Is the 40k just Fantasy in the future? They exists in parallel worlds or Fantasy is an enclosed space quadrant of universe in 40k?
Short answer : Games Workshop officially stated that the 2 universes are not linked in any way. (edit if you find a link to that statement, but i'm sure of it)
Long answer : the two worlds were previously kind of related but Games Workshop broke the link in the third edition of 40k
The context (and timeline) of this affirmation is important
This is a link I found with a quick search. (Maybe not canon, be careful with this info)
In earlier editions of both Warhammer Fantasy and Warhammer 40,000, there were many indications that the Warhammer World, the planet that is the primary setting for Warhammer Fantasy, was located in the same universe as Warhammer 40,000. In later editions of both games, Games Workshop moved to create more of a separation between the two fictional universes, although they have also never completely denied the earlier connections between the two settings. It is likely that the Warhammer World does exist in the same universe as Warhammer 40,000 as the Warhammer World is known to have been terraformed and reshaped by beings known as the Old Ones, the same alien entities as the Old Ones of the Warhammer 40,000 universe
IIRC, back when I played, I indeed heard that Warhammer Fantasy was a single planet in 40k's world.
Deeper search :
Fantasy came first, and when GW decided to make a sci'fi game, they threw warhammer's world in the future saying the two worlds might be related. So in the earlier editions, GW hinted that Warhammer Fantasy takes place in the Eye of Terror.
Later (40K 3rd edition), they broke the link they previously suggested. Officially denying it.
Now once in a while they make a quick easter-egg or whatever, but this is just teasing and fun.
Alright, before I give a detailed answer, here are some historical details about Games Workshop that are relevant to the issue :
Rick Priestley joined GW in 82 with a game he developed called Rogue Trader, which would in the future become Warhammer 40k (due to worrying about it being confused with Rogue Trooper, another game they published). From the ads of the era, the game had little in common with what would eventually come out in 87.
Rogue Trader was supposed to come out quite shortly, but eventually did not. I suspect that this was because back then GW had been purchased by Citadel Miniatures's Bryan Ansell, and Citadel mostly had a Fantasy range of miniatures, and so instead, Warhammer The Mass Combat Fantasy Role-Playing Game came out in 83 (the original rulebooks were quite full of direct references to Citadel miniatures). So no, it is not quite true that Warhammer Fantasy came first. More details of this whole process can be found in the White Dwarf 97.
Almost immediately, science fiction elements crept in the fluff. Five months after the game came out, the first Citadel Compendium came out with an article entitled "Warhammer and Science-Fiction!", which included ideas on how to mix Warhammer with science fiction elements. Among those were already Warhammer 40k weapons. The Second Citadel Journal included some details of the technological past of Warhammer, in the scenario "Rigg's Shrine", which mentionned the High Age where the Amazons benefited from the tutelage of the Old Slanns and such, and the scenario is full of technological leftovers from the era.
The second edition was the first to have a really fleshed out background, which already mentioned how the world was basically built by the Old Slanns, a race of space frogs, with more informations on the topic during the third edition.
In march 87, you can find the following announcement for Warhammer 40k in the White Dwarf :
Once the game came out, though, they did not do all that much with it. Here are a few of the most important interactions between the two universes :
- The WD 100 includes the scenario "The Floating Gardens of Bahb-Elonn", which mentions that the Pygmies are the descendants of a crashed spaceship before the Slanns arrived on this world.
- Realm of Chaos : The Lost and the Damned mentions that the Known World is a planet cut off from the rest of the Imperium due to important warp storms("The Warhammer World is bound by storms of magic so that it remains isolated form the other wolds of the human galaxy"). It also has some ambiguous mention of a scenario with a crashed spaceship, and another with "The Obsidian Crag", a mysterious place full of weird technology in the Chaos Wastes (obsidian is quite often associated to the Slanns). And of course one of the divine reward was to transport a Chaos band to and fro from the Known World and the galaxy.
- The WD 108 contains a scenario about a warp gate linking the Known World to some random planet, which has unleashed an Ambull in some cave in the Empire.
- The Star Boat, a novel by Stephen Baxter, has a Norse expedition to the Chaos Wastes to recover a Slann spaceship.
The Star Boat was supposed to have two sequels. One was actually written and approved, but this was during the era where Warhammer Books was collapsing on itself, and as a result was never published as a Warhammer novel. To not waste it too much, it was reworked somewhat and published under the title "Webcrash" (the original title was supposed to be "Wood and Iron"). The story details (as far as I can tell, I do not know exactly how much was changed) an imperial ship (with minimal crew) washing up on the Known World, one of them going rogue and trying to do villainous thing while the navigator and a norse lady (the wife of the hero of the previous story) team up to stop him.
The third sequel I don't know too much about except that it was apparently amazing, with the working title "Titan vs. T-Rex", about a giant robot ending up on the Known World and fighting one of the Lizardmen's dinosaurs or something, though I don't think it was ever actually written.
More details about all those books are here :
The two Warhammer RPG supplements released on the internet, Realm of Sorcery and Realm of Divine Magic, by Ken Rolston, also tried to expand somewhat on the links between the two universes, but were unfortunately never published either.
After the 3rd edition of Warhammer and the 1st edition of 40k, those references sort of disappeared. Perhaps they did not know too much what to do with it (they couldn't really have the Imperium invade or whatnot), or maybe it was the change of staff (Andy Chambers usually said he doesn't like to reveal too much the inner workings of the universe back when he was the head honcho). A few still remained, such as technological objects popping up (like in the Albion campaign), identical demons in both universes and the description of a Thousand Son in some chaos scholar's book.
As for the details of the interactions between the two, here's a rough sketch of what it looked like :
The position of the Warhammer world is never really given. The Slann empire is very poorly described in Warhammer 40k, the Slanns themselves only appearing a few times in it. Rogue Trader does mention that currently, it is in the galactic north, but that it used to cover the whole galaxy, and that the ex-worlds of the Slann empire sometimes have backward Slanns living on them (this is related to the portal's collapse as described in Realm of Chaos). So we can just really say that it is not in the north, probably. "Wood and Iron" describes it as being out of reach of the Astronomican. Best guess would be that it would be in the Ultima Segmentum, but that is on some very vague hints.
The history is roughly this one :
- 200.000 years before the present : The planet is in an ice age slowly drifting away from its star. From the description, it seems that for the most part, lifeforms are mostly reptilians (Lizardmen, cold ones, dragons, dragon ogres, various dinosaur-like critters).
- At an unknown date, the ancestors of the pygmies crash on the planet. Things do not end well for them.
- 10.000 years before the present (this is the old chronology, nowadays it would be 15.000), the Slanns arrive. They install the giant portals at the poles to connect the planet to the web of warp portals they use to travel, install a giant city at the north pole (described in some details in Star Boat), push the planet back close to the sun, rearrange the continents (that was GW's excuse to explain why it looked like Earth), install new life forms, murder some others (in particular the ancestors of the lizardmen, which they then modified to create servants, according to more recent fluff).
- 8.000 years before the present or so, they create the Amazons to help them out for some things, and that is why to this day they still have some nifty technology around.
- And then of course, 7.000 years before the present, the portals collapse. While the reason was never revealed in so many words at any time (it is described as maybe an accident, maybe the evil intent of some warp creature or maybe the Slanns themselves doing it), Realm of Chaos does hint at the fact that it may be related to the failure of the Slanns to be able to control the warp and the deities and odd creatures it generates, which has been one of the major theme of the various experiments they conducted through their history. An important part that is not mentioned ever again in RoC is that this collapsed happened on thousands of planets, not just the Warhammer World (this ties in to the Rogue Trader's localization of the planet)
After that you know the drill. Chaos invades, the North Pole is annihilated, the Slanns sort of give up. The WD 100 mentions that some tried to escape the Known World by fixing up the old Pygmy ship but they never succeeded.
I guess also of note is the fact that Warhammer Armies hinted that maybe the Zoats (the same type as in Warhammer 40k) were maybe not of this planet, but that's about it.
There's plenty of other tidbits involving Slann settlements and weird technological artefacts and I could probably go on about some problems and solutions to the link between the two universes (like when the humans and elves and dwarves arrived, or why the elven gods are alive while the eldar gods are dead, hint that's because in 3rd edition the two only shared Khaine and maybe the Harlequin god), but then things start to veer into being less relevant and too much personal interpretation, so I'll leave it at that.
I can give an aswer based on some collectible informations about the two worlds (now in 2013, since things have changed in the past): alternative universes.
1) We can be almost 100% sure WH40K is not WH in the future, just take the Dark Eldar Codex (5th edition) and it will tell you that the old Eldar race (before the schism) created Slaanesh with their actions, so at most we could say WH is a future regression from WH40K, but I'm quite sure some other book in the WH universe will say something else about this genesis, so they're just not time-related.
2) They could be one inside the other, more specific, WH could be a single planet in WH40K, but then, as stated in a comment, it would be hard to explain why this lucky (if a world in constant war can be lucky) planet never heard about Space Marines and so on, and justifying it with WH magic sounds a poor defence against the powers of Eldar, warp, Chaos, Emperor, and so on.
That's why the alternate universe theory becomes the most plausible, similar settings, similar races, like almost every bit of WH is a cameo of WH40K and almost every bit of WH40K is a cameo of WH.
The Book "Realm of Chaos - Slaves to Darkness" linked the Worlds in 1988. Therefor the WH world lies in between some Worpstorm in the 40k Universe. But later GW did everything to break that Link again.
But with the reincarnation of the Squats (drinking space Dwarfs) maybe they reconnect with the WH world.
At least in the "older" versions (including version 6 and 7 of Warhammer Fantasy) there were many clear links between the two universes, that the Warhammer World is a planet in Warhammer 40k. For example, look up the Albion campaign for Warhammer. When the campaign ended, each race were given extra magic weapons according to their number of victories. Several of these "magic" weapons are clearly from Warhammer 40k, including a flametrower, a powerfist and other stuff.
Also, it is clearly and often said in 40k lore that many feudal and even stone-age human societies exist on worlds untouched by most of the galactic wars. These were colonised in the dark age of technology (I think, at least before the great strife) and were simply not recovered in the great crusade before the Horus heresy.
I hate that GW chance the storylines so much that they often contradict themselves, for example, as a Wood Elves player I really really don't like that they changed Orion to being many different guys that died each year and were substituted for a new one, instead of being similar to Ariel and simply being reborn each spring. What story you "believe" is to some point a choice, yet some make more sense than others. And the simple fact is that so many of the warhammer mysteries make sense if you put the world in the 40k universe. The War in Heaven explains why the old ones left and the corruption of the immaterium explains the collapse of the polar gates that turned them into Chaos Gates. It should also be noted that many Chaos units, not only deamons, but also some Khorne warriors, are exactly identical in the two universes.
When I started GM'ing WFRP, back when it first came out, W40K was a bit of a joke. Space Orks? Really? But as time passed and the W40K universe got more fleshed out and developed, I started incorporating the setting in my campaign. It's actually pretty easy. I assumed that the Old World is a planet where a cataclysmic event has reverted society, while opening a warp gate on each pole. This probably happened at the fall of the Eldar, and the birth of Slaanesh. Incorporating this concept into a campaign, gave me a lot of possibilities. First of all, it represented a new challenge to my high-level players. Secondly, it gave me a chance to write more interesting scenarios.
Remember that the W40K setting is fluid, and has evolved beyond recognition. In the old versions, species like Orks were merry pirates and freebooters, trading with humans. A slightly comic relief, not the moronic berserkers they are today. The Eldar were far closer to the High Elves of the Old World than they are in recent versions of the game.
My answer is that as long as GW don't give you an answer, then feel free to assume that the Old World is just a forgotten planet on the fringes of the galactic Imperium, forgotten and left to it's own fate. It makes for a much more fun concept than separating the two settings.
Canonically speaking, there's more than just the Old Ones: -the existence and presence of the Slaan, yet the Old Ones have fled, which would suggest the Slaan have returned. This matches the ending of the War In Heaven, with the Old Ones either dead, in hiding, or on the run from the Necrons and the C'tan. The Slaan disappeared from the galaxy somewhere between before the War In Heaven and after the return to sleep of the Necrons. The existence of the Skaven as well is another hint. Back when Genestealers and Genestealer Cults were introduced to 40k, Skaven were the mutant byproducts and results of Genestealer Cults that ended up worshipping Chaos, and were actually a legitimate part of a force of Genestealers, which could also include Demons.... this was redacted. So, the WH Skaven are the only "Tyranid" presence in WH. The fact there are Chaos Gates at the poles suggests some massive construction took place to have them there in the first place, a capability far beyond that of any of the feudal nations fighting each other in WH. There are some other things.... the Elf and Eldar Pantheons are the exact same.... which would fit the prophecy of the rebirth of the Gods (which is also the only theory of the Eldar not necessitating the destruction of Slaanesh). Finally, it is not entirely clear how anyone got here besides the Slaan and the Elf/Eldar creation story.
According to the old fluff the big hint that the warhammer world is in the eye of terror is •Sigmar* as for games workshops official policy of denying this. One thing is Sigmars "birth" was signalled by a comet. This was his cryo vat tank entering the atmosphere. One thing GW will never coment on is the missing 40k primarchs. Omegon and alpharius were same size as average space marines. This is the deal clincher if Sigmar is one of the "lost" then the world is in the eye of terror and also time there dose not pass as it dose in real space so he arrived pre heresy in the warhammer world.