quantities. This is done seriously and this is the quantum field
theory in which the observables are localized, not only in time, but
also in space. From that point of view, it is quite consistent and
therefore, if one wants to relieve the non-relativistic nature of
the observation concept, one must say that every real observable is
something like a field strength at that point.
Now this sounds wonderful in principle. But if we think about
whether it satisfies the other criterion, whether it is realistic,
we come to a rather negative judgment. Bohr and Rosenfeld, as I am
sure many of you know, analyzed this. And they came to the conclusion,
"Yes, it is possible to make such a measurement provided we have an
arbitrarily big charge in an extremely heavy point concentrated in
an arbitrarily small space." Well, nobody has yet succeeded to do that!
Well, it has other problems too.
So the situation is really this: If I try to satisfy the
relativistic requirement — if I ask myself, "Is it relativistic?" —
I can happily answer, "Yes". But if I ask myself, "Is it realistic?"
Well, I'm afraid I must answer "No, it is not very realistic." The
measurement of field strength at points, with the accuracy required
to see quantum effects, not only has not yet been accomplished in
practice, but evidently runs into very grave difficulties.
The last question which I would like to ask is, "Is it enough?"
In other words, could I build up a theory only on this basis? And this
is satisfied, and in fact it is done. So the quantum field theories
operate only with the concept of field measurement, and they work.
Well, many people say — and, I think, correctly — that they're not
really terribly consistent in themselves. But, on the whole, the lack
of consistency surely does not arise because one does not have enough