A post doc in an area that differs from my PhD?

I am currently doing a PhD in fluid mechanics but want to do mathematical physics tbh. In another forum I got an answer about a user who had done a PhD in accelerator physics and went to do a post-doc in condensed matter, vice versa even, but in their case, it seemed like it was largely about there being specific connections between the two that were desired at the times, I've read around other cases, e.g one in biology where the areas did not relate in such a way (or so that's the impression that comes across on a first read of the story), and where in this case the student mentions how she read a lot more of this other subject she ended up going to do a post-doc in, than her current PhD topic. But my question is, despite everyone in this forum saying otherwise, that mathematical physics programmes are harder to get into than applied maths, and, if one wasn't considered a good enough candidate to be offered a position in mathematical physics to start with- for a PhD- why would they then be for the case of a post-doc? I mean with the biology story above, I do not know about the fields enough to guess whether the situation would have been this - her grades were strong enough in the first place. I mean having a PhD in a different area is definitely not going to make you a stronger candidate, would be my guess, would that be correct? In which case, would I be better of dropping out and going for a Mres in mathematical physics, hope to get a publication or two, which would make me a significantly stronger candidate I guess, and I would also guess with publications prior degrees would not matter as much or? Many thanks for your help

Of course, doing your PhD in the same field as your postdoc makes you stronger candidates compare to someone changing fields... but it comes down to how well you see your self to PIs.
My PhD was in biological and chemical wastewater treatment (background as a scientist)- For my post doc, I switched to biofuels/biogas production (lab full of engineers). Slightly different area but the underlying theory, skills and lab work are very similar, so it was an easy switch, but require a lot of reading in the first few months. It's doable but you need to show a big interested in the area your changing to, and able to highlight the skills you can bring over.

How far along your PhD are you? There are certain Mathematical Physics programs that are really prestigious, like the one at Oxford. You could apply now and see if you get into one of those programs, think about how you would fund it, and so on. However, I think the idea that "X subject is harder than Y" really stops mattering when you reach postdoc level. If you can't get into one of the very top MRes programs then dropping out really is pointless, and even if you did it will always be a step backwards. What matters at postdoc level is your publications and your networks. There are so many ways of strengthening your current project to become more theoretical. Most of these involve trying to get someone to sponsor your application to the top maths societies if you aren't already a member, where you can engage with the subjects, network, and see whether or not you really do enjoy it (a lot of people think they want to do X thing until they get there and realise it's difficult and hard). If your department has a mathematical physics group, make friends with them. Go to their seminars. Present your work to them, ask for their ideas on the underlying concepts. Share ideas: you might end up as a coauthor on someone else's paper, which will make it much easier to nudge your way into where you want to be. Being the one person who has a crossover set of experiences between pure and applied physics is going to be in a much stronger position than the person who only has one. *You* can find and make connections between what you're doing and what you want to do, even if they aren't immediately obvious. If you really hate your PhD and are only a few months in, and can get into somewhere really good, then changing fields might be worth it; if not, the thing that will probably help you most in the long run is continuing to go forwards rather than trying to go backwards. In any case you should definitely have a concrete plan in place before dropping out of your PhD. Remember that if you drop out then you'll always have a gap on your CV for the rest of your life that you'll have to explain.

It depends on what transferrable skills you have. You would have to look at each postdoc and see how many of the job responsibilities you can do. It will obviously vary but if you can learn certain skills as part of your PhD that you think you will need in a future postdoc role it would be great. The mathematical physics field could be different but most PIs I know care less about the degree and instead what you can contribute.

@Nead, I believe we are in very in similar fields, as I am doing a weird biofuels & biogas PhD

yes i would get into a top mres programme- my ug, msc, phd all at the same uni, as they dont require a scholarship or funding like PhDs. the application process is very similar to that of which it was MSc then I woudl guess, unless you are applying for scholarships which I don't plan to.

apologies also to update, I am a month away from being a year in. thanks

I would not recommend a weird solution like doing MRes and PhD at the same time or dropping a funded PhD for MRes unless you are not interested in the PhD topic. You have very little chance in academia without a PhD. In academia it is expected to switch fields very often. The Postdoc is some kind of "Joker" who is expected to work in several fields unlike the PhD student.