Exploring the Nature of Space and Time

[Tanelorn]

In the context of all the very latest theories of:

The expansion of the universe,
The effects of dark matter and energy,
Gravitation and the other fundermental forces of nature,
General and Special Relativity,
Black hole singularities,
Inflation,
The size of the whole universe and the decreasing size of the observable universe,
Time,
The family of fundermental particles,
The shape of the universe,
Increasing entropy,
The constants of nature eg. G, c, more here: http://math.ucr.edu/home/baez/constants.html

and probably much more.


I would like to ask a very simple question that my teacher asked me when I was just 9 years old and which I am finding I am now less able to answer more than ever:

What is Space?

I could probably also throw in there "What is Time?", because I suspect there will be some commonality.


I suspect that a full and correct answer of this question is necessary for a truly fundermental understanding of the Physics of the Universe.

 

[Tanelorn]

OK here is a kick off:

http://en.wikipedia.org/wiki/SpaceAnd perhaps we could further define the question with "What are the properties of space", but that is probably not the whole question.
I get the impression that the medium of space has not been given its full due in terms of Physics Research, and it sometimes seems as if space itself is allowed to take on any properties as is required to accommodate the latest theories of cosmology.

 

[BillSaltLake]

Definitions of space and time are not complete, and are still being tweaked. In a crude sense, the assignment of space and time labels to "events" like supernovas let's us easily distinguish one supernova from another, for example. However, there are other ways to define coordinates, such as using a Fourier transformed "wave number" domain. Certain questions are still not satisfactorily answered, like why travel in any direction in space is possible yet travel in both time directions at least appears to be not possible.

 

[marcus]

Maybe you should check out the thread just started yesterday in Special&General Relativty forum called "Spacetime doesn't really exist, does it?"

https://www.physicsforums.com/showthread.php?t=487794

If space has no objective existence we don't have to consider the question "What is...?"

Anyway, a lot of people got on board yesterday's thread, so maybe this one is extra. Raises much the same questions.

 

[Tanelorn]

Thanks for replies.

Perhaps the other thread is covering this already, but too late here to switch on my mind light..


Another one of those properties that has been attached to the properties of space is vacuum energy, and particles and anti particles seemingly popping into existence out of nothing.

One thing's for sure, space doesn't appear to be just a simple 3D pure vacuum.

Could almost write a book on the subject "The Comprehensive Properties of Space" (or space and time). I recall we had one called the properties of matter back then in Physics class.

Do we even know if the properties of space (and time) have remained constant since the BB? I suspect not because the properties of space and time vary with gravitational field strength and so maybe other things as well.
PS. Arent the permitivity and permeability of free space also fundermental constants of space (or nature)? I couldn't see them in this link:

http://math.ucr.edu/home/baez/constants.html

 

[jtbell]

PS. Arent the permitivity and permeability of free space also fundermental constants of space (or nature)?

No they're not. They're artifacts of a particular choice of electromagnetic units (SI). In Gaussian CGS and Heaviside-Lorentz CGS units, they don't exist.

http://en.wikipedia.org/wiki/Centimetre_gram_second_system_of_units

 

[Tanelorn]

Thanks jtbell!


I read a popular press article once that described space as being "a Field". I am not sure if this has any meaning though, I certainly don't understand the intended meaning. Perhaps in a pure vacuum the various fields of nature (or waves) are all that is present?

 

[khemist]

Thanks jtbell!


I read a popular press article once that described space as being "a Field". I am not sure if this has any meaning though, I certainly don't understand the intended meaning. Perhaps in a pure vacuum the various fields of nature (or waves) are all that is present?

I think the more correct term is that space is a "manifold," a mathematical description of a space.

 

[marcus]

...
I read a popular press article once that described space as being "a Field". I am not sure if this has any meaning though, I certainly don't understand the intended meaning. Perhaps in a pure vacuum the various fields of nature (or waves) are all that is present?

One of the challenges being grappled with in theoretical physics today is how to define a field without an underlying manifold. In particular how to handle geometry without first setting out an underlying continuum for the geometry to be defined on.

This may sound paradoxical to people who haven't followed the research, like trying to define a field of grass without any earth---no acreage of ground for the grass to grow on.

People are used to defining fields on conventional flat Euclidean space, essentially graph paper but can have more dimensions than just two. For instance assigning a vector to each point in the space, perhaps showing force or flow velocity. A manifold is just a slight generalization (allowing curvature) of flat Euclidean space.

Traditionally people are accustomed to do field theory ON something: a conventional uncurved space or (slightly generalizing) a manifold which has the potential to be curved.
==========================

The approach Einstein used in 1915, with General Relativity, is to start with a manifold, with coordinates x,y,z etc. and then define fields on it ----one of which serves to determine shape or curvature around each point in the manifold. This shape-determining field serves as a metric or distance-measure function that you can use to tell distances, areas, volumes...

Now you could say that what really matters is not the underlying manifold or whatever coordinates were chosen to start with----what really matters is the geometry itself: the web of geometric relationships (distances between things, angles you measure, light-paths, areas, volumes and how they are related).

You could say that the underlying manifold has no objective existence. Two different manifolds with two different metric fields and layouts of matter and observers might represent the same geometry. The observer could not tell which he was in. What is real and unique is the interrelationship of geometry+matter---the particular manifold is just an arbitrary descriptive convenience of no physical importance.

So Einstein's 1915 strategy was to set up a manifold for starters, define fields (including geometric, including distance functions) on it, and then go up one level of abstraction and throw away the manifold. The particular manifold with its fields was just one equivalent representation of the geometry.

That said, the problem now is, can you be less round-about? Can you define the geometry--the web of geometric relationships in which events occur and measurements are made---without first setting out a manifold?

Why set it out to start with if you are going to eventually discard it as an arbitrary mathematical convenience or "gauge artifact"? Why not skip that step?

This is at the heart of the current effort to "define a background independent quantum field theory" which is a QG program goal.

Next month a major conference will be held at Einstein's Alma Mater the Zurich ETH (ETH=Swiss Federation Tech). It will be funded by the ESF-QG agency, the European Science Foundation QG branch. The conference will bring together people from many fields working toward this, and related, goals. The first such conference. What does this mean? To me it means that even though this goal seems paradoxical to some, there is a research constituency that is motivated to tackle it.

In effect those participating are researchers taking a fresh look at the problem at the start of this thread: "what is space?" and also "what is space time?"

If this interests you, here is the conference website:
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start

 

[harrylin]

One of the challenges being grappled with in theoretical physics today is how to define a field without an underlying manifold. In particular how to handle geometry without first setting out an underlying continuum for the geometry to be defined on.

This may sound paradoxical to people who haven't followed the research, like trying to define a field of grass without any earth---no acreage of ground for the grass to grow on.

People are used to defining fields on conventional flat Euclidean space, essentially graph paper but can have more dimensions than just two. For instance assigning a vector to each point in the space, perhaps showing force or flow velocity. A manifold is just a slight generalization (allowing curvature) of flat Euclidean space.

Traditionally people are accustomed to do field theory ON something: a conventional uncurved space or (slightly generalizing) a manifold which has the potential to be curved.
==========================

The approach Einstein used in 1915, with General Relativity, is to start with a manifold, with coordinates x,y,z etc. and then define fields on it ----one of which serves to determine shape or curvature around each point in the manifold. This shape-determining field serves as a metric or distance-measure function that you can use to tell distances, areas, volumes...

Now you could say that what really matters is not the underlying manifold or whatever coordinates were chosen to start with----what really matters is the geometry itself: the web of geometric relationships (distances between things, angles you measure, light-paths, areas, volumes and how they are related).

You could say that the underlying manifold has no objective existence. Two different manifolds with two different metric fields and layouts of matter and observers might represent the same geometry. The observer could not tell which he was in. What is real and unique is the interrelationship of geometry+matter---the particular manifold is just an arbitrary descriptive convenience of no physical importance.

So Einstein's 1915 strategy was to set up a manifold for starters, define fields (including geometric, including distance functions) on it, and then go up one level of abstraction and throw away the manifold. The particular manifold with its fields was just one equivalent representation of the geometry.

That said, the problem now is, can you be less round-about? Can you define the geometry--the web of geometric relationships in which events occur and measurements are made---without first setting out a manifold?

Why set it out to start with if you are going to eventually discard it as an arbitrary mathematical convenience or "gauge artifact"? Why not skip that step?

This is at the heart of the current effort to "define a background independent quantum field theory" which is a QG program goal.

Next month a major conference will be held at Einstein's Alma Mater the Zurich ETH (ETH=Swiss Federation Tech). It will be funded by the ESF-QG agency, the European Science Foundation QG branch. The conference will bring together people from many fields working toward this, and related, goals. The first such conference. What does this mean? To me it means that even though this goal seems paradoxical to some, there is a research constituency that is motivated to tackle it.

In effect those participating are researchers taking a fresh look at the problem at the start of this thread: "what is space?" and also "what is space time?"

If this interests you, here is the conference website:
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start

Yes indeed. However, it looks to me that Einstein distinguished between the mathematical space-time relationships that we can measure and the physical reality underneath, which we can only infer.
See for example his Leiden inauguration lecture here:
http://www.tu-harburg.de/rzt/rzt/it/Ether.html

Of course that was purely in the context of GR, the combination with quantum theory provides more clues.

 

[rogerl]

One of the challenges being grappled with in theoretical physics today is how to define a field without an underlying manifold. In particular how to handle geometry without first setting out an underlying continuum for the geometry to be defined on.

This may sound paradoxical to people who haven't followed the research, like trying to define a field of grass without any earth---no acreage of ground for the grass to grow on.

People are used to defining fields on conventional flat Euclidean space, essentially graph paper but can have more dimensions than just two. For instance assigning a vector to each point in the space, perhaps showing force or flow velocity. A manifold is just a slight generalization (allowing curvature) of flat Euclidean space.

Traditionally people are accustomed to do field theory ON something: a conventional uncurved space or (slightly generalizing) a manifold which has the potential to be curved.
==========================

The approach Einstein used in 1915, with General Relativity, is to start with a manifold, with coordinates x,y,z etc. and then define fields on it ----one of which serves to determine shape or curvature around each point in the manifold. This shape-determining field serves as a metric or distance-measure function that you can use to tell distances, areas, volumes...

Now you could say that what really matters is not the underlying manifold or whatever coordinates were chosen to start with----what really matters is the geometry itself: the web of geometric relationships (distances between things, angles you measure, light-paths, areas, volumes and how they are related).

You could say that the underlying manifold has no objective existence. Two different manifolds with two different metric fields and layouts of matter and observers might represent the same geometry. The observer could not tell which he was in. What is real and unique is the interrelationship of geometry+matter---the particular manifold is just an arbitrary descriptive convenience of no physical importance.

So Einstein's 1915 strategy was to set up a manifold for starters, define fields (including geometric, including distance functions) on it, and then go up one level of abstraction and throw away the manifold. The particular manifold with its fields was just one equivalent representation of the geometry.

That said, the problem now is, can you be less round-about? Can you define the geometry--the web of geometric relationships in which events occur and measurements are made---without first setting out a manifold?

Why set it out to start with if you are going to eventually discard it as an arbitrary mathematical convenience or "gauge artifact"? Why not skip that step?

This is at the heart of the current effort to "define a background independent quantum field theory" which is a QG program goal.

Next month a major conference will be held at Einstein's Alma Mater the Zurich ETH (ETH=Swiss Federation Tech). It will be funded by the ESF-QG agency, the European Science Foundation QG branch. The conference will bring together people from many fields working toward this, and related, goals. The first such conference. What does this mean? To me it means that even though this goal seems paradoxical to some, there is a research constituency that is motivated to tackle it.

In effect those participating are researchers taking a fresh look at the problem at the start of this thread: "what is space?" and also "what is space time?"

If this interests you, here is the conference website:
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start

Markus. You shared this thread in a message previously:

https://www.physicsforums.com/showthread.php?t=166997

At the bottom of it. There was this statement by a poster called Mejennifer and she said (you didn't answer her):

"At any rate, I am not aware of any experiment that proofs or even suggests that reality is diffeomorphism invariant (whatever that might mean). And I think it is even doubtful if an experiment could ever determine that."

Is that true. No experiments can determine General Covariance (diffeomorphism invariance)? If not and experiments have proven it, is this proven categorically already?

 

[Tanelorn]

This is at the heart of the current effort to "define a background independent quantum field theory" which is a QG program goal.

Next month a major conference will be held at Einstein's Alma Mater the Zurich ETH (ETH=Swiss Federation Tech). It will be funded by the ESF-QG agency, the European Science Foundation QG branch. The conference will bring together people from many fields working toward this, and related, goals. The first such conference. What does this mean? To me it means that even though this goal seems paradoxical to some, there is a research constituency that is motivated to tackle it.

In effect those participating are researchers taking a fresh look at the problem at the start of this thread: "what is space?" and also "what is space time?"

If this interests you, here is the conference website:
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start

Thanks for reply Marcus.

What a coincidence that I asked this question just as some of the top researchers in the field also thought that it was time to take a fresh look at "what is space" and "what is space time". If I had independant means I would like to have attended, assuming interested parties are allowed to observe. Would the eventual results and conclusions of the conference be published on the same site? Presumably several papers would be presented as well as new ones written?



Harryfin, thanks for this link:
http://www.tu-harburg.de/rzt/rzt/it/Ether.html


I see that the idea of the ether is still with us, as a medium in which electromagnietic and gravitation waves can travel, and also static fields. This is really getting to the heart of the matter as to my original question. In a similar way in which the ocean allows the energy of ocean waves to travel between places, space is a medium in which matter can exist and interact using waves and fields with other matter. Time and finite speed of light are also necessary otherwise every possible event in the universe happens simultanously, which results in total chaos and instability, because cause and effect would be happening everywhere at the same instant. I suggest therefore that our 3D space time may be the simplest form of reality that can actually work, ie. producing continuing interacting complex systems. Also nature does not seem to like unnecesary complexity, so perhaps 3D space time is also the only type of universe that ever actually exists.

It is also noteworthy that the ether of space time has a velocity of propagation (c) in a similar way to the various velocities that waves propagate in other mediums.
Does the presence of mass change the velocity of light by changing Eo and Uo?

I hope that the above is considered on topic for a Cosmology forum, I would like to think so.

 

[marcus]

...
Is that true. No experiments can determine General Covariance (diffeomorphism invariance)? If not and experiments have proven it, is this proven categorically already?

Gen. Cov. is a feature of general relativity, which is currently our theory of geometry. GR has been tested over and over. So what you are asking boils down to asking if GR is "true".

Is our theory of the geometry of the universe, how it evolves and responds to matter, true?

I think in empirical science you never finally verify a mathematical theory. You test it and you come to trust it provisionally if it surprises you by making unexpected predictions that turn out right. And eventually you make an observation/experiment that shows the theory' limitations, and you improve it or replace it with a better theory.

GR is currently how we understand geometry. It explains why so much of the time oldfashioned Euclidean is approximately exact, and it explains why it sometimes isn't. it's an elegant, remarkably precise and welltested theory, so we trust it provisionally (except at its "singularities" or breakdown points where quantum versions are being developed to complete the picture.)

And GR say geometry is general covariant, so we accept that provisionally too, as part of the package.

Someday we might find that Mother Nature has a preferred system of coordinates for doing geometry (not attached to some particular solution but in general) My personal hunch is I don't expect that in the foreseeable future but if it happened GR would be falsified. Then general covariance would be falsified.

That's how it goes, I think, you don't verify things, you falsify them if you can (and then try to do better.)

 

[Tanelorn]

jtbell, regarding the question on Eo and Uo not being fundermental constants of space time or nature. Does this mean that they can be calculated from something more fundermental in nature and if so what is it? I use these numbers fairly regularly in my work, thanks!

PS I don't even see the speed of light in one of the 26 fundermental constants!?

http://math.ucr.edu/home/baez/constants.html

 

[jtbell]

In SI units, $\mu_0$ and $c$ are defined as having the values $4\pi \times 10^{-7}$ T-m/A and 299792458 m/s, exactly. $\epsilon_0$ is then calculated from those two via the relationship

$$c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}$$

 

[Tanelorn]

jtbell, thanks, I knew this relationship, but I just wondered which of these are considered fundermental characteristics or Physical properties of the universe (or space time), which cannot be derived from other more fundermental physical properties? Perhaps just c, but I don't see c in this list of 26 constants, I am not sure why? http://math.ucr.edu/home/baez/constants.html

Actually, if anything, I would have expected Eo and Uo to be more fundermental properties of space time than c, because the velocity of light in free space depends on those characteristics? http://en.wikipedia.org/wiki/Vacuum_permittivity

The actual numeric value and units used I agree are not important.


Perhaps these links will help answer which are the most fundamental of the physical properties of space time:

http://en.wikipedia.org/wiki/Fine_structure_constant
http://en.wikipedia.org/wiki/Fundamental_physical_constant


Is there an equivalent to Eo and Uo for the propagation of gravitational force?

 

[marcus]

jtbell, regarding the question on Eo and Uo not being fundermental constants of space time or nature. Does this mean that they can be calculated from something more fundermental in nature and if so what is it? I use these numbers fairly regularly in my work, thanks!

PS I don't even see the speed of light in one of the 26 fundermental constants!?

http://math.ucr.edu/home/baez/constants.html

read baez first paragraph

The speed of light is there throughout the 26 because they are expressed in terms of Planck units. The ingredients of the natural system of units are c, hbar, and Newton G.

Would be good to learn about Planck units---the set of quantities you get when hbar c and G are given the numerical value one.

that defines a natural unit of mass and then all the other masses (e.g. in the list of 26) can be written as fractions of that mass. That is, as simple numbers, with no "kilograms" tacked on.

Wiki has an article on Planck units.

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

Regarding epsilon-naught and mu-naught, they can surely be calculated from more fundamental stuff!

For instnace, as I recall, from the elementry charge e, the charge on the electron, and other basics like hbar and c.

The coulomb constant is (1/137) hbar c/e²

 

[Tanelorn]

Thanks Marcus, I think I must be blind sometimes for missing it there in black white. I'm a mildly dyslexic, with bad eyesight, and poor memory (especially maths), trying to catch up on the latest theories of Cosmology! As they say back in Wales I am not so tup as to not know that I am tup :)

 

[harrylin]

[..]
Harryfin, thanks for this link:
http://www.tu-harburg.de/rzt/rzt/it/Ether.html
[..]
I suggest therefore that our 3D space time may be the simplest form of reality that can actually work, ie. producing continuing interacting complex systems. Also nature does not seem to like unnecesary complexity, so perhaps 3D space time is also the only type of universe that ever actually exists.

It is also noteworthy that the ether of space time has a velocity of propagation (c) in a similar way to the various velocities that waves propagate in other mediums.
Does the presence of mass change the velocity of light by changing Eo and Uo?

I hope that the above is considered on topic for a Cosmology forum, I would like to think so.

Such a space does make sense to me, and I think that it fits rather well with the developments in QM as well.

Concerning Cosmology and space, in a parallel thread bcrowell linked to a very interesting paper concerning expansion of the universe:

Davis and Lineweaver, Publications of the Astronomical Society of Australia, 21 (2004) 97,
msowww.anu.edu.au/~charley/papers/DavisLineweaver04.pdf

I found it enlightening.

Harald

 

[rogerl]

Gen. Cov. is a feature of general relativity, which is currently our theory of geometry. GR has been tested over and over. So what you are asking boils down to asking if GR is "true".

Is our theory of the geometry of the universe, how it evolves and responds to matter, true?

I think in empirical science you never finally verify a mathematical theory. You test it and you come to trust it provisionally if it surprises you by making unexpected predictions that turn out right. And eventually you make an observation/experiment that shows the theory' limitations, and you improve it or replace it with a better theory.

GR is currently how we understand geometry. It explains why so much of the time oldfashioned Euclidean is approximately exact, and it explains why it sometimes isn't. it's an elegant, remarkably precise and welltested theory, so we trust it provisionally (except at its "singularities" or breakdown points where quantum versions are being developed to complete the picture.)

And GR say geometry is general covariant, so we accept that provisionally too, as part of the package.

Someday we might find that Mother Nature has a preferred system of coordinates for doing geometry (not attached to some particular solution but in general) My personal hunch is I don't expect that in the foreseeable future but if it happened GR would be falsified. Then general covariance would be falsified.

That's how it goes, I think, you don't verify things, you falsify them if you can (and then try to do better.)

Hmm... you mentioned "Someday we might find that Mother Nature has a preferred system of coordinates for doing geometry".. but how can that be if we have already proven that Mother Nature has no preferred system of coordinates for doing geometry by means of experiments. What experiments were done already if not yet done. What exact experiments can determine it?

 

[Tanelorn]

Thanks Marcus, mathematics aside, would you agree that the permitivity and permeability of free space are fundermental Physical properties of space time and are not effects of some more fundermental physical property of space time?

Thanks Harald, I will read this paper next week.

 

[Tanelorn]

I had another related question on the properties of space:

When space expands is new space being created? Could this new space be as a result of some form of matter or energy decaying or changing form? For example, I recall reading that after an extremely long time protons may eventually decay (or was it evaporate?) Could new space or an expansion of space be created in such a process? If so, perhaps matter itself could be viewed as tangled or twisted up space? I believe I have read something in the popular science press about something like this, I am not sure of the science content though.

 

[IsometricPion]

When space expands is new space being created? Could this new space be as a result of some form of matter or energy decaying or changing form? For example, I recall reading that after an extremely long time protons may eventually decay (or was it evaporate?) Could new space or an expansion of space be created in such a process? If so, perhaps matter itself could be viewed as tangled or twisted up space?

The (average) density of matter in the universe decreases as space continues to expand. There is more volume in a given (http://en.wikipedia.org/wiki/Binding_energy" ) box as the universe expands; so,insofar as volume is equivalent to space, yes more is coming into existence as the universe expands.

The expansion of space-time is not intrinsically tied to the existence of matter. There are http://en.wikipedia.org/wiki/Vacuum_solution_%28general_relativity%29" of expanding universes with only energy. As far as matter affects the expansion of space-time, it slows down the expansion.

I suppose if the decay of matter reduced the (average matter-based) energy density of the the universe, dark energy would eventually be able to cause parts of the universe to expand faster than otherwise possible. However, this is only because high enough (enough to be gravitationally bound) densities of mass-energy are immune to the expansion, if stable (note that this is only effective at increasing the expansion of volumes in formerly gravitationally bound systems if the velocity of the decay products is higher than the escape velocity of the matter concentration in which they reside (so the decay products are no longer gravitationally bound)).

The http://en.wikipedia.org/wiki/Geon_%28physics%29" sounds similar to your last question, but there is no experimental evidence to support their existence and they are no longer (if they ever were favoured) considered likely candidates descriptions of fundamental particles.

 

[Tanelorn]

Thanks IsometricPion. Geons sound interesting, although probably not what I am trying to recall, probably something from New Scientist in the 80s, or my imagination..

It is interesting to note that the universe ends with all matter and energy gone and an almost infinite size space time. One way or another all the matter and energy disappears and space time gets pretty big, at least that is what some are predicting. Are there other possible more appealing end scenarios?

 

[marcus]

... would you agree that the permitivity and permeability of free space are fundermental Physical properties of space time ...

I would not. Someone already pointed out that they are artifacts of the choice of system of units. they don't exist in some systems of units and do exist in others.
they may have merely conventional values.

It is interesting to note that the universe ends with all matter and energy gone and an almost infinite size space time. One way or another all the matter and energy disappears and space time gets pretty big, at least that is what some are predicting. Are there other possible more appealing end scenarios?

In a mathematical science based on observational data, what you look for is the simplest model with the best fit to the most data.

As an engineer you certainly realize this better than most: It is not a business of fantasizing pretty scenarios that appeal to the imagination.

The standard LambdaCDM model is an amazing good fit to millions of datapoints, with more data coming in all the time.

Eventually it will be proven wrong and replaced by something better, that fits better, and predicts more stuff to check observationally.

In the meantime it does not matter if it says that expansion continues indefinitely so that our galaxy (by then enlarged by merging with Andromeda) is left alone.
It doesn't say all matter disappears---that is a misstatement.
But the picture is cold and lonely in the sense that the matter of Milky and Andromeda and the rest of the local group of galaxies is gathered into one big humongous galaxy and if there are still any astronomers left they will not be able to see any other galaxies outside of our one big one. And the stars will die. It is very sad, if you like indulging your emotions on cosmology.
But that simply does not matter to the job of continually taking in data, checking the model, and if necessary modifying it to get a better fit.

 

[Tanelorn]

Marcus, I think that what I was trying to say was that my memory may be playing tricks on me when trying to recall that new scientist article.

I am correct though, am I not, that some think that eventually all black holes and protons decay (cant remember the right word), leaving an entirely empty universe. I do not know if this is predicted by the standard LCDM model. Also I merely state that to me that this is not a very appealing end, and hope that there may still be other possible predicted outcomes.

 

[marcus]

Penrose has a conjecture like that but he is in a tiny minority and his scenario has not been worked out in detail. In ordinary LambdaCDM cosmo you don't get total emptiness.

 

[Tanelorn]

Marcus thanks for clarifying that an empty universe is not a part of the LDCM! It would be a rather sad fate if true.

I am very interested in understanding how the permittivity and permeability of free space arise. Something in the Physics of the universe has to be responsible for these properties? If there is some more fundamental Physics then what is it? I use these Physical constants almost every day so it would be good to understand what is behind them. TIA!

 

[marcus]

There is an element of petty arbitrariness in the choice of fundamental quantities. Some people say h, and some people say hbar. Tomato tomahto.
Some people think that 8 pi G is more fundamental than plain Newton G.

Let's agree that these are fundamental quantities: Newton G, hbar, c, and the electron charge e.
More exactly e is the positive version of the electron charge, called the "elementary charge". In this view electric charge is a primitive quantity. It is not expressed in terms of other units like force. It is its own type of quantity. The unit is e. Electrons have -1 e. A type of quark has +1/3 of e. Can you accept that view of things, at least for the time being? Yes? Then we agree.

So here is a basic set of building blocks {G, hbar, c, e}. We could even use them as units in a system of units

On top of that let's add a famous fundamental number called alpha the fine structure constant. It is approximately 1/137. Maybe the most famous number in physics, after math constants like pi. You can google it and get a more exact value. It's value is independent of whatever system of units.

I can't tell you why these things are what they are. A lot of people consider them fundamental. It's partly a matter of taste.

You can build lots of other stuff with {G, hbar, c, e} and 1/137. (I mean the precise value, not 1/137, but I will write the approximate value for shorthand.)

For example Coulomb constant = (1/137) hbar c/e²

Anything you can build with {G, hbar, c, e} and 1/137 is not fundamental, according to this view.
So the Coulomb constant, let's call it k_C, is not fundamental.

Are you OK with this? Do you find it acceptable?

It looks as if hbar and c and 1/137 contain some information about how charge relates to geometry.
Because you can get k_C out of them, and it relates charge to geometry. It tells the force between two charges if you tell it the distance between.

Now as an engineer you know that you can easily write the permy constants in terms of k_C. So in this view they cannot be fundamental. You can build them from that set of "natural unitis" and the fine structure constant 1/137.

It depends on which direction you come at it. Some people come at it already thinking of permy constants as real properties of "space", and then for them the electron charge is not a natural unit of charge. The electron charge is then not fundamental because you can write it in terms of the fine structure constant 1/137 and hbar and c and "epsilon-nought" and square root and garbage algebra. It looks screwed up ugly---but you can start with permy constants and crank out the elementary charge e.

I say toMAAAHH-to. And for me the elementary charge e is Mother Pie and Applehood fundamental. So I refuse to countenance the permies. But you, of course can say toMAY-to and do it the other way round.

 

[Tanelorn]

Thanks Marcus, I have actually never used the coulomb constant so I will have to review how this can be used with E nought later.

http://en.wikipedia.org/wiki/Coulomb's_lawI believe that what I am trying to ask is not about the choice of fundermental quantities or units, but whether the real world physical effect of the permitivity of free space is itself a by-product of some more fundermental property about the Physics of space or spacetime?

I am trying to think of an example.. perhaps similar to the way in which classical Physics has been improved with a deeper understanding of spacetime with the theories of special and general relativity?

 

[marcus]

You are cordially welcome. And thanks to you for the discussion. It's very analogous I think to peoples' preferences for hbar versus h. In advance physics books you typically see hbar. So I can think, if I see a person calculating with h that he is "really" using
2 pi hbar. You can define h = 2 pi hbar.

It is useless to argue. they are really the same constant just surrounded by different words and a different human narrative.

In the case of k_c if I see you use epsilonought in a calculation, I just say that you are "really" using 1/(4 pi k_c)

Because in my world that is how epsilonought is defined. It simply equals 1 over 4pi times the basic EM constant. It is essentially the same constant but you decorate it with different words and tell a different story about it.

For me, the story with k_c is about the relation of charge force and distance ANALOGOUS TO THE STORY WITH NEWTON G. Newton G simply tells me the force between two unit masses placed a distance apart. Coulomb k simply tells me the force between two unit CHARGES placed a distance apart.

I can tell the story of either G or k_c without blathering about "space" being a "material" with certain "properties". All there is is charge and geometry (measuring distancees). Or in the case of G there is mass and geometry.

You probably know that in physics books past a certain point they don't necessarily use SI units. SI units are constructed in a peculiar way that grew out of some historical compromises. As an engineer you have to talk to other engineers and you do not have much freedom of units so you are stuck with SI and there is a certain way of thinking that goes with that. It would be a bad idea to try to get out of that way of thinking.

SI is periodically revised by committees in Paris and votes at international conferences backed by the legal force of international treaties. In present SI , CURRENT is based on force between parallel wires a certain distance apart and then charge is defined based on current. Or that was how when last I looked.

To me, the force between parallel wires is a relativistic effect that can be explained given the force between static charges. You transform taking into account relative motion and the magnetic effect falls out. So the basic fact is not parallel wires, the basic fact is like charges repel. Magnetism is a side-effect of the charge law that you get when charges move.

So SI is based on a funny way of thinking, at the very outset. But that's fine. It is all logical on its own terms. And I think eventually it will be reformed and the electron charge will be the basis of definition for electric units. We just need to wait patiently for the slow wheels of international committeehood to turn.

What I'm doing is sketching my attitude for you. You have your own way of thinking and there is no reason we should agree at the level of what we think is fundamental.
I don't believe there is an absolute "space" or that it is a material. Different observers slice spacetime differently. For me the basic venue of reality is not space but geometry---the measurements we make of distances and time durations. The speed of light is a feature of geometry, a fundamental geometric constant that relates the measurements of space and time (and also other stuff).
So for me the speed of light is not a property of some substance called "space". It is not something one measures but something one measures with.

I was pleased back in the 1980s when SI was changed to make it impossible to measure the speed of light. You may recall they redefined the meter so that the speed of light in vacuum had to besuch and such per second, by definition, and could not be measured. That seemed like progress. I suppose the same thing could happen to the elementary charge. We'll see.

 

[Tanelorn]

Thanks for reply Marcus I will read your message further when I get home. I am actually on vacation in Daytona area and was hoping to see the shuttle launch, but they delayed it again so unfortunately we will miss it. :(

I just wanted to further clarify my previous message when I mentioned something more fundamental as the source for the perm of free space:

If we consider how FR4 substrate, which has an Er of ~4.3 times the value of E naught interacts with EM waves, then can we consider free space itself as also being made of something, which has some kind of similar property, which also interacts with electromagnetic waves in a similar way? Perhaps this "something" might be the ether or a particle of some kind, I don't know what, but it would be a source for the various properties and interactions of free space, such us E naught, Mu naught, G etc. Is any research being carried out in this area? Am I correct that at least one model for the ether is still considered to be a part of modern Physics?PS. Another thing which also keeps suggesting the presence of an ether to me is that every other type of wave I can think of, needs some kind of a medium in which to propagate.

Another intriguing characteristic of free space is Vacuum energy, which also suggests to me at least, that a perfect vacuum is still made of something.

http://en.wikipedia.org/wiki/Vacuum_energyPerhaps this may be an interesting field of future research?

 

[Tanelorn]

Hi Marcus, I still haven't had time go over this yet, just too tired after shared driving for 22 hours and arriving to a lot of work.

At the heart of it, I really want to understand whether what we call a perfect vacuum really contains absolutely nothing, and if so, how can truly absolutely nothing have a permitivity of 8.854 × 10−12 F·m−1. Something here just does not seem to be logical. For space to have any Physical characteristics at all, it must exist and consist of something? I shall return to this!

 

[petm1]

Relative time.

 

[Chronos]

Permeability and permissivity are derived properties of electromagnetic fields. It is probably safe to say that all of 'space' is embedded in electromagnetic fields. I prefer the classical definition of 'space' - the volume between particles of matter. It is interesting to note that even particles of matter are vastly more empty than 'solid'.