[JP (OP)]

This is a tangent I'd like to take from a recent string theory thread.  As far as I can tell, string theory is basically a result of trying to unify quantum mechanics and gravity, and from that attempt have come the ideas of multiple extra dimensions (beyond 4) as well as multiple possible universe.  The main drawback is that its not offering much in the way of testable results, and there's been a lot of debate over whether it's more science or philosophy because of that.

Let's put aside debates on the testability of string theory for now.  I'm curious about some of the alternative attempts to unify quantum mechanics and gravity.  I've heard of at least one, quantum loop gravity.  I don't know much about it, but it also doesn't seem to offer testable results.  What are some of the others?  Do they offer testability that's lacking in the more publicized theories?

[Soul Surfer]

There are several popular books on the subject and several routes which would require too much time to go through in detail here.  The big problem with testability is that the energy levels at which gravitational effects get significant are way beyond any human experimentation.  There is however one route that could possibly produce a probabilistic model based verification that I tend to favour and that is what I call "Evolutionary cosmology".

We are I assume all familiar about the way evolution shapes life on Earth to fit its environment.  Processes similar to evolution also shape the physical world for example the recycling way in which Electrons "orbit" the nucleus. Planets orbit stars and the cycles of water and carbon that shape our landscape.

Most string theories consider that the symmetry breaking processes produce random results and that is why there are so many possible answers.

The uncertainty principle in physics shows that at high energies all the low energy processes are masked inside quantum chaos.  It is quite possible that as the universe cooled down and these laws slowly (in the context of interaction speeds at the time)  froze out of quantum uncertainty the selection was not entirely at random but determined by processes that encourage "recycling" because these are more probable and last longer so the physical laws in our universe may not be entirely random but selected physically to enable continuity and complexity.

This could vastly reduce the odds against our universe existing with the finely balanced physical laws that it has now.  in just the same way that carbon chemistry is set up to create molecular complexity that leads via recycling processes to life as we know it.

[Murchie85]

String theory itself evolved from other theories and has been chopped and changed and even recently big adjustments made to the theory. I believe it was once called super gravity theory and incorporated more dimensions this changed when it was discovered only 11 or 22 dimensions were possible.

There are of course other theories than string theory and quantum loop theory I am only aware of a few obscure ones such as Heim theory and Antony Garette Lisi's theory although I am not aware of the differences between them.

[Farsight]

I'm curious about some of the alternative attempts to unify quantum mechanics and gravity. I've heard of at least one, quantum loop gravity.  I don't know much about it, but it also doesn't seem to offer testable results. What are some of the others? Do they offer testability that's lacking in the more publicized theories?

That's a tricky one. There's a whole bunch of what you'd call "fringe" theories out there by a host of different people, some very partial, some very sketchy, some with some errors. But there's a common thread, focussing on electromagnetism and relativity with a form of gauge theory, quantum electrodynamics, and topological quantum field theory. Unfortunately their advocates tend to be competitive and disunited, and many seem to argue over bones of contention rather than recognising the common ground. Hence we don't have a single clear theory, and I imagine it will be some time before we do. I used to think it would be an extension of the standard model, but as I speak I'm thinking it won't be. As to what it will be called, I don't know that either, but I'm absolutely 100% confident that it's coming. Note that you don't actually unify quantum mechanics and gravity, you unify electromagnetism and the strong force with gravity. Electromagnetism has an innate quantum nature, but gravity doesn't. It cannot be quantized, because "a photon doesn't approach you in steps". Sorry to be vague, there is a reason, but I can't talk about it.   

[yor_on]

Hmm, what can I say

We all would like to see that theory that 'tells it all'. On the other hand, it would be quite boring if it came. Whatever would we argue about then?

That gravity isn't a quantum phenomena?

Maybe, it all depends on how you look at it. From the point of relativity it is, as I see it, the geodesics of SpaceTime. But from the point of Quantum mechanics? Well, String theory have one definition, then we have photons, vacuum energy, gluon's and quarks, knots and other geometric formations, and then we come to the evolutionary theories, and on the other side I guess we have the creationist theories. then we have the electro-magnetic universe, qbits and the Quantum super-computer idea, and of course, the holographic universe, as well as the photonic one. I'm sure I missed some though

You have to like physics

[JP (OP)]

Thanks, everyone.  I learned about a bunch of new models here.  But back to my original question, it sounds like none of the other models are really any more testable at the moment than string theory is.  I guess that makes sense--if one of them was testable it would get tested pretty quickly.

[Farsight]

I don't think it's quite like that, JP. For example, classical electromagnetism, general relativity, and QED and the Standard Model have all been well tested. A "new" model will show how they fit together, but it won't really be all that new in itself. Have a look at The Maxwell Wave Function of a Photon for the sort of thing I mean by this. 

We all would like to see that theory that 'tells it all'. On the other hand, it would be quite boring if it came. Whatever would we argue about then? That gravity isn't a quantum phenomena?

No, that will be settled, but don't worry, people will always find plenty to argue about.   

Maybe, it all depends on how you look at it. From the point of relativity it is, as I see it, the geodesics of SpaceTime. But from the point of Quantum mechanics? ...

They're like different languages, and making them fit together isn't easy. But when you do, the end product won't make GR or QM wrong at all. Yes, you will look at things differently, because things like the Copenhagen Interpretation will go out of the window. But that isn't "the theory", that's the interpretation of the theory. You can keep the theory whilst moving to a new interpretation that fits snugly with something else.   

[yor_on]

So we have explained entanglements, the new results from Cern showing strange effects in the vacuum as well as super positions, both in themselves and as spatial phenomena existing in two places simultaneously Farsight? Missed that one..

Don't think so myself, that I might give my idea a new name won't explain it. We are still building on our theories, and I'm sure that will continue when I'm dead and gone. Otherwise there wouldn't be all those arguments. And I agree JP, if you accept mathematics as a science with its own definitions of 'experimenting', and 'proofs'. Then it's as any other science.

My problem is with its 'universality', that it can find proofs for things we don't have, and never will, like parallel tracks always meeting at some point. I don't expect SpaceTime to do so myself, although it's possible to build a logical proof for it that is consistent. You can see that two ways, either they will exist, somewhere. Or math can prove things that doesn't necessary have to be true?

I don't know how to look at that one?
I'm too bad at math

[yor_on]

JP

String Theory and predictability, heh Maybe we are getting there?  String theory predicts an experimental result.
 
==
And a fluid too

So likable ..

[yor_on]

Here you have two lectures on modern string theory by the 2010 winner of the Isaac Newton medal, Professor Edward Witten, Institute for Advanced Study.

And as it's a non-technical explanation, it's almost understandable.

Part one (Duration: 33 minutes)
And Part two (Duration 31 minutes)

Tastefully 'lended' from IOP Institute of physics. Listen to them, take you an hour, but after that hour.. You're there, finally anchored in the newest mathematics.

[Soul Surfer]

Thanks for the links.  It is a most interesting and revealing lecture and although I am a member of the IOP and can log in there I seldom trawl through its content and did not realise that lectures like tis were available nowadays.  Maybe I should do a bit more exploring.  This lecture has helped me with some of the thinking sketched out in my first reply to the question here and to be found elsewhere in bits on this site.

[Farsight]

And as it's a non-technical explanation, it's almost understandable.

Don't be fooled, yor-on. It isn't understandable. If it was, you could explain it, only you can't.

[Geezer]

And as it's a non-technical explanation, it's almost understandable.

Don't be fooled, yor-on. It isn't understandable. If it was, you could explain it, only you can't.

I don't know how you are able to ascertain that Yor_on can't explain it.

Anyway, you seem to be saying that you have identified some major flaw. Can you enlighten us on that point, or are you proposing an alternative theory?

[imatfaal]

The lack of understandability could also come from the fact that witten is one of the few physicists to be a fields medallist as well - the maths in string theory is formidable in the extreme. 

Yor-On, if you are interested I-tunes university released a few months ago the latest in leonard susskind's lectures on the "theoretical minimum of physics".  he has finally made it round to new theories including string thoery.  I havent got round to watching yet - but maybe you will find stanfords finest string-wrangler can help you make head or tail of string-theories.  Even though he is a bit scatterbrained, and I think the course is pretty eccentric; he is a great teacher and he is teaching a group of mature amateur scientists - suits me fine.  I am currently watching his series on special relativity on the tube (trans. london underground train) every morning and evening - getting some pretty strange looks!