How could spacetime become discretised at the Planck scale?

[DJsTeLF]

In early 2010 I attended this inaugural lecture by string theorist- Prof. Mavromatos entitled 'MAGIC strings'. In it he proposes that some string theory models may violate Lorentz symmetry at the Planck scale resulting in a kind-of foamy spacetime that could be observed by differing arrival times of photons of different energies reaching us from distant astronomical sources. See http://www.kcl.ac.uk/news/events_details.php?year=2010&event_id=2178 or here for one of the papers: http://iopscience.iop.org/1742-6596/174/1/012016

Furthermore, in 'Cycle's of Time' that I read recently, Prof. Sir Roger Penrose mentions (page 203) that Wheeler and others have strongly argued that if we could examine spacetime at the Planck scale we would see a turbulent chaotic situation (from vacuum fluctuations of the quantum fields I suppose) or perhaps a discrete granular one. Penrose goes on to list some other approaches that may suggest how this discrete structure may manifest itself. Loosely transcribed these are: spin foams, casual sets, non-commutative geometry, Machian theories, twistor theory or strings and membranes existing in some higher-dimensional geometry...

I have studied some QM, introductory QFT and the Standard Model as well as some basic GR but I have no formal experience of string theory. My questions are therefore:

- What's involved with each of the above approaches? I.e. in what way does the spacetime become discretised? (Particularly in string theory)

- Are there any other popular(ish) approaches that should be added to the list?

- Supplementary query, with GR being a background-independent theory, I fail to see how one can end up with discretised spacetime without it being a pre-defined background onto which a theory of the dynamics would have to be 'bolted-on'??

Please forgive my ignorance if what I have said is misinformed, all comments and elucidations would be most welcome.

 

[mitchell porter]

See section 4 of the paper you link to. The visible world is a D3-brane in a higher-dimensional space, and there is a shower of point-like D0-branes passing through it. At the moment of intersection, a D0-brane shows up as a temporary point defect which can interfere with the propagation of an open string on the D3-brane. It's an imaginative picture, but it does not involve discretized space, and there's no evidence that this is actually happening.

 

[marcus]

... Penrose goes on to list some other approaches that may suggest how this discrete structure may manifest itself. Loosely transcribed these are: spin foams, causal sets, non-commutative geometry, ...

CDT (causal dynamical triangulations) is often mentioned along with those. It is one of the best presented and easiest to understand. To get some intuition you might have a look at a SciAm article. It is the "Loll QG SciAm"link in my signature at the end down at the bottom of the post.
Renate Loll (one of the coauthors) did her PhD at London Imperial, with Chris Isham, if I remember correctly. Good clear English style, good graphic illustration.

I'm not advising you that CDT is the best or most promising approach. It is a good place to start because it is well presented. Perhaps you will have a look and come back with more questions.

... with GR being a background-independent theory, I fail to see how one can end up with discretised spacetime without it being a pre-defined background onto which a theory of the dynamics would have to be 'bolted-on'??...

The confusion is with words like "discretized". What does it mean?

Our model of the H atom has discrete energy levels for the electron, and the spin of the electron takes discrete values. In the real world we see probability mixtures of discrete possibilities.

So geometry can be "discretized" without space being "MADE" of little bits.
Volumes, areas, angles could be quantized. MEASUREMENT of geometric quantitites could be governed by quantum rules----so that the result is analogous to the energy levels of an atom. Without space being "made" of little grains. Space is geometric relationships. It is not made of anything. Think of it as a web of relationships (of area, angle, ...) that we can in principle measure.

What we cannot in principle measure, does it mean anything? Can it be made of anything?

So e.g. in the spinfoam approach to quantized geometry, one of those you say Penrose mentioned, the focus is on how nature responds to measurement, not on what space is "made of". You take a finite set of geometric information and describe how it evolves.

You can say it is discretized because the area and volume operators are quantum operators with discrete spectrum----but probabilities can mix the outcomes so that they can vary continuously.

It's late at night here and I'm not being entirely clear. Maybe I can improve this tomorrow, or someone else will step in and say it better.

Maybe the main lesson is don't take it too literally when popular writers say "discrete".
There may not be anything material there to be discrete or continuous. Geometry might be about information, or making measurements.

 

[czes]

I am learning about Holographic Principle and Quantum Information. If the Holographic Universe is right, the space is a result of the information relation (geometric relation). A relation between 2 quantum information creates a Quantum Event. Each such Quantum Event causes (encodes in a program) time dilation = Planck time = about 10^-43 s.
Therefore a photon as a wavy distortion in that field of the Quantum Events counts these Events and when it passes 10^43 Events we call it one seconde of time.
In Holographic Universe it is just a simple program - something similar you have a game of the Virtual Reality in your Computer or in the CD.
There isn't space, distance, time in the CD but if you press a button of the computer the Virtual Reality runs due to program and Information written in the CD. The Quantum Information come to the relation and create the Events you observe on the screen of the computer.
There is also a simple calculation how the Planck length, Compton length, electromagnetic and gravitational interaction are related.
http://www.hlawiczes1.webpark.pl/gravastar.html
It is a hypothesis now but Holographic Principle support Hooft, Smoot (Nobel laureate), Suskind (String Theory), Verlinde, Smolin (LQG) and many other good physicists.

 

[Naty1]

Penrose describes twister theory in chapter 33 of THE ROAD TO REALITY, 2005. That's his baby...I think he has been working on it well over 20 years...and he spends almost 50 pages on it. The first section discusses "Theories where geometry has discrete elements".
Lots there if you are interested...I have not read this chapter as my sense is he has been unable to complete the theory.

Penrose has lots of concerns about string theory...a few mentioned around page 907.

Elsewhere, there seems to be some preliminary evidence...inferences...that spacetime becomes two dimensional...I think I saw some discussion here in the forums during the last few monthes but I've been away doing other things a lot. Don't know how this might relate to discreteness,

There are some good discussions in Lee Smolins THREE ROADS TO QUANTUM GRAVITY (2001)and I think they take a different perspective than many here, Marcus, for example if I understand his comments correctly.

page 95

There are good reasons to believe the continuous appearance of of space is as much an illusionas the smooth appearance of matter. When we look at a small enough scale, we see that space is made of things that we can count.

..(If) a photon travels through a discrete geometry it will suffer small deviations from the path that classical physics predicts...caused by the inteference effects that arise when the photons associated wave is scattered by the discrete nodes of the quantum geometry...

a computer image of quantum spacetime is pictured on page 144...reminds me of multiple space needles housing areas piled one upon another, varying diameters, and each quantized into tiny shapes.

Around page 163 Smolin writes how string theory describes a discrete structure...a string that is wrapped up with a radius R of one dimension...

What happens when R becomes very small is indistinguishable from what happens when R is very large

meaning it has a minimum size...

and 164

..a string going...very nearly the speed of light...would appear to contain a set of discrete elements...called string bits

and he goes on to say 166

..This tells us that M theory, if it exists, cannot describe a world in which space is continuous (nor) can one pack an infinite amount of information into any volume

He also has many references to hologrpahic principle constraints menitoned by others above. A number of these begin at page 169.

He does not have heavy math to describe the underpinnings of his comments...just introductory college level pieceparts from time to time for illustration.

 

[marcus]

Hi Naty, I could be wrong of course but I think Smolin does not speak for the main body of LQG opinion, and as you point out that popularization is 10 years old. Since around 2008 he sometimes doesn't go to the main workshops and conferences, or he goes and talks about something else off the beaten track. He does valuable exploratory work on several new proposals but does not produce LQG PhD students or mainstream LQG research papers.

"When we look at a small enough scale, we see that space is made of things that we can count." is not talking accurately about LQG----except as a impressionistic way of (mis)communicating at a popular level. Nor is it faithfully representing most other familiar approaches: CDT, AsymSafe gravity, NCgeometry.

Or else "things" must be something abstract and unfamiliar like a "potentialness to measure a volume".

It could be he is talking about Causal Sets. That is a rather small program. And there they prove Lorentz invariance IIRC. In causets spacetime really is a swarm of discrete pointlike events. I don't know how they do it but they prove Lorentz invariance. to be sure, one of us would have to check the recent series of PIRSA lectures by Rafael Sorkin and Fay Dowker, They gave a definitive course at Perimeter in 2010.

He might eventually turn out to be RIGHT! But what you quote him saying in 2001 is not representative of the greater part of the community.

Unless you explain carefully what you mean. Namely that geometric relationships (area, vol. angle...) are something you MEASURE and the measurement assumes a discrete set of values (i.e. the measurement operator has discrete spectrum.)
When you read the fine print you discover that this is what has always been meant, in LQG, by the "discreteness" of geometry.

In LQG the spin networks are one of several possible mathematical descriptions of a quantum state of geometry. A realistic state of geometry could be described by a superposition of those quantum states. Indicating what you would get if you defined a surface and measured its area or its enclosed volume. A quantum state represents information about reality.

The theory does not say that if you took a fantasically high power microscope and zoomed in you would actually see ball-and-stick model networks! the quantum state does not describe what IS. It describes the response to measurement operators.

However probably everybody at a certain popularization level will resort to phrases like "atoms of space and time" because they can't expect to communicate to broad audiences except by naive oversimplification.

I hope I'm not making a mountain of a molehill. I see a difference between the professional view of LQG and the popularized picture. Maybe you think there is no essential difference and one might as well forget the overfine distinctions.

So even in that case, where you have an approach that does have little bits of spacetime, Smolin's 2001 argument for some detectable effect on the speed of light would seem to fail.

 

[czes]

Thank you Naty 1 , for Penrose. I have to read THE ROAD TO REALITY at least. Have I to add Penrose to the list of Holographic Principle advocates ?
Thank you Marcus for Causal Sets Theory. They have a good connection to the Holographic Universe too, I think.
I am sorry for my obstinacy about Holographic Principle.
I do search what is the space for some years. The discretness is here very important and almost all physicists wrote about it. In LQG the discretness of the space is natural as the basic assumption. In String Theory they came to the same conclusion indirectly via Holographic Principle (Suskind, Maldacena, Hooft...).
I think, the right way is the Information Physics as wrote Archibald Wheeler.

May be we have enough experimental information about our reality. The problem is how to put it together. The simplified calculations in my website shows that everything might be explained (gravitational time dilation, holographic principle, Dark Matter effect) if we assume the discretness of the space as a result of the information interference.

We can't see the grain of the space as we can't see a bit of the information in a computer. In a Hologram we see a complete picture though we divide the screen. The picture will be less clear then, till it disappear when we come to a size of the wave length.

 

[marcus]

... In LQG the discretness of the space is natural as the basic assumption. ...

Actually not true. Discreteness of space is not assumed in LQG. The discreteness result must be proved as a theorem. It was a surprise when it was discovered, some years after theory first formulated.

You can read about that in any competent historical review of the subject, for example
http://arxiv.org/abs/1012.4707. If you need a page reference, please ask.

The main thing to realize is that with LQG there is no discreteness put in as an assumption.
Whatever discreteness is derived as a conclusion.

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

Since you are interested in the holog. principle, you might find this article interesting:
http://arxiv.org/abs/0804.0632

It addresses the problem of how to reconstruct the bulk from the boundary information, in simple case. Actually it studies a number of related problems like that.

 

[czes]

Thank you Marcus for the paper of Carlo Rovelli.
I did read about LQG some years ago and some articles about a discrete space were there already. I didn't know it wasn't at the begining.
I hope all the theories will meet together at the discrete Information Background at least.

 

[qsa]

In early 2010 I attended this inaugural lecture by string theorist- Prof. Mavromatos entitled 'MAGIC strings'. In it he proposes that some string theory models may violate Lorentz symmetry at the Planck scale resulting in a kind-of foamy spacetime that could be observed by differing arrival times of photons of different energies reaching us from distant astronomical sources. See http://www.kcl.ac.uk/news/events_details.php?year=2010&event_id=2178 or here for one of the papers: http://iopscience.iop.org/1742-6596/174/1/012016

Furthermore, in 'Cycle's of Time' that I read recently, Prof. Sir Roger Penrose mentions (page 203) that Wheeler and others have strongly argued that if we could examine spacetime at the Planck scale we would see a turbulent chaotic situation (from vacuum fluctuations of the quantum fields I suppose) or perhaps a discrete granular one. Penrose goes on to list some other approaches that may suggest how this discrete structure may manifest itself. Loosely transcribed these are: spin foams, casual sets, non-commutative geometry, Machian theories, twistor theory or strings and membranes existing in some higher-dimensional geometry...

I have studied some QM, introductory QFT and the Standard Model as well as some basic GR but I have no formal experience of string theory. My questions are therefore:

- What's involved with each of the above approaches? I.e. in what way does the spacetime become discretised? (Particularly in string theory)

- Are there any other popular(ish) approaches that should be added to the list?

- Supplementary query, with GR being a background-independent theory, I fail to see how one can end up with discretised spacetime without it being a pre-defined background onto which a theory of the dynamics would have to be 'bolted-on'??

Please forgive my ignorance if what I have said is misinformed, all comments and elucidations would be most welcome.

topos is the secret.my own theory looks at thing from similar perspective.


Topos started with Christopher Ishamh

https://www.physicsforums.com/newreply.php?do=newreply&p=3116315


here is a detailed paper:


http://arxiv.org/PS_cache/arxiv/pdf/0803/0803.0417v1.pdf

from page 8

In this context, a striking feature of the various current programmes for
quantising gravity—including superstring theory and loop quantum gravity—
is that, notwithstanding their disparate views on the nature of space and
time, they almost all use more-or-less standard quantum theory. Although
understandable from a pragmatic viewpoint (since all we have is more-or-less
standard quantum theory) this situation is nevertheless questionable when
viewed from a wider perspective.
For us, one of the most important issues is the use in the standard quantum formalism of critical mathematical ingredients that are taken for granted
and yet which, we claim, implicitly assume certain properties of space and/or
time. Such an a priori imposition of spatio-temporal concepts would be a
major category
5
error if they turn out to be fundamentally incompatible with
what is needed for a theory of quantum gravity.
A prime example is the use of the continuum6
by which, in this context,
is meant the real and/or complex numbers. These are a central ingredient in
all the various mathematical frameworks in which quantum theory is commonly discussed. For example, this is clearly so with the use of (i) Hilber...

 

[czes]

This topod of Isham is very interesting. Almost all the best physicists say the space is emergent.
My assumption is that the space is build of the interacting information. Each interaction causes the Planck's time dilation and we get the time. If the wasn't time dilation at the quantum level our Universe would be a perfect Black Hole with an infinity entropy. The program which has written time dilation creates our Observable Universe.
This fundamental programmed Planck time dilation causes the discrete space.
Therefore the order of events is possible:
While group theory has become an essential tool for theoretical physics, order theory remains entirely overlooked. (Kevin H. Knuth 2010)
http://arxiv.org/abs/1009.5161

 

[Careful]

This topod of Isham is very interesting. Almost all the best physicists say the space is emergent.

It depends upon what you mean with that. For example, it could very well be that space is ''emergent'' and ''continuous space'' is broken down but that the continuum description of spacetime still holds (this is the case in my work for example). It all has to do with non-integrable distributions in Finsler geometry.

 

[czes]

It depends upon what you mean with that. For example, it could very well be that space is ''emergent'' and ''continuous space'' is broken down but that the continuum description of spacetime still holds (this is the case in my work for example). It all has to do with non-integrable distributions in Finsler geometry.

Verlinde wrote "gravity is entropic force". It is not true at quantum level because quantum microstates are conserved (experiment with ultracold neutrons).
In Holographic Universe the space is build of the interacting information as Quantum Events.
In previous String Theory models the strings were moving in the space while in M-theory it is rather the branes create the space.
I think the physics is going toward the Wheeler's idea of the Information Background of the space.

 

[Careful]

Verlinde wrote "gravity is entropic force". It is not true at quantum level because quantum microstates are conserved (experiment with ultracold neutrons).
In Holographic Universe the space is build of the interacting information as Quantum Events.
In previous String Theory models the strings were moving in the space while in M-theory it is rather the branes create the space.
I think the physics is going toward the Wheeler's idea of the Information Background of the space.

I think it is not, there hasn't been a single physicist yet who has uttered something new and deep after he was 55 and I think Wheeler is no exception to that. The holographic principle has nothing to do with discreteness or whatever; all the holographic principle says is that ''information'' about dynamics is stored on a two surface. First of all, nobody knows what information is and second, the quantum part is merely an addition which may very well be wrong (and is wrong as far as I understand it). Verlinde has no gravity theory, he has plenty of ansatze which didn't lead to a coherent picture yet. Moreover, do you know of any coherent theory with emergent locality and causality where strings make up space? Please, give me its equations and some easy solutions.

 

[czes]

Lubos Motl's blog:

Quantum Mechanics guarantees that the concept of a completely smooth geometry is incompatible with quantum mechanics that make things fluctuate. But string theory goes much further. Geometric descriptions such as general relativity, are only approximations valid at very long distances. At very short distances, comparable to the "length of the string" (string scale) or "the smallest meaningful black hole" (the Planck scale), physics does not admit a simple description in terms of usual geometry. Geometry is generalized to something much more grandiose, and the difference between geometry and matter disappears - this is the content of unification of gravity with other forces and matter.

However, the effects in string theory do not say simply that "space is made of atoms of space". Instead, there are many new objects, fields, concepts appearing in this regime and all of them are "fuzzy" and mixed up in some way. This fuzziness also allows topology of space to change smoothly once a topologically nontrivial submanifold shrinks to very short, substringy distances.

http://motls.blogspot.com/2004/10/emergent-space-and-emergent-time.html

Due to Holographic principle the information is encoded on a surface of the Event Horizon as a Planck length squared. The space is an illusion then. The space is just a set of numbers due to a program like a space in a computer game.
In holography you are the creator of the space by discrete events of the interference.

 

[Careful]

Lubos Motl's blog:

Quantum Mechanics guarantees that the concept of a completely smooth geometry is incompatible with quantum mechanics that make things fluctuate. But string theory goes much further. Geometric descriptions such as general relativity, are only approximations valid at very long distances. At very short distances, comparable to the "length of the string" (string scale) or "the smallest meaningful black hole" (the Planck scale), physics does not admit a simple description in terms of usual geometry. Geometry is generalized to something much more grandiose, and the difference between geometry and matter disappears - this is the content of unification of gravity with other forces and matter.

However, the effects in string theory do not say simply that "space is made of atoms of space". Instead, there are many new objects, fields, concepts appearing in this regime and all of them are "fuzzy" and mixed up in some way. This fuzziness also allows topology of space to change smoothly once a topologically nontrivial submanifold shrinks to very short, substringy distances.

http://motls.blogspot.com/2004/10/emergent-space-and-emergent-time.html

Lubos can say whatever he wants, if he can make his proze precise I would love to see the evidence. There is nothing deep about wild speculations how nature should look on the smallest scales; I have myself entertained such fantasies for years and nobody has even come close to computing simple things in such picture. Moreover, if this is the picture Lubos has, then he knows he has to drastically modify quantum mechancs itself, even more so than I did. I haven't seen anything yet which even comes close to what I did, let along an even more wild way of thinking which deprives QM of all geometrical underpinnings. Also, I give very good philosophical and logical arguments why doing so would be utter nonsense.

Due to Holographic principle the information is encoded on a surface of the Event Horizon as a Planck length squared. The space is an illusion then. The space is just a set of numbers due to a program like a space in a computer game.
In holography you are the creator of the space by discrete events of the interference.

That is your very wild interpretation of this principle which is by no means necessitated by the work of Bousso et al. Again, it is very easy to say all these crazy things you cannot realize. The devil is in the details.

 

[czes]

In a larger and more speculative sense, the theory suggests that the entire universe can be seen as a two-dimensional information structure "painted" on the cosmological horizon, such that the three dimensions we observe are only an effective description at macroscopic scales and at low energies.

Bekenstein's topical overview "A Tale of Two Entropies" describes potentially profound implications of Wheeler's trend in part by noting a previously unexpected connection between the world of information theory and classical physics. Bekenstein summarizes that "Thermodynamic entropy and Shannon entropy are conceptually equivalent: the number of arrangements that are counted by Boltzmann entropy reflects the amount of Shannon information one would need to implement any particular arrangement..." of matter and energy. The only salient difference between the thermodynamic entropy of physics and the Shannon's entropy of information is in the units of measure; the former is expressed in units of energy divided by temperature, the latter in essentially dimensionless "bits" of information, and so the difference is merely a matter of convention.

The holographic principle states that the entropy of ordinary mass (not just black holes) is also proportional to surface area and not volume; that volume itself is illusory and the universe is really a hologram which is isomorphic to the information "inscribed" on the surface of its boundary.

 

[Careful]

In a larger and more speculative sense, the theory suggests that the entire universe can be seen as a two-dimensional information structure "painted" on the cosmological horizon, such that the three dimensions we observe are only an effective description at macroscopic scales and at low energies.

Bekenstein's topical overview "A Tale of Two Entropies" describes potentially profound implications of Wheeler's trend in part by noting a previously unexpected connection between the world of information theory and classical physics. Bekenstein summarizes that "Thermodynamic entropy and Shannon entropy are conceptually equivalent: the number of arrangements that are counted by Boltzmann entropy reflects the amount of Shannon information one would need to implement any particular arrangement..." of matter and energy. The only salient difference between the thermodynamic entropy of physics and the Shannon's entropy of information is in the units of measure; the former is expressed in units of energy divided by temperature, the latter in essentially dimensionless "bits" of information, and so the difference is merely a matter of convention.

The holographic principle states that the entropy of ordinary mass (not just black holes) is also proportional to surface area and not volume; that volume itself is illusory and the universe is really a hologram which is isomorphic to the information "inscribed" on the surface of its boundary.

Ok, let us play a game. How would you retrieve locality and causality in what you call a fundamentally holographic description of the universe ? How would you define ''entropy of ordinary mass'' in a quantum gravity context ? How do you define the inside of a surface if you deny the existence of a three dimensional space ? Ohw, and this is just for starters, I have plenty of more worries...

 

[czes]

Ok, let us play a game. How would you retrieve locality and causality in what you call a fundamentally holographic description of the universe ? How would you define ''entropy of ordinary mass'' in a quantum gravity context ? How do you define the inside of a surface if you deny the existence of a three dimensional space ? Ohw, and this is just for starters, I have plenty of more worries...

In 1997, Juan Maldacena gave the first holographic descriptions of a higher dimensional object, the 3+1 dimensional type IIB membrane, which resolved a long-standing problem of finding a string description which describes a gauge theory. These developments simultaneously explained how string theory is related to quantum chromodynamics, and afterwards holography gained wide acceptance.

The Holographic Universe demonstrates the same rules as the conventional physics. Therefore we can't distinguish what is true. Many scientists are engaging in the holographic principle like Nobel prize laureates Gerard 't Hooft and George Smoot.

http://arxiv.org/abs/1003.5952

We propose an alternative interpretation which takes into account the temperature intrinsic to the information holographically stored on the screen which is the surface of the universe. Dark energy is thereby obviated and the acceleration is due to an entropic force naturally arising from the information storage on a surface screen. We consider an additional quantitative approach based upon the entropy and surface terms usually neglected in General Relativity and show that this leads to the entropic accelerating universe."

http://arxiv.org/abs/1002.4278

 

[Careful]

In 1997, Juan Maldacena gave the first holographic descriptions of a higher dimensional object, the 3+1 dimensional type IIB membrane, which resolved a long-standing problem of finding a string description which describes a gauge theory. These developments simultaneously explained how string theory is related to quantum chromodynamics, and afterwards holography gained wide acceptance.

You did not answer my question: I asked you how locality and causality were retreived in a purely holographic description of the universe.

Many scientists are engaging in the holographic principle like Nobel prize laureates Gerard 't Hooft and George Smoot.

http://arxiv.org/abs/1003.5952

Sure, but I am not convinced that they interpret it in the same way as you do ! 't Hoofts first considerations were actually pretty modest and also the results of Bousso do not indicate by any means that the laws should have a holographic formulation. All they say is that classical reasonable thermodynamic entropy currents must satisfy the Bekenstein bound for special null hypersurfaces. So, this merely suggests that holography is an emergent property as it is in my theory.

We propose an alternative interpretation which takes into account the temperature intrinsic to the information holographically stored on the screen which is the surface of the universe. Dark energy is thereby obviated and the acceleration is due to an entropic force naturally arising from the information storage on a surface screen. We consider an additional quantitative approach based upon the entropy and surface terms usually neglected in General Relativity and show that this leads to the entropic accelerating universe."

http://arxiv.org/abs/1002.4278

But all these temperature effects attached to Rindler horizons only hold locally (not even quasi locally). Not all curvature effects are taken into account in this way which would destroy the canonical character of the Rindler vacuum state (indeed everything hinges upon local Killing fields). You cannot retrieve general relativity in its full glory in this way, no matter what some Indian relativist may claim (all his proofs contain severe errors/limitations).

 

[czes]

Jacob Beckenstein summarized a current trend started by John Archibald Wheeler, which suggests scientists may "regard the physical world as made of information, with energy and matter as incidentals”.
http://community.livejournal.com/ref_sciam/1190.html

Holographic Universe is an hypothesis yet. I do not claim it is true. I read some articles of good physicists suggesting the Holographic Universe.

When you play a computer game it is very real sometimes. The virtual reality may mimic all particles and interactions in physics. It is a question of the program and capacity of the computer.

We live between the minimum and maxtmum entropy Events Horizons. It is very simple program at least - the capacity is enormous.

 

[Careful]

Jacob Beckenstein summarized a current trend started by John Archibald Wheeler, which suggests scientists may "regard the physical world as made of information, with energy and matter as incidentals”.
http://community.livejournal.com/ref_sciam/1190.html

Holographic Universe is an hypothesis yet. I do not claim it is true. I read some articles of good physicists suggesting the Holographic Universe.

When you play a computer game it is very real sometimes. The virtual reality may mimic all particles and interactions in physics. It is a question of the program and capacity of the computer.

We live between the minimum and maxtmum entropy Events Horizons. It is very simple program at least - the capacity is enormous.

In short, I do not doubt holography; I just see no rational reason for the very strong form of it which you believe in. I guess I am just more conservative than you are...

 

[czes]

In short, I do not doubt holography; I just see no rational reason for the very strong form of it which you believe in. I guess I am just more conservative than you are...

yes. Though i am not sure if it is really a reality. I want just know everything about it. What if it is true ?

 

[Careful]

yes. Though i am not sure if it is really a reality. I want just know everything about it. What if it is true ?

I don't think it is true in the strong form. All it says to me is that the dynamics is more constrained than people usually imagine it to be the case.

 

[czes]

What do you mean by the dynamics ? The interaction between particles of matter ? What is vacuum and what is gravitational field ? Is it the same or completely differrent things ?

 

[Careful]

What do you mean by the dynamics ? The interaction between particles of matter ? What is vacuum and what is gravitational field ? Is it the same or completely differrent things ?

Quickly, because I have to go. By dynamics I mean the gravitational theory and the theory defining particle interactions. The vacuum state is uniquely defined by the dynamics and there is no ambiguity at this level.

 

[czes]

Thank you careful for exciting discussion.

 

[lucien86]

There is a somewhat simpler and slightly different answer to most of the ones given here (though maybe a little 'heretical').

We need to start with a few definitions for time -Lets be clear time isn't simple, classical Newtonian time is a discontinuity with two dimensional regions and a point in the middle. Each region is unique and totally different to the others and classical observation says that only the present or 'point time' actually exists. The future is a collapsing superposition of probabilities of the point time, and the past is the cumulative information state of the point time, neither actually exist separately.
You can roughly extrapolate point time (instantaneous time) down to the quantum limit or below, maybe down to Plank time itself. Now look at and add point time and space time together, within point time no region of space time can be much bigger than about an atom across (very roughly). Put very simply within this tiny region time actually can behave like a physical dimension, outside it time as observed behaves like a point.

Everything else beyond this is guess work but we can begin with a simple observation. Empty space can be empty (to Relativity) but distance is tied to energy by gravity and kinetic energy, this can be used to describe space time and ties it to three spatial dimensions. Now space time as defined is four dimensional and each dimension adds a power to the total energy density, in short physical space time can't stay stable over large regions which is why it breaks up into discontinuities. The region that breaks down as far as we can guess is time which is why we experience reality as we do.
Now the breakdown region is at the speed of light (where space and time actually unify) which excludes normal matter and is only accessible to objects of zero or 'imaginary' mass. This means that it forms an impenetrable barrier between us and what is outside, the big question is about what is beyond that barrier. Is there a greater physical space time or is space time just an abstract concept? don't expect relativity to give a definitive answer on this because the basic equations still work either way. The problem is that for point time the whole universe is basically in the FTL region. General Relativity solves this by saying that the universe does not exist in the present but only the past and future. (I won't comment on my opinion of that, but all of these ideas can be extended much much further.)