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Boundary between deterministic reality and probabilistic reality 
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#1
Oct1409, 04:20 AM

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Is there a firm boundary?
For example, if a scientist makes an observation which is probabilistically determined, his actions (for example recording data) are altered, although we would generally think of a scientist as a deterministic system. How do physicists deal with this conceptualy? 


#2
Oct1409, 10:59 AM

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It is not generally agreed that there is a deterministic reality in the first place. So can you be more specific in your question?



#3
Oct1509, 12:26 AM

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if I create a computer game that uses a random number generator (say a mersenne twister seeded with the game start time) to vary the action then is the game deterministic or probabilistic?



#4
Oct1509, 03:02 AM

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Boundary between deterministic reality and probabilistic reality
You can say about a theory whether it is deterministic or probabilistic, but you can't really say of nature whether it is one or the other, unless it is deterministic
Let me explain. A *theory* can be deterministic or probabilistic, whether the end results of the theory are probability distributions, or "fixed outcomes". Newton's mechanics is deterministic in its dynamics, but doesn't say anything about its initial conditions: if the initial conditions are fixed "precisely" (which is hard to do with real numbers...), then the dynamics is "precise". If the initial conditions are specified in a probabilistic way, then the outcomes are also probabilistic (classical statistical mechanics). Quantum theory as it is usually formulated is probabilistic of course: the results of it are probability distributions. Now, what is "deterministic" ? We could postulate that "deterministic" means: *if* we know the "state" of nature at a certain point in "time", *then* we can calculate, with certainty, the future "state" of nature. But that doesn't need to be the only form of determinism. *If* my theory is a "magic book" in which all successive states of nature as a function of time are *listed*, then that theory "the magic book" theory, would be also deterministic, although no "evolution" rules are given to go from one state at a given time to another state at a later time. In other words, the "state" of nature at a given time is in no way an indication of the "next" state, but nevertheless, with my magic book, I would know everything that happens, at all times, precisely. Now, given that it is always in principle *possible* to consider that magic book theory (call it "fatalism") as the "correct" theory of nature, it is impossible to say that nature is not deterministic in this sense. 


#5
Oct1509, 10:45 AM

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#6
Oct1509, 11:22 AM

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#7
Oct1509, 11:44 AM

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Isn't this idea called Eternalism or Btheory of time? 


#8
Oct1909, 03:34 AM

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A simple analogy would be a movie. In as much as you would take the movie "for real", anything can happen in the movie, and maybe there's no sensible "formula" at all that allows you, after having watched the first half hour or so, to predict what will happen in the last half hour, even after taking the movie data from the first half hour (which is ALL the "movie universe" contains in that time  there are no possible hidden variables in the "movie universe" apart from the data on the DVD). But nevertheless the movie is deterministic because the last half hour is also "already fixed" on the DVD, even though you cannot derive it from the first half hour. So in as much as you give some reality to the movie universe, and for the characters IN the movie, it seems that their universe seems pretty random in the middle of the movie, nevertheless there "exists" a DVD where everything of what will happen to their universe is already fixed. At no point, it has been said that the content of the DVD is in one way or another available to the characters of the movie. At no point it has been said that there is a way for the characters to FIND OUT what's the content of the DVD before they "live" it. But the DVD "exists". Again, this is just an analogy, to illustrate the difficulty of saying that the universe IS stochastic. Only about a theory, you can say whether it is deterministic or stochastic, and that's pretty simple to find out: does the theory crunch out probabilities or does it crunch out "for sure" states ? Our movie characters can only have stochastic theories about the rest of the movie, but nevertheless the movie is deterministic, because fixed on a DVD support. It's not to say that I think per se that we are characters in a cosmic movie where some deity is playing a cosmic DVD or so, but because this is not totally inconceivable, we can't say anything about the *real* nature of stochasticity of nature, simply because we seem not to be able to do any better than come up with stochastic theories of its workings. It might be that the workings of our universe are such that it is fundamentally impossible to do any better than that and that we will never have access to the "cosmic DVD", but there's even no way to know that. So the best we can say, is that for the moment, we only have a stochastic description of nature. Whether nature itself IS stochastic or not is a question to which we don't have any answer. 


#9
Oct1909, 03:50 AM

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I was always the minority (often of one) in this debate with my friends who played poker. 


#10
Oct1909, 07:36 AM

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On the other hand, maybe there are people who would define "deterministic" in a stricter sense, namely, the future is entirely fixed if you know the "state" of the universe at a certain instant. This is what one understands by it, intuitively: can I know/calculate/find out the future if I know the present "well enough" before the future arrives, if only I'm smart/fast/powerful enough. Classical physics is "deterministic" on this account, because there are deterministic evolution equations of the state of nature. However, what kills this idea IMO is the concept of hidden variables. If we understand by "hidden variables", a part of the "state" of nature which is, in principle, inaccessible to observation, then we have the problem of principle that our "magic book" could be part of those hidden variables. In that case, nature is deterministic, the laws of nature are very simple, namely "do what's on the current page in the book", and we can't access it. 


#11
Oct2009, 12:44 PM

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#12
Oct2009, 01:27 PM

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#13
Oct2109, 04:59 PM

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#14
Oct2109, 05:59 PM

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So it's more like they can't have accurate information about each other untill they're both going at the same speed. But as I have mentioned, I may be misunderstanding relativity, I'm not a physicist. EDIT: What I mean is, while there's no universal reference frame, that doesn't translate to the same thing as all time is happening at once. 


#15
Oct2209, 11:39 AM

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My problem is that I cannot see how time can be objective (i.e. the past being fixed, the future being potential/open, and now being the point between) if the experience of it is subjective. If I experience a specific point of time as "now" and you experience another specific point of time as "now", then who is right and who is wrong? And then again, if "my now" is t1 and "your now" is t2, and t1 is before t2, then somehow, I am experiencing "your past" (seen from your p.o.v.), and you are experiencing "my future" (seen from my p.o.v.). How can the future be open if you are "already" there? Maybe I'm just thinking in the wrong way... 


#16
Oct2209, 11:49 AM

P: 1,212

The experience of it is not subjective time is crucially not actually experienced differently, but the observations made will be different. There is no problem here, as causality is never violated, and you will never see someone's future before they see it. You'll just observe them moving slower and with contracted lengths, just as they will see you.
This is all objective and described by a purely deterministic theory. 


#17
Oct2209, 11:58 AM

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#18
Oct2209, 12:53 PM

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James Hartle says this about that: 


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