how would you do the following?

a) create/destroy/alter an idea.
b)or more generally affect probability e.g dice.
c)warp space(e.g baba yagas fabled hut which is bigger on the inside and stuff like that)

With a Hermetic magus?

a) CrMe, PeMe, MuMe (but this third can't be permanent) if in someone's mind. More generally, the same using a ritual with a huge Target so that it encompasses the world.
b) This can be done very directly with some hedge magic. Not sure with Hermetic magic.
c) Faerie Magic: Glamour Mystery - MuIm to make the inside of the building appear bigger than it really is, now it is bigger.

yeah, "luck" is difficult to define in Hermetic magic. You can implement the results of luck (ie, ReAn so that when you "roll the bones" you get the result you want), but that's essentially direct telekinesis.

However, that starts to get into an argument regarding whether or not Luck actually exists as a thing to affect, or else if it's just a series of events you don't perceive. Because if it's just effects on your life you don't notice, then you can affect those things with standard magic - probably in Rego, as that's "Normal things happening, only unnaturally".

However, to have a general "luck" field would be horribly complex, and would probably require the equivalent of Elementalism (no increases in difficulty for additional Forms) to have a hope of casting at a reasonable level.

for the purpose of this question let as define "affecting luck" as blasting something with entropy or removing said entropy. in other words increasing or decreasing order and disorder

Regarding c), in the Mercere section of True Lineage, it is mentionned that Mercere shared the secret of Portal that lead to the known Hermetic Portal (Mercere portal remains a secret of the House). It is also that Mercere knew more, but did not have time / did not consider interesting to share the knowledge to do "Bags of Holding" "... but he did not pass on the secret that made designing these devices possible".
Then, a futher down "... would love to make portable items using this magical concept...".

One possible way to go around that was discuss on this forum was to use Muto Imaginem to turn an item into its flat species in a book, to be carried and turn back once destination is reached.

However, there is so far no lead on how to do portable bottomless pouch. Compressing or cheating on dimension does not seems to fit any Form.

Cringe. I think I know where you're headed, but I don't think the whole second half is what you intended. Entropy is rather far from luck, and entropy isn't disorder, though it is frequently mis-taught that way. Chaos is much closer to what you seem to be thinking than is entropy. You're talking about making more predictable what is not very predictable, which would be making a chaotic system less chaotic.

Still, I know of no way to do this within Hermetic magic. The closest I've thought of would be Faerie Magic's Story Magic (HoH:MC p.98).

Glamour has been used in canon to cheat size in just such a way.

Oh, so you're playing Mage the Ascencion?

no, to the medieval mind, Entropy is not really a concept, and as such it has no place in Ars Magica. Similarly, 'blasts' of abstract force isn't really a thing in Ars Magica, nor something to be desired.

Absolutely. This concept was defined mid 1800, and was purely linked to a physical phenomenon. No correlation to luck.

Luck on the other hand is "very real", in the sense that Fortuna or Fors Fortuna as divinity of luck and fate exists in the Roman mythology.
Now, the interesting point is that Fortuna is not pure luck, it is also fate and destiny. So it seems that if a mage could integrate Fortunam from the Learned Magician, luck could have its own Form. The way Fortunam is described in Hedge Magic looks likes it is affecting mostly random event, but can be used to grant a bonus for specific activities.

"Chaos" exists in the Greek mythology, but is describded as a shapeless mass, where the first divinity came from.
It is not matching the modern concept of entropy, except for its shapeless status. Chaos is more akin to a primordial ooze where everything originated from. Closer to the vision of Chaos as Moorcock described it in his various cycles, without its destructive urge.
However, if a mage would look at improving Creo (creation of full-fledge human life with soul and everything ?), contacting a Daimon of Chaos could be an interesting lead - and of course would open the whole discussion about Creation of human and soul being the only perview of the Divine - but it would make an interesting - if not theological - Saga.

Canonically, the Glamour Mystery plus experimentation (that might, say, have persuaded a faerie to lend a hand, unbeknownst to the Hermetic magus) has been sufficient.
We do not know if the Glamour Mystery on its own is also sufficient.
We do not know if anything beyond vanilla Hermetic magic is necessary: making a hut larger inside than it is outside might well be covered by the Muto guidelines that change the target into something "highly unnatural".

First, this is experimentation, not any sort of original research. Scipio's scores were not high enough to make such a Lesser Enchanted Device without the bonus from experimentation. The rules for such experimentation are given in the core book. Then note that when effects have had special modifiers due to experimentation, it has been explicitly written with them. See Image of the Lady and Servant of Fire in that same section for examples, though there are other examples in the book as well. This particular effect has no such note at all. Other effects of experimentation have also been listed in the character notes. So we know the only effect on this item's creation from experimentation was a bonus to the lab total. Thus, if you have high enough scores, this can be created without experimentation.

right,the fortunam is more or less waht i had in mind. however curiously the book doesn't mention anything about integrating it. so i assumed, the implication was that the 10 forms can do it themselves?

concerning creatin/destroying ideas. i did not mean it in the strictly mental(i.e brain) sense. for instance a particular medieval silk weaving guild is an "idea" the mandate of heaven in chinese philosophy is an idea,which if attacked,can be withdrawn and can even have a domino effect of physical consequences.or increasing the idea of progressivism might help the democratic party win the next election. and so on

I'm fine with Hermetic Magic not being perfect, not being able to do everything.

Sure. But the results of experimentation can be unpredictable and unreplicable.

This is a fallacy.
We know that in some cases "special" effects have been explicitly attributed to experimentation.
We do not know if that's true in every case.

Thus, one cannot assume that the Glamour Mystery can replicate the feat on its own.
At the same time, one cannot assume that "vanilla" Hermetic magic cannot replicate the feat on its own.
In fact, I suspect that leaving this aspect undefined is a conscious decision of the author/editor, so as to leave all possibilities open - both to troupes who want "fully Hermetic" babayaga huts (which are, after all a staple of myth), and to troupes who want "space warping" out of their games.

I know what you mean from the pure logical standpoint, divorcing it from the context. But let's include the context, specifically what is written about these descriptions.

(underline mine)

You see, I'm not making an assumption based on a few cases. I'm noting that the descriptions are "full." If they are missing relevant R/T/D/base/similar information, they are not full. Thus the descriptions of their creation would not be full descriptions if they lacked notes about abnormalities due to experimentation; changes due to experimentation are considered part of a "full description," as evidenced by specific notes when there are abnormalities. So, according to the explanation in the very book we're referencing, we do know it's true in every case.

for the record, since misconceptions abound, entropy is defined as the heat energy divided by the temperature. So in actuality ignem magic could handle this quite nicely, though it wouldn't be described in those terms.

That would be the 'old', 'classical', or 'macroscopic' definition. But it'll do for any non-microscopic discussion.
However you define it though, the concept does not pre-date 1850, and so is irrelevant in mythic europe.

I hadn't wanted to go into too many technical things about entropy and chaos, but as things are headed there...

Eh, close. First, there isn't just one definition. What you've stated is roughly the macroscopic definition, though you've actually defined the change (usually necessarily infinitesimal as you've written it) in entropy, not the entropy itself. (And I might note for those who don't know, heat is a transfer of energy, not energy something possesses.) There is also a microscopic definition based on the number of microscopic configurations for a macroscopic state.

The best way I've heard to describe it outside of a formal definition I got from a chemistry professor ranting about how problematic it is that so many teachers teach that entropy is disorder. He was hoping teachers would switch to explaining that the second law of thermodynamics describes the tendency for energy to spread out, not the tendency for things to become more disordered.

yes, but the microstates definition is not established under thermodynamic laws (which by definition del with heat exchange) and are in effect nothing more than mental masturbation to try and fit misconceptions regarding entropy into a formal definition. The law of entropy is that entropy- as defined by heat energy divided by temperature- will increase. Since in a closed system energy is neither created nor destroyed this means that temperature will generally go down in order for entropy to go up, thus the concept of the heat death of the universe.
I can call a crowd of comedians standing on the top of a ship as a deck of cards, but that doesn't mean I can use them to play poker.

Since misconceptions are abounding, let me try to fix some, including addressing how some modern science can be used to an extent with ArM5.

The second law of thermodynamics does NOT state that the entropy in a closed system will increase. It states the entropy will not decrease. ∆S≥0 is not the same as ∆S>0.

It's dS=dQ/T or ∆S=integral of dQ/T, not S=Q/T nor S=∆Q/T nor S=E/T. These are VERY different mathematically.

No, just no. That is such a generalization that it's invalid way too often to be an acceptable generalization. Look at radioactive decay, for example. For a more readily doable case, pour lots of 0°C water on some really cold (well below 0°C) ice (water ice) in an insulated container. Afterward you will have 0°C ice water.

The problem here is the misuse of the formula as I noted above. If you only allow heat to flow between the two bodies, you get the system's incremental change in entropy as
∆S = dQ/Tc + (-dQ)/Th = dQ(Th-Tc)/(ThTc),
where Th is the hotter temperature and Tc is colder temperature. So this tells us we have heat flow from hot to cold.

So, sure, entropy is part of thermodynamics, and you can link that to Ignem. However, handling probability with Ignem based on this reasoning requires us to misunderstand what entropy is. Entropy is not randomness. If you want to use modern understanding to go along with luck, you really want to talk about chaos in many situations, not entropy.

Now, with ArM5 based on what we got from the Greeks, some things can be used better. The Greeks theorized atoms. Insert molecules, and things are closer. They also theorized the shape of the atom was related to its macroscopic behavior, and we see this in many ways. The microscope structure of molecules and atoms give rise to macroscopic behaviors. Also, Aristotle's "nature place" could be related to Hamiltonian dynamics, and Hamiltonian dynamics is used in classical mechanics as well as the Hamiltonian operator being fundamentally important in quantum mechanics. "Natural place" could also be connected to things like Pauli exclusion and neutron degeneracy pressure. So there are a lot of ways we could mesh things together for those who desire it. That's probably of more value if someone wants to work ArM5 into a modern setting, though.

The more on-topic bit would be chaos. Rego to reduce the chaos in a system would make it more predictable, essentially controlling luck. That comes back to non-general control of luck, but rather using Rego to control outcomes instead of relying on luck. I suppose you could just nudge with Rego instead of using full control, though.

Actually in my engineering class on thermodynamics it was described that entropy is defined as the heat energy over temperature but since it is impossible to know the actual heat energy of any given system the change in heat energy is used instead since they are, in practical terms, the same.
And yes there are exceptions, but for the most part those can be overcome by enlarging what you consider to be the system- radioactive decay being the exception since that is a non-reversible reaction. My point however was not the accuracy of the conclusion, but rather that the idea of temperature being a net decrease led to the idea of the heat death of the universe, which then began to spin into alternate non-scientific ideas as to what entropy meant, since there can be change in temperature within a single body, if that body is a gas. (PV=nRT), which is how we got work from heat in the first place which necessitated the development/understanding of these laws.
(note: by development I mean the human process of the discovery and codification of how things work, not implying that humans create reality or any such gobbledygook)