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KingofNoobs

Determinism

22 posts in this topic

Hello,

Are real life events run-time configurable, or are they deterministic? Is this determined by whether or not the real life events are stored in a fast accessible ram or if they are just stored to disk or lost?

I have often found that a ram constraint in one of my programs caused me to be more creative and make a much better program. Would this coincidence apply to real life events as well? The less memory one has the more effort one must put into using one's brain, thus one becomes smarter? Is that what smart is?

I promise this is really all video game related, because I found that when I started programming video games I had to (am having to would be the correct tense here) change my entire outlook on life about somethings such as what it means to 'exist.'

So, I am wondering if it is games that are causing me to see the true nature of things, or if I am changing my way of looking at things to better understand the nature of games. Because nothing outside the world frustum exists.

-Dave Ottley
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Thanks to Godel and others, it has been proven that we do not have the ability to determine if the Universe is deterministic. It is a property that we cannot measure from within.

So the debate is entirely philosophical.


For myself, I believe the Universe is not deterministic.

If the Universe is deterministic then you have no free will, so you might as well jump under a bus because that fate is predetermined. Edited by frob
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FRob,

I disagree with your edit. Just because "something" is determined to will have happened at some point in the future does not mean that that something is knowable by us. If your logic was true then it would be in everybody's best interest (or at least not in any negative interest to any individual) to jump under a bus, thus ending the human race. Certainly a deterministic world is better than none at all, and because we cannot prove it one way or the other, I would like to assume the minimum amount and believe that the Universe is deterministic, because that meshes with how we are able to interpret things sensually. I don't see a need for any non-determinism and that fact does not imply that I cannot (as a CPU) make decisions about my future and my effect on the future of the Universe, thus "altering" the fate of that Universe from some other state that would have emerged had my CPU been unavailable to make those calculations.

-Dave Ottley
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This is basically identical to something I said in that thread about consciousness but I think it bears repeating:

I think referring to determinism as if it is antithetical to free will is creating a false dichotomy and I think it leads to notion of "free will" that is ultimately more disappointing even than one in which our actions are fully predetermined.

It's intuitively "obvious" that if it's possible to predict exactly which actions I will take in which situations, then I don't really have "free will." That's fine, I guess, if we define "free" in such a way that doesn't allow for this kind of predictability, but what about "will"? Let's say we introduce non-determinism (if such a thing even exists) to give me back my "freedom." That means that, at least some subset of my actions are[i] completely unpredictable.[/i]

That is, nothing about the observable world has any bearing whatsoever on what I end up doing in such cases. This includes both the makeup of my brain itself as well as everything my brain knows. Is this "free"? Sure. But how can it be "will"? I claim that, at very least, my will has to have intentionality; that is, it has to be "about" something. It has to be about my environment, or it has to be about [i]me[/i]. My choices have to have at least [i]some [/i]basis in reality (as it is accessible to me) for them to be "choices" at all. And non-determinism completely strips me of this by making my choices [i]by definition unrelated[/i] to everything my brain knows, does, thinks, or feels.

After all, if [i]my [/i]"choices" are non-deterministic, how are they different from [i]your [/i]"choices"? My "will" is just as much your "will" or the "will" of the God of Randomness or whatever else you want to call it, because, by virtue of being devoid of any meaningful relationship to the visible world, my "choices" no longer have anything to do with [i]me[/i].

If this is what it takes for me to have "free will" then I'd rather go back to my old boring non-free will, because at least my choices will be [i]more[/i] related to my circumstances than flipping a coin, not less.
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[quote name='KingofNoobs' timestamp='1353361994' post='5002457']
I would like to assume the minimum amount and believe that the Universe is deterministic, because that meshes with how we are able to interpret things sensually. I don't see a need for any non-determinism and that fact does not imply that I cannot (as a CPU) make decisions about my future and my effect on the future of the Universe
[/quote]
That is the crux of the philosophical argument.


It is a very old debate. It is also a very common topic for college and university studies. I remember writing essays on it in my freshman English class, and again in a computer science class, and a third time in a debate class I took for fun.


The premise goes something like this:

If the Universe is fully deterministic, we can rewind it from the last moment of the Universe all the way back to the big bang. We can replay it infinitely many times and it will always progress exactly the same way, along the same paths, without any deviation. That is true by definition of deterministic.


Following that reasoning, a deterministic Universe is one where we have no free will. All the conclusions we reach and all the actions we take were determined by the state of the Universe at inception. We may believe that we have free will, but it is a logical error. This again is true by definition of determinism.

If we were able to alter the state of events, then we would not replay the Universe from beginning to end without any deviation. If that were true then the Universe would have some degree of non-determinism.


So do we have the ability to choose, and therefore introduce non-determinism, or do we not?

The answer generally reverts to the classic "Cogito ergo sum."
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[quote]
Certainly a deterministic world is better than none at all, and because we cannot prove it one way or the other, I would like to assume the minimum amount and believe that the Universe is deterministic, because that meshes with how we are able to interpret things sensually.
[/quote]
My understanding is that at the most fundamental levels yet probed, nature appears to be non-deterministic. The consensus is that the experimental evidence is overwhelming. It is merely an accident of scale (in both space and time) that our senses perceive determinism, as the statistical amount of non-determinism appears negligible from our perspective.

I find it interesting that you feel that determinism is the "minimum".

[quote]
I don't see a need for any non-determinism and that fact does not imply that I cannot (as a CPU) make decisions about my future and my effect on the future of the Universe, thus "altering" the fate of that Universe from some other state that would have emerged had my CPU been unavailable to make those calculations.
[/quote]
How could there be an alternative state? If the universe is perfectly deterministic, whether your "CPU" is either available or unavailable at a given moment of time, and the outcome of any computation is fixed with the initial conditions. Your affect on the future is a known quantity - if "affect" can even be used. You would be a consequence, not an antecedent.

It sounds like you haven't fully fleshed out the self consistency of these views.
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Here's a question: does non-determinism mean you have free will?

My answer: technically, no. It just means you can't predict the outcome, though you can still be totally free will-less. If things are non-deterministic, it's possible you have no control over the non-determinism/randomness, and thus, are just being acted upon by randomness rather than willfully acting yourself.

Of course, this doesn't mean you don't have free will in a non-deterministic universe. It's possible you do have free will. But I don't think we can say non-deterministic = free will, because, for example, you wouldn't randomly apply forces to a proton, and because it's moving in a non-deterministic way, conclude the proton is moving according to its own free will. It's not; it's being acted upon by random impulses. Hence, I rather think a better conclusion is non-deterministic = maybe free will, or maybe not.

Free will is a mind boggling notion, because it's hard to really define what "will" and "choice" is, regardless of whether you take a deterministic or non-deterministic stance.
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[quote name='frob' timestamp='1353358551' post='5002438']
If the Universe is deterministic then you have no free will, so you might as well jump under a bus because that fate is predetermined.
[/quote]That sounds like the same reason that people choose to believe creationism over evolution -- "I have a bad emotional reaction to those ideas, so they must be wrong".
It sounds like the only visible effects of your definitions of free-will vs non-free-will, is whether an omniscient obvserver can predict our actions, which, seeing there isn't one around here, means there's no impact at all. Maybe this particular way of thinking about will is a big red herring?

Why should we despair if some omniscient observer was able to predict our next move? Assuming there is no omniscient observer, then there's no real impact on our lives. Maybe if you know that your actions are going to repeat unchanged for all time, you'd think about being a nicer person to people around you -- some people who have had the "[i]life flash before your eyes[/i]" NDE have reported a huge increase in their empathy after realising that their choices can/will be replayed -- but of course, if that's the case then it's either going to happen or not, already [img]http://public.gamedev.net//public/style_emoticons/default/happy.png[/img]

It's not nearly as frightening an idea to other cultures. e.g. some religions make a strong distinction between your mind (the bit that makes choices) and the "inner-god", who has no control at all but is conscious and watching everything "you" do.
Depending on your background, the words "I" and "me" can mean very different things. It's typical though for many people to be very shallow in the way we define "[b]I[/b]".
If you don't believe that "you" are your body or your mind, but "you" are the watcher of your mind, then you already don't have any free will, determinism or not, and many people are very comfortable with that.

It's possible that the universe is deterministic, but it's also possible that there are infinitely many initial conditions, giving rise to infinitely many variations of deterministic universes. If "you" were only an observer that had no outwards interaction with the universe, then your presence in a universe wouldn't affect it, and let's say you could travel from deterministic-world to deterministic-world at will, this could give the observer the impression of having created change in a single mutable-universe. ...but that's outside of the realm of science so I guess it's all just about being emotionally comfortable at that point.
[quote name='Cornstalks' timestamp='1353375581' post='5002530']But I don't think we can say non-deterministic = free will[/quote]That's a really good point. We have randomly influenced will, for sure, but that's true in both a deterministic and non-deterministic universe. We are sufficiently complicated that it's near impossible to predict our choices (most of the time), making them seem like true choices. ...but what makes our choices different to the "choices" of a particle? Why should we believe that there's other mechanisms behind them?

Modern neuroscience is poking uncomfortable holes in our ideas of choice, with experiments showing that when asked to press a button at a random moment, immediately after making the decision to do so, it's possible to detect that the decision has been made up to 7 seconds before the participant is aware that they've made the decision. They believe they acted on the spur of the moment, but the data shows that their actions were pre-determined.
This doesn't mean the universe is or isn't deterministic, it just means that we're just big complex machinery like everything else.

If you really don't believe that your free will is actually controlled by a million random occurrences, watch Derren Brown's work -- he does a lot of "mind reading", some of which is for real ([i]e.g. careful observation of unconscious muscular cues[/i]), but most of which is just great showmanship combined with planting the "read" idea into the subjects mind minutes, hours or days in advance. These people believe they've chosen some particular thought using their free will, when actually, they've just been manipulated into thinking that particular thing. He's even manipulated people into "freely deciding of their own will", to commit bank robberies or confess to murders. Edited by Hodgman
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[quote name='Cornstalks' timestamp='1353375581' post='5002530']
Here's a question: does non-determinism mean you have free will?

My answer: technically, no. It just means you can't predict the outcome, though you can still be totally free will-less. If things are non-deterministic, it's possible you have no control over the non-determinism/randomness, and thus, are just being acted upon by randomness rather than willfully acting yourself.

Of course, this doesn't mean you don't have free will in a non-deterministic universe. It's possible you do have free will. But I don't think we can say non-deterministic = free will, because, for example, you wouldn't randomly apply forces to a proton, and because it's moving in a non-deterministic way, conclude the proton is moving according to its own free will. It's not; it's being acted upon by random impulses. Hence, I rather think a better conclusion is non-deterministic = maybe free will, or maybe not.

Free will is a mind boggling notion, because it's hard to really define what "will" and "choice" is, regardless of whether you take a deterministic or non-deterministic stance.
[/quote]

I think my post above addresses this: not only is it [i]possible[/i] that "you have no control over the non-determinism," it's [i]necessary[/i], except in cases of extremely counter-intuitive notions of "you." In any cases in which the final outcome of a "choice" is determined by some non-deterministic process, then there's simply no way to justify it as a choice made by [i]you [/i]that distinguishes "you" from "me" or "random-God" or that tree over there.
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Ah, armchair philosophers and the undying false dichotomy between determinism and non-determinism.

And of course the utter red herring of "Free Will".


It's kind of depressing how often "logic" and "computer science" are mentioned as the impetus for these discussions, and yet nobody seems to be actually educated as to the state of the art in those fields.

Goedel incompleteness has no bearing whatsoever on determinism, by the way. It only relates to formal systems, and the universe is quite clearly not required to be encompassed by any formal system.

Also, there is a third state known as [i]indeterminacy[/i] which is far more important to this topic than any of the rest.


The universe is not conclusively non-deterministic in any strictly meaningful sense because that mandates a deeper understanding of its operating principles than we actually have. It can, at best, be said to be [i]indeterminate[/i]. Also, the idea of a deterministic universe presupposes the existence of some formal axioms from which we could describe all operating phenomena of said universe; it is far from clear whether or not this is the case.

According to the best experimental models we have, the universe is [i]probabilistic[/i] in nature, which is far from saying it is [i]non-deterministic[/i]. We just don't have a complete model for explaining the observed distributions of probability.


Frankly, this is ground that has been trodden to death over the past few hundred years, and if any genuine insight is to be had from the whole affair, it's going to come from people who are deeply familiar with the existing body of thought on the subject. Everyone else is basically playing a game of catch-up with Romantic era philosophers who had a lot more time on their hands to think about this stuff. (If you can get all the way from Descartes to contemporary postmodernism by yourself without any additional insight, you're pretty damn smart.)
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[quote name='KingofNoobs' timestamp='1353358014' post='5002434']
I have often found that a ram constraint in one of my programs caused me to be more creative and make a much better program. Would this coincidence apply to real life events as well? The less memory one has the more effort one must put into using one's brain, thus one becomes smarter? Is that what smart is?
[/quote]
I would say that in Computing and in many other fields geniuses are found when he/she has achieved something from [i]nothing. [/i]I mean, he/she found out something when they had miminum resources at their disposal. So yeah, I think this is no coincidence.

On to the Determinism vs Non-Determinism:
There are evidences, in my vision, of both things occurring in our universe.

For example, when the sperm meets the egg meiosis may start. If it does, the outcome is totally unpredictable. The genes will be combined through cross-over and mutation at rates and conditions unmeasurable.
You could go deeper in that thought (which is heavy) and try to reason why each of our cells live. Why do they seek resources and multiply. We have been studying this and we understand how it works, but the motivation of nature in its biological processes is quite unpredictable.

An example of determinism would be the [i]laws[/i] of physics. Gravity, for instance, has many explanations for why it happens, but the outcome is known and single: things are attracted to a bigger body.

Now, just to heat up the discussion.
For those that believe that the universe has an inner formula (unknown to us) and that everything can be synthesised deterministically, do you believe that we will reach a deterministic-polynomial solution to the NP-Complete problems? Or is this an exception to the rule? [img]http://public.gamedev.net//public/style_emoticons/default/wink.png[/img]
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[quote name='ApochPiQ' timestamp='1353379945' post='5002548']
Frankly, this is ground that has been trodden to death over the past few hundred years, and if any genuine insight is to be had from the whole affair, it's going to come from people who are deeply familiar with the existing body of thought on the subject.
[/quote]
You are right that this has been discussed by far too long time and that we need an insight.
However I believe this insight will not come from people like us, in a forum, ensuring our already formed opinions, nor from people that study the matter. Because those questions, IMO, are not based on knowledge nor on linear progressions of thought.

We really need an insight from a genius (with little resource and humble thoughts) or from an exterior superior force (?alien? / ?Gods?).
I believe in many enlightened people that are born from time to time, since philosophers thousands years ago, to George Boole and Alan Turing. Edited by kuramayoko10
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[quote name='ApochPiQ' timestamp='1353379945' post='5002548']
Ah, armchair philosophers and the undying false dichotomy between determinism and non-determinism.
[/quote]

I'm going to stop you right there. Is that what you actually meant to say? Because I'm pretty sure most people wouldn't object to defining determinism of some outcome as something like "there exists some model that, given the appropriate inputs, will accurately predict the outcome" and non-determinism as "there does not exist such a model," in which case it's not a false dichotomy at all.
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[quote name='cowsarenotevil' timestamp='1353385028' post='5002573']
[quote name='ApochPiQ' timestamp='1353379945' post='5002548']
Ah, armchair philosophers and the undying false dichotomy between determinism and non-determinism.
[/quote]

I'm going to stop you right there. Is that what you actually meant to say? Because I'm pretty sure most people wouldn't object to defining determinism of some outcome as something like "there exists some model that, given the appropriate inputs, will accurately predict the outcome" and non-determinism as "there does not exist such a model," in which case it's not a false dichotomy at all.
[/quote]Well it's hard to see through the smug cloud [img]http://public.gamedev.net//public/style_emoticons/default/tongue.png[/img] but maybe it's a false dichotomy because a "dichotomy" when drawn as a Venn diagram is exactly two non-overlapping circles, but you can instead choose to step away from those two and say "[i]you know what, there's no way to tell either way, so I'm going to draw a new circle that overlaps both of yours and go do something useful[/i]".
Kind of like answering with [url="http://en.wikipedia.org/wiki/Mu_(negative)"]mu[/url] ([i]"unask the question"[/i]).

If we assume there's a multiverse, made up of all the universes from all the possible initial conditions, then it's possible to hypothesise that different laws of physics apply to different 'universes'. In that case, perhaps some of them could be deterministic, and others non-deterministic, which makes both positions are true.
[quote name='ApochPiQ' timestamp='1353379945' post='5002548']Everyone else is basically playing a game of catch-up with Romantic era philosophers who had a lot more time on their hands to think about this stuff.[/quote]To be fair, many of those philosophers also believed that humans were half material and half magic, that mice spontaneously came into existence from filth, knew nothing of probabilistic physics or the geometry of the universe, and fields like neuroscience ([i]which turns some of their philosophising into real, testable science[/i]) were centuries from being discovered.
Perhaps starting from scratch with a modern scientific view of reality would allow a new generation of philosophers to tread completely new ground, or at least skip past a few dead ends?
Half of Descartes "breakthroughs" were just describing fictional systems of integrating magic with biology.... which we should really laugh at today. Edited by Hodgman
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[quote name='Hodgman' timestamp='1353385566' post='5002575']
Well it's hard to see through the smug cloud [img]http://public.gamedev.net//public/style_emoticons/default/tongue.png[/img] but maybe it's a false dichotomy because a "dichotomy" when drawn as a Venn diagram is exactly two non-overlapping circles, but you can instead choose to step away from those two and say "[i]you know what, there's no way to tell either way, so I'm going to draw a new circle that overlaps both of yours and go do something useful[/i]".
Kind of like answering with [url="http://en.wikipedia.org/wiki/Mu_(negative)"]mu[/url] ([i]"unask the question"[/i]).
[/quote]

Yeah, that does make sense. I still think it's worth differentiating a false dichotomy from a useless one, though.
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I think one can ask the following questions:

1 - Can an external observer determine the state of the universe at any given time t given the initial state of the universe(at t=0) and the laws of physics?
Let's assume that the external observer cannot 'affect' the universe, he can only observe and conclude.
It all depends on, I think, the laws of physics. If we assume all the laws of classical mechanics are true, then the answer will be yes because none of the laws in classical mechanics have a random element. With the advent of quantum mechanics I am not sure if we can come to the same conclusion, because AFAIK quantum mechanics has random elements in it.
One way of looking at it is:
A function, say f(x) takes the current state of the universe x and returns the state of the universe at the next 'moment of time'
If the function is one to one then the external observer can determine the future state of the universe.
If the function is one to many then the external observer will not be able to determine the future state of the universe.
An example of a one to one function is f(x) = x+1, here every value of x will have only one corresponding value of f(x)(0->1 , 1->2 etc)
A one to many function would be something like f(x) = sqrt(x), here if x is say 9, f(x) can either be -3 or +3 therefore there is no way of determining what is the exact value of f(x) and therefore the external observer won't be able to determine the future state of the universe given a function like f(x).

2 - Can an internal observer determine the state of the universe at any given time t given the initial state of the universe and the laws of physics?
By internal observer I mean people like you, me or any random scientist. Can we predict the future given enough knowledge and computational ability?
If the laws of physics have random elements then ofcourse we can't using the same explanation as above.
But if the laws of physics are non-random?
What happens if I predict my own future then try to falsify it on purpose.
For example, I, knowing myself and all the laws of physics, predict that tomorrow morning I will pick up my cup of tea with my right hand. But tomorrow morning I will pick it up with my left hand instead, just to prove myself wrong. Ofcourse when I first predicted this, I would take that into consideration and conclude that because my future self would like to prove my past self wrong, so he will pick it up with the left hand instead. But then my future self, knowing I had predicted that he would pick it up with the left hand, would now pick it up with his right. And the loop goes on and on...
Imagine the following scenario:
There is a Robot that is programmed to predict his the future.
The robot knows the initial state of the universe and all the laws of physics.
What is the robot is going to do is simulate an 'alternate reality' using the initial state and the laws of physics.
In his alternate reality, he will have the simulate himself too(because he is a part of the universe).
His simulation of himself will also try to simulate the alternate reality.
His simulation of himself then will have a simulation of itself(which is the simulation of the simulation of the robot).
And the simulation of the simulation of the robot will again have a simulation of himself.
So using this sort of brute force algorithm of simulating the future, the robot will run into an infinite loop and he will not be able to predict the future.

This was only an example of how an algorithm for future-prediction can be made and does not prove that an internal observer cannot predict the future with a non-random ruleset for their might be another algorithm that can accurately predict the future. But until that algorithm is found, we cannot say that an internal observer can determine the future state of the universe.

All my thoughts on the subject in a nutshell.
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[quote name='MathAddict' timestamp='1353443140' post='5002745']
AFAIK quantum mechanics has random elements in it[/quote]
Too far, too fast.

How do we know that quantum events are random? Because we can observe no discernible patterns.

Is that sufficient to prove randomness? No, because we can't see the entire sequence (and even if we could, we might not be able to figure out the derivation - imagine looking at a random slice of 100 digits of pi...)
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[quote name='swiftcoder' timestamp='1353445568' post='5002757']
[quote name='MathAddict' timestamp='1353443140' post='5002745']
AFAIK quantum mechanics has random elements in it[/quote]
Too far, too fast.

How do we know that quantum events are random? Because we can observe no discernible patterns.

Is that sufficient to prove randomness? No, because we can't see the entire sequence (and even if we could, we might not be able to figure out the derivation - imagine looking at a random slice of 100 digits of pi...)
[/quote]
I refrained from using quantum mechanics because I don't know much about the subject.
What I meant to say was - if physics depends on randomness, determinism is not possible. The quantum mechanics thing was just a real life example.


EDIT:
[media]http://www.youtube.com/watch?v=dmX1W5umC1c[/media]
According to that, there is a way to check whether 'this sort of classical underlying explanation of quantum mechanics can exist even in principle' and it turns out there isn't. Edited by MathAddict
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Disclaimer: I haven't actually watched the video with the Einstein doll.

Quantum mechanics doesn't say that randomness exists; it still allows for hidden variables but puts constraints on how they can manifest physically. Bell's theorem, for instance, shows that quantum mechanics cannot be described in terms of local hidden variables, but this is a much stricter definition and says nothing about the kind of hidden variables we imagine in, say, a computer simulation (e.g. a global pseudo-random generator).

There are also constraints about the kind of predictions [i]we[/i] can make, related for instance to the fact that we can't make a simulation of the universe that a) fits inside the universe and b) contains a perfect simulation of itself. This too is sad but says nothing about whether the universe is actually deterministic.
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Its interesting that a couple of hours before finding this topic I was reading news about NASA big announcement from Mars mission that cant be announced yet, and from there, followed link about what is going on when we die.
The[url="http://www.robertlanza.com/what-happens-when-you-die-evidence-suggests-time-simply-reboots/"] link[/url] went to the site of Robert Lanza who has theory called Biocentrism which states among other things that conciosness use tools like time and space to decsribe the world around us.
He repeatedly says about experiment in quantum physic where [b]"measurements an observer makes can influence events that have already happened in the past[/b][b]. One experiment (Science 315, 966, 2007) confirmed that flipping a switch could retroactively change a result that had happened before the switch was flipped. Regardless of the choice you, the observer, make, it'll be you who will experience the outcomes ?- the universes ?- that will result.[/b][b]"[/b]

So the world is as we see it or "the world is in the eye of beholder". [img]http://public.gamedev.net//public/style_emoticons/default/wink.png[/img]
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[quote name='Zed McJack' timestamp='1353479581' post='5002854'][b]"measurements an observer makes can influence events that have already happened in the past[/b][b]. One experiment (Science 315, 966, 2007) confirmed that flipping a switch could retroactively change a result that had happened before the switch was flipped. Regardless of the choice you, the observer, make, it'll be you who will experience the outcomes ?- the universes ?- that will result.[/b][b]"[/b]

[/quote]

how exactly was that...measured?
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It's related to the double-slit experiment.

The basic version of the experiment shows that a single photon can go through both slits, and then "both" photons interfere with each other on the other side.

This was then extended to actually measure which slot it went through -- as soon as you do this, the photon stops going through both. It either goes through one or the other, and the self-interference disappears.

This was again extended to measure which slot it went through, but then erase that information [i]after[/i] the photon had gone through the slots. This discarding of the intermediate measurement causes the photon to retroactively go through both slots again, like in the first experiment, even though it was observed going through a single slot like in the 2nd experiment.
Further, you can rig the intermediate slot-detector to either discard the information or keep it, [i]after[/i] the photon has gone through the slots, and that later decision will change whether it goes through one or the other with 50/50 probability (like in the 2nd), or whether it goes through both and undergoes self-interference on the other side (like in the 1st).

Basically, the path taken by the photon is lazily evaluated. It only actually "decides" which path to take when the universe needs to "know". Until then, it's in a superposition of all possible paths ([i]including going through one, then the other, then orbiting Jupiter, then going through one again, then hitting the screen[/i]).

AFAIK, there are ways to explain this phenomenon that don't violate causality -- which means that the present hasn't actually changed the past -- but I'm no physicist.

[edit]Here's the specific experiment referenced above: [url="http://news.sciencemag.org/sciencenow/2007/02/16-04.html"]http://news.sciencem...7/02/16-04.html[/url] Edited by Hodgman
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