What is a field?

[mistermack]

I'm blissfully ignorant when it comes to fields. At school level physics, (a long time ago), the word was used widely, without any detail being thrown in.

 

I took it to mean "field of influence" but the physics world must have moved on since then.

 

A gravitational field is said to be the curving of space time. Is a magnetic field and electric field also due to curving of a different component of space time?

[Sriman Dutta]

Field is the sphere of influence of a non-contact force.

[Mordred]

A field is an abstract device to mathematically model any collection of objects. Typically with geometric coordinates.

[DimaMazin]

Field is place of time transformation into space.

[geordief]

A field is an abstract device to mathematically model any collection of objects. Typically with geometric coordinates.

So ,with no "detection" there is no field?

 

Your "objects" are physical objects -not mathematical objects?

 

Can they be "potential objects" ie the model/field shows the result if an object is placed at that position?

[Sensei]

Magnetic field lines can be visualized by cheap device called "compass array".

Anyone can build 2D compass array using f.e. 100 (10x10) compasses, attached on table with similar spacing between them.

There is also 3D version of this device.

 

Here are my photos. It has 117 compasses.

 

 

While moving magnet, they obviously spin.

While passing current through wire, or through electromagnet, we can see how they spin accordingly to direction of current.

 

Visualization of electric field we do using f.e. Electroscope.

https://en.wikipedia.org/wiki/Electroscope

 

Edited January 31, 2017 by Sensei

[Mordred]

So ,with no "detection" there is no field?

 

Your "objects" are physical objects -not mathematical objects?

 

Can they be "potential objects" ie the model/field shows the result if an object is placed at that position?

Objects can be events/coordinates physical objects etc. Even strictly mathematical objects such as vectors/spinors etc Edited January 31, 2017 by Mordred

[studiot]

So ,with no "detection" there is no field?

 

Your "objects" are physical objects -not mathematical objects?

 

Can they be "potential objects" ie the model/field shows the result if an object is placed at that position?

 

No detection is not a prerequisite.

 

No he gave a mathematical explanation (but see below) so they are mathematical objects ie objects recognised by mathematics.

 

Yes they can be potential objects - the most famous would be potential energy - and a PE field could conform to your if question, but it doesn't have to.

 

 

A field is an abstract device to mathematically model any collection of objects. Typically with geometric coordinates.

 

I think this definition is a tad too wide since all the 'objects' of the field must be of the same type.

 

OK so here is my take.

 

A field as defined in mathematics is not quite the same as a field as defined in physics.

 

Since this is a Physics question here is a Physics answer.

 

A field has (at least) two parts.

 

The field variable, which may, as Srinam says be a Force or it may be quite another quantity.

 

The type or name of the field is usually named after the field variable so we have magnetic fields, velocity fields, temperature fields and so on.

 

The field is just a catalog of the values of the field variable in some region of space at a particular point in time.

 

It may be a static field in which case the values do not change with time or it may be a dynamic field in which case the values change with time.

 

The values in a constant field do not change with position in the region. This is important for vector fields.

 

The coordinate system of position in the region is separate from the coordinate system describing the values of the field variable.

 

Note I said the field variable may have more than one value.

 

Field variables with only one value at any point are called scalars and their own coordinate system is called a scale.

 

All the values of a field variable in a scalar field must have be of the same type eg evrey value in a temperature field must be a temperature.

 

Field variables with more than one value may be called vectors or tensors depending upon the number of type of the values.

 

If a field variable has more than one value the different values need not be of the same type eg a vector field variable has two values, magnitude and direction which are different types of quantity.

Edited January 31, 2017 by studiot

[StringJunky]

A field describes what is happening between interacting objects.

[studiot]

A field describes what is happening between interacting objects.

 

Do you think this also applies to non-conservative fields, sources or sinks?

 

How do you describe the situation if I introduce an object eg a piece of wood or plastic into a magnetic or electric field?

Edited January 31, 2017 by studiot

[mistermack]

I'm no wiser.

The mystery for me, is the action of a force via nothing.

With gravity, they postulate a particle that transmits the force of gravity. That's after me struggling to accept that gravity is not a force.

 

Is there going to be a similar particle, that is responsible for magnetic force? And electrostatic force?

[StringJunky]

 

Do you think this also applies to non-conservative fields, sources or sinks?

 

How do you describe the situation if I introduce an object eg a piece of wood or plastic into a magnetic or electric field?

I appreciate it's likely not all-encompassing but is my definition generally correct?

 

By my definition, the wood and plastic objects are not part of the field; they are invisible wrt the field. Am I wrong?

Edited January 31, 2017 by StringJunky

[Bender]

I appreciate it's likely not all-encompassing but is my definition generally correct?

 

By my definition, the wood and plastic objects are not part of the field; they are invisible wrt the field. Am I wrong?

But the field is still there, even if there is no interaction.

 

I would describe a field a something that has a value in every position in space.

 

There is also a distinction between vector fields and scalar fields

- scalar fields only have a value at each point in space, e.g. temperature or gravitational potential energy

- vector fields have a value and a direction, e.g. velocity field in a fluid, temperature gradient or gravitational field

[geordief]

I'm no wiser.

The mystery for me, is the action of a force via nothing.

With gravity, they postulate a particle that transmits the force of gravity. That's after me struggling to accept that gravity is not a force.

 

Is there going to be a similar particle, that is responsible for magnetic force? And electrostatic force?

I am fairly sure I have heard that it is the photon that is responsible for the electro-magnetic force

 

Also I think I have learned that gravity is not modeled as a force in General Relativity but that is not to say that it is not modeled as a force (it causes acceleration) in Newtonian physics.

 

In quantum -gravity ,perhaps it will be eventually modeled as a force (I have no good idea if that is possible) . .

Edited January 31, 2017 by geordief

[studiot]

But the field is still there, even if there is no interaction.

 

 

 

Yeah, right on.

 

 

 

I would describe a field a something that has a value in every position in space.

 

 

 

Even if that value is zero or null at some points.

 

But not necessarily at every point, just in the region of space of interest.

IOW Fields can have boundaries.

 

 

 

 

There is also a distinction between vector fields and scalar fields

- scalar fields only have a value at each point in space, e.g. temperature or gravitational potential energy

- vector fields have a value and a direction, e.g. velocity field in a fluid, temperature gradient or gravitational field

 

That is why I said value or values.

It does not stop at vectors, though. Tensors etc also make fields.

 

I'm no wiser.

The mystery for me, is the action of a force via nothing.

With gravity, they postulate a particle that transmits the force of gravity. That's after me struggling to accept that gravity is not a force.

 

Is there going to be a similar particle, that is responsible for magnetic force? And electrostatic force?

 

Are you asking about Fields, mediating particles 'action at a distance' or what?

[StringJunky]

But the field is still there, even if there is no interaction.

Then that's describing a field just as an empty map; space with co-ordinates

Edited January 31, 2017 by StringJunky

[studiot]

Then that's describing a field just as an empty map.

 

Yes that's exactly what geordief asked and the answer is the same.

 

The 'Field map' of Potential energy for a 1kg test mass is the same whether the mass occupies a particular point or not.

That is there is only one point where the mass actually occupies at any one instant, the rest of the map is available if and when the mass choose to be there.

[StringJunky]

 

Yes that's exactly what geordief asked and the answer is the same.

 

The 'Field map' of Potential energy for a 1kg test mass is the same whether the mass occupies a particular point or not.

That is there is only one point where the mass actually occupies at any one instant, the rest of the map is available if and when the mass choose to be there.

OK. Ta.

[studiot]

OK. Ta.

 

 

Think of it this way.

 

Consider a point mass, m at point A in a potential (energy) field.

Moving it to a point B of lower energy yields some energy for some purpose eg accelerating the mass.

If we now complete the circle we have to re-supply that energy to the mass to return it to point A because the PE field is a conservative field.

 

But at all time the potential is there at both points A and B and all points in between.

[mistermack]

If I asked the question, "what is the wind?" , then mapping the wind, describing interactions between temp and pressure, density, velocity etc etc doesn't answer the question.

 

The question is what it IS. Sometimes, it seems that "field" is the mathematical description of "something".

What is that something?

 

Another thing that makes me curious, is that I just read in wikipedia " In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons."

Surely you can see photons, or detect them fairly easily. I've never heard of light streaming from a magnet. ??

[studiot]

If I asked the question, "what is the wind?" , then mapping the wind, describing interactions between temp and pressure, density, velocity etc etc doesn't answer the question.

 

The question is what it IS. Sometimes, it seems that "field" is the mathematical description of "something".

What is that something?

 

Another thing that makes me curious, is that I just read in wikipedia " In quantum physics, the electromagnetic field is quantized and electromagnetic interactions result from the exchange of photons."

Surely you can see photons, or detect them fairly easily. I've never heard of light streaming from a magnet. ??

 

Actually your question was answered.

 

Maybe my post#8 was too long or you did not understand it.

 

It should have at least help you realise that there are many fields, each one quite different.

So the question 'what is it?' is pretty meaningless

 

I'm sorry that the definition of a field does not match your idea of one.

 

That's life.

 

If you would like to know more, ask for further explanation rather than railing against those trying to help you.

Edited February 1, 2017 by studiot

[StringJunky]

 

Actually your question was answered.

 

Maybe my post#8 was too long or you did not understand it.

 

It should have at least help you realise that there are many fields, each one quite different.

So the question 'what is it?' is pretty meaningless

 

I'm sorry that the definition of a field does not match your idea of one.

 

That's life.

 

If you would like to know more, ask for further explanation rather than railing against those trying to help you.

He's probably asking ontologically. What everybody has described, one way or the other, is the best he's going to get, isn't it?

 

Mistermack: science descibes how things behave, not what they are.

Edited February 1, 2017 by StringJunky

[Mordred]

Well to demonstrate Studiots last reply lets name a few different fields.

 

scalar field

Vector field

Spinor field

Boson field (force field)

Fermion field (matter field)

Event field (relativity)

Tensor field

etc etc.

 

A field is as stated an abstract device to describe any collection of objects. (objects can be nearly anything as long as its the same type)

Edited February 1, 2017 by Mordred

[geordief]

Non ontologically ,does a moving electron or proton set up a magnetic field and does that field require an environment composed of

other EM fields to have effect?

Is there such a thing as a diagram of the field set up by one moving "elemental" EM charge (such as an electron or a proton ,I surmize)

Edited February 1, 2017 by geordief

[mistermack]

Ok, but it's pretty obvious that I'm asking what the field describes. I made that clear.

I don't buy the comment that science doesn't study what things are, only how they behave. The study of the Sun didn't just discover how it behaved. The most important discoveries were about what they Sun actually WAS.

 

I'm not trying to introduce ontological mystery to the debate.

Studiot, you say you answered the question. You didn't. You said a field was a catalogue of values of a variable.

It's the nature of the variable that I was inquiring about, and that was pretty clear in the OP.

ie , what is that variable, in seemingly empty space?

 

I'm getting the feeling that the answer is " I don't know " and that's fair enough.

As far as gravity goes, there seems to be confusion. A warping of space time? OK, but that seems to definitely assume a physical structure of space time, rather than nothing. Or is the variable a particle that is passing between bodies?

 

I asked the same question about a magnetic field. Is it a different kind of warping of space time, or the result of photons being constantly passed through the field?