SCIENCE AND THE UNOBSERVABLE — DINGLE 219 



The unobservability of absolute simultaneity, then, depends on the 

 fact that we cannot determine unambiguously how long light takes to 

 inform us of an event; or, more generally, how long after an event it is 

 possible for us to know of it. This ignorance, of course, would not be 

 necessary if we could know of an event immediately it occurred — if, 

 this is to say, we could observe it by some messenger which traveled 

 at an infinite speed. This is not pure fancy. Before the time of 

 Homer, in the seventeenth century, it was believed that light might 

 travel at an infinite speed, and before the theory of relativity arose, 

 it was believed that gravitational action was transmitted instantane- 

 ously. It is not uncommon, too, to imagine that there might be 

 instantaneous telepathic communication. We can at least, then, 

 conceive that an instantaneous messenger might exist, and therefore 

 absolute simultaneity is not logically unobservable. It is unobservable 

 simply and solely because, so far as our present survey of the universe 

 has gone, there is no evidence that it is possible to learn of a distant 

 event at the moment at which it occurs. In other words, absolute 

 simultaneity is physically unobservable. 



It is important to emphasize this because, as I remarked just now, 

 it has been claimed that absolute simultaneity is rejected because it 

 is logically unobservable, and this claim has been made the basis of 

 the philosophy of the logical positivists. Let us hear the late Prof. 

 Moritz Schlick, one of the foremost and most able of the founders of 

 this school of thought. He maintains that there are only two signif- 

 icant classes of unobservables, corresponding to what I have called 

 the "actually" and the "logically" unobservable. 



The distinction between impossibility of fact and impossibility of principle, is 

 absolute, without the slightest ambiguity; it is not of such and such a degree; 

 it is essential. 8 



He then cites Einstein's rejection of absolute simultaneity as an 

 example of the rejection of the logically unobservable, the "impossi- 

 bility of principle." He makes it quite clear that he does not dis- 

 tinguish between the physically and the practically unobservable; 

 they are all included in the "impossibility of fact." 



The statement, "There are mountains 3,000 meters high on the far side of the 

 moon," is perfectly sensible, although our present technical skill is insufficient 

 to assure us of its truth or falsity. It would still be sensible if it were unquestion- 

 ably established scientifically that man would never reach the far side of the moon 

 by any means. The verification remains conceivable; we are able to express what 

 it would be necessary to do to decide the question, what it would be necessary to 

 experience; verification is logically possible, and that is all that matters. 



I hope I have made it clear that if the conceivability of verification 

 is all that matters, absolute simultaneity is verifiable, and, therefore, 

 not logically unobservable. We can conceive of the possibility of 



* Erkcnntnis. French translation in Actualiies Sclentlflques et Industrlelles, No. 152, p. 25 (1934). 



