
A dialog on quantum gravity (2003) - smadaan
http://arxiv.org/pdf/hep-th/0310077.pdf 
======
Steuard
First, advice about arXiv.org links: for me, at least, it's nice to see a link
to the main abstract page rather than directly to the full PDF. Here, that
link would be <http://arxiv.org/abs/hep-th/0310077>

And second, this isn't "When physicists chat over lunch". This is "A nine year
old dialog by a loop quantum gravity specialist putting words into the mouth
of an imaginary string theorist." That may or may not be worth reading, but I
think the context is important. [The date "2008" on the final page is some
sort of typo: the arXiv time stamp on the first page makes the submission date
clear.]

~~~
pfedor
Worth noting that nine years and one LHC later, the supersymmetric particles
still haven't been found.

~~~
shardling
I don't think anyone really expected the LHC to find supersymmetric particles
by now.

------
jackgill
I quite enjoyed this, it's a nice piece of "inside baseball" for high energy
theoretical physics. For those who don't have time to read 20 pages of jargon
from another specialty, here's an excerpt from the end of the paper that I
think communicates the key point:

"I think that string theory is a wonderful theory. I have a tremendous
admiration for the people that have been able to build it. Still, a theory can
be awesome, and physically wrong. The history of science is full of beautiful
ideas that turned out to be wrong. The awe for the math should not blind us.
In spite of the tremendous mental power of the people working in it, in spite
of the string revolutions and the excitement and the hype, years go by and the
theory isn’t delivering physics. All the key problems remain wide open. The
connection with reality becomes more and more remote. All physical predictions
derived from the theory have been contradicted by the experiments. I don’t
think that the old claim that string theory is such a successful quantum
theory of gravity holds anymore. Today, if too many theoreticians do strings,
there is the very concrete risk that all this tremendous mental power, the
intelligence of a generation, is wasted following a beautiful but empty
fantasy."

I spent a few years during my undergrad working in a high energy experiment
lab, and this was a frequently debated subject. If the LHC does find
supersymmetry, it will be interesting to see the impact on the current
theoretical landscape.

~~~
tammer
I enjoyed this as well, and I know absolutely nothing about the field. I think
that's why the writing is so enjoyable, I understood the conversation without
understanding most of the details.

I for one would greatly enjoy more such writing on HN, it's a refreshing
change from some of the bloggier posts that make the majority of the
frontpage.

------
fhars
In case you wonder who will win the argument in the end: Sal is short for
Filippo Salviati, the alter ego of Galileo Galilei in his "Dialogue Concerning
the Two Chief World Systems", and Simp is Simplicio, the obviously stupid
defender of Ptolemaic and Aristotelian science.

[http://en.wikipedia.org/wiki/Dialogue_Concerning_the_Two_Chi...](http://en.wikipedia.org/wiki/Dialogue_Concerning_the_Two_Chief_World_Systems)

~~~
andrewcooke
while it's clearly an article intended to persuade, and not trying to present
a balanced view, i think the doubts and questions raised against string theory
are still commonly held (i cannot for a moment comment on particular problems,
but from the occasional things i hear from academia, string theory is not
considered that "good" in an "aesthetic" sense... it may be the best we have,
but it's kind of embarrassing how complex it is, and how little it gives (or
_fixes_ ) in return)

~~~
Steuard
Heck, many of the doubts and questions raised there are commonly held among
string theorists. (The days of people exclaiming "We're going to unify physics
within a few years!" are at least two decades behind us.) In my experience,
string theorists do consider those issues significant, but not problematic
enough to make them uneasy about the theory as a whole.

As a string theorist, the thing that bugs me about this dialog is mostly that
the string theorist character is _so_ befuddled by relativity. I'm sure there
must be real string theorists out there who are confused by the difference
between "general relativity" and "quantum fields in curved spacetime" or who
don't understand that GR introduces subtleties with things like globally
defined time coordinates, but I've never met one (at least not to know it).
And it's bizarre to see a string theorist character act so surprised by the
notion of spacetime as an emergent property of some very different underlying
physics, given that the same thing happens in string theory itself. (Certain
fields defined entirely on the two-dimensional worldsheet of a fundamental
string interact in specific ways that give them the structure of a spacetime
manifold.)

------
borgchick
1\. I am feeling rather smug that I read through the entire thing, and despite
understanding less than 1% of it, feel that it has in some way, opened my
eyes.

2\. I marvel at the world Sal and Simp live in. To understand all the details
they are going on about must make the world truly an amazing place to exist
in.

3\. Why can't all things in this world be worked out like these two? You know,
politics, war, religion, etc? Ah, I can dream...

~~~
mck-
That is exactly how I feel too! I felt like I was watching a movie, that was
compelling by the scene of it, despite not understanding what's going on for
the most part..

But it makes you dream.. and makes me want to dive deeper into it all..

Fabric of the Cosmos (Brian Greene) opened my eyes into the world of physics
(and string theory). Now I can't wait to find an equally compelling book on
the ideas of loop gravity

------
nickzarzycki
I know some of these words!

I'm surprised by how long I managed to keep reading/enjoying this despite
having almost no knowledge on the subject.

~~~
knieveltech
Likewise, and I expect it's due to the novelty of polite, reasoned discourse.

------
abecedarius
Can anyone explain loop quantum gravity to lazy programmers with a simple,
impractical Python or Haskell program computing a spectrum of amplitudes for
spin networks? (Or whatever the theory actually gives you, which I'm not
really clear on.) The closest I found poking around on Wikipedia was
<http://en.wikipedia.org/wiki/Spin_foam>

(Maybe it is clear from <http://en.wikipedia.org/wiki/Loop_quantum_gravity>
for someone who knows more math.)

------
jaequery
they should just get into web development

------
andrewcooke
<http://en.wikipedia.org/wiki/Loop_quantum_gravity>

_The deep origin of the problem is the fact that gravity is geometry. When the
quantum properties of gravity are not disregarded, spacetime itself becomes a
quantum object, and therefore the usual logic of conventional quantum field
theory, which requires the existence of a well defined classical geometry,
does not work anymore. This is the starting point of LQG. A quantum theory
defined without assuming a classical spacetime is called background
independent. LQG is essentially a technique for studying quantum field theory
when there is no spacetime background. The quantum states of the theory,
labelled by spin networks, should not be thought as living inside a physical
space, but rather to define physical space themselves. This is the quantum
version of the main property of Einstein's general relativity, where the
solutions of the theory are not gravitational field living inside a spacetime,
but are themselves defining spacetime._

 _The theory of LQG is one of the possible solutions of the problem of quantum
gravity, along with string theory. There are substantial differences, however,
with string theory. String theory addresses also another major open problem in
fundamental physics besides quantum gravity: the problem of unification,
namely understanding all known forces and particles as manifestations of a
single entity. To this aim, string theory postulates the existence of extra
dimensions and so-far unobserved particles and symmetries. LQG, on the
contrary, is based only on quantum theory and general relativity and its scope
is limited to understanding the quantum aspects of the gravitational
interaction. On the other hand, the consequences of LQG are radical, because
they change in depth our understanding of the nature of space and time and
provide a tentative but detailed physical and mathematical picture of quantum
spacetime._

my very rough take on that, and the linked paper, is that string theory is a
huge artifice, that tries to cover everything, "starting" in a sense, with
"generalising" quantum mechanics into something very abstract that can then be
used to "pull in" relativity. in contrast, loop theory goes back to the
geometric basics of relativity and tries to reconstruct those in a way that
would be consistent with quantum mechanics, in the hope that rebuilding
relativity on that will allow quantum mechanics to be pulled in naturally.

so they have very different styles. string theory is trying to do everything,
and is getting top-heavy. loop theory is a "back to basics" that is trying to
re-do relativity in the hope that qm can then be connected back on (perhaps
still with pieces of string theory, to make the qm more elegant).

i have no idea what the current take on all this is, though - the paper linked
here seems to be from back in 2003.
[https://www.google.com/trends/explore#q=string%20theory%2C%2...](https://www.google.com/trends/explore#q=string%20theory%2C%20loop%20theory&cmpt=q)
\- it looks like loop theory may have stalled?

evidence of current interest - [http://phys.org/news/2012-01-physicists-loop-
quantum-gravity...](http://phys.org/news/2012-01-physicists-loop-quantum-
gravity.html) (sounds like the kind of thing that atomic optics or whatever it
is called could help with - i think there was a link here recently showing how
those might improve gravitational wave detection).

