
Is the universe fundamentally deterministic? - shubhamjain
http://physics.stackexchange.com/questions/63811/is-the-universe-fundamentally-deterministic
======
spodek
> " _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.

~~~
Arjuna
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)

~~~
mtdewcmu
I'm probably taking Feynman out of context here, but I think philosopher is
almost right, and Feynman is being a tad pompous. I think it's necessary to
some extent to _assume_ that the same conditions always produce the same
result in order to do science. Otherwise, why do experiments? If the universe
deviated slightly from determinism, it probably wouldn't make science
impossible to do, and since I don't think science can actually test
determinism, it's outside the scope of science and must be assumed. It seems a
bit pompous to say that we've set up electrons the same way and observed that
they go in two different directions for no reason. You can't eliminate reasons
that you don't know about.

~~~
ByronT
_It seems a bit pompous to say that we 've set up electrons the same way and
observed that they go in two different directions for no reason. You can't
eliminate reasons that you don't know about._

Have you heard about the EPR paradox [1]? Einstein, Podolsky, and Rosen
believed there may have been "hidden variables" in the language of quantum
mechanics which were not yet accounted for. These hidden variables, if
discovered, would render quantum mechanics more deterministic.

This idea was essentially shattered by Bell's theorem [2]. The universe
probably _is not_ deterministic (at least in the sense we think about
determinism). That's a hard pill to swallow, and it goes against all human
intuition. But the majority of physicists do seem to accept this fact, however
uncomfortable it might initially feel.

[1]
[http://en.wikipedia.org/wiki/EPR_paradox](http://en.wikipedia.org/wiki/EPR_paradox)

[2]
[http://en.wikipedia.org/wiki/Bell%27s_theorem](http://en.wikipedia.org/wiki/Bell%27s_theorem)

~~~
mtdewcmu
"Probably is not deterministic" is not quite the same as " _provably_ is not
deterministic." Physicists have opinions on a lot of things, but I don't think
they can fundamentally answer this.

~~~
ByronT
In your original post, you state that we might as well assume determinism.
Why? Given the widely accepted nondeterminism of quantum behavior, why
wouldn't we assume nondeterminism instead?

I realize what you're grasping at is that deterministic behavior might somehow
underly this seemingly nondeterministic behavior. Bell's theorem, however, has
made it difficult to reject that the this fundamental aspect of the universe
is hard to explain without assuming nondeterminism (or superdeterminism).

I agree you can't prove it. You just haven't really explained why you're
leaning toward the deterministic side.

~~~
mtdewcmu
I didn't make myself clear. I meant assume in the mathematical sense, as in,
to take as an axiom. If you do a science experiment, you are assuming that you
will get back a repeatable result, and you won't be measuring something
perfectly random. Otherwise, people might assume you're crazy. :)

------
jblow
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?

~~~
no_gravity
2 things:

1) If we define "universe" as "everything that exists" then you cannot record
everything that happens. Because the record would be part of the universe and
would have to recorded and the record of the record would have to be
recorded...

2) The "first" and "second" universes would not be universes on their own. The
"second" is a continuation of what you call the "first". So the system you
describe would be random. Because the "deterministic" things that happen later
on are based on the random things that happened earlier.

~~~
jblow
Just pause for a moment on "everything that exists" because that is not
necessarily a useful distinction.

If you think of a universe as "everything I could see/touch/etc given infinite
time", someone outside the universe who was in control of it might record it
with no problem because they are not subject to the constraints of the
universe. e.g. imagine the universe as being like a computer simulation that
you programmed. You can probably pause it in a debugger any time you want,
look at whatever state variables you want, etc.

So in this case the definition cannot be "everything that exists" because you
have to define "exists" in the case where different sets of things may have
different levels of "reality".

~~~
no_gravity
Lets say we are in a "game of life" simulation on a computer. It has been
proven turing complete. So you would say its deterministic because "game of
life" has deterministic rules. But if the computer that runs the simulation is
in a random universe, then it might happen that bits of the computer switch in
a random fashion. Making our universe inside the "game of life" simulation
random too. And we _could_ touch the randomly flipping bits.

So there is no point in saying something is "outside" our universe and
something is "inside".

~~~
jblow
There is no point to saying something is "outside" and something is "inside"
_in the extreme cases of the example you just gave_ , which may or may not be
the case. What if the outer universe is not a random universe? What if its
laws of nature are so unlike ours that we do not know what to say about it?
(Maybe it does not have space or time, but it has other things instead.)

But I think this is a little bit beside the point, because as I mentioned in
my first post, all of this is predicated on us adopting the naïve view of time
in our own universe, which is maybe not the best idea, given that we have
plenty of evidence otherwise.

(And anyway, if you believe that "everything that exists" can be infinite,
then there is no sense in saying our universe is being produced by some kind
of simulation controlled in one particular other place, because of course it
is, but actually this is probably happening in an infinite number of different
ways from different places such that there is not really point any more in
claiming that this is happening at all; the situation becomes such that you
could just draw a relation between situation X and situation Y and could state
that one could be causal of the other from a certain point of view.)

------
asgard1024
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
>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
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.

------
joosters
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?

~~~
siddboots
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)

------
lnanek2
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.

~~~
cscurmudgeon
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-c...](http://www.spiegel.de/international/germany/scientists-use-computer-
to-mathematically-prove-goedel-god-theorem-a-928668.html)

[http://math.stackexchange.com/questions/248548/godels-
ontolo...](http://math.stackexchange.com/questions/248548/godels-ontological-
proof-how-does-it-work)

~~~
leephillips
No, it just requires understanding something about the nature of mathematics
and its relationship with reality. You can't prove the existence of god any
more than you can prove the existence of gravity. To make the connection
between the math and reality requires _interpretation_ , as Einstein
repeatedly pointed out.

Gödel's amusing exercise is just a formalization of St. Anselm's, and most of
the centuries of criticism of the latter can be applied to the former. In
particular, his little proof has three definitions and _six_ special-purpose
axioms, and, in addition to all this, depends on a property of goodness or
perfection that is only defined implicitly. Naturally, the existence of
something that he interprets as "god" falls out as a theorem. Big deal.

The nature of mathematics are that the theorems are contained within the
axioms. If the axioms are self-evident, and the conclusions are surprising, we
have some interesting math. But at least two of Gödel's axioms are not self-
evident. Therefore, this "proof" strikes me as a setup, and is not very
interesting.

~~~
cscurmudgeon
The same can be said for anything in life. I really hope you wake up everyday
praying that the laws of physics hold today because there is no proof that
will hold today.

If you claim that goodness is artificial/relative/subjective, then you are
hilariously self-refuting yourself. :)

[http://plato.stanford.edu/entries/relativism/#5.9](http://plato.stanford.edu/entries/relativism/#5.9)

~~~
leephillips
Well, you fooled me. I thought you were serious and might know what you were
talking about.

~~~
cscurmudgeon
>I thought you were serious and might know what you were talking about.

Thanks for the kind mature words.

You are no different from the Christian fundamentalists who accuse that the
scientists (including logicians like me) don't know what they are talking
about. Sigh.

I was being respectful, but you showed your deficiency by attacking my
intellect. Thanks for that.

I can only point in the right direction. I cannot rectify a highly deficient
education (ah, the US).

------
flibble
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
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.

~~~
flibble
"We must have just overlooked some distinguishing detail in that initial +z
state!" \- yes, or there is an external influence we have missed.

The fact that the x-axis spin results are 50/50 strongly suggests something
very specific and deterministic is going on. It would seem that we just aren't
able to (pre)determine it without influencing it.

~~~
Steuard
I completely agree that it _suggests_ that we have overlooked something in the
underlying state. However, if you take a course in quantum mechanics that
includes a detailed mathematical discussion of the Bell inequalities (for
instance), you will see that any possible deterministic system along the lines
that you're thinking of here will give predictions that are measurably
different from the predictions of quantum mechanics. These experiments have
been done, and the results are incompatible with any deterministic model
(regardless of how difficult we assume it is to measure those "hidden
variables" without influencing them).

I agree that this flies in the face of everything that seems sensible in the
world. I _loathed_ quantum mechanics for years for reasons along these lines.
Nevertheless, the universe is under no obligation to obey my intuition for
what is sensible, nor my sense of elegance and beauty. Much to my
disappointment, experiments make it absolutely clear that we do not live in a
universe where "common sense" arguments are at all trustworthy on the scale of
individual particles.

(Also, personally, I would say that the 50/50 result on the x-axis is purely a
consequence of symmetry: nothing in the initial state or the measurement
apparatus is set up to prefer +x over -x or vice versa, so it would be more or
less impossible for it to split any way _other_ than 50/50.)

~~~
flibble
We shall see :)

------
Tenoke
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).

------
gaze
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."

------
shubhamjain
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
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.

------
valtron
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-
be...](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/)

------
samatman
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.

------
codeulike
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.

------
novacole
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.

------
eterm
This reminds me of SMAC, on discovering (iirc) Quantum Computing:

"Not only does God play dice, the dice are loaded"

------
jhallenworld
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)?

------
stiff
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
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.

------
danbruc
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)

------
deletes
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.

------
Steuard
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.]

~~~
maaku
> 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.

I think maybe you'd want to reference Heisenberg uncertainty here, not
superposition? Superposition is weird, but deterministic except for the wave
function collapse, but you correctly deal with that later. Its the Heisenberg
uncertainty that prevents you from precisely measuring a particle's current
state, thereby preventing you from obtaining fully predictive information
except in philosophical hypotheticals.

~~~
Steuard
The (Heisenberg) uncertainty is a specific, computable quantity associated
with the expectation value of an operator for a given quantum state. If that
state is an eigenstate of the operator to be measured, the uncertainty is
exactly zero. The only way for a state to have a non-zero uncertainty in its
expectation value is if it is a superposition of more than one eigenstate with
different eigenvalues.

So I stand by my "superposition" phrasing. :) Taking a measurement of a system
that is in a superposition of distinct eigenstates of the observable being
measured will give a probabilistic distribution of measurement outcomes (with
probabilities derived from the amplitudes of the corresponding terms in the
superposition).

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

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

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

