Is the concept of strings still relevant in modern physics?

[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.

 

[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.

 

[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.

 

[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?

 

[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.

 

[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?

 

[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?

 

[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.

 

[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?

 

[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.

 

[diverz]

That means energy is fundamental according to the fact that it can't be broken down into simpler forms and particles aren't fundamental because they can be broken down further.

In string theory, it is established that strings are fundamental.

Hmm... Interesting.

Can 2 different things exhibit identical properties? Or maybe I should phrase it this way -----> Is it possible for strings and energy to be fundamental building blocks and yet be different at the same time since your view is that energy is a property of strings?

 

[diverz]

Ok then in the words of your argument, can you tell me what really is physical/object?

 

[FilipKunc]

i'm not a physicist, but i don't think that this question can be answered before we understand the whole string theory... in fact we might never answer this question, just like we can't explain "where did the uniwerse come from?"- we can only tell how it began, but not why it began, or where it came from...
i have a question: do you think that it's possible to unify space and matter (just like einstein unified space and time)...? we can't call something "the theory of everything" if it leaves space, in which everything takes place, and matter, that fills the space as two separate things. that's just my opinion.

 

[diverz]

Well I was just going to mention space.

What do you guys think of space? Standard human understanding/definition of space is that it is emptiness. Nothing. Absolutely empty.

Now my question to you is ----> How can something exist and yet is empty (no mass no substance no nothing... ie. a vacuum)?

This is getting interesting

I want to see what you guys think.

 

[diverz]

Perhaps I should phrase it this way ----> What makes space? Space just can't exist for the sake of existence isn't it? I know you guys are all rational beings who are into science. You don't just accept "its simply there" as an answer. Now try to answer my question and I'll see if I agree with your point of view.

 

[diverz]

Bump for Ambitwistor's point of view/comments

 

[pelastration]

Originally posted by diverz
Now my question to you is ----> How can something exist and yet is empty (no mass no substance no nothing... ie. a vacuum)?

1. See spacetime as unbreakable and elastic.
2. Let it penetrates itself.
3. A new quantum package (QP)is created with two separate but joined layers.
So it's still empty but the spacetime layers will oscillate locally in a different way.
4. Such QP's can interact and build up other QP's. They all are still empty.
5. A human is thus a house build by empty packages.
6. The human observer can see only other QP's that are resonant to his observing QP's system.

 

[hypnagogue]

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.)

Is there any theoretical limit to how small a string you can create by breaking a larger string? Or can the process theoretically proceed ad infinitum?

 

[hypnagogue]

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.)

This is an interesting take. However, it seems as if this position leaves us with a conglomeration of physical properties without a central object to "have" them in the first place. If there is no such thing as "substance," then what is the object that has physical properties, and by what mechanisms does it 'enforce' its ownership of these physical properties? (Sorry if that's a little metaphorical, but I can't think of a better way to phrase it for now.)

 

[Mike2]

Originally posted by diverz
Perhaps I should phrase it this way ----> What makes space? Space just can't exist for the sake of existence isn't it? I know you guys are all rational beings who are into science. You don't just accept "its simply there" as an answer. Now try to answer my question and I'll see if I agree with your point of view.

Of course reality must be reducible to logic itself. Physics must be derivable from the principles of reason alone. For otherwise, you are right, it only begs the question as to how the fundamentals came to be. A theory based on the existence of just some particle or field that is not itself justified only give us better engineering, but it is certainly not psychologically satisfying because it leaves questions unanswered. We will not stop until we can say that physics is the result of some description of logic. That is my effort here.

On http://www.sirus.com/users/mjake/StringTh.html#consider I show how the principles of logic and probabilities can be described graphically is some sort of "sample space". Then I show that we can impose a coordinate system on it. And then we can describe a type of string theory as being the propogation of some open "event" in sample space.

But this in itself does not answer your question, where did it all come from. The question reduces to how the manifold of space-time came into existence in the first place. I've read that no dimensionality can exist at a mathematical critical point where all partial derivatives are zero. But such a point is also unstable, any movement whatsoever will only accelerate in that direction. So it seems that the universe started from such a critical point. And the manifold of reality has been growing ever since. It's curious that general relativity predicts an expanding manifold of space-time.

 

[Jeebus]

I know its a bot of a "grey" answer but strings are not a predetermined definate in size so we should theorize that a string is in fact made up of smaller strings, and those strings are made up of even smaller strings and so forth. It just keeps going to an inherent fuzziness of open strings that automatically incorporate one of the key ingredients in string theory.

All of physical reality is made out of different states of the superstring. Roughly speaking, each vibrational mode of the string can be thought of as a point particle. Hence, one superstring gives rise to infinitely many local fermion and boson fields. All of the observed bosons and fermions can be cosidered as a vibrational mode of the fundamental superstring. It must be noted that the string is both constituent and interaction. Superstrings can be either open or closed.

I also think that strings could be made or from the inflation of the universe or Gravity differentiates and even symmetries -- but I use this term loosely because there is no given via axioms in theories.

 

[Mike2]

Originally posted by Jeebus
I know its a bot of a "grey" answer but strings are not a predetermined definate in size so we should theorize that a string is in fact made up of smaller strings, and those strings are made up of even smaller strings and so forth. It just keeps going to an inherent fuzziness of open strings that automatically incorporate one of the key ingredients in string theory.

It might be that closed strings can be made up of smaller closed string on some sort of membrane. If the smaller inside strings cancel at their boarders, then the result is the one larger string on the outside edge.

Maybe that's how strings interact, when one meet another they share a common boarder that cancels to leave only one larger string?

 

[S = k log w]

How can a string be elemental if it can be cut into smaller strings,
or for that matter if it can be cut? Wouldn't an absolute findamental be indivisable? If a string needed time or space in which to exist, then how can bit be fundamental? To say that it exists but is fundamental is rather like saying matter exists in the ether

 

[quartodeciman]

Strings would be fundamental if no matter what you did to them the only possible result were strings. Atoms were once considered the same way. It wasn't so much that atoms were pointlike in size. They simply were thought to be indivisible. As soon as it was realized that smaller bodies that were not atoms (electrons) could be removed from atoms, then atoms weren't fundamental any more.

 

[selfAdjoint]

But it is true that strings remain strings no matter what you do to them. If you cut them apart you have two strings. If you join two together you have one string. Twisting them, knotting them doesn't change their stringyness, and the whole of string theory accepts the unsplittible stringiness of strings. The theory may be wrong about that, but so far there has appeared no reason to think so.

 

[Mentat]

Hold on, I thought M-Theory postulated a minimum size (the Planck's size) for all physical entities...wouldn't this mean that you can't "cut" a string any smaller than it already is?

 

[quartodeciman]

According to Pat Schwartz:

""
The string tension in string theory is denoted by the quantity 1/(2 π a'), where a' is pronounced "alpha prime"and is equal to the square of the string length scale.
""
.
link -->
http://superstringtheory.com/basics/basic3a.html

T_string = 1/2πα' (string tension from length scale)

L_min ~ 2√α' (minimum length from length scale)

Then L_min ~ √(2/π)(1/√T_string)
.

So, is the minimum length bounded below or is the string tension bounded above?

------

bonus: lecture slides on branes.
The Physics of Branes by Sunil Mukhi -->
http://www.ias.ac.in/meetings/annmeet/68am_talks/smukhi/index.html

Branes are more fundamental than strings?

 

[Loren Booda]

String tension is to mechanical tension as Planck's constant is to angular momentum.

 

[FilipKunc]

isn't there something called "string coupling constant" which defines (correct me if I'm wrong) how easy it is for a string to divide itself into smaller strings? if strings have a tension, then it is possible to tear them apart, if one pulls hard enough ;).

 

[quartodeciman]

Also, open strings can wrap around compact dimensions, even multiple times. So I guess string length is variable.

The wrap count serves as a quantum number?

 

[lethe]

Originally posted by quartodeciman

The wrap count serves as a quantum number?

yup

 

[S = k log w]

Agnst

It has been historically true that one theory of what is the most fundamental conceptual unit of existence had later been challanged by another theory of something even smaller, (and/or larger). I would like to know an elequent 'theory of everything'. Is it possible that there is no fundamental elements/strings/whatever, nor 'theory of everything'? String theory poses one problem for me; when asked, why do strings 'exist'?, the answer, (please correct me if I am wrong), is that strings 'exist' because strings 'exist'. Before (I believe it was Einstein) all 'things' 'existed' in something called the ether (sp?). Einstein had asked, what is ether made from? Isn't saying that strings 'exist' because strings 'exist' like saying strings 'exist' in an 'ether'? That strings 'exist' because strings 'exist' is taking it on faith that strings 'exist' because strings 'exist'. For what reason does this not lead to existentalist angst?

Is there a mathematical proof that there is such a thing as a fundamental?

 

[Mike2]

Originally posted by S = k log w
It has been historically true that one theory of what is the most fundamental conceptual unit of existence had later been challanged by another theory of something even smaller, (and/or larger). I would like to know an elequent 'theory of everything'. Is it possible that there is no fundamental elements/strings/whatever, nor 'theory of everything'? String theory poses one problem for me; when asked, why do strings 'exist'?, the answer, (please correct me if I am wrong), is that strings 'exist' because strings 'exist'. Before (I believe it was Einstein) all 'things' 'existed' in something called the ether (sp?). Einstein had asked, what is ether made from? Isn't saying that strings 'exist' because strings 'exist' like saying strings 'exist' in an 'ether'? That strings 'exist' because strings 'exist' is taking it on faith that strings 'exist' because strings 'exist'. For what reason does this not lead to existentalist angst?

Is there a mathematical proof that there is such a thing as a fundamental?

We are looking for physical laws that are logical in every way, and which can be described by mathematics. You've seen Venn diagrams used to show how to construct AND's and OR's of logic. And these AND's and OR's can just as easily be describe in a sample space. These spaces can be parameterized with coordinates. And they look very much like the manifolds talked about in physics. AND's and OR's are included in both.

If we ever expect to find mathematical laws of physics that are logical in every way, then we should realize that they will be a description of how events grow in sample space.

We seem tantilizingly close to justifying the geometry of physics. The Action integral is proportional to the surface area of the world sheet. The Lagrangian is the generalized gradient and is equal to zero so that it describes a geodesic, etc. But they have no reason for this geometry other than to say it works. It might be possible to recognize these world-sheets as growing events in sample space, and the geodesics as the most probable direction of its growth. But this would take a leap of faith on their part to believe that there is a logical explanation for everything even if we don't know it yet. How can we escape the conclusion that physics is a mathematical description of logic when we impose the requirement of logic and mathematics on our physics to begin with?