A simple bench top experiment with GRAVITY . Is there anything there?

[imatfaal]

If ,as a thought experiment the mass of the earth was to change noticeably ,would an observer in another frame of reference ,say something free falling from the moon towards the centre of the Earth notice no acceleration?

 

So that from the FOR of the Earth the body approaching it , (if I am right) the body's acceleration would appear to vary with every change in mass (no idea how such a thing could be accomplished,if at all) but from the FOR of the infalling object or observer it would appear as if there was no acceleratiion "onboard" but the Earth would appear to be accelerating towards the observer or object....

 

 

I *think* someone in free fall would notice jerk (is that the right word: change in acceleration) but I don't really know...

 

I would hazard that barring tidal effects that locally there is no noticeable / measureable effect. Accelerometers measure zero in freefall - ie they measure acceleration relative to freefall and that would always be zero.

[geordief]

 

 

 

I would hazard that barring tidal effects that locally there is no noticeable / measureable effect. Accelerometers measure zero in freefall - ie they measure acceleration relative to freefall and that would always be zero.

Is my Binary Black Hole merger environment a realistic scenario? Would it be complicated by the presence of gravitational waves? Or would the gravitational waves be a /the central part of the experiment?

 

Actually I am not clear how long these gravitational waves last for . Are they just created "at the death" or would they be an important factor in the extremely close spacetime environment in a longer period leading up to the merger?

Edited September 19, 2016 by geordief

[Strange]

Is my Binary Black Hole merger environment a realistic scenario? Would it be complicated by the presence of gravitational waves? Or would the gravitational waves be a /the central part of the experiment?

Your thought experiment is rather unphysical because mass cannot just disappear. Also, my understanding is that gravitational waves have no effect in the Z axis (the direction of propagation).

 

 

Actually I am not clear how long these gravitational waves last for . Are they just created "at the death" or would they be an important factor in the extremely close spacetime environment in a longer period leading up to the merger?

 

Most of the signal is from the time when the black holes are spiralling in towards one another (the "inspiral"). Then there is a short "ringdown" after that as the black hole returns to its spherical shape.

[geordief]

Your thought experiment is rather unphysical because mass cannot just disappear. Also, my understanding is that gravitational waves have no effect in the Z axis (the direction of propagation).

No but the distribution of the matter is being rapidly changed.,isn't it?

 

Does that scenario allow us to ask the question whether or not the object would notice it was being accelerated.?

 

Is it wrong to describe that scenario as "free fall" if the thing that the object is supposedly "free falling" to is itself rapidly changing ?

 

Edit: hope I am not going too off topic Mike.

[Mike Smith Cosmos]

I would hazard that barring tidal effects that locally there is no noticeable / measureable effect. Accelerometers measure zero in freefall - ie they measure acceleration relative to freefall and that would always be zero.

.

We are in free fall !

 

But who or what ( by way of particle or field , or something else, is causing this free fall)

 

.OR

.

 

Mike

Edited September 19, 2016 by Mike Smith Cosmos

[geordief]

.

We are in free fall !

 

But who or what ( by way of particle or field is causing this free fall)

 

Mike

I keep hearing (I think) it is the geometry of spacetime that causes this movement.

 

I am not clear what role gravitons (they are only theoretical aren't they ) would play.

 

Do they just mediate changes in the gravity field -not cause the gravitational effects of the gravity field "as is" ?

Edited September 19, 2016 by geordief

[Mike Smith Cosmos]

I keep hearing (I think) it is the geometry of spacetime that causes this movement.

 

I am not clear what role gravitons (they are only theoretical aren't they ) would play.

 

Do they just mediate changes in the gravity field -not cause the gravitational effects of the gravity field "as is" ?

.

Yes that is fine by me , but what field is that , what stimulates or causes it , and what is the field ' IN ' ?

 

Mike

[Klaynos]

The field is gravity. It may have exchange particles called gravitons. It can be modeled as curved space time in many situations. It's not a field in anything.

[geordief]

.

Yes that is fine by me , but what field is that , what stimulates or causes it , and what is the field ' IN ' ?

 

Mike

Well my knowledge is more holes than substance but I am thinking lately that motion is the "natural" order of things for objects in the the universe and so we are looking for the direction these objects will take anyway . There is no such thing as a stationary object.Everything is moving in its direction. Apparently this "natural" direction is along a geodesic in spacetime.

 

Btw,I am not sure you can use the term "gravitational field" in the same way as as "EM field" (You know I am just parroting what I have heard so I can't back this up with the experience of a proper education)

 

Maybe both are referred to as "fields" but I think them properties of the two fields are completely different.

[Strange]

I keep hearing (I think) it is the geometry of spacetime that causes this movement.

 

That is one description. Another is Newton's force between two masses.

 

 

I am not clear what role gravitons (they are only theoretical aren't they ) would play.

 

Do they just mediate changes in the gravity field -not cause the gravitational effects of the gravity field "as is" ?

 

If a quantum theory of gravity is developed, then that will be another way of describing the asme thing.

 

So, gravity is what makes mass attract another mass. It can be described in different ways: a force, curvature of space-time, in future other things.

 

Of course, these are just descriptions of how it behaves. They can't tell you what it "really is". Nothing can. Ultimately what it "really is", is gravity.

[geordief]

 

That is one description. Another is Newton's force between two masses.

 

 

If a quantum theory of gravity is developed, then that will be another way of describing the asme thing.

 

So, gravity is what makes mass attract another mass. It can be described in different ways: a force, curvature of space-time, in future other things.

 

Of course, these are just descriptions of how it behaves. They can't tell you what it "really is". Nothing can. Ultimately what it "really is", is gravity.

But doesn't the spacetime theory have the Newtonian theory "in its back pocket".

 

Why bring up the mass attracting mass notion if it has been superseded? There is no going back to Newtonian force attraction is there except as a shorthand in special circumstances?

 

People looking for a quantum gravity theory are surely not trying to incorporate Newtonian gravity into their new theory are they?

Edited September 19, 2016 by geordief

[Mike Smith Cosmos]

The field is gravity. It may have exchange particles called gravitons. It can be modeled as curved space time in many situations. It's not a field in anything.

.

Ok.

We or I may be getting somewhere now !

If what you are saying is correct ( which I do not doubt , that you have a good chance of being correct . )

 

Then . :-

 

There is this Gravity Field there ( which I might have to come back to later , as to how in the first place it is generated as a field ) . Presumably by the mass of the earth?

 

And somehow or somewhere gravitons appear ( like little alien space ships , by the trillion ) encircle me and my travelling companion ( bunch of atoms in a saucer ) and drag us down at a set acceleration ( 9.81 meters second squared ) towards the centre of the earth.

Because that is the strength of the field in this immediate locality. ( again I might have to ask later where these gravitons were, before we arrived ) or are there trillions around everywhere , making up the field in the first place ?

 

(Forgetting the ' little alien space ships ' , for the moment ) . This is beginning to make some sort of loose sense .

 

 

In the presence of a lot of mass like the earth , there exists a gravity field composed of gazillion GRAVITONS. Whose sole purpose in life is to latch on to any and all , separated mass and drag it down to the centre of the generating gravitational field at an acceleration dictated to by the strength of the field at that particular location

( calculated by the Newtonian formula ( force of attraction ) F = G m1 xm2/r squared and then subject to F = m x a ) where ' a '. is the constant acceleration at that point in the field ?

 

Is that where we are at , or am I miles apart ?

 

 

Mike

Edited September 19, 2016 by Mike Smith Cosmos

[Strange]

But doesn't the spacetime theory have the Newtonian theory "in its back pocket".

 

Why bring up the mass attracting mass notion if it has been superseded?

 

Because it works. It is a valid description (in most cases).

 

And that is all science does: come up with good descriptions. If the description works, then use it.

 

 

People looking for a quantum gravity theory are surely not trying to incorporate Newtonian gravity into their new theory are they?

 

Any theory of quantum gravity will have to produce the same results as Newtonian gravity (in the cases where it is valid) and GR (in the cases where that is valid).

 

But it will still just be another description of the way masses appear to attract one another. It won't tell us what it "really" is.

[Mike Smith Cosmos]

 

 

I would appreciate someone checking out my post # 87 . As if it is correct , I am well on the way to an understanding of :-

 

HOW GRAVITY OPERATES AROUND LARGE MASSES ,in the astronomic environment , moons , like planets , stars etc I think ?

 

Mike

[Strange]

There is this Gravity Field there ( which I might have to come back to later , as to how in the first place it is generated as a field ) . Presumably by the mass of the earth?

 

I would say that the field is always there (and everywhere) and the presence of mass like the earth just "perturbs" the field.

 

 

And somehow or somewhere gravitons appear

 

If one could quantise the gravitational field, then gravitons would be how the quantised interaction would be described. They would be "virtual gravitons" so they are not particles and (you are not going to like this) don't really exist. It is just the way the mathematics describes quantum interactions.

 

Virtual photons are said to mediate the electromagnetic force, but magnets do not release trillions of photons that go out and capture bits of metal.

[geordief]

It won't tell us what it "really" is.

Not a problem with that . What reality is is a question in its own right and the one (circular) answer might well be along the lines of "Reality is what may not be defined"

 

Or less seriously "Reality ,like the poor will always be with us"

Edited September 19, 2016 by geordief

[Endy0816]

@Mike: Any mass attracts other mass. If you want to do something 'bench-top' might try repeating Cavendish experiment.

[studiot]

 

MSC post#82

Yes that is fine by me , but what field is that , what stimulates or causes it , and what is the field ' IN ' ?

 

Whilst I have no idea what the purpose of this thread is, I consider this a very reasonable question. +1

 

In Physics a field is a system that assigns a desired quantity to every point in some region of a space define by particular coordinate axes or even the entire space.

The desired quantity may be a scalar, vector, tensor, or other type of quantity in which case the field is a scalar, vector tensor etc field. An example of a field of another type of quantity is a direction field.

 

But in every case the quantity concerned has no extents along the coordinate axes, it is drawn from another set which may have its own coordinate system.

 

So we actually have two linked sets.

The set of 'values' of the quantity

The set of points in the coordinate system.

 

We perform mathematical processes, such as calculus, on these paired sets, by using the values from the first set and the change from one point to another in the second.

Edited September 19, 2016 by studiot

[swansont]

But doesn't the spacetime theory have the Newtonian theory "in its back pocket".

 

Why bring up the mass attracting mass notion if it has been superseded? There is no going back to Newtonian force attraction is there except as a shorthand in special circumstances?

 

 

Newtonian gravity works for almost all phenomena. It is GR that is needed for "special circumstances" and we would need quantum gravity for even smaller subset of special cases. Newtonian gravity got people to the moon and back, and many other space probes to their destinations. That's the criterion for a model: it describes how things behave.

 

The fact that you have another classical model (GR) and hope to have a quantum one, all using different descriptions, is an excellent indicator that none of them tell you what gravity "really" is.

@Mike: Any mass attracts other mass. If you want to do something 'bench-top' might try repeating Cavendish experiment.

 

Which will reveal only Newtonian behavior.

[geordief]

 

 

Newtonian gravity works for almost all phenomena. It is GR that is needed for "special circumstances" and we would need quantum gravity for even smaller subset of special cases. Newtonian gravity got people to the moon and back, and many other space probes to their destinations. That's the criterion for a model: it describes how things behave.

 

.

Is this semantics.?I thought I had learned that GR incorporated Newtonian Gravity theory (you only do not use GR for most cases because the differences are normally zero to negligible and the computational effort is so extreme.)

 

In the same way I anticipate that any new theory of Quantum Gravity that succeeds will incorporate both GR and Newtonian gravity in a Russian Doll-like way.

 

Is my pov badly wrong? Is the Russian Doll analogy wrong headed?

 

I had previously asked a question whether Newtonian Gravity was "something of a "special case" of General Relativity?"

 

http://www.scienceforums.net/topic/94702-why-is-newtonian-gravity-so-accurate/page-1#entry917866

 

and was not disabused of the notion then.

[Mordred]

Newtonian gravity can be derived from GR. Its a special treatment where the effects of gravity is significantly low. Mainly no time dilation.

 

Once you start having time dilation you depart from Newtonian gravity approximation.

Does that help or did it confuse ?

 

[math]g_{\mu \nu} = \eta_{\mu \nu} + h_{\mu \nu}[/math]

 

The [latex]\eta [/latex] specifies an approximately Euclidean geometry. More specifically Minkowskii geometry.

 

This doesn't hold true for geometry near a blackhole

Edited September 19, 2016 by Mordred

[geordief]

Newtonian gravity can be derived from GR. Its a special treatment where the effects of gravity is significantly low. Mainly no time dilation.

 

Once you start having time dilation you depart from Newtonian gravity approximation.

Does that help or did it confuse ?

No it didn't confuse . I manage that well on my own

[Mordred]

No it didn't confuse . I manage that well on my own

Excellent. Now the formulas you tend to see for GW waves is for an observer away from a BH in an Newton approximated geometry.

 

(An observer on Earth). A GW wave polarizes in the x and y axis. The direction of propogation is the z axis.

 

It doesn't alter the mass term per se, but it does induce pressure changes. So in this manner a GW wave through the energy/mass relation. Can induce additional gravity via the stress tensor.

 

In the formula above the stress tensor changes will be included under the "h" tensor. (I think of the h tensor as the permutation tensor)

Edited September 19, 2016 by Mordred

[geordief]

Excellent. Now the formulas you tend to see for GW waves is for an observer away from a BH in an Newton approximated geometry.

 

(An observer on Earth). A GW wave polarizes in the x and y axis. The direction of propogation is the z axis.

 

It doesn't alter the mass term per se, but it does induce pressure changes. So in this manner a GW wave through the energy/mass relation. Can induce additional gravity via the stress tensor.

 

In the formula above the stress tensor changes will be included under the "h" tensor. (I think of the h tensor as the permutation tensor)

So as not to unintentionally confuse you, I replied to the first part of your post (you added the second part with the equation a few minutes later)

 

I have not studied tensors yet and that equation is actually too hard for me too attempt to understand at this stage (maybe ever )

 

So the first part of your post did not confuse ,but the second would confuse me if I attempted to understand it properly...

Edited September 19, 2016 by geordief

[swansont]

Is this semantics.?I thought I had learned that GR incorporated Newtonian Gravity theory (you only do not use GR for most cases because the differences are normally zero to negligible and the computational effort is so extreme.)

 

In the same way I anticipate that any new theory of Quantum Gravity that succeeds will incorporate both GR and Newtonian gravity in a Russian Doll-like way.

 

Is my pov badly wrong? Is the Russian Doll analogy wrong headed?

 

I had previously asked a question whether Newtonian Gravity was "something of a "special case" of General Relativity?"

 

http://www.scienceforums.net/topic/94702-why-is-newtonian-gravity-so-accurate/page-1#entry917866

 

and was not disabused of the notion then.

 

 

Not so much incorporates it as they give the same answers. Newtonian gravity has no explanation for why masses attract. GR has geometry. I was interpreting "special circumstances" in terms of the scope of applicability, i.e. the boundary conditions. To me it has a different connotation than "special case", which is usually in terms of simplifying assumptions e.g. a special case might be a complex equation with some term(s) set to zero. But a special circumstance would be strong gravity, which is not the normal environment.

 

Sorry for the confusion.