The general goal of a Ph.D. defense (and to some extent a master defense) is to assess that the author of the manuscript is indeed the student. Of course, it has smaller over objectives, depending on the type of defense, like assess the candidate’s ability to talk in public and transmit knowledge, the capability of making concise and logic answers to scientific questions…
I have assisted to two different types of Ph.D. defenses, the one organized in Sweden and the one organized in Japan, in Physics/Engineering/Maths. I know of Ph.D. defenses in France, for Physics topics -through the testimony of friends and colleagues.
But since the main goal to verify that the student is the one who did the research – sometimes trying to see if the student is the one who wrote the manuscript too – is shared amongst all Universities, the general organization/elements are the same. They will always be an oral presentation given by the student (the defendant) and they will always be an evaluation committee (which can be called jury).
The oral presentation length and scope vary strongly between countries – and I suspect between disciplines. The Ph.D. defense talk is between 30 to 45 min in Sweden, while it is separated in two occasions in Japan, with a 30 to 45 min talk on the first defense day, followed by a 20 min talk on a follow-up defense (few weeks after the first defense). In France, the defender presentation varies from 25 to 45 min. Often, the Ph.D. student is asked to have an introduction comprehensible for high school level science – most importantly if the defense is public or not. A defense is by default public in most universities – the exception arises if private interests would be damaged if the thesis results were open to the public (usually a company is involved in the process).
The evaluation committee, in theory, should not have conflict of interests with the decision to validate/invalidate the defender’s thesis. Conflict of interests can be positives – for instance, the head of the department might want to increase the statistical success of the doctoral studies – or negatives, for example, a member of the committee has another Ph.D. student working on the same research. The establishment of the committee is ruled by university guidelines and current practices. For instance, in Sweden, the guidelines encourage the committee members to come from another University, to have several different fields of the discipline represented and, sometimes, encourage sex parity. Usually, the thesis supervisor is excluded by default from being a committee member – or at least in the debate following the defense to decide on the outcome. The evaluation committee members will usually have access to the manuscript in advance and will question the candidate to satisfy their evaluation.
A third member of the defense can sometimes be present – as is it for Sweden for instance, it is an opponent. An opponent is a recognized scientist (someone with a Ph.D. or equivalent) working on a similar field but who did not collaborate with the student (for the thesis research topics). Sometimes, the opponent is tasked with the large vulgarization presentation, of which the defendant can lean on to shorten the introduction on the defense presentation. The most important part, however, is the scientific discussion which should be conducted between opponent and defendant. Unfortunately, it can be difficult to have a reasonable opponent. I have assisted to defenses where: the opponent asks rhetorical questions (of which the answers are contained within the question formulation or of which the opponent answer immediately after) or polar questions (I literally heard in one defense a Ph.D. candidate says “yes” and “no” for more than 70% of the questions asked) – I dubbed those “the unprepared” (usually the opponent had not time to read exactly the manuscript and just ask questions parting to the field of research – of which the opponent is an expert). I have also had the “joy” of assisting to some defense where the opponent literally ask questions to “trick” the defendant, sometimes even insisting on asking questions on subjects the defendant had close to no-knowledge of (for instance: how did you operate the machine to measure this value for a Ph.D. student who makes theoretical models and just receive the data from another source) – I called them “the show-offs”. It is often (not always) young researchers either not sure of how the process works in the country where they act as an opponent, or often they do not want to feel inadequate and want to show that they “know” what they are talking about. The most interesting – and scientifically beautiful – are the defense in which a conversation occurs. The opponent will ask questions – but be receptive to questions from the defendant. Basically, a conversation from expert to expert – as equals. My best memory of Ph.D. defense is one where such conversation occurs between the opponent and defendant and lasted up to 3 hours – and because of the way it was conducted (and my basic knowledge), I was able to understand and follow the conversation for the first 2 hours.
The best defenses I have assisted have been the one in Sweden – particularly Uppsala University. The process is ceremonial and well codified: the opponent introduces the thesis subject for 20 to 30 min to the general audience, then the defender presents the manuscript for around 45 min, the open discussion between them follow (and has no delimited time frame – only that after 3 hours the defendant can ask for a break), the jury is then called to ask their own questions (the limit is 15 min par jury member, the number of juries can vary from minimum 3 up to 5), and, finally, the member of the public can ask questions. The jury then meet, first with the supervisor(s), then alone, to declare if the Ph.D. student and the manuscript meet the requirements for the diploma. I have witness only one time where the Ph.D. student did not obtain the diploma after the defense, during my 5 year stay, and even then, the student receive a 6 months extra time to re-try again.
Basically, once the manuscript has been written, the defense is a small hurdle to pass in comparison! I strongly encourage the prospective/actual Ph.D. students to write their own manuscript. It is stupid not to do so – even if your language skills are not up to par – you can always ask/pay someone to review your text. I think that, in the end, cheating in science brings you no-where. What will you do if you are employed to operate/fabricate something that you never bother to learn because someone did it for you? Writing a thesis can be hard but it brings experience that is valuable – even in everyday life (and to learn about yourself). I would remind as well that being caught on this falsehood can have real consequences, even years after.
For my own defense, I was “lucky” (I fabricated my luck by suggesting this opponent) to have a good opponent, who had an interesting discussion with me. To give friends and family the opportunity to experience my defense, I have used the “Live” option from Youtube, which can still be watched.