I What happens if you increase μ0 and decrease ϵ0? Or vice versa

[Dale]

If a car is going 100 mph and I change it so that it is moving 50 mph then that is not just a change of units. I didnt redefine 100 mph to be 50 mph. I actually changed the cars motion.

Yes, but let’s think a little more carefully about what this statement means. You have a reference speed 1 mph, and the dimensionless ratio of the car’s speed to the reference speed is changing from 100 to 50. Since that is a dimensionless change it can have physical meaning.

This is as opposed to changing from 100 mph to 45 m/s. There we have changed our dimensionful quantities, but nothing dimensionless has changed. We changed both our number and our reference speed by the same proportion, so all the dimensionless quantities are the same.

I am not talking about changing the definition of Planck's constant. I am asking about what would happen if we could magically change the actual value of Planck's constant itself.

You cannot do that without changing your units. So now all of your reference values have changed. Specifically your unit for mass and all units that depend on the kg have changed.

By doing this you have unavoidably done the equivalent of changing from mph to m/s, except that our new units are unknown. So we can no longer disentangle changes due to the value and changes due to the units.

How would the world change? I ask because I want to know what the constant represents so I can understand the equations better. Of course we can't actually change Planck's constant but its just a thought experiment.

I am not objecting to the thought experiment at all, but I am trying to explain that your question, as posed, is under-specified. There is not enough information to discuss it.

Because $$\mu_0=\alpha\frac{4\pi \hbar}{e^2 c}$$ it is not possible to change only $\hbar$. You must also specify how the other quantities change to keep this equation true. Once you have done so, the changes to $\alpha$ completely determine the physical changes and the changes to the other quantities determine the changes to your units.

 

[Dale]

For example if I say that I could change the speed of light to be $300\frac{m}{sec}$ you will again tell me that this would redefine meter and second? I don't think meter and second are defined through the speed of light in SI.

Yes. You can look up the definition for the meter on the official BIPM website:

“ The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency DeltanuCs.”

https://www.bipm.org/en/measurement-units/base-units.html

The new SI system is really clean and consistent. But to understand it you have to understand the fact that the dimensionful constants are arbitrary. So much so that the BIPM, a self appointed committee, can fix their values by decree.

 

[em3ry]

I get the impression that there's been a lot of arguing lately about changing systems of units.

 

[em3ry]

...the subject of this thread which is the physical significance of the Planck's constant.

usually you can just look at the units and it will tell you what it represents but in this case it doesn't seem to be that easy

It seems to represent an aspect of electromagnetic interactions that is not immediately obvious. I think the impedance of free space is a good place to start

On the subject of obscure electromagnetic phenomena, I think the emission of electromagnetic waves by oscillating charges is fairly well understood (Larmor formula) but that the reverse process of absorbing electromagnetic radiation, especially blackbody radiation, is less well understood. Perhaps this is why the physical significance of Plancks constant is obscure. It has something to do with absorbance

There doesn't seem to be a Larmor formula for absorbance

 

[Dale]

I get the impression that there's been a lot of arguing lately about changing systems of units.

The BIPM changed the SI quite substantially in 2019, but a lot of people are still not aware of the change. So it comes up often, usually in the context of questions like yours. If you are unaware of the SI change usually you are unaware of the difficulties I mentioned above regarding this type of question.

 

[Dale]

Perhaps this is why the physical significance of planks constant is obscure.

For the purpose of your question, have you decided how the other quantities in that equation will change to keep the equation true? Until you do so the question is incomplete.

Also, for your reference “The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 x 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and DeltanuCs.” So this is how Planck’s constant is tied to the SI units.

 

[hutchphd]

There doesn't seem to be a Larmor formula for absorbance

The microscopic processes of Electromagnetism and Mechanics care not about the direction of time. If you run time (including the boundary conditions) backwards, Larmor will describe absorbance. This forms the basis for the principle of detailed balance and Einstein's A and B coefficients for light in equilibrium.

It might be a good idea to try to understand something before opining.

.

 

[em3ry]

I guess the place to start would be to increase $\mu$ while leaving everything else the same. $\alpha$ would increase the same amount. If c is unchanged then $\epsilon$ would have to decrease

Of course that might not be possible if the other factors are themselves functions of $\mu$. I think some of them might be.

 

[em3ry]

The microscopic processes of Electromagnetism and Mechanics care not about the direction of time. If you run time (including the boundary conditions) backwards, Larmor will describe absorbance. This forms the basis for the principle of detailed balance and Einstein's A and B coefficients for light in equilibrium.

It might be a good idea to try to understand something before opining.

I understood that perfectly but unlike the emission process the absorption process depends on chance encounters with other particles at exactly the right moment when the incoming light wave arrives.

Breaking something apart is the exact opposite of putting it together but putting it back together tends to be much harder

 

[hutchphd]

Yes, and this is all in textbooks. So study them please. Your questions are not foolish but they are elementary. Do the work.

 

[em3ry]

That is what I am trying to do

 

[Dale]

I guess the place to start would be to increase $\mu$ while leaving everything else the same. $\alpha$ would increase the same amount. If c is unchanged then $\epsilon$ would have to decrease

Of course that might not be possible if the other factors are themselves functions of $\mu$. I think some of them might be.

That is, I think, the best approach.

One of the most important effects of this is to change the stability of nuclei. The repulsive forces between protons will increase relative to the nuclear forces, so that heavier nuclei that are currently stable will become unstable. This will vastly change chemistry.

Also, nuclear fusion will be more difficult since the Coulombic barrier will be relatively higher. So stars and the stellar life cycle will change. There will be less synthesis of heavier elements, like carbon. That will have obvious implications for life.

I have a table that I made once with other effects. I will see if I can find that tomorrow. One thing that I know changed was the length of physical objects relative to electromagnetic radiation, but I cannot remember which way it changed.

 

[hutchphd]

That is what I am trying to do

No you are expecting answers to questions you have not even thought about for a minute. Do the work. Why are you talking about $\mu_0$? What happened to Planck?

 

[em3ry]

Wow.

 

[Delta2]

No you are expecting answers to questions you have not even thought about for a minute. Do the work. Why are you talking about $\mu_0$? What happened to Planck?

I don't think his questions are so easy to answer. Furthermore the answers might not lie in his books, at least not explicitly.

 

[vanhees71]

I've not read the entire thread, but the answer was never as simple as today, because the International System of Units (SI units) have been finally defined by giving the fundamental constants a definite value. The only exception is the gravitational constant which cannot be measured accurately enough. That's why the only exception is the definition of the second, which still uses a material constant, namely the frequency of a certain hyperfine transition of the $^{133}\text{Cs}$ atom, i.e., one fixes this transition frequency $\Delta \nu_{\text{Cs}}$ to a certain value, defining the unit second for time.

All other units are defined by giving the fundamental constants $c$ (speed of light in vacuo), $h=\hbar/(2 \pi)$ (Planck's action), $e$ (elementary charge), $N_{\text{A}}$ (Avogadro number), and $k_{\text{B}}$ (Boltzmann constant) definite values. This shows that all the numbers in the SI units are just determined by defining the units.

In electromagnetism the constants $\epsilon_0$ and $\mu_0$ are now both derived quantities (in the SI before 2019 $\mu_0$ was fixed by the then valid definition of the Ampere, but now the Ampere is defined by the definition of the elementary charge and the second.

For details see

https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units

 

[etotheipi]

This is why we should just use Lorentz-Heaviside units when doing EM

 

[vanhees71]

Well, for electricians the HL units were pretty inconvenient ;-)). Of course, in theoretical physics HL units are the best choice.

 

[Delta2]

@vanhees71 do you agree with what Dale says, that changing the dimensionful constants just changes the system of units we are using and to see physical changes we have to change the dimensionless constants?

 

[vanhees71]

Yes, and it depends also on the system of units used, which "constants" you need. E.g., formulating classical (microscopic) electrodynamics in terms of the SI you apparently need two independent constants of this kind, for which usually $\mu_0$ and $\epsilon_0$. Both have no deeper physical meaning than to introduce an extra unit for electric charge or electric current in addition to the three basic units for time, length, and mass which is all you need in classical physics (besides temperature for thermodynamics). These constants (permeability and permittivity of the vacuum or magnetic and electric field constant) are related to the speed of light by $c=1/\sqrt{\epsilon_0 \mu_0}$.

Note that these constants have changed with the new system of units. This is due to the redefinition and the particular choice of the values for $\nu_{\text{Cs}}$, $c$, $e$, and $h$. These are, of course, chosen such that, as to the best of our abilities to measure them in terms of the older definitions of the SI units, these units change as little as possible. In fact afaik the largest change is indeed in the realm of the electromagnetic units, i.e., in the value for $\mu_0$ which was in the old system exactly $4 \pi \cdot 10^{-7} \text{N}/\text{A}^2$. The relative change is $\delta \mu_0/\mu_0 \simeq 1.5 \cdot 10^{-10}$.

From the point of view of relativity the older electromagnetic systems of units, the Gaussian system or its "rationalization" the Heaviside-Lorentz system, are more natural, and there you need only one fundamental constant. Here one usually uses the speed of light $c$. It's more natural, because you measure electric and magnetic field components (which relativistically are combined to the Faraday or field-strength four-tensor $F_{\mu \nu}$). The reason, why there's only one "conversion constant" is that one does not introduce an additional base unit for electric charge or current.

 

[Vanadium 50]

. If a car is going 100 mph and I change it so that it is moving 50 mph then that is not just a change of units. I didnt redefine 100 mph to be 50 mph. I actually changed the cars motion.

If you said you were changing μ (e.g. by picking another material) that would be an OK analogy. But you didn't. You said you were changing μ₀. That's not changing the speed of one car, that's changing the entire universe. And what people are telling you repeatedly is that this has to be done consistently. Furthermore, when you do it consistently you redefine your system of units, and in many cases you end up right where you started.

 

[em3ry]

You seem to think that you are telling me something that I don't know

I have been thinking about the physical significance of Planck's constant.

The effect of increasing Planck's constant on blackbody radiation is the red line below. Y-axis is frequency

Apparently if Planck's constant were infinite then there would be no blackbody radiation. But we know that as long as the speed of light is finite then accelerating charges will emit light.

Wikipedia says:
So that got me thinking. The speed of light is $$\sqrt{\frac{1}{μ0 ϵ0}}$$. So that means that if you increase μ0 and decrease ϵ0 by the same amount then the speed of light would be the same but surely something in the universe would change.

So what would change if you increased μ0 and decreased ϵ0 by the same amount so that the speed of light was the same as before? Surely this would have some effect on the universe.

My question is about μ0 and ϵ0 but if you have any insight into Planck's constant then I will be glad to hear it too.

edit: I see that the Larmor formula depends on ϵ0 and c but not μ0. Decreasing ϵ0 increases the energy radiated by an accelerating charge.

$$P = {2 \over 3} \frac{q^2 a^2}{ 4 \pi \varepsilon_0 c^3}$$

edit2: Dale mentioned the Fine structure constant in post 1 below. It is:
$$\alpha = \frac{1}{4 \pi \varepsilon_0} \frac{e^2}{\hbar c} = \frac{\mu_0}{4 \pi} \frac{e^2 c}{\hbar}$$

edit3: The force between two separated electric charges with spherical symmetry (in the vacuum of classical electromagnetism) is given by Coulomb's law:
$$F_\text{C} = \frac{1} {4 \pi \varepsilon_0} \frac{q_1 q_2} {r^2}$$

comparing to the equation for the Fine structure constant we get:
$$F_\text{C} r^2 = constant = \alpha \hbar c$$

 

[em3ry]

I can see what's going on here. You people aren't even discussing my thread anymore. You are just continuing some argument that you were having between yourselves before i got here. Maybe its time for me to leave.

 

[etotheipi]

No need to throw a hissy fit, please. I think @Dale's post #2 and @Vanadium 50's post #7 sum up pretty well the right response to your question, and it's not really clear what you're trying to achieve.

Maybe, it would be worth getting hold of a copy of Sommerfeld's lectures on E&M (I think Vol. 3), where there is some nice discussion of electromagnetic units, in order to clear up some misconceptions.

 

[Ibix]

I can see what's going on here. You people aren't even discussing my thread anymore. You are just continuing some argument that you were having between yourselves before i got here. Maybe its time for me to leave.

Everyone seems to me to be pointing out that you can't just change $\mu_0$ without changing at least one other constant. Depending which other constant you change you may see genuinely different physics (if it's a dimensionless constant) or just unit redefinition (if it's another dimensionful constant). It isn't clear to me, at least, that you've accepted that point.

 

[em3ry]

so your answer to my question is that nothing will change?

if that is your answer then why don't you just say that? Like I say it's clear that you are just continuing some argument that you were having before I got here

 

[Ibix]

so your answer to my question is that nothing will change?

No. The answer is "maybe". As I just said, it depends what else you change, and I don't believe you've yet specified that.

 

[em3ry]

I specified it here:

I guess the place to start would be to increase $\mu$ while leaving everything else the same. $\alpha$ would increase the same amount. If c is unchanged then $\epsilon$ would have to decrease

Of course that might not be possible if the other factors are themselves functions of $\mu$. I think some of them might be.

 

[em3ry]

I probably shouldn't say this since it's irrelevant to my question and it's just going to get me sucked into your arguing but it occurs to me that any equation can be turned into a dimensionless constant k

a = b becomes:
k = b/a where k=1

A less trivial equation is the Larmor formula

$$P = {2 \over 3} \frac{q^2 a^2}{ c^3}$$

$$ 1.5 = \frac{q^2 a^2}{ P c^3}$$

That particular constant might not be variable though

so I guess the defining feature of a dimensionless constant is not just that it is constant and dimensionless but also that it's particular value is arbitrary in the sense that it's value can't be determined by the other values in the equation

 

[Ibix]

I specified it here:

Ok - I missed that this morning, apologies.

If you vary $\alpha$ then this does have real physical effects. The fine structure constant governs how strong the EM field is compared to other forces, so you'd see different spectral lines in atomic emissions among other things. There's a (probably non-exhaustive) list on the fine structure constant Wikipedia page.