What Are the Philosophical Implications of M-Theory?

[fuzzyfelt]

Wow, is there anything you don't have an answer for? Thank you again, I'll work on understanding your answer,
Fi

 

[StarshipX]

If I'm not mistaken, Superstring/M-theory is an extension of general relativity. http://arxiv.org/PS_cache/hep-th/pdf/0504/0504089.pdf extends the methods of differential geometry.

 

[selfAdjoint]

If I'm not mistaken, Superstring/M-theory is an extension of general relativity. http://arxiv.org/PS_cache/hep-th/pdf/0504/0504089.pdf extends the methods of differential geometry.

Not quite right. Superstring theories have "flat" Minkowski spacetime as a background, and they produce a particle, the graviton, which couples to matter according to the same equations as Einstein's curvature. But there are difficulties with accepting this straight off as a theory of gravity.

Some approaches to M-theory construct a spacetime as part of the theory. This is "background independent" but I have never heard that the spacetime constructed was that of general relativity.

ADDED: The paper hep-th/0504089 does not discuss strings or M-theory, but complexifed general relativity, a completely different subject.

 

[fuzzyfelt]

Many caveats- I've been concentrating on the small picture, out of my depth, etc., but looking very simplisticly at the different theories above, with a quasi platonicish/zenish view, would it be correct to say that if this universe were some imperfect extension, the perfect point of departure in strings belongs at the beginning of this universe, and with twistors in complex space?

 

[selfAdjoint]

Many caveats- I've been concentrating on the small picture, out of my depth, etc., but looking very simplisticly at the different theories above, with a quasi platonicish/zenish view, would it be correct to say that if this universe were some imperfect extension, the perfect point of departure in strings belongs at the beginning of this universe, and with twistors in complex space?

I'm not sure I understand this question, but let me try, are you asking, given that there is something prior to spacetime, is the application of that priorness localized at the beginning of spacetime (the "big bang"), or is it being applied at every moment and point throughout the universe. Did the prior thing, strings or twistors or whatever, generate spacetime and then go idle, or does spacetime continually come out of strings or twistors or whatever.

I believe the theories I mentioned, would require the second option. The strings or twistors are evolving and as they do they generate the evolving spacetime. Or rather the spacetime we see is just a low energy approximation to the evolving prior physics, now and always.

 

[fuzzyfelt]

Yes, I like your answer, that it is an ongoing low energy approximation to the evolving prior physics. Thanks for humouring me, actually trying to verbalise questions helps as well as the answers, it all gives me a sense of direction. I'm keen now to learn about the different spacetimes in twistor theory and what they're suppossed to be, on top of everything else!
I must say, I probably won't be getting much computer time in the next month, my parents are visiting, so please enjoy a well deserved break from my questions!

 

[fuzzyfelt]

A little computer time! Just to clarify, I did mean to take the ongoing nature of spacetime generation into account, but I realize that it is a bit too significant not to stress. My aim had been to see if there were a distinction between the two ideas, that strings requires the big bang, whereas twistors, as far as I have read, do not seem to be dependent upon it, or at least not directly, is that so?
Also, is it important, should strings have a curved spacetime?

 

[fuzzyfelt]

Sorry, maybe this is a better question, what sort of spacetime should string theory have? And in fact, I think this thread is better off without my questions, maybe it could be just a thread about what Selfadjoint thinks, what do you think?

 

[selfAdjoint]

I'm BAAACK!

According to many but by no means all physicsts, string theory should be in a dynamic background independent spacetime. The great example of a theory like this is general relativity. Spacetime isn't just a background for physics in GR; "Matter tells spacetime how to curve, and curved spacetime tells matter how to move." "Matter", in this slogan, tacitly means anything that has momentum, but not necessarily mass, such as light, as well. It doesn't do any good to put string theory into a curved spacetime, if the curvature is fixed and not interactive; that's just another inert background.

 

[fuzzyfelt]

Great!
Thank you,I've been getting confused about matter as well as spacetime. Again, of course, it doesn't make a lot of sense to me, but I'm sure it will!

 

[fuzzyfelt]

Hi SelfAdjoint, Ok, I'm pretty sure I understand what you say about spacetime. I just finished 'Elegant Universe' (Yay! I'm a slow reader), and although I'm not really after solutions, more just understanding, is it just me, or was there a conclusion he seemed to want us to draw from the way it was written that looped string -gravitons, are event horizons of anti-black holes, and depending on what black holes do with the stuff they suck in would determine whether the stuff spat out by these strings was generated, or background dependent,( or just recycled?). Have I really gone right off-track here? I really haven't thought it through too carefully. I'll try 'Fabric of the Cosmos' next and see if that puts me straight.

 

[fuzzyfelt]

...I got carried away! On another note, I think I have read contradictions about the compacted dimensions -I'm sure I have read that they were extended and have compacted since the big bang, and elsewhere that they were never extended, which is more correct?

 

[selfAdjoint]

...I got carried away! On another note, I think I have read contradictions about the compacted dimensions -I'm sure I have read that they were extended and have compacted since the big bang, and elsewhere that they were never extended, which is more correct?

Those are both conjectures. Neither is correct as yet, they will have to be evaluated based on observation, if possible. For example a couple of years ago there was a conjecture, based on the Randall-Sundrum model in string theory (roughly, that our universe is the boundary of a higher dimensional space, called a brane), that some of the compacted dimensions could be small but large enough to affect gravity at very short ranges, like a millimeter. And an experimental program was undertaken to detect these effects, if they existed. But nothing ever showed up. This doesn't falsify the conjecture, but it does render it a little doubtful. It had a chance to shine, and didn't.

 

[fuzzyfelt]

Thanks Selfadjoint, sorry to be such a goose. Of course these are conjectures, I must remember to begin everything with 'hypothetically speaking'. I was suffering a momentary lapse of sense, elated by superficial comprehension and wired under the influence of my first cup of coffee in weeks. It does feel a bit of a roller coaster ride, one minute a little thing makes sense and it feels so good, followed by the realisation that is really hardly anything, which feels really bad! Sorry.
Rather than saying which is 'more correct', would you happen to know of any very good reason why one of these conjectures is favoured over or has been superceded by the other? The idea that dimensions were originally extended was from something dated 1988, and had gone on to say that, theoretically, the compactification process in itself could cause the gauge fields we have today. In the book I just finished, dated 2000, he says all spatial dimensions are completely symmetric, all curled in a multidimensional nugget until 3 spatial ones are extended. It just matters to me about at what stage and in what form - curled up or extended, they are symmetric, and for what good reason is there even speculation about this if the dimensions are pretty much the same thing, thus rather symmetrical, through duality, curled or not, unless compactification does play a role in the present state.

 

[fuzzyfelt]

But I see what you mean, Brane conjectures seem to supersede the others.

 

[fuzzyfelt]

or not so much supersede as present alternate ideas.