You speak and present your thesis without a slide presentation, probably with a pen to explain something on the board. This question came to mind when I was wondering "why do we need a slide presentation?" and "have all thesis defenses in the world been done using a slide presentation?" and "will you be penalized if you defend your thesis without a slide presentation?".
Of course, slide presentations are a very recent development. Even the humble overhead projector didn't exist when I defended. We wrote with chalk on a slate like material.
Yes, you can do a presentation without slides. In some ways you can make a better presentation unless you are very skilled with slide creation. Most slide presentations are incredibly boring. They have been described, somewhat accurately, as the private notes that a speaker would normally keep to themself rather than making them visible. I know a couple of people who can do a good job, but it is, in my view a poor technology in general.
However, the expectations of your audience should be taken into account. If they expect slides, then you should probably do it that way. If you distribute the slides, printed perhaps, before the presentation then the audience has a place to take "notes in context". So, the "penalized" question is an entirely local one. Make sure you understand the local expectations. And, don't be boring.
At one university where I studied, the chalk boards were actually slate and there was a chalk tray at the bottom. The classrooms were very seldom cleaned and after several weeks a small cone of chalk dust would be growing on the floor just below the end of the tray.
Since the boards were seldom washed, just dry wiped, it sometimes occurred that the writing on the board would start to slide down the board on the "lubrication" of the still remaining dust from earlier uses. It would get distorted as it slid, of course.
In the UK it is very rare to have slides in what we call a Viva, which serves the same purpose as a defense. In general, a viva last 2-3 hours, and for the first 10 minutes, the candidate will describe their "thesis" - i.e. what it is they are claiming we know now, that we didn't know before they started. The rest of the 2 hours 50 minutes is the examiners (panel members) challenging them, and asking questions about their work.
My PhD advisor demands a presentation without slides even though we are in a field that works with a lot of images, for example from electron microscopy. We only have a blackboard/whiteboard. There was only one exception where an industry partner wanted a powerpoint presentation. This was continued even during the pandemic, so that we have to draw on a tablet or something similar instead of showing prepared slides. My advisor's reason for doing so is that without images, the defendant has to focus more on the ideas and abstract concepts of his work instead of showing one sample after the other.
So I agree with the other answers, that this is very much a cultural thing, and not even necessarily determined be the field. Speak to your advisor.
A little bit of a different perspective here - public speaking in general vs. a thesis presentation. But I believe the same things that work in one type of speech work in another. Obviously the expectations of your particular field, your university/department and your advisor are most critical. But assuming that you are either expected or encouraged to have a slide presentation, here are a few things that I have learned from Toastmasters over the past few years. If you have not tried Toastmasters, I highly recommend it. Nearly all clubs are virtual (Zoom, etc.) right now and most encourage guests.
- Keep the number of slides to a minimum. I'd suggest no more than one per minute, and that is for a short (5 - 15 minute) speech. For a longer speech, such as a thesis, you might have a handful going over key points and then spend most of the time explaining your thesis with perhaps a slide every 5 - 10 minutes.
- Keep the slides simple. 1 - 3 images and/or 5 - 10 lines of text. Sometimes even just 1 picture and 1 line of text is enough. This helps accentuate key points and also keep people from spending their time trying to read everything when they should be listening to you speak.
- Do NOT hand out the slides (paper or email) in advance, unless you are required to do so. Hand them out (or email them or send a link to download them) at the end of the presentation. Otherwise, 1/2 the people will be distracted by the slides in their hands (reviewing old slides or jumping ahead to the next one) instead of listening to you speak.
- Pick a simple theme and stick to it. Colored border, simple graphic background, etc. Otherwise the background and other irrelevant stuff (unless your thesis is about graphic design) gets in the way of listening to you speak.
- Practice, practice, practice! It is too easy (been there, done that) to use the slides as a crutch. Don't let that happen, especially on something as important as your thesis. In fact, practice it at least a few times without the slides. That will (a) help make sure you really know the speech well and (b) take care of the unlikely, but possible, situation of technical failure with the presentation (e.g., projector dies just as you get started).
Yes, it is certainly possible. One of the good things about starting a viva with a brief talk is not to transfer useful information (we have already read the thesis very carefully) but to give the candidate a chance to get "warmed up" and deal with any nerves they may have. So I would be happy with whatever form of presentation suited the candidate, it is largely for their benefit. It is perfectly reasonable to have some nerves* and few people are ready to answer difficult questions right from the start. For similar reasons, I often make sure that early in the viva I have prepared a question that has an easy but non-trivial answer.
I also advise my students to pre-prepare some slides containing any diagrams etc. that they think they may need during the viva even if they are not intending to use them in the initial presentation.
* just occasionally** there has been a candidate for which I wasn't completely nerveless myself ;o)
** they usually end up being the best vivas ;o)