11 ms·

In a numerical coincidence, some see evidence for string theory

59 points by theafh 5y ago

kleiba 5y ago
The leading candidate for the fundamental theory of gravity and everything else, string theory ...unless you're in the "string theory == baloney" camp.
- deleted 5y ago
  [deleted]
PaulHoule 5y ago
My guess is that that quantum gravity is constrained by not just the holographic principle but similar invariants. Many things you calculate from string theory might well turn out to be also true in other theories not so much because string theory is right but because string theory is bound by the same constraints.
- me_me_me 5y ago
  Its worth mentioning that string theory went through quite few 'recalibrations', it feels like it string theorists are throwing stuff to see what sticks sometimes.
  - tejohnso 5y ago
    > sometimes Thank you for being generous :)
- rytill 5y ago
  Could such a statement be tested? What are the parts of string theory that are unique to it and not just invariants that apply to a large basket of theories?
  - PaulHoule 5y ago
    One thing I'd point to is unitarity, which is essential to any kind of quantum mechanics. https://en.wikipedia.org/wiki/Unitarity_(physics) https://en.wikipedia.org/wiki/Unitarity_(physics) (For my PhD thesis I found out that you didn't need an orthogonal basis set to do quantum mechanical calculations but rather you could do it just fine with an overdetermined basis set such as coherent states over p and q so long as you had resolution of unity over the basis set.) If there is unitarity than there is no information loss in a black hole. If that's the case, the controversy that Stephen Hawking promoted about information loss shouldn't have been controversial -- in fact it held back progress in quantum gravity for 20-30 years which is a serious tragedy. There is no idea at all how to do quantum mechanics without unitarity so if you believe in information loss you are rejecting the possibility of quantum gravity. My take is that the classical picture of black hole interiors is entirely wrong, even though it is a difficult concept to formulate exactly, I'm pretty sure that what's on the other side of the event horizon is something completely different though there is no way we can know
    - ianai 5y ago
      "For my PhD thesis I found out that you didn't need an orthogonal basis set to do quantum mechanical calculations but rather you could do it just fine with an overdetermined basis set such as coherent states over p and q so long as you had resolution of unity over the basis set" Isn't this just a fact from the definition of R^n topologies? i.e. There are only n-basis vectors possible and any transformation of them which is still R^n will be linearly related to those original n-basis vectors. (It's been over a decade since LA, though, so I'm not 100% on my verbiage.) Put another way, any non-zero determinate/linearly independent set of vectors within R^n can have at most n-elements.
      - PaulHoule 5y ago
        ... but also the vector space can be infinite (like position q or momentum p) which are very familiar but you can take the trace integrating over both q and p to get the density of states. There is a community in chemical physics that does this all the time and here is a run-of-the-mill paper that uses the method. https://www.researchgate.net/publication/263170709_Wavepacket_dynamics_and_quantum_mechanical_energy_densities_in_the_quartet_N_2_O_2_system https://www.researchgate.net/publication/263170709_Wavepacke... Roughly a wavepacket which is localized in both q and p propagates on a path similar to a classical particle located at q, p and you can use trick to do quantum mechanical calculations based on the classical mechanics. It is not a method of investigating the "classical quantum correspondence" but rather a method of calculating QM quantities.
- immibis 5y ago
  Are there other theories which meet the same constraints?
- civilized 5y ago
  Yes, and characterizing that common constraint seems more likely to be a fruitful path than assuming this coincidence validates string theory.
platz 5y ago
> They simply add a series of possible “corrections” to general relativity that might become important at short distances. Say you want to predict the chance that two gravitons will interact in a certain way. You start with the standard mathematical term from relativity, then add new terms (using any and all relevant variables as building blocks) that matter more as distances get smaller. How do you know what the correct values/labels are.
- peteradio 5y ago
  You can throw all sorts of interaction terms with unknown coefficients in there and the renormalization will pare down any inconsistent with large distance symmetries. The remaining unknown coefficients are equated to combinations of known and unknown physical constants, to find the unknown ones requires a new measurement (AKA massive international experiments or cosmological observation). edit: Weinberg explains: https://www.int.washington.edu/PROGRAMS/talent13/refs/weinberg_rg_good_thing.pdf https://www.int.washington.edu/PROGRAMS/talent13/refs/weinbe...
buscoquadnary 5y ago
My understanding was that has always been the basis for string theory the math works out very nicely and it is so beautifully self consistent, it is elegant, mathematically sound and ornate, it has everything anyone could ever want from a theory of everything, except for having any actual testable thesis, making any real predictions, and actually providing any real direction to anything besides funding committees.
- erdos4d 5y ago
  Perfect for long careers built on publication output.
- ChrisLomont 5y ago
  It has made some nice post-dictions: things spring out of that we do see, things not built into the model to start, which also lends credence.
- prvc 5y ago
  Without understanding any of the details whatsoever: if it does provide a simpler explanation of known phenomena only, shouldn't it then be adopted by Occam's Razor?
  - Out_of_Characte 5y ago
    Any theory has to have predictability, meaning your theory is more relyable in explaining every future phenomenon than the current working theory. without that your theory is as good as claiming 'god' or divine intervention at every corner.
  - ndsipa_pomu 5y ago
    With a similar lack of knowledge, I believe that although conceptually simpler, the maths is much harder to work with.
  - pfortuny 5y ago
    Yes but it does nothing of the sort…
TwoNineA 5y ago
Should be called "String hypothesis", not theory.
- civilized 5y ago
  This isn't how working physicists talk, though. The physics researchers who develop mathematical models for physics are called theorists.
- selectodude 5y ago
  A string hypothesis would imply that they've come up with anything that's actually testable.
- contravariant 5y ago
  I know people attempted to correct the definition of 'theory' to discredit the 'It's only a theory' argument, but claiming it's only called a theory when it's "proven" is an overcorrection. In fact no theories are "proven" per se, the better theories just have known limitations.
  - robocat 5y ago
    “A hypothesis is a tentative explanation that can be tested by further investigation. A theory is a well-supported explanation of observations. A scientific law is a statement that summarizes the relationship between variables.” - The Internet
    - contravariant 5y ago
      I think we can safely dismiss 'The Internet' as an authority on anything.
- dekhn 5y ago
  More like a String Idea.
jjgreen 5y ago
Woit's take on this: https://www.math.columbia.edu/~woit/wordpress/?p=12641 https://www.math.columbia.edu/~woit/wordpress/?p=12641
ogogmad 5y ago
A tear-down of this article by the Columbia physicist Peter Woit can be found here: https://www.math.columbia.edu/~woit/wordpress/?p=12641 https://www.math.columbia.edu/~woit/wordpress/?p=12641 It’s not about string theory or about conventional quantum gravity in four space-time dimensions. The topic is graviton scattering in maximally supersymmetric theories in ten flat space-time dimensions, and the argument is that the basic principles of supersymmetry, Lorentz invariance, analyticity and unitarity imply a bound on the coefficient of the lowest order correction term. The only relation to string theory is that a string theory calculation of this correction coefficient satisfies the bound (as expected, since string theory is supposed to satisfy the assumed basic principles). Much is made of the fact that in string theory one can get any value of the coefficient consistent with the bound. This is taken as evidence for the “inevitability” of string theory, but I don’t see this at all. It’s more accurately evidence for the usual problem with string theory: it’s consistent with anything. Also, see John Baez's comment: https://www.math.columbia.edu/~woit/wordpress/?p=12641#comment-239945 https://www.math.columbia.edu/~woit/wordpress/?p=12641#comme...
- dekhn 5y ago
  (I am a person who knows some physics but not a lot). If any of this stuff turned out to be true, what would the lab experiments and positive outcomes look like? I'm accustomed to particle accelerators and all sorts of other physical experiments, but I really struggle to relate these sorts of theories to anything that could be done in the lab, and if the theories were predictive, what sort of applied science could be done? No wide-eyed "we could make space elevators and travel FTL", just realistic extrapolation.
  - tsimionescu 5y ago
    One clear requirement for at least some versions of String Theory is Supersymmetry - the existence of supersymmetric partners to some of the particles in the SM at some energy level. This energy level is not really pinned down, except that it must be higher than anything we've tried so far (since we've been unable to find these particles). The sky is the limit on what is a maximal bound - probably all the way down to the Planck scale, or close to it. The same is true of the extra dimensions - those are detectable, but again only at energy levels that far exceed what is plausible not just for the near future, but probably for the entire future of life in the universe.
Diggsey 5y ago
> The trio specifically considered the range of values of α permitted by those two principles in supersymmetric 10D universes. The two main problems with string theory are supersymmetry and the fact that it requires 10 dimensions. This is a problem because we have no evidence for either of those things, and have to make up new physics to explain them away. So this paper says nothing more than string theory isn't inconsistent... with itself. Which it shouldn't be since the whole point of string theory was to come up with a consistent mathematical model that combines relativity and quantum mechanics.
r00tanon 5y ago
The more I read about string theory, including Brian Greene's books, the more it seems akin to arguing over how many angels can dance on the head of an n-dimensional pin. If anything good comes out of string theory research it will probably be the development of the mathematical machinery and the computational power that is needed to perform and analyze more experiments and the observational data we are gathering in the study of high energy events out in the cosmos.

59 points by theafh 5y ago

The leading candidate for the fundamental theory of gravity and everything else, string theory ...unless you're in the "string theory == baloney" camp.

deleted 5y ago
[deleted]

PaulHoule 5y ago

My guess is that that quantum gravity is constrained by not just the holographic principle but similar invariants. Many things you calculate from string theory might well turn out to be also true in other theories not so much because string theory is right but because string theory is bound by the same constraints.

me_me_me 5y ago
Its worth mentioning that string theory went through quite few 'recalibrations', it feels like it string theorists are throwing stuff to see what sticks sometimes.
- tejohnso 5y ago
  > sometimes Thank you for being generous :)
rytill 5y ago
Could such a statement be tested? What are the parts of string theory that are unique to it and not just invariants that apply to a large basket of theories?
- PaulHoule 5y ago
  One thing I'd point to is unitarity, which is essential to any kind of quantum mechanics. https://en.wikipedia.org/wiki/Unitarity_(physics) https://en.wikipedia.org/wiki/Unitarity_(physics) (For my PhD thesis I found out that you didn't need an orthogonal basis set to do quantum mechanical calculations but rather you could do it just fine with an overdetermined basis set such as coherent states over p and q so long as you had resolution of unity over the basis set.) If there is unitarity than there is no information loss in a black hole. If that's the case, the controversy that Stephen Hawking promoted about information loss shouldn't have been controversial -- in fact it held back progress in quantum gravity for 20-30 years which is a serious tragedy. There is no idea at all how to do quantum mechanics without unitarity so if you believe in information loss you are rejecting the possibility of quantum gravity. My take is that the classical picture of black hole interiors is entirely wrong, even though it is a difficult concept to formulate exactly, I'm pretty sure that what's on the other side of the event horizon is something completely different though there is no way we can know
  - ianai 5y ago
    "For my PhD thesis I found out that you didn't need an orthogonal basis set to do quantum mechanical calculations but rather you could do it just fine with an overdetermined basis set such as coherent states over p and q so long as you had resolution of unity over the basis set" Isn't this just a fact from the definition of R^n topologies? i.e. There are only n-basis vectors possible and any transformation of them which is still R^n will be linearly related to those original n-basis vectors. (It's been over a decade since LA, though, so I'm not 100% on my verbiage.) Put another way, any non-zero determinate/linearly independent set of vectors within R^n can have at most n-elements.
    - PaulHoule 5y ago
      ... but also the vector space can be infinite (like position q or momentum p) which are very familiar but you can take the trace integrating over both q and p to get the density of states. There is a community in chemical physics that does this all the time and here is a run-of-the-mill paper that uses the method. https://www.researchgate.net/publication/263170709_Wavepacket_dynamics_and_quantum_mechanical_energy_densities_in_the_quartet_N_2_O_2_system https://www.researchgate.net/publication/263170709_Wavepacke... Roughly a wavepacket which is localized in both q and p propagates on a path similar to a classical particle located at q, p and you can use trick to do quantum mechanical calculations based on the classical mechanics. It is not a method of investigating the "classical quantum correspondence" but rather a method of calculating QM quantities.
immibis 5y ago
Are there other theories which meet the same constraints?
civilized 5y ago
Yes, and characterizing that common constraint seems more likely to be a fruitful path than assuming this coincidence validates string theory.

platz 5y ago

> They simply add a series of possible “corrections” to general relativity that might become important at short distances. Say you want to predict the chance that two gravitons will interact in a certain way. You start with the standard mathematical term from relativity, then add new terms (using any and all relevant variables as building blocks) that matter more as distances get smaller. How do you know what the correct values/labels are.

peteradio 5y ago
You can throw all sorts of interaction terms with unknown coefficients in there and the renormalization will pare down any inconsistent with large distance symmetries. The remaining unknown coefficients are equated to combinations of known and unknown physical constants, to find the unknown ones requires a new measurement (AKA massive international experiments or cosmological observation). edit: Weinberg explains: https://www.int.washington.edu/PROGRAMS/talent13/refs/weinberg_rg_good_thing.pdf https://www.int.washington.edu/PROGRAMS/talent13/refs/weinbe...

buscoquadnary 5y ago

My understanding was that has always been the basis for string theory the math works out very nicely and it is so beautifully self consistent, it is elegant, mathematically sound and ornate, it has everything anyone could ever want from a theory of everything, except for having any actual testable thesis, making any real predictions, and actually providing any real direction to anything besides funding committees.

erdos4d 5y ago
Perfect for long careers built on publication output.
ChrisLomont 5y ago
It has made some nice post-dictions: things spring out of that we do see, things not built into the model to start, which also lends credence.
prvc 5y ago
Without understanding any of the details whatsoever: if it does provide a simpler explanation of known phenomena only, shouldn't it then be adopted by Occam's Razor?
- Out_of_Characte 5y ago
  Any theory has to have predictability, meaning your theory is more relyable in explaining every future phenomenon than the current working theory. without that your theory is as good as claiming 'god' or divine intervention at every corner.
- ndsipa_pomu 5y ago
  With a similar lack of knowledge, I believe that although conceptually simpler, the maths is much harder to work with.
- pfortuny 5y ago
  Yes but it does nothing of the sort…

TwoNineA 5y ago

Should be called "String hypothesis", not theory.

civilized 5y ago
This isn't how working physicists talk, though. The physics researchers who develop mathematical models for physics are called theorists.
selectodude 5y ago
A string hypothesis would imply that they've come up with anything that's actually testable.
contravariant 5y ago
I know people attempted to correct the definition of 'theory' to discredit the 'It's only a theory' argument, but claiming it's only called a theory when it's "proven" is an overcorrection. In fact no theories are "proven" per se, the better theories just have known limitations.
- robocat 5y ago
  “A hypothesis is a tentative explanation that can be tested by further investigation. A theory is a well-supported explanation of observations. A scientific law is a statement that summarizes the relationship between variables.” - The Internet
  - contravariant 5y ago
    I think we can safely dismiss 'The Internet' as an authority on anything.
dekhn 5y ago
More like a String Idea.

jjgreen 5y ago

Woit's take on this: https://www.math.columbia.edu/~woit/wordpress/?p=12641 https://www.math.columbia.edu/~woit/wordpress/?p=12641

ogogmad 5y ago

A tear-down of this article by the Columbia physicist Peter Woit can be found here: https://www.math.columbia.edu/~woit/wordpress/?p=12641 https://www.math.columbia.edu/~woit/wordpress/?p=12641 It’s not about string theory or about conventional quantum gravity in four space-time dimensions. The topic is graviton scattering in maximally supersymmetric theories in ten flat space-time dimensions, and the argument is that the basic principles of supersymmetry, Lorentz invariance, analyticity and unitarity imply a bound on the coefficient of the lowest order correction term. The only relation to string theory is that a string theory calculation of this correction coefficient satisfies the bound (as expected, since string theory is supposed to satisfy the assumed basic principles). Much is made of the fact that in string theory one can get any value of the coefficient consistent with the bound. This is taken as evidence for the “inevitability” of string theory, but I don’t see this at all. It’s more accurately evidence for the usual problem with string theory: it’s consistent with anything. Also, see John Baez's comment: https://www.math.columbia.edu/~woit/wordpress/?p=12641#comment-239945 https://www.math.columbia.edu/~woit/wordpress/?p=12641#comme...

dekhn 5y ago
(I am a person who knows some physics but not a lot). If any of this stuff turned out to be true, what would the lab experiments and positive outcomes look like? I'm accustomed to particle accelerators and all sorts of other physical experiments, but I really struggle to relate these sorts of theories to anything that could be done in the lab, and if the theories were predictive, what sort of applied science could be done? No wide-eyed "we could make space elevators and travel FTL", just realistic extrapolation.
- tsimionescu 5y ago
  One clear requirement for at least some versions of String Theory is Supersymmetry - the existence of supersymmetric partners to some of the particles in the SM at some energy level. This energy level is not really pinned down, except that it must be higher than anything we've tried so far (since we've been unable to find these particles). The sky is the limit on what is a maximal bound - probably all the way down to the Planck scale, or close to it. The same is true of the extra dimensions - those are detectable, but again only at energy levels that far exceed what is plausible not just for the near future, but probably for the entire future of life in the universe.

Diggsey 5y ago

> The trio specifically considered the range of values of α permitted by those two principles in supersymmetric 10D universes. The two main problems with string theory are supersymmetry and the fact that it requires 10 dimensions. This is a problem because we have no evidence for either of those things, and have to make up new physics to explain them away. So this paper says nothing more than string theory isn't inconsistent... with itself. Which it shouldn't be since the whole point of string theory was to come up with a consistent mathematical model that combines relativity and quantum mechanics.

r00tanon 5y ago

The more I read about string theory, including Brian Greene's books, the more it seems akin to arguing over how many angels can dance on the head of an n-dimensional pin. If anything good comes out of string theory research it will probably be the development of the mathematical machinery and the computational power that is needed to perform and analyze more experiments and the observational data we are gathering in the study of high energy events out in the cosmos.