# What are strings made of?

Herringbone
At one time, I read where strings were vibrating bits of space-time. In "The Elegant Universe" TV program, they were described as vibrating bits of energy. In "The Elegant Universe" book they are described as consisting of fundamental "string stuff" and that questioning their composition really has no meaning.

OK... so now I'm (even more) confused.

## Answers and Replies

Ambitwistor
The book description is the closest to the truth. They are certainly not vibrating "bits of spacetime"; I don't even know what that would mean. They aren't "bits of energy", either; they have energy.

Strings aren't made up of anything more fundamental; they are the fundamental building blocks --- everything is made up of them. Strings have a size and shape and a tension, and that's about it for classical physical properties (neglecting technicalities like conformal fields, Chan-Paton factors, etc.).

Herringbone
Yeah, "vibrating bits of spacetime or energy" never made sense to me either.

The string's (incredibly huge) tension is an interesting property however. Tension in a classical string is created by the electromagnetic force. What is the force creating tension in a superstring?

Is there another fundamental force? It seems that the vibration of a superstring would involve either the oscillation of this force or the oscillation of the Calabi-Yau space dimensions.

Ambitwistor
Tension in a classical string is created by the electromagnetic force. What is the force creating tension in a superstring?

There isn't any; tension is fundamental.

Is there another fundamental force? It seems that the vibration of a superstring would involve either the oscillation of this force or the oscillation of the Calabi-Yau space dimensions.

The vibration of a string is pretty much just like the vibration of a classical string, postulating the "tension" as an elementary property of the string. String vibrations aren't due to any oscillation of space; they occur even in flat background spacetimes.

Loren Booda
Strings could consist of probability space, rather than spacetime.

Mike2
Originally posted by Herringbone

The string's (incredibly huge) tension is an interesting property however. Tension in a classical string is created by the electromagnetic force. What is the force creating tension in a superstring?

Is there another fundamental force? It seems that the vibration of a superstring would involve either the oscillation of this force or the oscillation of the Calabi-Yau space dimensions.

Tension is a force. Force is the gradient of a scalar potential field. Perhaps this is where the "background" potential comes in. Now, they suppose that the tension is constant along the string. But that seem more of a stipulation than a derivation.

See comments in the thread:

diff EQ on strings, check out the math

madcat11
Doesn't your definition of tension (with regard to strings) necessarily have to rely on the way in which the term is used?
For example, tension can be looked at as coming from without or from within the thing... it can be a force applied or a force inherent, the act of movement or the condition of the already moved or moving...

Or - and I think this is probably the closest to the norm where strings are concerned - it can be a measure of that which something already contains, i.e., condition of stretch, tautness, elongation, position, measure of vibration or even balance.

And sometimes, it's only sematics... we all know what we're talking about; it's just getting that concept across to someone else, right?

Herringbone
tension

Doesn't your definition of tension (with regard to strings) necessarily have to rely on the way in which the term is used?

The classical definition of tension that I'm thinking of is the internal force within something that act against a set of external forces working to pull the thing apart. For example it's the EM force providing tension in my guitar string that prevents it's breaking from the pulling force the guitar and I exert on it.

Which raises the question: can superstrings break under their tension and what happens if they do? The sound my guitar makes when I break a string isn't very musical.

There isn't any; tension is fundamental.

This seems to be the key point. The string tension force would appear to be the fundamental force since it is from the actions of this force that gives rise to all the other forces.

Ambitwistor

Which raises the question: can superstrings break under their tension and what happens if they do?

Strings have have a uniform tension (perturbatively), but they can break.

This seems to be the key point. The string tension force would appear to be the fundamental force since it is from the actions of this force that gives rise to all the other forces.

There isn't a "string tension force" in the sense that there is, say, an electromagnetic force: there is no force field permeating space. Tension is just one physical property of a string, like its length.

quartodeciman
"String stuff"?? Shades of Sagan! Remember "star stuff" from Cosmos? "We are made of star stuff." So, I guess star stuff is made of string stuff.

------

Ed Witten at Santa Barbara in 1996 answered the question as follows:

(an approximation)
---
In our theory, all matter is explained in terms of strings. Without a better theory, it makes no sense to ask then what a string is. It will probably take another half century to understand the present theory in a sensible way.
---

Here, you can listen to his whole talk and look at his overheads.

UCSB KITP public lecture "Duality, Spacetime and Quantum Mechanics" --->
http://online.itp.ucsb.edu/online/plecture/witten/

lethe

Originally posted by Ambitwistor
The book description is the closest to the truth. They are certainly not vibrating "bits of spacetime"; I don't even know what that would mean.

i m not quite sure how to interpret that statement either, but i sometimes say it too.

the reason is that i consider the geometry of spacetime to be a coherent state of gravitons. gravitons are stringy excitations, therefore spacetime is somehow a coherent state of string.

in this sense, i can think of a string as a bit of spacetime, no? i, like you, am not quite sure what to make of the statement. so this mean that spacetime is no longer a manifold? that certainly doesn t appear from the math.

what do you think?

madcat11
Re: tension

Since you can't experimentally 'pluck' strings, like you can a guitar string, you have to look at them indirectly.

Swartz and Scherk used the postulated properties of a graviton and its messenger particle to calculate that the particle's transmitted force is inversely proportional to its string tension. So you have the direct relationship of 1/(2pa') where a' is alpha prime and is equal to the square of the string length scale.

And since the graviton is so weak, the tension is enormous (actually the Planck tension or around 1039 tons...) And this huge tension means the string contracts to the Planck length (very very tiny...)

Also, high tension means high energy string. So a string's energy is determined by two things: its vibration and its tension.

So, if you have tension directly related to both length and vibration of a string, it becomes an inate part of the string's nature and not some outside 'input.' So maybe the analogy of a guitar string is inaccurate, since you have to pluck a guitar string to make it vibrate, as well as having to first string it up to give it the proper tension...

Herringbone
There isn't a "string tension force" in the sense that there is, say, an electromagnetic force: there is no force field permeating space.

The string tension “force” sort of reminds me of a pre-Einstein description of a force, which got me thinking.

My guitar string is essentially held together by the exchanges of photons between the atoms of the metal. If I were to cut one end of the string while it were under tension, the other end wouldn’t “know” it until a later time determined by the speed of light.

Since the tension in a superstring is fundamental and is not transmitted by a boson limited to the speed of light, when a string breaks is there a limitation as to how fast that information is transmitted across the entire string?

pelastration
I see a rubber band (elastic) on my table. I am amazed about i's level of activity. Maybe it's due to the air pressure in Europe but it doesn't move or osccilates at all. The rubber band lays still. Like in a relaxed state.
Now on the most basic Witten level this must be different? Amazing! It's almost like magic.
Is it also different in the States?

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Ambitwistor
If you postulate a relativistic string, which is what is done in string theory, then influences cannot propagate faster than c.

In fact, effects on a string propagate at a speed

$$v = \sqrt{\frac{\tau}{\mu}}$$

where &tau; is the tension and &mu;=m/L is the mass per unit length. But in string theory, the tension is equal to the energy per unit length, so

$$\tau = E/L = mc^2/L = \mu c^2$$

and thus

$$v = \sqrt{\frac{\mu c^2}{\mu}} = \sqrt{c^2} = c$$

pelastration

Originally posted by Ambitwistor
The book description is the closest to the truth. They are certainly not vibrating "bits of spacetime"; I don't even know what that would mean. They aren't "bits of energy", either; they have energy.
And how do they get energy? For a mobile phone?

Originally posted by Ambitwistor
Strings aren't made up of anything more fundamental; they are the fundamental building blocks --- everything is made up of them.
So they ARE?
That's it?

lethe
Originally posted by Herringbone
The string's (incredibly huge) tension is an interesting property however. Tension in a classical string is created by the electromagnetic force. What is the force creating tension in a superstring?

the whole thing becomes a lot less mystifying if instead of calling it "tension", you call it mass per unit length.

a particle has mass (which labels its irrep of the Poincaré group), so the string (which is going to replace the particle) should have a mass too, and a mass per unit length.

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pelastration
Originally posted by lethe
the whole thing becomes a lot less mystifying if instead of calling it "tension", you call it mass per unit length.

a particle has mass (which labels its irrep of the Poincaré group), so the string should have a mass too, per unit length.
So - as Ambitwistor said - "strings aren't made up of anything more fundamental" there was mass from the beginning. Nice

pelastration
Originally posted by Herringbone
At one time, I read where strings were vibrating bits of space-time. In "The Elegant Universe" TV program, they were described as vibrating bits of energy. In "The Elegant Universe" book they are described as consisting of fundamental "string stuff" and that questioning their composition really has no meaning.

OK... so now I'm (even more) confused.
Now Herringbone I think this is one of the best - fundamental questions - on this forum about string.
I could ask also 'How strings are created?', but that's even more fundamental magic.
After 35 years ST the experts don't know. They even say that's a question for half of the 21 Century. See any logic?

Herringbone
So, if you have tension directly related to both length and vibration of a string, it becomes an inate part of the string's nature and not some outside 'input.' So maybe the analogy of a guitar string is inaccurate, since you have to pluck a guitar string to make it vibrate, as well as having to first string it up to give it the proper tension...

Well, I agree that the guitar might not be the best analogy but after all the pop representations of string theory its kind of hard not to go there.

And aside from the obvious difference of open vs closed loops and fundamental string stuff, I can see some analogies, albeit extremely superficial. Now of course we're talking about one of those perfect guitars you find in physics books where there are no frictional losses, etc...

You don't actually have to pluck a guitar string to make it vibrate. Just increasing the tension quickly will start it vibrating as the string length decreases. This could be considered analogous to the energy of the Big Bang (?) causing strings under their enormous tension to shrink down to be contained by the curled dimensions which could have given them their initial "input".

The analogy is that as I add energy into my guitar string by turning the tuning peg, I'm causing the string to "shrink" down to be constrained by the space defined by the guitar's body which also causes the string to vibrate.

With no losses the guitar string would indefinitely continue to produce a spectrum of vibrations determined by the shape of it's guitar "space" as well as the length and tension of it's strings just as a superstring produces a spectrum of vibrations determined by the string's length and tension as well as the shape of the tiny Calabi-Yau space that constrains it.

pelastration
Originally posted by Herringbone
The analogy is that as I add energy into my guitar string by turning the tuning peg, I'm causing the string to "shrink" down to be constrained by the space defined by the guitar's body which also causes the string to vibrate.
What 'force' turns the peg and where is it fixed to make 'turning' possible?

pelastration
Herringbone,

I will explain more: Have you ever put in a bath (filled with water of course) a piece of wood with a screw inside and tried to screw that screw deeper inside the wood? It will keep you busy for about 10 years till the water left. You can start this experiment today .

Herringbone
What 'force' turns the peg and where is it fixed to make 'turning' possible?

Like I said, not the best analogy but I see it like this:

The source of energy that turns the peg is me. I create the "lil' Bang" for my guitar string. The tuning peg is fixed to the "space" of my guitar string's 'universe' (the guitar) which serves to constrain the final size of the string.

When I turn the peg a certain direction, I'm changing the guitar string's universe in such a way that the string's tension increases, its size decreases and it begins to vibrate.

But all this analogy stuff is probably getting off topic so...

Herringbone
I will explain more: Have you ever put in a bath (filled with water of course) a piece of wood with a screw inside and tried to screw that screw deeper inside the wood? It will keep you busy for about 10 years till the water left.

Hi pelastration,

Not sure I get your analogy but no problem with the experiment.

I'll just turn up the air conditioner until the water freezes and boom, the screw is screwed.

Or (providing the conditions didn't prohibit it) I'll just use an electric screwdriver that provides enough speed and torque to make to water seem like it was frozen (due to friction and inertia).

pelastration
Originally posted by Herringbone
Hi pelastration,

Not sure I get your analogy but no problem with the experiment.

I'll just turn up the air conditioner until the water freezes and boom, the screw is screwed.

Or (providing the conditions didn't prohibit it) I'll just use an electric screwdriver that provides enough speed and torque to make to water seem like it was frozen (due to friction and inertia).

but in fact ? Is that a Black & Decker your using? Those Baltimore guys come always with something dangerous! Electric tolls in water!
But of course the strings will also be limited in oscillations. What do they do at Kelvin zero?
On your second point: I am rather sceptic. Maybe you should make from this 'gedanken' experiment a real bathroom experiment . To have more fun, you can add some extra soap to find some spacefoam bubble facts! Success. Keep us informed.

diverz
"Strings" are just manifestations of energy. Vibration is a form of energy in itself because energy by itself is formless and without mass.

All in all, everything in the universe is made up of energy. "Vibrations" and combinations of "Vibrations" make up the assortment of particles we see.

Thats my opinion.

What is mass? Mass feels solid isn't it? Why does it feel solid? Does solid itself have any meaning or is it just a word to describe the sensation we human beings feel when we touch something solid?

Ambitwistor
Originally posted by diverz
"Strings" are just manifestations of energy. Vibration is a form of energy in itself because energy by itself is formless and without mass.

All in all, everything in the universe is made up of energy. "Vibrations" and combinations of "Vibrations" make up the assortment of particles we see.

I really don't like it when people treat energy (or mass) as some kind of substance that things are made out of. Energy and mass are just some physical properties that objects can have, among many others (charge, momentum, angular momentum, etc.)

Thats my opinion.

Mass feels solid isn't it? Why does it feel solid?

Matter in solid form feels solid. That's because the matter in your hand doesn't interpenetrate the matter in an object. And that's because the matter in your hand repels the matter in an object (due to fermionic and electrostatic repulsion).

diverz
Originally posted by Ambitwistor
I really don't like it when people treat energy (or mass) as some kind of substance that things are made out of. Energy and mass are just some physical properties that objects can have, among many others (charge, momentum, angular momentum, etc.)

Thats my opinion.

You don't like it just because you feel that energy and mass are just some physical properties? Then what do you feel that strings are made of?

Ambitwistor
Originally posted by diverz
You don't like it just because you feel that energy and mass are just some physical properties?

I don't "feel" that energy and mass are physical properties. Energy and mass are physical properties. Particles and strings have energy, but it's not correct to say that they "are" energy or are "made up of" energy; energy is just one of many physical properties that a particle or string may have.

Then what do you feel that strings are made of?

String's aren't made out of anything more fundamental, because they are fundamental. This is just like how quarks and leptons in the Standard Model aren't made out of anything more fundamental, because they are fundamental.

diverz
How sure are you that strings are fundamental? I mean people used to think that atoms are fundamental but discovered there's something more to that.

Ambitwistor
Originally posted by diverz
How sure are you that strings are fundamental? I mean people used to think that atoms are fundamental but discovered there's something more to that.

There isn't any experimental evidence that strings even exist, let alone are fundamental. But in string theory, strings are fundamental. (If you try to break a string to see what it's made of, you just get two strings.)

quartodeciman
One has to start somewhere with something. Anything capable of generating all phenomena at higher levels must have an intricate set of attributes built into it, a spectrum of potentialities. Someone else can demand furthur explanation of the basis for all these attributes. But it isn't likely anyone can provide a significant theory that offers explanation all the way down.

------
"The world is held up by the trunks of three giant elephants."
"What holds up the elephants?"
"The elephants are standing on the shell of an even larger tortoise."
"What does the tortoise stand on?"
"It stands on the shell of yet an even larger tortoise."
"What does..."
"Sorry, it's tortoises from here on."

diverz
Originally posted by Ambitwistor
There isn't any experimental evidence that strings even exist, let alone are fundamental. But in string theory, strings are fundamental. (If you try to break a string to see what it's made of, you just get two strings.)

Ok I agree with you that strings are fundamental in string theory. The point I'm trying to make is if strings can be broken down further, its not fundamental anymore. However, we can stop this discussion at this point because there's no experimental evidence.

Now I have another question. Do you think energy can be broken down into smaller parts?

Ambitwistor
Do you think energy can be broken down into smaller parts?

Energy isn't an object that can be broken into parts; your question makes no more sense than asking whether, say, momentum can be "broken into parts".

diverz
What is defined as being fundamental? If I'm not mistaken, it means something that cannot be broken down further? Correct me if I'm wrong.