How should a student deal with personal criticisms from an assistant professor?

He may be competing with your supervisor. He may not like your general topic. He may not like you. Or, he really believes what he says. Or he wants to test you. You don't know.

You have to live with people who confuse what's going on with prejudice- or agenda-coloured "truth". Practically all successful researchers have encountered such put-downs.

Schechtman (later Nobel winner) got a "Introduction to Crystallography" book put on his table when he first reported that he got a 5-fold symmetry, hence quasicrystals, in his experiment. Feynman got put-down big time initially by Oppenheimer and was only saved by Dyson's aggressive intervention.

You think your progress is good? Your supervisor does, too? That's enough for you to go on, then. This guy may be a big shot, but for you, it doesn't count. Big shots are usually right when they think something is cool/great. They can be awfully wrong when they think something is not good. Trust your instincts.

Captain Emacs already gave a great answer, I'd like to address this point specifically:

I'm cautious to answer back to the professor, because he tells me I am being defensive, but then I am cautious of being too silent, because he tells me I am not contributing enough.

I suggest the following strategy:

• If he criticizes you personally: Ignore it.

• If he criticizes your work: Answer back, but use questions. Don't defend your work, try to find out why we criticizes it and what alternatives he suggests. If he is right, you get valuable feedback. If he is wrong (and others are present, like at your continuation), he will make himself look bad rather than you.

Example: "And you call yourself a scientist? This section is rubbish."

• Bad answer: (Get angry) "Of course I am a scientist! And this section is great because..."
• Good answer: (Take out a pen and paper to take notes) "Why do you think it is rubbish? How can it be improved?"

I have not spoken to my supervisor since my continuation or told him about my past encounters with this professor. Do I express my disappointment and confusion in our next meeting, or do I just leave it?

Sure, mention it! Your advisor's job is not only to help you write a great thesis, but also to help you find your way in academia and guide you through the process.

Kelly, if you have had a chance to meet with your supervisor since you asked this question, you may not need any further advice.

I'm male, so maybe not qualified to speak to this topic. You call yourself Kelly, which where I am (geographically) is usually but not exclusively female. A male professor is criticizing you personally and telling you that you are not suitable for academia, so it's even more likely that you are female. If I've guessed right on that, then since I see you also used a male pronoun for your supervisor, you should discuss this with women on the faculty in your department.

My other comments are gender neutral.

I'm worried because you said you prepared for three weeks and the exam went badly. If that's just because an assistant professor asked inappropriate questions, that doesn't reflect badly on you. But your supervisor is supposed to intervene for you if something inappropriate happens in an exam, and it sounds like he didn't. If the rudeness was coming from an assistant professor, then your supervisor should be at least of equal rank and it isn't obvious what power dynamic would keep him silent when you were being mistreated.

If the exam did go badly, your supervisor should bring it up without prompting, I think, but I wouldn't count on it. I think you need to know if your supervisor thinks it went badly, and if he shares your view on why it went badly. If you agree that this one assistant professor is the problem, then you supervisor should agree to keep him off your committee in the future, and all is good.

You say you view your PhD progress as very good and "as far as you know" your supervisor shares the same sentiment. I wish you were more confident in that. Maybe you and your supervisor need to agree on milestones and deadlines to reach the milestones so you can judge that you are making good progress.

It's 99.9% likely the other professor is wrong in his assessment of you. But you're probably not the next Feynman or the next Marie Curie, because they wouldn't need to ask the question you've asked. They succeeded no matter what anyone else said. The rest of us might be suitable for academia but still slip through the cracks without support from mentors and colleagues. In your question I see red flags that say your supervisor isn't supporting you.