Quantum theory of gravity.

[Amazing Random]

Well we all know what events gravity creates : an apple falling from a tree  , planets orbiting around the sun . Until the mid 1920's we thought that gravity was the attraction between objects with mass , but Einstein came and said nope : gravity is the bending of space-time around something with stress .  

 

Quantum mechanics is the theory that describes the world of subatomic particles , the foundations of you , me , everything . However at its original form quantum mechanics didn't describe how those foundation blocks interact with each other . 

 

So some physicists tried to unified Relativity and Quantum Mechanics to describe those interactions. Their efforts were successful by the creation of quantum electrodynamics and quantum chromodynamics. However when they applied their methods to gravity , they failed to describe the gravitational interaction of subatomic particles.

 

This is a brief history of our today's subject . Well I was wondering since some effects wave-matter duality can be applied only to the microscopic world of molecules , why cant something be applied only to "big" objects ?

Well I thought that the standard model of physics predicts a boson carrying the gravitational force . But what if gravity doesn't exist in the microscopic world? I mean the objects we study in a quantum system and their effects are very small . So  there wouldn't affect space-time at all . (And space-time is not easy to be bent).

 

So what if gravity just doesn't exist at the microscopic world ? This would agree with the experimental data until today .

 

 

 

 

[Strange]

8 minutes ago, Amazing Random said:

Well I thought that the standard model of physics predicts a boson carrying the gravitational force . But what if gravity doesn't exist in the microscopic world? I mean the objects we study in a quantum system and their effects are very small . So  there wouldn't affect space-time at all . (And space-time is not easy to be bent).

It is an interesting question. The next questions would be, at what level does it stop working and, more importantly, why?

We know gravity applies at the scale of individual atoms (even hydrogen atoms). And there may be evidence that it works at even smaller scales. But it gets increasingly hard to test at those scales. 

I'm also not sure that just saying it doesn't exist at the quantum scale solves all the problems. How do we get from there to being able to predict what happens indie a black hole, for example.

There are some theories where spacetime is "emergent" from some lower-level physical description. So, presumably, in those gravity also emerges at a particular scale. But I don't think any of these theories can yet be tested.

[uncool]

If gravity "doesn't exist in the microscopic world", then how can it affect anything in the macroscopic world?

The microscopic and macroscopic worlds aren't entirely separate; the macroscopic world has to "grow out of" the microscopic world. 

[Amazing Random]

4 minutes ago, uncool said:

If gravity "doesn't exist in the microscopic world", then how can it affect anything in the macroscopic world?

The microscopic and macroscopic worlds aren't entirely separate; the macroscopic world has to "grow out of" the microscopic world. 

A fluid as a whole has different properties than the molecules who make it...

[uncool]

2 minutes ago, Amazing Random said:

A fluid as a whole has different properties than the molecules who make it...

But its properties are based on the properties of those molecules. 

[Amazing Random]

26 minutes ago, Strange said:

It is an interesting question. The next questions would be, at what level does it stop working and, more importantly, why?

We know gravity applies at the scale of individual atoms (even hydrogen atoms). And there may be evidence that it works at even smaller scales. But it gets increasingly hard to test at those scales. 

I'm also not sure that just saying it doesn't exist at the quantum scale solves all the problems. How do we get from there to being able to predict what happens indie a black hole, for example.

There are some theories where spacetime is "emergent" from some lower-level physical description. So, presumably, in those gravity also emerges at a particular scale. But I don't think any of these theories can yet be tested.

How does it work on hydrogen atoms?

 

8 minutes ago, uncool said:

But its properties are based on the properties of those molecules. 

Gravity is the effect of stress . When an object doesn't have the required stress , gravity doesn't exist. Many small things can add up to something bigger . When something has an effect due to its population it doesn't mean the individual part of the population may affect . Look how democracy works . One person's opinion is never enough , well 1 thousands persons opinion makes the local authorities consider them and 1 million persons makes the national authorities consider them.:)

27 minutes ago, Strange said:

It is an interesting question. The next questions would be, at what level does it stop working and, more importantly, why?

We know gravity applies at the scale of individual atoms (even hydrogen atoms). And there may be evidence that it works at even smaller scales. But it gets increasingly hard to test at those scales. 

I'm also not sure that just saying it doesn't exist at the quantum scale solves all the problems. How do we get from there to being able to predict what happens indie a black hole, for example.

There are some theories where spacetime is "emergent" from some lower-level physical description. So, presumably, in those gravity also emerges at a particular scale. But I don't think any of these theories can yet be tested.

Do you think there is a limit before the space-time cannot be bent?

Edited August 1, 2019 by Amazing Random

[Mordred]

Ok don't treat spacetime as some materialistic fabric for starters. Those descriptives you read about are misleading analogies. 

 The problem with gravity on microscopic scales is that the effect of two individual particles would have on each other due to their mass terms are so insignificant we will likely never be able to measure any influence. There wouldn't be enough to cause an "action" of displacement. Action is equivalent to a quanta in units.

 Hence the expression a quanta of action. Anything below this scale can never be measured even with a hypothetical perfect detector.

 Yes significant mass however does affect individual particles, muon decay is one example. 

 Now even if we were to discover the graviton, this may or may not solve the divergence problem with renormalization of the Poincare group. This is one of the primary reasons we have not been able to unify gravity. The other being able to determine if gravity will unify by running of the coupling constants with regards to symmetry breaking.

 To explain this fully would require you to have a very strong understanding of gauge group theory in regards to particle physics. Ie the Standard model of physics had eighteen primary parameters. However MSSM had 110.

Edited August 1, 2019 by Mordred

[uncool]

2 hours ago, Amazing Random said:

Gravity is the effect of stress . When an object doesn't have the required stress , gravity doesn't exist. Many small things can add up to something bigger . When something has an effect due to its population it doesn't mean the individual part of the population may affect . Look how democracy works . One person's opinion is never enough , well 1 thousands persons opinion makes the local authorities consider them and 1 million persons makes the national authorities consider them.:)

As you said: many small things can add up to something bigger. But those small things, too, must exist. My point is that gravity must arise out of something that exists, even on the microscopic scale. 

[Mordred]

Here is a paper discussing the issue with a proposed methodology. It is simply one example of a plethora of proposed methods.

Quantization of gravity is still in debate. Nature keeps us away from direct experiments. Mathematics gives only a few hints. The key problem seems the ultraviolet fluctuations of space-time near the Planck scale. They cannot be renormalized through traditional field theoretic methods. Risks are enormous in attacking this problem, among which mathematical hyper-sophistication is perhaps not the least. Existing approaches have been developed for decades, intimidating de facto the newcomer. Nevertheless we propose here still another road to quantize gravity.

Now onto the comment made by Uncool..

The physical scenario, called geometrogenesis or emergence of space- time, is certainly not new: it has been discussed extensively in various quantum gravity approaches [6, 7, 2, 9, 10, 11, 12, 13, 14]. At the big bang (which could perhaps more accurately be called the big cooling), space- time would condense from a perturbative or dilute pre-geometric phase to an effective state which is large and geometric. This state is better and better described by general relativity plus matter fields as the universe cools further. Our particular trend in this scenario is to emphasize the role of the renormalization group (RG) as the guiding thread throughout the pre- geometric phase as well as afterwards

This above process involves the running of the coupling constants in terms of symmetry breaking.. as you can see gravity becomes emergent after the breaking. Another way this can be described is gravity drops out of thermal equilibrium.

Here is the article keep in mind it is but one example. PS on phone it is an arxiv article...

https://www.google.com/url?sa=t&source=web&rct=j&url=https://arxiv.org/pdf/1112.5104&ved=2ahUKEwjXnNqhz-DjAhVlHzQIHfzuAjgQFjABegQIBhAB&usg=AOvVaw2Q8l6DtEpIv_3n6OaoqHUh

Here is another relevant section with regards to action being involved it's an essential ingredient to QFT.

The essential features needed for renormalization are an action S, a scale decomposition, a notion of locality and a power counting theorem. As we shall see the notions of scale and locality can be adapted to quite exotic contexts (vectors, matrices, tensors). The scale decomposition separates the ultraviolet (”high fluctuation”) scales of the fluctuation fields from the infrared scales of the ”background field”. At fixed attributions for the scales of the lines of a graph, some subgraphs play an essential role. They are the connected subgraphs whose internal lines all have higher scale index than all the external lines of the subgraph. Let’s call them the ”high” subgraphs. The renormalization recipe is to check that the divergent high subgraphs satisfy the locality requirement. If by power counting their local divergent parts are of the form of the initial action, then the theory is renormalizable. In ordinary scalar just renormalizable theories such as φ 4 4 the power counting is summarized in a formula for the divergence degree ω such as ω = 4 − N

Hope that helps

[Mordred]

Let's clarify a bit on the reference to graphs and subgraphs in that quoted section. As noted this article loses a lot on technicalities.

 That section is referring the graphs in reference to the Feynman path integrals the internal lines being the propogators Ie virtual particles but in QFT it's simply field influence QFT treats all particles as field excitation.  Real particles are observable Ie a quanta of action minimal.

 The propogators propogators the operators and the Observable operators operate on the propogators. 

(Much like space tells matter how to move matter  tells  space how to curve lol)

Anyways this is an example. The internal wavy line is the propogators while the external lines the Operators.

[math]\array{e^+ \searrow &&\nearrow P^-\\&\leadsto &\\ e^-\nearrow &&\searrow P^+}[/math]

Gauge bosons  are offshell  they are part of the virtual particle family hence in the propogator integrals. The operators Ie real particles  (more accurately observable particles ) are the external line path integrals.

Edited August 1, 2019 by Mordred

[Strange]

7 hours ago, Amazing Random said:

How does it work on hydrogen atoms?

Same as it does on anything else!

There is an experiment at CERN to measure the gravitational effect of antimatter and compare it with normal matter. As it is hard to make antimatter, the experiment generates small numbers of anti-hydrogen atoms and measures their properties. (They haven't got a definitive answer to the gravity yet, though.)

[swansont]

11 hours ago, Amazing Random said:

  This is a brief history of our today's subject . Well I was wondering since some effects wave-matter duality can be applied only to the microscopic world of molecules , why cant something be applied only to "big" objects ?

Well I thought that the standard model of physics predicts a boson carrying the gravitational force . But what if gravity doesn't exist in the microscopic world? I mean the objects we study in a quantum system and their effects are very small . So  there wouldn't affect space-time at all . (And space-time is not easy to be bent).

 

So what if gravity just doesn't exist at the microscopic world ? This would agree with the experimental data until today .

 

How does gravity "turn on" once the system is big enough? And is a collection of gas one object or is it a bunch of small ones? How would a star form if hydrogen didn't exert gravity because it's just an atom (or molecule)?

There's also GR's equivalence principle, which tells us (in part) that gravity does not depend on the composition of the matter. I think you may run afoul of that.

[Amazing Random]

On 8/1/2019 at 1:41 PM, swansont said:

 

How does gravity "turn on" once the system is big enough? And is a collection of gas one object or is it a bunch of small ones? How would a star form if hydrogen didn't exert gravity because it's just an atom (or molecule)?

There's also GR's equivalence principle, which tells us (in part) that gravity does not depend on the composition of the matter. I think you may run afoul of that.

??? Gravity is created when large quantities of matter come close to each other . 

It is turned on when the stress of the matter is big enough.

[Strange]

16 minutes ago, Amazing Random said:

It is turned on when the stress of the matter is big enough.

What generates this stress? And how would you calculate it?

[Amazing Random]

20 minutes ago, Strange said:

What generates this stress? And how would you calculate it?

*Relativistic stress.

[swansont]

1 hour ago, Amazing Random said:

??? Gravity is created when large quantities of matter come close to each other . 

What constitutes "large"? Why doesn't gravity exist below this threshold, whatever it is? And what is that threshold?

This is your conjecture. You can't hand-wave your way around this. 

1 hour ago, Amazing Random said:

It is turned on when the stress of the matter is big enough.

What stress is there? Why would e.g. ten molecules exert gravity, if each individual molecule exerts no gravity?

[QuantumT]

Very interesting discussion!

Would it be totally wrong to consider QM as a singularity, and thereby accept that it might be incompatible with relativity?

Then gravity becomes an exclusively relativistic issue.

[StringJunky]

On 8/1/2019 at 3:15 AM, uncool said:

As you said: many small things can add up to something bigger. But those small things, too, must exist. My point is that gravity must arise out of something that exists, even on the microscopic scale. 

 

27 minutes ago, swansont said:

What constitutes "large"? Why doesn't gravity exist below this threshold, whatever it is? And what is that threshold?

This is your conjecture. You can't hand-wave your way around this. 

What stress is there? Why would e.g. ten molecules exert gravity, if each individual molecule exerts no gravity?

Could there not be a minimum energy for action or effect? I'm thinking along the lines where virtual particles individually have no action until a quantum is produced by combination.

[Amazing Random]

This is my opinion : Gravity is an effect only of the macroscopic world . The objects in the microscopic world don't have the required stress-energy to cause any bending to the space-time . So gravity isn't created . And yes there is a barrier of stress-energy below which gravity doesn't exist.Above the stress-energy barrier gravity exists.

[Strange]

1 hour ago, Amazing Random said:

*Relativistic stress.

What generates this stress? And how would you calculate it?

[Amazing Random]

Just now, Strange said:

What generates this stress? And how would you calculate it?

Relativistic stress have all object with mass and all objects moving at the speed of light.I don't understand.....

P = mc+h/λ

 

 

On 8/1/2019 at 6:51 AM, Mordred said:

Let's clarify a bit on the reference to graphs and subgraphs in that quoted section. As noted this article loses a lot on technicalities.

 That section is referring the graphs in reference to the Feynman path integrals the internal lines being the propogators Ie virtual particles but in QFT it's simply field influence QFT treats all particles as field excitation.  Real particles are observable Ie a quanta of action minimal.

 The propogators propogators the operators and the Observable operators operate on the propogators. 

(Much like space tells matter how to move matter  tells  space how to curve lol)

Anyways this is an example. The internal wavy line is the propogators while the external lines the Operators.

e+↘e−↗⇝↗P−↘P+

Gauge bosons  are offshell  they are part of the virtual particle family hence in the propogator integrals. The operators Ie real particles  (more accurately observable particles ) are the external line path integrals.

Omg you are very smart!

38 minutes ago, QuantumT said:

Very interesting discussion!

Would it be totally wrong to consider QM as a singularity, and thereby accept that it might be incompatible with relativity?

Then gravity becomes an exclusively relativistic issue.

What do you mean by stating that?

On 8/1/2019 at 5:39 AM, Mordred said:

Here is a paper discussing the issue with a proposed methodology. It is simply one example of a plethora of proposed methods.

 

 

Now onto the comment made by Uncool..

 

 

This above process involves the running of the coupling constants in terms of symmetry breaking.. as you can see gravity becomes emergent after the breaking. Another way this can be described is gravity drops out of thermal equilibrium.

Here is the article keep in mind it is but one example. PS on phone it is an arxiv article...

https://www.google.com/url?sa=t&source=web&rct=j&url=https://arxiv.org/pdf/1112.5104&ved=2ahUKEwjXnNqhz-DjAhVlHzQIHfzuAjgQFjABegQIBhAB&usg=AOvVaw2Q8l6DtEpIv_3n6OaoqHUh

Here is another relevant section with regards to action being involved it's an essential ingredient to QFT.

 

 

Hope that helps

I can't understand you . I know particle physics but clearly you are on a different level. Sorry:)

[Strange]

4 minutes ago, Amazing Random said:

Relativistic stress have all object with mass and all objects moving at the speed of light.I don't understand.....

P = mc+h/λ

That looks like (a rather garbled) equation for momentum. Where does it come from? What is the connection to stress? What is "relativistic stress"?

 

[QuantumT]

6 minutes ago, Amazing Random said:

What do you mean by stating that?

Not stating - suggesting!

I'm suggesting that there might not be any gravity in QM, because it operates under different rules than the macro/relativistic world.

[Amazing Random]

Just now, Strange said:

That looks like (a rather garbled) equation for momentum. Where does it come from? What is the connection to stress? What is "relativistic stress"?

 

Relativistic stress is relativistic momentum

[Strange]

Just now, Amazing Random said:

Relativistic stress is relativistic momentum

Can you provide a reference to support that claim?