I like your comments

I have not seen Gates vs Greene, but apparently Gates was talking about non-critical string theory, like Nanopoulos. String theory normally starts in 10 dimensions (the “critical dimension”) in order to avoid a quantum inconsistency (the “conformal anomaly”), and then 6 of the dimensions are assumed to be small. Non-critical string theory can start in any number of dimensions, and deals with the anomaly by adding an extra “Liouville” field along the string to compensate. It’s not a subject I know about and I don’t understand what its foundations are supposed to be. There are a few notable string theorists who take an interest in it, but most stick to the normal theory. Extra time dimensions are a similar story, it’s a minority interest and I can’t make any sense of it as physics. I can assure you that most string models only assume one dimension of time.

Anyhow to give another example of contradictory claims in string theory,
last year there was a panel debate involving Freese, Smolin, Levin, Gates, Greene, Gleiser (video
available on the web). Jim Gates claimed that string theory is consistent with no extra dimensions. Brian Greene claimed it does. so who is right and who is part of the “core string theory community”? Also in the same debate it was also pointed out that some models also predict extra time dimensions.
So what is the correct claim by ST regarding extra space and extra time dimensions?
Thanks

http://arxiv.org/abs/1209.5772

Shouldn’t it be a clue that exponential complexity is a possible signal there is a fundamental mis-conception? A la epicycles.

My candidate is that since physics treats time as a measure from one event to the next, it only re-enforces the impression of sequence as fundamental, rather than a measure of change, due to action. As in the earth isn’t traveling the fourth dimension from yesterday to tomorrow, but tomorrow becomes yesterday because the earth rotates.

Duration doesn’t transcend the present, but is simply the state of the present between events, so there is no physical dimensionality.

One would think that if time is a dimension from past to future, then a frame with a faster clock rate would travel into the future faster, but the opposite is true. It ages/burns quicker, so it moves into the past faster. Witness the twin in the faster frame has died when her slower twin returns.

The cat is not both dead and alive, because it is the collapse of future probability which yields current actuality. Time emerges from the effect of change, rather than is the cause of it.

Knowledge is created inductively, as future becomes past, but is used deductively, as the past is used to predict the future.

We reach for the future, but can only grasp the past.

I agree with Mitchell’s response. I’d be skeptical about some of the bullet points you’ve raised. They are definitely not the core of things discussed in the string community.

I will follow up on Mitchell’s nice comments too, though. String theory generically predicts that the world is described by a spontaneously broken non-abelian gauge theory. This is correct, of course, but still isn’t our world in a precise way. “The challenge will still remain of finding a string vacuum that matches experiment” is precisely the right idea, though. The first thing that must be done in being more precise is matching the “discrete” data we see in the world, such as the standard model gauge group and matter representations. This can and has been done in concrete, globally consistent string compactifications (I believe O(100) or O(1000) exist at this point). The next difficulty is one of moduli stabilization: within a compactification that realizes the discrete data appropriately, we must study the effective potential on the moduli space – i.e. what string vacuum we’re in – to determine the masses, mixings angles, and other similar data that we know exist in particle physics. This is a formidable task and has not been accomplished to date.

So, in short – Mitchell is absolutely right that any string vacuum is extremely predictive, and nearly all vacua have been ruled out since their low energy gauge group and matter content are not the standard model. However, there are O(1000) that are pretty close . . .

Cheers,
P

Conceptually, you can analyze what goes on in string theory as follows. There is a theoretical core, consisting of the “theories” linked by the dualities that were worked out in the mid-1990s. Within that theoretical core, there is a phenomenological core consisting of the vacua which people study as possible descriptions of the real world. Then outside the theoretical core, there are various heterodox versions of string theory.

So very briefly, I *think* that what Damour and Horava talk about is inside the theoretical core but way outside the phenomenological core; Nanopoulos is outside the theoretical core, in “noncritical string theory”; and Erik Verlinde is off on his own. His idea was *inspired* by work in AdS/CFT, but entropic gravity has severe problems. (There seems to be a revised version coming, not yet written up.)

As things stand, string theory is like field theory, it’s a framework which yields many models. To get unique predictions, you have to pick a specific vacuum. String cosmology is still a wildcard, we don’t really know how it works. It could turn out to be more predictive than just anthropic selection. But if it isn’t, the challenge will still remain of finding a string vacuum that matches experiment. Any individual vacuum is extremely predictive; if you can find one, and show that it predicts the measured particle masses and mixings, in principle you could use it to predict the next n decimal places, and then go do the measurements. But as I said in comment #34, string phenomenology is not at that level yet.

o does string theory predict violation of equivalence principle?
Thibault Damour and Cliff Will claim it does. Most others claim it does not.

o does string theory predict violation of lorentz invariance?
Nanopoulus claims it does. Most others claim it does not.

o Does string predict Horava gravity?
That’s what Peter Horava claimed in his colloquium at Berkeley(video available)

o Does string theory predict MOND?
Verlinde claimed so in his colloquium at Caltech(reported by a user on a woit’s blog)

Anyhow to a non-expert like me, it seems string theory is consistent with any observation(even
contradictory ones), which means it can never be falsified or proven.

Also do you agree with the grades given by Strominger in his colloquium at Harvard (Fall 2010)?
video again available online