Field Question

[MikeAL]

I have a small question that has been teasing me of late.

If the universe is expanding, and total energy can neither be created nor destroyed, and every inch of the universe is a field (eg magnetic field), then why doesn't field strength weaken as the universe expands?

[Strange]

The field itself doesn't have a strength. It is just a way of representing the effect of, say, an electric charge. There is a value of the electric field around a charge that decreases with distance (inverse square law). That relationship won't be changed because space is expanding. But if things are carried away from each other because of expanding space, then obviously the effect of the charge will decrease the same as if they were separated for any other reason.

1 hour ago, MikeAL said:

and total energy can neither be created nor destroyed

This is only true within a single frame of reference. The concept of energy is hard to define unambiguously in GR so the total energy of the universe is not well defined.

[MikeAL]

Hey Strange,

I think that the idea of a global field as merely a representation of a local effect is not quite accurate. I understand that you are saying the pertubation of the field is the strength, and that's fair enough, but as the universe expands, so too must the accommodating field. As there is no new energy entering the system (forget about Hawking for a moment), then the current field, unless it is independent of time and space itself, must stretch - unless other energy inside the universe is collapsing to be converted to the field. The total force that the field is capable of generating must therefore remain constant. If the universe is expanding, and total potential force cannot increase, then it must become diffuse, thus the local effects should decrease.

 

[studiot]

2 hours ago, MikeAL said:

I have a small question that has been teasing me of late.

If the universe is expanding, and total energy can neither be created nor destroyed, and every inch of the universe is a field (eg magnetic field), then why doesn't field strength weaken as the universe expands?

 

19 minutes ago, MikeAL said:

Hey Strange,

I think that the idea of a global field as merely a representation of a local effect is not quite accurate. I understand that you are saying the pertubation of the field is the strength, and that's fair enough, but as the universe expands, so too must the accommodating field. As there is no new energy entering the system (forget about Hawking for a moment), then the current field, unless it is independent of time and space itself, must stretch - unless other energy inside the universe is collapsing to be converted to the field. The total force that the field is capable of generating must therefore remain constant. If the universe is expanding, and total potential force cannot increase, then it must become diffuse, thus the local effects should decrease.

 

 

As I understand what you are asking and rephrasing it

 

The total energy in a given field, say electrostatic or magnetic, is being spread out over an increasing volume as the Universe expands.

Does that not mean that the measures of field strength diminishes?

 

The answer to this is yes it does.

This is known as Gauss's Law.

 

But matters are complicated because you have misunderstood some terminology.

 

There is no such thing as 'potential force'

There are various measures of field strength, appropriate to the particular type of field.

These measure are also called flux density or field intensity.

But you should not confuse this type of field with a quantum field, which is a different thing entirely.

Also I think you should be careful not to confuse force and energy.

These are different things that may exist independently.

Finally potential is yet another different thing that I recommend you leave out until you have a proper handle on the others.

 

So the first question is

 

Which type of field do you wish to discuss?

Edited May 4, 2018 by studiot

[Strange]

A field is just the way we describe the value of something at each point in space. It is not a "thing". It doesn't get stretched and it doesn't have any energy.

12 minutes ago, MikeAL said:

The total force that the field is capable of generating must therefore remain constant.

The force (in the case of the electric field, for example) is proportional to the product of the two charges divided by the square of the distance between them. This is not dependent on the expansion of space (unless it increases the distance between the two charges which, for all sorts of reasons, it won't).

 

Also, the total energy of the universe (in as much as that means anything) seems to be increasing: dark energy is proportional to the volume of space so as space expands, dark energy increases.

3 minutes ago, studiot said:

The total energy in a given field, say electrostatic or magnetic, is being spread out over an increasing volume as the Universe expands.

Does that not mean that the meausres of field strength diminishes?

 

The answer to this is yes it does.

That could sound as if you are saying something different to me, but I think we are both saying the same thing from a slightly different perspective.

You are describing the case where you have a sphere (for example) that gets bigger as space expands. Within (or at the edge of) that sphere the field strength will decrease.

I am looking at it from the point of view of two points a fixed distance apart (or a sphere of fixed size) in which case the field strength doesn't change.

These are both correct and valid descriptions. The reason I chose mine was that I wanted to emphasise that the field itself doesn't have any sort of  "inherent strength" that weakens with expansion. It is only the changing distance (as highlighted in studiot's description) that changes the field strength.

[studiot]

8 minutes ago, Strange said:

A field is just the way we describe the value of something at each point in space. It is not a "thing". It doesn't get stretched and it doesn't have any energy.

The force (in the case of the electric field, for example) is proportional to the product of the two charges divided by the square of the distance between them. This is not dependent on the expansion of space (unless it increases the distance between the two charges which, for all sorts of reasons, it won't).

 

Also, the total energy of the universe (in as much as that means anything) seems to be increasing: dark energy is proportional to the volume of space so as space expands, dark energy increases.

That could sound as if you are saying something different to me, but I think we are both saying the same thing from a slightly different perspective.

You are describing the case where you have a sphere (for example) that gets bigger as space expands. Within (or at the edge of) that sphere the field strength will decrease.

I am looking at it from the point of view of two points a fixed distance apart (or a sphere of fixed size) in which case the field strength doesn't change.

These are both correct and valid descriptions. The reason I chose mine was that I wanted to emphasise that the field itself doesn't have any sort of  "inherent strength" that weakens with expansion. It is only the changing distance (as highlighted in studiot's description) that changes the field strength.

Sort of.

 

We live in a three dimensional (of space) universe so I think the fruit cake analogy is better than the balloon one.

Gauss's Law applies in all cases not just spheres and surfaces.

 

The point of the fruit cake is that as the dough rises the cake expands in 3D.

As it does so all the sultanas etc move further away from each other.

So any interaction will take place of an increased distance.

[Strange]

14 minutes ago, studiot said:

So any interaction will take place of an increased distance.

If the objects move apart because of expansion, certainly. (In any practical case that doesn’t happen.)

But the important point is that this just due to the change in distance not a “weakening of the field” as the OP was thinking  

 

2 hours ago, MikeAL said:

I have a small question that has been teasing me of late.

If the universe is expanding, and total energy can neither be created nor destroyed, and every inch of the universe is a field (eg magnetic field), then why doesn't field strength weaken as the universe expands?

The usual question asked about this is: if light is redshifted by expansion, where does the energy of the photons go?

[MikeAL]

1 hour ago, Strange said:

The usual question asked about this is: if light is redshifted by expansion, where does the energy of the photons go?

I don't think it's a red shift question. Stuidot points out that it does happen if we consider Gauss' law:

1 hour ago, studiot said:

The total energy in a given field, say electrostatic or magnetic, is being spread out over an increasing volume as the Universe expands.

Does that not mean that the measures of field strength diminishes?

 

The answer to this is yes it does.

This is known as Gauss's Law.

So, back to the question, isn't there an inherent contradiction between an expanding universe and an undiminished field strength?

[studiot]

1 hour ago, MikeAL said:

I don't think it's a red shift question. Stuidot points out that it does happen if we consider Gauss' law:

So, back to the question, isn't there an inherent contradiction between an expanding universe and an undiminished field strength?

No there is no contradiction.

Strange touched on the why of it but only briefly.

2 hours ago, Strange said:

(unless it increases the distance between the two charges which, for all sorts of reasons, it won't).

 

It is space that expands, not matter.

Matter is not space.

 

Now consider our solar system.

The disposition  of the planets is governed by the balance of the gravitational forces within the solar system.

In turn these are governed by the amount of the matter in the system.

But matter does not change.

So the balance of forces will not change.

So the disposition of matter will not change.

That is the solar system will remain the same size.

However the distance between our solar system and the next will increase.

In the 'empty' space between solar systems there field strength is low.

The % of 'empty' space will increase in a given portion of the universe.

So the average field strength will go down.

You can make the same case for galaxy sized agglomerates and atomic scale assemblies (molecules), although at the atomic scale the fields and forces have a different origin.

 

Note I emphasised the phrase 'in a given portion of the universe'.

This is because of the peculair behaviour of infinity.

We have to discuss a limited volume, because the introduction of additional volume in an already infinite volume presents mathematical and philosophical difficulties.

We do not know if the universe is actually infinite or is finite.

 

 

Edited May 4, 2018 by studiot

[swansont]

3 hours ago, MikeAL said:

Hey Strange,

I think that the idea of a global field as merely a representation of a local effect is not quite accurate. I understand that you are saying the pertubation of the field is the strength, and that's fair enough, but as the universe expands, so too must the accommodating field. As there is no new energy entering the system (forget about Hawking for a moment), then the current field, unless it is independent of time and space itself, must stretch - unless other energy inside the universe is collapsing to be converted to the field. The total force that the field is capable of generating must therefore remain constant. If the universe is expanding, and total potential force cannot increase, then it must become diffuse, thus the local effects should decrease.

 

Why would the force change? F = kQq/r^2  

None of those variables are any different.

"total potential force" is not a thing, AFAIK.

[MikeAL]

 

17 minutes ago, studiot said:

In the 'empty' space between solar systems there field strength is low.

 

14 minutes ago, swansont said:

Why would the force change? F = kQq/r^2  

None of those variables are any different.

Hey, Studiot, when you say the field strength is low in interstellar space, you are referring to the distance between the objects. When Swansont said none of the variables changed, again, same thing. Same with Strange. I think you are both missing my point. The relationship between the physical components will not change, granted, but the intensity of the field, which is an all-universal permeating field should decrease, just like adjusting the light intensity on your monitor. It should reduce because of the larger volume that is now being encompassed. For this not to happen would surely imply the field is a space-time independent phenomenon, and not beholden to the rules governing the universe - ie not really generated from within our universe at all.

[Strange]

1 hour ago, MikeAL said:

So, back to the question, isn't there an inherent contradiction between an expanding universe and an undiminished field strength?

Only because you are considering a larger volume. That would be true even if the universe were not expanding. 

1 hour ago, MikeAL said:

I don't think it's a red shift question.

Correct. I was just pointing out that there are cases where energy does “disappear “

8 minutes ago, MikeAL said:

but the intensity of the field, which is an all-universal permeating field should decrease,

That is the point we are all making: there is no such thing as “the” intensity of the field. It is purely a function of charge and distance. 

Edited May 4, 2018 by Strange
Clarity

[MikeAL]

Just now, Strange said:

Only because you are considering a larger volume. That would be true even if the universe were not expanding. 

What do you mean by this Strange? Of course I am considering the larger volume. I find the last sentence particularly thought provoking. It raises another interesting facet.

[swansont]

31 minutes ago, MikeAL said:

 

 

Hey, Studiot, when you say the field strength is low in interstellar space, you are referring to the distance between the objects. When Swansont said none of the variables changed, again, same thing. Same with Strange. I think you are both missing my point. The relationship between the physical components will not change, granted, but the intensity of the field, which is an all-universal permeating field should decrease, just like adjusting the light intensity on your monitor. It should reduce because of the larger volume that is now being encompassed. For this not to happen would surely imply the field is a space-time independent phenomenon, and not beholden to the rules governing the universe - ie not really generated from within our universe at all.

Again, why? The field strength at any value of r is the same. 

If you are getting some total value, you integrate r from 0 (or epsilon) to infinity. Have those parameters changed?

[studiot]

41 minutes ago, MikeAL said:

I think you are both missing my point. The relationship between the physical components will not change, granted, but the intensity of the field, which is an all-universal permeating field

 

No you are missing my (our) point.

This is not Star Wars; Jedi knights and 'The Force' is a work of fiction.

4 hours ago, studiot said:

But you should not confuse this type of field with a quantum field, which is a different thing entirely

There is no such thing as 'an all-universal permeating field'.

So decide what you want to discuss, picking only from that we can observe.

[geordief]

18 minutes ago, swansont said:

 

If you are getting some total value, you integrate r from 0 (or epsilon) to infinity. Have those parameters changed?

Does it matter that we can't say whether the universe is infinite or not?

 

Is infinity(in this context)  an approximation,albeit as close an approximation to anything as can be imagined ?

 

 

Edited May 4, 2018 by geordief

[MikeAL]

12 minutes ago, swansont said:

Again, why? The field strength at any value of r is the same. 

This is a local phenomenon. Perhaps it is only proportionally the same. How do you know it is absolutely the same? I don't think you can. How do you know the entire field that it is operating in has not weakened?

Let me put it a different way. If I have an electromagnetic field in a box, then is it not possible that this field can be manipulated to generate a certain maximal calculable amount of electromagnetic force? If I enlarge the box, the total force that can be generated with this field surely does not increase, therefore it must become weaker per unit area.

If this does not happen with the universe, then I wonder why? 

[Strange]

59 minutes ago, MikeAL said:

What do you mean by this Strange? Of course I am considering the larger volume.

And you can always do that (and that Studiots' example) or you can consider a constant volume (my example). The expanding universe only affects this if you decide to use a coordinate system that expands. But again, your choice. 

There is nothing inherent in the field that can weaken. The only thing that matters is the charge and the distance.

8 minutes ago, MikeAL said:

This is a local phenomenon. Perhaps it is only proportionally the same. How do you know it is absolutely the same? I don't think you can. How do you know the entire field that it is operating in has not weakened?

If the strength of the electromagnetic interaction had changed we would see evidence of that when we look at distant galaxies. Everything we see suggests physics in the past was the same as today.

10 minutes ago, MikeAL said:

If I have an electromagnetic field in a box, then is it not possible that this field can be manipulated to generate a certain maximal calculable amount of electromagnetic force?

You can change the charge generating the field or the dimensions.

10 minutes ago, MikeAL said:

If I enlarge the box, the total force that can be generated with this field surely does not increase, therefore it must become weaker per unit area.

The force decrease with distance (with an inverse square law).

11 minutes ago, MikeAL said:

If this does not happen with the universe, then I wonder why? 

If you increase the distance, then the force decreases (with an inverse square law).

[MikeAL]

Just now, Strange said:

The only thing that matters is the charge and the distance.

At the risk of talking in circles or at cross purposes, do we not consider charge as the ability of a particle to alter the existing electric field- which is said to permeate the entire universe? Perhaps the underlying question I am asking is: how can we consider this existing universal field to be immutable given all that we know about everything else in said universe?

[swansont]

56 minutes ago, MikeAL said:

This is a local phenomenon. Perhaps it is only proportionally the same. How do you know it is absolutely the same? I don't think you can. How do you know the entire field that it is operating in has not weakened?

That would imply that Coulomb's law was incorrect, and by extension, it would have an impact on Maxwell's equations and how light behaves. And, as Strange has said, in interaction strengths (in the fine structure constant, for example)

Quote

Let me put it a different way. If I have an electromagnetic field in a box, then is it not possible that this field can be manipulated to generate a certain maximal calculable amount of electromagnetic force? If I enlarge the box, the total force that can be generated with this field surely does not increase, therefore it must become weaker per unit area.

Is there a limit on this total force that can be generated? Is there any basis for such an idea?

If it's infinite (and if this is a valid concept, it should be) then how does it get reduced?

It would be hard to do any of this with electrostatics, but we could try it with gravity. How does gravity get weaker because there are more masses out there in the universe? What deviations from Newton's law would you expect, and what data matches this?

Quote

If this does not happen with the universe, then I wonder why? 

My guess is because it's a made-up limitation.

1 hour ago, geordief said:

Does it matter that we can't say whether the universe is infinite or not?

 I don't think that matters.

[MikeAL]

Just now, swansont said:

How does gravity get weaker because there are more masses out there in the universe?

OK, let's play with gravity for a second. According to the proposition of weakening field strength because of universal inflation we would expect that the ability of mass to curve space around it lessens as space itself expands. Perhaps imperceptibly so on the scale by which we can measure. If we backtrack through time, however, we get the ability of mass to bend space (and slow time but lets keep it simple) increasing backward through time, until a very small mass can bend a huge amount of space. This seems to fit, at least in broad strokes, with suggestions around the big bang, does it not? Infinitely tiny yet somehow containing the entire universe, time not existing in any appreciable way.

[Strange]

1 hour ago, MikeAL said:

At the risk of talking in circles or at cross purposes, do we not consider charge as the ability of a particle to alter the existing electric field- which is said to permeate the entire universe? Perhaps the underlying question I am asking is: how can we consider this existing universal field to be immutable given all that we know about everything else in said universe?

Perhaps you are thinking of the vacuum permittivity. https://en.wikipedia.org/wiki/Vacuum_permittivity

This is sortof a measure of how much effect the electric field has in empty space. It is part of the 'k' in the equation swansont posted above.

But there is no reason to think that this will change because the distance between things has changed. And, the evidence suggests that physical constants had the same values in the past (or they all changed together in some implausible way so that the end result was identical).

28 minutes ago, MikeAL said:

This seems to fit, at least in broad strokes, with suggestions around the big bang, does it not?

No.

[MikeAL]

Just now, Strange said:

24 minutes ago, MikeAL said:

This seems to fit, at least in broad strokes, with suggestions around the big bang, does it not?

No.

Yes.

[Strange]

2 minutes ago, MikeAL said:

Yes.

Please show the mathematics that supports this claim.

And now you have gone from asking questions to proposing an alternative to GR and standard cosmology, perhaps this should be moved to the Speculations forum.

[studiot]

Expansion (and contraction) are actually peculiar processes.

Here is a simple block that expands (equally) in all directions, as shown by the arrows.

But it has a hole in the middle.

Can you tell which way the sides of the hole move?

That is does the hole get larger or smaller?