[Curious Cat (OP)]

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[dragontickler]

This is a rephrasing and reposting of my "What is gravity, really?",
which Halc seems to think belongs in the NTs. But I'm not proposing
anything new here. I am just exposing Youtube.
Maybe I should have said "What causes gravity, really"?

And of course I know it doesn't. I feel silly even asking it, rhetorically.
But YT and Prof.Brainy Greene/horn/e insist it does!?
Please have a look at my other post so I don't have to repeat it, here.
I think YT has gone crazy, around the bend.

Hi sir, you may want to check out a video by VOIBLE. It has a simple explanation regarding gravity.

[TommyJ]

1. Does gravitational time dilation/GTD really cause gravity?
2. "What causes gravity, really"?

1. Gravitational time dilation is a form of time dilation, an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass.

Gravity [..] is a consequence of masses moving along geodesic lines in a curved spacetime caused by the uneven distribution of mass.
(Wikipedia)

2. ‘Mass’ here is not necessarily a rest-mass.

Photons have energy, ‘relativistic mass’, which is the source of gravity, not ‘rest mass’. Hence, the energy is the source of gravity.

It is proposed that particles called gravitons cause objects to be attracted to one another. But gravitons have never actually been observed.

[Halc]

It is proposed that particles called gravitons cause objects to be attracted to one another.

It has not been thus proposed. The rest of the post was an excellent summary.
A hypothetical graviton is a quantum excitation in the gravitational field, and it conveys gravitational waves, not gravity, so it does not cause objects to be attracted. Gravity would be a force if it did, not a geometric effect.
LIGO detects gravitational waves, but not on the quantum level. These waves do not exert any force on Earth or LIGO. They just change the local (4 km?) geometry of the spacetime containing the device.

[TommyJ]

Thank you for amendment, Halc.
Right.

The LIGO gravitational waves detection verified that neutron stars merging produced gravitational wave signals during 2 seconds.
The first signal itself was detected as a vibration of the distance between mirrors four kilometres apart. Is it 4 km - for the amplification of the ripples?

[Eternal Student]

Hi.
 Well this is one of the more interesting and polite discussion happening here today, I hope you won't mind if I join in.

@Halc and in defence of @TommyJ ,    some sources of information do consider gravitons as the force carrying particle responsible for the force gravity.
   For example, in a school A level syllabus  (age 16-18 in the UK) this is exactly what pupils would be taught.

Wiki  also goes along these lines:
It is hypothesized that gravitational interactions are mediated by an as yet undiscovered elementary particle, dubbed the graviton. The three other known forces of nature are mediated by elementary particles: electromagnetism by the photon, the strong interaction by gluons, and the weak interaction by the W and Z bosons. All three of these forces appear to be accurately described by the Standard Model of particle physics. In the classical limit, a successful theory of gravitons would reduce to general relativity, which itself reduces to Newton's law of gravitation in the weak-field limit.

   However,  gravitons haven't been observed (for example at the LHC) and remain theoretical.  They are also slightly different in theories of quantum gravity.  It is difficult to reconcile gravitons as force carrying particles with General Relativity and geometric curvature.    Both versions of what a Graviton is (Halc's  or  TommyJ's version) are hypothetical and used in the literature.

Best Wishes.

[Halc]

As for ES's post, I can accept that the word graviton is also used to describe a hypothetical force-mediating particle, in which case any reference to it must be clear as to which they are talking about. Couldn't they have given them different names? I hardly ever see references to the force one.

Is it 4 km - for the amplification of the ripples?

The waves result in a change in the space between the two ends by -21 orders of magnitude, so making it longer means they have to detect a length change equal to a 1000th the width of proton instead of say a millionth of that width.
Plus, the waves to which the device is most sensitive becomes 8km, which is pretty typical for things like star mergers.

[Colin2B]

Hi.
 Well this is one of the more interesting and polite discussion happening here today, I hope you won't mind if I join in.

@Halc and in defence of @TommyJ ,    some sources of information do consider gravitons as the force carrying particle responsible for the force gravity.
   For example, in a school A level syllabus  (age 16-18 in the UK) this is exactly what pupils would be taught.

Wiki  also goes along these lines:
It is hypothesized that gravitational interactions are mediated by an as yet undiscovered elementary particle, dubbed the graviton.

I would agree with what you are saying, but in defence of @Halc I would disagree with what is said here:

It is proposed that particles called gravitons cause objects to be attracted to one another.

Agreed this has taken an interesting turn.
I think we have crossed this quagmire of what is taught before. How much do you simplify at an early age, how much do some teachers really understand their subject?

Differences in interpretation are interesting. @TommyJ says "caused by", wiki says “gravitational interactions are mediated by”
I would say the latter is closest.
In any quantum gravity field theory the graviton would be an elementary particle that mediates the force of gravitational interaction. As @Halc says it's not often used in this way. probably because the theory has real problems at the moment.

As you know QED treats photons as particles that carry or mediate the electromagnetic force. In this model charged particles interact by emitting and absorbing photons, but these photons don't experience the electromagnetic force themselves, and so they do not interact with each other, but the effects of electromagnetism are produced by the energy and momentum they carry. The photons that carry force are known as virtual particles in QED and are shown as internal interactions in the Feynman diagrams and basically are there to make the energy equations add up.

Clearly, the graviton would have to fit a similar model, but I wonder what would take the place of charged particle interactions when, as @Halc says, the gravitational wave causes a disturbance in spacetime. We would certainly see that as a movement of test particles (as a distant observer)and in a Newtonian system would describe it as a force. Also, that movement (oscillation) of the test particles is not in the direction of the wave propagation, so nothing is being pulled towards the source, but it is similar to light exerting an effect on electrons in the photoelectric effect. Does GR give a clue? This isn’t an area I’ve looked into.

[Eternal Student]

Hi.

In any quantum gravity field theory the graviton would be an elementary particle that mediates the force of gravitational interaction. As @Halc says it's not often used in this way. probably because the theory has real problems at the moment.

    Agreed.   When it is used it's used this way:

Upon qunatization, Einsteins equation predicts spin-two particles called gravitons.  We don't know how to carry out such a quantization consistently, but the existence of gravitons is sufficiently robust that it is expected to be a feature of any well-defined scheme      [Fig. 4.2 and text,  p. 167,  Introduction to Spacetime and Geometry,  Sean Carroll]

As you know QED treats photons as particles that carry or mediate the electromagnetic force. In this model charged particles interact by emitting and absorbing photons, but these photons don't experience the electromagnetic force themselves, and so they do not interact with each other, but the effects of electromagnetism are produced by the energy and momentum they carry. The photons that carry force are known as virtual particles in QED and are shown as internal interactions in the Feynman diagrams and basically are there to make the energy equations add up

    The analagous Feynmann diagrams with gravitons look like this:

Feynman.JPG (27.32 kB . 742x228 - viewed 2024 times)
    The vertical swirls are virtual gravitons.  The diagram on the left shows two electrons interacting, the one on the right shows a graviton interacting with another graviton by exchanging a virtual graviton.

Clearly, the graviton would have to fit a similar model, but I wonder what would take the place of charged particle interactions when, as @Halc says, the gravitational wave causes a disturbance in spacetime. We would certainly see that as a movement of test particles (as a distant observer)and in a Newtonian system would describe it as a force. Also, that movement (oscillation) of the test particles is not in the direction of the wave propagation, so nothing is being pulled towards the source, but it is similar to light exerting an effect on electrons in the photoelectric effect. Does GR give a clue? This isn’t an area I’ve looked into.

    See above.  Note that there aren't any consistent quantum theories of gravity that I am aware of.  The general properties are based on ....  well, in my case, whatever Sean Caroll was basing it on.
    There are some important features that would seem to be present in the scheme:
    Electromagnetism is mediated by virtual photons which only couple to (electrically) charged particles and, as you (Colin2B) said earlier, photons do not couple to each other and exchange a virtual photon - so they don't experience an electromagnetic force.  As far as the gravity is concerned, anything with energy-momentum is a charge.  Since gravity couples to energy-momentum, gravitons should interact with every kind of particle, including other gravitons. 
[reference: p.166-167,  Spacetime and Geometry]
    The interaction of gravitons with other gravitons is supported by the non-linear nature of the Einstein Field Equations.  A solution involving two masses as a source of gravitation is not the sum or superposition of the solutions for each mass individually.   This is completely different to anything in electrodynamics.

     The discussion of gravitons in Chapter 4 is not based around gravitational waves, actually gravitational waves aren't even mentioned anywhere in that chapter.  Carroll describes the virtual graviton quite directly as a quantised perturbation of the metric and is discussing the ability for the exchange of a virtual graviton to deflect the path of two particles that approach each other.

     You (Colin2B), Halc and most others (including myself) have generally referred to gravity as a geometric effect rather than any force.  Gravitons do not naturally fall out from classical GR and you are correct that LIGO has not been detecting any forces towards or away from any sources but just a disturbance in the metric.
    Section 4.7 on p. 177-181,  Spacetime and Geometry seems most relevant here:
In practice, it is common to invoke the equivalence principle to justify any of the following four ideas:
  1. ... (usual stuff)...
  2. There exists a metric on spacetime, the curvature of which we interpret as gravity.
  3. There do not exist any other fields that resemble gravity.
  4. The interactions of matter fields to curvature are minimal: they do not involve direct couplings to the Riemann tensor or its contractions.
 .........
   Which is enough to suggest that any quantum field theory for gravity would be unlike anything else you've seen.

Carroll goes on to state the following:
   ...the demands of eventually reconciling general relativity with quantum mechanics suggest to many that the metric will ultimately be revealed as a concept derived from a more fundamental collection of degrees of freedom.... 
    So that we are left with the possibility that curvature of the metric on spacetime may not ultimately be the cause of gravity and the "graviton" will not be a quantised perturbation of the metric field.  GR is an effective field theory only.
It is just one of those interesting things that in this effective field theory, the metric field doesn't just describe a fundamental particle (or approximately describe it),  the metric field also directly determines how we measure lengths and times.   Most of the time, such as in the LIGO experiments, it is the distortion of lengths that we measure.  A Gravitational wave may be indicative of a bundle of gravitons travelling through space but in most quantum theories of gravity, gravitons are still being exchanged even where the metric field appears to remain static (upto the limit of our ability to measure it or determine the absolute positions and lengths of separtion between any two objects).

It's late, I'm going to try and put the image in the right place and then sign off.  I don't know if any of the above helped.

Best Wishes. 

[TommyJ]

Everything above helped from all of the participants (Not for me only, I believe).

How much do you simplify at an early age, how much do some teachers really understand their subject?

I have no experience of my own that is enough to compare the oversimplification between generations in physics. But for other key disciplines I would confirm, it is there.

Which is enough to suggest that any quantum field theory for gravity would be unlike anything else you've seen.

Unfortunately, whatever it would be, it is pushed outside the physics’ focus (if I am correct). The vector of physics is about making more precise predictions, not understanding quantization of GR deeper.
It worked for electromagnetism with patches, but didn’t work for gravity.
And gravity is at some point simplified to unknown yet, with the list of ideas.