QG Troubles: Problems Connected with Quantum Gravity

[wolram]

http://relativity.livingreviews.org/Articles/lrr-1998-1/node12.html

Here is a list of problems that have been connected to quantum gravity in the past, but about which loop quantum gravity has little to say:

 

[marcus]

Hello Wolram,
you have found a piece of Rovelli's 1998 introduction to Loop Gravity.
It is the best online introduction and survey of the field so far. John Baez recommends it as a place to start whenever I've seen him asked about a good beginning LQG textbook. (there is no standard textbook, as such, so far---because loop gravity is too new)

It is a mistake to expect a physical theory to explain too much---Newton's theory of gravity explained the orbits of planets but did not try to explain the origin of the universe----Feynmann's theory of quantum electrodynamics explained the interaction of atoms with light but did not explain the workings of the mind---string theory may experience difficulty gelling into a finished version because it tries to be a TOE and embrace ALL the interactions.

I think Rovelli is being an honest writer and laying his cards on the table here---LQG (which he and a few others founded as a theory) is in some sense a cautious, modest, focused development. It tries to make one step in the unification: quantum mechanics and general relativity, and to preserve the essential geometrical features of GR (dynamic geometry, invariance under smooth deformations, backgroundlessness). This is already a hard problem just to make this one step successfully---because QM as we know it today is not background-independent!

So he frankly admits that the theory does not address the problem of the mind and the directionality of time and the origin of the universe! And I would imagine that this is without regret! Here is an exerpt from the section of Rovelli you linked to, in case other people are curious:
--------------------------------------
"5.4 Problems not addressed
Quantum gravity is an open problem that has been investigated for over seventy years now. When one contemplates two deep problems, one is tempted to believe that they are related. In the history of physics, there are surprising examples of two deep problems solved by one stroke (the unification of electricity and magnetism and the nature of light, for instance); but also many examples in which a great hope to solve more than one problem at once was disappointed (finding the theory of strong interactions and getting rid of quantum field theory infinities, for instance). Quantum gravity has been asked, at one time or another, to address almost every deep open problem in theoretical physics (and beyond). Here is a list of problems that have been connected to quantum gravity in the past, but about which loop quantum gravity has little to say:

Interpretation of quantum mechanics.
...

Quantum cosmology.
...


Unifications of all interactions or ``Theory of Everything''.
(A common criticism of loop quantum gravity is that it does not unify all interactions.)
...

Mass of the elementary particles.
...


Origin of the Universe.
..."


Arrow of time.


Physics of the mind.
------------------------------------

Since 1998 when Rovelli wrote that, there HAS been a good deal of progress in loop quantum COSMOLOGY but what he means here by cosmology is something grander. What Bojowald means by LQC is merely those cosmology issues which can reasonably be tackled by loop gravity methods, like studying the big bang singularity. LQC does not study the wavefunction of the whole universe for all time which is such a huge idea it might not make sense and only philosophers should consider it. Rovelli is trying to say that his field has a limited focus by listing stuff it does not get into---he didnt know in 1998 that Bojowald would make a sortee into big bang cosmology.

 

[wolram]

hi MARCUS.
do you know how difficult it is to stay "mainstream", when there
is so much anti progrestional rubbish on the net, i have posted
a few that would reverse all our understanding to date, the
problem is lack of proof, if the probes that are or soon or will
be looking for evidence do not bring back evidence for our
theories then we will be back to 1904, my plebarian view is
tainted when posts such as the DODECAHEDRON universe or
HOLEUMS are put to the forefront thees theories only proliferate
due to the weakness of what we hold to be true.

 

[selfAdjoint]

Originally posted by wolram
hi MARCUS.
do you know how difficult it is to stay "mainstream", when there
is so much anti progrestional rubbish on the net, i have posted
a few that would reverse all our understanding to date, the
problem is lack of proof, if the probes that are or soon or will
be looking for evidence do not bring back evidence for our
theories then we will be back to 1904, my plebarian view is
tainted when posts such as the DODECAHEDRON universe or
HOLEUMS are put to the forefront thees theories only proliferate
due to the weakness of what we hold to be true.

You bet. And that stuff is on the preprint arxive too. My way of handling this is to just let the wilder stuff ride. If there is anything to it, more papers will appear, and there will be announced conferences at professional venues and so on. If it just plays out in the public venue like Maglueno's (sp?) FTL, then I let it go.

For my money, the people who have been posting on the strings-and-QCD forum are straight shooters and not trying to sell a load of bull. They disagree, but they are on the same side versus crank physics. Of course it's disturbing to many that there isn't just one true way to go forward in physics right now, but that's the fact. I try to keep up with what the string people are doing and also what the LQG people are doing. The rest of the pretenders have to convince me that they are worth listening to.

 

[marcus]

Originally posted by wolram
hi MARCUS.
do you know how difficult it is to stay "mainstream", when there
is ...
...my plebarian view is
tainted when posts such as the DODECAHEDRON universe or
HOLEUMS are put to the forefront thees theories only proliferate
due to the weakness of what we hold to be true.

hello Wolram and selfAdjoint,

I am happy with the state of things in theoretical physics and science. (the demographic, economic, political and cultural problems of the world are horribly worse than the problems within Science)

It does not bother me if crackpots post in arxiv.org or if the definition of "mainstream" becomes obscured. It is like driving at night without headlights---one squints a little to see the white line and the edge of the road.

You, Wolram, serve us well by often hitting us wild fly balls or hard line-drive questions, for anyone to field (means "try to catch" in american baseball slang). I for one appreciate the concentration that you put into this---and the fact that your questions are from an individualistic point of view.

It is not in the cards for us humans to ever have an orthodox belief system where we know all the answers. "What we know" must always be weak. Dominant belief systems are merely a temporary delusion--and may even be dangerous.

I don't require assurance that what I am trying to understand is ultimately valid---some final answer. I struggle to understand in hope---and because that effort is the best I can offer.

I shall ignore Dodecahedrons because I am busy trying to understand covariant spin networks. And in the unlikely event that it turns out that after all Dodecahedrons or Colliding Brane-Worlds or some such actually proves correct (!) then I will be a good sport about it and laugh at the joke played on me by Nature.

She is a flirt. You have to pick something and go after it without any guarantee. If you turn out to have been wrong, well that is part of the comedy. Lots of long-beards have turned out to be wrong too---so plebs should not be afraid of that

 

[wolram]

good advice from SA,MARCUS, thanks.

maybe you could help with a question or should i start another thread?

i have a problem with the way gravitational radiation is given a
"frequency", as i understand space time is elastic but stiff,
if this is so i imagine that any oscillation in it will be
damped like the suspention on a car, one would see one big
bounce then progresivly smaller bounces reducing to zero, as the
light from these events last only a few seconds, i imagine the
gravity "signal" would also only last a few seconds, but unlike
light the gravity" signal" would be damped out very quickly due to the stiffness
of space time. sorry if this total rot.

 

[marcus]

bell metal, or spring steel, can be "stiff"
but the bell can keep ringing
and the springs can keep bouncing
people understand the idea of stiff in various ways

maybe spacetime geometry is hard like spring steel
and not "damped"

a shock absorber in a car drains energy out of the
vibration---turns it into heat, something like friction

my guess is that MATTER can absorb some energy from g. waves and
be pushed and pulled and heated up and deplete their
energy---so matter might damp them down some

but in open space with room to travel they wouldn't get
damped, only spread out

 

[wolram]

space time ringing like a bell, i like it
thank MARCUS.