the status of String Theory

[hyksos]

It is late June, 2018.

On this day, I will be investigating the status of String Theory. String Theory may not actually be a scientific "theory" , in the sense that it describes or predicts measured phenomena, but moreso acts as some sort of mathematical framework. String "Theory" started out as an attempt to describe hadrons, but interest in it exploded when it was realized (by accident) that it might be able to describe quantum gravity.

(more cynical tone ), String Theory is little more than mathematicians trying to convince each other that 11 dimensional spacetime with six extra 'compact' dimensions, is actually sensible. Doing such convincing requires "fiendishly complex mathematics" that easily covers rooms of chalkboards. When publishing, the papers come out as half-inch thick mathematical treatises mostly having something to do with "dualities" and other esoterica from algebraic geometry.

(Less cynical tone), String Theory may be the mystical gateway to a Theory of Everything or a TOE.

basic claims
String Theory initially required that spacetime have 26 dimensions. This was later reduced to 10, then bumped up to 11 with the discovery of D-branes. While we can easily confirm the 4 dimensions in regular life, we might ask where the other remaining six dimensions are. If the theory requires there be six more dimensions to space, then why can't we see them?

The question might sound like something that a high school kid might ask, who is unaware of the more nuanced aspects of the theory. This is wrong. "Why can't we see the extra dimensions?" is a rational question -- further it is in fact the entire pivot point about which String Theory turns as a discipline. The sweat and drama of String Theory is all about convincing one's colleagues that these extra six dimensions are "curled up" across distances near the Planck Scale -- i.e. that they are "compactified" dimensions curled into tiny geometrical objects called Calabi-Yau manifolds. A working string theorist is tasked with taking a piece of chalk , going to the chalkboard, and really explaining what that means, exactly. Such a task easily fills the chalkboard, fills the rest of the chalkboards in the room, and then spills across to the chalkboards in the room across the hall. (as we shall see later)

relation to existing Physics
String Theory bears little resemblance to quantum field theory or the Standard Model of particle physics. Instead, it is a framework that appears to have the power to describe a whole arboretum of various field theories. Or if, you will, a whole garden of varieties of Standard Models (plural). String Theorists bend over backwards in trying to find a limiting case which describes the universe we inhabit. In almost every scenario this involves presuming the universe we live in obeys supersymmetry, and the limiting case is invariable the low-energy limit of the theory. The test of bending, pruning, and preening String Theory to make it look like our universe is called String Phenomenology.

String Theory strongly suggests that no new physics arises between the scales of 10^-17 meters and down to 10^-35 meters. This claim is outlandish, and sours the theory with incredulity.

One might hope that a number of outstanding unsolved problems in physics would be addressed, or even laid-to-rest by string theory. Unfortunately, string theory is agnostic about which universe it is referring to in any of its 'varieties'. Thus, we don't get a straightforward answer about what Dark Energy is -- or which particle makes up Dark Matter, or why neutrinos have a tiny mass -- or what an inflaton is or the properties of the inflaton field. It says even less about what a Sterile Neutrino is. Ironically, sterile neutrinos are alleged to exist from experiment alone.

String Theory is surprisingly successful as a tool for describing the physics around black holes.

relation to Experiment
String theory has no experimental verification to date.

Worse, string theory has weaknesses even when formulating a valid experiment to falsify it. One way to test string theory (in principle) would be to discover lots of magnetic monopoles. If none of them are lighter than the Planck Mass, that would be indirect evidence that String Theory is correct. The problem is that 1. Planck Mass = 0.02 milligrams, and 2. only such monsters would be created during the Big Bang, and even then so rarely that there would be maybe 1 monopole in our local galactic group, if we're lucky. Such an experiment is duly ruled out.

relation to Mathematics
Ben Allanach is post-doctoral research physicist. He has been working at CERN for 20 years. His time there was dedicated almost exclusively to sifting the data from the apparatuses there and determining the signatures of supersymmetric particles. Learn more about him here http://www.damtp.cam.ac.uk/user/bca20/

Dr. Allanach has described string theory as being , quote "fiendishly complex mathematics". People of Mr. Allanach's education level do not toss phrases like that around for lunch. The take-away is that even the post-docs at CERN feel that String Theory is "fiendishly complex".

This ties back in to my earlier claim above, that String Theory (as a day-to-day task) is convincing your colleagues that curled-up 6-dimensional manifolds are perfectly reasonable things to be concentrating one's energies onto. Explaining that this makes sense at all is by itself difficult.

If we consider what Roger Penrose's criticism, it is possible to claim that String Theory is mathematics, period. Even its strongest acolytes admit that it's findings have bled over into pure mathematics, particularly algebraic geometry. Mathematicians work on String theory, coming particularly from backgrounds in topology and geometry. In the case of Mirror Symmetry, the physicists and mathematicians raced against each other to churn out theorems before the other 'side' found their own proof.

relation to Cosmology
We could reasonably expect that a Theory-of-Everything would go a long way in resolving aspects of our cosmos and its origins and the results of the Big Bang. For String Theory, the case is more nuanced and complex.

The universe we inhabit is excruciatingly flat, geometrically. It is also unusually uniform in direction. Cosmologists call this "Flat and isotropic". The flatness and isotropy of our universe is so bizarre that an equally bizarre explanation was floated called Cosmic Inflation. In that scenario a field called an Inflaton Field drove rapid expansion in the early universe. The situation we see now is that there are 3 very large dimensions of space, and string theory chiming in, 6 more dimensions who are all compactified at the Planck scales.

Inflation demands that the obvious 3 Ds were expanded at a rapid pace, but what of the other curled up, compactified Ds? Why were they not also expanded during inflation?

This sounds like another question asked by a high school kid who is not up-to-speed on the nuances. Again, no. This question is highly sound and rational. Like a thorn in its side, this question haunts String Theory, and heaps incredulity onto the existing pile of doubts.

The difference between a flat dimension 10²⁶ (at least) meters and a dimension curled up into a circle at 10^-35 meters is excruciating. This becomes even more excruciating if you accept Inflationary Cosmology at face value. In that case the second number is multiplied again by a factor of 3 x 10²³

A Theory Of Everything would explain these two numbers in a natural way.

(3 regular D) 3.0 x 10⁴⁹ meters
(6 compact D) 1.616 x 10^-35 meters

No explanation is forthcoming from String Theory, even while it requires this exact scenario to actually be manifest. In recent years, some have tried to wrangle the mathematics to make this disparity seem more reasonable, but the results are wonky.


Physics, science, or ??
I've already hinted that String Theory may not be even be physics. Worse -- is it even science?

Could String Theory even be something worse, like philosophy? Sabine Hossenfelder has sneered at it calling it "multiversal math magic".

How will String Theory be looked at 100 years from now?

250 years later, Albert Einstein, who was Newton's true successor, was able to seriously suggest that in this vast ocean , all the laws of nature might be reduced to a few fundamental ideas , expressed by a handful of mathematical symbols.

[mitchellmckain]

You do make a good point. To be sure, it is an hypothesis rather than a scientific fact.

It is far reaching enough to be called a theory though. Mathematical frameworks are generally given the name of theory, so that is nothing new. But you would be absolutely right to say that this is more of an extremely attractive idea than anything established by the objective evidence, which is what the work of science generally depends upon.

String Theory strongly suggests that no new physics arises between the scales of 10^-17 meters and down to 10^-35 meters. This claim is outlandish, and sours the theory with incredulity.

This claim however is rather dubious. The opposite can also said to be the case by the simple fact that string theory is badly failing to live up to its promise. The mathematics doesn't actually work, which strongly suggests that we are missing something important. In retrospect, therefore, the opposite conclusion may be considered reasonable, that string theory is telling us that there is more physics between those two scales.

[DragonFly]

From an old note that I have from somewhere many years ago:

In 3 (or fewer) dimensions, two wrapped (around the curled-up dimensions) strings will likely collide, half of which will be sting/antistring annihilations which lessen the usual dimensional constriction, allowing 3 of the dimensions to continue to expand and become the normal (uncurled up) 3-D space that we reside in.


Does this make any sense?

[hyksos]

From an old note that I have from somewhere many years ago:

In 3 (or fewer) dimensions, two wrapped (around the curled-up dimensions) strings will likely collide, half of which will be sting/antistring annihilations which lessen the usual dimensional constriction, allowing 3 of the dimensions to continue to expand and become the normal (uncurled up) 3-D space that we reside in.


Does this make any sense?

This is consistent with how this is usually treated. Suppose that at some time prior to the Big Bang, all dimensions were curled up, and then only three of them expanded. But this is not as simple as this note suggests.

[Watson]

Re: Math Quotes
Postby Watson on Tue Dec 16, 2014 11:27 pm

"Anyone can claim to do science.
Math is the proof it is done correctly."

Seems to me, the math is telling you something about the STRING THEORY science?

[mitchellmckain]

Seems to me, the math is telling you something about the STRING THEORY science?

String theory is complicated, of course. Some things work beautifully, other things do not.

For example, the vibrational modes do reproduce the observed particle spectrum. That is what is so hard to ignore.

However, the renormalization methods used in the Standard Model have never worked for any attempts to quantize gravity and the same applies to string theory -- even though that is precisely what string theory was developed to do. Therefore, the part where we would expect string theory to give the same kind of calculations as in the Standard Model but for gravity do not work. AND even to get as much of the renormalization as does work, we need these supersymmetric particles which have never been observed.

Other problems have been dealt with. At one time, a problem was that there were too many string theories. And then M-theory showed how most of them could be seen as parts/limits of a single theory.

So it is like having a puzzle where many pieces fit together nicely and form a pretty picture while other pieces don't fit and we still have a big gap right in the place we need in order to see if the theory is correct.

[mitchellmckain]

String Theory strongly suggests that no new physics arises between the scales of 10^-17 meters and down to 10^-35 meters. This claim is outlandish, and sours the theory with incredulity.

This claim however is rather dubious. The opposite can also said to be the case by the simple fact that string theory is badly failing to live up to its promise. The mathematics doesn't actually work, which strongly suggests that we are missing something important. In retrospect, therefore, the opposite conclusion may be considered reasonable, that string theory is telling us that there is more physics between those two scales.

Ok, so the fact that the vibrational modes of the strings reproduce the current point particle spectrum doesn't support the idea that these point particles such as electrons, neutrinos and quarks are really composites. But they are after all point particles in the Standard Model which works to an extremely high degree of precision, and so making these out to be composite entities is already hard to justify. String theory alters this to say they are not really point particles after all, but strings which are real really small. All in all, this reproduction of the particle spectrum just means string theory agrees with the physics of the Standard model.

BUT if you could somehow get around this and imagine some twist that made physics in an intermediate scale possible then I don't see how you can rule rule out this altering both the Standard Model and string theory. The real problem here, is that the whole suggestion of intermediate physics is rather nebulous without an actual working theory to test. Seems to me what you have here is an idea which is just as untestable as string theory but with far less going for it. Also since the mass paradox tells us that preons would have to be far more massive that the particles they "make," this would call into question whether they really should be considered more elementary after all -- they frankly sound more like some kind of resonance to me.

[hyksos]

For example, the vibrational modes do reproduce the observed particle spectrum. That is what is so hard to ignore.

All in all, this reproduction of the particle spectrum just means string theory agrees with the physics of the Standard model.

The Standard Model is not reproduced as cleanly as your claim suggests. String Theory is a large framework which contains our familiar Standard Model as a reduced case ( a "redux"). Our universe's Standard Model is one bush in a sprawling orchard of models. In situations in which String Theory is "agnostic" about some problem in physics, it is agnostic for this reason.

[Event Horizon]

I have a theory about strings. It could mean a fixed discreet size within a definable range.
Instead of being strings, I think they may be loops. It conserves energy, gives a definable minimum and maximum size, and they could then also rotate on their axis giving longitudinal spin allowing for more effects to investigate. It makes more sense than an open-ended packet of an unknowable quanta of energy due to being unquantifiable, having no particular size. A do-nut shaped string would also allow it to exist in all the classical dimensions, whereas a string may only have two dimensions.
I'm glad people are still thinking about this stuff.

[mitchellmckain]

I have a theory about strings. It could mean a fixed discreet size within a definable range.
Instead of being strings, I think they may be loops. It conserves energy, gives a definable minimum and maximum size, and they could then also rotate on their axis giving longitudinal spin allowing for more effects to investigate. It makes more sense than an open-ended packet of an unknowable quanta of energy due to being unquantifiable, having no particular size. A do-nut shaped string would also allow it to exist in all the classical dimensions, whereas a string may only have two dimensions.
I'm glad people are still thinking about this stuff.

There were 5 different string theories, some with closed loop strings and some with both closed and open strings, but all were shown to be compactifications of M theory which has higher dimensional objects than just strings. This also can reduce to an 11-dimensional supergravity (GR/Kaluza Klien with supersymmetry in 11 dimensions). The implication I see is that we may just have the vibrational modes of an 11-dimensional space-time itself. In other words, I suspect there may be a purely geometrical version in which all the particles, strings, and branes turn out to be mathematical artifacts of trying to quantify and construct equations for the vibrations of space-time.

[mitchellmckain]

For example, the vibrational modes do reproduce the observed particle spectrum. That is what is so hard to ignore.

All in all, this reproduction of the particle spectrum just means string theory agrees with the physics of the Standard model.

The Standard Model is not reproduced as cleanly as your claim suggests. String Theory is a large framework which contains our familiar Standard Model as a reduced case ( a "redux"). Our universe's Standard Model is one bush in a sprawling orchard of models. In situations in which String Theory is "agnostic" about some problem in physics, it is agnostic for this reason.

I am actually aware of that. But you cannot have it both ways. You cannot say the reproduction of the particle spectrum is both contrary to your feeling that there should be intermediate physics AND doesn't constitute agreement with the Standard Model. Yes, as a theoretical framework it is flexible enough to put the Standard Model in it, but then that cannot be a reason to also say it excludes intermediate physics. This is why I called your claim dubious, for there is no reason to say it isn't also flexible enough to put in some theory of intermediate physics you like to imagine.

[hyksos]

Yes, as a theoretical framework it is flexible enough to put the Standard Model in it, but then that cannot be a reason to also say it excludes intermediate physics. This is why I called your claim dubious, for there is no reason to say it isn't also flexible enough to put in some theory of intermediate physics you like to imagine.

We are now talking to each other in a vacuum of an internet forum.

Either string theory is

1. "flexible enough for intermediate physics"

or

2. string theory outright demands there cannot exist new physics from 10^-17 m to 10^-35 m.

(Given we are talking to each other in a vacuum, comma) my intuition tells me to lean strongly on 2 being true. Allow me to buttress my intuition with the following points. Imagine that string theory cannot accomodate new physics in that range of distance due to the fact that compactification puts huge constraints on the theories we could possibly produce.

These huge constraints are likely related to masses of the particles, so that theoreticians are stuck between a rock and a hard place. That is, either you compactify the extra dimensions down to the planck scale, and the familiar Standard Model pops out , with the masses of the particles matching experimental values

-- OR --

you don't compactify those extra dimensions, in which case you must now explain to your colleagues why we can't see them.

If those dimensions must be compactified at scale X, or else the "particle spectrum" is not recreated. Wherein that procedure is any accommodation for new physics? At the end of the day, you can't have it both ways. So you may be correct, that in spirit, the fundamental assumptions of string theory do not de novo remove such possibility of new physics -- but in practice, the theory must do so. Because the alternative would be explaining to your colleagues the obvious fact that these extra dimensions are invisible to us. In other words, it brings you back to square one.


I'm not being an irrational person here. Remember what I said about String theory. The bulk of the blood-sweat-tears that goes into this mathematics is the nagging explanation for why the 6 (or 7) extra dimensions are utterly invisible to scientific instruments. This task is not easy. You can't pull it off with a flick of a hat. You cannot do it with 5 lines of math -- or even 100. It takes 30+ odd pages of algebraic geometry to demonstrate the soundness of this idea.

I will further buttress my intuition with the following. A few working physicists have actually opened their mind to the extra dimensions being large (Lisa Randall for instance) And oh lookie! There is even a wikipedia article on it.

https://en.wikipedia.org/wiki/Large_extra_dimension

Now the next thing you're going to say is "Well I don't trust wikipedia so get that link outta my face.". That's fine. I feel ya. However, the said article contains 32 fully-fleshed citations. So ignore it at your own peril.

[hyksos]

For example, the vibrational modes do reproduce the observed particle spectrum. That is what is so hard to ignore.

For those of you passing through this thread, the above observation is by mitchellmckain.

By no means does String THeory neatly and cleanly reproduce the Standard Model. Below is a page taken from a recent textbook on string theory.

To be able to read this you need a small glossary
MIPF
modular invariant partition function. These are interesting to theorists because they are related to a matrix formulation of String Theory found by Leonard Susskind et al. The textbook is currently discussing how computers were used to brute-force search a bunch of matrices, in hopes of turning up something that matches the real world.

brane configuration
String theorists believe that the particles in the room you are sitting in are open-strings, whose ends are attached to these weird objects called D-branes. More exotic particles ( gravitons, inflatons, instantons) move freely around in the extra dimensions and are considered to be closed loops.

tadpole cancellations
Any good TOE will not contain divergences. These are places where theory explodes to infinity, or divides by zero, or other mathematical fouls. Tadpole cancellations are situations in which a Feynman diagram will contain a finely-balances cancellation, avoiding the divergence. Feynman diagrams are used to perform integrations in Quantum Field Theory, of which the Standard model is one example.

In other words, what you are reading is that the computer turned up about 124 million possible matrices that (may or may not) reproduce the Standard Model of particle physics. And even then, none of these contain tapdpole cancellations. Because that is so difficult they have to leave it for a different chapter!

In summary : String Theory does not neatly reproduce the Standard Model spectrum in any straightforward and obvious way. It is widely believed that it could reproduce the SM , but only with much mathematical sturm and drang. Theorists bend over backwards through a flaming hoop.

[hyksos]

(the exact textbook above will be furnished upon request.)

[mitchellmckain]

Like I said, you cannot have it both ways.

The same degree with which string theory has all these constraints which disallow a lot of intermediate physics is ALSO the degree to which it agrees with the Standard Model, in which the elementary particles are POINT particles not composite particles and therefore does not agree with the preon hypothesis which hyksos is so attracted to. But if you turn around and and push this idea that string theory is just a theoretical framework which could be made to fit anything then saying it opposes the idea of preons doesn't make sense.

AND I never made the claim that string theory "cleanly reproduces the Standard Model," that was a strawman hyksos has put up. All I said was that it reproduces the point particle spectrum of the Standard Model and this is what makes string theory so compelling. I can agree this doesn't sound so consistent with the idea of preons... unless you push the idea that this just means string theory is flexible enough to fit anything.

Regardless... let us remember from before we got sidetracked by hyksos' obsessive defensiveness against the possiblity that any part of what he says could ever be criticized in any way, that I quite agreed that string theory doesn't work (not renormalizable, i.e. cancelling divergences such as in tadpoles) and is little more than an attractive hypothesis -- but far far far far far more attractive than the preon idea of endlessly breaking things down into smaller and smaller (and more massive) parts.

[mitchellmckain]

P.S. What hyksos dug up shows that there is some flexibility in string theory as far as choosing the brane configuration. Thus it is not a matter of string theory predicting the Standard Model but one of choosing the setup so that it agrees with the Standard Model. D-branes are a way of using the open string versions of string theory, but remember that this does not mean scientists are asserting open strings are more correct than closed string, for these different versions of the theory are equally valid. To clarify, the D-brane picture would mean that in some of the 11 dimensions, the universe consists of the very small (plank scale) space between D-branes and all the particles in the universe would be represented by open strings which connect the two. In the usual closed string picture, the universe simply wraps around in those dimensions in a very small (plank scale) loop and thus the particles are represented by the closed loop type strings. In both cases it is a matter of dealing with the extra-dimension of the 11 in string theory which we do not see -- for we only see 4, that is 3 of space and 1 of time. The others are thus believed to be too small for us to see in one of the two ways suggested. Regardless, it is the basic thesis of M-theory that these different ways of doing string theory are mathematical simplifications of the same brane theory.

[mitchellmckain]

Before the unification into M-theory the five versions of string theory were as follows:

Type I Both open and closed strings which are unoriented. This one used Dbrane configurations to deal with anomalies.

Type IIA Oriented closed strings with left-right symmetry, involving areas of mathematics known as simplectic topology and algebraic geometry. The cancellation of anomalies was trivial in this case.

Type IIB Oriented closed strings but left-right asymmetric, involving the area of mathematics known as deformation theory. The cancellation of anomalies was non-trivial in this case.

The two Heterotic string theories have closed strings with left and right waves decoupled including both superstring waves in 10d and bosonic string waves in 26d. This requires compactification for dimensional consistency using a gauge group in 10d and the two possibilities give the two types of heterotic strings: SO(32) and E8xE8. Both gauge groups made the theories anomaly free.

As you can see only one of the five used open strings and D-branes. But since M-theory made a mathematical connection between all five theories, then that made the open string picture more interesting to scientists than they were before. But like I said it is not about saying that the open strings is the way thing really are, but simply that looking at them in this way might be useful.

I think what we are seeing in the choice of brane configurations of M-theory is that this unifying theory has considerably more flexibility than the five string theories it brought together.

[hyksos]

But like I said it is not about saying that the open strings is the way thing really are, but simply that looking at them in this way might be useful.

Yes.

Herein is an explosive example of communicating mathematical physics to a lay audience, wherein something crucial is lost in translation. mm is not suffering from this problem at all, since he just goes ahead and states what the physics says, which is fine. Rather the offender here is Brian Greene and his Elegant Universe.

This issue where "well if we look at it this way , it could be useful" is all over this type of unification physics. Brian Greene, in talking down for a lay audience, communicates rather like "It says reality is like X, period. Now shut up and accept it." This is unrealistic, but it is a tone that one must adopt to try to communicate it to a lay audience.

String Theorists decided to throw a long pass to win the football game in the final seconds. They threw a "Hail Mary Pass" (if you will) deep into the most abstract endzone they could find. The idea was to go extremely abstract, and then carefully and slowly step backwards into concreteness. The hope was stepping backwards would be like chipping away on a block of ivory to get a statue. If we consider "All possible universes which could exist" then chip away the detritus, we hoped that we would be left with a beautiful statue that is our universe.

The carving of the statue did not proceed directly to our universe. Worse, Sabine Hossenfelder feels the procedure is still hopelessly disconnected from reality -- she even called it "multiversal math magic".

P.S. What hyksos dug up shows that there is some flexibility in string theory as far as choosing the brane configuration. Thus it is not a matter of string theory predicting the Standard Model but one of choosing the setup so that it agrees with the Standard Model. D-branes are a way of using the open string versions of string theory, but remember that this does not mean scientists are asserting open strings are more correct than closed string, for these different versions of the theory are equally valid.

It's too flexible. If we exclude SU(3) and ignore tadpole cancellations, we can start with 124 million possible brane configurations to whittle those down to 115 million. Progress?

The fear is -- a theory which can predict anything, predicts nothing.

[mitchellmckain]

Sometimes talk about "the way things really are" is nothing but hot air. For example, is it the Ptolemaic system or the Copernican system which right. It is ultimately a meaningless question (without any implication of positivism either). These are just two different non-inertial frames of reference and thus we can look at the sun, Earth, and planets either way. The Ptolemaic system is more useful for seeing where the planets are in the sky of Earth and the Copernican system is more useful for calculations, general principles and when you leave the Earth surface. This is because they really are equivalent.

Likewise, after M-theory showed that the 5 different string theories were mathematically connected as simplifications of a single theory, then talk about which is "the way things really are" became meaningless. Then it only a question of which is more useful for what purpose. This is quite a different thing from philosophical position of instrumentalism. It has nothing to do with a claim that none of the theories have anything to do with reality. It is just that when different ways of doing things are shown to be equivalent as far as the physics is concerned, then their usefulness is all that really matters anymore when doing physics.

Oh... and a theory which cannot predict anything also cannot exclude anything. If it DOES excludes anything then that constitutes a prediction.

[hyksos]

Fine. But raw String Theory must be picked, shelled, washed, pruned, preened and primped before it even begins to look like a Field Theory.

Given that, would you agree that String Theory is more like a framework than a solid "theory"?

[mitchellmckain]

Fine. But raw String Theory must be picked, shelled, washed, pruned, preened and primped before it even begins to look like a Field Theory.

Given that, would you agree that String Theory is more like a framework than a solid "theory"?

The full term is "theoretical framework" which is synonymous with theory and it has more to do with scope than anything else. String theory has plenty of scope, what it lacks is completion and workability. So say rather that it is not a finished theory or a working theory. Also the real theoretical framework here is QFT, which makes no predictions at all without a model -- i.e. the Standard Model. String theory seems to be in between the two -- part theoretical framework and part model.

I guess it has much the same status as abiogenesis, which is also not a finished or working theory either. They are both great ideas and a work in progress. I certainly hope we will get there with abiogenesis very soon. String theory? Hard to say. Clearly something crucial is missing and there is no telling when someone might stumble across it. It might be tomorrow and it might be a very very long time. It is also quite possible that the breakthrough will come from an entirely new unexpected direction and even as a completely different theory (and only in retrospect will we realize that this new theory also resolves the problems of string theory). But notice this possibility totally supports the idea of setting string theory aside and looking for answers elsewhere.

[hyksos]

possible that the breakthrough will come from an entirely new unexpected direction and even as a completely different theory (and only in retrospect will we realize that this new theory also resolves the problems of string theory). But notice this possibility totally supports the idea of setting string theory aside and looking for answers elsewhere.

I read recently that any good TOE will address and lay-to-rest the problem of granularity of spacetime. The same article claimed that the strings of string theory are equal to the alleged spacetime granules.

I was skeptical of this claim. My understanding of this issue is that string theory has associated the strings with extant particles in the same way that certain local perturbations were associated with "particles" in QFT. In the opposing case of Loop Quantum Gravity, the granule is a "quantum of volume" in a spin network. Nevertheless, I am aware that in many publications inside string theory, there is less so concentration on recreating the particles, and instead researchers are far more interested in having the theory recreate the Vacuum State of QFT.

Posting history suggest that Lincoln is still active on this forum. It would be nice if he could chime in here. ( Well, a man can dream. )

[mitchellmckain]

I haven't studied as much of LQT as I have of string theory, but the basic idea sounds very reasonable -- it looks like a very logical way to go.

[hyksos]

( I don't mean to speak for Lincoln ... ) I expect that Lincoln would say something like this: There is nothing about theories of quantum gravity that suggest that there are little "space pixels" at some smallest resolution. Likely a popular misunderstanding.

( Hopefully this will elicit his participation.. )

[dandelion]

For a quick comparative view including consideration about empirical evidence, these slides may be helpful here-
https://pitp.phas.ubc.ca/confs/gravity2 ... ovelli.pdf

[hyksos]

For a quick comparative view including consideration about empirical evidence, these slides may be helpful here-
https://pitp.phas.ubc.ca/confs/gravity2 ... ovelli.pdf

The two most important slides here are, 1. the very last slide. and 2. slide 23 showing a grey "quantum region" sandwiched between two pink triangles of Minkowski.

If you will, imagine a situation in which a black hole is evaporating and losing mass. There will come a point in the far future where the black hole will reach some 'critically small mass' wherein it can no longer be a black hole anymore. Its remnants will decay away as regular particles of the standard model. There is an interval of mass where a transition from black hole ---> baryonic matter will happen.

What is this critically small mass? Do we know?

We do. It is the Planck Mass. You might think "Well that's super tiny isn't it?". It is definitely not. The Planck Mass is 0.02 milligrams. Literally the air molecules you are breathing now have mass less than Planck Mass. Salt grains smaller than this can be seen with the naked eye.

When an evaporating black hole gets nearby 0.1 milligrams and less something will happen to it. Something mysterious. The black hole will turn into something, of which we know not its nature. (Some physicists have dubbed this object a Liminal Being). General Relativity cannot tell us what happens. Quantum Mechanics cannot tell us what happens.

Only a genuine bonafide theory of quantum gravity can describe the physics of that object and what it does.


( I wanted to say some other things about "space pixels" and what we know about what that would mean in practice. But I will wait until these posts are digested. I still want Lincoln to post something here about space pixels. )

[Event Horizon]

If proven, it ceases to be a theory anyway. More immediately, if strings exist we should be able to develop a test so we know if we're all spending useful time on this or not.

[mitchellmckain]

If you will, imagine a situation in which a black hole is evaporating and losing mass. There will come a point in the far future where the black hole will reach some 'critically small mass' wherein it can no longer be a black hole anymore. Its remnants will decay away as regular particles of the standard model. There is an interval of mass where a transition from black hole ---> baryonic matter will happen.

What is this critically small mass? Do we know?

We do. It is the Planck Mass. You might think "Well that's super tiny isn't it?". It is definitely not. The Planck Mass is 0.02 milligrams. Literally the air molecules you are breathing now have mass less than Planck Mass. Salt grains smaller than this can be seen with the naked eye.

When an evaporating black hole gets nearby 0.1 milligrams and less something will happen to it. Something mysterious. The black hole will turn into something, of which we know not its nature. (Some physicists have dubbed this object a Liminal Being). General Relativity cannot tell us what happens. Quantum Mechanics cannot tell us what happens.

Only a genuine bonafide theory of quantum gravity can describe the physics of that object and what it does.


( I wanted to say some other things about "space pixels" and what we know about what that would mean in practice. But I will wait until these posts are digested. I still want Lincoln to post something here about space pixels. )

Where are you getting this? I don't think it makes any sense.

The rate of mass loss is inversely proportional to the mass. So if there was such a lower limit as you describe, it would have no relevance because the black hole would lose its remaining mass too fast for anything to happen. Thus it ceases to be a black hole because it ceases to have any mass and thus ceases to be anything at all. But I don't think what you say can be right, anyway. As far as we know (i.e according to GR), a black hole is a singularity. The mass density is infinite. Thus how can reducing the mass by any finite amount affect anything?

The suggestion of loop quantum gravity that black holes never form a singularity at all, is a much more interesting idea. In this case, the black holes "bounce," which means they are in a state of time dilation delayed collapse followed by explosion. Of course as they swallow more mass then this explosion gets delayed more and more. Also they still have something like an event horizon but it a little altered from what is described in General Relativity. In any case, it paints a very different picture of the end state of the universe with this added stage where all the black holes are exploding. I have a hard time envisioning is what sort of matter or energy would come out in such an explosion.

[hyksos]

Where are you getting this? I don't think it makes any sense.

http://rantonels.github.io/is-the-planck-length-the-minimum-possible-length/

[mitchellmckain]

Where are you getting this? I don't think it makes any sense.

http://rantonels.github.io/is-the-planck-length-the-minimum-possible-length/

Very good.

MP is the threshold between particle physics and gravity, being an upper bound on the mass of fundamental particles and a lower bound on the mass of a black hole. As a black hole evaporates down to the Planck mass from Hawking radiation, you would expect it to morph into particle(s) as it jumps this fence.

You should take note that Hawking radiation is already a case of the mass-energy of the black hole "jumping the fence," so this final disappearance of the black hole at this limiting mass isn't really a different process. But the identification of this mass limit is an interesting addition to the physics of Hawking radiation.