A dialog on quantum gravity (2003)

by smadaan • 14 years ago

65 19 14 years ago

Steuard3• 14 years ago

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• 14 years ago

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

shardling2• 14 years ago

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

shardling2• 14 years ago

>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.

The latex source (another reason to link the abstract and not PDF) just has a macro for "todays" date there -- perhaps arxiv recompiled the paper in 2008. (This is actually pretty common to see in papers on the arxiv.)

mck-2• 14 years ago

I would love to see sequel: "A chat over lunch, 9 years later."

Steuard3• 14 years ago

I doubt it would be any different. LQG folks still seem to resent the greater attention that string theory gets. String theorists still mostly don't spend any time thinking about LQG. Neither has made any great breakthroughs or grand new predictions for experiments in that time. And it's just as easy to make up a straw-man character today as it was in 2003.

jackgill• 14 years ago

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• 14 years ago

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.

nickzarzycki• 14 years ago

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• 14 years ago

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

andrewcooke2• 14 years ago

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... - it looks like loop theory may have stalled?

evidence of current interest - http://phys.org/news/2012-01-physicists-loop-quantum-gravity... (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).

borgchick• 14 years ago

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-2• 14 years ago

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

fhars2• 14 years ago

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...

andrewcooke2• 14 years ago

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)

fhars2• 14 years ago

Well, yes. I do remember when someone put a copy of an article on the pin board of the library of the local high energy physics lab titled "String Theory Draws Theorists Together" and someone added "String-" in front of the word "Theorists" with a red marker sometime in the early 90ies. So it wasn't without distractors then, too.

Steuard3• 14 years ago

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.)

jaequery• 14 years ago

they should just get into web development

abecedarius• 14 years ago

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.)

A dialog on quantum gravity (2003)

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