String theory predicts nothing?

[DaveC426913]

I've read a lot of things to this effect, that string theory predicts nothing, or that it can't be disproven, and as such, many theorists argue it is doomed.

How is this so? Is it because the scales proposed are so incredibly tiny that we can't measure that small, even in the future? Is there some principle that prevents the theory from making predictions or being testable?

 

[MistyMountain]

String theory predicts both everything and nothing at the same time.

It would be nice if it predicted something, but it doesn't predict anything. :)

Those who say they understand it must understand everything and nothing at the same time.

All this is giving me a hedache, so I am going to let someone smarter answer your question.

 

[DaveC426913]

Yes, you didn't really shed much light on my question... :(

Any takers?

 

[selfAdjoint]

If you read the popular books by string theorists, you will see that they claim that the reason string theory doesn't predict anything is that the energy scale is so high we couldn't observe what they predict.
But this is actually not the end of the story. Foes of string theory, who have become very vocal recently, assert that string theory predicts nothing, period. That there is no physical fact, at any energy, which if observed would confirm or falsify string theory. String theory is all about how strings and branes and so on behave, but efforts to connect this with other kinds of physics have come up short. And with the recent discovery that string theory predicts googles or infinities of inequivalent vacua, only one of which coreesponds to our world, and gives no recipe for finding which one that is, you can see that the non-prediction stakes have been raised. This has led the critics to jeer louder and the string physicsits, unwisely in my opinion, to circle the wagons.
So there you have it.

 

[marcus]

Hi selfAdjoint, hello DaveC

that puts the situation in a nutshell---I would agree with sA about the wagons.

I'm becoming increasingly aware that there are alternatives to string which (although possibly less grand, elegant, attractive to some people) nevertheless DO make predictions.

so that we have the opportunity of proving some of these other theories wrong, if future empirical observations go against them. It is something to consider. the following is just my opinion.

PREDICTION ONE: Laurent Freidel and co-workers are developing a Quantum Gravity model that only works if there is a slight energy dependence of the speed of gammaray photons. Freidel's spinfoam QG is at the forefront of the spinfoam approach and (at least in the 3D case where it has been fully worked out) it REQUIRES this slight dispersion in photon speed. This should be observable by the GLAST satellite to be launched in 2007. I would say that, if GLAST does not observe a dispersion in Gammaray Burst photons, this will shoot down the leading spinfoam QG candidate.

A number of other QG approaches have also led their developers to expect some energy dependence---some type of DSR (deformed special rel)---and would be tend to be refuted along with Freidel's approach. I won't try to list all the authors and approaches that would be involved.

PREDICTION TWO: Loop Quantum Cosmology (LQC) as developed by Martin Bojowald has recently reached the stage where both the Bang and the Hole singularities have been removed and many of the researchers are saying that it LOOKS LIKE our Bang was really a Bounce.

On the BH side, they are saying that gravitational collapse leads to a Bounce. And on the BB side they are saying that our Bang was preceded by a contraction that looks mathematically similar to a BH collapse.

This was discussed a lot by the Friday papers at the October Loops '05 conference. It is looking like, in a Loop Gravity context, there is really only ONE thing, a bounce. And it makes a new region of the universe BRANCH OFF everywhere there is a black hole.

What this indicates to me is that a TYPICAL region of the universe is going to be one whose parameters are optimized for producing lots of black holes since such regions have more offspring. So this is NOT a hard and fast prediction, but it is looking increasingly likely that Loop Gravity implies that the parameters of cosmology/standard model are typically optimal for BH formation

So it challenges you: Can you find any parameter which if it were slightly changed would make BHs more abundant? Loop Gravity suggests that you can NOT find such a parameter. If you can, it would tend to refute Loop Gravity.

THIS IS CALLED cosmological natural selection, or the black hole natural selection hypothesis and IT IS ALREADY BEING TESTED. Because it predicts a nice low limit on the mass of neutron stars. So every time another neutron star is found, and astronomers measure its mass, we have a test. If they find one that is 2 solar masses or more, that would kill the idea (judging by what I've read) and even less, like 1.7 solar masses, if they were very sure of it.

-------------------------------------

so it is like deciding what girl to take to the prom. String is rich and has a nice car. But with Loop, even though she might not be so impressive at first sight, there might be more chance of empirical verification.

 

[alfredblase]

Hmmm, ok, String Theory will eventually come up with something that QM does not as they are different theories.

If ST is not correct I should still think that QM is a false theory because one of QM's axioms is that all fundamental particles are point particles, i,e that they have no spatial dimension. This cannot be true as then massive fundamental particles would have infinite density. Rather what QM really states is that fundamental particles are so small that their dimensions can never be measured. This is no different to what ST does.

In conclusion, in choosing between taking a beautiful, if mysterious and rather unaproachable girl to the prom over an ugly, hypocritical slut, I choose the former (being ST) every time. =)

 

[DaveC426913]

I appreciate the info Marcus, but with all due respect, this thread is not about alternative theories. I'm interested in the failing of string theory.


SelfAdjoint:
"...string theory predicts nothing, period. That there is no physical fact, at any energy, which if observed would confirm or falsify string theory. "

How is this possible? Surely with enough probing and enough time (decades? centuries?) we should be able to confirm of falsify aspects if it?

Are opponents saying " there's no practical way in the near future" or are they saying "there is no way in principle" that we will be able to confirm or falsify string theory.

 

[Careful]

**
If ST is not correct I should still think that QM is a false theory because one of QM's axioms is that all fundamental particles are point particles, i,e that they have no spatial dimension. This cannot be true as then massive fundamental particles would have infinite density. Rather what QM really states is that fundamental particles are so small that their dimensions can never be measured. This is no different to what ST does. **

You do not have to await the final failure of ST to know that that hypothesis is (in the end) incorrect. By the way, string theory should first reproduce the full predictions of the standard model implying that it could be falsified at the level where QFT is still happy.

**
In conclusion, in choosing between taking a beautiful, if mysterious and rather unaproachable girl to the prom over an ugly, hypocritical slut, I choose the former (being ST) every time. =) **

Nah, the beautiful, mysterious and unaproachable girl will not want to go with you to the prom. It is better to sit at a table from which you can eat as opposed to one which is only covered with golden plates

 

[alfredblase]

You guys are so impatient All you think about is sex, as long its got two legs u want to do it now, "any hole's a goal" right? But man its well worth putting in the effort and waiting for that more special girl, cos u won't enjoy it otherwise. Let me explain:

You do not have to await the final failure of ST to know that that hypothesis is (in the end) incorrect. By the way, string theory should first reproduce the full predictions of the standard model implying that it could be falsified at the level where QFT is still happy.

You see, this illustates the whole thing, ST is still a very fledgling theory, despite all the funding it recieves, despite the decades people have been working on it. And that is because it is a very ambitious, complex and difficult theory. That doesn't mean its not well worth working and waiting for. Give it a proper chance man. U see this girl could show u tricks u never even dreamed of, she's got it all dude! hahaha

 

[Careful]

**You guys are so impatient All you think about is sex, as long its got two legs u want to do it now, "any hole's a goal" right? But man its well worth putting in the effort and waiting for that more special girl, cos u won't enjoy it otherwise.**

You seem to forget that in order to appreciate when a girl is good, you first have to ride in many test models.

**
You see, this illustates the whole thing, ST is still a very fledgling theory, despite all the funding it recieves, despite the decades people have been working on it. And that is because it is a very ambitious, complex and difficult theory. That doesn't mean its not well worth working and waiting for. Give it a proper chance man.**

This is like the most silly excuse I have ever heard: (a) string theory is over thirty (b) did not produce one single physical result that is open to falsification (c) did not manage to reproduce the standard model (d) only reproduces GR perturbatively (while the strong field region is actually the interesting one) (e) an enormous amount of people are looking for it (f) despite of this lack of succes needs (i) extra dimensions (ii) supersymmetry (iii) calabi yau compactifications (with of course huge ambiguity here) ... (g) the landscape to cure the latter illness...

It seems to me that the girl you are waiting for isn't pretty at all, but an ugly tart which is covered inside a beautiful wedding cake

 

[alfredblase]

ok, I am going to stop discussing this with you, because your sentences are crap and dumb, and show a lack of intelligence and understanding.

 

[Careful]

ok, I am going to stop discussing this with you, because you don't seem to understand, and your arguments are ... failing to think of a better expression, your sentences are crap and dumb.

Really, what is objectively wrong with them ? Perhaps you could explain to us what *your* justification is for telling to people that ``string theory is the big catch´´ while it hasn't clearly lived up to any of the standards a physical theory requires. Actually, my view is that it is ok to do string theory, but the reason for it is negative alas (the lack of *clearly* superior alternatives).

 

[DaveC426913]

Please do not digress

Hello? I would like this thread to not be hijacked if you don't mind - at least until I've gotten an answer to my question. This is not about theories in competition, or which one os better. I just want to understand how string theory predicts nothing.

So far, the only thing I've seen that has addressed that is selfAdjoint's statement that "...the energy scale is so high we couldn't observe what they predict..."

Doesn't this just mean it's out of our reach? That's not the same thing thing as 'never'.

 

[selfAdjoint]

Hello? I would like this thread to not be hijacked if you don't mind - at least until I've gotten an answer to my question. This is not about theories in competition, or which one os better. I just want to understand how string theory predicts nothing.
So far, the only thing I've seen that has addressed that is selfAdjoint's statement that "...the energy scale is so high we couldn't observe what they predict..."
Doesn't this just mean it's out of our reach? That's not the same thing thing as 'never'.

Did you see my other statement? String theory has been accused of predicting NOTHING that can ever be checked! The string physicists have not really defended themselves against this charge.

 

[DaveC426913]

Did you see my other statement? String theory has been accused of predicting NOTHING that can ever be checked!

I did see that, but that is not answering my question since it is part of my question.

How does the theory not predict anything about anything? The theory requires particles, their interactions and energies to behave certain ways, does it not? Predictions would put numbers to them, which can, in principle be verified or refuted.

By the way, if it doesn't predict anything, and its not modeled after observable physics, does that not mean it is not even a theory by definition? Wouldn't it merely be a hypothesis? Or a conjecture?

 

[DaveC426913]

Did you see my other statement? String theory has been accused of predicting NOTHING that can ever be checked!

I did see that, but that is not answering my question since it is part of my question.

How does the theory not predict anything about anything? The theory requires particles, their interactions and energies to behave certain ways, does it not? Predictions would put numbers to them, which can, in principle be verified or refuted.

By the way, if it doesn't predict anything, and its not modeled after observable physics, does that not mean it is not even a theory by definition? Wouldn't it merely be a hypothesis? Or a conjecture?

 

[DaveC426913]

Did you see my other statement? String theory has been accused of predicting NOTHING that can ever be checked!

I did see that, but that is not answering my question since it is part of my question.

How does the theory not predict anything about anything? The theory requires particles, their interactions and energies to behave certain ways, does it not? Predictions would put numbers to them, which can, in principle be verified or refuted.

By the way, if it doesn't predict anything, and its not modeled after observable physics, does that not mean it is not even a theory by definition? Wouldn't it merely be a hypothesis? Or a conjecture?

 

[DaveC426913]

Did you see my other statement? String theory has been accused of predicting NOTHING that can ever be checked!

I did see that, but that is not answering my question since it is part of my question.

How does the theory not predict anything about anything? The theory requires particles, their interactions and energies to behave certain ways, does it not? Predictions would put numbers to them, which can, in principle be verified or refuted.

By the way, if it doesn't predict anything, and its not modeled after observable physics, does that not mean it is not even a theory by definition? Wouldn't it merely be a hypothesis? Or a conjecture?

 

[DaveC426913]

Did you see my other statement? String theory has been accused of predicting NOTHING that can ever be checked!

I did see that, but that is not answering my question since it is part of my question.

How does the theory not predict anything about anything? The theory requires particles, their interactions and energies to behave certain ways, does it not? Predictions would put numbers to them, which can, in principle be verified or refuted.

By the way, if it doesn't predict anything, and its not modeled after observable physics, does that not mean it is not even a theory by definition? Wouldn't it merely be a hypothesis? Or a conjecture?

 

[DaveC426913]

Did you see my other statement? String theory has been accused of predicting NOTHING that can ever be checked!

I did see that, but that is not answering my question since it is part of my question.

How does the theory not predict anything about anything? The theory requires particles, their interactions and energies to behave certain ways, does it not? Predictions would put numbers to them, which can, in principle be verified or refuted.

By the way, if it doesn't predict anything, and its not modeled after observable physics, does that not mean it is not even a theory by definition? Wouldn't it merely be a hypothesis? Or a conjecture?

 

[DaveC426913]

Did you see my other statement? String theory has been accused of predicting NOTHING that can ever be checked!

I did see that, but that is not answering my question since it is part of my question.

How does the theory not predict anything about anything? The theory requires particles, their interactions and energies to behave certain ways, does it not? Predictions would put numbers to them, which can, in principle be verified or refuted.

By the way, if it doesn't predict anything, and its not modeled after observable physics, does that not mean it is not even a theory by definition? Wouldn't it merely be a hypothesis? Or a conjecture?

 

[alfredblase]

hahaha Dave... your question cannot be answered. attempting to answer how string theory doesn't predict anything is like attemptin to answer how not to make a cup of tea... u just don't make it, plain and simple!

and yes strictly speaking, you are right, string theory is a hypothesis only, and yet it is referred to as a theory because of the large ammounts of compelling circumstantial evidence, that cannot be ignored, suggesting it it is true. It works too well and too elegantly and has too much promise not to be worth the effort of researching it further and not be taken seriously.

 

[marcus]

this was post #1

I've read a lot of things to this effect, that string theory predicts nothing, or that it can't be disproven, and as such, many theorists argue it is doomed.
How is this so? Is it because the scales proposed are so incredibly tiny that we can't measure that small, even in the future? Is there some principle that prevents the theory from making predictions or being testable?

this was post #4

If you read the popular books by string theorists, you will see that they claim that the reason string theory doesn't predict anything is that the energy scale is so high we couldn't observe what they predict.
But this is actually not the end of the story. Foes of string theory, who have become very vocal recently, assert that string theory predicts nothing, period. That there is no physical fact, at any energy, which if observed would confirm or falsify string theory. String theory is all about how strings and branes and so on behave, but efforts to connect this with other kinds of physics have come up short. And with the recent discovery that string theory predicts googles or infinities of inequivalent vacua, only one of which coreesponds to our world, and gives no recipe for finding which one that is, you can see that the non-prediction stakes have been raised. This has led the critics to jeer louder and the string physicists, unwisely in my opinion, to circle the wagons.
So there you have it.

I believe that selfAdjoint is referring in part to a series of papers beginning with the famous KKLT paper of January 2003 which uncovered "googles or infinities of inequivalent vacua" with which string is compatible----the so-called "string theory Landscape".

The number of different vacua, which the theory is believed incapable of excluding and from among which it is thought to have no way of selecting, has been variously estimated since the KKLT paper appeared---as 10^100, or 10^200 or 10^500 or (one hears) infinity.

An influential segment of the string population, led by Leonard Susskind, has reacted to this, since January 2003, by advocating that theorists GIVE UP on prediction and revise their idea of the scientific quest. It is proposed that we accept that all these vacua are possible, that they may even physically exist (!) and that we just happen to live in one of them---one which has physical properties making it habitable.

DAVEC, this is not something that interests me---I am just mentioning it by way of background to what selfAdjoint said in answer to your question.
Here are the 491 papers which have been written which cite the KKLT paper:

http://arxiv.org/cits/hep-th/0301240

here is an alternative list
http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:hep-th/0301240

the red line on the graph shows steady linear growth at a rate of about 200 papers per year

the paper which cites KKLT and which (save one) is itself most heavily cited is by Susskind:
http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:hep-th/0302219

This appeared in February 2003 (just a month later) and has 203 citations.
=============================

DAVEC, what they are talking about has nothing to do with not being able to probe at some unattainably high energy. It is perceived as a difficulty in principle.

Stringology is seen (by some major string theorists like Susskind) as being UNABLE IN PRINCIPLE TO SELECT from among a huge number of groundstates or versions of physics.

And for the past 3 years there has been a huge noise about this. Indeed as time goes on more and more influential people seem to show up in the Anthropic Landscape ranks. It seems that now David Gross is the only prominent VOCAL HOLDOUT.

=========================

Anyway this is just background to what selfAdjoint said. I think he answered your question. NO the inability to unpredict or discard any from among this huge set of vacua is seen by the theorists as inherent---not dependent on some high energy threshhold.

I think the question is answered unless you want someone to EXPLAIN the thought that went into the KKLT paper and the Susskind paper and all the subsequent furor.
There are still people engaged in "counting the string theory vacua".
Michael Douglas is prominent in that line of research.

This is not an interest of mine---other people could explain why string is compatible with a vast plethora of alternative versions of the physical vacuum. I will just say that I believe it has to do with the embarrassing "extra dimensions". There are millions upon millions of different ways one can imagine "curling them up". Googles (a google is 10^100 or some such number) upon googles of different "compactifications".

============================

My feeling is that this is all pretty much irrelevant. The difficulties only affect the string approach. I see no indication that non-string researchers like QG theorists are inclined to GIVE UP ON PREDICTION of the fundamental constants of physics and cosmology.

Just because string does not explain why the universe is the way it is does not mean people will not stop trying!

The disappointment of a few hundred string theorists is hardly a sign that the overall human enterprise to explain nature's basic proportions is doomed to failure

(It only looks like that to THEM. And many of the younger ones seem to be getting out.)

What interests me is to watch and see what new directions people go in, as string interest dwindles.

However you have specifically requested that we not talk about developments outside string so I will refrain.

 

[alfredblase]

hang on.. emmm does this mean string theory is over? that it should no longer be considered as the way things might work? that really sucks! I just badgered my two main referees into sending me sealed references for doctoral studies that are not worth undertaking??! will someone please clear this up? one minute string theory looks like the most promising line of research, the next it looks like it should be thrown in the waste paper bin??!

 

[selfAdjoint]

At this moment, string theory is in flux. Nobody can give you gaurantees it will still be a rich source of jobs, let alone results, for ten years or more to come. Note that the experimental particle physics budget is being stiffed by the US government too.

You should of course follow your tastes and talents, but if you have room to wiggle yet, and haven't completely made up your mind, I might suggest theoretical condensed matter physics. It never stops giving.

 

[Sauron]

Someone said in this thread that loop quantum gravity had probed bouncing of black holes ¿anybody has a link to a paper explaining it?

 

[DaveC426913]

What the hey? It looks like the 'me' from no less than seven alternate universes have gotten our replies quantum entangled in one thread...

 

[DaveC426913]

(What the hey? It looks like the 'me' from no less than seven alternate universes have gotten our replies quantum entangled in one thread...)

The number of different vacua, which the theory is believed incapable of excluding and from among which it is thought to have no way of selecting, has been variously estimated since the KKLT paper appeared---as 10^100, or 10^200 or 10^500 or (one hears) infinity.

DAVEC, what they are talking about has nothing to do with not being able to probe at some unattainably high energy. It is perceived as a difficulty in principle.

Stringology is seen (by some major string theorists like Susskind) as being UNABLE IN PRINCIPLE TO SELECT from among a huge number of groundstates or versions of physics.

Thank you. This satisfies my question.

However you have specifically requested that we not talk about developments outside string so I will refrain.

Not, at least, until I'd gotten the initial question answered. :)

 

[marcus]

Someone said in this thread that loop quantum gravity had probed bouncing of black holes ¿anybody has a link to a paper explaining it?

Hey Sauron, it is good to see you back. I remember reading your posts in 2003----and those of Instanton, who has been absent a long time.

Here is what I said in post #5 about BLACK HOLE BOUNCE

...
...
PREDICTION TWO: Loop Quantum Cosmology (LQC) as developed by Martin Bojowald has recently reached the stage where both the Bang and the Hole singularities have been removed and many of the researchers are saying that it LOOKS LIKE our Bang was really a Bounce.
On the BH side, they are saying that gravitational collapse leads to a Bounce. And on the BB side they are saying that our Bang was preceded by a contraction that looks mathematically similar to a BH collapse.
This was discussed a lot by the Friday papers at the October Loops '05 conference. It is looking like, in a Loop Gravity context, there is really only ONE thing, a bounce. And it makes a new region of the universe BRANCH OFF everywhere there is a black hole.
What this indicates to me is that a TYPICAL region of the universe is going to be one whose parameters are optimized for producing lots of black holes since such regions have more offspring. So this is NOT a hard and fast prediction, but it is looking increasingly likely that Loop Gravity implies that the parameters of cosmology/standard model are typically optimal for BH formation
So it challenges you: Can you find any parameter which if it were slightly changed would make BHs more abundant? Loop Gravity suggests that you can NOT find such a parameter. If you can, it would tend to refute Loop Gravity.
THIS IS CALLED cosmological natural selection, or the black hole natural selection hypothesis and IT IS ALREADY BEING TESTED. Because it predicts a nice low limit on the mass of neutron stars. So every time another neutron star is found, and astronomers measure its mass, we have a test. If they find one that is 2 solar masses or more, that would kill the idea (judging by what I've read) and even less, like 1.7 solar masses, if they were very sure of it.

SAURON, I THINK "PROBE" IS TOO STRONG A WORD. It claims too much. What they have done is remove the BH and BB singularities and found indication that gravity causes REPULSION at high densities, so that it LOOKS LIKE there is a bounce in each case.

Ashtekar's latest paper with Bojowald is fairly representative

http://arxiv.org/abs/gr-qc/0509075

there are a lot of papers by Bojowald related to this, sorry I have to let you choose:
http://arxiv.org/find/gr-qc/1/au:+Bojowald_M/0/1/0/all/0/1

Ashtekar's talk on Friday at the October Loops '05 conference is also respresentative
http://loops05.aei.mpg.de/

http://loops05.aei.mpg.de/index_files/abstract_ashtekar.html

Quantum Geometry and Space-time Singularities

"... I will present examples which strongly suggest that the *physical* space-time does not end at singularities. Quantum geometry can serve as a bridge..."

the key thing is that at this point they are saying STRONGLY SUGGEST instead of claiming to have rigorously derived the result.

there are also some online seminar talks by Parampreet Singh from Fall 2005, in the Penn State seminar collection. He is a postdoc interested in stuff related to Loop gravity bounce and inflation---including numerical calculation work, and phenomenology (possible observable signatures of QG in the CMB and graviational collapse.) So he tends to be up to date.

I will get the Penn State online seminar menu.

 

[marcus]

Here is Penn State seminar menu
http://www.phys.psu.edu/events/index.html
select "Fall 2005" and look for talks by Param Singh
(there are also ones about this by other people too)

Also on arxiv there are papers about QG removing BH and BB singularities by Leonardo Modesto, Oliver Winkler, Viqar Husain.

And there were talks at the Loops '05 conference by them too.

Basically at this point there is active research and they leave it to YOU to connect the dots.

1. Loop gravity may of course be wrong. It is beginning to be testable and it may or may not survive the first test(s) and even if it survives there is lots of room for it to simply be wrong.

2. if it isn't wrong, then----they tell us----it looks like spacetime EXTENDS through a (planck density) quantum regime at the pit of a BH.
there is no classical singularity that stops it from evolving. mathematically it looks like the BB, which Bojowald found to be a bounce back in 2001

3. in Loop cosmology, inflation appears to be generic (e.g. Ganashyam Date's paper), it comes with the bounce. Loop needs no special paraphernalia or fine tuning, to do inflation.

4. the Loop researchers will not as yet SAY that BH leads to BB, they only have results suggestive of this. But for me personally this has now reached the point where it could serve, if not to FALSIFY, at least to reduce the credibility of Loop models if I saw evidence that there is no BH to BB connection.

This now is my own personal judgement. If, next month, I see evidence, like reliable reports of a two-solar-mass neutron star in a binary pulsar configuration, that the observable universe is not optimal for BH formation, then this would make Loop gravity less interesting and believable for me personally.

Lee Smolin might find this attitude deplorable, but that is my subjective feeling. It looks to me as if Loop models point to a universe that branches thru BH (regions 'reproduce' this way, and proliferate). So a typical piece of the universe is apt to be a prolific one, and appealing to the principle of mediocrity: if our section is typical then it is evolved to produce abundant BH---parameters should have evolved to a local peak---they should be at or near a local maximum.

If I found that parameters AREN'T at a local maximum in BH abundance then (even though they haven't actually predicted this) I would take this as a negative for Loop. I know this is pretty vague. the situation hasnt settle out clearly yet.