Is string theory dead or still valid

[potato123]

Didn't someone invent a bunch of dimensions in order to make the theory compatible with other theories but are the extra dimensions right or correct.

 

[phinds]

Didn't someone invent a bunch of dimensions in order to make the theory compatible with other theories but are the extra dimensions right or correct.

Have you done any research on this? The short answer is that string theory is still as alive as it ever, although not being worked on as vigorously as it was some time back, mainly because after 40+ years it still has not produced any physics, just math.

 

[JorisL]

Didn't someone invent a bunch of dimensions in order to make the theory compatible with other theories but are the extra dimensions right or correct.

The dimension issue can be "easily" addressed through dimensional reduction (compactification).

Let me sketch some things that are essential to know about string "theory". ^[1]
First of all there isn't a single string theory. What we call string theory is basically a framework that can be used to build a lot of theories.
The 5 (superstring) theories are Type I, Type IIA and IIB, Heterotic SO(32) and $E_8 \times E_8$.

Superstring theory which includes fermionic states (an example of a fermion is an electron in the standard model) needs 10 space-time dimensions for consistency.
It is one of the first things one learns when looking at a textbook. ^[2]
Why did we start looking into string theory one might ask? String theory came about when researchers were looking for a theory describing the strong force. ^[3]
It was soon found out that the theory included a spin 2-particle which was believed to be the graviton.

Clearly it seems wrong to look at 10 space-time dimensions while every observation we make tells us there are 3 space dimensions + 1 time dimension. How can we resolve this? The idea was actually an old (around 1920) idea of Kaluza and Klein recycled.
Suppose the 6 extra dimensions are so tiny we cannot perceive them then we effectively have a 4D theory.
There are a few difficulties, first of all how do we know what the tiny dimensions look like (can it be a 6-dimensional sphere for example)?
Second, how does this influence the observable fields we derive within string theory?

At this last part we are stuck for a while now. The landscape of string theory contains a lot of possible solutions, the hard part is find the right one (or a family of those, who knows).

A lot of work is being done not with full blown string theory but with super gravity which is basically the low energy effective field theory following from string theory.
It seems that the solutions we are looking for break so much symmetry that we are unable to find fully explicit solutions (again really rudimentary) that are related to our universe.

[1] Forgive me being a little crude in terminology, however I don't believe its necessary to be as exact as possible.
[2] If you know about quantization etc. you can check one of the first chapters in Zwiebach's book. It first looks at bosonic strings which require 26 space-time dimensions, the (in)famous result $\sum_{n\geq 0} n =-\frac{1}{12}$.
[3] There's a lecture by Lenny Susskind in the video section in which he touches on this if I'm not mistaken.

 

[MacRudi]

Well spoken @JorisL
I think the most important in stringtheory is that we get now a vision and imagination of the nature of mathematics behind. We can see the light at the end of the tunnel how to describe mathematically the true nature of the universe and the smallest. And this with a covariant background and locality. Lagrangian physics will be then only mathematics for engineers in future.
I would go much further in the future:
We get now with stringtheory our intuitive thinking back like in classical physics and don't have to interprete anymore.
But this will take some more 20 years and 2 more revolutions like S duality and T duality I guess

 

[JorisL]

I want to add an addendum to my post by the way.

If I had the time to do so I would try to learn more about the "contenders" in the "race" to find a theory of quantum gravity. (the recent insight put Garrett Lisi's work on my radar for LQG the situation is a bit better as there's a full-blown textbook now)
What I mean by this is that we shouldn't bet all our money on one candidate theory.
Unfortunately there is little time (or that's the idea I get looking at the faculty over here) for professional physicists to expand in this direction.
It isn't made easier when you consider the level of mathematical sophistication involved.

We get now with string theory our intuitive thinking back like in classical physics and don't have to interprete anymore.

I'm more familiar with the work in supergravity which leads me to be cautious about the intuitive thinking.
For example orientifolds which are somewhat natural in full string theory become hard to grasp in supergravity as far as I can tell.
I haven't found a really good way to think about them so far.

Once you are entering the world of fields things get murky (for me at least). So intuition in the classical physics sense is not what I see in string theory.
What I do see is a framework that naturally includes gravity and is very powerful.

An example proponents often use are the extra dimensions (which is somewhat viable). But if you honestly think about it, isn't it a weird idea?
I'm working on finding an explicit dS solution of supergravity from compactification(in 6 dimensions as a toy model).
If I find such a solution one could wonder, could we have found the D=6 sugra without invoking these extra dimensions?
What would the difference be other than a simpler description in D=10?

 

[MacRudi]

For example orientifolds which are somewhat natural in full string theory become hard to grasp in supergravity as far as I can tell.
I haven't found a really good way to think about them so far.

I understand what you mean. I try to make my mind free of lagrangian Yang Mill imagination, when in Calabi-Yau orientifolds. (for me perhaps easier, because i was socialised with GR and not QT) I'm not really so far, as I hope to be in this intuitive thinking. I think there must be some more revolutions like S duality and T duality. But I "feel" that there will be in some years the point, where it will look easy and intuitive. It is only a feeling. I cannot prove or say why.

 

[kodama]

I want to add an addendum to my post by the way.

If I had the time to do so I would try to learn more about the "contenders" in the "race" to find a theory of quantum gravity. (the recent insight put Garrett Lisi's work on my radar for LQG the situation is a bit better as there's a full-blown textbook now)

can you elaborate on Lisi's work ? which text boook?

 

[JorisL]

can you elaborate on Lisi's work ? which text boook?

You can find some info on Garrett's work in the interview in the insights, all I know at this time is that it has to do with the $E_8$ Lie group.
There are some topics in the forum as well.

The textbook is on LQG, there is a pdf version on Rovelli's website. http://www.cpt.univ-mrs.fr/~rovelli/IntroductionLQG.pdf

 

[CB90]

Just before i start i just want you to know that i am just a simple mechanical tradesman and not a very intelligent person when it comes to these things, which is why i want to ask this question and to see your thoughts so be nice because it may seem silly to you all but its just an idea that i had.

So to my understanding that from this String Theory they found that everything in the universe is made up of these certain strings. Would it be plausible that everything has a certain string combination that forms it into what it is, such as a simple cup. Now if they had found a way to somehow map these combinations or make ups and obviously had the technology/ machines to be able to de construct a object/thing and re assemble it some where else clone it out of the string in the air.

Also if this was plausible and say it had/could be done i believe they wouldn't mess with it to much because if they alter to much string within our universe it could lead to an imbalance leading to maybe a black hole or even worse something that our minds cannot comprehend or they might use it but with a law of equivalent exchange, where you cannot clone/duplicate something but break something down and recreate it somewhere else but using the strings from the old object to replicate what you use to remake that object.

Also if this was one day possible it could change the world, we could even recreate our Ozone layer or prevent animal extinction. There would be endless possibilities.

It was just a random thought i had and i just wanted to get it out there and it most likely is impossible but the world wasnt made without dreams and ideas.

 

[phinds]

So to my understanding that from this String Theory they found that everything in the universe is made up of these certain strings. Would it be plausible that everything has a certain string combination that forms it into what it is, such as a simple cup.

What you misunderstand is that not "everything" is made from strings but rather fundamental particles are made from string (in string theory). Then atoms are made from fundamental particles, then molecules are made from atoms, THEN the things you are talking about (everyday objects) are made from molecules.

 

[CB90]

What you misunderstand is that not "everything" is made from strings but rather fundamental particles are made from string (in string theory). Then atoms are made from fundamental particles, then molecules are made from atoms, THEN the things you are talking about (everyday objects) are made from molecules.

Sorry if i haven't grasped it yet but like what you just said everything is started from string in theory because if things like everyday objects are made up of molecules, which are made up of atoms , which are made up of fundamental particles , which are made up of the strings. So wouldn't that make the strings the universes DNA and everything you see and feel made up of its own DNA (string uniquely pieced together to make the object or thing)

 

[phinds]

Sorry if i haven't grasped it yet but like what you just said everything is started from string in theory because if things like everyday objects are made up of molecules, which are made up of atoms , which are made up of fundamental particles , which are made up of the strings. So wouldn't that make the strings the universes DNA and everything you see and feel made up of its own DNA (string uniquely pieced together to make the object or thing)

Only in the same sense that a diamond and a lump of coal are identical because they are both made up of carbon atoms. I think we're just playing with words here. What I'm saying is that a diamond and a lump of coal are NOT the same but they are both made up of identical atoms and identical strings. There is no "diamond" string or "coal" string.

 

[CB90]

Only in the same sense that a diamond and a lump of coal are identical because they are both made up of carbon atoms. I think we're just playing with words here. What I'm saying is that a diamond and a lump of coal are NOT the same but they are both made up of identical atoms and identical strings. There is no "diamond" string or "coal" string.

i still don't a hundred percent get it because to me if its a physical object like a diamond its got to be made up of something, like i get that there isn't a diamond or coal string and iv never seen what strings look like under a microscope but in my simple mind your right that diamonds and coal arent the same but wouldn't it be that the atoms that form them are a different atom structure. which would mean the strings that make the atoms all be different to make the different atoms.

Thank you for making that a bit clearer and letting me express my mind. i probably should just stick to fixing stuff haha

 

[phinds]

i still don't a hundred percent get it because to me if its a physical object like a diamond its got to be made up of something, like i get that there isn't a diamond or coal string and iv never seen what strings look like under a microscope but in my simple mind your right that diamonds and coal arent the same but wouldn't it be that the atoms that form them are a different atom structure. which would mean the strings that make the atoms all be different to make the different atoms.

Thank you for making that a bit clearer and letting me express my mind. i probably should just stick to fixing stuff haha

No, you're still not getting it. First strings, if they exist, are WAY too small to see with any human device now or in the future. Second it is irrelevant to the strings what form the atoms take at a larger level, even a molecule, to say nothing of somethings a large as a thing that people can see with the naked eye.

Also, you skipped a step. Strings don't form atoms anyway, they form the elementary particles such as protons, neutrons, and electrons, that DO form atoms. The strings don't care how the elementary particles aggregate to form atoms and then molecules and so forth. Your whole attempt to tie strings to anything other than the formation of elementary particles is doomed. I suggest you read up on the "Standard Model".

EDIT: and by the way, I didn't even go into quarks, which are the elements of protons and neutrons, since I think this is all too complicated for you. As I said, read up on the Standard Model for some clarity.

 

[MacRudi]

Molecules are out of atoms
atoms are out of proton, neutron and electron
proton is out of quarks, gluons and axion
neutron is out of quarks, gluons and axion
here we have the first stage, where we come to strings as tiny tiny little wiggeling things
very small buildingblocks, so tiny that no one can see under any microscope
in the opposite think of the whole universe as one brane, which we cannot see behind the horizont in full size. And now think of many big branes that are many universes in a multiverse. All the tiniest and the biggest can be decribed in strings and branes, which can be the same. a string can be a brane and a brane can be a string. What a buildingblock is in tiniest can be the biggest thing like a whole universe. In the end we have lost the thinking of reductionism, when we describe mathematically the smallest and the biggest
that's stringtheory

 

[phinds]

Molecules are out of atoms
atoms are out of proton, neutron and electron
proton is out of quarks, gluons and axion
neutron is out of quarks, gluons and axion
here we have the first stage, where we come to strings as tiny tiny little wiggeling things
very small buildingblocks, so tiny that no one can see under any microscope
in the opposite think of the whole universe as one brane, which we cannot see behind the horizont in full size. And now think of many big branes that are many universes in a multiverse. All the tiniest and the biggest can be decribed in strings and branes, which can be the same. a string can be a brane and a brane can be a string. What a buildingblock is in tiniest can be the biggest thing like a whole universe. In the end we have lost the thinking of reductionism, when we describe mathematically the smallest and the biggest
that's stringtheory

I really think it is not helpful to this discussion to bring in branes and speculative multiverse theory when the OP doesn't even understand the Standard Model yet. You're just getting too far ahead of where the discussion is.

 

[MacRudi]

I really think it is not helpful to this discussion to bring in branes and speculative multiverse theory when the OP doesn't even understand the Standard Model yet. You're just getting too far ahead of where the discussion is.

To get an impression of what we are talking in string theory it is good to talk about the smallest and the biggest and not only about the smallest, because then you only think in a buildingblock system like a particle physicist is doing. String theory is not a reductionistic buildingblock system like quantummechanics/particle physics. Every child should learn this at school from the beginning like different algebra we have - and not only one algebra. This is the revolution in thinking for mankind not to think in reductionism like it was with Einstein not to think euclidic and with relativity.

 

[Flyx]

Didn't someone invent a bunch of dimensions in order to make the theory compatible with other theories but are the extra dimensions right or correct.

We cannot currently detect the extra dimensions, but M-theory makes a lot of predictions that can be tested. At the moment, it is still valid.

 

[haushofer]

And if we don't detect these predictions, we explain this by adjusting the parameters of the theory :P

 

[MacRudi]

And if we don't detect these predictions, we explain this by adjusting the parameters of the theory :P

Same problem was with Einsteins theories. Completely out of sight to detect that the clock of big ben 20 meters above is running faster than the watch on the ground in big ben. 1 second in a million years.
It is now a much powerful framework we have and directly attached following to the work of Einstein-Schrödinger theory in the 1930-1954 years and Kaluza Klein in the same time. If Richard Feynman would have lived much earlier in this time, then we wouldn't have all the lost years inbetween from the 1960s to now with QM as framework. We have now only a correction in physics with string theory as framework in the tradition of Einstein thinking. That's all.

 

[haushofer]

I'm not sure what you mean with "Einstein's theories". GR? In that case I don't agree; that theory only contains one parameter kappa, which is fixed by the correspondence principle.

 

[MacRudi]

I'm not sure what you mean with "Einstein's theories". GR? In that case I don't agree; that theory only contains one parameter kappa, which is fixed by the correspondence principle.

oh I understand. You mean because of the 10 ^500 universes as parameters. Parameters are different to these 10 ^500 universes. If we would want a SUSY for QM then we would have over 200 constants as parameters and not less than 50. In stringtheory we have only very few, if any at least. I was talking about predictions. It was impossible and unimaginable to detect, that the clock 20 meters above is running faster than the clock on the ground. For the physicists in this time highly speculative

 

[rootone]

... It was impossible and unimaginable to detect, that the clock 20 meters above is running faster than the clock on the ground

but not impossible to detect that something strange and inexplicable with current theories was happening to the orbit of Mercury.
GR successfully explained this.

 

[JorisL]

Moduli stabilization remains an active issue.
When scanning for dS vacua in string theory one problem is that we don't keep any SUSY complicating the result.
Another is that often there are quantum corrections which we cannot calculate.

Also the 10^500 is an old, very crude number based on Calabi-Yau, flux compactifications. This is a small subset of solutions which cannot be de Sitter as a matter of fact since CY three-folds are ricci flat. (Ricci curvature scalar of internal manifold should be negative)
More on the landscape

haushofer's first reply was a bit tongue-in-cheek by the way (if I'm not mistaken)

 

[MacRudi]

Moduli stabilization remains an active issue.
When scanning for dS vacua in string theory one problem is that we don't keep any SUSY complicating the result.
Another is that often there are quantum corrections which we cannot calculate.

Also the 10^500 is an old, very crude number based on Calabi-Yau, flux compactifications. This is a small subset of solutions which cannot be de Sitter as a matter of fact since CY three-folds are ricci flat. (Ricci curvature scalar of internal manifold should be negative)
More on the landscape

haushofer's first reply was a bit tongue-in-cheek by the way (if I'm not mistaken)

we can have different opinions on that. Witten is not of fan of landscape topologies. It is an open field, we need to examine more detailed. But that we have the possibilities to think in that ways is altogether stringtheory, we wouldn't have in QM. This is showing the powerfull framework we have now

 

[JorisL]

I agree, I'm working with group manifolds and the coolest thing happened, I reduced the possible manifolds that might allow dS vacua to 4 (compactification to D=6, trying to find a toy model).
It turns out I get almost exactly the same equations (unsolved for now) for each manifold. This suggests something deeper could be happening but I haven't seen it yet. (my advisor and his collaborators has a hunch but its far from conclusive)

That's the deal with string theory and supergravity, often we have a feeling and need to investigate more but going deeper takes time (which I no longer have).

I'd say the answer to the OP is that it's not dead but there are some issues we don't fully understand.

And my opinion wrt other candidates for a theory of quantum gravity is that we need some authorities that know two of the theories very well.
Who knows one could use ideas from other theories in whatever flavour they like best.

 

[MacRudi]

Similar to my opinion. We need a 3rd and fourth revolution. I guess also, we will get these inspirations from complete different fields to string theory. We will see.

 

[haushofer]

I agree, I'm working with group manifolds and the coolest thing happened, I reduced the possible manifolds that might allow dS vacua to 4 (compactification to D=6, trying to find a toy model).
It turns out I get almost exactly the same equations (unsolved for now) for each manifold. This suggests something deeper could be happening but I haven't seen it yet. (my advisor and his collaborators has a hunch but its far from conclusive)

That's the deal with string theory and supergravity, often we have a feeling and need to investigate more but going deeper takes time (which I no longer have).

I'd say the answer to the OP is that it's not dead but there are some issues we don't fully understand.

And my opinion wrt other candidates for a theory of quantum gravity is that we need some authorities that know two of the theories very well.
Who knows one could use ideas from other theories in whatever flavour they like best.

Then you surely must have seen this one,

https://arxiv.org/abs/1804.01120

We present a brief overview of attempts to construct de Sitter vacua in string theory and explain how the results of this 20-year endeavor could point to the fact that string theory harbors no de Sitter vacua at all. Making such a statement is often considered controversial and "bad news for string theory". We discuss how perhaps the opposite can be true

To me, it becomes a bit suspicious how every time something initially bad for string theory turns out to be good. Just sayin'.

 

[jtlz]

Is it true non-perturbative string theory or M-theory cannot be reduced to QFT except in some limits? Why?

What are the most complicated forces/new fields of nature that QFT can be valid and where it is not (then M-theory or something more complicated takes over)? Any reference of the limitations of our vintage QFT?

 

[phyzguy]

We cannot currently detect the extra dimensions, but M-theory makes a lot of predictions that can be tested. At the moment, it is still valid.

Can you name any testable predictions made by M-theory and tell us how the results compare to observations? I would not consider it "valid" until it is an actual theory where one can do calculations and compare the results to observations. I do not think that this is yet the case.

 

[Fra]

To me, it becomes a bit suspicious how every time something initially bad for string theory turns out to be good. Just sayin'.

I think this is another manifestation of "duality". There is no such thing as, good or bad, it is all just a matter of choice of background context ;-)

/Fredrik

 

[phyzguy]

It seems to me that the name "string theory" is a misnomer. A theory in physics is a model where I can do calculations and compare the results of these calculations withe experiment. SInce this isn't (yet) the case in string theory, I think it would be more appropriate to call it the "string concept" or the "string idea".

 

[PeroK]

It seems to me that the name "string theory" is a misnomer. A theory in physics is a model where I can do calculations and compare the results of these calculations withe experiment. SInce this isn't (yet) the case in string theory, I think it would be more appropriate to call it the "string concept" or the "string idea".

... or "string thing"?

 

[haushofer]

It seems to me that the name "string theory" is a misnomer. A theory in physics is a model where I can do calculations and compare the results of these calculations withe experiment. SInce this isn't (yet) the case in string theory, I think it would be more appropriate to call it the "string concept" or the "string idea".

Well, string theory is a 2-dimensional (superconformal) quantum field theory. But quantum field theory in itself is a paradigm, not a theory. So I guess that's where the bad naming starts.

Regarding experiments: I think people doing AdS/CFT would disagree. String dualities are testable.

 

[phyzguy]

Well, string theory is a 2-dimensional (superconformal) quantum field theory. But quantum field theory in itself is a paradigm, not a theory. So I guess that's where the bad naming starts.

Regarding experiments: I think people doing AdS/CFT would disagree. String dualities are testable.

Quantum Field Theory can do detailed calculations that can be compared to experiment in exquisite detail. Things like scattering amplitudes and the electron or muon magnetic moment. It's not at all like string theory in that respect.

Can you name a "string duality" that has been tested against observations?