How quickly does a magnetic field propagate in a vacuum?

[Hardik Batra]

Why can't the vacuum be magnetized?

Another question...

Magnetic field induced in a substance, depends on Magnetic intensity (H) and Magnetization (M).
If H is too much strong then what happens in a substance.

 

[Simon Bridge]

1. No magnetic zones.
2. depends what you mean by "too strong".

 

[Hardik Batra]

2. depends what you mean by "too strong".

I don't know.
But in my textbook written as,
if H is not too much strong, then the magnetization M induced in the substance is proportional to magnetic intensity H

M = X_mH

1. No magnetic zones.

means magnetic field won't produced in vacuum due to magnet.

 

[Simon Bridge]

But in my textbook written as,
if H is not too much strong, then the magnetization M induced in the substance is proportional to magnetic intensity H

Oh right - the textbook is making a weak-field approximation.
$M\propto H$

When H is "too strong" then the proportionality does not hold and things get complicated.

Its a bit like Hook's law only works so long as you don't exceed the elastic limit.

means magnetic field won't produced in vacuum.

... no: it means that the vacuum itself cannot be magnetic, although objects in the vacuum can be.
Consider: what is the definition of "a vacuum"?

Note: it is possible to have magnetic fields in a vacuum.

 

[Hardik Batra]

Oh right - the textbook is making a weak-field approximation.
$M\propto H$

When H is "too strong" then the proportionality does not hold and things get complicated.

Its a bit like Hook's law only works so long as you don't exceed the elastic limit.

I want to know if H is too strong then what kind of complication occur in a substance.

 

[snedkliv]

Would it not be considerably correct to state that "vacuum" rather then beeing "empty of matter" is "full of EM-fields"?

Hence creating a more permanent magnetization (which is mostly characterised by the crystal structure, molecular alignments and the elements present in the substance beeing magnetized) requires matter, which by definition is not considered in the studied system when studying "vacuum".

However, the effect of magnetism and the definitions of para-, dia- and ferro-magnetism might not match for much longer, considering freaky results of experiments with DNA and quantum systems are teaching us.

Hence, the vacuum clearly does not seem to be empty considering that the EM-fields contain potential energy that can interract with matter. (at least no more empty then the "space-time curl" related to matter, beeing reponsible for the effect of gravity)

Saturation is normally occurring in physical systems and at "extreme energies" systems tend to encounter relativistic effects in addition to this.So to answer the orignial question of why vacuum cannot be magnetized I would like to form a answer where the definition of magnetism and vacuum are not really compatible as described above.

Edit: About what happens in the material when the field strength becomes high I believe that the resulting eddies and small field variations create a more "turbulent" environment within the field. And if no negative feedback loop exist within the system (as I believe the case of magnetic fields in the vacuum is) small fluctuations will spread easily.

If anyone wants to comment, please do!

 

[Simon Bridge]

Welcome to PF snedkliv;

Would it not be considerably correct to state that "vacuum" rather then being "empty of matter" is "full of EM-fields"?

Classically, a vacuum is "nothingness": the container for other things - so a vacuum "full of EM fields" is not a vacuum. The vacuum is the bit "in between" the fields.

However - take a closer look at the Feynman diagram for a vacuum.

Note: a full reply requires an answer in terms of Field Theory - the short answer is that there is no way to align the magnetic terms in the Feynman sum that corresponds to a vacuum so as to make a magnet out of it.

@Hardik: what happens depends on the material.
The relationship between M and H stops being linear ...
For the general case: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/magpr.html

 

[crispbee]

In another thread there was a similar discussion about magnetic fields in a perfect vacuum. It was an interesting exchange between the engineers that were happy with understanding the rules for what happens and the theorists that wanted to "how" it happens. It is said that magnetic fields are not emissive, which I get, and that they are closed and potentially infinitely looped but at what speed does a magnetic field line get established? Imagine a giant electromagnet in space that generates a large field where the field lines can be detected at distance. If we had a number of very fast detectors spaced out along one of the field lines and turned off the current to our electromagnet then switched it on again, does the field line collapse along its entirety instantly and re-establish itself instantaneously along it's entire length? If the field is not emissive, then it should. But that poses other problems... I appreciate that the field strength would diminish and grow as the current varied, but if assume that all detectors detect the same level of flux instantaneously, then it proves the same thing regardless of how quickly the electromagnet can be activated and deactivated.
There's something peculiar to magnetism that just doesn't sit right with me and to date I haven't seen an explanation of how it works that I feel comfortable with. I'm using the question about speed to point to the aspect that I think gets to the nub of my problem.

 

[Simon Bridge]

Welcome to PF;

at what speed does a magnetic field line get established?

... in a vacuum, changes to the magnetic field propagate outwards at the speed of light as with the electric field.

Always keep thought exeriments as the dirt-simple as you can:
You have a permanent magnet at some place, and an ideal gaussmeter a distance r a long way away.
You quickly shift the permanent magnet to a new location, say - a short distance d<<r towards the gaussmeter.
The it takes at least a time of (r-d)/c for the gaussmeter to register the change.

If this was not the case then shifting the magnet could be used to send messages faster than light, which would violate relativity.