String theory on the way out?

[arthwollipot]

Okay, now we're way beyond my level of graspingness.

 

[Tim Thompson]

String Theory IV

If you're familiar with Feynman's path integral, the issue is whether one integrates over all field configurations and all metrics (that's quantum gravity) or merely over all field configurations with fixed metric (that's quantum field theory).

That's the secret handshake that clarifies the key point. I didn't realize the real difference between gravity and quantum field theory.

Okay, now we're way beyond my level of graspingness.

At some point this always happens to everyone, we just have different levels of graspingness. The closer we get to the frontiers of physics, the farther we get from things that can be readily explained to non-physicists. Popular level explanations, in order to avoid complication, usually imply that physicists actually know things that are in reality not necessarily well known.

String theory is as far out on the frontiers of physics as you are going to get. Most people who have opinions about whether or not string theory is worth pursuing are far short of the level of graspingness required to actually know what they are talking about. This is true even for people with a physics background, if they have not studied string theory in considerable detail. My own expertise is in physics, but mostly atmospheric physics. I have no direct experience with quantum field theory, quantum gravity or string theory. I am dealing with my own level of graspingness here and myself learning things as I go.

So welcome to the "limited level of graspingness" crowd.

 

[Hercules Rockefeller]

Clifford Johnson debated Smolin and Woit on his blog Asymptotia under Scenes: Storm In A Teacup. You have to google it (I can't post links yet).

I don't think you need to be a physicist to see that the critics of ST lost that argument.

 

[Tim Thompson]

String Theory V

Clifford Johnson debated Smolin and Woit on his blog Asymptotia under Scenes: Storm In A Teacup ...

Scenes:Storm In A Teacup
Clifford Johnson (Personal USC home page)
Clifford Johnson (Faculty USC home page)

I met Johnson at a talk he gave on string theory and had him autograph my copy of his book D-Branes (Cambridge Monographs on Mathematical Physics; Cambridge University Press, 2003). When I handed him the book he expressed some surprise that anyone actually bought it. It's a good book, I think, but you really have to be into that mathematical physics thing or it looks like a foreign language. I am not really surprised that Johnson would come out on top of such a debate, as I think I could probably out debate either Woit or Smolin on string theory and I know far less than any of them do about it on a technical level. But I understand the history and I think I understand the real weakness of the positions that Woit & Smolin take, and that weakness is philosophical more than technical, both of them being in the "give up and quit" camp for insufficient reasons.

 

[BenBurch]

My opinion is that we will probably need to focus on cosmic ray physics if we want to find a way to prove or disprove string theory. YES it takes a lot of time and a lot of data and a lot of processing to use cosmic rays to do physics, but we will never be able to make a machine to turn out particles with energies as high as that reached by some cosmic rays.

 

[Dancing David]

That should be easier with satelites than the old photo stacks on mountaintops.

 

[BenBurch]

That should be easier with satelites than the old photo stacks on mountaintops.

True that.

Though we have other terrestrial and airborne methods now;

http://en.wikipedia.org/wiki/High_Resolution_Fly's_Eye_Cosmic_Ray_Detector

http://en.wikipedia.org/wiki/BESS

 

[Beerina]

Ah, many thanks, this is something of great interest I was unaware of. I am particularly intrigued by the requirement of a string theory on a spacetime background. It certainly seems that background dependence in string theory had been considered a weakness, at least up to now. Carlo Rovelli, in his book Quantum Gravity (Cambridge University Press, 2004), asserts that quantum field theory discards the spacetime background altogether (e.g., "Thus, there is no background 'spacetime', forming the stage on which things move. There is no 'time' along which everything flows. The world in which we happen to live can be understood without using the notion of time." - page 31). He is not the only source indicating that quantum field theory (and string theory) can & should be background independent.

Now the AdS/CFT duality requires a string theory on a spacetime background, so this would appear to be indeed a "revolutionary" new direction to take things. But what about the equivalent field in one less dimension, is it background independent? If not, that would seem to be not just a departure from the norm for string theory, but a departure from the norm for quantum field theory in general.

At least that's the way it looks at first glance from where I sit. What are your thoughts on background independence and spacetime in quantum field theory?

I admit straight away I am not well-versed in physics at this level, so my objection might not actually be an objection.

However, time not really existing could be a problem from a computational point of view. We have examples of computational problems for which there is no known faster solution than trying all possibilities -- you basically have to run the computation itself, rather than using some clever shortcut to get the answer. One example might be using multiplication you learned in grade school to multiply 123 x 352, rather than adding up 123 352 times.

That such problems have no shortcut has not been proven, but it is believed true currently, IIRC. So...the universe, which can run such computers which can run such simulations "can't get there from here" if there is no time interacting. No stuff bouncing around, so to speak. Yet such states, in the real universe, actually exist. If there is no shortcut that exists, as we have reason to believe, how did such a state get here?

Alternatively, it suggests an equally amazing thing, that the universe is either finite in computational speed, yet superior to a Turing machine or there is something infinite in nature to the universe-as-computational-device. This is similar to the arguments about a human mind being able to solve problems a Turing machine cannot, with its identical implications for the universe.

 

[SusanB-M1]

Clifford Johnson debated Smolin and Woit on his blog Asymptotia under Scenes: Storm In A Teacup. You have to google it (I can't post links yet).

I don't think you need to be a physicist to see that the critics of ST lost that argument.

Sounds interesting, thank you; I'll google that later.

On a flippant note - in Terry Pratchett's 'I Shall Wear Midnight', there is mention of 'the elasticated string theory'!

 

[Skamandros]

The thoery of the string is based on the three methods of the stringss, only as three partickles fumdamental is based on the methods of quarks. The science of physics has discoveredd this like all contaning the three.