[HN Gopher] In a numerical coincidence, some see evidence for st...
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In a numerical coincidence, some see evidence for string theory
Author : theafh
Score : 49 points
Date : 2022-01-21 15:01 UTC (7 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| platz wrote:
| > 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 wrote:
| 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/weinbe...
| Diggsey wrote:
| > The trio specifically considered the range of values of a
| 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.
| kleiba wrote:
| _The leading candidate for the fundamental theory of gravity and
| everything else, string theory_
|
| ...unless you 're in the "string theory == baloney" camp.
| [deleted]
| jjgreen wrote:
| Woit's take on this:
| https://www.math.columbia.edu/~woit/wordpress/?p=12641
| PaulHoule wrote:
| 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.
| civilized wrote:
| Yes, and characterizing that common constraint seems more
| likely to be a fruitful path than assuming this coincidence
| validates string theory.
| immibis wrote:
| Are there other theories which meet the same constraints?
| me_me_me wrote:
| 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 wrote:
| > sometimes
|
| Thank you for being generous :)
| rytill wrote:
| 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 wrote:
| One thing I'd point to is unitarity, which is essential to
| any kind of quantum mechanics.
|
| 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
| tsimionescu wrote:
| > 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.
|
| I always understood that one of the hopes for quantum
| gravity is exactly to find a new theory that replaces
| quantum mechanics, hopefully a fully deterministic (and
| single-world) theory. So, if some possible property of
| quantum gravity somehow forced us to abandon unitarity,
| that actually seems like it might have been an enticing
| aspect to explore, not some obvious dead end. (Note: I'm
| using the term "quantum gravity" here to refer to any
| theory of gravity that matches the observed results of
| quantum mechanics, which might be a replacement for
| standard QM, not necessarily a theory consistent with QM).
| ianai wrote:
| "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 wrote:
| ... 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_Wavepa
| cke...
|
| 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.
| ianai wrote:
| Which is what makes simulating things within QM compute-
| able in finite time, too, as I recall.
|
| I wonder if unitarity could be tested with analogs to
| black holes like the sonic analog. Otherwise this seems
| testable in a weak sense.
|
| [edit] I just want to point out that, to my intuitive
| idea of how a black hole would work in string theory - it
| seems all the information would not be lost. It would be
| compressed to the event horizon along with everything
| else.
| civilized wrote:
| The article mentions unitarity and Lorentz invariance as
| the key constraints used in the "bootstrapping" method they
| used to calculate alpha "from accepted truths" and without
| relying on any new theory.
| TwoNineA wrote:
| Should be called "String hypothesis", not theory.
| civilized wrote:
| This isn't how working physicists talk, though. The physics
| researchers who develop mathematical models for physics are
| called theorists.
| contravariant wrote:
| 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 wrote:
| "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 wrote:
| I think we can safely dismiss 'The Internet' as an
| authority on anything.
| selectodude wrote:
| A string hypothesis would imply that they've come up with
| anything that's actually testable.
| dekhn wrote:
| More like a String Idea.
| buscoquadnary wrote:
| 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.
| ChrisLomont wrote:
| 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.
| pontus wrote:
| The entire point of string theory not being testable is not
| specific to string theory but rather to quantum gravity. Any
| theory of quantum gravity will only deviate from our current
| understanding (general relativity) at the Planck Scale. So, if
| one has an issue with string theory not being testable, one
| actually has an issue with pursuing any version of quantum
| gravity.
|
| The interesting thing about string theory that sets it apart
| from all other theories of quantum gravity is that it's
| actually not just a theory of gravity. Because of this, there
| is actually hope that string theory could produce nontrivial
| testable predictions at energy scales much lower than the
| Planck Scale. However, it turns out that teasing out these
| predictions is very nontrivial, primarily because of a
| structure called the string landscape. Basically it turns out
| that the equations of string theory, while being tidy at high
| energy, produce a multitude of solutions at low energy and,
| unless you know which solution to look at, making definitive
| predictions is hard.
|
| Normally the way around a situation like that is to make some
| observations and try to constrain which solution we should look
| at. Unfortunately our current best understanding of the
| landscape is only one-way; we can't currently constrain which
| solution to look at based on observational evidence. It's sort
| of like a one-way hash. If we can invert this 'hash function',
| we could in principle make measurements to constrain which
| solution we're in and at that point string theory would be
| falsifiable.
| tsimionescu wrote:
| The bigger problem is that some of String Theory's few
| predictions have in fact been falsified. Supersymmetry has
| been predicted twice already, once at LHC energy levels and
| once before. Those predictions turned out wrong, but instead
| of abandoning the theory, it was easily tweaked to make a new
| prediction at a slightly higher energy level that LHC can
| predict.
|
| The very fact that such tweaks can be made is a known problem
| with string theory as a component of physics.
| prvc wrote:
| 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 wrote:
| 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.
| naasking wrote:
| > if it does provide a simpler explanation of known phenomena
| only, shouldn't it then be adopted by Occam's Razor
|
| Yes, but it doesn't describe known phenomena only, it
| describes things we haven't seen, things we can't ever see,
| and can be tweaked so much that it can accommodate phenomena
| that we know can't be true.
| ndsipa_pomu wrote:
| With a similar lack of knowledge, I believe that although
| conceptually simpler, the maths is much harder to work with.
| pfortuny wrote:
| Yes but it does nothing of the sort...
| mypastself wrote:
| Prior to reading Brian Greene's _The Elegant Universe_ , I
| was unfamiliar with the details, but under the assumption
| that the string theory is sound. Greene is one of its chief
| popularizers (as well as a minor researcher), and writes with
| great clarity on the topic. I enjoyed the book.
|
| Yet I ended up becoming agnostic on the string theory after
| reading it. There seem to be some genuine contributions to
| mathematics, but those are side products. The theory has yet
| to make any correct predictions regarding physics and our
| understanding of the universe, and it is likely completely
| unfalsifiable.
|
| To this layman, a lot of it seem like a bunch of
| extraordinarily smart people twisting their minds (and our
| conception of the universe) into a pretzel attempting to make
| post hoc explanations of existing phenomena.
| ogogmad wrote:
| The maths isn't rigorous and well-defined AFAICT. There aren't
| any definitive equations either, as far as I know. However,
| there are some cool geometric observations like AdS/CFT.
|
| There are some very polemical critics of string theory like
| Peter Woit and Sabine Hossenfelder, who suggest that it's a
| complicated philosophy that started off simple and neat,
| encountered some problems, and then grew in complexity and
| vagueness.
| bopbeepboop wrote:
| erdos4d wrote:
| Perfect for long careers built on publication output.
| marcosdumay wrote:
| > math works out very nicely and it is so beautifully self
| consistent
|
| AFAIK, nobody knows if it's self consistent yet.
|
| Beauty is on the eye of the beholder. But I count "unsolvable"
| as a very ugly trait.
| ogogmad wrote:
| 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
|
| _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#comme...
| dekhn wrote:
| (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 wrote:
| 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.
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