Is this a Deterministic Universe

[Tanelorn]

Does the seemingly random nature of QM mean that the Universe is very non deterministic and does this mean that our existence is completely as a result of random chance, or am I significantly misunderstanding the nature of QM?

I am new to QM and the idea of determinism, and I just wanted to get an idea for the current mainstream view. I also read a quote recently, I forget who, that said that no one really understands QM and wondered if that plays a part also..

 

[DrChinese]

You can really go all over the place with this topic, a lot of it being more philosophical than about physics.

A deterministic universe is a viable possibility even with QM as it is. There is a non-local theory called Bohmian Mechanics in which QM's predictions are replicated, and everything is fully deterministic too.

Also, a deterministic universe means "our existence is completely as a result of random chance" just as much as an indeterministic one does. The difference being that in one, our future has just one (random) possible outcome. In other interpretations of QM, there are many possible outcomes and none is more favored over another (although I guess some outcomes might be more likely in a sense).

 

[Tanelorn]

Thanks Dr, please let me ask this in a slightly different way. If the exact same universe were to be reproduced or started over n times from t=0 i.e. the BB, what are the chances that I would be here typing this in every one? I have heard answers from other Physicists ranging from n to 1 and even 0.

Since I didn't do well in my QM class and it is so long ago, I am unable to judge which answer is most likely and so I wondered what most experts in modern QM think?

I am not looking for a philosophical take or bias, just an understanding of the Physics and QM facts, which perhaps are not sufficiently well understood to be certain of the answer to a question like this?

 

[DrChinese]

If the exact same universe were to be reproduced or started over n times from t=0 i.e. the BB, what are the chances that I would be here typing this in every one?

As I follow the more indeterministic branch: zero.

But as mentioned, there are those pesky Bohmians too!

 

[Tanelorn]

And not 1? Am I not really here? :) My answer remains 1.

Anyway thanks for replying. I hope that other QM people can weigh in on this, perhaps a poll? - again, on the Physics only.

 

[lyncsta]

#Limited knowledge noob response warring#
​

Well if the position an electron in space is based on probability and is somewhat random, and the big bang started on the quantum level, then the outcome of that big bang is a probability.

On a more science fiction side of things, Backwards time travel is theoretically possible, so..

Deterministic universe backwards time travel = paradox
undermined universe backwards time travel = paradox free :), except for maybe running into your self.

 

[Tanelorn]

lyncsta, I don't believe in HG Wells time travel. You can't travel to a place that does not exist and the wave functions of everything in the past have all collapsed and do not exist. Let's leave this for the Sci-Fi thread. :)

 

[StevieTNZ]

Thanks Dr, please let me ask this in a slightly different way. If the exact same universe were to be reproduced or started over n times from t=0 i.e. the BB, what are the chances that I would be here typing this in every one? I have heard answers from other Physicists ranging from n to 1 and even 0.

You would have a probability associated with what you typed, but also a probability to reply differently.

Whether what you actually typed was predetermined or not (i.e. QM supplemented with hidden variables (non-local, mind you)) is really hard to say right now. I don't even know if an experiment can be performed that can differentiate between Bohmian Mechanics (deterministic version of QM) and standard QM, but there are a few articles published that try and do such.*

*For clarity: QM = Quantum Mechanics

 

[EricJRose83]

Ever notice the correlation between logic, computability and causality? Basically the idea that every cause has a preceding cause that 'caused' it into existence is the basis for logic, which we use to discern the natural of the universe. In other words, it's the guiding principle (along with observation) through which we discover scientific truth. The idea that something can be 'uncaused' is a little ridiculous in my opinion because it not only goes against the very tenants we use to discern truth, but because it also holds the unfortunate reality of being logically impossible.

No one in the world can explain to you how an uncaused cause could possibly come into existence, because it would be impossible and our logical minds simply can't comprehend it as a result. Given that, I think it's fair to say that either the observations made in regards to 'spooky' action at the sub-atomic level are incorrect, or there is a non-local, superluminal deterministic system at work. Or, perhaps the universe simply isn't inherently logical. Being of rational/logical minds though, we instinctively rebel against anything that isn't.

With that said, the idea of inherent probability without an underlying deterministic system is rubbish in my opinion. If the quantum universe were inherently probable, what then makes one effect more probable than another? What's the cause of it?

Any system of probability necessitates an underlying deterministic system.

 

[lyncsta]

lyncsta, I don't believe in HG Wells time travel. You can't travel to a place that does not exist and the wave functions of everything in the past have all collapsed and do not exist. Let's leave this for the Sci-Fi thread. :)

Its not predicted by "HG Wells time travel", but by Albert einsteins theory of relativity - time dilation.

http://en.wikipedia.org/wiki/Time_dilation


If all matter exists at once, then so too must all time. hence spacetime.

 

[lyncsta]

Ever notice the correlation between logic, computability and causality? Basically the idea that every cause has a preceding cause that 'caused' it into existence is the basis for logic, which we use to discern the natural of the universe. In other words, it's the guiding principle (along with observation) through which we discover scientific truth. The idea that something can be 'uncaused' is a little ridiculous in my opinion because it not only goes against the very tenants we use to discern truth, but because it also holds the unfortunate reality of being logically impossible.

.

There is the multiverse theory :) That would explain the cause. But I suppose that just kicks the can down the road. We would be left asking what caused the multiverse.

 

[Tanelorn]

Its not predicted by "HG Wells time travel", but by Albert einsteins theory of relativity - time dilation.

http://en.wikipedia.org/wiki/Time_dilation


If all matter exists at once, then so too must all time. hence spacetime.

I see time as a thin slice (Planck time) of the present in which particles interact. Other past Planck times have no further effect and thus no longer exist. To go back to a point in the past would require recreating a whole new Universe in full, which is quite impossible.

 

[Tanelorn]

There is the multiverse theory :) That would explain the cause. But I suppose that just kicks the can down the road. We would be left asking what caused the multiverse.

I agree, every time you think you have found first cause you can always ask what caused that?
First cause and final effect therefore do not exist, just like a place at infinity cannot.

 

[Tanelorn]

Any system of probability necessitates an underlying deterministic system.

Did you not mean the opposite? If you can't tell what the outcome of one single random event is then the Universe is already non-deterministic.



I hope those that know QM better than me can weigh in on trying to understand whether QM means that the Universe is non deterministic with probabilities involved at every cause and effect, and at every Planck time step.

 

[DrChinese]

I see time as a thin slice (Planck time) of the present in which particles interact. Other past Planck times have no further effect and thus no longer exist. ...

There is no evidence to support this statement, and plenty of evidence to indicate it is false. Are you familiar with delayed choice quantum erasers? These appear to change the past. There are a number of interpretations of QM in which past time is treated no differently than future time. These are the "time symmetric" class. The Transactional Interpretation and Relational Blockworld are examples.

 

[EricJRose83]

Did you not mean the opposite? If you can't tell what the outcome of one single random event is then the Universe is already non-deterministic.
I hope those that know QM better than me can weigh in on trying to understand whether QM means that the Universe is non deterministic with probabilities involved at every cause and effect, and at every Planck time step.

No, I meant what I said. I think you're confusing predictability with determinism. Just because something might be unpredictable and 'seemingly' random, doesn't mean it isn't deterministic. No worries though, pretty much everyone gets these two ideas mixed up. True randomness is impossible.

You really have to ask yourself what makes one probable outcome more likely than another in a probability system.

 

[Naty1]

As Dr. Chinese hinted, there are a lot of interpretations about what QM means. Books upon books upon books. We have the math and it works, but what does it mean? [See the Penrose quote below] What does superposition, action at a distance, delayed choice, uncertainty, 'mean'...??

For interesting insights, check out prior discussions in these forums on Heisenberg Uncertainty Principle.

I forget who, that said that no one really understands QM and wondered if that plays a part also..

Sounds like Richard Feynman.

A corollary is "Shut up and calculate" which means the math of QM as laid out is clear [See Penrose below]...what it means, how to interpret it is 'not so clear".




http://en.wikipedia.org/wiki/Deterministic_system

In quantum mechanics, the Schrödinger equation, which describes the continuous time evolution of a system's wave function, is deterministic. However, the relationship between a system's wave function and the observable properties of the system appears to be non-deterministic…. A deterministic model will thus always produce the same output from a given starting condition or initial state.

for a prior discussion, which I liked:

...quantum theory is a causal but indeterministic theory: Even the complete knowledge about its state, which develops by a causal dynamical equation, doesn't imply that all observables take determined values.

[They take a statistical range of values.]

more detail on this:

from Roger Penrose celebrating Stephen Hawking’s 60th birthday in 1993 at Cambridge England...this description offered me a new insight into quantum/classical relationships:

Either we do physics on a large scale, in which case we use classical level physics; the equations of Newton, Maxwell or Einstein and these equations are deterministic, time symmetric and local. Or we may do quantum theory, if we are looking at small things; then we tend to use a different framework where time evolution is described... by what is called unitary evolution...which in one of the most familiar descriptions is the evolution according to the Schrödinger equation: deterministic, time symmetric and local. These are exactly the same words I used to describe classical physics{!}.

However this is not the entire story... In addition we require what is called the "reduction of the state vector" or "collapse" of the wave function to describe the procedure that is adopted when an effect is magnified from the quantum to the classical level...quantum state reduction is non deterministic, time-asymmetric and non local...The way we do quantum mechanics is to adopt a strange procedure which always seems to work...the superposition of alternative probabilities involving w, z, complex numbers...an essential ingredient of the Schrödinger equation. When you magnify to the classical level you take the squared modulii (of w, z) and these do give you the alternative probabilities of the two alternatives to happen...it is a completely different process from the quantum (realm) where the complex numbers w and z remain as constants "just sitting there"...in fact the key to keeping them sitting there is quantum linearity...

and he goes on to relate this linearity and superposition to the double slit experiment.
You can associate the imaginary aspect of complex quantum numbers with virtual particles as an initial perspective.

 

[Naty1]

Upon rereading all the above, seems like briefly discussing HUP would be of interest to you.

Regard 'uncertain' as 'non deterministic' in descriptions below. [I bet someone will object to that!]

These are from my notes, so, thankfully, I can just copy and paste:

Synopsis: If you search HUP in these forums you can rummage through many pages of disagreements and clarifications. The quotes below are slightly edited posts [for brevity, clarity] from those discussions. [I had little idea myself what HUP REALLY meant until arguments/discussions/and some research papers were dissected in these forums.]My own single sentence explanations :

A] Get a better instrument and you'll get better measurement results to any accuracy.

B] Quantum theory does not predict the outcomes of single measurements, it only predicts the ensemble [statistical] properties of multiple measurements.

C] In classical mechanics we can predict with absolute precision, to arbitrary accuracy, the future position and momentum [for example] of a single particle; The HUP says no you can't: you can only make a statistically based prediction! Summary Details:

It IS possible to simultaneously measure the position and momentum of a single particle. The HUP doesn't say anything about whether you can measure both in a single measurement at the same time. That is a separate issue.

It is possible to measure position and momentum simultaneously…a single measurement of a particle. What we can't do is to prepare an identical set of states… such that we would be able to make an accurate prediction about what the result of a position measurement would be and an accurate prediction about what the result of a momentum measurement would be….for an ensemble of future measurements.

What we call "uncertainty" is a property of a statistical distribution. The HUP isn't about a single measurement and what can be obtained out of that single measurement. It is about how well we can predict subsequent measurements given the ‘identical’ initial conditions. The commutativity and non commutivity of operators applies to the distribution of results, not an individual measurement. This "inability to repeat identical measurement results" is in my opinion better described as an inability to prepare a state which results in identical observables.

The uncertainty principle results from uncertainties which arise when attempting to prepare a set of identically prepared states…from identical initial conditions. The wave function is not associated with an individual particle but rather with the probability for finding particles at a particular position.

What we can't do is to prepare an identical set of states [that yields identical measurements]. NO STATE PREPARATION PROCEDURE IS POSSIBLE WHICH WOULD YIELD AN ENSEMBLE OF SYSTEMS IDENTICAL IN ALL OF THEIR OBSERVABLE PROPERTIES. [instead, identical’ state preparation procedures yield a statistical distribution of observables [measurements].]

Fredrik:

To prepare a state is to bring a particle on which we intend to do a measurement to the measuring device. Different ways of doing that may give us different average results. Two ways of doing it (two preparation procedures) are considered equivalent if no series of measurements can distinguish between them (i.e. if they give us the same wavefunction, or more generally, the same state operator/density matrix). These equivalence classes are often called "states".

The uncertainty principle restricts the degree of statistical homogeneity which it is possible to achieve in an ensemble of similarly prepared systems. A non-destructive position measurement is a state preparation that localizes the particle in the sense that it makes its wavefunction sharply peaked. This of course "flattens" its Fourier transform, so if the Fourier transform was sharply peaked before the position measurement, it isn't anymore.

The Uncertainty Principle finds its natural interpretation as a lower bound on the statistical dispersion among similarly prepared systems resulting from identical state preparation procedures and is not in any real sense related to the possible disturbance of a system by a measurement. The distinction between measurement and state preparation is essential for clarity.

A quantum state (pure or otherwise) represents an ensemble of similarly prepared systems. For example, the system may be a single electron. The ensemble will be the conceptual (infinite) set of all single electrons which have been subjected to some state preparation technique (to be specified for each state), generally by interaction with a suitable apparatus.

 

[EricJRose83]

I also read a quote recently, I forget who, that said that no one really understands QM and wondered if that plays a part also..

No one completely understands QM simply because they can't find an underlying deterministic system at work. Physicists can offer probable predictions based off of the data they've gathered, but that it's. They can't describe exactly how everything works because they simply don't have accountability of all of the variables involved.

No offense to anyone, but if someone says they have a complete understanding of QM... Well, let's just say they have an extremely large, self-assuaging ego.

 

[DrChinese]

...True randomness is impossible. ...

Not sure how you get this. As far as anyone knows, every quantum measurement of a previously unknown observable yields a completely random outcome.

It looks random, tastes random, smells random, so it must be a duck.

The idea that "true" randomness does not exist in nature is purely by assumption.

 

[Naty1]

No one completely understands QM simply because they can't find an underlying deterministic system at work.

Depends on just what you have in mind... I have given several quotes that seem to contradict what this appears to state. If you replace 'find' with 'observe'...that would seem consistent with the quotes I posted. On the other hand, the quotes I posted are also open to varied interpretation!

QM seems especially sensitive to particular words and for me at least 'saying what I mean' in QM is not easy...That's why I save clear and concise posts and quotes verbatum when I find ones I like...

 

[EricJRose83]

Not sure how you get this. As far as anyone knows, every quantum measurement of a previously unknown observable yields a completely random outcome.

It looks random, tastes random, smells random, so it must be a duck.

The idea that "true" randomness does not exist in nature is purely by assumption.

If the measured results were truly random, then they would offer zero chance of predictability. The fact that we're able to form a system of probability in regards to QM necessitates an underlying deterministic system at work, because without one there is nothing to 'cause' one event from being more probable than another.

 

[EricJRose83]

Depends on just what you have in mind... I have given several quotes that seem to contradict what this appears to state. If you replace 'find' with 'observe'...that would seem consistent with the quotes I posted. On the other hand, the quotes I posted are also open to varied interpretation!

QM seems especially sensitive to particular words and for me at least 'saying what I mean' in QM is not easy...That's why I save clear and concise posts and quotes verbatum when I find ones I like...

True, unfortunately when trying to get an idea across to someone, the other party may not interpret some of the chosen words the same way you do. What I meant by, "No one understands QM" is that no one can give an explanation as to why everything in QM happens. When it comes to seemingly random events, they simply say it is the way it is, rather than offer an explanation as to how it happens. A truly complete theory offers not only a degree of prediction, but exact causal explanations as well, in my opinion at least. Without a complete understanding of an underlying deterministic system, they really don't actually understand much at all.

Which isn't to say that quantum theory isn't useful, it's extremely useful obviously. I don't believe it to be complete though.

 

[DrChinese]

If the measured results were truly random, then they would offer zero chance of predictability.

Not sure what you mean by this. Their outcome is not (correctly/consistently) predictable in individual trials. If you measure a pair of polarization entangled photons at 0 degrees (one) and 45 degrees (the other), each outcome (+, -) will be random and the combined results (same or different polarization) will also be random. The results will tend towards 50-50 in the long run. There is no demonstrable "cause" for any of these outcomes.

I would welcome you to provide one scintilla* of experimental evidence to support the existence of ANY cause of this apparently PURELY random behavior. Similar for timing of radioactive decay and other quantum processes. The ONLY way to reach your conclusion is by circular reasoning, ie assuming that there IS an underlying cause and then explaining why you cannot demonstrate it scientifically.


* not that I would recognize a scintilla if I saw it...

 

[Tanelorn]

Thanks for replies, getting a little lost. You are right I did not realize there is a difference between determinism and probability and I still don't know what it is. What is it?

So let me ask the question instead using probability, if the BB started over n times what is the probability that I would be around to be able to type this in each case?

 

[DrChinese]

A truly complete theory offers not only a degree of prediction, but exact causal explanations as well, in my opinion at least. ... I don't believe it to be complete though.

I agree that it is... your opinion.

I assume you are already familiar with the entire EPR completeness argument. They felt the same way... in 1935.

 

[DrChinese]

So again if the BB started over n times...

1. If the initial conditions were X each time (identical), in my view no two would lead to identical universes at a later date. I would say this is a majority view.

2. However, as I mention, there are some physicists who think EVERY universe would be the same. The downside (if you want to call it that) is that you accept the existence of non-local (faster than light) influences.

There is no current means to select 1 over 2 or vice versa. Simply a matter of opinion.

 

[EricJRose83]

lol Well I think any discussion in regards to causal systems will end up employing circular reasoning, given the natural of causality.

Are the results exactly 50/50 given enough run time? If these events were truly random they would have to be. If they were off by the tiniest fraction of a percent, then I would argue that it isn't true randomness at work.

Also, where then does the probability aspect of prediction come from in QM? Are there other observations made who's results are not consistently 50/50?

Thank you for replying, Dr. Chinese. I'm simply trying to further my understanding of QM myself.

 

[EricJRose83]

I agree that it is... your opinion.

I assume you are already familiar with the entire EPR completeness argument. They felt the same way... in 1935.

I'm vaguely familiar with it. I seem to recall bell's theorem disproving any local, non-superluminal influence at work in regards to some of the spooky events in QM.

 

[EricJRose83]

Thanks for replies, getting a little lost. You are right I did not realize there is a difference between determinism and probability and I still don't know what it is. What is it?

Well, determinism basically states that every cause has a preceding cause. It's simple causality 101. The counter point of determinism is randomness, which says things just... happen randomly for no reason at all without preceding causes.

Probability is used in regards to deterministic systems who's complexity is too severe to make exact prediction plausible.

Some people who work in QM though think that the universe is inherently probable at the quantum level instead of deterministic, due to seemingly random observed events. The problem I have with this is that an underlying deterministic system is necessary for any degree of probability.