The Wave Function Of The Universe

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The discussion centers on the question of what caused the collapse of the universe's wave function, particularly in the context of different interpretations of quantum mechanics. It explores whether the universe can have a wave function under standard quantum mechanics, with the Copenhagen interpretation suggesting that a classical measurement is necessary for collapse. Participants highlight that decoherence may only account for apparent collapse, while the Many Worlds interpretation (MWI) posits that the wave function evolves without collapse. The conversation also touches on the implications of inflation and quantum tunneling in the universe's formation, questioning whether a wave function for the universe is meaningful. Ultimately, the debate reflects ongoing complexities and uncertainties in understanding the quantum state of the universe.
Crazy4Physics
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This may be a bizarre question, but if the entire universe has a wave function, and wave function collapse for real (unlike in Bohmian mechanics or the Many Worlds interpretation), then what caused the collapse of the wave function of the universe? It must have been collapsed before we came about, the universe didn't just exist as a quantum superposition. Wouldn't this presuppose there is some external classical system, collapsing the quantum system? If so, what collapsed that classical system?! This seems to be a problem for me, and I am having a hard time understanding it.

What collapsed the wave function of the universe?
 
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Well we know decoherence causes apparent collapse.

The universe itself doesn't have to collapse - simply stuff inside it - and decoherence seems to handle that pretty well.

But there is zero doubt MW is the most elegant interpretation as far as the wavefunction of the entire universe is concerned.

Thanks
Bill
 
bhobba said:
Well we know decoherence causes apparent collapse.

The universe itself doesn't have to collapse - simply stuff inside it - and decoherence seems to handle that pretty well.

But there is zero doubt MW is the most elegant interpretation as far as the wavefunction of the entire universe is concerned.

Thanks
Bill

"Well we know decoherence causes apparent collapse."

Yes, but by an outside system; what was outside the universe to collapse its wave-function? Also, if it is only "apparent", then this means that something like Bohmian mechanics, or the MW is true. If I am not mistaken, in standard (the Copenhagen interpretation) Quantum Mechanics; the wave-function does collapse. Can dechoherence account for real collapse, or just apparent collapse?

"The universe itself doesn't have to collapse - simply stuff inside it - and decoherence seems to handle that pretty well."

Well, if the universe has a wave-function; it must have collapsed. It didn't just exist as a superposition until the point when we came along.

"But there is zero doubt MW is the most elegant interpretation as far as the wavefunction of the entire universe is concerned."

How so? It seems to me that the MW interpretation comes with too much baggage.
 
Collapse -or its appearance- happens when a measurement is performed on a system. By definition, no measurement can be performed on the whole universe. So if it makes sense to assign a state to the universe as a whole, this state never collapses. It simply evolves according to Schrödinger's equation.

Edit: The question is - does it make sense to assign a state to the whole universe? This is a matter of interpretation. The MWI and the Bohmian interpretation say yes, the Copenhagen interpretation says no.
 
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Crazy4Physics said:
Well, if the universe has a wave-function; it must have collapsed. It didn't just exist as a superposition until the point when we came along.

Cant follow that at all. In MW the wavefunction simply evolves - no collapse - nothing. Other interpretations have a bit of an issue - but not MW.

If you consider inflation it started with the wavefunction of the false vacuum that via quantum tunneling started our universe. It grew to our current universe - no observation of the entire universe as far as I can see was required.

Crazy4Physics said:
How so? It seems to me that the MW interpretation comes with too much baggage.

The issue here isn't the baggage MW comes with - its applying it to the whole universe. No collapse - no collapse issues at all with MW. Yes you can debate MW on other fronts - but the wavefunction of the entire universe trivial for it.

Thanks
Bill
 
kith said:
Collapse -or its appearance- happens when a measurement is performed on a system. By definition, no measurement can be performed on the whole universe. So if it makes sense to assign a state to the universe as a whole, this state never collapses. It simply evolves according to Schrödinger's equation.

Edit: The question is - does it make sense to assign a state to the whole universe? This is a matter of interpretation. The MWI and the Bohmian interpretation say yes, the Copenhagen interpretation says no.

Are you saying that if standard Quantum Mechanics (The Copenhagen interpretation) is true, then the universe cannot have a wave-function?

What about objective collapse theories, like GRW or Penrose?
 
kith said:
The question is - does it make sense to assign a state to the whole universe? This is a matter of interpretation. The MWI and the Bohmian interpretation say yes, the Copenhagen interpretation says no.

That I agree with.

Thanks
Bill
 
bhobba said:
Cant follow that at all. In MW the wavefunction simply evolves - no collapse - nothing. Other interpretations have a bit of an issue - but not MW.

Ya, I'm talking about standard QM here; the Copenhagen interpretation.

bhobba said:
If you consider inflation it started with the wavefunction of the false vacuum that via quantum tunneling started our universe. It grew to our current universe - no observation of the entire universe as far as I can see was required.

The universe tunelled into being? From what?! Also, if nothing collapsed the universe's wave-function, then it would still exist as a superposition.

bhobba said:
The issue here isn't the baggage MW comes with - its applying it to the whole universe. No collapse - no collapse issues at all with MW. Yes you can debate MW on other fronts - but the wavefunction of the entire universe trivial for it.

Thanks
Bill

I'm talking about standard QM Though. If The Copenhagen interpretation is true, then something must have collapsed the universe's wave-function (unless the universe doesn't have a wave-function under standard QM, which I am not sure of).
 
Crazy4Physics said:
Are you saying that if standard Quantum Mechanics (The Copenhagen interpretation) is true, then the universe cannot have a wave-function?

I think he is talking about, in interpretations like the Ensemble interpretation, and Copenhagen, a state is associated with a state preparation procedure - which most of the time is really an observation in another guise.

You will find a discussion of this interesting issue in Chapter 9 of Ballentine - QM - A Modern Development.

Thanks
Bill
 
  • #10
Crazy4Physics said:
Ya, I'm talking about standard QM here; the Copenhagen interpretation.

Then that has an issue - a state is a state preparation procedure - what prepared the state of the entire universe?

Crazy4Physics said:
The universe tunelled into being? From what?! Also, if nothing collapsed the universe's wave-function, then it would still exist as a superposition.

The false vacuum. In MW no collapse ever occurs - superposition's remain in superposition's. But that's not an issue - a superposition is simply another state - big deal.

Crazy4Physics said:
I'm talking about standard QM Though. If The Copenhagen interpretation is true, then something must have collapsed the universe's wave-function (unless the universe doesn't have a wave-function under standard QM, which I am not sure of).

Copenhagen has issues. As far as I know those into applying QM to the entire universe use MW.

Thanks
Bill
 
  • #11
Crazy4Physics said:
Also, if it is only "apparent", then this means that something like Bohmian mechanics, or the MW is true. If I am not mistaken, in standard (the Copenhagen interpretation) Quantum Mechanics; the wave-function does collapse. Can dechoherence account for real collapse, or just apparent collapse?

This is getting a bit off-topic and has been addressed in many threads devoted to it so what I am going to briefly outline is not the whole story of this controversial issue.

To understand apparent collapse you need to understand the concept of proper and improper mixed states:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Now what decoherence does is transform a superposition into an improper mixed state Ʃpi |bi><bi|, the |bi><bi| being the possible outcomes of the observation.

This has exactly the same form, and is observationally indistinguishable from, a proper mixed state. A proper mixed state is the state |bi><bi| randomly presented to be observed. If it was a proper mixed state measurement problem solved - no actual collapse occurred and what was there was revealed by measurement.

The debate however is if that's good enough. We will not reach a conclusion on that in this thread. Best to read the link I gave above and form you own view.

Thanks
Bill
 
  • #12
Crazy4Physics said:
what caused the collapse of the wave function of the universe? It must have been collapsed before we came about, the universe didn't just exist as a quantum superposition. Wouldn't this presuppose there is some external classical system, collapsing the quantum system? If so, what collapsed that classical system?! This seems to be a problem for me, nd I am having a hard time understanding it.

What collapsed the wave function of the universe?

Your questions might be at/beyond the edge of our knowledge, that said...

The wave-function of the universe does not have to collapse. It can change shape, and form, continually.

Wave-functions can break into smaller waves or join to form larger waves.

Super-positions and entanglements continue to exist -- even when there is an apparent "collapse" of the wavefunction . They simply change form/partners.

It's a question of semantics on what one considers as "collapse".

The change in shape, form etc, of the wave-function, might/could have been triggered by the uncertainty/random fluctuations.

As objects get bigger the effect of their overall wave-functions gets diminished.
 
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  • #13
Crazy4Physics said:
This may be a bizarre question, but if the entire universe has a wave function, and wave function collapse for real (unlike in Bohmian mechanics or the Many Worlds interpretation), then what caused the collapse of the wave function of the universe? It must have been collapsed before we came about, the universe didn't just exist as a quantum superposition. Wouldn't this presuppose there is some external classical system, collapsing the quantum system? If so, what collapsed that classical system?! This seems to be a problem for me, and I am having a hard time understanding it.

What collapsed the wave function of the universe?

There's also the possibility in quantum theory that the universe isn't described by a wave-function (a pure state) at all, but rather just a density matrix (a mixed state) and the time-evolution isn't given by a Schrödinger equation, but rather by a Lindblad equation or even by a non-Markovian generalization of it. There needn't be a "surrounding" theory such that this density matrix comes from tracing over some degrees of freedom of a pure state (contrary to what happens in decoherence). (That's not just a crazy idea, but could actually happen in black hole evaporation for example as pointed out by Unruh and Wald.)
 
  • #14
Crazy4Physics said:
I'm talking about standard QM Though. If The Copenhagen interpretation is true, then something must have collapsed the universe's wave-function (unless the universe doesn't have a wave-function under standard QM, which I am not sure of).

In "standard QM", eg. Landau & Lifshitz, there cannot be a wave function of the universe, because a classical apparatus is needed to collapse the wave function. In L&L's view, QM is not a complete theory, because it requires classical measuring devices.

I once asked a question about a similar topic, and had helpful answers from Demystifier and martinbn: https://www.physicsforums.com/showthread.php?t=667996 .
 
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  • #15
atyy said:
In "standard QM", eg. Landau & Lifshitz, there cannot be a wave function of the universe, because a classical apparatus is needed to collapse the wave function.

I don't get it (not that it's problematic - it certainly is - but cannot I think is a bit strong). There is no theory of the early universe I am aware of that requires the universe to be observed - so that it of course can't be observed is not an issue in that sense. As explained below its a bit strange considering what the wavefunction is in Copenhagen, but I don't think it's actually ruled out.

I certainly agree applying the concept of state to the entire universe in the usual interpretations like Ensemble or Copenhagen is highly problematical because they associate a state with a preparation procedure.

In Copenhagen the state encodes the results of observations if it was to be observed - and since it can't be observed one has to question how it can have meaning. However, logically that doesn't preclude it existing - you do not have to observe a quantum system for it to have a state. Still its a bit of a fine point in logic, and since a state in those interpretations require some kind of preparation its highly problematical for sure. But I don't think it's completely ruled out.

MWI however handles it with ease, and most guys I have read that talk about the wavefunction of the universe seem to adhere to that.

Thanks
Bill
 
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  • #16
Crazy4Physics said:
This may be a bizarre question, but if the entire universe has a wave function, and wave function collapse for real (unlike in Bohmian mechanics or the Many Worlds interpretation), then what caused the collapse of the wave function of the universe? It must have been collapsed before we came about, the universe didn't just exist as a quantum superposition. Wouldn't this presuppose there is some external classical system, collapsing the quantum system? If so, what collapsed that classical system?! This seems to be a problem for me, and I am having a hard time understanding it.

What collapsed the wave function of the universe?
Those who prefer some variant of the orthodox (Copenhagen) interpretation often believe that the enire universe does NOT have a wave function. According to such a view, wave function is only a tool useful for description of microscopic systems, and wave-function collapse is not real.
 
  • #17
Crazy4Physics said:
Also, if nothing collapsed the universe's wave-function, then it would still exist as a superposition.
You seem to have a misconception about the concept of superposition. Being a superposition is not a property of the state. A particle which has a sharply localized position is at the same time in a superposition of momentum states. Whether you use a superposition description or not depends whether you are going to measure the position or the momentum.

In QM, a state is a vector in Hilbert space. It is your choice of basis which decides whether you look at it as a superposition or not. This choice is arbitrary until something singles out a certain basis. This happens in measurements, where the final states need to be eigenstates of the physical quantity you are measuring.
 
  • #18
bhobba said:
I don't get it (not that it's problematic - it certainly is - but cannot I think is a bit strong). There is no theory of the early universe I am aware of that requires the universe to be observed - so that it of course can't be observed is not an issue in that sense. As explained below its a bit strange considering what the wavefunction is in Copenhagen, but I don't think it's actually ruled out.

Perhaps "cannot" is indeed a bit strong. However, we do observe the universe and it doesn't seem to be in a superposition, but we do have a quantum theory which seems to match observations as to how cosmic microwave background fluctuations came about. So if we collapsed the wave function, then we would be excluded from the wave function as the classical apparatus.
 
  • #19
I think we should differentiate between standard QM and the standard interpretation of QM. Standard QM is simply QM as it is now. It seems standard QM has become a synonym of the phrase "standard interpretation of QM" in this thread.
 
  • #20
Demystifier said:
Those who prefer some variant of the orthodox (Copenhagen) interpretation often believe that the enire universe does NOT have a wave function. According to such a view, wave function is only a tool useful for description of microscopic systems, and wave-function collapse is not real.

I thought wave function collapse was real under Copenhagen?
 
  • #21
Crazy4Physics said:
This may be a bizarre question, but if the entire universe has a wave function, and wave function collapse for real (unlike in Bohmian mechanics or the Many Worlds interpretation), then what caused the collapse of the wave function of the universe? It must have been collapsed before we came about, the universe didn't just exist as a quantum superposition. Wouldn't this presuppose there is some external classical system, collapsing the quantum system? If so, what collapsed that classical system?! This seems to be a problem for me, and I am having a hard time understanding it.
What collapsed the wave function of the universe?

not a bizarre question, is an usual question in quantum cosmology.

Wave function of the Universe
http://prd.aps.org/abstract/PRD/v28/i12/p2960_1

"The quantum state of a spatially closed universe can be described by a wave function which is a functional on the geometries of compact three-manifolds and on the values of the matter fields on these manifolds. The wave function obeys the Wheeler-DeWitt second-order functional differential equation."
 
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  • #22
Crazy4Physics said:
I thought wave function collapse was real under Copenhagen?

There are a few variants and in some it is.

The most common view however is as detailed in the Wikipedia article:
http://en.wikipedia.org/wiki/Copenhagen_interpretation

'The Copenhagen Interpretation denies that the wave function is anything more than a theoretical concept, or is at least non-committal about its being a discrete entity or a discernible component of some discrete entity.

The subjective view, that the wave function is merely a mathematical tool for calculating the probabilities in a specific experiment, has some similarities to the Ensemble interpretation in that it takes probabilities to be the essence of the quantum state, but unlike the ensemble interpretation, it takes these probabilities to be perfectly applicable to single experimental outcomes, as it interprets them in terms of subjective probability.

There are some who say that there are objective variants of the Copenhagen Interpretation that allow for a "real" wave function, but it is questionable whether that view is really consistent with some of Bohr's statements. Bohr emphasized that science is concerned with predictions of the outcomes of experiments, and that any additional propositions offered are not scientific but meta-physical. Bohr was heavily influenced by positivism. On the other hand, Bohr and Heisenberg were not in complete agreement, and they held different views at different times. Heisenberg in particular was prompted to move towards realism.

Even if the wave function is not regarded as real, there is still a divide between those who treat it as definitely and entirely subjective, and those who are non-committal or agnostic about the subject. An example of the agnostic view is given by Carl Friedrich von Weizsäcker, who, while participating in a colloquium at Cambridge, denied that the Copenhagen interpretation asserted "What cannot be observed does not exist." He suggested instead that the Copenhagen interpretation follows the principle "What is observed certainly exists; about what is not observed we are still free to make suitable assumptions. We use that freedom to avoid paradoxes.'

If the wavefunction is simply a subjective mathematical tool to calculate probabilities like the Bayesian view of probabilities then that it collapses is of zero concern. Only if you consider it in some sense real is it an issue.

Its actually quite similar to the Ensemble interpretation, I personally hold to, which is more akin to the frequentest view of probability - Copenhagen is more Bayesian like:
http://en.wikipedia.org/wiki/Ensemble_interpretation

Thanks
Bill
 
  • #23
I'm trying to understand some things that have been told... but I feel lost...
First of all, the collapse does not happen when a HUMAN makes the measurement...The measurement can be done anytime by the nature itself... The particles don't understand a human is looking at them, they just interact with some matter (a photon let us say) and their "wavefunction" collapses...
 
  • #24
A clear exposition of a modern Copenhagen interpretation with the view that:

- there is no such thing as wave function collapse because the wave function is not real
- there is no such thing as a wave function of the universe

is given in: http://arxiv.org/abs/1308.5290

In no way do I believe this settles all questions around quantum theory, but it does seem to me a particularly clear exposition of a modern "Bohr style" interpretation. In this author's view:

"Is there not the one largest system, the universe, with
the physical system of interest as a small part of it? Yes,
there is the universe, but no-one has a wave function for
the universe, and there is no wave function of the universe."
 
  • #25
PAllen said:
"Is there not the one largest system, the universe, with
the physical system of interest as a small part of it? Yes,
there is the universe, but no-one has a wave function for
the universe, and there is no wave function of the universe."

This sounds very much like, "If the barber only shaves those who don't shave themselves, who shaves the barber?"
 
  • #26
bahamagreen said:
This sounds very much like, "If the barber only shaves those who don't shave themselves, who shaves the barber?"

No, as Bhobba suggests, wave function of the universe implies the universe is a pure prepared state (in modern Copanhagen interpretations). If you disagree with that proposition, then you can't assume the universe has a wave function.
 
  • #27
bahamagreen said:
This sounds very much like, "If the barber only shaves those who don't shave themselves, who shaves the barber?"

other barber.
 
  • #28
Crazy4Physics said:
This may be a bizarre question, but if the entire universe has a wave function, and wave function collapse for real (unlike in Bohmian mechanics or the Many Worlds interpretation), then what caused the collapse of the wave function of the universe? It must have been collapsed before we came about, the universe didn't just exist as a quantum superposition. Wouldn't this presuppose there is some external classical system, collapsing the quantum system? If so, what collapsed that classical system?! This seems to be a problem for me, and I am having a hard time understanding it.

What collapsed the wave function of the universe?

rembember all the propaganda, "all is quantum""


.
 
  • #29
PAllen said:
No, as Bhobba suggests, wave function of the universe implies the universe is a pure prepared state (in modern Copanhagen interpretations). If you disagree with that proposition, then you can't assume the universe has a wave function.

Exactly. I don't think it is totally ruled out - but it most certainly seems at odds with it.

To lay my cards on the table I don't think you can have a wave function of the universe within Copenhagen - you need something like MW, and I believe Decoherent Histories can also accommodate it, but I am not expert enough in it to know for sure.

Thanks
Bill
 

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