FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups 
 ProfileProfile   PreferencesPreferences   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Forum index » Science and Technology » Physics » Research
Are either GR or QFT more fundamental than the other?
Post new topic   Reply to topic Page 1 of 1 [13 Posts] View previous topic :: View next topic
Author Message
markwh04@yahoo.com
science forum Guru Wannabe


Joined: 12 Sep 2005
Posts: 137

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

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.
Back to top
Arnold Neumaier
science forum Guru


Joined: 24 Mar 2005
Posts: 379

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

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
Back to top
markwh04@yahoo.com
science forum Guru Wannabe


Joined: 12 Sep 2005
Posts: 137

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

<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
non-commutative 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 theory-space, 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!
Back to top
I.Vecchi
science forum Guru Wannabe


Joined: 05 May 2005
Posts: 124

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

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
Back to top
Phillip Helbig---remove C
science forum addict


Joined: 06 May 2005
Posts: 88

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

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 stress-energy 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.
Back to top
a student
science forum beginner


Joined: 06 May 2005
Posts: 39

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

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 stress-energy 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 mean-field couplings -
http://xxx.lanl.gov/abs/quant-ph/0509134 - but it seems to involve an
ensemble of rocks and spacetimes.
Back to top
tryggth
science forum beginner


Joined: 18 Jun 2006
Posts: 1

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

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"?
Back to top
John Baez
science forum Guru Wannabe


Joined: 01 May 2005
Posts: 220

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

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 stress-energy 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!
Back to top
Oh No
science forum addict


Joined: 06 Apr 2006
Posts: 82

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

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
Back to top
Starbles@Earthlink.net
science forum Guru Wannabe


Joined: 11 Sep 2005
Posts: 172

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

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...)
Back to top
Oh No
science forum addict


Joined: 06 Apr 2006
Posts: 82

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

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
Back to top
Phillip Helbig---remove C
science forum addict


Joined: 06 May 2005
Posts: 88

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

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.
Back to top
Starbles@Earthlink.net
science forum Guru Wannabe


Joined: 11 Sep 2005
Posts: 172

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

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?
Back to top
Google

Back to top
Display posts from previous:   
Post new topic   Reply to topic Page 1 of 1 [13 Posts] View previous topic :: View next topic
The time now is Sat Jun 24, 2017 10:29 am | All times are GMT
Forum index » Science and Technology » Physics » Research
Jump to:  

Similar Topics
Topic Author Forum Replies Last Post
No new posts Fundamental region for the modular group Timothy Murphy Math 5 Mon Jul 17, 2006 8:32 pm
No new posts *unique* prime factorizations; the fundamental theorem of... DGoncz@aol.com Math 5 Sun Jul 16, 2006 9:53 am
No new posts Scientists Question Nature's Fundamental Laws glbrad01 Relativity 10 Fri Jul 14, 2006 1:22 pm
No new posts The Fundamental Theorem of Calculus Maury Barbato Math 19 Tue Jul 11, 2006 10:06 pm
No new posts Irrationality and the Fundamental Theorem of Arithmetic J. B. Kennedy Math 30 Mon Jul 10, 2006 12:02 pm

Copyright © 2004-2005 DeniX Solutions SRL
Other DeniX Solutions sites: Electronics forum |  Medicine forum |  Unix/Linux blog |  Unix/Linux documentation |  Unix/Linux forums  |  send newsletters
 


Powered by phpBB © 2001, 2005 phpBB Group
[ Time: 0.0483s ][ Queries: 20 (0.0058s) ][ GZIP on - Debug on ]