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Is the universe fundamentally deterministic?

103 points by shubhamjain 13y ago

joosters 13y ago
A good related discussion on determinism and free will: http://www.bbc.co.uk/programmes/b00z5y9z http://www.bbc.co.uk/programmes/b00z5y9z (BBC Radio 4 - In Our Time) - no idea if this is blocked outside the UK?
- agravier 13y ago
  Thanks. Not blocked (Malta).
- siddboots 13y ago
  Definitely not blocked here in Australia. In Our Time's archive is one of the Great Wonders of The Internet, as far as I'm concerned. http://www.bbc.co.uk/radio4/features/in-our-time/archive http://www.bbc.co.uk/radio4/features/in-our-time/archive
jblow 13y ago
In the answers I do not see any mention of the Second Time Around Problem, which conclusively answers the question with "there is no way to know". The idea is: suppose there is a universe that has elements that are fundamentally random. It is nondeterministic. Well, let that universe run for its lifetime, and record everything that happens. Then make a deterministic universe that just plays back the recording (this is the "second time around"). From the viewpoint of someone living inside the universe, there is no way to tell whether it is the first time around or the second time around. ... But the other thing to point out is that this question presumes an old idea about the passage of time, which is that things happen in a sequence A, B, C, D, ... and that if you are at C then D "has not happened yet". But if you look at relativity, this appears to be a naïve viewpoint. In relativity, the time at a faraway point in space that you would consider "simultaneous" with your own clock depends on the relative speed between you and that point. As you speed up and slow down, you can make a faraway point "go forward or backward in time" with regard to which moment there you would consider "now". The crazy thing is that for angular movements the relative speed is amplified by distance, so when you are moving around at everyday speeds the "now" on planets across the galaxy is going back and forth by thousands or millions of years. (This is hard to observe because you are viewing tiny amounts of light from very very far away that have been traveling for a very long time, and the light that you are about to see was very close to you when you did the angular movement so it will not be much affected, etc, but hey, the math says what it says, you either believe what physics tells you or you don't.) So when you make a distant "now" go forward, then backward, then forward again, do you expect the two forwards to be the same, or not?
- siddboots 13y ago
  That argument is, roughly, Humean scepticism. Science is a game we play where we do experiments, build theories, do more experiments, and so on. Any scientific answer to how the universe works has to start with the assumption that the nature of universe can be understood in the first place (otherwise we'll never get anywhere.) Edit: This was in response to your comment prior to edit. Up to "..."
  - eli_gottlieb 13y ago
    Well yes, which is why we can't really ever know. It's somewhere between difficult and impossible to prove the axiom of your entire epistemological process.
- paraschopra 13y ago
  This is weird. If the observer in the universe is able to make accurate predictions consistently and repeatedly, doesn't that give confidence to his beliefs that the universe is deterministic. What you might be referring to is another way of saying that "any theory can be proven wrong if and when we observe a data point negating it". So, of course, there is no way of knowing if the universe is __truly__ deterministic or not, but then again we can say the same thing about sun rising from the east (unlikely as it may be, it might well just not rise tomorrow at all and then we will have to revise our theories).
  - jblow 13y ago
    Did you read the OP? The whole question is about whether the apparent randomness in quantum mechanics means the universe is fundamentally nondeterministic. We cannot make accurate predictions of measurements of quantum state apart from saying things like "A will happen with some probability, otherwise B will happen". It is that "with some probability" that is the question. (Yes, at the level of the macroscopic world we can make consistently accurate predictions, but physicists would say this is because we are operating at a scale where the statistical nature of quantum mechanics averages out, etc. Unless you are Carver Mead or one of the other wave guide kinds of guys who believe there actually is no randomness in QM and it is deterministic all the way down.)
deletes 13y ago
I think most people think it isn't because you can't measure something without influencing it and thus changing the state. Not being able to record the state doesn't mean it isn't deterministic.
eterm 13y ago
This reminds me of SMAC, on discovering (iirc) Quantum Computing: "Not only does God play dice, the dice are loaded"
shubhamjain 13y ago
I have been reading Godel, Escher, Bach by Douglas Hofstadter. In the section, formal system and reality, the author talks about universe as a very big formal system with all sets of rules governing it. Though, author doesn't answer the determinism question itself but I am aligned to towards it being deterministic because I think there rules at every level, quantum or physical. Everything must occur because of a factor or the other.
- nhaehnle 13y ago
  The interesting point here is that even though the universe may be entirely deterministic --and quantum theory says that it is, if you consider the universe as a single gigantic "wave function" -- that may still be irrelevant for our lives because we cannot actually observe this gigantic wave function. In fact, we're a part of this function, in an unusual sense for which there is no everyday life equivalent.
allard 13y ago
friend's book — http://www.amazon.com/Free-Will-Joseph-Keim-Campbell/dp/0745646670/ http://www.amazon.com/Free-Will-Joseph-Keim-Campbell/dp/0745...
codeulike 13y ago
If its deterministic, but cannot be simplified and modelled by anything smaller than itself (imagine a simulator that modelled every atom - it would have to have more atoms than it was modelling) ... then does it matter? That is, it may be deterministic but fundamentally unpredictable in practice. See Dennets 'freedom evolves' for a long discussion of this.
asgard1024 13y ago
I would say that the determinism or non-determinism doesn't really matter. It seems to me that determinism is a feature of the observer too. For example, roll of dice is non-deterministic for a human being, even in perfectly deterministic universe. It's just too complicated to simulate, so it appears random, and we can pretty much declare it to be non-deterministic. And pseudo-random generators, purely idealized systems, can generate non-determinism from determinism. You can also have systems that do the opposite - that are itself based on non-determinism, but are in the effect deterministic. Classical computers use non-deterministic movement of electrons (or better say, they don't rely on their determinism), but can still produce repeatable (deterministic) results. So you can move away from determinism or non-determinism on lower level (like the level of elementary particles) by various methods of aggregation. Considering there is still about 20 orders of magnitude between the "size" of elementary particles and Planck length, I would dare say that reality may still switch between determinism and non-determinism several times beyond the what we know. Also, people may find this lecture by Dan Dennett interesting (which inspired the above argument): http://www.youtube.com/watch?v=uxup7sxIUmg http://www.youtube.com/watch?v=uxup7sxIUmg
- wyager 13y ago
  >I would say that the determinism or non-determinism doesn't really matter. It's kind of (very) important for information theory and computability theory. If the universe is deterministic, a sufficiently small physical system can be exactly simulated by a sufficiently powerful Turing machine.
  - asgard1024 13y ago
    I am not sure I understand how is that important or helpful. In simulations of physical systems, there already is a barrier of what we can do (chaotic behavior) regardless whether or not the underlying physics is deterministic or not. In other words, even if the question ("is universe deterministic") remains unanswered, we can already simulate all we need to the way it's possible to do it.
lnanek2 13y ago
The idea that the universe is anything more than physical laws playing out has just as little evidence as religion and the existence of omniscient godly beings. It is pretty much based entirely on wishful thinking by people who want to believe free will exists. They have no hard evidence and are basically making up an invisible pink fire breathing dragon just as much as the theists.
- dasil003 13y ago
  Ah, but why must a physical law be deterministic? The problem here is that we're all human and thus certain things are probably unknowable. You have disdain for someone who makes something up to fill that void, but I have disdain for someone who creates axioms in order to affirm their faith in science as the arbiter of truth. I am agnostic in the viewpoint that philosophy has a lot more to say on the subject than the either the worshiper of religious gods or the worshiper of science.
- cscurmudgeon 13y ago
  Except that really serious people have given arguments and proofs for the existence of God. Try attacking Godel's ontological argument without contradicting yourself. (This would require a year or more of study of formal logic culminating in quantified modal logic. Sadly, formal logic is something most scientists don't learn.) http://www.spiegel.de/international/germany/scientists-use-computer-to-mathematically-prove-goedel-god-theorem-a-928668.html http://www.spiegel.de/international/germany/scientists-use-c... http://math.stackexchange.com/questions/248548/godels-ontological-proof-how-does-it-work http://math.stackexchange.com/questions/248548/godels-ontolo...
  - Anderkent 13y ago
    The logic itself might be sound, but it doesn't matter if the assumptions are false, or possibly self contradictory. A false statement implies any other statement.
    - cscurmudgeon 13y ago
spodek 13y ago
> "Is the universe fundamentally deterministic?" The best answer we have to date is "Nobody knows," to which I would add, "nor do we know how to find out." I see no shame in that answer. I'm surprised nobody suggested that answer so far, unless I missed it. That doesn't mean we'll never find out or make any progress, but we don't know now. Personally, I see value in such a direct answer. If the question is "Are the laws of physics as we know them deterministic?" that's a question about human knowledge and the most effective equations and interpretations of them we have. We can answer that question definitively, but that doesn't answer the original question about the universe itself. If the question is about the universe itself, all we can offer is the laws of physics we know, which brings us back to the first point. We have incomplete knowledge of the universe. No experiment has been done yet to conclusively answer the question, so all we have is opinion. Anyone suggesting otherwise either knows of an experiment I don't (I'd be overjoyed to find out), or stating mere opinion. Opinion is nice, but it answers the question "Do you think the universe is deterministic," which is not the question asked. Edit: I see someone put this answer on the original site.
- sillysaurus2 13y ago
  The best answer we have to date is "Nobody knows," to which I would add, "nor do we know how to find out." I see no shame in that answer. I'm surprised nobody suggested that answer so far, unless I missed it. Well, it's stack exchange. You can't answer in a way that implies you don't know. They force answerers to act like everything is known, and that they personally know everything. Example: http://gamedev.stackexchange.com/questions/65768/how-can-i-deal-with-actor-translations-and-other-noise-in-third-party-motion-c/65769#65769 http://gamedev.stackexchange.com/questions/65768/how-can-i-d...
- Arjuna 13y ago
  Until someone can explain otherwise, I defer to Dr. Feynman, who essentially tells us that probability is intrinsic to the universe itself, and our experience of reality. I love the line that says (from the quote, below), "Someone has said it this way: 'nature herself doesn't know which way the electron is going to go.'" When discussing the double-slit experiment, he said: "It is not a lack of unknown gears - a lack of internal complications - that makes nature have probability in it; it seems to be in some sense intrinsic. Someone has said it this way: 'nature herself doesn't know which way the electron is going to go.' A philosopher once said (a pompous one): 'it is necessary for the very existence of science that the same conditions always produce the same result.' Well, they don't: if you set up electrons in any way - I mean, you set up the circumstance here, in the same conditions every time, and you cannot predict behind which hole you'll see the electron. They don't - and yet the science goes on in spite of him." -- Watch the full lecture here: https://www.youtube.com/watch?v=hUJfjRoxCbk https://www.youtube.com/watch?v=hUJfjRoxCbk I posted a transcribed portion previously: https://news.ycombinator.com/item?id=3756989 https://news.ycombinator.com/item?id=3756989
flibble 13y ago
I would propose that common sense rather than a deep understanding of quantum mechanics is sufficient to answer this question. If we can agree that if a two systems are in the exact same state then they will continue to be in the same state in future, then we can say deduce that the universe is deterministic. If a player hits a snooker ball exactly the same way two times, and all other balls are in the same position, the temperature is the same, humidity is the same, rotation of the earth is the same, rotation of electrons and unknown state of quarks are the same -- everything is the same -- then the result will be the same. To have a different outcome from the same inputs in a closed system is not possible. The same goes for human thought. If your brain is in the same state twice, all neuro chemicals are in the same state and all external forces are the same, and you are posed with a problem twice, you will make the same decision twice. There is no other possible outcome, for if there were another outcome it would be due to a differing influence. Thus your decisions are effectually predetermined. This the universe in its current state can only move forward in one way -- effectually predetermined. If anyone can come up with a case that two closed systems that are in the same state can diverge, I'd love to hear it.
- Steuard 13y ago
  On the first day of my upper level quantum mechanics class, I discuss exactly the sort of systems that you've asked about here. Unfortunately, it turns out that "common sense" is not in fact an accurate guide to the behavior of our universe (especially on an atomic scale). This is deeply frustrating, but in the end, the universe doesn't care. Specifically, I introduce the idea of a Stern-Gerlach experiment to measure the spin of an electron along specific directions. If you prepare an electron in (say) a "+z" state (which is absolutely unique and unambiguous) and then measure its spin along the perpendicular x-axis, the results are 50/50 (and completely unpredictable). At this point, your intuition is crying out, "We must have just overlooked some distinguishing detail in that initial +z state!" But it turns out that there are experimentally testable predictions of that sort of "hidden information" theory that differ from the predictions of quantum theory (look up "Bell inequalities"), and experiments always come back saying that the quantum description is correct.
  - 13y ago
Steuard 13y ago
Like so many things, the answer to this question depends on what exactly you mean by it. There are certainly philosophical games you can play to show that either answer is "possible": brains in a jar, malicious trickster deities, recordings on repeat, and the like. But if you're asking about what the best available scientific evidence supports, there is absolutely compelling evidence that this question must be answered in the framework of quantum mechanics. (There are experimental tests that can distinguish between quantum and classical predictions, and their results are unambiguous.) Within that framework, the most natural interpretation of this question might be, "Would it be possible in principle for a conscious being with perfect knowledge of the current state of the universe to predict the outcome that they would observe for any possible experiment?" Quantum mechanics answers this with an definitive "No." Even very simple systems can be prepared in a "superposition" of two possible states, in a sense that represents not just our ignorance of some underlying truth but a true mixture with measurable consequences. On the other hand, you might intend the question to mean "Given complete knowledge of the current state of the universe, would it in principle be possible to predict the future state of the universe?" If we leave aside issues of "observers" and "collapse of the wave function" for the moment, quantum mechanics clearly says that the answer is "Yes". (The mathematical property involved is "unitarity of time evolution".) If you begin with an electron in a superposition of "spin up" and "spin down" states and make a measurement, then a few moments later your experimental apparatus will be in a specific corresponding superposition of "detected spin up" and "detected spin down" states. But where you go from there depends on your preferred interpretation of quantum mechanics. The traditional Copenhagen interpretation says that once an observer looks at the measurement, the superposition state (the wave function) "collapses" to one answer or the other probabilistically, so determinism is lost. A "many-worlds" style interpretation would instead imply that the superposition of both outcomes persists in the state of the universe, but any particular instance of a given observer will see an outcome consistent with one measurement or the other (and the phenomenon of "decoherence" prevents those instances from interfering with each other in any meaningful way in the future). [Edit to add a disclaimer: There are weird open questions related to quantum gravity and black holes that call the unitarity of quantum mechanics into some doubt: a study of effects like Hawking radiation that arise when studying quantum fields in curved space-time might suggest that information about the state of the universe is lost over time. My impression as someone working in this field is that most people expect deep down that unitarity will be preserved in an ultimate theory, but there's no way to be sure until we have one. Also, LQG folks may be less confident of how their model would ultimately answer that question than string theory folks like me tend to be.]
Vektorweg 13y ago
i think: if the even smaller components starts to cycle, its deterministic.
danbruc 13y ago
I would love if it turns out, that the De Broglie–Bohm theory [1] is correct making the universe deterministic. [1] http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory
Tenoke 13y ago
Well, the very short answer that I've arrived is that: If you believe in the Copenhagen interpretation (and similar interpretations) then the universe isn't deterministic - there are many events that can happen but only one does happen in the end (when the set of probabilities assume specific values - the wavefunction collapse). If you subscribe to the Many-worlds Interpretation then the universe is deterministic in a way - all events that could happen will happen in some branches (or in a portion of the branches equal to the probability of the event happening, as much as this makes sense) and it was all going to be that way since 'the beginning of time'. However, from another point of view (ours) - it isn't determined which branch you will experience yourself observing and this can make things seem non-deterministic (but in reality you or equivalents of you will experience yourself observing all branches in which you exist so I don't think that this would be the correct way to think about it).
deleted 13y ago
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samatman 13y ago
Since this is Hacker News, it's worth mentioning that John Conway, of Game of Life fame, has proposed a Strong Free Will Theorem: http://arxiv.org/pdf/0807.3286.pdf http://arxiv.org/pdf/0807.3286.pdf Basically, a foremost expert on deterministic cellular automata is of the opinion that Universe is likely a cellular automaton, but not a deterministic one. Which I find intriguing.
valtron 13y ago
If you're interested in this, you should read up on Bell's theorem. [1] [2] The explanation of it I like best is this one (I don't remember where I originally read it): It involves a two player game, the players completely separated from each other. Each is independently shown X or Y. Each player must then give either answer A and B. They win the game if: - when both are shown X, they choose different answers - otherwise, they choose the same answer They can choose a strategy to follow before, but after they're physically separated, each player's decision can only depend on what they're shown (X or Y). Given this restriction, even allowing the players to have (pseudo-)non-deterministic strategies, they'll win with a chance of 75%. However, if the players' strategy involves each having half of an entangled pair of photons, they can make measurements of its spin at one of two predetermined angles (P1X = 0, P2X = 67.5, P1Y = 45, P2Y = 22.5), based on if they see X or Y. Their answer is based on whether they get a spin-up or spin-down measurement. With this strategy, they win about 85% (cos^2(tau/16)) of the time. [1] http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem [2] http://www.askamathematician.com/2010/06/q-how-it-is-that-bells-theorem-proves-that-there-are-no-hidden-variables-in-quantum-mechanics-how-do-we-know-that-god-really-does-play-dice-with-the-universe/ http://www.askamathematician.com/2010/06/q-how-it-is-that-be...
ffrryuu 13y ago
Einstein would turn in his grave. Not only does God play dice, the dice are loaded. Chairman Sheng-ji Yang
gaze 13y ago
Ayyo. I work for a quantum information group. Wavefunction collapse is a way of dealing with measurement, or having information spill from a closed system into a system with a huge number of degrees of freedom. If I was able to solve the schroedinger equation on the system that consisted of the "quantum machine" and the observer all in a perfectly sealed insulated box, there'd be nothing nondeterministic about this. I'll go further. Wavefunction collapse is just a system with a few quantum degrees of freedom getting itself all entangled with a system with a HUGE number of degrees of freedom. Hence the "randomness."
stiff 13y ago
I find it a bit unsettling that those question often center on finding out whether the universe is "in principle" deterministic. Can the universe be deterministic in principle but at the same time be indeterministic in that it is not possible to practically compute some things (because of computational complexity or other practical limitations)? Is practical and "theoretical determinism" different? Is "theoretical determinism" even a thing? Am I just being stupid, or has anybody serious already explored it?
- protonfish 13y ago
  I think that using the term "in principle" is the only way to pretend this question is difficult to answer. We observe what appears to be inherent unpredictability at the quantum level. The butterfly effect claims that imperceptible differences in starting conditions can cause significant effects (over time.) If this is an accurate model of how our universe works, we would expect that not only is nothing deterministic, but it would be extremely difficult to predict the state of reality over even short time periods. This is exactly what we observe on the macroscopic level.
jhallenworld 13y ago
Well in sequential state machines (computers), true randomness has to come from the outside. I have the feeling that it's the same for the universe as a whole: there is no mechanism which can make true randomness- it has to be a source of information from outside of the system. I suppose it makes no real difference if that's the case, or if you just call randomness an intrinsic property of some parts of the system. BTW, related questions: is the universe finite? Is the universe discrete or continuous (or better, do you need infinite information to represent it)?
novacole 13y ago
The answer to this question is easy. Of course the universe is fundamentally deterministic. Simply look at the current state of any given system (the in this case the universe) at any given point in time. If we were to look at all of the elements of "now" could indeterminate randomness (fully or partial) account for the current state of all things in the present moment? Of course not! If we say that universe if non-deterministic, but is instead probabilistic all we are saying is that "we do not understand the system well enough to know the causal relations of all elements within the system, but we can tell you how the system as a whole will work most of the time." Probability is not different from determinism, it is simply a way of measuring outcomes for systems for which we do not or cannot have enough information about in order to understand the causal relations of all of the elements within the system.

103 points by shubhamjain 13y ago

A good related discussion on determinism and free will: http://www.bbc.co.uk/programmes/b00z5y9z http://www.bbc.co.uk/programmes/b00z5y9z (BBC Radio 4 - In Our Time) - no idea if this is blocked outside the UK?

agravier 13y ago
Thanks. Not blocked (Malta).
siddboots 13y ago
Definitely not blocked here in Australia. In Our Time's archive is one of the Great Wonders of The Internet, as far as I'm concerned. http://www.bbc.co.uk/radio4/features/in-our-time/archive http://www.bbc.co.uk/radio4/features/in-our-time/archive

jblow 13y ago

In the answers I do not see any mention of the Second Time Around Problem, which conclusively answers the question with "there is no way to know". The idea is: suppose there is a universe that has elements that are fundamentally random. It is nondeterministic. Well, let that universe run for its lifetime, and record everything that happens. Then make a deterministic universe that just plays back the recording (this is the "second time around"). From the viewpoint of someone living inside the universe, there is no way to tell whether it is the first time around or the second time around. ... But the other thing to point out is that this question presumes an old idea about the passage of time, which is that things happen in a sequence A, B, C, D, ... and that if you are at C then D "has not happened yet". But if you look at relativity, this appears to be a naïve viewpoint. In relativity, the time at a faraway point in space that you would consider "simultaneous" with your own clock depends on the relative speed between you and that point. As you speed up and slow down, you can make a faraway point "go forward or backward in time" with regard to which moment there you would consider "now". The crazy thing is that for angular movements the relative speed is amplified by distance, so when you are moving around at everyday speeds the "now" on planets across the galaxy is going back and forth by thousands or millions of years. (This is hard to observe because you are viewing tiny amounts of light from very very far away that have been traveling for a very long time, and the light that you are about to see was very close to you when you did the angular movement so it will not be much affected, etc, but hey, the math says what it says, you either believe what physics tells you or you don't.) So when you make a distant "now" go forward, then backward, then forward again, do you expect the two forwards to be the same, or not?

siddboots 13y ago
That argument is, roughly, Humean scepticism. Science is a game we play where we do experiments, build theories, do more experiments, and so on. Any scientific answer to how the universe works has to start with the assumption that the nature of universe can be understood in the first place (otherwise we'll never get anywhere.) Edit: This was in response to your comment prior to edit. Up to "..."
- eli_gottlieb 13y ago
  Well yes, which is why we can't really ever know. It's somewhere between difficult and impossible to prove the axiom of your entire epistemological process.
paraschopra 13y ago
This is weird. If the observer in the universe is able to make accurate predictions consistently and repeatedly, doesn't that give confidence to his beliefs that the universe is deterministic. What you might be referring to is another way of saying that "any theory can be proven wrong if and when we observe a data point negating it". So, of course, there is no way of knowing if the universe is __truly__ deterministic or not, but then again we can say the same thing about sun rising from the east (unlikely as it may be, it might well just not rise tomorrow at all and then we will have to revise our theories).
- jblow 13y ago
  Did you read the OP? The whole question is about whether the apparent randomness in quantum mechanics means the universe is fundamentally nondeterministic. We cannot make accurate predictions of measurements of quantum state apart from saying things like "A will happen with some probability, otherwise B will happen". It is that "with some probability" that is the question. (Yes, at the level of the macroscopic world we can make consistently accurate predictions, but physicists would say this is because we are operating at a scale where the statistical nature of quantum mechanics averages out, etc. Unless you are Carver Mead or one of the other wave guide kinds of guys who believe there actually is no randomness in QM and it is deterministic all the way down.)

deletes 13y ago

I think most people think it isn't because you can't measure something without influencing it and thus changing the state. Not being able to record the state doesn't mean it isn't deterministic.

eterm 13y ago

This reminds me of SMAC, on discovering (iirc) Quantum Computing: "Not only does God play dice, the dice are loaded"

shubhamjain 13y ago

I have been reading Godel, Escher, Bach by Douglas Hofstadter. In the section, formal system and reality, the author talks about universe as a very big formal system with all sets of rules governing it. Though, author doesn't answer the determinism question itself but I am aligned to towards it being deterministic because I think there rules at every level, quantum or physical. Everything must occur because of a factor or the other.

nhaehnle 13y ago
The interesting point here is that even though the universe may be entirely deterministic --and quantum theory says that it is, if you consider the universe as a single gigantic "wave function" -- that may still be irrelevant for our lives because we cannot actually observe this gigantic wave function. In fact, we're a part of this function, in an unusual sense for which there is no everyday life equivalent.

allard 13y ago

friend's book — http://www.amazon.com/Free-Will-Joseph-Keim-Campbell/dp/0745646670/ http://www.amazon.com/Free-Will-Joseph-Keim-Campbell/dp/0745...

codeulike 13y ago

If its deterministic, but cannot be simplified and modelled by anything smaller than itself (imagine a simulator that modelled every atom - it would have to have more atoms than it was modelling) ... then does it matter? That is, it may be deterministic but fundamentally unpredictable in practice. See Dennets 'freedom evolves' for a long discussion of this.

asgard1024 13y ago

I would say that the determinism or non-determinism doesn't really matter. It seems to me that determinism is a feature of the observer too. For example, roll of dice is non-deterministic for a human being, even in perfectly deterministic universe. It's just too complicated to simulate, so it appears random, and we can pretty much declare it to be non-deterministic. And pseudo-random generators, purely idealized systems, can generate non-determinism from determinism. You can also have systems that do the opposite - that are itself based on non-determinism, but are in the effect deterministic. Classical computers use non-deterministic movement of electrons (or better say, they don't rely on their determinism), but can still produce repeatable (deterministic) results. So you can move away from determinism or non-determinism on lower level (like the level of elementary particles) by various methods of aggregation. Considering there is still about 20 orders of magnitude between the "size" of elementary particles and Planck length, I would dare say that reality may still switch between determinism and non-determinism several times beyond the what we know. Also, people may find this lecture by Dan Dennett interesting (which inspired the above argument): http://www.youtube.com/watch?v=uxup7sxIUmg http://www.youtube.com/watch?v=uxup7sxIUmg

wyager 13y ago
>I would say that the determinism or non-determinism doesn't really matter. It's kind of (very) important for information theory and computability theory. If the universe is deterministic, a sufficiently small physical system can be exactly simulated by a sufficiently powerful Turing machine.
- asgard1024 13y ago
  I am not sure I understand how is that important or helpful. In simulations of physical systems, there already is a barrier of what we can do (chaotic behavior) regardless whether or not the underlying physics is deterministic or not. In other words, even if the question ("is universe deterministic") remains unanswered, we can already simulate all we need to the way it's possible to do it.

lnanek2 13y ago

The idea that the universe is anything more than physical laws playing out has just as little evidence as religion and the existence of omniscient godly beings. It is pretty much based entirely on wishful thinking by people who want to believe free will exists. They have no hard evidence and are basically making up an invisible pink fire breathing dragon just as much as the theists.

dasil003 13y ago
Ah, but why must a physical law be deterministic? The problem here is that we're all human and thus certain things are probably unknowable. You have disdain for someone who makes something up to fill that void, but I have disdain for someone who creates axioms in order to affirm their faith in science as the arbiter of truth. I am agnostic in the viewpoint that philosophy has a lot more to say on the subject than the either the worshiper of religious gods or the worshiper of science.
cscurmudgeon 13y ago
Except that really serious people have given arguments and proofs for the existence of God. Try attacking Godel's ontological argument without contradicting yourself. (This would require a year or more of study of formal logic culminating in quantified modal logic. Sadly, formal logic is something most scientists don't learn.) http://www.spiegel.de/international/germany/scientists-use-computer-to-mathematically-prove-goedel-god-theorem-a-928668.html http://www.spiegel.de/international/germany/scientists-use-c... http://math.stackexchange.com/questions/248548/godels-ontological-proof-how-does-it-work http://math.stackexchange.com/questions/248548/godels-ontolo...
- Anderkent 13y ago
  The logic itself might be sound, but it doesn't matter if the assumptions are false, or possibly self contradictory. A false statement implies any other statement.
  - cscurmudgeon 13y ago

spodek 13y ago

> "Is the universe fundamentally deterministic?" The best answer we have to date is "Nobody knows," to which I would add, "nor do we know how to find out." I see no shame in that answer. I'm surprised nobody suggested that answer so far, unless I missed it. That doesn't mean we'll never find out or make any progress, but we don't know now. Personally, I see value in such a direct answer. If the question is "Are the laws of physics as we know them deterministic?" that's a question about human knowledge and the most effective equations and interpretations of them we have. We can answer that question definitively, but that doesn't answer the original question about the universe itself. If the question is about the universe itself, all we can offer is the laws of physics we know, which brings us back to the first point. We have incomplete knowledge of the universe. No experiment has been done yet to conclusively answer the question, so all we have is opinion. Anyone suggesting otherwise either knows of an experiment I don't (I'd be overjoyed to find out), or stating mere opinion. Opinion is nice, but it answers the question "Do you think the universe is deterministic," which is not the question asked. Edit: I see someone put this answer on the original site.

sillysaurus2 13y ago
The best answer we have to date is "Nobody knows," to which I would add, "nor do we know how to find out." I see no shame in that answer. I'm surprised nobody suggested that answer so far, unless I missed it. Well, it's stack exchange. You can't answer in a way that implies you don't know. They force answerers to act like everything is known, and that they personally know everything. Example: http://gamedev.stackexchange.com/questions/65768/how-can-i-deal-with-actor-translations-and-other-noise-in-third-party-motion-c/65769#65769 http://gamedev.stackexchange.com/questions/65768/how-can-i-d...
Arjuna 13y ago
Until someone can explain otherwise, I defer to Dr. Feynman, who essentially tells us that probability is intrinsic to the universe itself, and our experience of reality. I love the line that says (from the quote, below), "Someone has said it this way: 'nature herself doesn't know which way the electron is going to go.'" When discussing the double-slit experiment, he said: "It is not a lack of unknown gears - a lack of internal complications - that makes nature have probability in it; it seems to be in some sense intrinsic. Someone has said it this way: 'nature herself doesn't know which way the electron is going to go.' A philosopher once said (a pompous one): 'it is necessary for the very existence of science that the same conditions always produce the same result.' Well, they don't: if you set up electrons in any way - I mean, you set up the circumstance here, in the same conditions every time, and you cannot predict behind which hole you'll see the electron. They don't - and yet the science goes on in spite of him." -- Watch the full lecture here: https://www.youtube.com/watch?v=hUJfjRoxCbk https://www.youtube.com/watch?v=hUJfjRoxCbk I posted a transcribed portion previously: https://news.ycombinator.com/item?id=3756989 https://news.ycombinator.com/item?id=3756989

flibble 13y ago

I would propose that common sense rather than a deep understanding of quantum mechanics is sufficient to answer this question. If we can agree that if a two systems are in the exact same state then they will continue to be in the same state in future, then we can say deduce that the universe is deterministic. If a player hits a snooker ball exactly the same way two times, and all other balls are in the same position, the temperature is the same, humidity is the same, rotation of the earth is the same, rotation of electrons and unknown state of quarks are the same -- everything is the same -- then the result will be the same. To have a different outcome from the same inputs in a closed system is not possible. The same goes for human thought. If your brain is in the same state twice, all neuro chemicals are in the same state and all external forces are the same, and you are posed with a problem twice, you will make the same decision twice. There is no other possible outcome, for if there were another outcome it would be due to a differing influence. Thus your decisions are effectually predetermined. This the universe in its current state can only move forward in one way -- effectually predetermined. If anyone can come up with a case that two closed systems that are in the same state can diverge, I'd love to hear it.

Steuard 13y ago
On the first day of my upper level quantum mechanics class, I discuss exactly the sort of systems that you've asked about here. Unfortunately, it turns out that "common sense" is not in fact an accurate guide to the behavior of our universe (especially on an atomic scale). This is deeply frustrating, but in the end, the universe doesn't care. Specifically, I introduce the idea of a Stern-Gerlach experiment to measure the spin of an electron along specific directions. If you prepare an electron in (say) a "+z" state (which is absolutely unique and unambiguous) and then measure its spin along the perpendicular x-axis, the results are 50/50 (and completely unpredictable). At this point, your intuition is crying out, "We must have just overlooked some distinguishing detail in that initial +z state!" But it turns out that there are experimentally testable predictions of that sort of "hidden information" theory that differ from the predictions of quantum theory (look up "Bell inequalities"), and experiments always come back saying that the quantum description is correct.
- 13y ago

Steuard 13y ago

Like so many things, the answer to this question depends on what exactly you mean by it. There are certainly philosophical games you can play to show that either answer is "possible": brains in a jar, malicious trickster deities, recordings on repeat, and the like. But if you're asking about what the best available scientific evidence supports, there is absolutely compelling evidence that this question must be answered in the framework of quantum mechanics. (There are experimental tests that can distinguish between quantum and classical predictions, and their results are unambiguous.) Within that framework, the most natural interpretation of this question might be, "Would it be possible in principle for a conscious being with perfect knowledge of the current state of the universe to predict the outcome that they would observe for any possible experiment?" Quantum mechanics answers this with an definitive "No." Even very simple systems can be prepared in a "superposition" of two possible states, in a sense that represents not just our ignorance of some underlying truth but a true mixture with measurable consequences. On the other hand, you might intend the question to mean "Given complete knowledge of the current state of the universe, would it in principle be possible to predict the future state of the universe?" If we leave aside issues of "observers" and "collapse of the wave function" for the moment, quantum mechanics clearly says that the answer is "Yes". (The mathematical property involved is "unitarity of time evolution".) If you begin with an electron in a superposition of "spin up" and "spin down" states and make a measurement, then a few moments later your experimental apparatus will be in a specific corresponding superposition of "detected spin up" and "detected spin down" states. But where you go from there depends on your preferred interpretation of quantum mechanics. The traditional Copenhagen interpretation says that once an observer looks at the measurement, the superposition state (the wave function) "collapses" to one answer or the other probabilistically, so determinism is lost. A "many-worlds" style interpretation would instead imply that the superposition of both outcomes persists in the state of the universe, but any particular instance of a given observer will see an outcome consistent with one measurement or the other (and the phenomenon of "decoherence" prevents those instances from interfering with each other in any meaningful way in the future). [Edit to add a disclaimer: There are weird open questions related to quantum gravity and black holes that call the unitarity of quantum mechanics into some doubt: a study of effects like Hawking radiation that arise when studying quantum fields in curved space-time might suggest that information about the state of the universe is lost over time. My impression as someone working in this field is that most people expect deep down that unitarity will be preserved in an ultimate theory, but there's no way to be sure until we have one. Also, LQG folks may be less confident of how their model would ultimately answer that question than string theory folks like me tend to be.]

Vektorweg 13y ago

i think: if the even smaller components starts to cycle, its deterministic.

danbruc 13y ago

I would love if it turns out, that the De Broglie–Bohm theory [1] is correct making the universe deterministic. [1] http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory

Tenoke 13y ago

Well, the very short answer that I've arrived is that: If you believe in the Copenhagen interpretation (and similar interpretations) then the universe isn't deterministic - there are many events that can happen but only one does happen in the end (when the set of probabilities assume specific values - the wavefunction collapse). If you subscribe to the Many-worlds Interpretation then the universe is deterministic in a way - all events that could happen will happen in some branches (or in a portion of the branches equal to the probability of the event happening, as much as this makes sense) and it was all going to be that way since 'the beginning of time'. However, from another point of view (ours) - it isn't determined which branch you will experience yourself observing and this can make things seem non-deterministic (but in reality you or equivalents of you will experience yourself observing all branches in which you exist so I don't think that this would be the correct way to think about it).

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samatman 13y ago

Since this is Hacker News, it's worth mentioning that John Conway, of Game of Life fame, has proposed a Strong Free Will Theorem: http://arxiv.org/pdf/0807.3286.pdf http://arxiv.org/pdf/0807.3286.pdf Basically, a foremost expert on deterministic cellular automata is of the opinion that Universe is likely a cellular automaton, but not a deterministic one. Which I find intriguing.

valtron 13y ago

If you're interested in this, you should read up on Bell's theorem. [1] [2] The explanation of it I like best is this one (I don't remember where I originally read it): It involves a two player game, the players completely separated from each other. Each is independently shown X or Y. Each player must then give either answer A and B. They win the game if: - when both are shown X, they choose different answers - otherwise, they choose the same answer They can choose a strategy to follow before, but after they're physically separated, each player's decision can only depend on what they're shown (X or Y). Given this restriction, even allowing the players to have (pseudo-)non-deterministic strategies, they'll win with a chance of 75%. However, if the players' strategy involves each having half of an entangled pair of photons, they can make measurements of its spin at one of two predetermined angles (P1X = 0, P2X = 67.5, P1Y = 45, P2Y = 22.5), based on if they see X or Y. Their answer is based on whether they get a spin-up or spin-down measurement. With this strategy, they win about 85% (cos^2(tau/16)) of the time. [1] http://en.wikipedia.org/wiki/Bell%27s_theorem http://en.wikipedia.org/wiki/Bell%27s_theorem [2] http://www.askamathematician.com/2010/06/q-how-it-is-that-bells-theorem-proves-that-there-are-no-hidden-variables-in-quantum-mechanics-how-do-we-know-that-god-really-does-play-dice-with-the-universe/ http://www.askamathematician.com/2010/06/q-how-it-is-that-be...

ffrryuu 13y ago

Einstein would turn in his grave. Not only does God play dice, the dice are loaded. Chairman Sheng-ji Yang

gaze 13y ago

Ayyo. I work for a quantum information group. Wavefunction collapse is a way of dealing with measurement, or having information spill from a closed system into a system with a huge number of degrees of freedom. If I was able to solve the schroedinger equation on the system that consisted of the "quantum machine" and the observer all in a perfectly sealed insulated box, there'd be nothing nondeterministic about this. I'll go further. Wavefunction collapse is just a system with a few quantum degrees of freedom getting itself all entangled with a system with a HUGE number of degrees of freedom. Hence the "randomness."

stiff 13y ago

I find it a bit unsettling that those question often center on finding out whether the universe is "in principle" deterministic. Can the universe be deterministic in principle but at the same time be indeterministic in that it is not possible to practically compute some things (because of computational complexity or other practical limitations)? Is practical and "theoretical determinism" different? Is "theoretical determinism" even a thing? Am I just being stupid, or has anybody serious already explored it?

protonfish 13y ago
I think that using the term "in principle" is the only way to pretend this question is difficult to answer. We observe what appears to be inherent unpredictability at the quantum level. The butterfly effect claims that imperceptible differences in starting conditions can cause significant effects (over time.) If this is an accurate model of how our universe works, we would expect that not only is nothing deterministic, but it would be extremely difficult to predict the state of reality over even short time periods. This is exactly what we observe on the macroscopic level.

jhallenworld 13y ago

Well in sequential state machines (computers), true randomness has to come from the outside. I have the feeling that it's the same for the universe as a whole: there is no mechanism which can make true randomness- it has to be a source of information from outside of the system. I suppose it makes no real difference if that's the case, or if you just call randomness an intrinsic property of some parts of the system. BTW, related questions: is the universe finite? Is the universe discrete or continuous (or better, do you need infinite information to represent it)?

novacole 13y ago

The answer to this question is easy. Of course the universe is fundamentally deterministic. Simply look at the current state of any given system (the in this case the universe) at any given point in time. If we were to look at all of the elements of "now" could indeterminate randomness (fully or partial) account for the current state of all things in the present moment? Of course not! If we say that universe if non-deterministic, but is instead probabilistic all we are saying is that "we do not understand the system well enough to know the causal relations of all elements within the system, but we can tell you how the system as a whole will work most of the time." Probability is not different from determinism, it is simply a way of measuring outcomes for systems for which we do not or cannot have enough information about in order to understand the causal relations of all of the elements within the system.