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Starbles@Earthlink.net science forum Guru Wannabe
Joined: 11 Sep 2005
Posts: 172

Posted: Tue Apr 11, 2006 7:11 pm Post subject:
Are either GR or QFT more fundamental than the other?



Why does everybody assume that the gravitational field must be
quantized in some sense? Why not simply derive a spacetime independent
version of the Schrodinger equation, and derive an indeterministic
gravitational field based on the indeterminism of other quantum fields?
Wouldn't that be much simpler? 

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Phillip Helbigremove C science forum addict
Joined: 06 May 2005
Posts: 88

Posted: Wed Apr 12, 2006 7:46 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



In article <1144640507.697351.267050@z34g2000cwc.googlegroups.com>,
Starbles@Earthlink.net writes:
Quote:  Why does everybody assume that the gravitational field must be
quantized in some sense?

Penrose is one of the few who believe that quantum gravity will require
more changes to QM than to GR. 

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Oh No science forum addict
Joined: 06 Apr 2006
Posts: 82

Posted: Wed Apr 12, 2006 7:46 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



Thus spake Starbles@Earthlink.net
Quote:  Why does everybody assume that the gravitational field must be
quantized in some sense?

Various reasons. One of the main ones is a thought experiment by Eppley
and Hannah showing that the assumption that it is not quantised leads to
conflict with one of: conservation of momentum, the uncertainty
principle, or relativistic causality.
Quote:  Why not simply derive a spacetime independent
version of the Schrodinger equation, and derive an indeterministic
gravitational field based on the indeterminism of other quantum fields?
Wouldn't that be much simpler?
I would describe that as a mainstream approach, much tried without 
success over the last 30 or 40 years.
Regards

Charles Francis
substitute charles for NotI to email 

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Starbles@Earthlink.net science forum Guru Wannabe
Joined: 11 Sep 2005
Posts: 172

Posted: Sun Apr 16, 2006 9:42 am Post subject:
Re: Are either GR or QFT more fundamental than the other?



Oh No wrote:
Quote:  Thus spake Starbles@Earthlink.net
Why does everybody assume that the gravitational field must be
quantized in some sense?
Various reasons. One of the main ones is a thought experiment by Eppley
and Hannah showing that the assumption that it is not quantised leads to
conflict with one of: conservation of momentum, the uncertainty
principle, or relativistic causality.
That's because of the currently held view that when particles collide 
their actions are instantaneous. IE when an electron in orbit around a
hydrogen nucleus absorbs a photon, the overall field of the nucleus +
electron + photon is not to our knowledge a smooth transition. The
electromagnetic field at any point changes sharply.
Quote:  Why not simply derive a spacetime independent
version of the Schrodinger equation, and derive an indeterministic
gravitational field based on the indeterminism of other quantum fields?
Wouldn't that be much simpler?
I would describe that as a mainstream approach, much tried without
success over the last 30 or 40 years.
I would like to know all of the attempted approaches and what went 
wrong with them. Is there a site for that?
Quote:  Regards

Charles Francis
substitute charles for NotI to email

(...Starblade Riven Darksquall...) 

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Oh No science forum addict
Joined: 06 Apr 2006
Posts: 82

Posted: Sun Apr 16, 2006 5:55 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



Thus spake Starbles@Earthlink.net
Quote:  Oh No wrote:
Thus spake Starbles@Earthlink.net
Why does everybody assume that the gravitational field must be
quantized in some sense?
Various reasons. One of the main ones is a thought experiment by Eppley
and Hannah showing that the assumption that it is not quantised leads to
conflict with one of: conservation of momentum, the uncertainty
principle, or relativistic causality.
That's because of the currently held view that when particles collide
their actions are instantaneous. IE when an electron in orbit around a
hydrogen nucleus absorbs a photon, the overall field of the nucleus +
electron + photon is not to our knowledge a smooth transition. The
electromagnetic field at any point changes sharply.

I would put it a little more strongly. The change is, to our knowledge,
not a smooth transition. The point of the Eppley and Hannah thought
experiment is to show that the same must be true of interaction with a
gravitational wave. This is normally taken to mean, that since in the
case of e.m. the e.m. wave is quantised, i.e. really consists of
photons, then likewise the gravitational field really consists of
gravitons. I don't go along with that. I think it merely shows that the
classical description of gravity as a smooth field breaks down, not that
the break down takes the form of gravitons, which are hugely problematic
mathematically.
Quote: 
I would like to know all of the attempted approaches and what went
wrong with them. Is there a site for that?

That I don't know. All I can say is that a great deal has been tried,
that theories are typically mathematically very difficult and that they
fail to churn up correct answers.
Regards

Charles Francis
substitute charles for NotI to email 

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John Baez science forum Guru Wannabe
Joined: 01 May 2005
Posts: 220

Posted: Thu Jun 15, 2006 9:22 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



In article <1144640507.697351.267050@z34g2000cwc.googlegroups.com>,
<Starbles@Earthlink.net> wrote:
Quote:  Why does everybody assume that the gravitational field must be
quantized in some sense?

How do you want to couple the spacetime curvature to the matter
fields when the former is classical and the latter (we know) is
quantum? You can couple the curvature to the *expectation value*
of the stressenergy tensor of the matter fields, but this has
been studied and is known to lead to problems: a rock in a superposition
of two position states would have a gravitational field like that of
two smaller rocks.
Quote:  Why not simply derive a spacetime independent
version of the Schrodinger equation, and derive an indeterministic
gravitational field based on the indeterminism of other quantum fields?
Wouldn't that be much simpler?

Go for it! 

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tryggth science forum beginner
Joined: 18 Jun 2006
Posts: 1

Posted: Sun Jun 18, 2006 12:19 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



John Baez wrote:
Quote:  In article <1144640507.697351.267050@z34g2000cwc.googlegroups.com>,
Starbles@Earthlink.net> wrote:
[snip]
Why not simply derive a spacetime independent
version of the Schrodinger equation, and derive an indeterministic
gravitational field based on the indeterminism of other quantum fields?
Wouldn't that be much simpler?
Go for it!

Don't do it! Its a trick. :)
Whatever became of Unruh's commutation relation between the metric and
G, the "Einstein tensor"? 

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a student science forum beginner
Joined: 06 May 2005
Posts: 39

Posted: Mon Jun 19, 2006 8:41 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



John Baez wrote:
Quote:  In article <1144640507.697351.267050@z34g2000cwc.googlegroups.com>,
Starbles@Earthlink.net> wrote:
Why does everybody assume that the gravitational field must be
quantized in some sense?
How do you want to couple the spacetime curvature to the matter
fields when the former is classical and the latter (we know) is
quantum? You can couple the curvature to the *expectation value*
of the stressenergy tensor of the matter fields, but this has
been studied and is known to lead to problems: a rock in a superposition
of two position states would have a gravitational field like that of
two smaller rocks.

Someone has claimed to go one better than meanfield couplings 
http://xxx.lanl.gov/abs/quantph/0509134  but it seems to involve an
ensemble of rocks and spacetimes. 

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Phillip Helbigremove C science forum addict
Joined: 06 May 2005
Posts: 88

Posted: Mon Jun 19, 2006 8:47 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



In article <e6rq6v$1at$1@glue.ucr.edu>,
baez@math.removethis.ucr.andthis.edu (John Baez) writes:
Quote:  In article <1144640507.697351.267050@z34g2000cwc.googlegroups.com>,
Starbles@Earthlink.net> wrote:
Why does everybody assume that the gravitational field must be
quantized in some sense?
How do you want to couple the spacetime curvature to the matter
fields when the former is classical and the latter (we know) is
quantum? You can couple the curvature to the *expectation value*
of the stressenergy tensor of the matter fields, but this has
been studied and is known to lead to problems: a rock in a superposition
of two position states would have a gravitational field like that of
two smaller rocks.

Conventional wisdom is that quantum gravity will leave quantum theory
relatively intact, while modifying gravity in some way. Of course, the
name "quantum gravity" implies this. On the other hand, Penrose things
that quantum gravity will require a modification of quantum theory (this
does not imply that GR will be unchanged, just that QM needs to be
modified as well). Specifically, his idea is that superposition breaks
down at the level where gravity cannot be ignored. 

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I.Vecchi science forum Guru Wannabe
Joined: 05 May 2005
Posts: 124

Posted: Mon Jun 19, 2006 8:47 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



John Baez ha scritto:
Quote:  In article <1144640507.697351.267050@z34g2000cwc.googlegroups.com>,
Starbles@Earthlink.net> wrote:
Why not simply derive a spacetime independent
version of the Schrodinger equation, and derive an indeterministic
gravitational field based on the indeterminism of other quantum fields?
Wouldn't that be much simpler?
Go for it!

Yes! And when you are done don't forget to tell us how it relates to
our actual observations.
IV

"Mensionis quaestionem expellas furca, tamen usque recurret"
"You may chase the measurement problem away with a pitchfork, yet in
the end it will come back".
Horace&IV 

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markwh04@yahoo.com science forum Guru Wannabe
Joined: 12 Sep 2005
Posts: 137

Posted: Fri Jun 23, 2006 10:05 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



<Starbles@Earthlink.net> wrote:
Quote:  Why does everybody assume that the gravitational field must be
quantized in some sense?

Not everybody does. The general issue is discussed in some depth in the
opening chapters of Claus Kiefer's recent Quantum Gravity book. Kiefer,
comes out on the "pro" side, but the general argument used is flawed.
(Nonetheless, Kiefer's book is the best I've ever seen in the field, of
the dozens of books in the field that I've reviewed).
Quote:  From John Baez:
How do you want to couple the spacetime curvature to the matter
fields when the former is classical and the latter (we know) is
quantum?

I'm surprise to see you ask that. You know, as well as anyone, that a
classical and quantum theory can be combined as one; and that there are
certainly ways that a theory of gravity may arise (almost even as an
afterthought) as an effective field theory.
Jacobson's 1995 paper is a prime example of this; where the Einstein's
equations emerge as a consequence of the Bekenstein bound coupled to
the laws of thermodynamics.
Another example is a more recent series of papers which simply puts the
two algebras (the classical algebra of manifold theory and the
noncommutative algebra of a quantum theory) together, alongside one
another  consistently.
On the latter note, we know that quantum degrees of freedom *exist*.
But it is a serious leap to pose, from there, that *all* degrees of
freedom of *all* systems are therefore quantum. There is a broad range
of theories, in theoryspace, spanning the gap between pure quantum and
pure classical, in which both types of degrees of freedom exist
together.
There is nothing that says that anything at all need to be done to
incorporate gravity into quantum theory, in the way of "quantizing
gravity". To use Jacobson's characterization: gravity is no more a
fundamental field than phonons, and to quantize it as a fundamental
field, as if it were one, would be just as fallacious.
The unification of quantum theory and general relativity has nothing
per se to do with "quantum gravity", it need not entail quantizing
gravity, and the very prospect of doing so will probably, itself, turn
out to be as much a red herring as Aether was in the 19th century.
Quote:  From Starbles:
Why not simply derive a spacetime independent
version of the Schrodinger equation, and derive an indeterministic
gravitational field based on the indeterminism of other quantum fields?

Rovelli's approach in his Quantum Gravity treatise has closely followed
that general track. In fact, all of Chapter 3 in his treatise was
concerned with reformulating the entire framework of mechanics on a
purely spacetime, timeless, foundation. Chapters 4 (and I think 5)
discuss the general issues of how Quantum Theory is to be interpreted
(and defined) in this general setting.
As he has correctly pointed out, the real repairs that need to be done
to make general relativity and quantum theory fit together fall more on
the side of fixing quantum theory  to get rid of its notion of time
"as a flow", to find a way of intrinsically defining time that
applicable even in settings where there is no usual notion of time (the
method described would potentially even work in a Euclidean space where
there's 4 spatial dimensions and no temporal dimension), how to write
down Lagrangian and Hamiltonian mechanics that extricates from its
formulation any notion of "evolution" or "equations of motion", etc.
It's ironic that the most valuable part of his treatise ... and the one
that will longest stand the test of time ... is the one that PRECEDES
the main topic of his book, which is loop quantum gravity! 

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Arnold Neumaier science forum Guru
Joined: 24 Mar 2005
Posts: 379

Posted: Sat Jun 24, 2006 9:13 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



markwh04@yahoo.com wrote:
[he = Rovelli]
Quote:  As he has correctly pointed out, the real repairs that need to be done
to make general relativity and quantum theory fit together fall more on
the side of fixing quantum theory  to get rid of its notion of time
"as a flow", to find a way of intrinsically defining time that
applicable even in settings where there is no usual notion of time

In standard relativistic quantum filed theory there is no notion of time
as a flow. All there is are vacuum expectation values.
Time as a flow appears only when one contracts the formulation to a
dynamkical description.
So Rovelli's analysis of ordinary quantum mechanics is somehow off
the mark since quantum field theory borrows little than an abstract
framework from ordinary quantum mechanics.
Arnold Neumaier 

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markwh04@yahoo.com science forum Guru Wannabe
Joined: 12 Sep 2005
Posts: 137

Posted: Fri Jun 30, 2006 8:21 pm Post subject:
Re: Are either GR or QFT more fundamental than the other?



Arnold Neumaier wrote:
Quote:  markwh04@yahoo.com wrote:
As he has correctly pointed out, the real repairs that need to be done
to make general relativity and quantum theory fit together fall more on
the side of fixing quantum theory  to get rid of its notion of time
"as a flow", to find a way of intrinsically defining time that
applicable even in settings where there is no usual notion of time
In standard relativistic quantum filed theory there is no notion of time
as a flow.

Your "notion of time as a flow" has little relation to what's being
discussed in what you're replying to, and there is not pertinent.
It might do well to actually review Chapter 3 of Rovelli, where the
timeless formulation of mechanics is laid out and explain how the
element it IS getting rid of is not a notion of time flow. There is,
indeed, a substantial difference between the timeless formulation posed
and that the usual formulation of mechanics, which requires a t
coordinate and only works in globally hyperbolic spacetimes. 

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