Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Magnetic Fields and Relativity
Name: Muntaseer Al K.
Status: student
Age: 20s
Location: N/A
Country: N/A
Date: 10/25/2005


Question:
Suppose a spaceship is moving with a constant velocity with respect to an observer. Also imagine that an electron in the spaceship is at rest with respect to the spaceship. therefore an observer in spaceship suppose to detect no magnetic field around the electron since it is at rest with respect to him and according to postulate of special relativity laws of science are same in all inertial frame. But an observer who notices both the spaceship and the electron are moving with the same velocity with respect to him must detect a magnetic field perpendicular to the motion of the electron since moving charge creates magnetic field.

What should be the answer of this paradox? Is magnetic field is a relative phenomenon?


Replies:
Dear Al,

You undoubtedly have received excellent answers to this question already, but I cannot help myself from putting in my two cents, though I am VERY late.

You show wisdom in saying magnetism is a "relative phenomenon". In fact magnetism can be understood by combining electrostatics and relativity.

My favorite example (which I believe led Einstein to his theory of special relativity) is to consider two protons (or any two objects in space each carrying a positive charge) at rest relative to you. As you know, they will repel each other with a force given by coulomb's law.

Now run past the charges. Since they are moving as seen by you, you will see them each generate a magnetic field and feel an attractive force due to moving through the magnetic field generated by the other charge. In fact, if you could run at the speed of light (you cannot but you could get close, in principle) the magnetic force would exactly cancel the electric force!

How could your running past the charges possibly change the force they exert on each other? Nonetheless your running does! This can be explained by relativity since if you see the charges moving past you (all motion is relative so its the same whether you are running past the charges or they are moving (in the opposite direction) past you), special relativity says time in the system moving relative to you is dilated as measured by you. Therefore the charges move apart more slowly. In fact, if the relative speed is the speed of light, the time dilation is infinite and so you never see the charges separate.

You could equally well explain this by saying the magnetic force just cancels the electric force and so the charges never separate.

I hope this helps. It is a weird world!

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory