o

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Revista Brasileira de Ensino de Fsica, vol. 18, n 3, setembro, 1996 143

Estrelas, Ci^encia e a Bomba:

Uma Entrevista com Hans Bethe

Renato Ejnisman

Department of Physics and Astronomy,

University of Rochester, Rochester NY 14627 USA

Trabalho recebido em 12 de julho de 1996

Hans Bethe fala de trechos de sua vida como fsico, sobre a fabricac~ao da b omba at^omica,

desarmamento e a situac~ao atual da ci^encia.

Abstract

Hans Bethe talks ab out pieces of his life as a physicist, ab out the making of the atomic

bomb, disarmament and the current status of science.

sido convidado p elo editor. Alem disso, a entrevista Hans Bethe faz parte de um grup o restrito de fsicos

publicada na Folha foi traduzida, o que a faz menos que caram para a Historia. O curioso, no caso de

precisa, e leve, em alguns casos, a ordem das p erguntas Bethe, e que, ap esar de seus 90 anos, completados em

invertida p or quest~oes editoriais. julho de 1996, ele ainda faz Historia. Atualmente, ele e

Antes de continuar, vale descrever um p ouco como um dos lderes em estudos sobre o mecanismo de explo-

foi a entrevista. Eu cheguei na sala de Hans Bethe na s~ao em sup ernovas, tendo comecado a trabalhar nesta

Universidade de Cornell em Ithaca (EUA) numa terca- area cerca de vinte anos atras, numa idade em que a

feira a tarde (21/05/96) e quei surpreso em v^e-lo rme maioria dos fsicos se ap osenta.

e forte em seu trabalho: Bethe estava consultando uma

Po de-se dizer que Bethe tornou-se uma gura

tab ela com a abund^ancia dos elementos no Universo

lendariadafsica, sendo o protagonista de uma serie

para comparar com alguns de seus resultados. Ap e-

de historias passadas de gerac~ao em gerac~ao de estu-

sar de morar nos Estados Unidos desde 1935, ainda

dantes. Na presente entrevista, Bethe conta o que ha

mantem um forte sotaque alem~ao. Ele fala devagar,

de verdade em alguns destes mitos. Ele da tambem sua

dando a impress~ao de escolher cuidadosamente cada

opini~ao sobre como deve ser feito o desarmamento nu-

palavra. Bethe me pareceu uma p essoa extremamente

clear, conta como foi a fabricac~ao da b omba e comenta

mo desta e simpatica e a entrevista transcorreu como

o que deve ser feito em a ci^encia ho je em dia.

num b om \bate-pap o".

Minha ideia original era entrevistar Hans Bethe e

No que segue, apresento uma p equena biogra a de

mandar o trabalho para algum jornal de grande cir-

Hans Bethe e dep ois a vers~ao completa da entrevista

culac~ao para que o publico  leigo pudesse conhecer me-

com este renomado cientista.

lhor uma imp ortante gura da fsica. De fato, trechos

desta entrevista foram publicados no jornal AFolha de

Breve Biogra a

S~ao Paulo em 28/07/96. No entanto, como foram dei-

xados de lado os trechos de maior interesse para fsicos Hans Bethe nasceu na Alemanha em 2 de julho de

e historiadores da fsica, acredito ser op ortuno publicar 1906. Obteve seu diploma universitario p ela Universi-

aentrevista completa num veculo mais voltado para dade de Frankfurt e seu doutoramento p ela Universi-

fsicos, como no presente artigo, esp ecialmente p or ter dade de Munique, aos 22 anos de idade. O tema era

144 Renato Ejnisman

Bethe:Not to do anything for my health. difrac~ao de eletrons em solidos, sob a orientac~ao de

ninguem menos que Sommerfeld. Em 1932, jaumreco-

Prof. Bahcall from Princeton University

nhecido lder da fsica, obteve uma p osic~ao na Universi-

once said that you did so many things in ph-

dade de Tubingen.  No ano seguinte, com a ascens~ao do

ysics that if he did not knowyou, he would b e

nazismo, Bethe p erdeu seu cargo devidoa  sua origem

tempted to think that you were in fact many

judaica.

p ersons conspiring to sign pap ers with a single

Com a a juda de Sommerfeld, Bethe obteve uma

name. If this were true, whichonewould b e

p osic~ao na Universidade de Manchester na Inglaterra.

your preferred Hans Bethe?

Em 1934, receb eu uma oferta de emprego na Universi-

Bethe:First of al l, I am a physicist. But, probably, my

dade de Cornell (EUA), que ateent~ao n~ao fazia parte

most productive years were when I was a nuclear physi-

do circuito da fsica mundial. Em fevereiro de 1935,

cist, especial ly when I wrote a long comprehensive paper

Bethe chega a Cornell onde p ermanece ate ho je.

in Reviews of Modern Physics in 1935 and 1936. Then,

Bethe trabalhou em diversas areas da fsica, muitas

Icontinued in nuclear physics and, in particular, I got

vezes com contribuic~oes fundamentais. Ele a judou a

interestedinnuclear matter in the 1950's. I triedto

formar as bases da fsica nuclear teorica (principalmente

prove that the forces between two nucleons are just right

atraves de seu artigo na Reviews of Modern Physics nos

to explain the binding energy and the density of heavier

anos 30), trabalhou em fsica do estado solido, foi pi-

nuclei. Wel l, I failed. But, at least, I came somewhat

oneiro na criac~ao da Eletro din^amica Qu^antica (Bethe

close to it. This is the theory of nuclear matter which

explicou o deslo camento Lamb), fez p esquisas em fsica

was started by Keith Brueckner. I think I simpli edit

aplicada (p or exemplo, estudou materiais que pudessem

greatly and then extended it to include the correlation

ser utilizados em veculos espaciais), explicou o meca-

between threenucleons.

nismo de pro duc~ao de energia em estrelas (que lhe ren-

deuopr^emio Nob el de 1967), entre outros feitos. Atu-

People like to tell stories of hero es and some

almente, Bethe trabalha com explos~oes de sup ernovas

of them are very fantasized versions of what re-

e astrofsica de neutrinos.

ally happ ened. In the history of physics there

Durante a Segunda Grande Guerra, Bethe traba-

are many such stories that involveyou as a main

lhou no desenvolvimento do radar e mais tarde na

character. Let's talk ab out a few of them to see

bricac~ao da b omba at^omica em Los Alamos. Como

how they really were. For instance, it is told

chefe da divi~ao de fsica teorica do Pro jeto Manhattan,

that you havesolved the problem of hownuclear

Bethe foi um dos pioneiros na utilizac~ao de computa-

disintegrations to ok place in a trip by train and

dores, ap esar de ele proprio utilizar p oucas vezes este

then in a subway.Howwas that?

equipamento. Apos a criac~ao da b omba, Bethe tornou-

se um dos lderes da luta p elo desarmamento. Bethe

Bethe: I got interested in nuclei in 1933, which was es-

teve ativa participac~ao em areas so ciais, combinando

sential ly fol lowing the discovery of the neutron by Chad-

sua impressionante carreira em ci^encia com imp ortan-

wick, and I was working together with Rudolph Peierls.

tes participac~oesnavidapoltica, tendo participado de

We worked very wel l together at Manchester. We once

diversos comit^es norte-americanos e internacionais.

went to visit the Cavendish Laboratory in Cambridge

where Chadwick and Goldhaber have discovered the di-

AEntrevista

sintegration of the deuteron by gamma rays. Chadwick

th

chal lenged us and said: \I bet you can not make a the- You are now close to your 90 birthday and

ory of this process". So, on the train back from Cam- you are still doing research, you are still one of

bridge to Manchester, which took a long, long time, we the most imp ortantinvestigators in your eld.

produced the theory, which was mainly based on the idea What is your secret?

o

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Revista Brasileira de Ensino de Fsica, vol. 18, n 3, setembro, 1996 145

who knowwhy it shines. Is that true? In any that the forces between nucleons are short range. Using

case, how did it feel to b e then the only p erson that theory we then also calculatedthescattering of neu-

to knew why stars shine? trons by protons. That was a beautiful theory, very sim-

ple. The only problem with it was that it was wrong. It

gave a cross-section of 3 or 4 barns and, two years later,

Bethe:The story you arereferring to is not mine. There

it was shown that it is real ly about 20 barns. Now, this

is a great confusion with respect to it. It was Houter-

was solved by Wigner in the subway between Columbia

mans who told his girlfriend that. I did not. But, wel l,

University and Penn Station. Wigner suggestedthat

how did it go? The ideaoftheproton-proton reaction

the deuteron also had a singlet state but nobody knew

was, I think, original ly due to Weiszacker but he didn't

where that might belocated. One could deduce that this

know how to do it. But Critch eld, who was a gradu-

singlet state had a very smal l binding energy. I then put

ate student at Ceorge Washington University took it up

that into my Reviews of Modern Physics article and la-

and said he was going to do it. Indeed, he did it and

ter it was extendedbyTel ler and Schwinger and a very

Gamow, who was his thesis advisor told him: \Wel l,

beautiful theory came out of it. So, this could be inves-

there is this Hans Bethe who knows a lot about Nuclear

tigatedexperimental ly by using para-hydrogen to scatter

Physics. Why don't you send it to him and seeifhe

neutrons.

can con rm it and maybemakeitmoreaccurate". So,

Then you made another imp ortant discovery

that's how I got into the proton-proton reaction. Then,

on a train which basically triggered the develop-

a little later, in the spring of 1938, Gamow and Tel ler

ment of QED. Do you think trains are a go o d

organizeda conference in Washington after threeother

place for inspiration?

conferences which had al l been on theoretical physics,

but they decided that this time it should beonastrop-

Bethe:(laughter) No, I don't think so. In both cases,

hysics and in particular in energy production in stars.

and special ly for QED, I was very stimulated and wan-

So, that conference was held and was very interesting

tedbad ly to get that theory worked out. I was at the

and in particular one of the lecturers was Stromgren,

Shelter Island conferenceandIhadtheidea that I pro-

aSwedish-Danish astrophysicistwhotoldusthatthe

bably could solve the problem of the Lamb shift. So, the

temperatureinthecenter of the Sun was about 20 mi-

rst occasion that I had to sit by myself and work on

l lion degrees rather than 40 mil lion degrees, which was

this was on the train.

much too much for proton-proton reactions. So, Crit-

Was it easy?

ch eld and I felt happy because with 20 mil lion degrees

Bethe:Yes, it was fairly easy.

we could do very wel l. By now, it is actual ly believedto

be 15 mil lion degrees, which is stil l OK. Then, Gamow Let me nowaskyou a question related to

gave a talk, partly reporting on the work by Weizsacker your work on energy pro duction in stars that led

and both of them were not very interested in the energy to your Nob el Prize in 1967. There were strong

production, but on building up elements. Thereisa evidences that lead to the idea that stars were

great diculty here. You can get to helium very easily fueled bynuclear reactions. But, nob o dy knew

but then there is a gap. You can't add a proton because whichwere those reactions. You were the rst

lithium-5 is not stable and you can't add an alpha par- to p oint them out and verify that they pro duced

ticle because beryl lium-8 is unstable. So, they had the the correct amount of energy.Now, students in

most weirdideas about how to build up the elements. I physics havemany stories they tell themselves

decided after that conference that one should separate ab out famous physicists. There is one in which

the problem of energy production from the problem of your girlfriend tells you how b eautiful the sun-

building up elements. I went to work on energy produc- shine is and you resp ond that it is indeed b eau-

tion and there was the problem that while Critch eld's tiful, sp ecially b ecause you are the only p erson

146 Renato Ejnisman

idea worked very wel l for the Sun, it wouldn't work for Bethe: It is indeedcontradictory. The Second World

Sirius or any heavier star because they had much greater War was a very serious matter and the Nazi doctrine

energy production per unit mass and not much higher upset al l western civilization. So, it was obvious that

center temperature. I then searched through the perio- one had to resist it and work as much as possible for

dic table, just stupid ly going one by one. So, without the Al lies to win the war. So, at a very early time I vo-

any inspiration, I got to carbon and, in this case, you lunteeredtoworkontheRadiation Laboratory at MIT

get a cycle leading backtocarbon. That was done about on radar. I was happily doing that when Oppenheimer

the same time by Weizsacker, but he didn't do anything asked me to join the atomic bomb project. So, I was

except to write down these reactions, whereas I wrote a among the founders of Los Alamos. When Los Ala-

paper in which I calculated the rate of these reactions at mos got together in April of 1943, Oppenheimer cal led

various temperatures and found out that these reactions in two more experiencedpeople, namely Rabi and Ro-

gave about the correct rate and center temperature. bert Bacher. They had experience at the Radiation Lab

and said: \Oppie, you should make divisions of your

You mentioned Gamow and there is another

lab, you can't just run it single handed and, being the

story in which Alpher and Gamowinvited you

director, you can't run the theoretical division". They

to sign a pap er that turned out to b e very im-

said the obvious person to run it was Hans Bethe. That

p ortant for the establishment of the Big Bang

was very simple. So, I had a division which was impor-

theory. The reason they invited you, however,

tant but relatively smal l. The lab as a whole, I guess,

was just b ecause the author's list would form the

had three or four thousand employees, most of them

rst three letters of the Greek alphab et. How

scientists or engineers. The theory division had only

was that?

one hundred, but we got into al l areas of the work. In

Bethe:Alpher, Bethe and Gamow. That was real ly just

particular, I knew nuclear physics but I had two group

a joke. The paper was not obviously wrong so I said

leaders in my division who knew just as much, namely

\OK, put me on". Now we know the paper is obviously

Victor Weisskopf and Robert Serber. Most of the nu-

wrong. Elements are not build up in the Big Bang, but

clear physics was done on those two groups. I got most

in big stars.

concernedabout the problem of implosion by which the

bomb was assembled, so I worked mainly on that. Most

You not only contributed in a decisiveway for

of it was done by other people, but I did what I could.

various elds in physics, but also, in some cases,

you were a pioneer, helping to create the eld it-

Howwas the atmosphere in this place where

self. During the pro cess of developing new ideas

the b est physicists of that time where con ned?

and even discover new elds of research, do you

have doubts?

Bethe: They werealltremendously engagedinthispro-

Bethe: Wel l, I have no doubts. Gettings a new theory is

blem, we al l wantedittosucceed. We al l workedtoget-

a wonderful experience and you arecompletely involved

her and one thing that \Oppie" instituted was a wee-

in it, nothing else matters, it is just a wonderful state

kly meeting with the senior people, about fty of them,

of mind.

which was cal led the Coordinating Council. Therewere

no secrets, everything could bediscussedthere. There Let us now talk ab out Los Alamos and your

was no division of know ledge, contrary to the wishes of work with the atomic b omb. Do es it feel con-

General Groves and that was essential because we told tradictory to you that you were one of the main

each other what we have learned and what troubles we gures on the creation of the b omb and then

had. Often somebody from a di erent group would have you were also one of the main gures on the

agood suggestion to solve it. ght against nuclear arms race?

o

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Revista Brasileira de Ensino de Fsica, vol. 18, n 3, setembro, 1996 147

In the meantime, did you talk ab out physics Bethe: I don't think that's possible, because making a

in general with your colleagues? nuclear weapon is so easy and therearelotsofcountries

which are without consciousness and ought to make

Bethe: Never. Some people did. Feynman, for ins-

troubles. Iraq is an obvious one and North Koreais

tance. But, I felt that this was a big job and we had

another. Iraq put lots of e ort into making nuclear

to do it as fast as possible. There was no time left for

bombs, but didn't get there. But then, thereare so-

anything else.

mewhat moreresponsible countries, India for instance.

Did you also take it as a p ersonal task since

India knows how to make an atomic bomb and I think

you were obliged to escap e Germany b ecause of

could do so if they want it. There wil l be now a new go-

the Nazis?

vernment and you can't tel l what they wil l do. So, there

aretoo many countries around the world which areir-

Bethe:No, I was part of a team. I did think at the time

responsible and could make nuclear weapons. And there

that the bomb was probably going to beused against Ger-

is nothing to prevent them, except the existenceofmore

many, but victory over Germany was achievedbefore

stable countries, like the Unites Sates and, I hope, pre-

the bomb was ready.

sent day Russia which have a certain number of stored

What did you think when the b ombwas

nuclear weapons which could beusedinanemergency

dropp ed?

to defeat such a wrong country. I believe something like

Bethe: I think it was right to do so. The main think it

a hundred nuclear weapons have to beretainedbythe

saved was Japanese lives. If it hadn't been for the bomb,

responsible countries.

the rebombing of the Japanese cities would have con-

Brazil has not signed the Treaty for Non-

tinued and the number of casualties would have been

Proliferation of Nuclear Weap ons b ecause it

much, much greater than it was in Hiroshima and Na-

claims that the treaty not only keeps developing

gasaki.

countries from obtaining technology for building

Did the scientists know b eforehand that the

nuclear weap ons, but more imp ortant, b ecause

bombwas going to b e dropp ed?

it even keeps these countries from obtaining any

kind of nuclear technology, including the ones

Bethe: We assumedit. We wereasked to send two

for p eaceful purp oses. Do you think that is

bombs to the mid-paci c, so we did. We had a thirdin

true?

readiness. After the Japanese surrender, we got orders

not to ship it out.

Bethe: No, the use of nuclear power reactors is perfec-

Howdidyou feel after the rst test? tly permissible. In fact, the treaty went together with

Eisenhower's cal l for atoms for peace. And I think part

Bethe: The rst feeling was one of accomplishment. We

of the treaty says that countries shoukl general ly be sup-

did it. And the second feeling was horrible.

ported when they try to build nuclear reactors for power.

In what sense?

But, in your case, Brazil and Argentina for years were

preparing nuclear weapons against each other, but then

Bethe: This destructive power would now exist and

morereasonable governments came into play and they

would make wars much worse than it had been. I think

signed an agreement.

it is important to realize that in Japan we dropped two

bombs, but at a time the United Sates and the Soviet

After the end of the Second World War, mai-

Union each had ten thousand of them.

nly b ecause of the success of the atomic b ombs,

Do you think the ultimate goal of the ght science and physics in particular got huge fun-

against nuclear weap ons should b e to wip e them ding. Now, after the end of the Cold War, the

all out? situation has changed again and there is not a

148 Renato Ejnisman

lot of money anymore for science. Howdoyou How ab out your recentworkwithneutrinos?

think science should b e done nowadays?

Bethe: We do the best we can. If we have less money,

Bethe: I have worked mainly with solar neutrinos. Bah-

wel l, we can't do as much, but we wil l stil l do whatever

cal l and I are of the same opinion that the peculiar ob-

we can.

servations of solar neutrinos are explainedbytheMSW

theory which says that electron neutrinos areconverted

Do you think certain felds of science should

into mu neutrinos that you can not observe with radio-

stop b eing funded now?

active detectors. The crucial experiment is in prepara-

Bethe: No, I think al l science should be funded. Of

tion. It is cal led SNO [Sudbury Neutrino Observatory].

course, we should not fund crazy science, like cold nu-

Last Friday,wehadatalkfrom one of the experimen-

clear fusion. But, whenever thereisrespectable scien-

ters and he said it wil l take about a year before SNO gets

ti c basis, it should be funded.

into operation. We're very curious about what they wil l

Is there anything in particular that you see as

say.

the most imp ortant thing in science nowadays?

Howisyour daynowadays?

Bethe: I usual ly come to my oce. Today, for ins-

Bethe: Let's put it this way: if I wereagraduate stu-

tance, I came just at noon, yesterday I came at 10:00,

dent now, I would probably study molecular biology. I

but then I left early. I just do what is necessary.

think that's where the most intportant discoveries wil l

be made in the next twenty years.

I noticed there is a sign close to your black-

b oard that says: \SAVE". Is there any sp ecial

That brings the question: what are you doing

meaning for it?

nowadays?

Bethe: In the last twenty years, I have worked with Bethe: Of course, it is very special. If that sign is on, it

supernova explosions and I think I have a good unders- means that whatever I have written on the black-board

tanding of the mechanism of the explosion. should be saved. The other side says: \ERASE".