LHC - the last chance for all theories of everything?

[Adrian59]

The LHC is probably the last of the large accelerators so should the main contenders for the title of the theory of everything (if such a thing exists) now, at the dawn of the LHC start up, clearly state what findings would a) prove their theory b) support their theory & c) eliminate their theory? Physics, after all, is still an empirical science & at some point even the most elegant mathematical theories need to connect with experimental fact.

 

[Finbar]

The LHC is probably the last of the large accelerators so should the main contenders for the title of the theory of everything (if such a thing exists) now, at the dawn of the LHC start up, clearly state what findings would a) prove their theory b) support their theory & c) eliminate their theory? Physics, after all, is still an empirical science & at some point even the most elegant mathematical theories need to connect with experimental fact.

I don't think it will be the last. Why do you think this? The LHC is unlikely to find conclusive evidence for any theory of everything. Of coarse one would like to prove or disprove theories but in reality this isn't likely. Finding support for a theory such as strings is probably the best we can hope for. For conclusive evidence we will have to wait for a larger accelerator or some novel way of testing fundamental physics that has not yet been thought up. Another hope would be more cosmological data that supports theories.

I think the things that the LHC should be able to prove or disprove, at least to some degree, is the Higgs boson and supersymmetry. And even if in these cases there may well be ways out if they don't find them at the energy levels of the LHC (theorists who love SUSY will just claim its at a higher energy scale). All we can really do is hope that they find something significant so that governments continue to fund fundamental physics experiments otherwise we may enter an age where there is relatively little empirical evidence to guide new theories.

 

[marcus]

... an age where there is relatively little empirical evidence to guide new theories.

A lot of evidence is coming from astrophysics.
This was part of the message in a recent talk that Edward Witten gave at Cern, called something like "Physics away from the high energy frontier".

The notes are on line. If you move away from the collider energy frontier, you still have high energy phenomena to observe. Cosmic rays, astrophysical gamma radiation. Some interesting things can be deduced from such data.

 

[Naty1]

All we can really do is hope that they find something significant so that governments continue to fund fundamental physics experiments

Yes. Don't forget that perhaps something totally UNEXPECTED may arise...Wouldn't that be most exciting of all possibilities!...something to set science off in new directions, perhaps towards a different theory of everything from current forms. Something that really alters our perceptions...like Hubbles work, or the discovery of dark energy and dark matter...

 

[Adrian59]

I don't think it will be the last. Why do you think this?QUOTE]

I thought it is unlikely that anything to match the LHC will happen soon especially after the Superconducting Super Collider was cancelled. I admit implying never was an overstatement but I would contend that nothing remotely as powerful is likely in even the medium term.

 

[marcus]

Adrian the future of bigger colliders is in part a political question.
There are alternative ways of deducing facts about the universe (Witten discussed a bunch, but one sees evidence of this all the time) so who builds the next big machine, and when they build it, is likely to be determined at least as much by politics---somewhat like the race to the moon there is an element of national prestige and proving to the world one's command of the most challenging technologies.

So I would say that you have to think about the aspirations of the Chinese People's Republic.
Think what it would mean to the Chinese if they could build a more successful collider than the French and Swiss! It would demonstrate scientific, economic, and engineering strength of major proportions. And also would make China the center of particle physics activity and the host to a great international collaboration of minds.

Given the political motives, I would imagine that it is almost certain that a post-LHC collider will be built, regardless of the strictly scientific need for it, and that it will very likely be reliable ("with a vengeance" so to speak). That is, will experience a less eventful start-up than LHC.

Just a guess though.

 

[Naty1]

So I would say that you have to think about the aspirations of the Chinese People's Republic.

Sounds quite possible. In fact, it would not be surprising to see India and China in a race to build the most powerful collider.

 

[marcus]

National aspirations can be achieved within the context of international collaboration. How about an International Collider which the Chinese manage to get built on their soil. This would be an amazing boost to their universities, technology companies, and scientific establishment. Also their prestige in the EastAsian region. Doubtless there is an initiative to do this and they are already working on it. I wonder if something like that will happen.

 

[humanino]

So if we do not build large colliders anymore, Science will have to wait for powerful tabletop accelerators. We'll not see that in our lifetime, but there is no reason why it should be impossible in principle.

 

[marcus]

In case anyone wants to look at it, here are slides for Witten's talk about the prospects of physics "away from the high energy frontier":
http://indico.cern.ch/getFile.py/ac...onId=1&resId=0&materialId=slides&confId=51128
And here is a video of the talk itself:
http://cdsweb.cern.ch/record/1176909/
It is in the first session of the workshop, which is the default. And it is minute 4:30 to minute 32:20 (followed by questions from audience until 37:30)
So you just start the first session and drag the button to 4:30 to skip the chairman's introduction.
A lot of the talk is about how to get new physics out of astrophysical observation and cosmology, without relying on colliders or high energy machines in general.

 

[RUTA]

Has anyone knowledge of a unification approach that doesn't require larger accelerators? I saw a popular program last week about superstring theory in which they said we'd need an accelerator as big as the galaxy to find evidence of strings. I'm not aware of a GUT or SUT that doesn't require much larger accelerators. Have you?

 

[marcus]

Has anyone knowledge of a unification approach that doesn't require larger accelerators? ...

Ruta, why don't you watch the first few minutes of Witten's talk? He is talking about unification (GUTs) almost every slide, but the focus is on what can be discovered and tested away from the high energy frontier. That is by other means than machines like the LHC.

I think it is unrealistic to imagine that there will not be some development of colliders beyond LHC. But it is also unrealistic to suppose that the whole progress in fundamental physics depends on making larger and larger colliders. Listen to the talk, which is number one talk in a well-attended CERN workshop. It illustrates that there are already clever people thinking about how physics can advance by other avenues.

 

[MTd2]

I would just like to know if there is any progress towards using high temperature superconductors for the magnets in particle accelerators.

 

[humanino]

The Qweak experiment at Jefferson Lab will run around 1 GeV, but measuring parity violation with precision and provide orthogonal (complementary)to very high energy colliders constraints on physics BSM.

High precision parity violation is not new, it just can be done better today, and it's quite sensitive to tiny vacuum fluctuations.

 

[RUTA]

Ruta, why don't you watch the first few minutes of Witten's talk? He is talking about unification (GUTs) almost every slide, but the focus is on what can be discovered and tested away from the high energy frontier. That is by other means than machines like the LHC.

I think it is unrealistic to imagine that there will not be some development of colliders beyond LHC. But it is also unrealistic to suppose that the whole progress in fundamental physics depends on making larger and larger colliders. Listen to the talk, which is number one talk in a well-attended CERN workshop. It illustrates that there are already clever people thinking about how physics can advance by other avenues.

Thanks much, Marcus. Of course I've been hearing about low energy predictions of GUTs and SUTs, but these, as you know, are not definitive given the extremely high unification energies. I'm not asking for indirect, secondary consequences of high energy unification schemes. I'm wondering if someone has any ideas which don't entail high energies to unify physics in the first place.

 

[Adrian59]

In case anyone wants to look at it, here are slides for Witten's talk about the prospects of physics "away from the high energy frontier":
QUOTE]

Thanks Marcus. I watched the video & looked at the lecture notes. The bit about low frequency strings appeared interesting. However, one area I was trying to invite comment about when starting this thread was the exact evidence needed to support any current GUT or TOE. For instance what do we expect supersymetry will look like & if we do see it can we be sure it is supersymetry & not some unexpected exotic particle?

 

[tom.stoer]

I think the things that the LHC should be able to prove or disprove, at least to some degree, is the Higgs boson and supersymmetry. And even if in these cases there may well be ways out if they don't find them at the energy levels of the LHC (theorists who love SUSY will just claim its at a higher energy scale). All we can really do is hope that they find something significant so that governments continue to fund fundamental physics experiments otherwise we may enter an age where there is relatively little empirical evidence to guide new theories.

That's the very core of the discussion. The LHC can find evidence for entities like Higgs, SUSY, etc. iff they are in the LHC's energy range.

For the SM Higgs it's pretty clear: if the LHC does not prove its existence it automatically disproves the SM. For all other topics it can only push the limits of their "existence" to higher energies.

So that automatically means the LHC will be absolutely mute about any "ToE". It can support theories with respect to their low energy regime, but nothing else.

For example string theory (with small extra dimensions): as its low energy limit is a certain SUGRA, the LHC cannot distinguish between the two scenarios "ST is the ToE" and "SUGRA is the ToE".

My conclusion is that something like a ToE cannot exist in a physical sense:
a) you can only "prove", support or disprove a theory in a very restricted sense = in certain regimes
b) a ToE should be able to tell you why it is the ToE; I don't think that ST does only because it is allowed to exist in a 10-D spacetime: it does not tell you why other approaches must fail
c) in a certain sense the discussions regarding duality show that there may be not one fundamental theory but only certain dual descriptions of something we like to call "reality" - whatever that means;
look at QFT: what are the fundamental entities of a "quantum ontology"? state vectors in Fock space and field operators - or path integrals and "trajectories in field configuration space";
look at ST: there are dualities between very different string theories, compactifications etc.
so in terms of ontology this question is undecidable;
d) any mathematical approach to a ToE is not able to prove why this approach must be mathematical at all

 

[marcus]

Tom
How does your perspective compare with the one Weinberg presented towards the end of his 6 July Cern talk?

I expect you may have watched it but in case not I'll get the link.
http://cdsweb.cern.ch/record/1188567/

I mentioned in this 11 July post
https://www.physicsforums.com/showthread.php?p=2268482#post2268482
that to save time you can drag the time button to minute 58 (if you want to skip the historical part) and get to the core message
which is in the last 12 minutes.

The slides are here:
http://itpworkshop.unibe.ch/MaKaC/getFile.py/access?contribId=126&sessionId=19&resId=0&materialId=slides&confId=2

There was further discussion here:
https://www.physicsforums.com/showthread.php?t=324841

The title of the Cern talk was "The Quantum Theory of Fields--Effective or Fundamental?"

 

[tom.stoer]

Marcus, which statement or perspective do you mean?

The first one regarding the LHC energy range?

The LHC can find evidence for entities like Higgs, SUSY, etc. iff they are in the LHC's energy range.

The second one regarding the low energy limit?

So that automatically means the LHC will be absolutely mute about any "ToE". It can support theories with respect to their low energy regime, but nothing else.

Or my general conclusion regarding possible existence (or non-existence) of a ToE

My conclusion is that something like a ToE cannot exist in a physical sense: ...

 

[marcus]

Tom I often find your informed opinion on things very helpful but in this case what you say does not make sense to me and I contrast it with the cautious optimism in Weinberg's talk.

...
My conclusion is that something like a ToE cannot exist in a physical sense:
a) you can only "prove", support or disprove a theory in a very restricted sense = in certain regimes
b) a ToE should be able to tell you why it is the ToE; I don't think that ST does only because it is allowed to exist in a 10-D spacetime: it does not tell you why other approaches must fail
c) in a certain sense the discussions regarding duality show that there may be not one fundamental theory but only certain dual descriptions of something we like to call "reality" - whatever that means;
look at QFT: what are the fundamental entities of a "quantum ontology"? state vectors in Fock space and field operators - or path integrals and "trajectories in field configuration space";
look at ST: there are dualities between very different string theories, compactifications etc.
so in terms of ontology this question is undecidable;
d) any mathematical approach to a ToE is not able to prove why this approach must be mathematical at all

I think Weinberg was using a very pragmatic idea of a ToE when he referred to "how nature is". Talking about a predictive theory that appears good to arbitrary high energy.
The commonsense view of science as a process and a community---not as an ultimate.
A theory is a ToE if it acts like a ToE and most of the community accepts it provisionally as such.
And he was saying that string may be irrelevant. String may not turn out to be "how nature is". He suggested an alternative line he is currently pursuing based on asymsafe qg and quantum field theory.

Therefore I think your references to string theory (ST) in your quote may be irrelevant and distracting. Maybe I will take them out and look at the bare bones skeleton of the argument, minus the flesh of that example.

==skeleton argument==
My conclusion is that something like a ToE cannot exist in a physical sense:
a) you can only "prove", support or disprove a theory in a very restricted sense = in certain regimes

b) a ToE should be able to tell you why it is the ToE

c) in a certain sense the discussions regarding duality show that there may be not one fundamental theory but only certain dual descriptions of something we like to call "reality" - whatever that means;
look at QFT: what are the fundamental entities of a "quantum ontology"? state vectors in Fock space and field operators - or path integrals and "trajectories in field configuration space"

d) any mathematical approach to a ToE is not able to prove why this approach must be mathematical at all
==end of excerpt==

Let me think a bit about this
d) is right of course. physical theories are mathematical. the fact that math works to describe regularity in nature is a mystery. but we do not have to address that. A ToE is just a physical theory that works predictively to arbitrary high energy, it does not have to explain all the mysteries.
Like "why does existence exist?" That is a good question but ToE does not have to address it.

c) does not have to worry us. There can be alternative equivalent formulations of the regularities in nature. Sometimes we eventually find that one is better. Sometimes we find a more general mathematics that comprehends both. It is OK. There can still be a ToE even if it comes in several equivalent formulations.

b) pragmatically, I do not see why any physical theory has to explain why it is an adequate theory. If something works and acts like a ToE then it is a ToE.
It does not have to contain a "theory of theories". Science is a communal process governed in part by tradition---and the tradition says that a theory is accepted provisionally until and if a better one is found. That is as good as it gets, in the tradition. All acceptance by the community is provisional. No acceptance is ultimate. So b) is no problem.

a) What you say here is absolutely right, except that we do not know in advance the ultimate limit that we can probe empirically.
Clever ways of testing a model may be invented which surprise us and which go beyond the domain of verifiability that we would have expected.
Pragmatically, the meaning of infinity is "way beyond what anybody expected".
If an theory predicts accurately way beyond what anybody ever thought we would be able to test, because of some clever ways to test that people think up, then it will be acting like a ToE. Predictive out to arbitrarily high energies. It's possible that humans will get such a theory. I don't think that on purely logical grounds you can deny us the possibility.

But I like your argument and think that it is one worth making.

 

[tom.stoer]

Marcus,

thanks for the very first sentence of you post :-)

Of course I will try to respond carefully. First of all I was trying to go beyond a purely physical (positivistic) concept of a theory. If we restrict the meaning of ToE to "it predicts experimental results accurately" the all you comments are right - or the other way round - my comments a) to d) are somehow irrelevant.

What I was trying to do is to enlarge the concept of a ToE towards an ontology. That does not mean that we must decsribe nature in terms of things, objects and so an, but at least in terms of structures, laws, relations etc. So w/o knowing in detail what a quantum object "really is", we have a gut feeling, that there is some underlying "quantum ontology".

However quantum mechanics does not tell us what this really is and why it is as it is.

The only theory that tries to go into that direction is string theory. It provides a rather large (and rather un-explored) theoretical framework, which allows one to restrict its application due to certain consistency conditions. Of course there is still a long way to go - and to be honest I doubt that string theory will do the job - but is is definitely the first theory that at least tries to answer such questions.

Regarding the details:

... Weinberg was using a very pragmatic idea of a ToE when he referred to "how nature is". Talking about a predictive theory that appears good to arbitrary high energy.

OK; here Weinberg restricts himself more than I would prefer; Of course he is right in terms of a scientific program, but I would not call the result a "ToE".

And he was saying that string may be irrelevant. String may not turn out to be "how nature is". ...
Therefore I think your references to string theory (ST) in your quote may be irrelevant and distracting.

see above

d) is right of course ... but ToE does not have to address it.

OK; let's drop d) as it may be too metaphysical and focus on a) - c)

c) does not have to worry us. There can be alternative equivalent formulations of the regularities in nature. Sometimes we eventually find that one is better. Sometimes we find a more general mathematics that comprehends both. It is OK. There can still be a ToE even if it comes in several equivalent formulations.

That's not the point. In terms of purely physical applications you are right, but not in terms of a more fundamental ontology. Look at Maxwells equations: let us assume for a moment that the 4D Minkowski formulation is not known. Then assume that somebody explores the 4D formulation together with Lorentz symmetry, fieldstrength tensor, 4-potentials and gauge symmetry. I would call that a step towards the knowledge what "nature really is".

b) pragmatically, I do not see why any physical theory has to explain why it is an adequate theory. If something works and acts like a ToE then it is a ToE.
It does not have to contain a "theory of theories".

See above; it's especially here where my comments regarding ST apply. As another example I could use Bell's theorem; it is negative in the sense that it tells was what "nature not is". But it reveales some deeper knowledge about the classical and the quantum world and excludes a huge set of "theoretical approaches".

a) What you say here is absolutely right, except that we do not know in advance the ultimate limit that we can probe empirically.

Here we have to be rather careful. If we believe in a theory to be the ToE just because of its support from the LHC, than our belief goes beyond the experimental knowledge.

Assume for a moment that we do not know that GR + QFT are incompatible. Then we could call GR + SM a ToE, simply because all experimental predictions are correct (the problem with 3 generations, Higgs etc. does not apply in the context of argument a). Nevertheless it is no ToE, because:
- we will find inconsistencies at higher energies (argument a)
- it does not explain the particle content, symmetry etc. (arguments b - c)

Of course the LHC could provide mechanisms to test the theory beyond the LHC's energy scale, e.g. due to loop effects that already allowed us to restrict the allowed mass range of the Higgs even if this range is not covered my already operating accelerators. That is not my point.



Predictive out to arbitrarily high energies is a very good starting point. For such a theory I would agree to call it a ToE (of course still with some limitations from b - c). So I think we can agree on a common understanding of a ToE restricted to a pragmatic perspective.

Nevertheless I would like to go beyond that perspective. I know that this is a fundamental clash within the community.
A) we all "agree" that we should use the QM / QFT framework in a pragmatic way - and must not / cannot try to describe nature "as it really is"; photons "are" not the lines within Feynman diagrams, ...
B) All researches trying to understand (e.g.) how QCD works are true believers that quarks, gluons etc. are "real physical entities" - whatever that means - and I am sure sure they would deny that these entities are only mathematical tools to describe scattering cross sections w/o any underlying realistic interpretation.

Of course nobody as of today knows what these "quantum entities / quantum ontology really is", but nevertheless all major research programs are driven by the idea to "understand more about their existence / being".

 

[marcus]

I think I understand what you are saying.
There is no clear connection that I can point to, but it made me think of the philosophical seriousness of the physicists of the 1900-1925 generation. What would some of them say to you, if you could talk with them?

There is a tradition of wanting more from physics than a predictive mathematical model and wanting an ontology, which says what really is really there, really.

This is an appealing quest, and probably as old as Anaximander. There is no cure for this desire. I hope there will always people who want this. I am happy to differ with you as to the basic goal and admit you have a good idea.

I will simply tell you mine, for comparison.
If it were possible I would like to see an accurate nonsingular testable dynamics of geometry-and-matter.

In this dynamics, geometry and matter would be (different aspects of) the same thing. So it would be completely natural that they interact and influence each other.

This dynamics would not break down or blow up at singularities. It would continue on through.

I am unprejudiced as to what "dynamics" means. For me, I do not care what the mathematical formalism is. Mathematics evolves in a kind of Darwinian way to meet the needs of physicists and satisfy the rational demands of mathematicians. It mutates, proliferates, survives---the future course of its evolution is not predictable. I do not insist on a Lagrangian although that would be nice. There will be some adequate formalism.

And I find that I do not care if such a thing is final. I kind of hope it is not. And that the people who have it will not see it as final or complete.

For me there is only one revolution, and that is the one we are in. I would love to see it finally carried through, and I relish every step along the way. But since scientific revolutions are so much fun, I trust that this one will not be the last.

 

[tom.stoer]

Hey Marcus,

I don't think we disagree; we are just discussing on different levels. I fully agree with your concepts and requirements for a ToE. But as soon as the dust settles I would like to understand more about the Why - not only the How.

Why is spacetime a 4D manifold with SO(3,1)?
What is the reason / origin for the gauge symmetries of the SM
What are the reasons for three fermion generations, flavour symmetry and Higgs?
What are the underlying / unifying concepts?
Why is it This way - and not That way?

So this can be seen as a second level with explanations, restrictions, consistency conditions etc.

Maybe we should not focus on ontology, or rather restrict it to a negative approach telling us what nature Not is.

 

[Dmitry67]

Why is it This way - and not That way?

Do you know how Max Tegmark answered that question?

 

[tom.stoer]

no; please explain!

 

[Dmitry67]

no; please explain!

http://arxiv.org/abs/0704.0646

Stephen Hawking famously asked “what is it that breathes fire into the equations and makes a universe for them to describe?” [93]. In the context of the MUH, there is thus no breathing required, since the point is not that a mathematical structure describes a universe, but that it is a universe

...

As a way out of this philosophical conundrum, I have suggested [12]) that complete mathematical democracy holds: that mathematical existence and physical existence
are equivalent, so that all mathematical structures have the same ontological status. This can be viewed as a form of radical Platonism, asserting that the mathematical
structures in Plato’s realm of ideas, the Mindscape of Rucker [6], exist “out there” in a physical sense [9], casting the so-called modal realism theory of David Lewis [92]
in mathematical terms akin to what Barrow [7, 8] refers to as “ in the sky”. If this theory is correct, then since it has no free parameters, all properties of all parallel universes (including the subjective perceptions of SAS’s in them) could in principle be derived by an infinitely
intelligent mathematician.

 

[tom.stoer]

OK, I did neither study nor understand the paper within a few hours, but one first comment is in order: personally I do not think that mathematical and physical "existence" have the same ontological status nor do I believe that (all) pure mathematical entities are physically real. Example: I would not say that the SU(3) "exists" and that this can be proved or derived from QCD.

All I believe is that there is "something out there" independent from our mind - a "physical reality" - simply because of the fact that we all agree on some fundamental experiences / phenomena / measurements which is hardly thinkable with radical idealism = w/o any mind-independent entities or structures. Why should totally independent human minds agree on some structures if not because a mind-independent reality? We all agree on the SU(3) structure fundamental to QCD because there is "something" = a "quark" that forces a human brain to form an "SU(3) idea". We cannot neither separate the physical existence of "quarks" from the "SU(3) idea", nor can we identify them.

I do not know what a quark really "is", nor do I know what the ontological status of the SU(3) "is". I just think that the "physical existence of quarks" is represented by the human mind (or brain - if you want to avoid another discussion regarding mind :-) by something like an "SU(3) structure".

 

[Dmitry67]

I strongly recommend reading it. Last summer I was really shocked... I was thinking about it over and over again...

Regarding the "physical reality". Even the either is gone, people are still trying to find tooth wheels behind the curtain. Max Tegmarks hypothesis is not only that ALL mathematical systems have the same physical existence (and therefore an answer to the famous question “Did God have a choice when he created a Universe" is negative) but also that TOE can be expressed in a form of pure equations, it is a pure mathematical system. Everything: the number of dimensions, space, time, particles must emerge from these formulas. No words are required.

 

[tom.stoer]

Before I start reading: one central question is if and how the ToE explains its own uniqueness.

Assume for a moment that the SM + a UV complete version of GR would be the ToE. How would this theory e.g. explain the SU(3) symmetry of QCD? the 3 families of fermions? the Higgs boson? Why not SU(4)? why not 4 generations? ...

So if the ToE is a mathematical framework - why is she exactly THIS framework and not something else?

Or do you say that all mathematical structures have the same "degree of existence". Then why do we observe in our universe an SU(3) structure and not SU(4)? Doesn't this idea of structures "having equal rights" lead to something like a mathematical multiverse w/o any explanation which structure comes to "true physical existence"?

 

[marcus]

Bee Hossenfelder's comment on Tegmark MathUniverse idea:
http://backreaction.blogspot.com/2007/09/imaginary-part.html

If you are looking to understand the reason why NORDITA institute at Stockholm University hired Bee Asst. Prof. right out of Perimeter postdoc,
then you just have to read that one blog post. Even without other reasons it is enough by itself.