# Thread: What are fields ? i.e Electromagnetic and Gravitational Fields..

1. ## What are fields ? i.e Electromagnetic and Gravitational Fields..

I understand that particles are discreet forms of energy with certain values depending on what one you are looking at. But i do not really understand what fields are. I have been looking on the internet but i can not find anything that breaks it down into a way i can understand.

I know that the Electromagnetic field is pervade throughout space but i do not understand the mechanisms of how sunlight actually travels to us for example.

So i was hoping some people on this board could break it down and explain in laymans terms to me what a field is please.

2. Originally Posted by kevin1981
I understand that particles are discreet forms of energy with certain values depending on what one you are looking at. But i do not really understand what fields are. I have been looking on the internet but i can not find anything that breaks it down into a way i can understand.

I know that the Electromagnetic field is pervade throughout space but i do not understand the mechanisms of how sunlight actually travels to us for example.

So i was hoping some people on this board could break it down and explain in laymans terms to me what a field is please.
Let's talk classically about electromagnetic fields -- so we won't talk about photons or particles.

Fields are distributed objects that handle what would otherwise be "action at a distance".

For instance take two charged bodies, say your hair and comb on a dry day. If you move the comb towards your hair, you hair stands on end with no apparent contact between the comb and your hair. Fields are the method by which this force can be explained, and they work very well.

That is a static field. If you shake an electron (i.e. accelerate it) an electromagnetic wave is produced. That is a dynamics change in the electromagnetic field, and that change propagates as a wave. The field consists of two components, an electric component and a magnetic component. An electromagnetic wave includes both components, at an angle with respect to each other that depends on the medium through which the wave propagates. In a vacuum the angle is ninety degrees.

Light is an electromagnetic wave. It propagates just fine through a vacuum, and that is how light from the sun reaches us.

To really understand this you need to read an introductory book on electrodynamics, but that will take some mathematics. A good introductory book is Classical Electromagnetic Radiation by Marion.

Classical Newtonian gravity is a field that is similar to the static electric field, except that it is always attractive and does not depend on electric charge. In a sense the "charge" is mass. Gravity waves are part of general relativity and that is a far more complicated issue.

3. I understand how light travels through space as the electromagnetic field propagates the whole of it. But i do not understand what the actual field is made of or how it works.

4. Originally Posted by kevin1981
I understand how light travels through space as the electromagnetic field propagates the whole of it. But i do not understand what the actual field is made of or how it works.
It is not made of anything. It is the field. It has no inner workings. It is a mathematical description of electromagnetism.

Quantum mechanically the electromagnetic field is a whole bunch of photons, but that takes a different perspective.

Similarly, the gravitational field, in some future quantum theory of gravity, will probably consist of a whole bunch of photons, and again that is different and at this stage speculative animal.

5. Sunlight is EM radiation radiating out from the surface of the sun, travelling through space in the form of elementary particles called photons. I actually find it easier to think of it this way, rather than as an electromagnetic field. The EM field of the sun is essentially the region of space around the sun in which it can affect objects electrically or magnetically. That's really what a field is, a region in which something can influence something else at a distance.

The photons themselves just travel through space at the speed of light.

6. Originally Posted by EDG
Sunlight is EM radiation radiating out from the surface of the sun, travelling through space in the form of elementary particles called photons. I actually find it easier to think of it this way, rather than as an electromagnetic field. The EM field of the sun is essentially the region of space around the sun in which it can affect objects electrically or magnetically. That's really what a field is, a region in which something can influence something else at a distance.

The photons themselves just travel through space at the speed of light.
Nice, simple, and wrong.

One of the major points of classical fields is in fact to avoid having to talk about "action at a distance". The field is viewed as a real physical thing, and it is through the field that energy is transferred and force is exerted.

A field is not a region, although it does exist as a function defined over a spatial extent. The technical term is a vector field. In classical electromagnetism it is the field that carries energy. Even in circuits it is the field that carries the energy of electricity and not the kinetic energy of the electrons. The field is itself quite physical and that is an important point of electrodynamics and something that needs to be appreciated to really understand how to think about fields. In the classical picture the energy of the field is an essential characteristic, and if one uses some vector analysis (see Poynting vector) one can describe the power transmission accomplished by the field. The picture is quite a bit different from the quantum mechanical perspective with photons. It is important to understand the classical field picture as well as the quantum picture and usually the classical picture is understood first.

Photons hardly "just travel through space at the speed of light". Photons are in fact packets of energy and they interact with other packets of energy (aka particles) to create physical phenomena. Their traveling is only a small piece of the picture. But the correspondence between the photon picture (quantum electrodynamics) and the classical Maxwellian field picture is extremely involved and requires even more advanced mathematics. In the end the field picture and the photon picture are both about energy. But the OP asked about fields.
Last edited by DrRocket; 2010-Jun-13 at 01:31 PM.

7. It's correct for the level of understanding that the OP is after. You'll note that he asked for a response in layman's terms.

8. Oh for goodness sake, stop it right here and now!
If this continues I will ask the admins to just close Q&A as apparently simple layman's explanations cannot be given without turning into a discussion of deeper and deeper philosophy.
If a person ask for a explanation in layman's terms THEN GIVE IT IN LAYMAN'S TERMS (and if you want to go deep for your own satisfaction, take it to astronomy or scitech.

9. Order of Kilopi
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Tusenfem, what part of free and open discussion
do you not understand? I have got some extra
insight here into long held questions on this
subject. I would hope the original poster has
too. Thanks DrRocket and EDG.

Any real attempt to answer these questions
inevitably touches on some philosophy. We
are always at the limits of knowledge.

10. Originally Posted by peteshimmon
Tusenfem, what part of free and open discussion
do you not understand? I have got some extra
insight here into long held questions on this
subject. I would hope the original poster has
too. Thanks DrRocket and EDG.

Any real attempt to answer these questions
inevitably touches on some philosophy. We
are always at the limits of knowledge.

If you have a problem with moderation, then report the post, and do not start a meta discussion in the thread.

11. Order of Kilopi
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Jan 2010
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3,688
Originally Posted by DrRocket
Similarly, the gravitational field, in some future quantum theory of gravity, will probably consist of a whole bunch of photons, and again that is different and at this stage speculative animal.
(my bold)

You do mean gravitons, right?

12. Originally Posted by EDG
It's correct for the level of understanding that the OP is after. You'll note that he asked for a response in layman's terms.
Which he recreived. He then asked a follow-up question which requires another step in sophistication to answer, but still an elementary explanation.

Most importantly, he received a correct answer.

13. Thanks for trying to keep the post's simple tusenfem, but i do not mind if some are a little complex as i am asking complex questions. It is inevitable that some answers will be a little harder to understand than others but i enjoy reading and learning from them. Since joining this board i have extended my knowledge of the quantum world and now i will nick pick at DrRocket's post and go read up on some stuff to try and wrap my head around "fields" !!

1st off i will go and read about electrodynamics and see where that takes me

14. No more Meta-Discussion

15. Established Member
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Let me try at my level of understanding:

Fields result from a source; namely, a force acting upon some object.

so for example,

I will try to abstract my ideas from Volume II of Feynman's Lecture series, Chapter 1 ----Section 1.5

Loosely paraphrasing Feynman:

"Fields result from an accelerated movement of one object upon another. Given an electromagnetic field --they result from an electric charge being accelerated in space . . . so in this case the charge being accelerated--it must have "some" other electric charge on the other end to "effect" the field"

Now quoting more Feynman directly:

"A major reason why we don't perceive Fields (--readily) is because we need to be able to step out of our personal frame of reference--the reference frame that Einstein saw when he imagined what would happen if one traveled at the speed of light"----

(my own understanding)--->it is at this juncture when one might realize that at traveling at the speed of light you would not readily recognize the light itself as being a part of the E/M field--which it is!

I will try to post on the nature of light in a subsequent post!--but------>

Light was first recognized as a part of the E/M field when J.C. Maxwell was able to deduce it theoretically (from the laws he derived and named after him) ---

When one is directly effected by a field; i.e. gravity ( we do not feel it because each individual has certain amount of mass ) we are in the reference frame of the field itself---(the field of the Earth's gravity)
Last edited by John Jaksich; 2010-Jun-13 at 03:54 PM. Reason: meaning and interpretation of gravity and mass

16. If you've ever seen a bar magnet surrounded by iron filings, you'll notice that the filings arange themselves in lines. These are referred to as "field lines", and show the charactaristics of the magnetic field around the magnet. If you were to release a 'test charge' in the field, it would (ignoring any other forces) be accelerated in the direction that the field lines point from its initial position. So, while a field is as described previously, you can also think of it as the characteristic of a given set of coordinates that dictates how certain particles behave within those coordinates.

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Good job Roobydo !

There are some very good sources which attempt to explain the nature of light---> the three of which I am personally familiar are:

The Feymnan Lecture Series and the book by

Silvan Schweber: QED and the Men Who Made It: Dyson, Feynman, Schwinger, and Tomonaga

The latter is not for the faint of heart (in mathematics) and Feynman probably does one of the better jobs in another book called:

QED The Strange Theory of Light and Matter

To me at least: Light is a very tricky thing to attempt to explain because it is one of our most coveted senses of perception---->

My very simplest explanation is this

(and I realize it is going to sound overly cryptic--and I apologize) When light from an object enters one's eye it comes from a source that has its electrons elevated to an "excited" state of radiation--so when one sees an apple we will see it red, shiny and on the tree because that is the perception we have from it . . .

18. Originally Posted by kevin1981
Thanks for trying to keep the post's simple tusenfem, but i do not mind if some are a little complex as i am asking complex questions. It is inevitable that some answers will be a little harder to understand than others but i enjoy reading and learning from them. Since joining this board i have extended my knowledge of the quantum world and now i will nick pick at DrRocket's post and go read up on some stuff to try and wrap my head around "fields" !!

1st off i will go and read about electrodynamics and see where that takes me
If you have access to them here are some recommended books, in increasing order of sophistication and mathematical difficulty on the topic of electrodynamics.

1. Engineering Electromagnetics -- Hayt : introductory text for electrical engineers.

2. Classical Electromagnetic Radiation -- Marion : intermediate text for physicists

3. Classical Electrodynamics -- Jackson : classic graduate text for physcists, complete and difficult for some

19. Originally Posted by jaksichj

The latter is not for the faint of heart (in mathematics) and Feynman probably does one of the better jobs in another book called:

QED The Strange Theory of Light and Matter

.
\

Here is a link to Feynman giving lectures on which that book is based. Watch and learn from the master.

20. Thanks for that link... 'Looks like we are eating steaks for tea'... It explains this so well and complete.
How can we add more. , and thats 30 years old...
and back here at the OP. 'Your field' is that place where those particles can reach.

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