..period clothes	One would have to say: what a good beginning to the last act - given the character of the play.  Stoppard's sense of theatre  depends upon surprise (that is, of the audience's expectations).  That sense of surprise extends from the detailed level - that of individual lines and words (and how much of the comic, witty effect of Stoppard's dialogue depends upon the character not saying what was expected) and that of the overall shape of the narrative: things do not work out in an expected way (who sees Thomasina's attraction to Septimus coming?) But that surprise not only is a source  of the play's comic effect - it is also the foundation for its tragedy (Thomasina's death is not where her 'story' should be heading: surprise becomes shock in this scene.  The audience has by now got used to the alternation of time periods - despite the playing with that simple alternation at the break between the two acts ( when Stoppard deliberately delays the return to the 1800 scenes).  By having the 'modern' characters in the clothes of the other period, Stoppard no doubt momentarily confuses the audience.  But there is something else to say about the element of playing with the audience's expectations.  Surprise is no doubt a very theatrical quality ( and that is important: you need to bear in mind that this is first and foremost a play - especially as you know a whole mass of the essays written on it will be earnest 'discussions' of the 'ideas' - as though Arcadia was some illustrated physics text book - which it isn't.)  But that quality also connects with the thematic side of the play: it has much to do with the limits of knowledge and prediction (retrodiction, also).  A playwright by intention must do what Fate does by nature -constantly deliver what was not foreseen and looked for.  For an audience that brings pleasure; in 'real life' it is much more likely to bring misery, or at best, a sense of not being in control - despite all our pretensions to knowledge.  The clothes are piled in a large laundry hamper; those bearing worn by Chloe and Valentine are described as 'workday' and 'unkempt'.  The past is being pillaged for what it has left over - rather than recreated as it was, or even idealised.  (That our ideas about the past might be very skewed is comically and nicely shown in Chloe's outfit: she imagines herself to be dressed like Jane Austen - but appears to more resemble Bo-Peep.) Sidley Park seems to be littered with the leftovers of the past: the grouse books, documents, now these clothes appear around the place not as treasured objects, but as lumber.  Chloe is reading the newspapers of today, Valentine is on the computer - both show as if momentarily past and present have fused.  The effect is comic, confusing - but of course this scene will end with a kind of fusing of past and present - in the two couples dancing to the same music - which by then will be asked to be taken seriously, will even be moving.  The scene begins with an idea used comically: it will end with the same idea in quite a different mode. (Again, that strikes me as a feature of Stoppard's practice: looking at an 'idea' from as many different angles, with resulting different effects, as posible.)
..Even in Arcadia..	The subheadline -'Sex, Literature and Death at Sidley Park' could serve as a subheading for the play itself - except, of course, being a comedy, death has been averted ( the duel came to nothing). But then again - by the end of this scene there will be a death - but quite unforesen by the audience at this point - confident in their knowledge they know what type of play they are watching - quite different to the overheated 'dramas' of the newspapers and the overheated imagination of Bernard.
the first person to think of this..	Remember Thomasina's exactly the same line -in scene one.( and Valentine's reaction is exactly the same as Septimus's then; first an unthinking negative, then an assent as the nature of the idea becomes clear.) Several points about this deliberate repetition: *   this picks up again what might be considered the relation between the two different times: repetition with difference.  *  compare Thomasina and Chloe:they are now similar in age, with Thomasina in this scene almost 17.  Thomasina is intrigued by the nature of desire, but naive as to its real nature as yet, perhaps - certainly inexperienced; Chloe certainly is not inexperienced.  Think of Chloe's reactions to Bernard's lecture - where the different concerns of three characters are neatly characterised by their different keywords for where the lecture stopped at (scene 5).  Different times, different morals - and Chloe, no doubt, is meant to show something about our sexually freer times.  But Stoppard avoids making the distinction between Chloe and Thomasina one of the simple contrast of 'innocence' and experience - sex, desire, is as important a concern for Thomasina as it is for Chloe.  In this scene it will perhaps result, indirectly, in Thomasina's death. *   the different theories that this phrase introduces can stand as marking the kind of range that the play has in its ideas and elements.  Thomasina's insight: If you could stop every atom in its position and direction, and if your mind could comprehend all the actions thus suspended, then if you were really, really good at algebra  you could write the formula for all the future..versus Chloe's:The universe is deterministic all right, just like Newton said, I mean it's trying to be, but the only thing going wrong is people fancying people who aren't supposed to be in that part of the plan.  From a Physical insight to a different kind of physical concern; from a sense of (theoretical) control and prediction, to something 'chaotic'(in the normal sense) and unplannable; from something highly abstract, to something earthy and human.  *  in other plays (one thinks especially of the plays of Shakespeare - think of King Lear and words like 'nature' and 'patience' and 'nothing') repetitions of words/phrases/images are part of the play's 'character' - its distinctive themes and atmosphere.  In Arcadia repetition is less 'poetic', more thematic - the repetitions and echoes show the relation between the different times - which I have tried to sum up in the phrase 'the same but different'.
..it's all because of sex	Which of the two theories does the play itself have most 'sympathy' with - that is, which is written into the play itself, rather than just mentioned? That Thomasina has anticipated Laplace - by eight years (the passage that Thomasina is almost, anachronistically, quoting was published in 1820) is a sign of her genius.  She will also anticipate Carnot, Kelvin, Lotka and Volterra, Mandelbrot - that is, right up to what is almost contemporary concerns in Mathematics and Physics.  She, herself, will not accept Laplace's simple determinism - she just anticipates the expression of that position.  Such theories do not become active within the nature of the play itself (as opposed to the mere content of its dialogue) - apart from theories concerning the 'arrow of time' - the irrevocable loss of heat which gives time a direction, which cannot be reversed.  That theory becomes 'active', not because it anticipates Kelvin's Second Law of Thermodynamics - but because it stands as a metaphor for what will be the loss of Thomasina herself.  Then Physics will become written into the play as pathos. Arcadia, like an equally brilliant play by Michael Frayn -Copenhagen, about the 20th century physicists Niels Bohr and Werner Heisenberg, uses the scientific theories as metaphors for more concrete, 'ordinary' concerns: loss, memory, the uncertain future, the lost past, etc. Chloe's theory is certainly more applicable to literature - and might be truer of ordinary human life (you must decide about that for yourself). Think of almost any comedy - for a 'story' to exist something 'out of the ordinary' i.e. unexpected must happen - and incontinent desire is a great force for getting characters out of their routines or away from their ordered images of themselves: sex is a great reducer of pretensions - hence its role in comedy.  But it also has a leading role in more 'serious' literature: think of Romeo and Juliet, Tristan and Isolde, Anna Karenina, Madame Bovary, even of any Jane Austen novel (desire without the sex, maybe).  In Arcadia itself, whatever the characters talk about, their actions are likely to be lead by sexual desire. (Social ambition is also important for Bernard and Septimus, Valentine has ambition in his science, and Hannah seems quite above attraction to anyone on stage - it gives her a kind of disengagement - which is part of the play's interest in her, that sense of her being sure-footed in her judgments)  Even Thomasina, as soon as she comes of age, as she does in scene 7, juggles as a character between a concern for her science and her interest in what attraction is.
..the attraction that Newton left out	This seems to me as good as any phrase for summarising the play's concerns - but also for characterising the play's dealings with science: there's a gap between life and theory (scientific, but also high-minded humanistic) into which comedy - and tragedy - intrudes.
the apple in the garden	The simple stage property - the apple - has gone from Newton's provocation towards his theory of gravity, an example of the natural world to which Euclidian geometry can only approximately apply, a gift for a teacher, to this: Eve's apple, traditionally believed to have brought sexual desire into the world (and that, for at least a sizable current in Christian thinking, was the Fall) This use of the apple can stand as an example of Stoppard's economy:a limited number of objects and situations are endlessly re-viewed and given different meanings.  The setting is unusually fixed - just one set, for two different time frames; the 'meanings' enacted on that one set constantly change (even words shift their meanings - hence, Stoppard's interest in puns and misunderstandings).