What are strings made out of?

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What are strings made out of?

Postby hyksos on August 11th, 2017, 4:39 pm 

Kneejerk answer:
They are made out of mass, you silly goose. Why are you asking such a stupid question?

Not so fast.

As it turns out, in superstring theory, the masses of fundamental particles are said to scale with the tension of the string. It is not the case that various particles are made out of heavier and lighter strings (like thick and thin wires), and therefore their mass is the "amount of string" present. Instead, the mass is determined by converting the amount of potential energy in the tension (T) of the string into mass using the conversion ,

m = E/c2

The various masses which accompany each fundamental particle are the various tensions that string takes on dependent on its mode of vibration.

To visualize this, imagine winding a rubber band around the thin part of a doorknob, which is also a circle. If the string has a tension T = 1/alphaprime , (the conventional notation for the string tension), then winding the string once, twice, three times ... around a circle of size R, costs an energy:

Image

This is because the tension is defined as the mass per unit length of the string; and if we wind the string n times around the circle, it has a length which is n times the circumference of the circle. Just as a 9D experimentalist cannot see momentum in the 10th dimension, she also cannot see this string's winding number. Instead, she sees each of the winding states above as new elementary particles in the 9D world, with discrete masses that depend on the size of the compactified dimension and the string tension.


So what are strings made of? Since mass is the tension of the string (and not the "stuff which the string is made of") the question is pushed to the backburner and not answered directly.

We can now ask whether or not the mathematical dilettantes at Princeton ever bother themselves with this ontological question : what substance are these strings composed of?

Further, we might ask whether this question makes any sense at all to begin with.

Your thoughts?
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Re: What are strings made out of?

Postby Braininvat on August 11th, 2017, 5:44 pm 

I thought strings were postulated as fundamental. And thus, not composed of any deeper structural stuff. They are braids of space itself, curls of geometric nothing. String theories rely on compactification, where space curls up in tiny extra-dimensional conformations that, from the observer's vantage, show themselves as "point" particles. For me, it's all a bit difficult to swallow, and I find myself preferring to think of most of those 11 dimensions as mathematical conveniences rather than genuine snapshots of reality. There are areas of physics where the string does seem to enhance a visual model, though, as with the graviton.
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Re: What are strings made out of?

Postby hyksos on August 11th, 2017, 9:28 pm 

Braininvat , people on other websites have expressed misgivings about this 'tension' concept in super strings, considering the strings are not composed of regular matter. For instance, Lubos Motl points out that tension must exist in a string in order to sustain a coherent oscillation at all. Another way of putting it is that a string will not oscillate unless and until it is under tension -- just like what is seen in usual guitar strings.
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Re: What are strings made out of?

Postby BadgerJelly on August 12th, 2017, 1:29 am 

This is exactly the area where I feel many physicists are ill equipped to express theories to the layman. I have mentioned this before with how they teach at schools from a young age. Doing this embeds the idea of the "atom".

It is also a problem of language structure in general. We have to use prepositions and this further confuses things. For instance when we talk about electrons at different "energy levels" and being of "higher" or "lower" energy. The commonality of such terms often leads us down a blind alley where we think we can see the physical nature of things beyond our immediate physical exposure.

I remember when I was about 15-16 yrs old my sister asked me what I wanted for Christmas. I told her about a book I wanted about String Theory. She thought I was weird! I read it but admit it was very hard going, but the one thing I did realise was the ease with which analogies of physical phenomenon can be deeply misleading (in comparison with how Michio Kaku attempted to explain things in Hyperspace it exposed his lack of understanding of those who lack understanding!)
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Re: What are strings made out of?

Postby hyksos on August 12th, 2017, 1:49 am 

BadgerJelly ,

String theory was developed under the framework of the Standard Model. So (no.1) it is still a relativistic theory and (no.2) it is still a quantum field theory. So while points are replaced by 'strings' in the Feynman diagrams, all the usual hocus pocus of relativistic field theory remains intact. The strings still act according to path integrals, and still only do things probabilistically. That these are quantum mechanical strings is completely left out in Brian Greene's Elegant Universe. Better to just tell the audience that these strings really do move around in space like regular macroscale rubberbands. (of course they don't)
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Re: What are strings made out of?

Postby BadgerJelly on August 12th, 2017, 2:35 am 

At least today we have the means of the internet to present diagrams and visual presentations of topology to help people understand the underlying abstract nature of what we're talking about.

Here is a good example of the use of math (highly recommend this channel):

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Re: What are strings made out of?

Postby hyksos on August 12th, 2017, 7:16 pm 

BadgerJelly,

There is a funny section in Penrose's book where he is talking about the very early part of string theory. He said for a few years, they were taking seriously the idea that the manifold the strings vibrate in was a 5-dimensional torus.
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Re: What are strings made out of?

Postby mitchellmckain on August 14th, 2017, 3:00 pm 

I think the implication of M-theory especially when formulated as a 11 dimensional supergravity is that this can be eventually be cast into a perfectly geometric form like GR and thus we are ultimately talking about the vibrational modes of space-time itself.


BJ.... fascinating video posted, thanks.
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