I General Relativistic Quantum Theory?

[jake jot]

Sort of. See my post #29 just now in response to @bhobba's earlier post.
Sure they can. Just not at linear order. See my post #29.

When I wrote "I guess spin 2 particles can't handle back reaction?" What I meant was "I guess spin 2 particles can't handle the symptoms of QFT that can't not take into account back reaction?". I know we so far do not have a version of QFT in which we can dynamically solve for the QFT and the background spacetime at once. And this includes spin 2 particles. But then spin 2 particles were not proven to exist so far yet. In case they exist. I was asking (asking for confirmation) we can't still dynamically solve for the QFT and the background spacetime at once even with spin 2 particles?

In String theory. We have spin 2 particles and this gives rise to GR by some kind of linearized gravity tricks? meaning the GR is not fundamental in string theory but emergent from the behavior of spin 2 particles? If so, then spin 2 particles were the fundamental. And there is nothing that would prevent the existence of anti-spin 2 particles or other modes of strings. This means GR not being fundamental can be over ridden by whatever string modes would produced particles or forces that would counter spin 2 particles? Just asking.

Einstein, AFAIK, never considered what a quantum theory of gravity might look like. Most of the work on the QFT of a massless spin-2 field, including figuring out what I described in post #29, was done after he died, in the 1960s and early 1970s.
AFAIK nobody knew about that until after Einstein died. Einstein did not develop GR by the route I described in post #29. When he developed GR, quantum field theory didn't even exist at all; non-relativistic QM was still in its infancy.

 

[PeterDonis]

I know we so far do not have a version of QFT in which we can dynamically solve for the QFT and the background spacetime at once. And this includes spin 2 particles.

No, that's not correct. The QFT of a massless, spin-2 field is General Relativity in the classical limit. That means it is a QFT that dynamically gives you a curved spacetime geometry. It's just not renormalizable.

What we don't have is a way of combining the QFT of a massless, spin-2 field with a QFT containing other fields that are not spin-2, such as the Standard Model of particle physics (which contains spin-1/2 and spin-1 fields), to get a self-consistent model that dynamically determines both the spacetime geometry and the "matter" content. So the problem is that the QFT that tells us the "matter" content does not include a massless, spin-2 field; that QFT can only be solved if we first assume a fixed, background spacetime.

Note, btw, that the above does not mean "back reaction" can't be included at all. One can still include, for example, the expectation value of the stress-energy tensor associated with the "matter" fields in the Einstein Field Equation that you then solve for the background spacetime geometry. What one can't do is have a fully quantum theory including both the "matter" content and the spacetime geometry, in which, for example, we could have a superposition of different spacetime geometries corresponding to a superposition of different matter field configurations.

GR is not fundamental in string theory but emergent from the behavior of spin 2 particles?

More precisely, it's emergent in string theory from the fact that one of the fundamental string modes appears as a massless, spin-2 field in the low energy limit.

there is nothing that would prevent the existence of anti-spin 2 particles or other modes of strings

What do you mean by "anti-spin 2 particles"? The graviton (the massless, spin-2 field) is its own antiparticle, since it does not carry any conserved charges.

As for other modes of strings, yes, string theory has lots of them. Too many, in fact; that's one of the major problems with string theory, that nobody knows how to write down a string theory that just contains the string modes that would produce the actual Standard Model particles we see. (In fact, nobody knows for sure that there even are string modes that would produce the actual Standard Model particles we see.)

 

[jake jot]

No, that's not correct. The QFT of a massless, spin-2 field is General Relativity in the classical limit. That means it is a QFT that dynamically gives you a curved spacetime geometry. It's just not renormalizable.

What we don't have is a way of combining the QFT of a massless, spin-2 field with a QFT containing other fields that are not spin-2, such as the Standard Model of particle physics (which contains spin-1/2 and spin-1 fields), to get a self-consistent model that dynamically determines both the spacetime geometry and the "matter" content. So the problem is that the QFT that tells us the "matter" content does not include a massless, spin-2 field; that QFT can only be solved if we first assume a fixed, background spacetime.

Note, btw, that the above does not mean "back reaction" can't be included at all. One can still include, for example, the expectation value of the stress-energy tensor associated with the "matter" fields in the Einstein Field Equation that you then solve for the background spacetime geometry. What one can't do is have a fully quantum theory including both the "matter" content and the spacetime geometry, in which, for example, we could have a superposition of different spacetime geometries corresponding to a superposition of different matter field configurations.
More precisely, it's emergent in string theory from the fact that one of the fundamental string modes appears as a massless, spin-2 field in the low energy limit.
What do you mean by "anti-spin 2 particles"? The graviton (the massless, spin-2 field) is its own antiparticle, since it does not carry any conserved charges.

Since GR is emergent in string theory. By other string modes I meant just like we have Pauli Exclusion principles that prevent matter from occupying same space. It is not impossible to have string modes that can shield against spin 2 particles? This is just asking.

As for other modes of strings, yes, string theory has lots of them. Too many, in fact; that's one of the major problems with string theory, that nobody knows how to write down a string theory that just contains the string modes that would produce the actual Standard Model particles we see. (In fact, nobody knows for sure that there even are string modes that would produce the actual Standard Model particles we see.)

Maybe (just for sake of arguments) one can just think of string theory as programmable, meaning you can input any arbitrary constants of nature? We will don't know where our constants of nature comes from so we may have to think of string theory as the numbers put it. At least we have the device (string theory) to make it happen.

 

[PeterDonis]

By other string modes I meant just like we have Pauli Exclusion principles that prevent matter from occupying same space.

First, that's not what the Pauli Exclusion principle says. It says no two fermions can be in the same quantum state, but what spatial region the particle occupies is not all there is to the quantum state.

Second, the Pauli Exclusion principle doesn't tell you anything about which particular types of fermions can exist. That's just as true if you think the fermions are "string modes".

It is not impossible to have string modes that can shield against spin 2 particles?

I don't know what you mean by "shield against". If you mean "not interact with", the answer is no: anything that has energy interacts with the massless spin-2 field. That's just as true in string theory.

Maybe one can just think of string theory as programmable, meaning you can input any arbitrary constants of nature?

I don't know what would make you think this. String theory only has one constant whose value can be chosen, the string tension. Everything else is a prediction of the theory, not an input to it. That includes all of the things that in our current theories we call "constants of nature" and have to put in values for from experiments.

 

[Fra]

I would be careful still to mix discussions of conceptual structures and founding principles, with mathematical methods, such are perturbation theory. For this reasons, perturbative approaches to gravity is of course interesting, as i provides a probe, but I do not consider terms popping out from such expansions to necessarily have any profound meaning beyond a mathematical tool.

/Fredrik

 

[Fra]

I don't know what would make you think this. String theory only has one constant whose value can be chosen, the string tension. Everything else is a prediction of the theory, not an input to it. That includes all of the things that in our current theories we call "constants of nature" and have to put in values for from experiments.

But let's not forget that string theory assumes the existence of an exotic space, and defining this backgound at least in the perturbative formulations, where no additional rules "determines" this space, and in particular the compactifications onto 4D + X, requires a lot of "choices".

And as its the idea of string theory to introduces this exotic space, doesn't it lies upon the same theory to determine it?

/Fredrik

 

[Vanadium 50]

It's sad that in spite of being year 2021 already. We were still discussing old 1950s physics. The last quantum revolution occurred in 1927. Hope in 2027 we will have another revolution. We are long overdue for it.

If we have "revolutions" all the time, they aren't really "revolutionary", are they?

Further, we are "discussing old 1950s physics" because one of us hasn't learned it yet. You might think about that before being so critical.

 

[jake jot]

But let's not forget that string theory assumes the existence of an exotic space, and defining this backgound at least in the perturbative formulations, where no additional rules "determines" this space, and in particular the compactifications onto 4D + X, requires a lot of "choices".

And as its the idea of string theory to introduces this exotic space, doesn't it lies upon the same theory to determine it?

/Fredrik

That's right. Something puzzling.
Traditionally, a choice of Calabi-Yau (or more generally, a choice of string vacuum) should determine specific values for the particle masses, etc.

But in String Theory. Spacetime is supposed to be emergent from behavior of spin 2 particles or string modes. Yet a Calabi-Yau that determines the string mode is made of Spacetime (?)

What kinds of concepts where the Planck scale is composed neither of spacetime nor matter (branes)? What do you call something that doesn't belong to matter nor to spacetime? (Exception. Don't mention about spin networks in Loop Quantum Gravity.) I just want to know others. Especially one that arbitrary constants can be put in by hand determining all constants of nature. This is more logical than evolution of laws. Fra. That's why people don't like it generally. As the saying goes. "There are more things in Planck scale than are dreamt of by your physics or philosophy, Horatio". Ones that can directly write the constants of nature without evolution of laws. Others just called it Naturalness, a very complex kinds of Naturalness is more intuitive than evolution of laws (where laws evolved in early universe as Smolin put it).

 

[PeterDonis]

in String Theory. Spacetime is supposed to be emergent from behavior of spin 2 particles or string modes

More precisely, the 4-dimensional spacetime we observe is emergent from those particular string modes that appear at low energy as a massless spin-2 field.

Yet a Calabi-Yau that determines the string mode is made of Spacetime (?)

No, it isn't. The four spacetime dimensions that emerge from the string modes are different from the dimensions that are contained in the Calabi-Yau spaces.

 

[jake jot]

More precisely, the 4-dimensional spacetime we observe is emergent from those particular string modes that appear at low energy as a massless spin-2 field.
No, it isn't. The four spacetime dimensions that emerge from the string modes are different from the dimensions that are contained in the Calabi-Yau spaces.

Aren't the dimensions that are contained in the Calabi-Yau spaces part of spacetime? I read this for example:

pdf (iop.org)

"Extra dimensions are indeed a known fundamental ingredient for String Theory, since all versions of the theory are naturally and consistently formulated only in a space-time of more than four dimensions (actually 10, or 11 if there is M-theory). For some time, however, it was conventional to assume that such extra dimensions were compactified to manifolds of small radii, with sizes about the order of the Planck length, P ∼ 10−33 cm, such that they would remain hidden to the experiment, thus explaining why we see only four dimensions."

So you have our normal spacetime.. then the rest of the 4 dimensions were compactified. Since our 4D spacetime are solutions to the EFE and the EFE can handle many dimensions (not just 4). What is the boundary when suddenly GR is no longer applicable as the dimensions got compactified? In the same paper (actually this is basic of string theory so all references or books it mentioned the same thing):

" Recent developments, based on the studies of the non-perturbative regime of the E8 × E8 theory by Witten and Horava [4], have suggested that some, if not all, of the extra dimensions could rather be larger than P . Perhaps motivated by this, some authors started to ask the question of how large could these extra dimensions be without getting into conflict with observations, and even more interesting, where and how would this extra dimensions manifest themselves. The intriguing answer to the first question point towards the possibility that extra dimensions as large as millimeters [5] could exist and yet remain hidden to the experiments [6, 7, 8, 9, 10, 11]. "

So is this millimeters size space described by GR? It's the same space as the compactified ones only theoretically assumed to be bigger. What size as it goes down when it's no longer described by GR? I presume the dimensions contained in the Calabi-Yau space were the same as the compactified ones.

(the millimeters size thing above may already been excluded by LHC experiments so the above is just mentioned for sake of discussions).

 

[PeterDonis]

Aren't the dimensions that are contained in the Calabi-Yau spaces part of spacetime?

It depends on what you call "spacetime". If you call all 10 (or 11) dimensions postulated by string theory "spacetime", then 6 of those dimensions are the ones in the Calabi-Yau spaces, yes. But if you only call the 4 dimensions we actually observe "spacetime", then no, the 6 dimensions in the Calabi-Yau spaces are different from those.

My understanding (which might possibly be mistaken; perhaps other experts on this forum can weigh in here) of how "spacetime emerges" in string theory is that the string mode that looks like a massless spin-2 field at low energy only affects the 4 dimensions we actually observe, not the others. If that is correct, then the dimensions contained in the Calabi-Yau spaces do not emerge that way; and that was what you were asking about.

So you have our normal spacetime.. then the rest of the 4 dimensions were compactified.

No. You have our normal spacetime of 4 dimensions, and then 6 (or 7) other dimensions that are compactified.

our 4D spacetime are solutions to the EFE

Yes.

the EFE can handle many dimensions (not just 4)

Not the EFE of GR, no. That EFE is specifically for 4 dimensions.

What is the boundary when suddenly GR is no longer applicable as the dimensions got compactified?

The paper isn't investigating when "GR is no longer applicable". It is investigating how large the compactified dimensions could be without conflicting with observations. That has nothing to do with GR not being applicable. See below.

is this millimeters size space described by GR?

Not as geometry, no. As above, GR describes only the geometry of the 4 ordinary spacetime dimensions we observe. As far as GR is concerned, these "extra dimensions", or more precisely their effects as manifested in things like new particles or fields beyond the ones we already know of (in the Standard Model of particle physics) would appear as part of the stress-energy tensor not the spacetime geometry. That doesn't mean GR is "not applicable"; it just means GR doesn't describe the "extra dimensions" as spacetime geometry.

What size as it goes down when it's no longer described by GR?

There isn't one. Why do you think there is? I don't see anything saying this in the paper you linked to.

 

[PeterDonis]

@jake jot Please note that the subject we are discussing is a very advanced one, and this thread should probably be labeled "A" instead of "I". If you do not already have a firm grasp of standard GR and standard quantum field theory and the Standard Model (and it doesn't seem like you do), you really should develop a firm grasp of those subjects first before trying to tackle this one.

 

[Fra]

Especially one that arbitrary constants can be put in by hand determining all constants of nature. This is more logical than evolution of laws. Fra. That's why people don't like it generally. As the saying goes. "There are more things in Planck scale than are dreamt of by your physics or philosophy, Horatio". Ones that can directly write the constants of nature without evolution of laws. Others just called it Naturalness, a very complex kinds of Naturalness is more intuitive than evolution of laws (where laws evolved in early universe as Smolin put it).

The obvious problem with "putting things in by hand", in models that are effectively chaotical or so large is that it requires an extreme fine tuning. This fine tuning is a problem from the explanatory point of view. A model that is not robust, will also probably have little practical predictive power. This is another argument against the eternal law + initial conditions view; because an insided observer can never have enough processing power, to simulate something larger than itself. So if we really want to understand the origin of the effective "laws" I think what smalling calls Newtonian paradigm (eternal law+initial conditions) is necessarily outdated.

What is an "explanation" worth, that requires an a priori improbable presumptions or initial conditions?
(Assuming we manage to find the finely tuned parameters in the first place? which is the other "practical" problem)

Edit: I know this is subtle but IMO an important point but that may seem silly. When we try to understand the nature of how physical law is implemented in nature, HOW does an electron "know" what laws to obey? An electron didnt change in behaviour due to progress of human science. How can we understand, frome the inside view, what really guides an electron? How does an electrove "view" the world? Questions like this may also change the preferences in what research paths you choose. What kind of answers are we really looking for?

/Fredrik

 

[jake jot]

The obvious problem with "putting things in by hand", in models that are effectively chaotical or so large is that it requires an extreme fine tuning. This fine tuning is a problem from the explanatory point of view. A model that is not robust, will also probably have little practical predictive power. This is another argument against the eternal law + initial conditions view; because an insided observer can never have enough processing power, to simulate something larger than itself. So if we really want to understand the origin of the effective "laws" I think what smalling calls Newtonian paradigm (eternal law+initial conditions) is necessarily outdated.

What is an "explanation" worth, that requires an a priori improbable presumptions or initial conditions?
(Assuming we manage to find the finely tuned parameters in the first place? which is the other "practical" problem)

Edit: I know this is subtle but IMO an important point but that may seem silly. When we try to understand the nature of how physical law is implemented in nature, HOW does an electron "know" what laws to obey? An electron didnt change in behaviour due to progress of human science. How can we understand, frome the inside view, what really guides an electron? How does an electrove "view" the world? Questions like this may also change the preferences in what research paths you choose. What kind of answers are we really looking for?

/Fredrik

About the electron. I tried to google "fine tuning electron". I read these:

Is the Universe Fine-Tuned for Life? | NOVA | PBS

"Take, for instance, the neutron. It is 1.00137841870 times heavier than the proton, which is what allows it to decay into a proton, electron and neutrino—a process that determined the relative abundances of hydrogen and helium after the big bang and gave us a universe dominated by hydrogen. If the neutron-to-proton mass ratio were even slightly different, we would be living in a very different universe: one, perhaps, with far too much helium, in which stars would have burned out too quickly for life to evolve, or one in which protons decayed into neutrons rather than the other way around, leaving the universe without atoms. So, in fact, we wouldn’t be living here at all—we wouldn’t exist.

Examples of such “fine-tuning” abound. Tweak the charge on an electron, for instance, or change the strength of the gravitational force or the strong nuclear force just a smidgen, and the universe would look very different, and likely be lifeless. The challenge for physicists is explaining why such physical parameters are what they are."

Other example is from the Fine-tuning article at Stanford site:

"

• The strength of the strong nuclear force, when measured against that of electromagnetism, seems fine-tuned for life (Rees 2000: ch. 4; Lewis & Barnes 2016: ch. 4). Had it been stronger by more than about 50%50%, almost all hydrogen would have been burned in the very early universe (MacDonald & Mullan 2009). Had it been weaker by a similar amount, stellar nucleosynthesis would have been much less efficient and few, if any, elements beyond hydrogen would have formed. For the production of appreciable amounts of both carbon and oxygen in stars, even much smaller deviations of the strength of the strong force from its actual value would be fatal (Hoyle et al. 1953; Barrow & Tipler 1986: 252–253; Oberhummer et al. 2000; Barnes 2012: sect. 4.7.2)."

Do you seriously think there are physics framework that can derived the constants of nature?? You kept mentioning about evolution of laws or self-inference. Did you mean the initial Big Bang had other laws of nature? Or was it in line with Smolin previous Universe before this? Remember Inflation itself is fine tuned. Without the initial condition, inflation won't produce flatness too.

The growth of inflation | symmetry magazine

"It was two fine-tuning problems, two such implausible balancing acts, that inflation was supposed to have solved. “You’re trying to explain away certain features of the universe that seem fine-tuned—like its homogeneity, or its flatness,” says Steinhardt, now at Princeton University, “but you do it by a mechanism that itself requires fine tuning. And that concern, which was there from the beginning, remains now.”

 

[Fra]

Do you seriously think there are physics framework that can derived the constants of nature?? You kept mentioning about evolution of laws or self-inference. Did you mean the initial Big Bang had other laws of nature?

One can object to fine tuning at different levels.

- To the extent a theory requires some fine tuning, but the fine doing is doable without getting divergence computations; then fine. After all, there is a reason you can not just set up a gigantic set of differential equations to model the human brain. Even if it would be principally possible, it would end up requiring increadonble computer power, and it would still end up with chaos - and with it we loose the predcitive power.

- But one should also differentiate between an explanation and a description. Of course, something that aspires not to explain mechanims, but merely describe things. Then a mathematical framework to which we fit experimental data is fine. The the parameters follows from the experimental fitting. Such models does not attempt to answer "why".

The issues comes with you seek to increase explanatory power (which i think we need to do unification of forces) and the issues also comes when the fine tuned parameters are inside a black box not directly accesible to experiment. We then have an "inverse problem", where the inverse may not even exist. But this is a question that Smolin did a good job beating to death in several books. If you read them you probably know the arguments, even if you disagree.

The future will kmpw what is actually possible to find. But I do think that the evolutionary perspective on law, gains us deep insight on the nature and emergence of "law" in a set of interacting system. This does not necessarily mean we can "derive" all constant, but i expect to reduce the number of parameters. It seems perfectly reasonble to me. Thinking the laws require no explanation is what is unreasonable to me.

About Smolins view, in some of his older papers, he for example entertained the concept of cosmological natural selection - It simply speculates that the laws of nature "mutates" during the big bang, so he imagines a history of universes spawned from each other via collapses and big bangs. The idea is that as we do not so far observe that laws of physics changed during the history of our universe, the idea is that any mutations would have to happened like at the very beginning of big bang. From this he had some ideas that limits the mass or neutron stars, in order to have an optimized black hole production.

What the "DNA" is law is etc, is of course now know. That question is a parallell to postulating structures, relations and evolutionary mechanisms. ie. what is the ultimate "abstractions" of matter, observer, relations between observers etC? what mathematical or logical formalism do we need? So far all of physics is basically fixed differential equations and boundary and initial value problems.

I share the idea that the "evolution of law" we are discussing here, in our universe for all practical purposes took place in the very earl big bang. Before the notion of spacetime. either you may believe in mutations like smolin, maybe a very QUICK eveolutionary process during the big bang, that evolved the laws to "perfection" at an amazing speed. One the mad phase is over, the laws are cooled down to what we see. But I think there is explanatory possibilities to ponder abotu this process, and then spacetime and primordal observer (first particles or matter) musy have evolved at the same time. In the early phase, i think some kind fo causal ordering and emergent structure is all we have.

About self-organisation. As smoling also mention in books. The idea of evolution of law, first generates the idea; is there a meta-law that determines the evolution of law? IF so we are back at the same question. The answer is No, there is no meta-law. The evolution is not deterministic. Its a random process, and this is why we instead need to think in terms of evolution and self-organisation. This is the paradigm shift Smoling talks about.

/Fredrik

 

[PeterDonis]

Do you seriously think there are physics framework that can derived the constants of nature??

That's what a lot of physicists are trying to find, yes.

Your sources are not textbooks or peer-reviewed papers. They do not state any kind of mainstream physics when they talk about fine-tuning. They just state the opinions of certain physicists that the authors talked to. Other physicists have different opinions.

We are getting rather far away from the original topic of this thread. Has the original question been answered?

 

[Vanadium 50]

Do you seriously think there are physics framework that can derived the constants of nature??

You know, this is the second time you've been critical, and it's not coming from a place of experience or knowledge.

This has already happened. In the 19th century it was discovered that c^-2 = ε₀ μ₀. Know any two and you can predict the third. In the 20th and 21st centuries people have been predicting the electron magnetic moment - and the more we know, the better the prediction. It started out as "1" (in appropriate units), and in 1928 was calculated to be 2, and in 1947 was calculated to be 2 + α/2π, and today this is known to better than 13 digits.

 

[jake jot]

I think you may be interested in articles by Thomas Andersen. I suggest you to check them out.

Let me get on topic.

A few days ago I actually googled about "Thomas Andersen relativistic" based on your suggestion, I found these among others:

https://www.researchgate.net/project/Emergent-Quantum-Mechanics-from-General-Relativity

"Goal: Einstein's classical general relativity theory has the flexibility and strength to be the field from which electromagnetism and quantum mechanics emerge from. "

and

https://www.researchgate.net/lab/NSciR-General-Relativity-as-the-foundational-field-Tom-Andersen

Fractal matter. Why do you think it's related to General Relativistic Quantum Theory? In your own words, can you share more what Thomas Andersen mean? Because I couldn't log in and read the articles.

 

[jake jot]

No, that's not correct. The QFT of a massless, spin-2 field is General Relativity in the classical limit. That means it is a QFT that dynamically gives you a curved spacetime geometry. It's just not renormalizable.

What we don't have is a way of combining the QFT of a massless, spin-2 field with a QFT containing other fields that are not spin-2, such as the Standard Model of particle physics (which contains spin-1/2 and spin-1 fields), to get a self-consistent model that dynamically determines both the spacetime geometry and the "matter" content. So the problem is that the QFT that tells us the "matter" content does not include a massless, spin-2 field; that QFT can only be solved if we first assume a fixed, background spacetime.

Wait. Since as you put it above "we don't have is a way of combining the QFT of a massless, spin-2 field with a QFT containing other fields that are not spin-2, such as the Standard Model of particle physics (which contains spin-1/2 and spin-1 fields)...". This means using the perspective of QFT hasn't solved it either. So Einstein original problem or challenge remained, how to create something where matter and spacetime evolve (the stage moves along with actor analogy).

Just to emphasize. You wrote above that "So the problem is that the QFT that tells us the "matter" content does not include a massless, spin-2 field; that QFT can only be solved if we first assume a fixed, background spacetime". Hence even when better QFT was developed after Einstein was gone. We still haven't solved it. Although we can at least use QFT and the spin 2 issue to tackle it. And still use QFT in general (QM married to SR).

By the way, how do you marry QM to SR without fields? What did Einstein propose to unify QM and SR?

And up to now. What is the progress or attempt in QFT how to do "a self-consistent model that dynamically determines both the spacetime geometry and the "matter" content"?

Isn't the approach of Loop Quantum Gravity the Einstein preferred approach? Smolin kept emphasizing the thing about stage and actor moves (background independence and diffeomorphism invariance). Does LQG have QFT in it?

I'm trying to read the book "The Road to Relativity" that studies the Einstein manuscript in his discovery of SR and GR.

I'm doing this because I'm so perflexed over something. The vacuum is said to have 120 magnitude more energy than predicted by GR. Why doesn't the universe just bend into itself?

Note, btw, that the above does not mean "back reaction" can't be included at all. One can still include, for example, the expectation value of the stress-energy tensor associated with the "matter" fields in the Einstein Field Equation that you then solve for the background spacetime geometry. What one can't do is have a fully quantum theory including both the "matter" content and the spacetime geometry, in which, for example, we could have a superposition of different spacetime geometries corresponding to a superposition of different matter field configurations.
More precisely, it's emergent in string theory from the fact that one of the fundamental string modes appears as a massless, spin-2 field in the low energy limit.
What do you mean by "anti-spin 2 particles"? The graviton (the massless, spin-2 field) is its own antiparticle, since it does not carry any conserved charges.

As for other modes of strings, yes, string theory has lots of them. Too many, in fact; that's one of the major problems with string theory, that nobody knows how to write down a string theory that just contains the string modes that would produce the actual Standard Model particles we see. (In fact, nobody knows for sure that there even are string modes that would produce the actual Standard Model particles we see.)

 

[PeterDonis]

Einstein original problem or challenge remained, how to create something where quantum matter and spacetime evolve (the stage moves along with actor analogy)

See the bolded addition above. GR already solves the problem of how to have classical matter and spacetime evolve dynamically together. The only part of the problem that isn't (yet) solved is how to do that for cases where the matter has to be described using quantum mechanics.

However, in a practical sense, that problem is a non-problem, because there are no practical cases where we need to describe the interaction between matter and spacetime geometry using quantum mechanics. The problem is purely a theoretical one. And it's not 100 percent certain even that it is a theoretical problem; some physicists (for example, Freeman Dyson) have proposed that maybe there is no such thing as a "quantum" theory of gravity, that quantum fields on a classical curved spacetime really is as deep as we need to go.

how do you marry QM to SR without fields?

You don't. All efforts to do that other than QFT failed back in the 1920s and 1930s.

What did Einstein propose to unify QM and SR?

He never proposed anything along these lines at all. The unification he was preoccupied with for most of his life after discovering GR was a classical unified theory of gravitation and electromagnetism. He was never convinced that QM was a necessary part of a fundamental theory.

What is the progress or attempt in QFT how to do "a self-consistent model that dynamically determines both the spacetime geometry and the "matter" content"?

String theory. String theory is a quantum field theory of strings instead of point particles.

Isn't the approach of Loop Quantum Gravity the Einstein preferred approach?

LQG wasn't even begun as a research program until well after Einstein's death. I don't think it's fruitful to speculate about what he would have thought of it.

Does LQG have QFT in it?

No. It's a different kind of framework. The only connection between the two is that LQG will need to show why QFT works in the regime in which it works--i.e., that a QFT like our Standard Model emerges from LQG in some appropriate limit.

The vacuum is said to have 120 magnitude more energy than predicted by GR.

No, that's not correct.

First, the actual observed value for the cosmological constant (aka dark energy) is 120 orders of magnitude smaller than the theoretical value, not larger.

Second, the 120 order of magnitude difference is between what QFT appears to predict and what we actually observe. GR makes no prediction whatsoever about the value of the cosmological constant; it just says that you can have a cosmological constant term in the field equation and the theory will still work.

 

[PeterDonis]

the 120 order of magnitude difference is between what QFT appears to predict and what we actually observe.

This article by John Baez might be helpful:

https://math.ucr.edu/home/baez/vacuum.html

 

[jake jot]

See the bolded addition above. GR already solves the problem of how to have classical matter and spacetime evolve dynamically together. The only part of the problem that isn't (yet) solved is how to do that for cases where the matter has to be described using quantum mechanics.

However, in a practical sense, that problem is a non-problem, because there are no practical cases where we need to describe the interaction between matter and spacetime geometry using quantum mechanics. The problem is purely a theoretical one. And it's not 100 percent certain even that it is a theoretical problem; some physicists (for example, Freeman Dyson) have proposed that maybe there is no such thing as a "quantum" theory of gravity, that quantum fields on a classical curved spacetime really is as deep as we need to go.
You don't. All efforts to do that other than QFT failed back in the 1920s and 1930s.
He never proposed anything along these lines at all. The unification he was preoccupied with for most of his life after discovering GR was a classical unified theory of gravitation and electromagnetism. He was never convinced that QM was a necessary part of a fundamental theory.
String theory. String theory is a quantum field theory of strings instead of point particles.
LQG wasn't even begun as a research program until well after Einstein's death. I don't think it's fruitful to speculate about what he would have thought of it.

I meant LQG could or might (?) be what Einstein wanted even though LQG was discovered much later after he died (I Know Einstein and Smolin belong to different generations).

No. It's a different kind of framework. The only connection between the two is that LQG will need to show why QFT works in the regime in which it works--i.e., that a QFT like our Standard Model emerges from LQG in some appropriate limit.
No, that's not correct.

First, the actual observed value for the cosmological constant (aka dark energy) is 120 orders of magnitude smaller than the theoretical value, not larger.

I just wrote "The vacuum is said to have 120 magnitude more energy than predicted by GR." Why did you attribute what I wrote to cosmological constant? I meant by vacuum as the ground state of QFT. Why. If one wrote "vacuum", it refers to cosmological constant. And if one wrote "quantum vacuum", it refers to the ground state of QFT? So I can be more careful with words next time.

Second, the 120 order of magnitude difference is between what QFT appears to predict and what we actually observe. GR makes no prediction whatsoever about the value of the cosmological constant; it just says that you can have a cosmological constant term in the field equation and the theory will still work.

What "QFT appears to predict" is related to quantum vacuum. So can't we think of nature as actually having quantum vacuum which is the ground state of the quantum fields? So there is a distinction between "quantum vacuum" and "cosmological constant" (which perhaps one refers simply as "vacuum"?)?

Also what we observe has to do with GR. If we ignore or don't use GR, the 120 magnitude problem predicted by QFT is still a problem or it goes away since if we don't use GR, spacetime doesn't bend?

 

[PeterDonis]

I just wrote "The vacuum is said to have 120 magnitude more energy than predicted by GR." Why did you attribute what I wrote to cosmological constant?

Because that's what "vacuum energy" means. "Vacuum energy", "cosmological constant", and "dark energy" are all just different names for the same thing.

I meant by vacuum as the ground state of QFT.

Yes. That is the meaning I understood you to be using. It doesn't change anything I said. The fact that "vacuum energy" ultimately comes from the ground state of QFT doesn't mean it can only be described in a quantum theory. The cosmological constant is how that same vacuum energy appears in GR.

what we observe has to do with GR.

What we observe of the accelerated expansion of the universe, which is how we know there is a nonzero cosmological constant/vacuum energy/dark energy, requires GR for its analysis, yes.

If we ignore or don't use GR

Then we would be doing an incorrect analysis of cosmological observations.

the 120 magnitude problem predicted by QFT is still a problem

The "problem predicted by QFT" is a problem in other contexts than cosmology, yes. If the energy of the vacuum were what QFT predicted, everything we observe would be very, very different. (It might even be impossible for creatures like us to exist at all.)

if we don't use GR, spacetime doesn't bend?

You are confused. You can't change what spacetime does by not using some particular theory developed by humans.

 

[PeterDonis]

So there is a distinction between "quantum vacuum" and "cosmological constant" (which perhaps one refers simply as "vacuum"?)?

I strongly suggest reading the Baez article I linked to. I think it will help you to overcome some evident confusions that you have.

 

[Fra]

So there is a distinction between "quantum vacuum" and "cosmological constant" (which perhaps one refers simply as "vacuum"?)?

Not sure if this makes it more clear?

1) The cosmological constant is the classical term in the classic GR equations.
In GR is interpreted as a energy density of empty space. (ie. empty classical space).
In principle this has nothing to do with quantum mechanics framework.

2) The other value is the "expectations values" as per QM/QFT of the energy density of the lowest energy levels in the quantum fields. As fields fluctuates with particle-antiparticle creations, the consequence is that even empty space - on average - contains energy. The problem is the formal computations typically diverge and are infinite. Sometimes one can make it finite be renormalization procedures, but its still HUGE, like discussed. In principle this has nothing to do with GR.

Its when you try to merge the two theories and think they are the same, that it doens't add up. But in principe, energy in GR and energy in QM is defined differently.

I personally think this will be solved automatically along with the progress of unification, so it is IMO not a "primary conceptual problem", I see it it's more symptomatic problem of a deeper issue.

Divergent formal expressions is abundant in QM, while renormalization helps tame it in a mathematical sense, it is IMO not a satisfactory way of handling it; its more an engineering trick of curing formal problems, than fundamentally fixing it. One could argue that the divergences are related to the background depencens as well. As the "expectations" actual refer to background structures, that are FIXED. And understanding constructing principles of GR the association "FIXED background ~ infinite inertia" seems natural. But these external backgrounds make no sense in GR. So the problem is not unexpected.

/Fredrik

 

[jake jot]

Peterdonis and others,

Related to this thread. I'm reading the great relativist John Stachel book Einstein from B to Z.

Einstein from 'B' to 'Z' - John Stachel - Google Books

In it is said that "Even relativists have not yet fully adopted the point of view, "no metric, no nothing". As relativists, why have you guys adopted the "no metric, no nothing" view. What is your problem with it?

In the next page where it wasn't shown above is written these:

"space shows. But one does not start out with global topology and then look for all the metrics solving the field equations that are compatible with it. The important point is that if one wants to consider all solutions to the field equations - or even a subclass wide enough to include solutions on topologically inequivalent manifolds - one must have a mathematical structure that allows this to be done.

Although the two ideas are not exactly the same, the moral I want to draw from this final point may be related to something that Chris Islam talked about. Supposed you take really seriously the point of view that there is something fundamentally local about the way general relativity approaches a problem. Then there is another fundamental tension between the basic approaches of general relativity and of quantum mechanics since quantum mechanics, in a deep sense, is fundamentally global in its approach to problems. It doesn't make much sense to talk about the wave function in one path of space-time, or the sum over all paths on one path of space-time. In solving a quantum-mechanical problem, you have to consider the whole manifold from the beginning. The conventional mathematical approach to general relativity, which starts with a manifold, masks this tension. If we develop a mathematical formulation of general relativity that emphasizes the element of locality from the beginning, it would emphasize this contrast more sharply. Such an emphasis on the tension may be a necessary stage in finding its ultimate resolution.

Indeed, this way of thinking also suggests new possibilities. Perhaps the existence of a global manifold is just a special case. Maybe in the ultimate theory all the patches won't always fit together to form a manifold, except in the classical limit. Here is where my point of view may connect some of Isham's ideas. If you don't build in a global manifold at the beginning, perhaps you are better off, because you may have to get rid of it in the end anyway.

What does these lines mean "The conventional mathematical approach to general relativity, which starts with a manifold, masks this tension.", why does it mask the tension?

And continued "If we develop a mathematical formulation of general relativity that emphasizes the element of locality from the beginning, it would emphasize this contrast more sharply." What would it take to emphasize element of locality from the beginning in GR?

And lastly. Concerning the last paragraph and all. It was written in 2000. Are there developments that already disprove some of the ideas?

In the last sentence " If you don't build in a global manifold at the beginning, perhaps you are better off, because you may have to get rid of it in the end anyway."
Can this be made for the case of QM where you don't consider the whole manifold from the beginning?

I want to see ideas where both QM and GR are just effective field theories that you can override in the more fundamental theories. Meaning you are not bounded (binded or whatever) by QM or GR in the ultimate framework.

 

[PeterDonis]

As relativists, why have you guys adopted the "no metric, no nothing" view.

I assume you mean why haven't relativists adopted this view? The answer is simple: they have. Stachel's claim is wrong.

Even on the basis of what Stachel himself says further down on the same page, his claim is at least questionable. After describing the way he claims GR is taught in textbooks (the way that he says does not adopt the "no metric, no nothing" view), he goes on to say that nobody actually does GR that way in practice--the way people actually do GR in practice is the "no metric, no nothing" view. So he's contradicting himself.

Further, I'm not sure even the way GR is taught in textbooks is as Stachel describes. The GR textbooks I'm most familiar with are MTW and Wald; I'm also fairly familiar with Carroll's online lecture notes. None of those sources present GR the way Stachel describes it. A more detailed discussion of why I think that would probably be too technical for an "I" level thread. But suffice it to say I don't agree with Stachel's claims in this regard.

 

[jake jot]

I assume you mean why haven't relativists adopted this view? The answer is simple: they have. Stachel's claim is wrong.

Even on the basis of what Stachel himself says further down on the same page, his claim is at least questionable. After describing the way he claims GR is taught in textbooks (the way that he says does not adopt the "no metric, no nothing" view), he goes on to say that nobody actually does GR that way in practice--the way people actually do GR in practice is the "no metric, no nothing" view. So he's contradicting himself.

Further, I'm not sure even the way GR is taught in textbooks is as Stachel describes. The GR textbooks I'm most familiar with are MTW and Wald; I'm also fairly familiar with Carroll's online lecture notes. None of those sources present GR the way Stachel describes it. A more detailed discussion of why I think that would probably be too technical for an "I" level thread. But suffice it to say I don't agree with Stachel's claims in this regard.

You mentioned above that a more detail discussion would probably be too technical for an "I" level thread, that's why I created an "A" thread in the relativity forum.

But without discussing the technical details (I opened that new thread for the relativity discussion without all this BSM stuff). I think here is the way to resolve the seemingly conflicts. Remember this passage by Einstein himself in 1954.

"It required a severe struggle to arrive at the concept of independent and absolute space, indispensable for the development of theory. It has required no less strenous exertions subsequently to overcome this concept - a process which is by no means as yet completed (Einstein 1954b)"

Looking at background of John Stachel. He was born in 1928, so maybe his statement is because he lived those years Einstein was contemplating it.

https://www.encyclopedia.com/arts/educational-magazines/stachel-john-jay-1928

"Lehigh University, Bethlehem, PA, instructor in physics, 1959-61; https://www.encyclopedia.com/social-sciences-and-law/education/colleges-us/university-pittsburgh, Pittsburgh, PA, instructor in physics, 1961-62, research associate, 1962-64; Boston University, Boston, MA, assistant professor of physics, 1964-69, associate professor, 1969-72, professor, 1972-97, professor emeritus, 1997—, Center for Einstein Studies, director, 1985—. Visiting research associate, Institute for Theoretical Physics, Warsaw, Poland, 1962; visiting professor, King's College, London, 1970-71, University of Paris, 1990-91, https://www.encyclopedia.com/people/science-and-technology/physics-biographies/max-planck Institute for History of Science, Berlin, 1994—, and https://www.encyclopedia.com/social-sciences-and-law/education/colleges-us/california-institute-technology, 1998; visiting senior research fellow, Department of Physics, https://www.encyclopedia.com/social-sciences-and-law/education/colleges-us/princeton-university, 1977-84; editor, Einstein papers, 1977-89; research associate, https://www.encyclopedia.com/social-sciences-and-law/education/colleges-us/university-california—Berkeley, 1994. "

"As director of Boston University's Center for Einstein Studies, Stachel conducts conferences on general relativity with other leading scholars, and with Don Howard edits the resulting collections of papers in the "Einstein Studies" series. In reviewing Volume One in this series, Einstein and the History of General Relativity, Wolfgang Drechsler wrote in Physics Today that the volume "is a true and valuable source of information on the origin and development of Einstein's theory. The reader may only wonder why such a book had not been written decades ago." "

So the answer is after 70 years, with new generations born, the no metric, no geometry is already not doubted.

Going to Einstein United Field Theory. You wrote in message #50

He never proposed anything along these lines at all. The unification he was preoccupied with for most of his life after discovering GR was a classical unified theory of gravitation and electromagnetism. He was never convinced that QM was a necessary part of a fundamental theory.

But at that time, there was already the electron. How come Einstein didn't add electron to unification of gravity and electromagnetism? Whereas in our present String Theory, the electron is included along with gravity, electroweak, and strong force, and matter particles like quark?

Whatever. I know GR and QFT explain our world. And the ultimate framework is only said to be valid at higher energies (like Planck scale), right? But why is it impossible to have the ultimate theory occurring right at the low energy scale, where with another mode. GR and QFT theories are not used by a more powerful theory? The physicists reasonings might be that they are no phenomena at low energy not explainable by our GR and QFT (even dark matter). I couldn't state the reasons why they are wrong, But just theoretically. Is it not at all impossible to formulate a theory where the ultimate framework can also occur at low energy by bridging some kind of barrier whereby GR and QFT are just effective field theory at a patch. Here the last two paragraphs of Stachel passages may be relevent.

(others, kindly give models where the ultimate framework occurs at low energy, and not the Planck scale)

 

[PeterDonis]

You mentioned above that a more detail discussion would probably be too technical for an "I" level thread, that's why I created an "A" thread in the relativity forum.

As I noted in that thread, though, I'm not sure you have the background for an "A" level discussion. You would probably be better served by taking the time to study some GR textbooks or course notes first. Then you might be able to evaluate for yourself whether Stachel's description is accurate. Sean Carroll's lecture notes on GR are freely available online:

https://arxiv.org/abs/gr-qc/9712019

 

[PeterDonis]

How come Einstein didn't add electron to unification of gravity and electromagnetism?

It's right there in what you quoted from me: "He was never convinced that QM was a necessary part of a fundamental theory." He thought a classical unified field theory was the way to go, so that's what he worked on.