Why all the clinging to locality?

[bhobba]

The real difference is that we believe QFT describes everything (except gravity)

Exactly why do you believe QFT doesn't describe gravity:
http://arxiv.org/abs/1209.3511

It isn't valid beyond about the Plank scale - but then again neither is QED, the Electroweak theory, or QCD.

Thanks
Bill

 

[stevendaryl]

Exactly why do you believe QFT doesn't describe gravity:
http://arxiv.org/abs/1209.3511

It isn't valid beyond about the Plank scale - but then again neither is QED, the Electroweak theory, or QCD.

Thanks
Bill

What I have heard said about non-renormalizable theories is that they require an infinite number of experimentally determined parameters (as opposed to QED, which has only the charge and mass of the fermions).

 

[bhobba]

What I have heard said about non-renormalizable theories is that they require an infinite number of experimentally determined parameters (as opposed to QED, which has only the charge and mass of the fermions).

But they can be approximated and valid up to a certain scale by theories with no such problems. That is the EFT approach. But we know that theories that are renormalisable without such shenanigans are only valid up to a certain scale anyway eg QED is only valid up to where the elecrtroweak theory takes over and we are pretty certain that the electroweak theory and QCD break down about the Plank scale as well.

The issue isn't that gravity isn't compatible with QFT, the issue is its only valid up to a certain scale - big deal - so are all our other theories.

Thanks
Bill

 

[Demystifier]

Exactly why do you believe QFT doesn't describe gravity:
http://arxiv.org/abs/1209.3511

It isn't valid beyond about the Plank scale - but then again neither is QED, the Electroweak theory, or QCD.

That's a good point! But then again, a funny thing about quantum gravity is that its effects become significant precisely at the scale at which the theory ceases to be valid. There is no regime in which the theory is both valid and significant.

 

[bhobba]

That's a good point! But then again, a funny thing about quantum gravity is that its effects become significant precisely at the scale at which the theory ceases to be valid. There is no regime in which the theory is both valid and significant.

Yea - noticed that to.

When I was into GR I chatted to Steve Carlip about this.

He agreed - but maddeningly its only of pedagogical value in keeping the issue in perspective - in practice its useless.

Thanks
Bill

 

[kaplan]

Exactly why do you believe QFT doesn't describe gravity:
http://arxiv.org/abs/1209.3511

It isn't valid beyond about the Plank scale - but then again neither is QED, the Electroweak theory, or QCD.

Thanks
Bill

The standard model would be valid to arbitrarily high energy scales if it wasn't coupled to gravity. In other words it's gravity that causes all the problems.

How do I know gravity isn't a QFT? I don't, for sure - for example gravity (or really string theory) in asymptotically anti-de Sitter spacetimes is a QFT. But if gravity is a QFT it has to be in some very non-trivial sense like that one. For one thing, cross-sections in gravity grow like the center of mass energy s (because the radius of a black hole is proportional to its energy), and QFTs don't do that.

 

[stevendaryl]

The standard model would be valid to arbitrarily high energy scales if it wasn't coupled to gravity. In other words it's gravity that causes all the problems.

How do I know gravity isn't a QFT? I don't, for sure - for example gravity (or really string theory) in asymptotically anti-de Sitter spacetimes is a QFT. But if gravity is a QFT it has to be in some very non-trivial sense like that one. For one thing, cross-sections in gravity grow like the center of mass energy s (because the radius of a black hole is proportional to its energy), and QFTs don't do that.

I thought that there were high energy problems that were not gravity-related. For example, QED has the Landau pole, which implies that it is inconsistent at high enough energy.

It's been too long since I've looked at this stuff, but as I understand it, asymptotically free theories (like QCD) have well-defined high-energy limits, but theories like QED where the coupling constant grows with energy do not.

 

[bhobba]

The standard model would be valid to arbitrarily high energy scales if it wasn't coupled to gravity.

You are sure the Standard Model is valid to arbitrarily high energies including the Higgs region that evidently has a Landau pole?

GR is not a QFT - its a classical theory. It is not incomparable with QFT because an EFT can be developed from it that is valid up to the Plank scale. The issue is its not valid to all energies - big deal - neither is the standard model - to the best of my knowledge anyway.

Thanks
Bill

 

[bhobba]

I thought that there were high energy problems that were not gravity-related. For example, QED has the Landau pole, which implies that it is inconsistent at high enough energy.

The Landau pole in QED is a non issue in the standard model because long before that energy is reached its replaced by the Electroweak theory.

My understanding is the Higgs has a Landau pole however - but I am not knowledgeable enough in that to know for sure.

Thanks
Bill

 

[kaplan]

I thought that there were high energy problems that were not gravity-related. For example, QED has the Landau pole, which implies that it is inconsistent at high enough energy.

It's been too long since I've looked at this stuff, but as I understand it, asymptotically free theories (like QCD) have well-defined high-energy limits, but theories like QED where the coupling constant grows with energy do not.

That's true - but the standard model does not have a Landau pole. Generally non-Abelian gauge theories are asymptotically free (or at least can be, depending on the matter content).

You are sure the Standard Model is valid to arbitrarily high energies including the Higgs region that evidently has a Landau pole?

It doesn't have a Landau pole. It could have had one if the Higgs were sufficiently heavy, but we now know it's not. Instead, it may have the problem that the quartic coupling runs negative, which I suppose invalidates the theory in the UV as well. But I think an asymptotically free fixed point (i.e. zero coupling at very high energies) is still possible given the experimental constraints. And in any case such problems are easy to fix with the addition of some extra massive matter.

GR is not a QFT - its a classical theory. It is not incomparable with QFT because an EFT can be developed from it that is valid up to the Plank scale. The issue is its not valid to all energies - big deal - neither is the standard model - to the best of my knowledge anyway.

It's a lot worse than that. We know how to construct field theories that are valid to arbitrarily high energies. We don't know how to do that with gravity, at least apart from string theory.

 

[bhobba]

It's a lot worse than that. We know how to construct field theories that are valid to arbitrarily high energies. We don't know how to do that with gravity, at least apart from string theory.

I dispute we know how to construct field theories valid to arbitrarily high energies eg we have the Landau pole in QED and even you admit the Higgs has an issue.

That's the whole point of the EFT program - we suspect our theories are simply low energy approximations to theories that don't have issues like renormalisation which means you have to have a cutoff to make sense of the results - its just that renormalisable theories are rather nice - once you fix the renormalised constants then low and behold you get results that are cutoff independent. But you still need a cutoff - if it was valid to all energies you wouldn't.

Thanks
Bill

 

[audioloop]

"pre-existing properties"

'pre existing properties' are values, values of who or what ? OBJECTS.
then there are 'existent things' without values, just that.
you can't talk about values without objects.

example, electrons without spin value.
consequently there is a reality without a precise value.


.

 

[bhobba]

you can't talk about values without objects.

The axiomatic approach to mathematics says otherwise.

Thanks
Bill

 

[audioloop]

The axiomatic approach to mathematics says otherwise.

Thanks
Bill

we are talking about physics.

 

[USeptim]

I dispute we know how to construct field theories valid to arbitrarily high energies eg we have the Landau pole in QED and even you admit the Higgs has an issue.

That's the whole point of the EFT program - we suspect our theories are simply low energy approximations to theories that don't have issues like renormalisation which means you have to have a cutoff to make sense of the results - its just that renormalisable theories are rather nice - once you fix the renormalised constants then low and behold you get results that are cutoff independent. But you still need a cutoff - if it was valid to all energies you wouldn't.

Thanks
Bill

I agree with you, renormalisation looks like a patch to fix a theory that adjusts some results at low energy scales with experiments but blows up when it's used with high energies.

 

[kaplan]

I dispute we know how to construct field theories valid to arbitrarily high energies eg we have the Landau pole in QED and even you admit the Higgs has an issue.

You can dispute it all you want, but it's true. QCD is a good example.

That's the whole point of the EFT program

No, it's not.

we suspect our theories are simply low energy approximations

That's true. In fact we more than suspect, we know - because of gravity.

 

[kaplan]

I agree with you, renormalisation looks like a patch to fix a theory that adjusts some results at low energy scales with experiments but blows up when it's used with high energies.

Renormalisation is a perfectly well-defined procedure, and it does not "blow up when it's used with high energies" - at least not in renormalisable field theories (hence the name). Not only that, we know experimentally that it works, because we've measured the runnings (the change of coupling constants with energy scale).

Don't forget - we're talking about the most precisely tested theories in the history of science, no chopped liver. It's irrational to ignore that.

 

[bhobba]

we are talking about physics.

Yes we are - and physical theories, being axiomatic systems, with parts mapped to stuff out there, can also contain things not necessarily mapped to objects. For example in renormalisation a regulator is introduced to allow finite answers to be extracted but some regulators, such as dimensional regulation, are not physically realizable.

Thanks
Bill

 

[bhobba]

I agree with you, renormalisation looks like a patch to fix a theory that adjusts some results at low energy scales with experiments but blows up when it's used with high energies.

Don't get me wrong.

Renormalisation is a perfectly valid process especially since Wilson clarified it with the EFT approach.

Its just that approach views renormalisable theories differently than in the past - they are not viewed as fundamental but merely as low energy approximations to theories that may not need the trick of renormalisation.

However we are getting way off topic and this really needs its own thread.

But as a warm up to that - if anyone want's to pursue it - the best paper I have come across at explaining what's going on is the following:
http://arxiv.org/pdf/hep-th/0212049.pdf
'The cut-off, first introduced as a mathematical trick to regularize integrals, has actually a deep physical meaning: it is the scale beyond which new physics occur and below which the model we study is a good effective description of the physics. In general, it involves only the renormalizable couplings and thus cannot pretend to be an exact description of the physics at all scales. However, if it is very large compared with the energy scale in which we are interested, all non-renormalizable couplings are highly suppressed and the effective model, retaining only renormalizable couplings, is valid and accurate'

Thanks
Bill

 

[stevendaryl]

You can dispute it all you want, but it's true. QCD is a good example.

Well, the standard model is not just QCD. I guess it's basically QCD + electroweak theory. The latter is not asymptotically free. I don't know whether electroweak has the same problem of a Landau pole that QED does. Superficial googling has not answered the question for me.

 

[bhobba]

I don't know whether electroweak has the same problem of a Landau pole that QED does. Superficial googling has not answered the question for me.

That's an interesting one.

We had a thread discussing it a while back and the thought was it was an open question.

However that's not what my concern is - its purely to do with the modern EFT view of renormalisation I quoted - and I have seen in a number of sources.

Thanks
Bill

 

[stevendaryl]

That's an interesting one.

We had a thread discussing it a while back and the thought was it was an open question.

However that's not what my concern is - its purely to do with the modern EFT view of renormalisation I quoted - and I have seen in a number of sources.

Thanks
Bill

Well, there are two different claims floating about. One is about EFT. The other is the question of whether the standard model breaks down at high enough energy. It is almost certainly incorrect at high energy (since it doesn't take into account gravity), but is it even consistent at high energy?

 

[bhobba]

Well, there are two different claims floating about. One is about EFT. The other is the question of whether the standard model breaks down at high enough energy. It is almost certainly incorrect at high energy (since it doesn't take into account gravity), but is it even consistent at high energy?

I don't think anyone is doubting the standard model breaks down at high enough energies - there was a claim made it was because it didn't include gravity. That indeed is quite possibly the reason it breaks down.

But I don't think its established for sure that is the reason - I think the modern view is because it relies on renomalisability to extract finite answers, that such theories are best viewed as effective theories valid up to a certain cutoff.

Its a view i have read all over the place eg:
http://cds.cern.ch/record/1281952/files/p145.pdf

Thanks
Bill

 

[stevendaryl]

I don't think anyone is doubting the standard model breaks down at high enough energies - there was a claim made it was because it didn't include gravity. That indeed is quite possibly the reason it breaks down.

I'm not sure what you mean by "breaks down". There are two different issues: (1) Does it become inaccurate at high energies? (2) Does it become inconsistent at high energies?

The answer to number (1) is certainly "yes", because of gravity. But I'm not sure if gravity has anything to do with (2). If the standard model has a Landau pole, then it is actually inconsistent at high enough energies.

 

[stevendaryl]

I don't think anyone is doubting the standard model breaks down at high enough energies - there was a claim made it was because it didn't include gravity. That indeed is quite possibly the reason it breaks down.

But I don't think its established for sure that is the reason - I think the modern view is because it relies on renomalisability to extract finite answers, that such theories are best viewed as effective theories valid up to a certain cutoff.

Its a view i have read all over the place eg:
http://cds.cern.ch/record/1281952/files/p145.pdf

Thanks
Bill

I just remembered another issue that is important for the high-energy behavior of QFT: Even if the theory is renormalizable, the perturbative expansion in powers of the coupling constant may not converge.

 

[bhobba]

I'm not sure what you mean by "breaks down"

By breaks down I mean is valid ie its predictions are true.

Even if there is no issues with things like Landau poles and one can make predictions to any energy scale the question is is it valid to push the SM that far. Well it doesn't include gravity so obviously not.

But a question is - can a renormalisable theory be considered fundamental?

I don't think there is an actual answer to that question in the sense of experiment deciding anything, but the modern EFT view of renormalisable theories is the fact they require a cutoff to extract finite answers means it is viewed as an approximation to a more fundamental theory.

That being the case the SM is really in the same boat as an EFT of gravity - its really only valid up to some energy scale by the inherent fact it's renormalisable.

That's all I am claiming. Its simply keeping the issue of a QFT of gravity in perspective. Its of zero value in actually doing anything useful.

Thanks
Bill

 

[audioloop]

Yes we are - and physical theories, being axiomatic systems, with parts mapped to stuff out there, can also contain things not necessarily mapped to objects. For example in renormalisation a regulator is introduced to allow finite answers to be extracted but some regulators, such as dimensional regulation, are not physically realizable.

Thanks
Bill

how can exist a spin without electrons ?

 

[craigi]

how can exist a spin without electrons ?

One answer is that other fermions and some bosons have spin too, but I don't think that is what you meant.

Did you mean, how can spin exist if not mapped to a particle?

 

[Maui]

how can exist a spin without electrons ?

Isn't this a relic of the inadequacies of the particle model of matter? The fact that physics has managed to experimentally separate spin from point particles seems to give further evidence for the field model of matter.

http://www.nature.com/nature/journal/v485/n7396/full/nature10974.html

 

[audioloop]

Did you mean, how can spin exist if not mapped to a particle?

to a quantum entity