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Author Topic: What are fields, such as electromagnetic and gravitational fields?  (Read 5126 times)

Offline DoctorBeaver

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No, Neil - not the sort you keep sheepies in. I mean electromagnetic fields etc. I know the definition, but what actually are they? Is there anything tangible in space with a field that makes it different from space without any fields present?
« Last Edit: 12/07/2008 11:56:49 by chris »


 

Offline ukmicky

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No, Neil - not the sort you keep sheepies in.
  >:(Spoil sport
 

Offline neilep

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No, Neil - not the sort you keep sheepies in.
  >:(Spoil sport

Grrrrr !!

I thought I'd best check before I upload some cute piccys of sheep grazing on da 'erb !

Ok....I'll finish this now...don't wanna destroy a thread where Eth is actually serious about klevur stuff !!
 

Offline JP

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Fields in physics occur when you assign a physical value to every point in space.  If I wanted to measure the properties of the air in a room, I could define a pressure field or a density field over the room.  In empty space, these two fields would have a value of 0.

What you're probably asking about is fields associated with forces.  This gets a bit more tricky.  These fields tell you what happens if you introduce a test particle into the field.  A gravitational field tells you how gravity will move a test mass, and an electric field tells you how the electromagnetic force will move a test charge.  Classically speaking, in empty space where a test particle feels no forces, these fields are also zero.

When you get to modern physics, the very fabric of space gets tied to fields.  In general relativity, space-time is curved due to gravity, which is the modern version of the classical gravitational field.  In quantum electrodynamics, there is an electromagnetic field present everywhere in the universe.  Due to the uncertainty principle, you can't say that it's at zero energy at any instant, so even in "empty space" you can have the field spontaneously spit out energetic particles for short periods of time.
 

Offline Soul Surfer

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Many people have the idea that maintaining an electrical magnetic or gravitational field requires the continuous input of energy.  this is not the case.  OK it takes energy to set one up for example the creation of a positron electron pair out of nothing requires a certain amount of energy which appears mostly as the mass of the particles but it does not require a continuous input like switching on an electromagnet.

One way of looking at a field is that it is the potential for something to happen and it is only when a particle that is sensitive to the action of that particular field moves through it that anything actually happens.
 

Offline DoctorBeaver

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jpetrucelli & Ian - I'm aware of what you've said. When Ian said "One way of looking at a field is that it is the potential for something to happen and it is only when a particle that is sensitive to the action of that particular field moves through it that anything actually happens." he was heading in the direction of what I'm asking.

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One way of looking at a field is that it is the potential for something to happen...

I'm thinking of the classic iron filings in a magnetic field demonstration. Obviously, the iron filings will only be affected if they are within a certain distance of the magnet; so what is there within that distance that causes the effect? Is it some kind of ripple in spacetime?

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...it is only when a particle that is sensitive to the action of that particular field moves through it that anything actually happens.

What is it that actually happens? What is the "action" that causes iron filings to arrange themselves the way they do? Is there bosonic interaction of some kind (I know that photons mediate the electromagnetic force)?
 

Offline Soul Surfer

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You place a magnet on the table and bring a flat board evenly scattered with iron filings smoothly and vertically down towards the magnet from about one metre away over a period of a couple of seconds. As you approach the magnet the iron filings rearrange themselves to show the field.  As they move these iron filings emit electromagnetic radiation at extremely low frequencies of a few cycles per second because of the interaction.  these extremely low frequency radio waves could in theory be detected to indicate what is happening as the filings approach the magnet.

This may sound a bit silly, but it is precisely what is happening when you go to hospital to get an NMR (nuclear magnetic resonance)image of something in your body except that the iron filings are the protons in the nuclei of the water molecules in your body  A fixed magnetic field polarises the water molecules in your body but this fixed magnetic field has a precise gradient in it so it is slightly stronger in some parts of your body than others.  A pulsed magnetic field disturbs this arrangement and after the pulse the molecules go back to their tidy arrangement and emit tiny radio signals as they do this.  The precise frequency of radio waves they emit depends on the exact value of the magnetic field and the strength of the signal depends on the number of molecules  this can be received and analysed using very sensitive radio receivers if you do this repeatedly with different values of the field gradient this can be used to form an image of what's going on inside your body
« Last Edit: 10/07/2008 10:26:56 by Soul Surfer »
 

Offline JP

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I'm thinking of the classic iron filings in a magnetic field demonstration. Obviously, the iron filings will only be affected if they are within a certain distance of the magnet; so what is there within that distance that causes the effect? Is it some kind of ripple in spacetime?
A generic field isn't a ripple in spacetime (gravity, however, could be), but is rather an invisible "thing" that overlays spacetime.  Another way of looking at fields (there's a lot of ways) is to think of them as stores of energy.  When you introduce a test particle, it interacts with the field to gain or lose energy.  Iron filings line up in a magnetic field because they're trying to get the lowest possible energy, which comes when they line up with the field.


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What is it that actually happens? What is the "action" that causes iron filings to arrange themselves the way they do? Is there bosonic interaction of some kind (I know that photons mediate the electromagnetic force)?
Classically, if a particle can decrease its potential energy by moving, that indicates it will experience a force in that direction (force is the negative spatial derivative of potential energy).  If you want to go to QM, fields are no longer smooth "things," but are made up of tons of particles, which act to give forces to things placed in the fields--so yes, you could think of these forces as mediated by bosons, which are in turn just QM speak for the "tiniest bits of the field."
 

Offline DoctorBeaver

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If you want to go to QM, fields are no longer smooth "things," but are made up of tons of particles, which act to give forces to things placed in the fields--so yes, you could think of these forces as mediated by bosons, which are in turn just QM speak for the "tiniest bits of the field."

That's what I was after. Would the particles be virtual particles?
 

Offline syhprum

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I think an understanding of fields is what separates the GCSE,s from the PhD,s
 

Offline DoctorBeaver

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In that case I'm 11+  :-X
 

Offline JP

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If you want to go to QM, fields are no longer smooth "things," but are made up of tons of particles, which act to give forces to things placed in the fields--so yes, you could think of these forces as mediated by bosons, which are in turn just QM speak for the "tiniest bits of the field."

That's what I was after. Would the particles be virtual particles?

Yes, at least as far as I know.  Other folks who are more expert in this area might have more to add.
 

Offline DoctorBeaver

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Hmmm... I've got a problem with that. QM says virtual particles can be of any type and any mass (except the actual mass of the real particle). So, why is it that the particular virtual particles are produced that are responsible for the type of field? Surely, it is the case that if any type of virtual particle can be produced, then all types of virtual particle will be produced thus causing all types of field to be produced everywhere. That, obviously, cannot be right.

I know QM is counter-intuitive, but my poor little brain is telling me that there must be something emanating from a magnet that causes the right type of virtual particles (virtual photons) to be produced in order for there to be a magnetic field around it.
 

Offline JP

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I think all types of fields are present to some degree everywhere.  If you have no objects producing or interacting with the fields, they'll all be at their lowest energy state, and will still be spitting out virtual particles.  When you put some object in space, its properties determine how strongly it interacts with each field.  For example, charged objects interact with the electromagnetic field, while matter with mass interacts with gravity (I don't know too much about the strong or weak nuclear forces, but I do know they're fields of a similar type).

Your magnet will generate a magnetic field since its interacting with the electromagnetic field by virtue of the motion of the charges within it.  Your magnet is also interacting with the gravitational field, but much more weakly (I'm assuming that there is some form of gravitational quantum field theory here).  However, with your magnet, I'm not sure at what point the photons become non-virtual.  At some point you could use charges to generate real photons that you can send off as part of a light wave.  I don't know off the top of my head where the line's drawn between virtual and non-virtual.
 

Offline DoctorBeaver

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I think all types of fields are present to some degree everywhere.  If you have no objects producing or interacting with the fields, they'll all be at their lowest energy state, and will still be spitting out virtual particles.  When you put some object in space, its properties determine how strongly it interacts with each field.  For example, charged objects interact with the electromagnetic field, while matter with mass interacts with gravity (I don't know too much about the strong or weak nuclear forces, but I do know they're fields of a similar type).


That makes sense.

I don't know off the top of my head where the line's drawn between virtual and non-virtual.

I thought that virtual particles were created in particle/antiparticle pairs that could take any mass except that of the classic particle/antiparticle. Don't they also anihilate each other? I'm not sure how that works with photons as they are their own antiparticles.
 

Offline Atomic-S

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I think this subject goes over most of our heads.  If we knew the answer to this stuff, we would know what space is, what time is, why the value of the universal gravitational constant is what it is, and a lot of other things. I understand this stuff is straining the best brains in the business.

There is currently a theory floating around that all this is ultimately explained by the "zero point energy" in space, the irreduceable minimum energy that quantum mechanics says must be present in a vacuum.

 

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