Why is Quantum Gravity theory still not "finished"?

[DrChinese]

I am familiar with the expression "not my cup of tea", but after a google search the best I can come up with for "no cup of tea" is that one is not consenting to having sex.

Perhaps you mean "no piece of cake".

I have the cake after my tea.

 

[atyy]

I don't understand why QED is not finished.

QED does not make sense at very high energies (above the Planck scale). However, QED makes sense at low energies, which is the regime that current experiments test.

See tom.stoer's reply about the Landau pole (#24), and bhobba's reply about effective field theory (#27), which state the same thing.

 

[PhysicsExplorer]

Concepts like force and pseudo-force are purely classical reasoning.

Indeed, but you seem to be missing the point. The point is from first principles, gravity is not a quantum field but relativity treats it specifically like a pseudo force. So nothing implies from first principles quantization even makes sense.

 

[PhysicsExplorer]

Neither does any other quantum field theory. As said above: the mediator particle is an artifact of pertubative quantization only.

But this is counter-intuitive. It's not the kind of physics I am accustomed with: particles do seem to exist and it seems the consensus is that they share mediator particles. To exchange forces, you need the quantization particle of the field, which we already know for each field. Unlike gravity, gravity doesn't need a particle - maybe history will prove me wrong, but I'm willing to bet on it.ps. a good question would be if the quantum fields don't require mediator particles, why have we found the quantization of each field, or are you suggesting we have misinterpreted something?

 

[PeterDonis]

But this is counter-intuitive. It's not the kind of physics I am accustomed with

So what?

particles do seem to exist

Real particles (external lines in Feynman diagrams, on shell) do "seem to exist", certainly--we observe them in experiments. But we're talking about virtual particles here (internal lines in Feynman diagrams, can be off shell). We don't directly observe them, so they don't "seem to exist" the way real particles do.

it seems the consensus is that they share mediator particles

Science doesn't work by consensus. It works by making accurate predictions.

 

[PhysicsExplorer]

So what?
Real particles (external lines in Feynman diagrams, on shell) do "seem to exist", certainly--we observe them in experiments. But we're talking about virtual particles here (internal lines in Feynman diagrams, can be off shell). We don't directly observe them, so they don't "seem to exist" the way real particles do.
Science doesn't work by consensus. It works by making accurate predictions.

First thing, the so what comment? The point is, we must chose a rational picture of physics which explains not only the dynamics, but the observed phenomenon as well. As for the Feynman diagram thing, I am sick of hearing about it, to be honest. People rabbit away on it without any extended knowledge of the rich history of the fluctuation and how physicists actually, generally, think about it and have done since 1955.

 

[PhysicsExplorer]

The fluctuation has been theoretically looked into, right up into modern times. Take some time to check out Prof. L. Crowell who has dedicated, much work on the fluctuation theory. Yes I know its all theory, but its the only one that really makes sense to me - especially within the context I have studied it in.

I even know of ways of giving spacetime an intrinsic uncertainty - which may be important because scientists have a few times asked what the mechanism/origin is behind the fluctuation in the ground state. It is of conjecture of some other physicists, that the matter you see around you is simply but longer lived fluctuations of the vacuum.

 

[PeterDonis]

the so what comment?

It was quite clear, I thought: saying that something is counterintuitive or not what you are accustomed to is simply irrelevant in a physics discussion. Particularly in a discussion on quantum physics, which is the most counterintuitive and most unlike what most people are accustomed to of any field of physics.

The point is, we must chose a rational picture of physics which explains not only the dynamics, but the observed phenomenon as well.

You have this backwards. The observed phenomenon--the actual data, the experimental results--are what "a rational picture of physics" has to explain. The "dynamics" is a tool we use as part of that explanation. But you have to be very careful attributing "reality" to an internal feature of a particular model that is not directly observed.

People rabbit away on it without any extended knowledge of the rich history of the fluctuation and how physicists actually, generally, think about it and have done since 1955.

So give us some actual references--textbooks and peer-reviewed papers--that show this "rich history" and what it tells us about quantum fluctuations and virtual particles.

ake some time to check out Prof. L. Crowell who has dedicated, much work on the fluctuation theory.

Please give a specific reference or references, with links.

I even know of ways of giving spacetime an intrinsic uncertainty

Please give a specific reference or references, with links.

 

[tom.stoer]

Back to the very beginning:

Arguably, there is no quantum theory of gravity, because we do not have a complete picture of gravity in the context of quantum theory.

This obviously circular. We do not understand A + B b/c we do not understand B in the context of A.

This may be for a number of reasons, one reason may be because ... Things ... may not be fully understood ...

Maybe it's the method of quantization...

Perhaps ... There may be ...

Anything else you may suspect we could perhaps ...

I even know of ways of giving spacetime an intrinsic uncertainty - which may be important because ... It is of conjecture of some other physicists ...

I haven't seen any single fact so far. It's pure guesswork, isn't it?

 

[PhysicsExplorer]

So give us some actual references--textbooks and peer-reviewed papers--that show this "rich history" and what it tells us about quantum fluctuations and virtual particles.

You know what, I might take you up on this offer, but not here and not now. I will create another thread with links and entice a debate on it, because its a real shame the thread was closed the other day, when people where fresh discussing the subject.

As for the rest, you are very demanding aren't you? Yes I will provide the references, when I have time. I am a bit busy right now.

 

[PhysicsExplorer]

Back to the very beginning:

This obviously circular.

What I said was ''Arguably, there is no quantum theory of gravity, because we do not have a complete picture of gravity in the context of quantum theory.''

Sorry if it seemed circular - what I meant is there are some who think we have the complete picture (in terms of gauge theory) resulting in graviton particles. So when I say there is no complete picture of gravity in the context of quantum theory, I mean the approaches we are using may be in error. It is surprising how many physicists have spent years on one single subject.

 

[PhysicsExplorer]

I haven't seen any single fact so far. It's pure guesswork, isn't it?

Depends on what you mean by guesswork?

There are real theoretical reasons to go one route, its not entirely in the dark.

 

[tom.stoer]

I think your arguments are strawmen.

We we have never insisted on gravity requiring "mediator particles". We never denied that we need a different perspective and maybe (!) a different quantization method. We never claimed that we have "the complete picture".

 

[PeterDonis]

I will create another thread with links and entice a debate on it

That's fine; but it probably belongs in the Beyond the Standard Model forum, not this one, just to make clear the nature of the subject matter.

you are very demanding aren't you?

Asking for references is not being "demanding".

 

[PeterDonis]

The OP question has been answered. Thread closed.