String Theory

[Shaguar]

OK so I have just reread Stephen Hawking's "Brief History of Time" for the unmpteenth time and have finally got Curved space/ time into my head when I hear on the radio that we now have a new theory that requires 11 (yes 11) dimensions !!!!

I am I alone in suspecting that some of the boffins just make it up, when asked by the interviewer if he could explain this in laymans terms he replied that "not even some of my most esteemed colleagues" can understand it , the rest of them flatly disagree any of it exsists !

So I now officially give up on my own tireless effort to find a unified theory, I know, I know, all you scientist realise it will be a great loss but I am now concentrating solely on researching the change of state brought about on my body by alcohol.

[JoeP]

If we recall the "three pieces of string walk into a bar" joke, we'll see that your new pursuit is still related to string theory.

I have yet to find any material anywhere that tries to help a layman understand string theory. The little strings are supposed to generate space and time, but how can you visualise them except as loops in space and time?

The 11 dimensions part is slightly easier. Image a drinking straw. It exists in 3 dimensions, but (if we ignore the thickness of the plastic) is really a 2-dimensional sheet of plastic. However one of the dimensions is tightly rolled up - it doesn't have an edge, it curls back on itself - and from a distance it looks one-dimensional. Put another way, large objects (including subatomic particles!) in the straw space can only exist with length - they can't move in the direction of the width of the straw.

Who knows why there need to be 11 (or 10 in some theories; I think there a 6-dimensional versions too) dimensions, but supposing there are, the idea is that all the other ones are rolled up tightly, much more tightly than the size of any physical particles or waves we know, so they have no differentiation.

Happy now? :cheers: :chug: :drunk3:

[Dragar]

Who knows why there need to be 11 (or 10 in some theories; I think there a 6-dimensional versions too) dimensions

I can't claim to know anything about String Theory in detail, but 11 are needed in order for the mathematics to be self consistent, I do believe.

Right now, it makes no predictions (though describes gravity perfectly, I'm told - which is no surprise, since that's half the point of the theory). It's very pretty, I'm also told. It's also very, very abstract, complicated, and a lot of it involves equations we can't yet solve. We have to wait till they finish it, and it makes some predictions, before we can start assessing it as a viable theory.

[slimshady2357]

I have yet to find any material anywhere that tries to help a layman understand string theory. The little strings are supposed to generate space and time, but how can you visualise them except as loops in space and time?

When I get home, I'll tell you the name of the book I'm reading right now, it does just that and it is fucking fantastic at doing it. I mean, it's fucking FANTASTIC :D

I'm really excited about this book and I highly suggest getting it to anyone interested in string theory. I just wish I could remember the name :(

The 11 dimensions part is slightly easier. Image a drinking straw. It exists in 3 dimensions, but (if we ignore the thickness of the plastic) is really a 2-dimensional sheet of plastic. However one of the dimensions is tightly rolled up - it doesn't have an edge, it curls back on itself - and from a distance it looks one-dimensional. Put another way, large objects (including subatomic particles!) in the straw space can only exist with length - they can't move in the direction of the width of the straw.

An EXCELLENT example and exactly one that is given in the book I mentioned. In fact in the book, they note that there are 4 extended dimensions (3 space and one time) and that the other 6 dimensions are all folded up into what's called a Calabi-Yau shape. The eleventh dimension has been mentioned, but I haven't gotten far enough yet to see how it is incorporated.

Adam

[slimshady2357]

Who knows why there need to be 11 (or 10 in some theories; I think there a 6-dimensional versions too) dimensions

I can't claim to know anything about String Theory in detail, but 11 are needed in order for the mathematics to be self consistent, I do believe.

Right now, it makes no predictions (though describes gravity perfectly, I'm told - which is no surprise, since that's half the point of the theory). It's very pretty, I'm also told. It's also very, very abstract, complicated, and a lot of it involves equations we can't yet solve. We have to wait till they finish it, and it makes some predictions, before we can start assessing it as a viable theory.

I'm not so sure that it descibes gravity perfectly as that it manages to unify the general relativity view of gravity with quantum mechanics, something that is considered "The greatest problem of physics right now" by many physicists :)

The problem with getting GR and QM together is that the 'smooth' space needed for GR to work is not present on ultramicroscopic scales, due to the uncertainty principle in QM. This, and the fact that QM and physics in general treats particles as point particles causes the GR version of gravity to break down. However, when you use a string instead (which has definite, though very small, extension) it 'smoothes' out the quantum chaos of space on that ultramicroscopic scale. This allows GR to operate again. As well, the versions of string theory which are being explored at the moment all predict a massless particle of spin-2, which apparently coresponds to what there should be for a graviton, the gravity carrying force particle.

Man, that book is soooooooooooooo much better at explaining this stuff than my reconstructed from memory version.

Someone above said that strings compose space and time, this isn't true, they exist in space-time.

I'll be back with that book name, I really can't recommend it enough.

Adam

[Shaguar]

Dragar, Joe, Slim et al, thankyou for replies that were more considered than my post deserved but I have to say that as of 7pm this evening I will be testing the straw by drinking Strongbow cider through it in the bar of the Bayshill pub Cheltenham.

[slimshady2357]

It's also very, very abstract, complicated, and a lot of it involves equations we can't yet solve.

I meant to say something about this. Not only do they only have approximate solutions to the equations involved, they only have approximations of the equations! :eek:

We have to wait till they finish it, and it makes some predictions, before we can start assessing it as a viable theory.

Due to what I mentioned above, we could be waiting quite a while :D I hope not though, it would be great if they could get some testable predictions soon. But man, you have to find some place that it disagrees with either (or both) of QM and GR, and they appear to be the two most successful physical theories ever.

Adam

[Skep]

Adam,

Are you reading Brian Greene's Elegant Universe (http://www.amazon.com/exec/obidos/ASIN/0375708111/103-3840362-3575006)? Excellent book. There's also a Nova special with the same name and hosted by Greene.

Wow, only $10.85 at amazon... I paid $29.95 when it first came out.

[slimshady2357]

Adam,

Are you reading Brian Greene's Elegant Universe (http://www.amazon.com/exec/obidos/ASIN/0375708111/103-3840362-3575006)? Excellent book. There's also a Nova special with the same name and hosted by Greene.

Wow, only $10.85 at amazon... I paid $29.95 when it first came out.

You bastard, I just talked to my sweetie on the phone and she gave me the book and author... you guessed it, Brian Greene, The Elegant Universe.

What did you think, Skeptoid? Like I said, I love it. But don't let that stop you from trashing it, if that's how you feel ;)

Oh, and I paid something like $40 Cnd when I bought it, similar to you. Is that $10.85 for the hardcover version?

Adam

[Dragar]

I'm not so sure that it descibes gravity perfectly as that it manages to unify the general relativity view of gravity with quantum mechanics, something that is considered "The greatest problem of physics right now" by many physicists :)

True, that. That's the goal of string theory, though; it was designed to produce GR gravity in a QM framework.

The problem with getting GR and QM together is that the 'smooth' space needed for GR to work is not present on ultramicroscopic scales, due to the uncertainty principle in QM. This, and the fact that QM and physics in general treats particles as point particles causes the GR version of gravity to break down. However, when you use a string instead (which has definite, though very small, extension) it 'smoothes' out the quantum chaos of space on that ultramicroscopic scale. This allows GR to operate again. As well, the versions of string theory which are being explored at the moment all predict a massless particle of spin-2, which apparently coresponds to what there should be for a graviton, the gravity carrying force particle.

Very nice. But, er...we haven't actually found gravitons, have we? ;)

I meant to say something about this. Not only do they only have approximate solutions to the equations involved, they only have approximations of the equations!

Well, yes. But don't let that worry you; most of quantum mechanics relies on computational approximations. The Schrödinger equation can only be solved analytically in a very few, idealised scenarios.

[Skep]

You bastard, I just talked to my sweetie on the phone and she gave me the book and author... you guessed it, Brian Greene, The Elegant Universe.

What did you think, Skeptoid? Like I said, I love it. But don't let that stop you from trashing it, if that's how you feel ;)

Oh, and I paid something like $40 Cnd when I bought it, similar to you. Is that $10.85 for the hardcover version?

Adam
It's an excellent book. I've read it at least twice. Greene does a good job of explaining the incompatibility of the current Standard Model of particle physics and General Relativity and how String Theory might tie the two together (pun intended :) ).

Greene is very enthusiastic about string theory and you can't fault him for that but you need to keep in mind that there is no experimental evidence showing that ST is the long sought after Theory of Everything. However, some theorists like Steven Weinberg have hit nothing but dead ends using conventional methods to reconcile SM and GR and concede that ST is the only game in town. Lee Smolin throws his support in the direction of Loop Quantum Gravity and critiques ST in his book, Three Roads to Quantum Gravity (http://www.amazon.com/exec/obidos/ASIN/0465078354/103-3840362-3575006). Other theorists think that ST and LQG are different ways of looking at the same underlying but as yet unknown principle. It's all very interesting and I'm keeping an open mind. :popcorn:

After looking at the page I linked, I see that the $10.85 is for the paperback edition, but still.

[ceptimus]

If you watch Roger Penrose's lectures from this site:

link (http://www.princeton.edu/WebMedia/lectures/)

...you'll see he does a pretty good job of debunking string theory.

(That site has lots of interesting lectures available, by the way - not all of them about physics).

His argument basically comes down to, "String theory requires at least ten dimensions, and the universe we live in doesn't have that many." Penrose has something of a vested interest, as his rival 'theory of twistors' is also a candidate to unify General Relativity with Quantum Theory. Twistor theory is fairly unfashionable at the moment - apparently the math in string theory is more interesting; but interesting math, while a good thing in itself, doesn't mean that string theory is actually correct. Unless and until string theory can make a few predictions about things that can be, or already have been tested, I place it in the 'merely interesting' category.

[Dragar]

His argument basically comes down to, "String theory requires at least ten spatial dimensions, and the universe we live in doesn't have that many."

:D

On the other hand, we suddenly detected 11 dimensions, ST would really be the new game in town. But yes; pretty maths gets physicists excited. I'm sceptical string theory really is the 'Theory of Everything'. But...I don't know enough yet to comment really.