# Magnetism & relativity

1. Jun 16, 2011

### lovetruth

Magnetism is said to arise due to relativity. When an extended charged object move, length contraction occurs which increases the objects charge density thus increasing electric field which is then perceived to be caused by magnetism (if we do not account for relativity). Thus, magnetism is a relativistic effect.

My Question is : Will point charges show magnetism as they do not have any length which can contract?

2. Jun 16, 2011

### Mentz114

Your explanation is not quite right. The magnetic field appears in a moving frame to counter-act the change in the electric field, so all observers will agree on the path of a charge in a field.

From the article

Yes, a moving point charge has a magnetic field.

3. Jun 16, 2011

### lovetruth

Please elaborate wat u mean by " counter-act the change in the electric field, so all observers will agree on the path of a charge in a field." I didnt quite get it.

4. Jun 16, 2011

### Mentz114

Suppose there's an experiment where an electron is slung through an electric field, which deflects it so it hits a certain spot on a screen.

An observer, moving inertially relative to the lab, would see the electric field as changed, and therefore would not expect the electron to hit the same spot, if they consider only the electric field. But they must see the electron hit the same spot as the observers in tha lab, and indeed they do. The reason is that a magnetic field would also be present for the moving observer, and this (in their calculations) would do just enough to steer the electron through the hole. It is almost as if the magnetic field is there to ensure that no causal paradox happens.

5. Jun 17, 2011

### lovetruth

The link i have provided proves that : Magnetism is just a Relativistic effect and not a fundamental force like Electric force; just as centrifugal is a pseudoforce. Now, magnetism arises due to length contraction of extended charged bodies which increases charge density thus leading to magnetism. Electron is an extended body i.e. it has finite volume but what about a point charge? It is just a point and cannot be squeezed further.

6. Jun 17, 2011

Staff Emeritus
I'm afraid it doesn't. One statement is "a moving charge produces a magnetic field". Another statement is "Given an arbitrary E and B field, there always exists a frame where the field is purely electric." Statement 2 does not follow from Statement 1.

7. Jun 17, 2011

### kmarinas86

Every inertial observer will agree with the first and higher derivatives of the path of the charge, but not every observer will agree on how much distance the charge traveled over time. Nor will they agree on the electric and magnetic components of the field. Nor will they agree on the angle between the acceleration $\mathbf{x}''$ of the charge and the velocity $\mathbf{x}'$ of the charge. The acceleration $\mathbf{x}''$ of the charge is the same for all inertial observers, but the velocity $\mathbf{x}'$ of the charge is the same only for co-moving inertial observers.

8. Jun 17, 2011

### kmarinas86

Whether a particle experiences a gain of magnetism, or a loss, is relative to the observer.
Whether a particle experiences a gain of charge density, or a loss, is relative to the observer.
Whether a particle experiences a gain of length contraction, or a loss, is relative to the observer.

...if Lorentz ether theory is not correct.

9. Jun 17, 2011

### lovetruth

The concept of magnetism is only invoked when relativistic effect like length contraction are ignored. When Maxell wrote electromagnetic equation, he didn't kno about relativity. Thus, magnetism was used as a scapegoat to explain the relativistic phenomena.

10. Jun 17, 2011

### lovetruth

Lorentz ether theory is correct excluding the concept of ether. Lorentz transformation is correct and is incorporated in SR.
So there is no doubt in the 3 statements u stated. Remove the word 'whether' as the 3 statements r absolutely correct.

11. Jun 17, 2011

### Blackforest

Could you please elaborate more on the last affirmations (preservation of the acceleration in a change of inertial frame) or suggest references to learn more about these items?