What's the universe collapsing into?

[friend]

So as time goes on there are more and more interactions in the world. That means that there are more wavefunction collapses going on. Some think that there is a universal wavefunction that is guiding all the individual wavefunctions. So I have to wonder if all these smaller, individual, instantaneous wavefunction collapses are somehow a process describing a very slow collapsing of the universal wavefunction. If so, then can we tell what state the universal wavefunction is collapsing into?

 

[mfb]

That means that there are more wavefunction collapses going on.

Only in interpretations of quantum mechanics with collapses.

Some think that there is a universal wavefunction that is guiding all the individual wavefunctions.

Depends on the interpretation.

So I have to wonder if all these smaller, individual, instantaneous wavefunction collapses are somehow a process describing a very slow collapsing of the universal wavefunction.

No.

 

[friend]

No.

We are certain of the past but not sure of the future. That means that as time goes on, more events enter the past. And there are more events we are certain of. So it seems we are getting more evidence to give us more certainty about... what? This sounds like a slow process of wave function collapse.

 

[mfb]

We are certain of the past but not sure of the future. That means that as time goes on, more events enter the past. And there are more events we are certain of.

That does not mean we would have more knowledge about the present. We do not.

 

[member 11137]

That does not mean we would have more knowledge about the present. We do not.

We perhaps don't have more knowledge about the present because the total number of events is increasing permanently (entropy) but we certainly accumulate experience on how the next and local future will look like. Leaving the darkness of ignorance is, at a fundamental level, the deep purpose of a scientific approach.

 

[mfb]

but we certainly accumulate experience on how the next and local future will look like.

Do we really know more about the quantum state in 10 minutes, than we knew yesterday about the quantum state 10 minutes in the future back then?

I'm not counting scientific progress here, because the discussion is about a theoretical maximum knowledge.

 

[friend]

We can use the wave function to calculate the probabilities of various alternatives from some starting point to some ending point in the future. But if there are interactions along the way that we observe, then the original wave function must be updated to reflect the interaction we see. We learn more about the trajectory, and we can narrow the possible outcomes of the original alternatives. Is this not what is going on as we learn more about the past? Doesn't that tend to narrow the alternatives for the future so that the final state that it collapses to is more certain than if there were no interactions in the past of which we take account? So this tells me that history seems to be a process of a slow collapse of a wave function to the ultimate state. What does that tell us about the state to which we are headed?

 

[andrewkirk]

I have to wonder if all these smaller, individual, instantaneous wavefunction collapses are somehow a process describing a very slow collapsing of the universal wavefunction.

This is an excellent example of why 'wavefunction collapse' is such a misleading term, which we are unfortunately stuck with for historical reasons. It would be much better to call it 'eigenket selection'.

The reason it is misleading is that it implies getting smaller and more precise - that somehow there is more information than before. But there isn't. When we measure location, the range of possible locations 'collapses' to something very small, and the range of possible momenta correspondingly blows out, and vice versa. The system is as much in superposition as it was before, it's just that the widest superposition is now in a different basis - the conjugate one.

PS: I love the title of this thread!. It is sweetly conjugate to that of all those threads in GR and Cosmology that ask what the universe is expanding into.

 

[rootone]

... Some think that there is a universal wavefunction that is guiding all the individual wavefunctions...

Please elaborate
(.
develop or present (a theory, policy, or system) in further detail.
)

 

[bhobba]

That means that there are more wavefunction collapses going on.

Since collapse is only part of some interpretations the above is not correct.

Thanks
Bill

 

[bhobba]

We are certain of the past but not sure of the future. That means that as time goes on, more events enter the past. And there are more events we are certain of

That does not follow eg see eternal inflation.

Thanks
Bill

 

[bhobba]

We perhaps don't have more knowledge about the present because the total number of events is increasing permanently (entropy)

That's not entropy.

As I tried to allude in my previous post that the total number of events is increasing may not be true - even if it was of any relevance - which it isn't.

Thanks
Bill

 

[bhobba]

We can use the wave function to calculate the probabilities of various alternatives from some starting point to some ending point in the future. But if there are interactions along the way that we observe, then the original wave function must be updated to reflect the interaction we see.

You need to investigate many worlds. No collapse, no update - everything evolves deterministically.

Thanks
Bill

 

[friend]

You need to investigate many worlds. No collapse, no update - everything evolves deterministically. Bill

I don't want to get hung up on the terminology. I'm referring to the math of quantum mechanics where a measurement or interaction results in only one of the possible eigenstates actually being realized in nature. Once an event/interaction/measurement occurs, it is part of the past which we know with certainty. Before it happens it is part of the future about which we can only calculate probabilities. Can we even separate the concepts of future/past from wave function prediction/measurement? Wave function prediction refer to measurements that haven't taken place yet. And once a measurement/interaction takes place, wave function predictions are no longer relevant for that measurement.

 

[friend]

We can use the wave function to calculate the probabilities of various alternatives from some starting point to some ending point in the future. But if there are interactions along the way that we observe, then the original wave function must be updated to reflect the interaction we see. We learn more about the trajectory, and we can narrow the possible outcomes of the original alternatives. Is this not what is going on as we learn more about the past? Doesn't that tend to narrow the alternatives for the future so that the final state that it collapses to is more certain than if there were no interactions in the past of which we take account? So this tells me that history seems to be a process of a slow collapse of a wave function to the ultimate state. What does that tell us about the state to which we are headed?

Would the updated wave function have the same eigenvalues as the original wave function? Are interactions along the way telling us which of the original eigenvalues it's getting closer too? Or do interactions determine a whole new set of eigenvalues to be considered?

 

[ddd123]

Collapse isn't a permanent fixing of an observable, see the Stern-Gerlach experiment: two successive collapses of a type of observable don't add more information than a single one, and the past information on an observable can always be erased by a new collapse on a conjugate observable.

 

[atyy]

So as time goes on there are more and more interactions in the world. That means that there are more wavefunction collapses going on. Some think that there is a universal wavefunction that is guiding all the individual wavefunctions. So I have to wonder if all these smaller, individual, instantaneous wavefunction collapses are somehow a process describing a very slow collapsing of the universal wavefunction. If so, then can we tell what state the universal wavefunction is collapsing into?

There are two sorts of collapse.

(1) The standard Copenhagen-style interpretation. Here neither the wave function nor the collapse of the wave function are necessarily real, and they are just tools to calculate the probabilities of measurement outcomes. It is not known whether the wave function of the universe has any meaning. Since the wave function and collapse are not necessarily real, and there is no known meaning to the wave function of the universe, the question is meaningless.

(2) GRW or CSL. Here the wave function is real and collapse is real, and the wave function of the universe does make sense. These are not pure interpretations and eventually predict deviations from quantum mechanics. It is unknown at present whether these interpretations can explain the full range of quantum phenomena. However, there are discussions about testing these theories, eg. http://arxiv.org/abs/1410.0270. The wave function is in Hilbert space, and the collapse is in Hilbert space, but there is a link to the ordinary space we see. When the collapse occurs in Hilbert space, an event occurs in ordinary space.

 

[friend]

There are two sorts of collapse.

Actually, I'm not sure about the usability of a universal wave function. Wouldn't it have to interact with something else to measure its state. Yet there is noting outside the whole universe to interact with it. We could at best only calculate probabilities of future states, never know with certainty which state it collapses to. But if past events help at all in knowing its trajectory, maybe that helps us know where we are going.

The other issue is what portion of the probabilities do past events represent? We don't know how many total events there are in the complete destiny of the universe. If we already know with probability 1 the past events, how can a probability of 1 enter a calculation for a new probability. More events entering the past does not change the probability of 1 for them. So I'm not sure these ideas go anywhere.

Perhaps there are other constraints that would help with using these ideas. For example, maybe it's true that there is a particular total amount of information in the universe no matter what happens in the universe. Though, I'm not sure how that helps.

 

[Derek Potter]

I don't want to get hung up on the terminology. I'm referring to the math of quantum mechanics where a measurement or interaction results in only one of the possible eigenstates actually being realized in nature.

That's why bhobba says you should look at MWi. In MWi there is no collapse so your statement that "a measurement or interaction results in only one of the possible eigenstates actually being realized in nature" is not true. What is true is that in a standard statement of QM, this probabalistic aspect of measurement is simply postulated. However MWi derives or claims to derive, all the important (observable) features of the postulate from unitary, deterministic, evolution. Obviously, since there is no actual collapse and no definite outcomes in this scheme, one of MWI's claims is that it can fully account for the appearence of collapse and the appearence of definiteness.

Once an event/interaction/measurement occurs, it is part of the past which we know with certainty.

Not necessarily. If we get entangled with the system - which is inevitable - then the system and ourselves enter a superposition of outcome-states. What is more, each entangled term comprises an observer state whose observation is 100% consistent with the associated system state. But all the outcome states for the observer-plus-system remain in superposition exactly as the system states were before the interaction.

Before it happens it is part of the future about which we can only calculate probabilities. Can we even separate the concepts of future/past from wave function prediction/measurement?

If you insist on wavefunction collapse then it is irreversible and there is a true past and a true future. But since MWi and possible some other interpretations of QM, manage perfectly well without collapse, the most charitable thing one can say about it is that it is added by hand to a theory that doesn't need it.

Yes, without irreversibility there is a profound cosmological question - why does the universe appear to be time-asymmetrical? Someone here may be able to tell you whether there are any viable theories around that either manage without quantum collapse irreversibility or else rely on it.

Wave function prediction refer to measurements that haven't taken place yet. And once a measurement/interaction takes place, wave function predictions are no longer relevant for that measurement.

If you are referring to the wavefunction of the system being measured then it enters a mixed state. However the wave function you should, perhaps, be considering is that of the system and the observer together. See above.

 

[Derek Potter]

Actually, I'm not sure about the usability of a universal wave function. Wouldn't it have to interact with something else to measure its state.

No. Just factorize the state space and see what goes on "inside" the universe rather than fretting about what it looks like from the outside.

Perhaps there are other constraints that would help with using these ideas. For example, maybe it's true that there is a particular total amount of information in the universe no matter what happens in the universe. Though, I'm not sure how that helps.

Everything will grind to an abrupt halt when the information runs out.

 

[Derek Potter]

the past information on an observable can always be erased...

No, that is not right. The observation has been made and it's written down on paper. The second measurement may randomize the state with respect to the original observable, but the information is not erased, the ink marks on the paper don't suddenly disappear.

... by a new collapse on a conjugate observable

...assuming you can actually guarantee wavefunction collapse! All you can really do is make a measurement and take note that this creates a mixed state. Whether there is actual collapse is highly problematical - the mixed state can be explained in terms of entanglement without collapse of the larger system comprising you and the system you're looking at.

Indeed, if a mixed state is produced by a simple interaction it can often be reversed. Interestingly not only does this erase the second measurement it will also restore the original measured state. This is perhaps the strongest argument against irreversible collapse.

 

[friend]

That's why bhobba says you should look at MWi.

I can't accept MWi. As I understand it, universes split off when an interaction takes place and an eigenstate is selected. If an interaction forces a split, then by definition those alternate universes can no longer interact. If we cannot interact with it, then we cannot observe it. So MWi becomes an inherently unprovable hypothesis by definition. We cannot even write a theory for things we cannot interact with.

All I am relying on is the math that stipulates that eigenvalues are selected. I don't care what the interpretation is.

 

[mfb]

So MWi becomes an inherently unprovable hypothesis by definition.

That is true for all interpretations. That's why we call them "interpretations" and not "theories".

We cannot even write a theory for things we cannot interact with.

Of course we can.

Actually, we are quite sure that there are things we cannot interact with any more, even without quantum mechanics. The matter that emitted the radiation we see as cosmic microwave background today is at a distance of more than 40 billion light years today. It will never be able to interact with us any more. We saw this matter in the past, however, and there is no reason to assume that the matter magically vanished. It is still there, just not observable any more.

 

[Khashishi]

We are certain of the past but not sure of the future.

Are we certain of the past? I don't think so! There are many past states that could have evolved into our present state. Wheeler's delayed choice experiment suggests that the past isn't determined until we measure it in the present. (http://www.nature.com/nphys/journal/v11/n7/full/nphys3343.html). As far as I'm concerned, the past is random like the future; there are just fewer possibilities in the past (due to lower entropy).

 

[friend]

Are we certain of the past? I don't think so! There are many past states that could have evolved into our present state. Wheeler's delayed choice experiment suggests that the past isn't determined until we measure it in the present. (http://www.nature.com/nphys/journal/v11/n7/full/nphys3343.html). As far as I'm concerned, the past is random like the future; there are just fewer possibilities in the past (due to lower entropy).

Do wave functions propagate to the past from the future? Isn't the complex conjugate of the wave function one that propagates from the end point in the future to the starting point in the past? Both are necessary to compute the probabilities.

 

[Khashishi]

Well, Schrodinger's equation is unitary so you can propagate it in either direction. And complex conjugate is just part of the mathematics. The information in the wavefunction and the complex conjugate is the same.

 

[atyy]

Actually, I'm not sure about the usability of a universal wave function. Wouldn't it have to interact with something else to measure its state. Yet there is noting outside the whole universe to interact with it.

That's the standard Copenhagen view that I mentioned.

The other issue is what portion of the probabilities do past events represent? We don't know how many total events there are in the complete destiny of the universe. If we already know with probability 1 the past events, how can a probability of 1 enter a calculation for a new probability. More events entering the past does not change the probability of 1 for them. So I'm not sure these ideas go anywhere.

That's exactly what collapse allows one to do: calculate the probability of a measurement given the result of an earlier one.

 

[bhobba]

And once a measurement/interaction takes place, wave function predictions are no longer relevant for that measurement.

Yes - so? Its got nothing to do with for example entropy that has been mentioned previously. Its a trite observation of no actual import whatever - at least I am not aware of any.

The universe doesn't collapse into anything because the universe can't be observed which would requre something outside the universe interacting with it that by the definition of the universe is impossible.

Thanks
Bill

 

[Derek Potter]

I can't accept MWi. As I understand it, universes split off when an interaction takes place

Then you don't understand it. Universes splitting off would be massive new physics. Entangement can be formulated as relative states which can be interpreted as different worlds in the sense I just explained.

and an eigenstate is selected.

There is no selection of an eigenstate in MWI. You can put your mathematical hat on and analyse the relative states in terms of eigenstates if you want to but nature does not do anything of the kind, the system just rolls along unaware that its observing itself and not making any kind of selection. That's the whole point of MWI.
MWI is, of course, augmented by decoherence theory, and this explains a preferred basis and einselection. But this is a result of modern MWI, it is not a fundamantal supposition.

If an interaction forces a split, then by definition those alternate universes can no longer interact. If we cannot interact with it, then we cannot observe it. So MWi becomes an inherently unprovable hypothesis by definition. We cannot even write a theory for things we cannot interact with.

Well, there is no split in MWI. The popularist picture of parallel universes was, I believe, added by de Witt, but in any case is superfluous baggage - an interpretation of an interpretation that only serves to detract from the simplicity of MWI.
It is true that we cannot interact with the other worlds. However, we can certainly interact with other worlds which cannot interact with each other - and they produce effects in our world. The apparent contradiction here is that there is not just a single set of alternative worlds but many. "Inhabitants" (to pursue the metaphor) of one world in a set cannot interact with other worlds in the set. But they can and do interact with worlds in other sets. Fun for logicians - each world in a set contains all the other sets of worlds...

All I am relying on is the math that stipulates that eigenvalues are selected. I don't care what the interpretation is.

The maths says no such thing. Such selection is postulated in some formulations of QM. It can be safely dropped in MWI, added by hand or a similar result though not identical can be derived.

 

[ddd123]

No, that is not right. The observation has been made and it's written down on paper. The second measurement may randomize the state with respect to the original observable, but the information is not erased, the ink marks on the paper don't suddenly disappear.

What if you don't register it? A Stern-Gerlach apparatus can run even if no-one's looking. In that case I don't see any trace of that information anymore.

 

[Derek Potter]

What if you don't register it? A Stern-Gerlach apparatus can run even if no-one's looking. In that case I don't see any trace of that information anymore.

You may not see it but it is there, scrambled in a gazillion entangled microscopic changes in the apparatus and environment. You must distinguish between classical erasure which is just deleting (readable) data and quantum erasure which undoes *all* of the microscopic entanglements. This is why they do quantum erasure experiments with photons, photons don't interact with the environment very much so observations made with simple observers, *can*, with immense effort and care, be erased.

 

[lowkee]

Hey guys new to this forum sorry for derailing the thread, but can some of you guys suggest any books or websites on this type of stuff or quantum physics in general. Trying to get my head around this stuff so layman's terms might help. Cheers

 

[Derek Potter]

Hey guys new to this forum sorry for derailing the thread, but can some of you guys suggest any books or websites on this type of stuff or quantum physics in general. Trying to get my head around this stuff so layman's terms might help. Cheers

This *is* all in layman's terms!
Sorry, couldn't resist.

 

[friend]

The universe doesn't collapse into anything because the universe can't be observed which would requre something outside the universe interacting with it that by the definition of the universe is impossible.

Even if we can't measure anything because there is nothing outside the universe to interact with it to cause a collapse to a particular value,... If all we have is the universal wave function, that would still allow calculation of possible eigenstates and their probabilities, right? The question is: do we have enough information to derive in theory a universal wave function? If not, what do we need?

 

[bhobba]

Even if we can't measure anything because there is nothing outside the universe to interact with it to cause a collapse to a particular value,... If all we have is the universal wave function, that would still allow calculation of possible eigenstates and their probabilities, right?

The probabilities thus obtained would be meaningless.

Its an open question if the state of the universe is a valid concept.

Thanks
Bill