I believe I have figured out what Zarkov has been attempting to convey when he insists that gravity pushes, rather than pulls. I am not sure exactly where this leads, but the point may be entirely relevant in understanding and defining gravity. Gravity has been compared to “forces”, but is not grouped with the accepted energetic forces for reasons that might get looked at later. How much might be understood about gravity if it were to be quantified as a fundamental pressure?

Riff-in-Thyme said:

I believe I have figured out what Zarkov has been attempting to convey when he insists that gravity pushes, rather than pulls. I am not sure exactly where this leads, but the point may be entirely relevant in understanding and defining gravity. Gravity has been compared to “forces”, but is not grouped with the accepted energetic forces for reasons that might get looked at later. How much might be understood about gravity if it were to be quantified as a fundamental pressure?

Gravity is neither push nor pull. It is due to the curvature of spacetime. Locally, in a purely gravitational field, the trajectory of an object in spacetime is a straight line, as it would be if there were no forces acting on the object. Gravitation has also been shown not to be entropic by quantum interference experiments in a gravitational field.

Gravity is neither push nor pull. It is due to the curvature of spacetime. Locally, in a purely gravitational field, the trajectory of an object in spacetime is a straight line, as it would be if there were no forces acting on the object. Gravitation has also been shown not to be entropic by quantum interference experiments in a gravitational field.

I understand is neither push nor pull, but I believe it’s nature can be definable as a pressure with mass as a lens of sorts?

Riff-in-Thyme said:

I understand is neither push nor pull, but I believe it’s nature can be definable as a pressure with mass as a lens of sorts?

If you’re going to say that there’s ‘pressure’ of any sort, then you’re saying there’s a push of some sort. You can’t say there is no push (or pull) and at the same time say that there may be.

I always think of it as “matter tells space how to curve, and space tells matter how to behave”.

There’s no ‘pressure’ applied, or felt.

captain_spalding said:

If you’re going to say that there’s ‘pressure’ of any sort, then you’re saying there’s a push of some sort. You can’t say there is no push (or pull) and at the same time say that there may be.

I always think of it as “matter tells space how to curve, and space tells matter how to behave”.

There’s no ‘pressure’ applied, or felt.

I am not assuming pressure is applied or felt. My understanding of pressure on a fundamental quantum level is that it is absorbed or exerted. The application either is achieved through is a particular mechanism. I am not sure how “felt” would come into the equation.

KJW said:

Gravitation has also been shown not to be entropic by quantum interference experiments in a gravitational field.

With gravity’s role in fusion I could suppose gravity as fundamentally neg-entropic?

Riff-in-Thyme said:

I am not assuming pressure is applied or felt. My understanding of pressure on a fundamental quantum level is that it is absorbed or exerted. The application either is achieved through is a particular mechanism. I am not sure how “felt” would come into the equation.

Please explain the difference between “applied” and “exerted” and between “felt” and “absorbed”.

Gravitation can only be distinguished from the absence of gravitation by its tidal effects and non-local effects. Point objects can’t experience gravitation in any way.

KJW said:

Point objects can’t experience gravitation in any way.

mind blown…

but yes that makes sense…

gravity is an influence on mass that causes mass to have a force

The Rev Dodgson said:

Please explain the difference between “applied” and “exerted” and between “felt” and “absorbed”.

Felt. You know – the stuff they make hats out of.

captain_spalding said:

The Rev Dodgson said:

Please explain the difference between “applied” and “exerted” and between “felt” and “absorbed”.

Felt. You know – the stuff they make hats out of.

Hats KFF Khaki Fur Felt. AKA as the slouch hat.

The Rev Dodgson said:

Please explain the difference between “applied” and “exerted” and between “felt” and “absorbed”.

Look at it this way. Is a current an exertion or an absorption?

KJW said:

Point objects can’t experience gravitation in any way.

So to un-confuse me on this point, how does an electron have mass?

Riff-in-Thyme said:

The Rev Dodgson said:

Please explain the difference between “applied” and “exerted” and between “felt” and “absorbed”.

Look at it this way. Is a current an exertion or an absorption?

I have no idea, I wouldn’t say it was either.

But I didn’t ask for the difference between “exerted” and “absorbed” anyway.

Why not just explain what you mean by these words?

Riff-in-Thyme said:

KJW said:

Point objects can’t experience gravitation in any way.

So to un-confuse me on this point, how does an electron have mass?

Mass is a source of gravitation. Point masses aren’t affected by gravitation.

A key point in understanding what gravitation is as a curvature of spacetime is to appreciate the distinction between local and non-local effects. Local means “at a point”; non-local means “over a region”. Although curvature can be defined at a point by considering an infinitesimal neighbourhood of the point, the effects of gravitation can only be measured non-locally because locally a curved spacetime is indistinguishable from a flat spacetime in the sense that at any single point, the metric can be coordinate-transformed to the Minkowskian metric.

KJW said:

Riff-in-Thyme said:

KJW said:

Point objects can’t experience gravitation in any way.

So to un-confuse me on this point, how does an electron have mass?

Mass is a source of gravitation. Point masses aren’t affected by gravitation.

Note that the trajectory of a point mass object in a gravitational field is independent of the mass of the object (a feather falls just as fast in a vacuum as does a hammer). This is the equivalence principle. The equivalence principle applies to gravity but not to the other “forces”.

KJW said:

Note that the trajectory of a point mass object in a gravitational field is independent of the mass of the object (a feather falls just as fast in a vacuum as does a hammer). This is the equivalence principle. The equivalence principle applies to gravity but not to the other “forces”.

Ok. This is about where my thinking has got to. Will be back with the bones picked out. Ta

Note that the trajectory of a point mass object in a gravitational field is independent of the mass of the object (a feather falls just as fast in a vacuum as does a hammer). This is the equivalence principle. The equivalence principle applies to gravity but not to the other “forces”.
——————
But is this really true or only approximately true? When most people talk about feathers and hammers falling they are only really taking about one siode of the equation. Gravity is observable as the attration between two masses. The earth will be attracted to, and fall towards, the hammer slight more than the feather I think.

Stealth said:

But is this really true or only approximately true? When most people talk about feathers and hammers falling they are only really taking about one siode of the equation. Gravity is observable as the attration between two masses. The earth will be attracted to, and fall towards, the hammer slight more than the feather I think.

When the earth falls towards the feather or the hammer, the earth’s trajectory is in the curvature generated by the feather or hammer as sources of the gravitation. Because the hammer has a greater mass, its gravitation is larger and the earth will fall faster towards the hammer than the feather. But the moon will fall no faster towards the hammer than the earth does. It’s important not the confuse the role of mass as a source of gravitation with the response of mass to external gravitation which doesn’t really exist due to the equivalence principle.

It should also be noted that the notion of attraction between two masses is non-local, and distinct from the absence of any local effect of gravitation on an object.

KJW said:

It should also be noted that the notion of attraction between two masses is non-local, and distinct from the absence of any local effect of gravitation on an object.

Perhaps the biggest difficulty people have in understanding the nature of gravitation as a curvature of spacetime is the failure to recognise that notions that they take for granted in flat geometries no longer apply in curved geometries. In flat geometries, local notions translate simply to global notions. For example, if two lines are parallel to a third line, then they are parallel to each other, and this applies irrespective of how far away the lines are from each other. In curved geometries, this is no longer true. For example, if one parallel-transports a line from one point to another point along two different paths, then the resulting two lines will not coincide in general. Thus, the attraction between masses is a different notion to the existence of a force acting on the masses because two different notions of parallelism are being applied, notions that are equivalent in flat geometries but not in curved geometries.

KJW said:

Perhaps the biggest difficulty people have in understanding the nature of gravitation as a curvature of spacetime is the failure to recognise that notions that they take for granted in flat geometries no longer apply in curved geometries.

Another property of flat spacetime is that straight spacetime trajectories project onto flat 3D-spaces as straight lines, whereas this is no longer true for curved spacetime. The orbit of a satellite around a central mass is a projection of its spacetime trajectory onto an approximately flat 3D space. Even though the spacetime trajectory is a straight line, the projection onto the (approximately) flat 3D space is an ellipse, and this is a consequence of the spacetime curvature.

Stealth said:

Note that the trajectory of a point mass object in a gravitational field is independent of the mass of the object (a feather falls just as fast in a vacuum as does a hammer). This is the equivalence principle. The equivalence principle applies to gravity but not to the other “forces”.
——————
But is this really true or only approximately true? When most people talk about feathers and hammers falling they are only really taking about one siode of the equation. Gravity is observable as the attration between two masses. The earth will be attracted to, and fall towards, the hammer slight more than the feather I think.

One other point I should mention with regards to this is that in calculating the geodesic trajectory of an object in the gravitational field of a massive object, it is usually assumed that the object itself has negligible effect on the spacetime curvature. This is because combining the effects of two gravitational sources into a single spacetime curvature is very difficult to do exactly as only in the weak field limit does gravitation combine linearly.

Riff-in-Thyme said:

KJW said:

Gravitation has also been shown not to be entropic by quantum interference experiments in a gravitational field.

With gravity’s role in fusion I could suppose gravity as fundamentally neg-entropic?

The weight that is exerted on the central core of a star by the remainder of the star is not a local property of gravitation as it depends on the star as a whole and not on the gravitation at any location within the star, in particular not on the gravitation at the core.

My statement refers to a hypothesis that objects fall because they increase in entropy as they fall. This has been shown to be false.

The gravitation that we experience is a consequence of what happens to the time direction in spacetime, specifically the gravitational time-dilation that produces the gravitational redshift. It isn’t just that gravitation produces the gravitational redshift (as if gravity were a force acting on photons), but that the time-dilation produces the gravitation that we experience. This is also true for artificial gravity, which occurs in an accelerated frame-of-reference in flat spacetime, the accelerated frame-of-reference has an associated time-dilation that produces the artificial gravity.

The 3D space surrounding the earth, while not exactly flat, is so close to being flat that it can be regarded as flat without significantly affecting the description of the gravity of the earth. For this reason (apart from the more obvious reasons), the rubber sheet description of gravity is wrong as it misrepresents the nature of the curvature associated with gravitation. That is, the gravitation that we experience is not due to the curvature of the 3D space, but the time dilation in 4D spacetime.

The difference between true gravitation and artificial gravity is that true gravitation has a source. While both true gravitation and artificial gravity are due to time-dilation (and are indistinguishable in this sense), the existence of a source forces the spacetime to be curved because changing to an accelerated frame-of-reference in flat spacetime cannot create a source. It is the existence of a source that is responsible for the tidal effects that are associated with true gravitation and not present in artificial gravity. Thus, the Pound-Rebka experiment, which demonstrates the existence of the time-dilation, combined with the existence of a source, inside the surface of the earth, proves that spacetime is curved. I feel that some people are looking for an alternative to general relativity to explain gravitation, perhaps thinking that spacetime curvature is some fictional notion that is used to model gravitation but without any real existence. But spacetime curvature is real and is the only explanation of gravitation (because spacetime is curved and that curvature explains the gravitation).

KJW said:

I feel that some people are looking for an alternative to general relativity to explain gravitation, perhaps thinking that spacetime curvature is some fictional notion that is used to model gravitation but without any real existence. But spacetime curvature is real and is the only explanation of gravitation (because spacetime is curved and that curvature explains the gravitation).

I’m not looking for any alternative, only greater explanation. The contributions here are hitting on what I am trying to pick out. Mildly distracted by the legalities of reality at the moment, but I’m getting at this thought gradually with your help.

I will come back to the equivalency principal, but somewhere to begin an explanation of what I am trying to get at is within the uncertainty principal. Heisenberg refers to position and velocity/speed. When examining the energetic evolution of a particle, the spatial position of a particle seems irrelevant unless it is placed relative to an origin or a destination within the evolution of that particle. When examined as such, the speed of the particle becomes part of the definition of where in it’s evolution the particle is and it’s velocity becomes the distance said particle is from it’s destination. About here is where I need further KJW type clarification to sum the thought up.

Riff-in-Thyme said:

KJW said:

spacetime curvature is real and is the only explanation of gravitation (because spacetime is curved and that curvature explains the gravitation).

I’m not looking for any alternative, only greater explanation.

The only “greater” explanation comes from quantum theory, but even then, quantum theory must still manifest gravitation classically as a spacetime curvature.

KJW said:

The only “greater” explanation comes from quantum theory, but even then, quantum theory must still manifest gravitation classically as a spacetime curvature.

I’m not quite sure I am talking about manifesting it?

Riff-in-Thyme said:

KJW said:

The only “greater” explanation comes from quantum theory, but even then, quantum theory must still manifest gravitation classically as a spacetime curvature.

I’m not quite sure I am talking about manifesting it?

The point is that any explanation of gravitation, be it at the classical or quantum level, must involve spacetime curvature at the classical limit.

KJW said:

The point is that any explanation of gravitation, be it at the classical or quantum level, must involve spacetime curvature at the classical limit.

Neither have I attempted to dis-involve curvature. I am simply of the opinion that there is a little more to the picture.

Riff-in-Thyme said:

I will come back to the equivalency principal, but somewhere to begin an explanation of what I am trying to get at is within the uncertainty principal. Heisenberg refers to position and velocity/speed. When examining the energetic evolution of a particle, the spatial position of a particle seems irrelevant unless it is placed relative to an origin or a destination within the evolution of that particle. When examined as such, the speed of the particle becomes part of the definition of where in it’s evolution the particle is and it’s velocity becomes the distance said particle is from it’s destination. About here is where I need further KJW type clarification to sum the thought up.

I included this to illustrate that often what we measure is only relevant to our perception and not necessarily to the nature, origin and destination of a particle.

Riff-in-Thyme said:

Neither have I attempted to dis-involve curvature. I am simply of the opinion that there is a little more to the picture.

I fully agree with that in that quantum theory requires that reality be viewed as a multiverse and that spacetime is only a classical notion.

Riff-in-Thyme said:

I included this to illustrate that often what we measure is only relevant to our perception and not necessarily to the nature, origin and destination of a particle.

The Heisenberg uncertainty principle and quantum theory in general is best understood in terms of a “multiverse” (mathematically, a function space).

KJW said:

The Heisenberg uncertainty principle and quantum theory in general is best understood in terms of a “multiverse” (mathematically, a function space).

Can that function space be defined without reference to a particle?

Riff-in-Thyme said:

Can that function space be defined without reference to a particle?

Here’s the way I see it (correctly or incorrectly):

What you are asking about is a quantum field theory. Physics provides two distinct ways to describe reality: mechanics and field theory. Mechanics describes reality in terms of particles, describing their position in spacetime in terms of a single parametric variable:

x = x(s)
y = y(s)
z = z(s)
t = t(s)

where s is the parameter. Although one usually encounters three equations describing spatial position in terms of the time variable, the above places all spacetime variables on equal footing as required by relativity.

By contrast, field theory describes reality in terms of a field function over all spacetime variables:

F = F(x,y,z,t)

Note the difference: in mechanics, the spacetime variables are the dependent variables, whereas in field theory, they are the independent variables. Field theory is the more powerful description as it can readily describe particles, but mechanics can’t readily describe fields. Also, field theory automatically places all spacetime variables on equal footing without the need to invoke an extra parameter.

Physics also describes two distinct realms: classical and quantum. Combined we have the four combinations:

Classical Mechanics
Classical Field Theory
Quantum Mechanics
Quantum Field Theory

Here’s the difficulty: the nature of quantum physics demands that quantum mechanics be described as if it were a classical field theory. So how is quantum field theory to be described? My solution (not necessarily the standard solution) is to force quantum mechanics into a mechanical description. Then quantum field theory becomes the corresponding field theoretical description. However, the resulting quantum mechanics must still be equivalent to a classical field theory. This is done by extending the domain from spacetime to a function space. Then, classical field theory (quantum mechanics) becomes a point in this space, and quantum field theory becomes a field function over this space. In other words, reality is being described as a quantum superposition of entire spacetimes.

KJW said:

Then, classical field theory (quantum mechanics) becomes a point in this space, and quantum field theory becomes a field function over this space. In other words, reality is being described as a quantum superposition of entire spacetimes.

Ok, now I’m comprehending your solution. Makes me think that the most powerful object that can exist is a measuring tool-if you can measure it you can overcome it, I guess. What is the ultimate measuring device?

Riff-in-Thyme said:

What is the ultimate measuring device?

I’m guessing either a supernova or Douglas Adam’s towel.

Riff-in-Thyme said:

What is the ultimate measuring device?

A measuring device is a classical object that quantum entangles each component of a quantum superposition with a distinct classical state. Although all of the distinct classical states remain in quantum superposition, we only observe one of the classical states because all of the classical states are mutually orthogonal and othogonal states do not interfer with each other. For example, in the double-slit experiment, when the particular slit the photon passed through is unobserved, the component wavefunctions from each of the slits are not orthogonal and an interference pattern forms. But when the particular slit the photon passed through becomes identifiable, the component wavefunctions from each of the slits become orthogonal and the interference pattern does not form.

KJW said:

Riff-in-Thyme said:

What is the ultimate measuring device?

A measuring device is a classical object that quantum entangles each component of a quantum superposition with a distinct classical state. Although all of the distinct classical states remain in quantum superposition, we only observe one of the classical states because all of the classical states are mutually orthogonal and othogonal states do not interfer with each other. For example, in the double-slit experiment, when the particular slit the photon passed through is unobserved, the component wavefunctions from each of the slits are not orthogonal and an interference pattern forms. But when the particular slit the photon passed through becomes identifiable, the component wavefunctions from each of the slits become orthogonal and the interference pattern does not form.

It would be interesting to record in detail the placing of a respectably sized supa-novae in near orbit to a generously endowed BH.

> Gravity is neither push nor pull.

I strongly disagree. Like positive charges repel – that makes electromagnetism a push. Positive masses attract – that makes gravity a pull. The two can be compared directly, and A Zee “Quantum field theory in a nutshell” proves that a graviton must specify an attraction, not a repulsion.

We’ve all toyed with the idea of gravity as being the manifestation of a space-based repulsion interrupted by the presence of masses in analogy with the way that the Casimir attraction between two plates is caused by the interruption of repulsion within the quantum vacuum. But it doesn’t work, either on normal distance scales or cosmologically. Gravity is a pull.

i was just thinking

i’m sure i’ve asked this question before

is all energy inside a blackhole converted to mass?

if light can’t escape does it get converted to mass or just bounce around forever? ie a black hole is actually bright but we can’t see it because all light thats fallen in can’t escape?

mollwollfumble said:

> Gravity is neither push nor pull.

I strongly disagree. Like positive charges repel – that makes electromagnetism a push. Positive masses attract – that makes gravity a pull. The two can be compared directly, and A Zee “Quantum field theory in a nutshell” proves that a graviton must specify an attraction, not a repulsion.

We’ve all toyed with the idea of gravity as being the manifestation of a space-based repulsion interrupted by the presence of masses in analogy with the way that the Casimir attraction between two plates is caused by the interruption of repulsion within the quantum vacuum. But it doesn’t work, either on normal distance scales or cosmologically. Gravity is a pull.

But gravity is not a force, so how can it be a pull (or push)?

KJW said:

mollwollfumble said:

> Gravity is neither push nor pull.

I strongly disagree. Like positive charges repel – that makes electromagnetism a push. Positive masses attract – that makes gravity a pull. The two can be compared directly, and A Zee “Quantum field theory in a nutshell” proves that a graviton must specify an attraction, not a repulsion.

We’ve all toyed with the idea of gravity as being the manifestation of a space-based repulsion interrupted by the presence of masses in analogy with the way that the Casimir attraction between two plates is caused by the interruption of repulsion within the quantum vacuum. But it doesn’t work, either on normal distance scales or cosmologically. Gravity is a pull.

But gravity is not a force, so how can it be a pull (or push)?

gravity is an influence not a force – it is the mass that has force due to the acceleration

mollwollfumble said:

A Zee “Quantum field theory in a nutshell” proves that a graviton must specify an attraction, not a repulsion.

Sure we can come up with various properties that gravitons have if they exist. But we don’t yet have a working theory of quantum gravity, so we don’t really know whether gravitons exist or not.

And if they do exist, it may be extremely difficult if not impossible to detect them directly; individual gravitons are a lot harder to detect than gravitational waves, and we haven’t been able to directly detect gravitational waves, yet.

PM 2Ring said:

mollwollfumble said:

A Zee “Quantum field theory in a nutshell” proves that a graviton must specify an attraction, not a repulsion.

Sure we can come up with various properties that gravitons have if they exist. But we don’t yet have a working theory of quantum gravity, so we don’t really know whether gravitons exist or not.

And if they do exist, it may be extremely difficult if not impossible to detect them directly; individual gravitons are a lot harder to detect than gravitational waves, and we haven’t been able to directly detect gravitational waves, yet.

don’t you shoot light between two points and measure the bend?

KJW said:

Stealth said:

But is this really true or only approximately true? When most people talk about feathers and hammers falling they are only really taking about one siode of the equation. Gravity is observable as the attration between two masses. The earth will be attracted to, and fall towards, the hammer slight more than the feather I think.

When the earth falls towards the feather or the hammer, the earth’s trajectory is in the curvature generated by the feather or hammer as sources of the gravitation. Because the hammer has a greater mass, its gravitation is larger and the earth will fall faster towards the hammer than the feather. But the moon will fall no faster towards the hammer than the earth does. It’s important not the confuse the role of mass as a source of gravitation with the response of mass to external gravitation which doesn’t really exist due to the equivalence principle.

It should also be noted that the notion of attraction between two masses is non-local, and distinct from the absence of any local effect of gravitation on an object.

But that explaination still seems one sided.
The earth and moon may fall towards the hammer at the same rate, but when combined with the hammer falling towards the earth or moon the combined answer is different.

wookiemeister said:

is all energy inside a blackhole converted to mass?

if light can’t escape does it get converted to mass or just bounce around forever? ie a black hole is actually bright but we can’t see it because all light thats fallen in can’t escape?

All trajectories in a BH go to the centre, stuff can’t bounce around inside a BH. Once you’re inside the event horizon of a BH, avoiding moving towards the centre is as impossible as avoiding moving towards the future.

We don’t know what form stuff takes once it gets to the centre of a BH; discussing such extreme conditions really requires a theory of Quantum Gravity. But it’s quite likely that normal matter can’t exist at the centre, there’s not enough room.

Everything at the centre of a BH “wants” to occupy the same position state, but the Pauli exclusion principle prevents multiple fermions of the same type from occupying the exact same quantum state. And all the fundamental particles of matter are fermions.

But not all particles are fermions. Photons and other similar particles of energy are bosons, which are not subject to Pauli exclusion, so if anything like normal stuff exists at the centre of a BH it’s most likely to be in the form of bosons and not fermions.

PM 2Ring said:

And if they do exist, it may be extremely difficult if not impossible to detect them directly; individual gravitons are a lot harder to detect than gravitational waves, and we haven’t been able to directly detect gravitational waves, yet.

Err, sorry, that may be my fault :)

I installed a rather large UPS in the UWA physics building, back in about 1990. Right above their gravitational experimental lab. Jaysus, was the bloke in charge rather fucking irate :)

Stealth said:

The earth will be attracted to, and fall towards, the hammer slightly more than the feather I think.

Certainly. See http://en.wikipedia.org/wiki/Reduced_mass

PM 2Ring said:

Stealth said:

The earth will be attracted to, and fall towards, the hammer slightly more than the feather I think.

Certainly. See http://en.wikipedia.org/wiki/Reduced_mass

fall towards the feather grasshopper!

wookiemeister said:

PM 2Ring said:

individual gravitons are a lot harder to detect than gravitational waves, and we haven’t been able to directly detect gravitational waves, yet.

don’t you shoot light between two points and measure the bend?

Sort of, but it’s not easy. From http://en.wikipedia.org/wiki/LIGO
Wikipedia said:

Each observatory supports an L-shaped ultra high vacuum system, measuring 4 kilometers (2.5 miles) on each side. Up to five interferometers can be set up in each vacuum system.

…

The primary interferometer at each site consists of mirrors suspended at each of the corners of the L; it is known as a power-recycled Michelson interferometer with Fabry–Pérot arms. A pre-stabilized laser emits a beam of up to 200 Watts that passes through an optical mode cleaner before reaching a beam splitter at the vertex of the L. There the beam splits into two paths, one for each arm of the L; each arm contains Fabry–Pérot cavities that store the beams and increase the effective path length.

When a gravitational wave passes through the interferometer, the space-time in the local area is altered. Depending on the source of the wave and its polarization, this results in an effective change in length of one or both of the cavities. The effective length change between the beams will cause the light currently in the cavity to become very slightly out of phase with the incoming light. The cavity will therefore periodically get very slightly out of resonance and the beams which are tuned to destructively interfere at the detector, will have a very slight periodically varying detuning. This results in a measurable signal. Note that the effective length change and the resulting phase change are a subtle tidal effect that must be carefully computed because the light waves are affected by the gravitational wave just as much as the beams themselves.

…

Based on current models of astronomical events, and the predictions of the general theory of relativity, gravitational waves that originate tens of millions of light years from Earth are expected to distort the 4 kilometer mirror spacing by about 10⁻¹⁸ m, less than one-thousandth the charge diameter of a proton. Equivalently, this is a relative change in distance of approximately one part in 10²¹.
A typical event which might cause a detection event would be the late stage inspiral and merger of two 10 solar mass black holes, not necessarily located in the Milky Way galaxy, which is expected to result in a very specific sequence of signals often summarized by the slogan chirp, burst, quasi-normal mode ringing, exponential decay.

Stealth said:

But that explaination still seems one sided.
The earth and moon may fall towards the hammer at the same rate, but when combined with the hammer falling towards the earth or moon the combined answer is different.

I also said this:

One other point I should mention with regards to this is that in calculating the geodesic trajectory of an object in the gravitational field of a massive object, it is usually assumed that the object itself has negligible effect on the spacetime curvature. This is because combining the effects of two gravitational sources into a single spacetime curvature is very difficult to do exactly as only in the weak field limit does gravitation combine linearly.

wookiemeister said:

gravity is an influence not a force – it is the mass that has force due to the acceleration

There’s no acceleration. A test mass does not accelerate in a gravitational field.

mollwollfumble said:

Like positive charges repel – that makes electromagnetism a push. Positive masses attract – that makes gravity a pull. The two can be compared directly

The comparison is superficial and misleading. Gravity satisfies the equivalence principle, electromagnetism does not. This makes the two completely different as the consequences of the equivalence principle are far-reaching.

KJW said:

mollwollfumble said:

Like positive charges repel – that makes electromagnetism a push. Positive masses attract – that makes gravity a pull. The two can be compared directly

The comparison is superficial and misleading. Gravity satisfies the equivalence principle, electromagnetism does not. This makes the two completely different as the consequences of the equivalence principle are far-reaching.

Seem to remember this as a piece of graffity, about 40 years ago,

“There’s no such thing as gravity, the Earth sucks” that explanation suits me.

mollwollfumble said:

A Zee “Quantum field theory in a nutshell” proves that a graviton must specify an attraction, not a repulsion.

Let’s not forget that the theory of gravitons is non-renormalisable, which I see as an indication that something is wrong with the theory.

KJW said:

mollwollfumble said:

A Zee “Quantum field theory in a nutshell” proves that a graviton must specify an attraction, not a repulsion.

Let’s not forget that the theory of gravitons is non-renormalisable, which I see as an indication that something is wrong with the theory.

If I were to pursue applying the word pressure to the nature of gravity I would neither call it an attraction or repulsion. My definition sought to identify the nature of gravity as the most fundamental causative element of the energetic universe. Another overly complex task I thought I might play with in between custody hearings and trying to maintain a decent income with injuries. It’s a pity I only get to pursue this stuff for distraction.