I wonder how different Ars Magica would be if it was set in the modern era which would it be similar to "Mage: The Ascension" or would it be different? Has anyone tried this before?
Whitewolf's mage (especially the oWoD version) IS Ars Magica set today. That becomes clear when you look at the history of both games. In fact, there are or were issues who owns the right to use the name of some early edition magi and covenants (which I do not name on this forum because I am not sure what became of that rights question).
The covenant to which I believe you are refering, it is teh one that got it's own book back in the 3rd edition, right?
Which hasn't been refered to once in any 5th ed material, but has been refered to in a Whte Wolf-publshed book, crossing explicitly ovr between VtM and MtA, yes?
Mistridge, grimgroth and tremere, yep. Doissetep might be intellectual property of WW as well. No need to be so misterious about that. 
Xavi
Aaaah, thi'old Grimgroth ex Miscellanea and the Tremere that Old Ancient @#}*, and the Tower of Discord and excuse to Ascension war, an all for Grimgroth... how transforma one League of Rusticani, alchemists, learned Wizards, Verditius and Bonisagi on the Iron Fist of the World...
Well, it's been tried many, many times with Mage the Ascension being one of notable published efforts. Personally, I never liked the backstory and mythology of Mage with the paradigm and paradox concepts. If I were to run modern Ars Magica it would more closely resemble the world of Harry Dresden books.
Mage: The Ascension was, supposedly, designed to represent a modern-era version of Ars Magica, but for many players this will introduce a rather major problems: Reality in the World of Darkness works by consensus which allows for the Technocracy and thus a modern era recognizable to the player characters. In Ars Magica, however, Mythic Europe operates on Aristotlean physics which means that in its modern incarnation space travel and explosions would be very different due to the fact that vacuums don't exist. I'm not sure, but I don't think firearms or electricity could work in the setting either. Likewise, a world where nobility and clergy benefit from MR would make many modern institutions impossible while Galileo & Newton being raving lunatics would make our own world unrecognizable.
Despite the pessimistic comments above, I think a Modern Era Ars Magica could work, but feel it would more or less have to be designed as a stand-alone game...
I don't think that is a central or very interesting feature of ArM. You can drop that without really creating any problems.
I think if you were going to play ArM modern, the first thing that you need is the Hermetic magic system (Arts, rituals, spontaneous and formulaic spells, spell-guidelines, Penetration, Magic Resistance, etc). I don't see any problems with using that unchanged. You might need to make some decisions about how it interacts with machinery. But, that's already an issue in 1220; from the point of view of Hermetic magic a computer is really just a very complicated water-wheel (made of Terram and maybe Auram).
The second thing you need is the basic character system (Characteristics, Virtues + Flaws, and Abilities). There doesn't seem to be anything problematic there. You will need some additional Abilities and some existing ones are not likely relevant. And you'll some additional Virtues and Flaws to represent the modern world.
Third, melee combat, wounds, deprivation, etc seems to be fine. You'll need additional rules for firearms. But you can probably get away with them being long-range, high damage missile weapons (with one or more new associated Abilities).
Long term events (study, lab activities, etc) seem fine as is.
Next, the Order of Hermes. It's just a pan-European (or pan-Global) "secret" society, with several factions and with rules for members. Not a problem.
The basic idea of the realms and regios and vis seems easy to implement. They are inaccessible to, and unknown by, most mundanes.
The thing that you will need to think about is what impact Hermetic magic (and all the other supernatural stuff) has had on world history. If the answer is "effectively none", then you need to explain (to the satisfaction your players at least) why that is plausible. Of course, finding out why the Order of the Hermes has had little impact could be a good saga idea.
That's a somewhat depressing statement coming from an Ars Magica author. I agree the idea of medieval physics needs to be set aside for a "modern setting" Ars Magica game... because a modern fantasy game demands a modern setting which demands certain concepts like modern phsyics. However, I personally find the idea that Ars Magica is set in "medieval Europe the way the medieval Europeans viewed it" as being a very central and interesting feature of ArsM...
Of course, there's nothing wrong with that. And I do really like that --- for example, the idea that diseases are caused by demons / bad humors is a great idea and very interesting and flavourful. But that's not really "Aristotelian Physics" anyway.
The Aristotelian physics bits around motion etc are the most problematic. One problem is that is not really the way "medieval Europeans" viewed it, as historically real philosophers in 1220 argued that parts were wrong, and other people (i.e. not philosophers) didn't view the world like that at all. The other problem is that it just seems pointless; it just seems to create arguments among the players about what would happen if something that the players understand poorly was actually true. I'd rather (as a player and an author) spend time worrying about how Hermetic magic affects the natural world, than be worrying about what happens in the natural world.
One thing i´ll add is that it may be a good idea to use the idea of an additional Form Ether roughly like we´ve done, basically it´s used as a catchall for all kinds of unseen energy, electricity that isnt lightning, magnetism, gravity, IR, UV, EM etc etc... Wherever it overlaps another form directly our standard is to just have it 1 or rarely 2 magnitudes higher base levels, balancing out that it becomes a rather wide Form with needing a bit higher scores to use it well.
Quite. There´s also sometimes a big difference between philosophers and scientists even if both proclaim to be the same at the time. "Flat earth" is still used as an expression today, quite often, perhaps even more often than "round/spherical earth", does that mean we think the earth is flat today?
Also important is the fact that they´re not beliefs, they´re attempts at creating explanations, and sometimes even though the explanation could be utterly silly, it could still generate a workable solution.
A perfect example of that is an artillery table from slightly later than "gametime", it´s mathematics has absolutely zero to do with gravity, but it could still be used to calculate fairly accurate firing angles/loadings.
Someone had sat down and come up with a theory of ballistics based on observation, and while his calculations had no relation to reality, because the whole thing was based on observing cannonfire in reality, it still gave very good results.
"Real physics plus" for the win. :mrgreen:
Which opens the question how can you discuss how magic effects the world if you don't understand how the world works. 
... but actually I don't disagree entirely. I honestly think the whole Art of Imaginem illustrates your point very well... trying to make Imaginem magic work within the framework of species really did not help matters and, if anything, just made it far more confusing. That said, I think there is a lot of good, flavorful and inspirational material found in Arts & Academy that has help my thinking about the setting and about the "science of magic", especially the experimental philosophy chapter. I would love to see Hermetic magic incorporate those ideas more expressly.
All this, of course, is off topic. In a "Modern Ars Magica" game, we would have to assume a modern world and modern physics. One wonders, would any changes need to be made to Hermetic magic in light of modern science (would lightning still be Auram for instance) or would we just assume that "magic" and "science" don't follow the same rules?
So, setting off a fusion reaction from some seawater... Rego Aquam? At what base per megaton? 
I think the only viable solution is indeed some kind of hidden-world-of-magic scenario, and agree with Richard Love's analysis. Personally, I would make the super-natural simply exceedingly rare in the Real World (there'd be lots of magic in regios and the Realms, though), as the explanation for why history doesn't really mention wizards and such.
A simple-ish way to explain why the Order (and other magical stuff) hadn't changed the world much would be to declare that a magical war (like the Schism War, but bigger) attracted too much attention from the Divine.
God pushed almost all magic into regios, etc and banished magic wielders there too. Result: very hidden magic, carefully scouting the 'real world' and doing very little in order to avoid God's Wrath.
Now, in modern times, people have stopped believing in the Divine as much so it seems to be safer to venture out...hmm, can we find some volunteers (read: expendable PCs) to investigate?
Seawater with ReAq? Not a chance i would allow that. First you use a ReAq(Au) to split the molecules and divide up hydrogen and oxygen, THEN you can use a high level ReAu to force a reaction. And i would use a base level high enough to equal a lot of other "doomsday" spells.
That and there may be an issue of not enough magi around that are even interested in affecting the "outside world".
Magic can give health, longevity and wealth, why would they want to interfere too much with the world? Any megalomaniacs would probably have to be handled forcefully though.
I've been thinking about a modern Ars Magica game. One of the things I would do is cut down on the base time it takes to do lab stuff.
Maybe a month (or even a week) instead of a season?
The whole thing about a nuclear power plant is that it ends up moving water to have some turbines spinning. No more, no less. A plain ReTe effect (por ReAq if you want the unnecessary water is enough to have the same final effect producing electricity. A covenant posing as a nuclear power plant is a nice trick to pass unnoticed while having some nice stuff (like bound griffins) in your wannabe reactor et al.
Cheers,
Xavi
Richard and I have opften had rows on this board, so I'm thrilled to be able to agree with him completely about something. Aristotlean physics adds very little to the game, beyond standing there as a gatekeeper to say "Sure, other people call fling balls of flame, but don't dare try to make gunpowder." and says to potential new authors "Oh, no, you just don't get it".
Medieval Europeans, for the most part, had no cosmological view of why objects moved as they did one way or the other, and worked from observed principles. Aristotlean physics was a view held by a tiny educated elite, who argued about its permutations.
I'm not saying it shouldn't have been included, but I ma saying that for me Ars is a game about magicians doing interesting things in a medeival European setting, and the only time I hear about AP is when playtesters say to me "You know cool thing X you wanted? You can't have that.", so for me, its basic effect is to make ME more mechanical and less interesting.
Richard and I have opften had rows on this board, so I'm thrilled to be able to agree with him completely about something. Aristotlean physics adds very little to the game, beyond standing there as a gatekeeper to say "Sure, other people call fling balls of flame, but don't dare try to make gunpowder." and says to potential new authors "Oh, no, you just don't get it".
Medieval Europeans, for the most part, had no cosmological view of why objects moved as they did one way or the other, and worked from observed principles. Aristotlean physics was a view held by a tiny educated, European elite, who argued about its permutations. In 1220, a lot of Arabs knew that there was something wrong with the model, and by 1290 or so, when they got into rocketry, they knew there were fringe examples which showed that AP was a useful, but not actually true, model.
I'm not saying it shouldn't have been included, but I am saying that for me Ars is a game about magicians doing interesting things in a medeival European setting, and the only time I hear about AP is when playtesters say to me "You know cool thing X you wanted? You can't have that.", so for me, its basic effect is to make ME more mechanical and less interesting.
In my game, there's an implied +0 tecnhology modifer for labwork, and as time goes on and you can get glassware that used to be blown by mastercraftsmen first from a factory, then from a supermarket, you get a higher and higher bonus.