Parts of an Interview with Werner Heisenberg on Nuclear Energy Development in
Germany during World War II
Conducted and edited by Joseph J. Ermenc, Professor Emeritus, Dartmouth
College, from tapes recorded at Urfeld in southern Bavaria, on August 29, 1967.
(cf. references)
(...)
HEISENBERG:
Very soon after that event there was a meeting in
Berlin at which we had to tell the whole story to the
officials. Irving has described this in his book, "The
Virus House".
You know the Irving book, don't you? I think it
has been very carefully done. He studied all kinds of
sources: documents and so on. I think he did it pretty
well.
ERMENC: I think the conclusions are rather superficial
but it is full of fact.
HEISENBERG: I would say that ninety-nine percent of all
facts are correct. It is a very careful book.
I wonder if you have read the interview I had with
representatives of Der Spiegel?
When Irving's book first came out, Der Spiegel had
a rather extensive report on it. Afterwards they asked
me whether I would be willing to give them an interview
about the book and say where I approved of it and where
I did not.
I think that Irving's book is a very good book in
the facts.
But it has one deficiency.
When he tries to determine motives he does not do very
well because
he can not really think himself into the atmosphere of a
totalitarian country
making war. That atmosphere was so totally different from
what he had experienced in his life. He could then not
quite avoid the error of using cliche thinking.
He thought it was obvious that there was competition
to make an atomic bomb - there was a race between Germany
and America to be the first to make an atomic bomb. This
is so obvious to him that he cannot get away from this
idea.
But this was not the situation. The situation
psychologically was different. It was a new situation for
us scientists in Germany. Now for the first time we could
get money from our government to do something interesting
and we intended to use this situation.
The official slogan of the government was: We must
make use of physics for warfare. We turned it around for
our slogan: We must make use of warfare for physics.
It was a very natural reaction. everybody likes to
make interesting experiments.
We felt already in the beginning that if it were pos-
sible at all to actually make explosives it would take such
a long time and require such an enormous effort that there
was a very good chance the War would be over before that
could be accomplished.
When we came to the point of the successful L-4
experiment, when we knew that we could make reactors, and
when we knew from
Weizsacker' s paper that you could in this way make
plutonium or something like it, we knew that in principle
we could make atomic bombs.
But still we did not make a serious effort in this
direction.
Now let's talk seriously about this.
If we wanted to make the necessary heavy water it would
take one to three years to get a sufficient amount.
To make enough plutonium would also take another three
years. So with the best conscience in
the world we could tell our government, "It will not be
pos sible to make a bomb within five years or so" .
We knew already that they would forbid any new developments
which couldn't be used within the next year or so. It was
then quite clear that they would say, "No, no! No effort
for the atomic bomb".
That was what happened.
That made us very happy. I would say this was a very fa-
vourable situation for us. First of all we could get money
for doing experiments. Then we could probably make
reactors because that
should now be possible by simply enlarging L-IV .
ERMENC: I have noted that as early as December 6, 1939, in
your report to the War Office, you practically outlined
the whole program of uranium utilization.
HEISENBERG: That was the first theoretical paper.
You simply had to use the figures that were available.
Most of the figures were nearly correct and you could see
how things would go. But then we had this drawback: two
figures had to be checked; one was the neutron absorption
coefficient of heavy water and the other was the
absorption coefficient of pure carbon.
There was perhaps a bit of competition to do this.
Döpel fought for doing the heavy water experiment
and Bothe fought for doing
the carbon experiment. Bothe's Heidelberg people thought
it was nicer to do experiments with a larger quantity of
material; if you only have a few liters of substance, as
was the case with the heavy water, you
will have a very small absorption and the chance that you
will make a mistake is rather big.
We got the heavy water; there was not more than roughly
eight liters of heavy water available in the whole of
Germany at that time .
the carbon experiment. Bothe's Heidelberg people thought it was nicer
to do experiments with a larger quantity of material; if you only have a
few liters of substance, as was the case with the heavy water, you
will have a very small absorption and the chance that you will make a
mistake is rather big.
We got the heavy water; there was not more than roughly
eight liters of heavy water available in the whole of
Germany at that time.
The Heidelberg people got about a ton of carbon.
Then we thought very hard in Leipzig about the kind of
measuring equipment we would need to get a pretty
accurate determination of the absorption coefficient even
with a very small amount of heavy
water. It took quite some effort to think out how things
should be
arranged to obtain reliable values.
It was an unusual experiment insofar as we put the sources
of neutrons in the middle of a sphere of heavy water; then we measured
the absorption of the neutrons into the middle of a sphere.
The experiment, which was carried out by Döopel very carefully
was a success. The results which we got with this small amount of
heavy water were practically the same as the values which your people
obtained later on with very large amounts.
ERMENC: How closely could you duplicate the results of this experi-
mental work? One percent?
HEISENBERG: I think it was better than that. We gained an accuracy
of the order of one per mil. That was necessary in order to get a
useful result. It was a very refined measurement.
We had two spheres - two bottles of aluminum. One bottle
was covered with cadmium and the other was not. Otherwise they were
completely equal in size. We had the source of neutrons in the center.
It was the diffusion of neutrons from center to
the outside which we measured .
That was a very sensitive measurement. We got quite
good values as I told you.
ERMENC: How were these reports received by your colleagues
in other institutions?
HEISENBERG: Of course they were interested. Everybody
said, "This is all very nice. Now we can try to build
reactors." So the next steps were to im-
prove the geometry and to improve the purity of the
material.
ERMENC: Were more funds made available to you? Was there
any pressure put on you to go faster?
HEISENBERG: At this meeting held during the summer of '42,
we hesitated very much to ask for too much money. We felt
that if we did it would create too great excitement.
If we had said that we could make atomic bombs then
it is possible that we would have received orders to build
factories to make atomic bombs. This would have been the
last thing we wanted. We knew this could not be done within the next
two or three years and the War would be lost by that time anyway.
So we were very careful in speaking about atomic bombs.
But we did say that they could be made.
This we had to say, to be safe, in case the Americans would
actually drop atomic bombs; this was a possibility.
Actually I think it was Mr. Milch, during the meeting, who
asked, "How long w ill it take the Americans to make this thing if they
go for it? "
After some discussions among our group, we mentioned the
following figures to Milch: The Americans could not possibly have a
reactor going before the end of '42 - which was a very good guess -
and an atomic bomb not before the summer of ' 44.
This was on the optimistic side . We wanted it definitely
to be on the short side, because if they actually would drop an atomic
bomb it would be better if we had given warning. Among ourselves we
believed that they would not be able to make atomic bombs before three
or four years.
ERMENC: Why wasn't there more interest in graphite knowing that
heavy water was very scarce ?
HEISENBERG: It was because of the experiment of Bothe's on graphite
which was not correct. Bothe had made a measurement of the neutron
absorption coefficient of pure carbon and an error had slipped into his
experiment. His values were too high but we assumed they were cor-
rect and so we did not think carbon could be used.
Afterwards we knew what his error probably was. He had
built a pile of graphite pieces but in between the graphite pieces there
was always some air and the nitrogen of the air has high neutron ab-
sorption. Somehow he must have forgotten this. I don't know why
but it's understandable .
ERMENC: Didn't others run similar experiments on carbon to check Bothe ?
HEISENBERG: There were so few groups that we never did
an experiment twice. Every group had some job to do. We
in Leipziq had done a measurement of heavy water and after
that time everybody believed our figure for heavy water;
nobody checked it. Nobody checked the measurement of Bothe.
We intended in the later development of the thing
to use carbon for shieldinq around the reactors.
ERMENC: This is what you did at Leipzig?
HEISENBERG: No, we did this at Haigerloch in South Germany.
There we wanted to get carbon as shieldinq for the outside
because we realized that carbon was much better than light
water.
But then there was not enough carbon and it was difficult to
get. So not much was done with it. The carbon line was really ruled
out by the experiment of Bothe.
But still if we would have gotten a carbon reactor going what
would have happened next? Would we then have aimed for the atomic
bomb? There the answer was also clearly, no. We would not have
had this carbon reactor going before say the end of 1943 or the beginning
of 1944.
If we could have had the reactor going one could say, "You
could have made plutonium". But to build a reactor which actually
produces enough material for bombs would have been an effort again of
two or three years at least. At the beginning of '44 nobody would have
thought of doing such a thing.
Already in the summer of ' 42 the leading people in Germany
knew that they had to win the war within the next half year if they
could win it at all. Their most serious mistake was that they didn't
give in after that time.
(...)
ERMENC: After '42 some of yiu in Germany began to think of the
development of the uranium reactor as being very important in
postwar development as a source of energy.
Was this a driving force at that time or was it still in the curiosity
stage?
HEISENBERG: We knew such a chain reaction could be made. Therefore
we could plan to use this reaction for power plants for submarines and
other applications.
We felt that this was now a very important development in
technics, engineering, economics and so on. Whatever the outcome of
the War, we felt that we should be in this development after the War.
We felt this was really a nice task which we could do during the War.
We didn't know what you people would do but we felt it was
quite possible that during the War the Americans also would say that
atomic bombs are not interesting because they can't be ready before the
end of the War, and that working on energy production would be a very
interesting development for peaceful use. The explosive side of the
problem could be done after the War when one would have more time.
ERMENC: I suppose that at this time talking about post-war develop-
ment and nuclear power wasn't a very effective argument for supporting
wartime science ?
HEISENBERG: Yes, but why not ?
Apparently, in your country, one didn't think of a quick end
of the War.
I must say I always felt that the War would end earlier than
it actually did. I also was convinced that Germany would lose the War
and so the problem of the War didn't interest me too much any more.
I was interested in what came afterwards.
But in your country, apparently one had the impression that
one still could use the bombs during the War. This did not work out
for the War between your country and Germany, but it did work out with
Japan. This is a point which I also made in the interview with Der
Spiegel.
The decision of our government not to make bombs was a
very sensible decision. It would have been sensible even for your
government because you would have won the War against Germany
earlier if you had made no atomic bombs . There 's no doubt about
that because then you would have put this whole effort into airplanes
and tanks and whatever else, and the War would have been ended
earlier. This may not be true for Japan since the war against Japan
was a different matter. But speaking only for the War against Ger-
many I think this is a fact.
I can also understand that during war one argues in different
ways. One wants to be as strong as possible at the end of the war.
One can very well argue for trying such a thing. But we felt there
was a fifty-fifty chance that perhaps the Americans would also argue
in the same way as we did when we said that during the War these
things could not be developed any way and we would leave it for a
later time.
ERMENC: I think Irving mentioned that at the meeting, when you and
Weizsacker were asked how much you would need to continue your work,
you mentioned a sum which the officials thought was ridiculously low.
In consequence they said that this work could not be important because
it doesn't cost very much. Is that correct?
HEISENBERG: This is perfectly correct. Yes. It was mainly Speer
who reacted this way.
This was of course a very clear intention of ours. We had
to avoid being committed to make a big effort making atomic bombs.
What we wanted was to get just enough money to go on with our re-
actor project, but not more than that. We were very much afraid that
otherwise someone would say, "Now let's go for the atomic bomb".
Also I read in Der Spiegel - you really should read these
editions of Der Spieqel - an interview they had with Speer.
You know he has been released from Spandau. He is now
an old man.
He was asked by Der Spiegel people:
"You took part in this meeting when the physicists told you
about the probabilities of atomic bombs . What was your reaction ? "
He said,
"We listened when they told us that in principle atomic bombs
could be made, but they also emphasized that it would take a number of
years; certainly not before five years or so. So I felt - I think he ex-
pressed it in a funny way - there was not much music in the thing.
Therefore, I didn't report the whole thing to the Fuehrer until two weeks
later or so and then in a very casual way because I did not want the
Fuehrer to get so interested that he would order great efforts immediately
to make the atomic bomb".
Speer felt it was better that the whole thing should be dropped,
and the Fuehrer also reacted that way.
This side of the problem clearly worked out as we had hoped
it would. We definitely did not want to get into this bomb business.
I wouldn't like to idealize this; we did this also for our per-
sonal safety. We thought that the probability that this would lead to
atomic bombs during the War was nearly zero. If we had done other-
wise, and if many thousand people had been put to work on it and then
if nothing had been developed, this could have had extremely disagree-
able consequences for us.
ERMENC: Considering this development beyond the demonstrations of
a sustainable chain reaction, did you consider that after this it prob-
ably would have been taken out of your hands very quickly ?
HEISENBERG: After the War?
ERMENC: No, even during the War if you had gone ahead.
In our country I know there was some resentment among the
scientists in Chicago because they didn't control the project any more
after they had proved the principle.
HEISENBERG: This was also one of the main points we discussed es-
pecially among a small group including Weizsacker, Wirtz and myself.
We felt it as very important then these things must remain in our hands,
then we could always keep control of what goes on. That we did
achieve .
Of course, we could only achieve this by not making bombs.
If we would have said, "Now let's make a big effort for the atomic
bomb", it certainly would have been taken out of our hands.
Michael.Thorwart@physik.uni-augsburg.de
(Jul-13-2001)
