Theories without Fundamental Space and Time

AgentSmith said:

What is the difference between spacetime and fundamental space and time?

• Asymptotic safety in quantum gravity
• Euclidean quantum gravity
• Causal dynamical triangulation[52]
• Causal fermion systems
• Causal Set Theory
• Covariant Feynman path integral approach
• Group field theory
• Wheeler–DeWitt equation
• Geometrodynamics
• Hořava–Lifshitz gravity
• MacDowell–Mansouri action
• Noncommutative geometry
• Path-integral based models of quantum cosmology[53]
• Regge calculus
• Scale relativity
• Shape Dynamics
• String-nets and quantum graphity
• Superfluid vacuum theory a.k.a. theory of BEC vacuum
• Supergravity
• Twistor theory[54]
• Canonical quantum gravity
• E8 Theory
• Quantum holonomy theory[55]

kiki_danc said:

Fundamental space and time is indeed the classical spacetime. And I guess all quantum gravity theories are going beyond fundamental space and time. The following list is taken from the quantum gravity entry of Wikipedia (they are in addition to the leading candidates of string theory and loop quantum gravity). There are just so many possibilities and models. I'm specifically looking (or filtering) for ones where the quantum degrees of freedom (of whatever) that makes up or create emergent space and time can be directly influenced or manipulated. Anyone care to share what models can do this? With so many models and possibilities. We need guidance from any data (no matter how unconventional) to have any chance to arrive at the right quantum gravity theory.

• Asymptotic safety in quantum gravity
• Euclidean quantum gravity
• Causal dynamical triangulation[52]
• Causal fermion systems
• Causal Set Theory
• Covariant Feynman path integral approach
• Group field theory
• Wheeler–DeWitt equation
• Geometrodynamics
• Hořava–Lifshitz gravity
• MacDowell–Mansouri action
• Noncommutative geometry
• Path-integral based models of quantum cosmology[53]
• Regge calculus
• Scale relativity
• Shape Dynamics
• String-nets and quantum graphity
• Superfluid vacuum theory a.k.a. theory of BEC vacuum
• Supergravity
• Twistor theory[54]
• Canonical quantum gravity
• E8 Theory
• Quantum holonomy theory[55]

kiki_danc said:

Einstein tried to unify general relativity with electromagnetism but failed because he missed the weak and strong forces. Could we say that our present attempt to unify the forces can only fully succeed if we take into account other possible forces or dynamics besides that in the Standard Model? How many acknowledge this scenario to be the possible case.. or is our attempt to unify spacetime with the quantum is regardless of whether they are other unknown forces, etc.?

Fra said:

The most famous example is of course string theory.

The general idea is that
- in order to unify or explain what are currently fixed or theoretically "unexplained" or to explain something in a coherent way as opposed to patchwork of effective theories -
one needs to put these things in a larger context which relaxes them. This is the idea behind explaining law as evolution or negotiation.

The idea of strings is in tradition of kaluza klein to consider compactified small extra dimension. Ie in each 4d point there is a 6d manifold that can varying topplogy etc. Then try to "explain" and unify all forces from the interaction propertirs of this manifold in 4d space. This increase the complexity and sometimes this has more than one solution.

The story is that in string theory is that people conjectures that there are so many ways to compactify the 6d manifold while constraining the cosomlogical scale of 4d part to deSitter space that one speaks of the string landscape. Now a RECENT debatw in string community is that larger part of this landscape is rather wrong (they call it swampland)

Originally there was some hope that there would not be a landacape. Once realized even string theorists startew to talk abouy evolution of law. Smolins evolving law was originally proposed as a conceptusl solution to string landacape problem. But right now i think it transcends itm

There are real string theorists on here that can give you better answers.

I just relate to other researc programs and have opniona on their logic but my own view does not contain strings or no continuum in the starting points.

I am not sure how you want to combine lqg and st. I have no meaningful comments.

/Fredrik

Fra said:

I don't think its rational for science to expect to learn about something without interacting with it, or without having access to observational data.

That said, there is plenty observational data including the condensed form of the patchwork of well corroborated theories we have for physics. But things does not only concern HEP or probing space, we also have more ready access to complex systems here on earth, whose relevance for understanding IMO is underestimated. Mainly due to the common reductionist philosophy of physicists. To speak for myself, I gained a lot of insight from observing and trying to understand how complex systems organise, interact and evolve. This involves biological systems, economical systems, and social systems. If you ponder about evolution of law at the level i do, all these things are potentially at some level described by the same mathematical abstractions as is evolution of physical law. This is indeed a kind of generalisation of probability theory. After all, logic of science has common roots with probability theory as it has to do with quantifying degrees of believe in a rational way.

My imprecise writings concerns first of all these abstractions, and the exact mathematical imlpementation of this is something i work on. Unfortunately i am not aware of an existing research program that is in line with mine. But several are related to it. Also once my arguments are precise and actually accomplishes something current programs dont, they will be published but not sooner.

So to speak for myself, the data today is decent enough to feed me trying to find a coherent framework instead of a patchwork of effective theories. A researcher can "experiment" with its own ideas, by exploring logical implications of conjectures analytically/formally and numerically by using computers. I do not blame too low energies at LHC for my lack of progress, I think there are "cheaper sources" of observational data that are relevant. So what is the probability that someone like me will succeed in my lifetime? Surely the vision is an a priori hard and difficult, and looking at history, a human once in a few hundred years will succed at such a task at best. So the odds are skyhigh, but its enjoyable nonetheless and there is no shame for not succeeding ;-)

The creative process of research or learning about anything for that matter can IMHO not be described as a deductive process, where you use only precise steps and arguments. The more precise describtions emerge when progress is made. During the process things may remain "somewhat precise" only to people within the same research community, or alike thinkers.

Precise things can be communicated to the outside of the community only when substantinal progress is made. But at this point the creative part of the problem will be largely done ;)

/Fredrik

kiki_danc said:

Are you familiar with Shape Dynamics..

kiki_danc said:

Are you familiar with Shape Dynamics.. which uses Weyl Transformation which is a shape-preserving transformation. Shape-preserving means that angles are unchanged, but overall scale can change. This is in contrast to General Relativity where length is preserved. Is this related to the idea of the relativity of observers (and how to relate the concepts)?

kiki_danc said:

Here is a new question.
...
First. It is just analogue so classical general relativity has no fundamental gauge symmetry.
But is it a requirement that all our ultimate theories need to reflect redundancy as in "Both gauge invariance and diffeomorphism invariance reflect a redundancy in the description of the system.". In the case of General Relativity what if there is no redundancy in the description of the system?
...
If redundancy and gauge principle need to be obeyed by any fundamental theory. If spacetime was really emergent from more primary constituents or ingredients (like spin networks, etc.). Can the redundancy instead be reflected in the more primary ingredients meaning the theory is no longer sensitive to shapes, scales, lengths or time? In other words, can the gauge transformation be made instead to these more primary ingredients (like spin networks, etc)?

Fra said:

Shape dynamics is a development from ideas of Julian Barbour, which essentially focues on the nature of time at its core, arguing time as dimension has no place in physics, going back to Machs ideas. I have not read any of his books, but some of his papers. Smolin also mentions Barbour in his own talks and papers as someone that explain the view of "end of time" clearly, in constrast to Smolins reality of time. But Smolins "reality of time" is not anything like Newtonian time. He seem to think of it more like an evolution parameters of laws.

https://fqxi.org/community/forum/topic/360
http://www.platonia.com/FQXi_Full_Proposal_2011.pdf

Barbour says time shall be operationally defined in terms of relative change of position in space, and given initial conditions (state and tangent) the timeless dynamical laws yield the future state. This is why he considers the shapes in 3D and their tangent changes as the right starting point. This corresponds to singling out a preferred 3D+1 of 4D; where the next 3D layer deductively follows from a timeless law.

Paradoxally it seems Smolin was Barbours phd supervisor, but today they seem to hold competing views on the nature of physical law and time.

While I symphatise with the relational ideas (I could even argue that they are not take far enough), set aside issues that SD disagrees with GR except for special cases, i have several conceptual issures, a couple are:

1) 3D space is a non-trivial starting point, that just like time, also begs an explanation. Just like 4D space ca be thought of as evolving 3D structures, why not see 3D space as an evolving 2D space, and 2D as evolving 1D, and 1D as an evolving point? Exactly where does the dimensionality 3 come from?

I will just say that i symphatise partly with this, but stopping at 3D breaks the beauty. I envision that you can keep reducing it further.

2) Another problem is that 3D space and "points" are classical concept. What is the generalization of the "point configuration space", into something that makes sense in a modern inference perspective, which i take to be the founding core of QM. Well represented by this famous quote

"It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."
-- https://en.wikiquote.org/wiki/Niels_Bohr

"What we can say about" to me, means what an observer can infer (measure + abduce) from interactions. This implies also preconditions such as formulating questions (~ preparing measurement devices etc). This is the reason why i analyse the inference.

So what is the minimal starting point for this scheme? Is it positions in classical 3D space? If not, what?

3) Julian argues that things should be operationally defined in terms of relative changes of positions, by why not argue that laws should also be operationally defined. If the argument is that physical law is there even if we do not know about it, then this lacks perspective. An external observer can easily predict what understanding we have of physical law, by observing the technology our civilisation uses. This the way in which is does make a difference. Then translate this to subatomic particles, explain if there is any principal difference beyond complexity scale?
This is related to my critique against Rovellis relational QM as well. It starts out well, but the failure is that at some points in the hierarcy the "relations" are detached from observation and gets places outside the inference process.

See old thread: https://www.physicsforums.com/threa...able-in-classical-and-quantum-gravity.220841/

This is a good focus but a big one. About questions on gauge symmetries vs observer relativity.I commented on this in a way that I suspect no one understood either here (explaining this properly requires no less than a paper and backing it up with other stuff, and this isn't the place)
https://www.physicsforums.com/threads/ed-witten-on-symmetry-and-emergence.927897/

So I would not look at SD in order to understand evolving law. Much deeper grips are needed.

/Fredrik

kiki_danc said:

I'll digest the above and reflect.

Fra, are you also adept in math besides theoretical exposition? Do you know how to manipulate the math of spacetime (or quantum gravity) to make the theory no longer sensitive to angles, scales, length or time by shifting the gauge transformations of basic elements elsewhere (perhaps to more basic degrees of freedom like spin networks (or others that can work))?

Can anyone else please comment more about this?

kiki_danc said:

Do you know how to manipulate the math of spacetime (or quantum gravity) to make the theory no longer sensitive to angles, scales, length or time by shifting the gauge transformations of basic elements elsewhere (perhaps to more basic degrees of freedom like spin networks (or others that can work))?

Fra said:

What "will work" in the sense of succeding with unification is an open question, no one knows.

But what i would can say about something that's more primary than spacetime and that is in line with an observer-centered perspective - without diverging into details of my own hypothesis - is to ask some questions without adding my personal answers. Then ponder how/if the various research programs (lqg, strings, causal sets etc) address these.

First: Spacetime embeds events (sets of n real numbers) in a topological space where the key is a distance metric.

What is the observational basis for these events? What is the basis for the continuum? Can an observer can represent a real number with infiite precision?

What is the observational meaning of the distance metric? What does it mean for events to be close vs beeing widely separated?

If we define also a geometry on this space what is the observational meaning to straight lines? What is the meaning of volume?

And when answering this, what is the relatiion between the originally distinguishable events and the continuum embedding and volumes of the emedding? For example when the observer does statistical inference, how do we ensure proper counting of events and prevent arbitrary integration measures in the continuous embedding?

Can a finite bounded observer of mass m count and resolve arbitrary numbers? If not, what happens when the observational process is constrained by the limitations of the observer? (Lets even think of it as a Planck scale observer)

From an single observer-perspective, what is the difference between information in general and classical information?

So what are the more basic degrees of freedom that may preceede spacetime?

Some hunches may emerge once you tried to answer the questions above. I would say that if spacetime are inderstood as relations between parta of matter or between communication observers then one can not answer this question without also answering what is the primary microstructure of matter. They unaviodably go hand in hand, which i why i think the evolutionary thinking is needed, where spacetime and matter emerge together. Here the step to evolution of law is also small. At least in my head.

This is why i think fundamwntal theories about empty space are bound to not be fruitful. Also vice versa, because without lorentzian spacetime the whole of HEP and its standardmodel would loose its backbone.

I see none of the big programs as satisfactory here. But string theory is at least most ambitious and worked on.

/Fredrik

kiki_danc said:

What is the difference between observer in quantum mechanics and observer in relativity? Are they not equal? Should they be made equivalent by some kind of transformation? does this already exist in our physics?

And when you say observer.. do you mean a human or a machine observer?

Also I'm looking for the model where these degrees of freedom that precede spacetime can form some kind of structure that can hold the observer.. do you know of any models that explore this?

• In classical mechanics - both special and general relativity - the physical process of observation, is assumed to be possible to make arbitrary small so that you can extract the desired information without distorting it.
• In quantum world, this is not possible becauase the process of acquiring information, changes the system. And not just at the classical way (which can be arbitrary small), but in a fundamental logical way. Ie. in quantum world you can not "copy" information without disturbing it.

• The laws of physics must be same for all interacting observers, meaning they should be able to communicate their observations and agree.