The observation of a magnetic field can differ between observers in relative motion.

• A. False
• B. Sometimes
• C. Never
• D. True

Answer: (D)

In special relativity, electric and magnetic fields are not fixed properties; they transform when you switch between observers moving relative to each other. A magnetic field is not absolute. What one observer measures as a magnetic field can change for another observer moving with respect to the charges producing the field, and sometimes an electric field appears where another observer would see a magnetic field, or vice versa.

A simple way to see this is with a moving charge: in the charge’s own rest frame, there is only an electric field. But in a frame where the charge is moving, the fields mix and you observe both electric and magnetic components. Another classic example is a current-carrying wire: in the lab frame you see a magnetic field around the wire, yet in a frame moving with the electrons, the charge densities shift due to length contraction, producing an electric field as well. These illustrate that observers in relative motion can disagree on the presence and strength of a magnetic field.

So the statement that the observation of a magnetic field can differ between observers in relative motion is true.