just a general discussion.

correct me if I'm wrong but I'm sure it was once stated that The Doctor couldn't physical interact with himself as it would damage the universe or something to that effect, but in Day of the Doctor, 10 and 11 very clearly shake hands when they say farewell to one another with no effect what's so ever.

can someone clear this up? was I dreaming that they couldn't physically touch one another?

As far as i am concerned anymore paradoxes are all talk these days.

Nothing ever happens anymore.

i agree totally. its like when 10 said "three of us together will create some anomalies"

well let us see the anomalies, see what happens if they are together long enough!

I think it has to be the same incarnation not different ones for the paradox to happen.

You may be thinking of the Blinovitch Limitation Effect, which is somewhat hand-wavy and not applied consistently. Even if contact between two versions of the same time-traveller doesn't create a paradox, it supposedly still generates a burst of shorted-out potential energy. I think that has been mostly ignored recently - the closest I suppose is Rose holding her baby self in Father's Day.

The other rule is that a time-traveller can't change events they've already been part of, can't go back for another go, that's the sort of thing that supposedly causes disastrous causality rips. Since they make the point in Day of the Doctor that the earlier incarnations forget what happens, it implies that the events always happened that way, just that the Doctor didn't remember that until the last time round.

It can be argued that the occasions where time-travellers have made contact with themselves were outside the normal rules of the universe, either by location or by technological tools, or even that Time Lords just aren't subject to the effect.

There's some great slash fiction involving Doctors 10 and 11.

I don't know if its considered canon or not, but there doesn't seem to be any issues with them physically interacting

it's always the same man, so it wouldn't matter if it was the same incarnation or different one's.

It is mentally but he renews his cells and biology so dosen't that mean technically hes not the same so differnt versions meeting naturally, would not cause a paradox.

I think it would only be a problem if it was the same incarnation. If it's two different ones, then it's basically just two different bodies coming into contact with each other. No different to normal physical contact.

They've been interacting together since 1973!

But then, we do that too, so someone touching themselves from 10 years ago wouldn't conflict. Even less, if it's only skin touching. Even when one thing turns into another, they're surely temporally linked?

Yeah but on a smaller rate compared to regeneration when it happens all at once also they don't just change cells isn't it everything except blood type?

This is mind boggling lol but does his DNA change too? If so maybe thats why there was no paradox when the two met?

The two Brigadiers couldn't interact in Mawdryn Undead, but it seems regenerated Doctors can. IIRC Doctor 9 was being careful about meeting himself in Father's day.

I've always had a problem with the 'every single cell' line after 9 regenerates into 10 and any other suggestions that all cell's have changed when the doctor regenerates, because I always think that if every cell had changed that would include having an entirely new brain with no memories in it, which blatantly isn't the case.

I see it more as the same DNA repairs then rearranges itself into a new form (like how Jenny was created from the doctors DNA being rearranged into a new form), but perhaps the brain controls it (like a brain controls most bodily functions) so it in itself doesn't change.

Memories aren't stored in the cells, though. The human brain is constantly adding new brain cells as old ones die off, but that doesn't contribute to memory loss - the brain rewires itself to compensate. Assuming a Time Lord brain works the same way, it's just a matter of having a brain that can rewire itself fast enough - the implication is that post-regenerative confusion is this process in action, although we've seen some Time Lords recover immediately.

Interestingly, the Eleventh Doctor had failing cognitive abilities as an old man, but the information and memories must have still been in there in some form. Perhaps only damaged brain cells regenerate, and the 'every cell' thing is just a simplification.