How force at a distance works?

[curious friend]

I wish to know if there is any attempt to describe physical nature of electric field. Magnetic field seems to be due to the relativistic effect when the charge is in motion. What is physical picture and difference between electric field, magnetic field and electromagnetic waves? Also if there is vacuum between two charges then what leads to force at a distance and how? May be there is no perfect answer to these fundamental questions but I wish to know if anybody has attempted to answer.

[ajb]

The electric and magnetic fields are really two sides of the same entity, the electromagnetic field. The two halves are related by Lorentz transformations. It is the frame used that determines if the phenomena is electrostatic or magnetic.

[DrRocket]

I wish to know if there is any attempt to describe physical nature of electric field. Magnetic field seems to be due to the relativistic effect when the charge is in motion. What is physical picture and difference between electric field, magnetic field and electromagnetic waves? Also if there is vacuum between two charges then what leads to force at a distance and how? May be there is no perfect answer to these fundamental questions but I wish to know if anybody has attempted to answer.

 

In addition to what ajb told you, you might consider that, given that matter is composed of atoms and molecules that are themselves composed of elementary particles, ALL forces are force at a distance at a sufficiently small scale. Fields are the means by which force at a distance is understood.

 

Also the fact that the resolution of the electromagnetic field into magnetic and electric components is dependent on the reference frame of the observer was one of the things that guided Einstein in his development of the theory of special relativity. You can find a good treatment of this aspect of electrodynamics in any good text on the subject. Classical Electrodynamics by J.D. Jackson is a good source.

[curious friend]

In addition to what ajb told you, you might consider that, given that matter is composed of atoms and molecules that are themselves composed of elementary particles, ALL forces are force at a distance at a sufficiently small scale. Fields are the means by which force at a distance is understood.

 

Also the fact that the resolution of the electromagnetic field into magnetic and electric components is dependent on the reference frame of the observer was one of the things that guided Einstein in his development of the theory of special relativity. You can find a good treatment of this aspect of electrodynamics in any good text on the subject. Classical Electrodynamics by J.D. Jackson is a good source.

 

Thanks…What I wish to know is classically if there is any difference between electric field and electromagnetic waves (photons)? Also electric field seems to be of massless nature but spread all over the space and still passing a message from one charge to another in vacuum at speed of light(photons). I feel surprised that such field can exert a force of attraction on a particle having mass. Therefore I was trying to find out if there is any opinion or study regarding the physical nature of fields.

[ajb]

Photons are the quanta of the electromagnetic waves, they are the particle nature of the field. The electromagnetic waves are ripples in the electromagnetic field.

 

Electromagnetic interactions you can think of as exchanges of momentum between two bodies that is mediated by virtual photons. Two points here to make. One is that although massless photons do have momentum and this is the energy divided by the speed of light. The second point is that the photons are virtual and thus are not constrained to exactly satisfy the relation between energy and momentum.

[curious friend]

Photons are the quanta of the electromagnetic waves, they are the particle nature of the field. The electromagnetic waves are ripples in the electromagnetic field.

 

Electromagnetic interactions you can think of as exchanges of momentum between two bodies that is mediated by virtual photons. Two points here to make. One is that although massless photons do have momentum and this is the energy divided by the speed of light. The second point is that the photons are virtual and thus are not constrained to exactly satisfy the relation between energy and momentum.

 

Thanks….Kindly consider a situation: A positive charge is at O(0,0). A negative charge is at P(+ x,0). Suppose the positive charge sends a virtual photon in +X direction towards the negative charge. Now this virual photon shall impart some momentum to the negative charge during collision and then the negative charge should get further pushed towards +X direction. But we know that the negative charge gets attracted towards the positive charge and should move in –X direction. I feel that the 'force of attraction' may not be fully explained by this concept of virtual photons based communication. Kindly guide.

[ajb]

This is a common misconception that exchange of virtual particles can only lead to a repulsive force. The problem lies in thinking too classically.

 

One way to think about this is using the Heisenberg uncertainty principle. We assume the virtual photon has an exact momentum, and thus it is described by a plane wave. That is we cannot know where it is, it is spread out everywhere. It does not matter which direction the momentum points. So the momentum can be in the direct of the receiving particle to the emitting particle and thus producing an attractive force.

[curious friend]

This is a common misconception that exchange of virtual particles can only lead to a repulsive force. The problem lies in thinking too classically.

 

One way to think about this is using the Heisenberg uncertainty principle. We assume the virtual photon has an exact momentum, and thus it is described by a plane wave. That is we cannot know where it is, it is spread out everywhere. It does not matter which direction the momentum points. So the momentum can be in the direct of the receiving particle to the emitting particle and thus producing an attractive force.

 

Thanks... This explains to some extent. But see the above situation again. If this experiment is repeated large number of times then classically in almost all cases the negative charge should travel –X direction towards the positive charge. I do not know if using Heisenberg uncertainty principle we are able to show maximum probability of finding the negative charge along –X direction between (0,0) to (+x,0) and minimum in +X direction.

[ajb]

You can justify this by looking at the wave functions. Have a look here.

Edited August 12, 2011 by ajb