D O C U M E N T S 4 8 , 4 9 M A Y 1 9 2 3 4 5

The fraction in the last brackets lies between 0 and 1, thus the brackets lies be-

tween –1 and 0, thus the whole expression between –c and 0. Your error is that you

related the relative motion of the second particle to the first one in respect to coor-

dinate system K instead of system K′. The theory does not state that the difference

in velocity between two particles cannot be above c, but that the velocity of one par-

ticle in respect to a legitimate coordinate system can never exceed c.

48. To Philipp Biach[1]

[Berlin,] 29 May 1923

Esteemed Sir,

Your message is quite amusing, but misleading nevertheless. For, this business

essentially has nothing to do with the theory of relativity. That motion, seen from

the purely kinematic point of view, is to be understood only as relative motion has

always been known (i.e., by sharp minds). The question that matters, however, is

whether there are any preferred states of motion also from the standpoint of physics.

Only experiment can reply to this, not general considerations of any sort.

Very respectfully,

A. Einstein.

49. To Heinrich Zangger[1]

[Berlin,] 29 May 1923

Dear Zangger,

You are entirely right. Unfortunately, we lack a firm factual basis. Everything

that Weyl, Eddington, and I have been doing recently is purely mathematical spec-

ulation and perhaps entirely

erroneous.[2]

The sole point of view is internal

consistency; but as I said, it may well be a charming wrong track.

I hope Tete’s condition isn’t worrisome. His glands are supposedly not right; I

know what that

means.[3]

c 1 –

1

q2

c2¹

----·

-+

©

§

2--

q

c

- –

1

q2

c2

---- -+

-------------------------------- - +=

1

-–--¹·cq

©

§

2

1

q2

c2

---- -+

-+=-------------------1–c