no, I'm not speaking about inertia in this case

But you seem to think that "inertia" and "gravity" are analogous, since you ask:

And if now 'inertia' is 'gravity'?

But inertia and gravity are not analogous, so this question doesn't make sense. What Einstein meant (bear in mind that you are reading an English translation of something he wrote in German, so you can't go by the exact words used) was that "inertial mass" and "gravitational mass" were the same thing--which is just the equivalence principle: proper acceleration due to being in a rocket in empty space with its engine firing, is the same (locally) as proper acceleration due to standing on the surface of a planet, at rest in a gravitational field. He did not mean that "inertia" and "gravity" were the same thing.

if I imagine a perfectly spherical restmass, non rotating, evenly distributed, in a presumed empty space. Then 'gravity' points inwards, doesn't it?

If by "gravity" you mean "the direction a freely falling object would fall if released from rest by a static observer", then yes.

If I now somehow shrink this restmass into a 'black hole' then the direction still will be the same, right?

Yes.

 

[harrylin]

[..]
If you are [..] standing on the surface of a planet, then yes, you feel acceleration, and you "notice inertia acting on you"--but the reason you feel those things is not gravity; it's [..] the planet's surface pushing on you. In short, something other than gravity is acting on you, preventing you from being in free fall. So feeling acceleration, noticing inertia acting on you, is not what gravity does to you; it's what something other than gravity does to you.

(emphasis mine)
Without gravity, the surface of the planet would not push against you. What you feel is the force of the planet's surface that is countering the action of gravitation. Apart of that, you meant of course "proper acceleration" [EDIT I notice that you clarified that in your next post]; relative to a free falling reference system you accelerate at g, but not relative to others such as the ECI frame.

 

[A.T.]

Without gravity, the surface of the planet would not push you back ...you feel is the force of the planet's surface that is countering the action of gravitation...

(emphasis mine)

How do you determine what is merely "pushing back" / "countering" as opposed to "action"?

 

[harrylin]

(emphasis mine)

How do you determine what is merely "pushing back" / "countering" as opposed to "action"?

Good question! How does one determine that if one feels a force between oneself and the floor, that it is not that one's body has an active force that is pushing downward on the floor, and the floor is merely passively pushing back with inertial reaction? I suppose that it is by the method of establishing cause and effect; indeed, that is a main scientific method. Try to stand on the floor of a free floating spaceship and you will feel no force.

PS. it's really too far off topic to elaborate further in this thread about Poincare's space imagination; but it could be an interesting discussion topic for the general physics sub-forum where by chance a similar discussion is going on now, e.g. #2 "[..[ mass causing [..]" and the link to the wiki with a paper on "active gravitational mass".

 

[A.T.]

Try to stand on the floor of a free floating spaceship and you will feel no force.

Switch the rockets on, and you feel the force. So the "cause" here is the thrust from the engines, while for the Earth's surfcace it's the pressure forces acting on it from below.

 

[harrylin]

Switch the rockets on, and you feel the force. So the "cause" here is the thrust from the engines, while for the Earth's surfcace it's the pressure forces acting on it from below.

In both cases "there are pressure forces acting on it from below".
The Earth's gravity has an equivalent effect as the rocket engines; that's the equivalence principle. If you disagree with the concept that gravitation is causative or active, please bring up your disagreement in the other thread to which I already linked. I will not elaborate more on that in this thread.

 

[A.T.]

The Earth's gravity has an equivalent effect as the rocket engines

The pressure forces pushing the Earth's surface from below are equivalent to the pressure forces in the rocket engine, pushing the rocket.

 

[stevendaryl]

(emphasis mine)
Without gravity, the surface of the planet would not push against you. What you feel is the force of the planet's surface that is countering the action of gravitation

But what you feel isn't any different in the case where the ground is pushing up on you to counteract gravity or the case where the ground is pushing up on you because it is being accelerated upward by rockets. So the point is that you aren't "feeling" gravity; you're feeling the ground pushing up on you.

We can make the situations even more similar by assuming that there is a platform that is held up by rockets firing. You are standing on that platform. Then in both cases, what you feel is the platform pressing up on your feet. In both cases, the reason the platform is pressing up on your feet is because there are rockets attached to the platform. So it is even more inappropriate to say that you are in any way feeling gravity, or feeling a force that is caused by gravity. Gravity isn't causing the rockets to fire.

What gravity does is to make it possible for the rockets to fire continuously without going anywhere.

 

[stevendaryl]

The Earth's gravity has an equivalent effect as the rocket engines; that's the equivalence principle.

No, that's not the equivalence principle. The equivalence principle doesn't equate gravity and the effect of rockets. It equates (1) using rockets (or other means) to stay stationary in a gravitational field, and (2) using rockets to accelerate in gravity-free space. It equates (1) falling in a gravitational field and (2) drifting at constant velocity in gravity-free space.

 

[PeterDonis]

Without gravity, the surface of the planet would not push against you.

The way we put this in GR is, without spacetime curvature, the surface of the planet would not push against you. It pushes against you because "free fall", i.e., zero proper acceleration, in your vicinity does not mean being at rest with respect to the planet. This is not due to any "force" of gravity; it is due to the curvature of spacetime in your vicinity. If you equate "gravity" with spacetime curvature, then yes, you could say it is "gravity" that explains why the surface of the planet has to push against you to keep you at rest relative to it. But on this interpretation, again, "gravity" is not a force and is not felt as a force.

What you feel is the force of the planet's surface that is countering the action of gravitation.

But "action of gravitation" here just means "the natural state of free fall"--the state every object is in if it is not being pushed on by something. In other words, according to GR, free fall does not have to be explained; it's the default state. What has to be explained is proper acceleration, and "gravity" does not cause proper acceleration; some non-gravitational interaction must be taking place.

 

[PeterDonis]

The Earth's gravity has an equivalent effect as the rocket engines; that's the equivalence principle.

No, it is not. The EP says that the Earth's surface, pushing up on you, has an equivalent effect to the rocket engines. It also says that free fall--the state you would be in if the rocket engines were not firing--has an equivalent effect to "gravity". (I see stevendaryl has made this same point.)

The reason this is so important is that people so often try to view the "acceleration due to gravity"--the coordinate acceleration that a freely falling body experiences relative to either the rocket with its engine firing, or the Earth--as equivalent to the proper acceleration imposed by the rocket engine. As many, many, many threads here on PF bear witness, that confusion is both common, and a cause of much further confusion. So we try very hard to prevent it from ever starting in the first place. Even if you understand the difference, other readers of this thread might not, unless we make it crystal clear: free fall and proper acceleration are not equivalent.

 

[harrylin]

[..] you're feeling the ground pushing up on you to counteract gravity [..]

Almost correct: the ground is passive, it's countering your downward push - as I explained in #42. If any further discussion, I will simply quote and reply in the linked thread as it's off-topic here and on-topic there.

 

[harrylin]

No, it is not. The EP says that the Earth's surface, pushing up on you, has an equivalent effect to the rocket engines. It also says that free fall--the state you would be in if the rocket engines were not firing--has an equivalent effect to "gravity". [..]

Yes it is. As I cited, the EP according to Einstein also says that a gravitational field exists for the man in the chest; that is, a gravitational field has an equivalent effect on the man in the chest as a rocket engine that is pulling the chest (he did not mention a rocket engine but stressed that is immaterial by what means the chest is accelerated). This is really standard knowledge, and again off topic... however the "action" aspect is on topic in the other thread which I now follow, waiting for your comments there.

The reason this is so important is that people so often try to view the "acceleration due to gravity"--the coordinate acceleration that a freely falling body experiences relative to either the rocket with its engine firing, or the Earth--as equivalent to the proper acceleration imposed by the rocket engine. As many, many, many threads here on PF bear witness, that confusion is both common, and a cause of much further confusion. So we try very hard to prevent it from ever starting in the first place. Even if you understand the difference, other readers of this thread might not, unless we make it crystal clear: free fall and proper acceleration are not equivalent.

I find it difficult to imagine people confounding a free fall experience with an accelerating rocket experience.

 

[A.T.]

the ground is passive

So am I.

it's countering your downward push

I can just as well say that I'm countering the ground's push on me. Newtons 3rd Law is symmetrical, so this is an arbitrary choice.

 

[A.T.]

a gravitational field has an equivalent effect on the man in the chest as a rocket engine that is pulling the chest

The equivalence is:

chest floor ~= Earth's surface
rocket thrust ~= pressure force supporting the Earth's surface

It's these interaction forces that result in proper acceleration which can be felt.

 

[stevendaryl]

you're feeling the ground pushing up on you to counteract gravity.

The way you quoted that makes it seem that you are quoting me, but that is not what I said. My sentence ended with the words "pushing up on you"

 

[stevendaryl]

I find it difficult to imagine people confounding a free fall experience with an accelerating rocket experience.

That's what it appeared that you were doing, when you equated gravity with rockets. As I said, the equivalence principle claims the equivalence of:

1. Standing on a platform hovering above the Earth using rockets.
2. Standing on a platform accelerating through empty space using rockets.

In both cases, what you "feel" is the platform pressing up against you, and the reason the platform is pressing up against you is because it has rockets attached.

 

[harrylin]

The way you quoted that makes it seem that you are quoting me, but that is not what I said. My sentence ended with the words "pushing up on you"

I cited the part on which we agree; the other part is the part that I comment on, as promised, in the appropriate thread. Enough hijacking!

 

[stevendaryl]

I cited the part on which we agree; the other part is the part that I comment on, as promised, in the appropriate thread. Enough hijacking!

Well, when you use quotes, you have to be careful that it's actually a quote.

 

[harrylin]

That's what it appeared that you were doing, when you equated gravity with rockets. As I said, the equivalence principle claims the equivalence of:

1. Standing on a platform hovering above the Earth using rockets.
2. Standing on a platform accelerating through empty space using rockets.

[..].

Strange! See again the post that you cite; you overlooked, once more, according to Einstein also:
1a. Standing in a chest that is in rest on the surface of the Earth
2a. Standing in a chest that is pulled with a rope.

 

[stevendaryl]

Strange! See again the post that you cite; you overlooked, once more, according to Einstein also:
1a. Standing in a chest that is in rest on the surface of the Earth
2a. Standing in a chest that is pulled with a rope.

You think a rope is different from a rocket, in this regard?

My point is that in neither case are you feeling the force of gravity.

 

[A.T.]

1a. Standing in a chest that is in rest on the surface of the Earth

It doesn't make a difference what supports the chest on Earth. The upwards contact force (for a standing chest) and the upwards rocket thrust (for a hovering chest) are both interaction forces that result in 1g upwards proper acceleration, that the man on the chest floor can feel.

2a. Standing in a chest that is pulled with a rope.

You can have the chest in 1a. also hang on a rope, from a tree.