22 RELATIVITY 



they are all related to the reckoning of length, time and 

 mass. If you alter the reckoning of length you alter the 

 reckoning of other physical quantities. 



Consider an electrically charged body at rest on the 

 earth. Since it is at rest it gives an electric field but no 

 magnetic field. But for the nebular physicist it is a 

 charged body moving at iooo miles a second. A moving 

 charge constitutes an electric current which in accordance 

 with the laws of electromagnetism gives rise to a mag- 

 netic field. How can the same body both give and 

 not give a magnetic field? On the classical theory we 

 should have had to explain one of these results as an 

 illusion. (There is no difficulty in doing that; only there 

 is nothing to indicate which of the two results is the one 

 to be explained away.) On the relativity theory both 

 results are accepted. Magnetic fields are relative. 

 There is no magnetic field relative to the terrestrial 

 frame of space; there is a magnetic field relative to 

 the nebular frame of space. The nebular physicist will 

 duly detect the magnetic field with his instruments 

 although our instruments show no magnetic field. That 

 is because he uses instruments at rest on his planet and 

 we use instruments at rest on ours; or at least we correct 

 our observations to accord with the indications of instru- 

 ments at rest in our respective frames of space. 



Is there really a magnetic field or not? This is like 

 the previous problem of the square and the oblong. 

 There is one specification of the field relative to one 

 planet, another relative to another. There is no abso- 

 lute specification. 



It is not quite true to say that all the physical quan- 

 tities are relative to frames of space. We can construct 

 new physical quantities by multiplying, dividing, etc.; 

 thus we multiply mass and velocity to give momentum, 



