Are the beliefs in contradiction to the perceived objectivity of physics?

This is one of the most important points to be clarified since it is felt by many to be the biggest obstacle, preventing them from understanding the Bayesian approach: is there a place for beliefs in science? The usual criticism is that science must be objective and, hence, that there should be no room for subjectivity. A colleague once told me: ``I do not believe something. I assess it. This is not a matter for religion!''

As I understand it, there are two possible ways to surmount the obstacle. The first is to try to give a more noble status of objectivity to the Bayesian approach, for example by formulating objective priors. In my opinion the main result of this attempt is to spoil the original nature of the theory, by adding dogmatic ingredients[22]. The second way consists, more simply, in recognizing that beliefs are a natural part of doing science. Admitting that they exist does not spoil the perceived objectivity of well-established science. In other words, one needs only to look closely at how frontier science makes progress, instead of seeking refuge in an idealized concept of objectivity.^8.6

Clearly this discussion would require another book, and not just some side remarks, but I am confident that the reader for whom this report is intended, and who is supposed to have working experience in frontier research, is already prepared for what I am going to say. I find it hard to discuss these matters with people who presume to teach us about the way physics, and science in general, proceeds, without having the slightest direct experience of what they are talking about.

First of all, I would like to invite you to pay attention to the expressions we use in private and public discussions, and in written matter too.^8.7 Here are some examples:

• ``I believe that ...'';
• ``We have to get experience with ...'';
• ``I don't trust that guy (or that collaboration, or that procedure)'';
• ``Oh yes, if this has been told you by ..., then you can rely on it'';
• ``We have only used the calorimeter for this analysis, because we are not yet confident with the central detector'';
• The evening before I had to talk about this subject, I overheard the following conversation in the CERN cafeteria:
  • Young fellow: ``I have measured the resistivity, and it turns out to be $10^{11}\,\Omega$'';
  • Senior: ``No, it cannot be. Tomorrow I will make the measurement and I am sure to get the right value. ...By the way, have you considered that ...?''

The role of beliefs in physics has been highlighted out in a particularly efficient way by the science historian Peter Galison[37]:

``Experiments begin and end in a matrix of beliefs. ...beliefs in instrument type, in programs of experiment enquiry, in the trained, individual judgments about every local behaviour of pieces of apparatus.''

Then, taking as an example the discovery of the positron, he remarks:

``Taken out of time there is no sense to the judgment that Anderson's track 75 is a positive electron; its textbook reproduction has been denuded of the prior experience that made Anderson confident in the cloud chamber, the magnet, the optics, and the photography.''

This means that pure observation does not create, or increase, knowledge without personal inputs which are needed to elaborate the information.^8.8In fact, there is nothing really objective in physics, if by objective we mean that something follows necessarily from observation, like the proof of a theorem. There are, instead, beliefs everywhere. Nevertheless, physics is objective, or at least that part of it that is at present well established, if we mean by `objective', that a rational individual cannot avoid believing it. This is the reason why we can talk in a relaxed way about beliefs in physics without even remotely thinking that it is at the same level as the stock exchange, betting on football scores, or ...New Age. The reason is that, after centuries of experimentation, theoretical work and successful predictions, there is such a consistent network of beliefs, it has acquired the status of an objective construction: one cannot mistrust one of the elements of the network without contradicting many others. Around this solid core of objective knowledge there are fuzzy borders which correspond to areas of present investigations, where the level of intersubjectivity is still very low. Nevertheless, when one proposes a new theory or model, one has to check immediately whether it contradicts some well-established beliefs. An interesting example comes from the 1997 HERA high $Q^2$ events, already discussed in Section . A positive consequence of this claim was to trigger a kind of mega-exercise undertaken by many theorists, consisting of systematic cross-checks of HERA data, candidate theories, and previous experimental data. The conclusion is that the most influential physicists^8.9 tend not to believe a possible explanation in terms of new physics [64,65]. But this has little to do with the statistical significance of the events. It is more a question of the difficulty of inserting this evidence into what is considered to be the most likely network of beliefs.

I would like to conclude this section with a Feynman quotation [66].

``Some years ago I had a conversation with a layman about flying saucers - because I am scientific I know all about flying saucers! I said `I don't think there are flying saucers'. So my antagonist said, `Is it impossible that there are flying saucers? Can you prove that it's impossible?' `No', I said, `I can't prove it's impossible. It's just very unlikely'. At that he said, `You are very unscientific. If you can't prove it impossible then how can you say that it's unlikely?' But that is the way that is scientific. It is scientific only to say what is more likely and what less likely, and not to be proving all the time the possible and impossible. To define what I mean, I might have said to him, `Listen, I mean that from my knowledge of the world that I see around me, I think that it is much more likely that the reports of flying saucers are the results of the known irrational characteristics of terrestrial intelligence than of the unknown rational efforts of extra-terrestrial intelligence'. It is just more likely. That is all.''