# What does 'emergent' mean in a physics context (split from Information Paradox)

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Here's an example of emergence I like: a Mexican wave as an emergent phenomenon.

Edited by StringJunky

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47 minutes ago, StringJunky said:

Here's an example of emergence I like: a Mexican wave as an emergent phenomenon.

That's a very inefficient way to turn a handle

8 hours ago, studiot said:

Not if the blocvking object was big enough.

There is a strong link between emergence, complexity and catastrophe theory.

You should read up on Rene Thom

OK ,first sunny day we get I will look for the caustic light   light caustics on my milky coffee.

Thom was the name of our PE teacher.Maybe he was French.

Edited by geordief
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10 hours ago, StringJunky said:

Here's an example of emergence I like: a Mexican wave as an emergent phenomenon.

Lovely pic +1

Perhaps you would like to consider and list the characteristics that make it emergent ?

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2 hours ago, studiot said:

Lovely pic +1

Perhaps you would like to consider and list the characteristics that make it emergent ?

A single figure will not make a Mexican wave, but a multiplicity will. You cannot  deduce  that phenomenon from a single figure. It is much like a  murmuration.

Edited by StringJunky
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Emergence reminds me of Lee Smolin and his emergent space ideas.

A blog of his, in SciAm...

Quote

In the same sense that a liquid is just a description of the collective motions of myriads of atoms, space and spacetime will turn out to be just a way of talking about the collective properties of the large number of atomic events. Their constant coming in and out of being, causing the next ones as they recede into the past, make up the continual construction of the world—also known to us as the flow of time.

The aim of a quantum theory of gravity is then first to hypothesize the laws that govern the elementary events, by which they continually come into being and then recede into the past. Then we must show how a large-scale picture emerges, in which these discrete events become subsumed in an emergent description of a smooth and continuous spacetime—as described by Einstein’s 1915 general theory of relativity.

Initially there is no space—just a network of individual elementary events, together with the relations expressing which of these were the direct causes of which other events. The notion of the flow of events collectively giving rise to a smooth description in terms of the geometry of a spacetime must emerge...

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23 minutes ago, TheVat said:

Initially there is no space—just a network of individual elementary event

How is there no space?Does any network not presuppose  distances between members,and a  temporal ordering?

But yes the passage from the very small and discrete to the large and seemingly continuous seems apt to be described as an "emergence",although it it is not "puff like" instantaneous  since quantum effects persist into very large structures

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4 hours ago, StringJunky said:

A single figure will not make a Mexican wave, but a multiplicity will. You cannot  deduce  that phenomenon from a single figure. It is much like a  murmuration.

OK so a single oscillator is not a wave and you cannot have a wave without a collection of oscillators.

But does a collection of oscillators always make a wave  or is there more to it ?

2 hours ago, TheVat said:

Emergence reminds me of Lee Smolin and his emergent space ideas.

A blog of his, in SciAm...

So what is an 'event' and how are they distinguished ?

And how does Smolin's definition conform or reduce to the standard definition of an event in spacetime ?

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2 hours ago, studiot said:

OK so a single oscillator is not a wave and you cannot have a wave without a collection of oscillators.

But does a collection of oscillators always make a wave  or is there more to it ?

So what is an 'event' and how are they distinguished ?

And how does Smolin's definition conform or reduce to the standard definition of an event in spacetime ?

I don't know. I don't think it can be random, it has to be sequential to make that moving wave; A static wave, like in the double slit experiment, can be randomly distributed. Just guessing here really. I appreciate that a lot of these terms are defined by consensus, but I don't know if my understanding of this  is formally correct.

Edited by StringJunky
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A quick Gedankenexperiment.

Consider H2O molecules. The interactions between such molecules are of the electromagnetic and quantum mechanics kind, and are thus well enough described by Maxwell and/or Schrödinger equations. A very large ensemble of such molecules on the other hand becomes a fluid, which is described by the Navier-Stokes equations and exhibits dynamics and properties that aren’t present (or meaningful) on the scale of individual molecules. That’s an example of emergence. So far so good.

Now instead of molecules imagine very massive objects, like stars eg, which interact approximately via gravity only, instead of EM and QM. The interactions are now well described by standard GR, there’s little mystery (but considerable computational effort) involved. So what happens if you have a very large ensemble of such objects, such as the stars in a galaxy, or the galaxies in the universe? It’s a lot like an ordinary fluid, except that the constituents now interact via gravity instead of EM - I propose the term gravitational fluid here. The big question then is - are there emergent dynamics in a gravitational fluid, just as there are emergent dynamics in an ordinary molecular fluid, given a sufficient number of constituents per volume? Could that be a potential model for dark matter? That way, DM wouldn’t require neither new particles nor modified gravity, but simply emergence.

The caveat I see here is that the ~100 billion stars of our galaxy are a very small number of constituent elements, when compared to fluids like water; so perhaps no emergence can happen here, because there aren’t enough parts. On the other hand though the interactions between constituents in a gravitational fluid are non-linear and much more complex, due to being governed by GR.

What are people’s thoughts on this?

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8 hours ago, Markus Hanke said:

The caveat I see here is that the ~100 billion stars of our galaxy are a very small number of constituent elements, when compared to fluids like water; so perhaps no emergence can happen here, because there aren’t enough parts. On the other hand though the interactions between constituents in a gravitational fluid are non-linear and much more complex, due to being governed by GR.

What are people’s thoughts on this?

IMO, and as @studiot very shrewdly pointed out, having however many elements is not a sine qua non in order to have a pattern of behaviour that merits the name of emergent.  You could have just a pair of 'particles' and see something appear that is not qualitatively equivalent to the addition/yuxtaposition of the parts (potential energy, entanglement,...). Quite simply, it's not present there when you consider a situation in which the elements are isolated (non-interacting, non-correlated,...) with high enough accuracy.

Your example that terms like dark energy in the Einstein equations were to be 'collaboratively caused', so to speak, is a distinct possibility, the way I see it. Of course, you would need a suitable set of hypotheses to show how this could be done. But I see no reason why something like this could not be achieved, in principle.

Within that (hypothetical) theoretical framework, it's possible to conceive that a model with just 1000 galaxies could give you some kind of crude picture of a cosmological constant.

OTOH:

If the level at which an idea like that is articulated were not to be that of galaxies, but stars, and if that many stars is conceived as a logical necessity, 1011 galaxies, each equipped with 1011 stars really make a big bunch of them. That would make an overall number of stars of 1022 stars within a cosmic horizon which is, now that I think about it, nearly an Avogadro's number worth of galaxies.

So I guess what I'm saying is, why not?

Edited by joigus
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9 hours ago, Markus Hanke said:

What are people’s thoughts on this?

Putting aside that i know no physics - perhaps we can think of it in terms of how we model phenomena. Taking the Mexican wave as an example, each person could be understood in terms of simple up and down motions, but to capture the wave we might want to use sinusoids (ignoring that we might use sinusoids to understand the initial up and down motion). So a property is emergent when we decide, for whatever reason, to apply a new model to understand it. Emergence then isn't a property of the universe, but a property of how we understand it.

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9 hours ago, Markus Hanke said:

A quick Gedankenexperiment.

Consider H2O molecules. The interactions between such molecules are of the electromagnetic and quantum mechanics kind, and are thus well enough described by Maxwell and/or Schrödinger equations. A very large ensemble of such molecules on the other hand becomes a fluid, which is described by the Navier-Stokes equations and exhibits dynamics and properties that aren’t present (or meaningful) on the scale of individual molecules. That’s an example of emergence. So far so good.

Now instead of molecules imagine very massive objects, like stars eg, which interact approximately via gravity only, instead of EM and QM. The interactions are now well described by standard GR, there’s little mystery (but considerable computational effort) involved. So what happens if you have a very large ensemble of such objects, such as the stars in a galaxy, or the galaxies in the universe? It’s a lot like an ordinary fluid, except that the constituents now interact via gravity instead of EM - I propose the term gravitational fluid here. The big question then is - are there emergent dynamics in a gravitational fluid, just as there are emergent dynamics in an ordinary molecular fluid, given a sufficient number of constituents per volume? Could that be a potential model for dark matter? That way, DM wouldn’t require neither new particles nor modified gravity, but simply emergence.

The caveat I see here is that the ~100 billion stars of our galaxy are a very small number of constituent elements, when compared to fluids like water; so perhaps no emergence can happen here, because there aren’t enough parts. On the other hand though the interactions between constituents in a gravitational fluid are non-linear and much more complex, due to being governed by GR.

What are people’s thoughts on this?

My thoughts are that drawing on Navier Stokes as a comparison is not enough to posit dark energy or dark matter.

N-S has the characteristic that, although difficult to insoluble, it arise purely from conventional dynamics.

49 minutes ago, joigus said:

IMO, and as @studiot very shrewdly pointed out, having however many elements is not a sine qua non in order to have a pattern of behaviour that merits the name of emergent.

Bringing the numbers game into consideration;

The suggestion to messers Navier and Stokes that a large enough lump of metal could spontaneously explode would have been fanciful.
But their work did predate knowledge of radioactivity.

Later (and therefore additional Physics) can be used to predict the size of a lump of Uranium that will spontaneously explode as a fission bomb.
The size is not of astronomical proportions.

Astronomical sized bodies of lighter elements such as hydrogen, helium etc will spontaneously ignite in self sustaining fusion dynamics.

I would suggest that both of these activities are emergent phenomena.

So I suppose that N & S might have thought such phenomena emergent but with the addition of new Physics.

49 minutes ago, joigus said:

You could have just a pair of 'particles' and see something appear that is not qualitatively equivalent to the addition/yuxtaposition of the parts (potential energy, entanglement,...). Quite simply, it's not present there when you consider a situation in which the elements are isolated (non-interacting, non-correlated,...) with high enough accuracy.

Yes Physicists love to separate out or isolate effects.

But can this always be achieved ?

Consider adding 3 + 4 = 7

Yet if we add a vector of magnitude 3, separated out from a vector of magnitude 4 by orthogonality we get a vector of magnitude 5.

Is this an 'emergent' pair ?

More numbers

The number of bricks to form my arch is irrelevant.

The pieces may be much smaller than a brick - pebble sized - or much larger as large blocks of stone.

Obviously many more small pieces will be needed to form the arch than from large pieces. It is even conceivable to form the arch of a single piece - you do not actually require two or more interacting pieces !

There is no emergent structural action in a corbel as there is in an an arch.
Corbels and arches act in structurally different manners.
Corbels work by supporting a bending moment in the bulk material , which has small but not zero tension stength
The configuration is such as to allow the tension remain within these low limits.

But in a true arch there is zero bending moment and zero tension.

This really is an interesting thread, which is showing up some interesting answers.

Edited by studiot
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Very interesting points are cropping up. Later, I would like to have a go at @StringJunky's example of a wave as an example, perhaps, of something we don't tend to think of* as 'emergent', but it really satisfies the essential criteria. Also, I would like to have a go at @TheVat's mention of Lee Smolin's efforts to realise space-time as 'emergent'. This, IMO, would require to construct space-time, as built up of more elementary building blocks, which might just be a matter of the description --see below.

But now I would like to contribute the following observation. Sometimes emergence appears as a result of correlation:

1) StringJunky's Mexican wave (many elements and strict correlation in behaviour)

But sometimes not:

2) Ideal gas: Pressure, temperature, and a host of thermodynamic variables, are relevant precisely because the particles are non-correlated in their behaviour (actually, completely non-correlated).

$\left\langle x_{i}x_{j}\right\rangle =0$

$\left\langle p_{i}p_{j}\right\rangle =0$

In this case,** it is precisely because the individual dynamical variables are completely uncorrelated (statistically flattened-out, so to speak), that we can talk about the system in terms of variables that do not bear any relationship whatsoever with the microstates.

It is entirely possible that this question of emergence is one that has to do with the description we wish to do --or find ourselves bound to do for practical reasons-- than with any condition that the system imposes upon us.

*This is possibly because the wave equation in terms of differential calculus was worked out by some of the Bernouilli's mathematicians so long ago, when the concept of 'emergence' was not in the toolkit of scientists.

**Now that I think about it, this is a very special case of correlation --very much in the vein of what Hanke said before.

Edited by joigus
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4 hours ago, Prometheus said:

Putting aside that i know no physics - perhaps we can think of it in terms of how we model phenomena. Taking the Mexican wave as an example, each person could be understood in terms of simple up and down motions, but to capture the wave we might want to use sinusoids (ignoring that we might use sinusoids to understand the initial up and down motion). So a property is emergent when we decide, for whatever reason, to apply a new model to understand it. Emergence then isn't a property of the universe, but a property of how we understand it.

This is my little hunch, it's perhaps a  function of the way our brain processes information, and what it mentally creates from that. I will add I'm more biologically minded in this subject than physics. I kind of feel it would be a good idea for cognitive neuroscientists and the other sciences to consult each other on this, to understand how the human observer affects the phenomenon. I don't think it is a rigidly objective phenomenon that you can solely apply numeric/quantitative anaysis to properly describe it.

Edited by StringJunky
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26 minutes ago, StringJunky said:

This is my little hunch, it's perhaps a  function of the way our brain processes information, and what it mentally creates from that. I will add I'm more biologically minded in this subject than physics. I kind of feel it would be a good idea for cognitive neuroscientists and the other sciences to consult each other on this, to understand how the human observer affects the phenomenon. I don't think it is a rigidly objective phenomenon that you can solely apply numeric/quantitative anaysis to properly describe it.

If Markus was right and DM was an emergent phenomenon would it  by the same token be "observer related"?

Seems absurdly improbable.

(unless I have absurdly misunderstood,as per usual)

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34 minutes ago, geordief said:

If Markus was right and DM was an emergent phenomenon would it  by the same token be "observer related"?

Seems absurdly improbable.

(unless I have absurdly misunderstood,as per usual)

You realize Markus is speculating himself? I don't know enough if my post contradicts his idea.

Edited by StringJunky
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On 12/10/2021 at 8:13 PM, joigus said:

The way I understand it is like this: A phenomenon is emergent when it can be studied in terms of elementary parts and their relationships, while the phenomena to be explained are not present in the parts, appearing instead as a consequence of certain relationships between them.

The example par excellence is pressure, which appears as a result of miriads of molecules hitting the walls of the container in so smooth and regular a way that all we experience is a consistent and continuously-varying resistance to reduce their volume if we try to push the system into a smaller volume.

I don't see this as emergent.

Pressure is related to a force, and the individual collisions also relate to a force, via conservation of momentum. It's a matter of scale and collective behavior. But I don't see how you can say that the force is not present in the parts.

IOW, it's collective behavior, but this is not emergent behavior.

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1 hour ago, StringJunky said:

This is my little hunch, it's perhaps a  function of the way our brain processes information, and what it mentally creates from that. I will add I'm more biologically minded in this subject than physics. I kind of feel it would be a good idea for cognitive neuroscientists and the other sciences to consult each other on this, to understand how the human observer affects the phenomenon. I don't think it is a rigidly objective phenomenon that you can solely apply numeric/quantitative anaysis to properly describe it.

There are perhaps lessons we can learn from cellular automata - take rule 30 for instance. The generating process is perfectly known and simple, yet the manifestations of it are anything but. The central column the rule produces can only be modelled as a stochastic process (currently - £10k prize if you prove otherwise), so much so it is used as the random number generator in Mathematica. We could say that, in this case, the randomness is emergent from the rule.

There was an interesting Royal Institute lecture on time - the lecturer ended by speculating that time was an emergent property based on our psychology - or something close to that effect..

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18 minutes ago, Prometheus said:

There was an interesting Royal Institute lecture on time - the lecturer ended by speculating that time was an emergent property based on our psychology - or something close to that effect..

Since time come into play in phenomena that do not involve human psychology, though, how can that be?

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Just now, swansont said:

Since time come into play in phenomena that do not involve human psychology, though, how can that be?

I've not the foggiest. Check out the lecture - The Royal Institution has a good reputation so i presume he's not talking total quack. As i remember he just mentions it near the end without many details.

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57 minutes ago, StringJunky said:

You realize Markus is speculating himself? I don't know enough if my post contradicts his idea.

Oh yes,I did realize that

I was just putting it out there,if "it" amounted to anything at all.

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4 minutes ago, geordief said:

Oh yes,I did realize that

I was just putting it out there,if "it" amounted to anything at all.

OK

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4 hours ago, swansont said:

I don't see this as emergent.

Pressure is related to a force, and the individual collisions also relate to a force, via conservation of momentum. It's a matter of scale and collective behavior. But I don't see how you can say that the force is not present in the parts.

IOW, it's collective behavior, but this is not emergent behavior.

Myriads of directional vectors have as only effect, to all intents and purposes, just one scalar value. These forces are directional; pressure is a scalar. These forces are wildly time-varying; pressure is constant or smoothly-varying. Etc.

OTOH:

When a system of the order 1024 variables can be effectively handled with 3 variables for all that matters, I call that 'emergence'. This pressure satisfies qualitatively different laws that cannot be thought of directly in terms of Newton's laws. It satisfies different laws, like the Van der Waals law, or the ideal-gas, the Curie law or any other equation of state. So it's not the same, it doesn't behave the same way, and it cannot be described in similar mathematical terms.

I see your point that there is kinda like a microscopic version of pressure, but I think that laws that go with it are an important criterion when it comes to judge when some quantity is emergent. In any case, it's a matter of defining a boundary.

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14 hours ago, studiot said:

My thoughts are that drawing on Navier Stokes as a comparison is not enough to posit dark energy or dark matter.

N-S has the characteristic that, although difficult to insoluble, it arise purely from conventional dynamics.

True enough.
Just to make this extra clear though, what I wrote is simply an idea - not even a hypothesis, and certainly not a claim. I’m just curious if a large system, the parts of which only interact gravitationally (GR), could - at least in principle - exhibit unexpected dynamics akin to DM observations. This requires no new physics at all, it’s GR - a generally relativistic n-body problem with large n

Or to put it differently - what is the error introduced if one treats such an n-body system as continuous to make the computation possible, as compared to keeping is discrete? Are there global dynamics inherent in a discrete n-body system (again, based on GR) that aren’t found if the system is treated as a continuum? If so, DM would simply be due to our not using GR correctly, rather than any new physics. Again, just an idea. Due to the non-linearity of GR I think it is very difficult to estimate the error introduced by simplifying assumptions.

Let us leave dark energy aside for now, since I think that is a different issue.

Edited by Markus Hanke
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5 hours ago, joigus said:

the Curie law

Sorry, that's nothing to do with pressure. Mistake.

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