Question about Quantum mechanics

[randman]

So what kind of "super time" measures the "non-static" changing of the past? This seems almost like the universe would have another time coordinate in addition to the one it already has...

I don't see that as necessary but would have to think about it. Maybe I don't follow. Why would does something need to "measure"?

I guess you could say God and His eternal reference point outside space-time but that gets us out of the topic a bit.

 

[randman]

Hartle and Hawking had a scheme in which the history of the universe is the sum (Feynman path-integral style) of all compact geometries in the past that match the current geometry of the universe in the present. In some sense, the past is "formed by" the present (if they're right, anyway), though I don't think it makes sense to say that is "changed".

I'd have to hear more. If it's just 2 plus 2 equals 4, and so 4 determines 2 plus 2, then that's not so interesting.

 

[sol invictus]

Hartle and Hawking had a scheme in which the history of the universe is the sum (Feynman path-integral style) of all compact geometries in the past that match the current geometry of the universe in the present. In some sense, the past is "formed by" the present (if they're right, anyway), though I don't think it makes sense to say that is "changed".

In any path integral formulation of quantum theory, the quantum state at a given time is equal to a weighted integral over all past configurations with specified boundary conditions on the present hypersurface. That's because the path integral (with appropriate boundary conditions) is a propagator - it tells you how states evolve.

So there's nothing about the H-H prescription that differs from standard quantum theory, except that they also try to integrate over space-time metrics as well as field configurations.

 

[Vorpal]

In any path integral formulation of quantum theory, the quantum state at a given time is equal to a weighted integral over all past configurations with specified boundary conditions on the present hypersurface. That's because the path integral (with appropriate boundary conditions) is a propagator - it tells you how states evolve.

I'm not saying the math is different from standard QM. Normal propagators have a sums over histories that have a boundary condition with both an initial and final state, and this is not true in the H-H scheme, up to an extremely broad class. I thought this could be interpreted as universe history "formed by" the present, because one has a final condition only. Or is this still illegitimate?

This sort of thing seems to be a motivation for Hawking's strongly anthropic stance.

 

[sol invictus]

I'm not saying the math is different from standard QM. Normal propagators have a sums over histories that have a boundary condition with both an initial and final state, and this is not true in the H-H scheme

In some spacetime geometries no boundary condition (other than smoothness) is necessary in the past, because the spacetime rounds off smoothly.

 

[randman]

A wiki reference to Hawking that be relevant here. Note: I don't generally like wiki but just as reference point.

Stephen Hawking, along with Thomas Hertog of CERN, proposed that the universe's initial conditions consisted of a superposition of many possible initial conditions, only a small fraction of which contributed to the conditions we see today.[25] According to their theory, it is inevitable that we find our universe's "fine-tuned" physical constants, as the current universe "selects" only those past histories that led to the present conditions. In this way, top-down cosmology provides an anthropic explanation for why we find ourselves in a universe that allows matter and life, without invoking the current existence of a multiverse.

http://en.wikipedia.org/wiki/Fine-tuned_Universe

Obviously a brief quote doesn't explain it and I am not sure I know this particular theory but thought it's worth fine-tuning can mean a little more than some that dismiss realize.

Note the comment on selecting "past histories."

 

[Dancing David]

What if that concept is actually borne out to be true in science?

That'd be something.

When they demonstrate it, then we can talk about it.

 

[sol invictus]

Bohmian mechanics does away with much of the seemingly irrational aspects of quantum theory. As a layman, I find it more intuitive than the mainstream approach.

LINK

The Bohm interpretation explicitly violates locality, which makes it almost certainly inconsistent with relativity. Since special relativity (specifically, relativistic quantum field theory) is the most precisely tested idea in the history of science, that's a pretty serious problem.

And as a bit more sidetrack, would sol (or anyone else) have the math and physics smarts to know if Cramer's Transactional Interpretation violates any known constraints or conditions?

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

I've never studied it, but it looks like it either agrees with standard quantum field theory - in which case I don't see how it can be different from many worlds - or it doesn't, in which case it's almost certainly wrong.

 

[randman]

No, because no energy is required. It's not really that new universes "pop up" - it's better to think of there having been an ensemble of identical universes prior to the measurement, after which one half (or other fraction) of the ensemble diverged from the other.

So what do they do? Reduce their mass and energy in half with each division?

 

[randman]

No, it doesn't. We've discussed that at some length here before. In a nutshell, many worlds is nothing more or less than unitary evolution of the wavefunction according to the Schrodinger equation, and therefore if the interactions in the Hamitonian are local, so is the evolution.

So how does the many-worlds hypothesis explain non-locality in entanglement within just one universe (no splitting off)?

In other words, the particles are entangled regardless of distance and time. So if there is a splitting off with each collapse so to speak, that doesn't explain how they are connected in the first place.

 

[Vorpal]

So how does the many-worlds hypothesis explain non-locality in entanglement within just one universe (no splitting off)?

In other words, the particles are entangled regardless of distance and time. So if there is a splitting off with each collapse so to speak, that doesn't explain how they are connected in the first place.

The state of an isolated system evolves according to the Schrödinger equation for some Hamiltonian. MWI simply claims that the universe is an isolated system (a kind of obvious claim, but one that's inconsistent with Copenhagen interpretation under the assumptions that the universe is both quantum-mechanical and we're capable of making measurements in it), and has decoherence to simulate state collapse due to measurement instead. Therefore, MWI it is just as local (or fails to be) as any isolated physical system is (or fails to be), and since the standard model is local, there's that.

MWI's "worlds" are the projections of the state onto some particular subspaces of the Hilbert space it lives in, and to say that worlds "branch" is just another way of saying that the state evolved to have components along those particular axes.

The whole "universe"/"multiverse" business in some explanations of MWI is a bit wooly. Everywhere else in QM, an isolated system is one physical system with some (possibly complicated) state. That's exactly what it still is in MWI. There's no reason to call projections to some subspaces "universes".

As to your last question, they're "connected" because the state is still in the same Hilbert space. In fact, worlds can "merge", which is a fancy way of interpreting the case in which the state evolves to longer have components along the relevant subspaces.

 

[randman]

As to your last question, they're "connected" because the state is still in the same Hilbert space. In fact, worlds can "merge", which is a fancy way of interpreting the case in which the state evolves to longer have components along the relevant subspaces.

I am talking about the implications of that. 2 particles become entangled and so the connection between them is independent of distance and time (outside space-time really). Since this is observed, there is no need for the MWI.

In other words, entanglement happens regardless of MWI. It's not dependent on it, and entanglement explains what MWI. MWI is not observed but entanglement is. So why is MWI considered needed in the first place?

It's redundant. Could be true but doesn't mean entangled particles are not connected outside space-time (independent of distance and time).

 

[sol invictus]

So what do they do? Reduce their mass and energy in half with each division?

Again, it's best not to think of there being any "divisions". Think of starting with an infinite ensemble, some members of which go in one direction and some in the other. There is no change in the total mass or energy of any of them at any time.

So how does the many-worlds hypothesis explain non-locality in entanglement within just one universe (no splitting off)?

There isn't really any non-locality in entanglement. Non-locality only shows up when you add something to QM, like a collapse postulate (a la Copenhagen) or insist on determinism (a la Bohm).

I am talking about the implications of that. 2 particles become entangled and so the connection between them is independent of distance and time (outside space-time really).

That they are entangled simply means they are correlated. The classical analog would be to produce (say) an electron and a positron. No matter how far apart they get in space or time, if one is an electron, the other is a positron. The difference in QM is that you can produce a state that is electron at location A and positron at location B plus electron at location B and positron and location A. That's true in all interpretations of QM - they differ only in what happens when you perform a measurement on such a state.

Since this is observed, there is no need for the MWI.

It's not that there's a "need" for MWI. The MWI is a theory about the world, one that's consistent with all data, and (in my view) it's the simplest such theory. But the other interpretations are (mostly) also consistent with data, so in that sense you don't "need" the MWI.

 

[randman]

Well, let me try to clarify a little based on my limited understanding. I am open to some ideas on MWI or a many-minds interpretation. I just don't think it preserves causality and locality in the same way.

Take the 2-slit experiment. You measure to see what way the photon went and so it travels in particle-like. It seems to know before you measure it that you are going to and so takes a particular path.

The MWI is that it still takes a wave-like path as well, but not in our universe but in another and so tries to explain away how it seems to know the measurement. That's my understanding at least.

The problem is in the other universe, either the people there would have to not be measuring for a which-way path, or in that universe, when they measure for the which-way path, it travels like a wave. It'd be the opposite of how quantum mechanics works here. The more you try to determine the path, the less we can know of it, and when you don't measure to determine the path, the path would be clear.

That begs the question. If the path is clear, isn't that so because it was measured? This idea doesn't work so we're left with the idea that the people there are not measuring the path.

Well, that means every time a photon takes a which-way path, there has to be a whole different universe with people acting differently. They have to be there already or perhaps the argument is there are no people there at all. Regardless, it has to preexist the collapse. The collapse doesn't cause it. As you say, there is no split. So MWI is more of a theory that all possible universes exist in all likelihood.

There are those making that argument, but if so, then universes exist where causality and locality are violated, and so why not this one? MWI does not really solve the problem though it could be true.

On the entanglement issue, I don't think you are doing it justice. The delayed-choice quantum eraser and related experiments suggest photons can collapse into a particle-like which-way path and then back again, and the entangled particles do the same.

So in MWI, are we able to switch between universes back and forth? Plus, the connection is still independent of space and time. Even under MWI, the entangled particles react to the measurement of those not entangled. Maybe the argument is the universe, multiverse reacts or rather preexists, but there is still a mechanism independent of distance and space involved, and so there is still a violation of locality and likely linear causality too.

If you are in one universe where the photon is particle-like, but you erase that and make it wave-like, then you have switched from one past into an alternate past. MWI may say, no, the alternate past was already there, but from our vantage point, we are changing the past, and in our universe then we can change the past, which violates causality.

So MWI just does not preserve locality and causality. If it preserves locality, it violates causality. Zeilinger, I believe, makes this point in one of his papers.

I think whatever the mechanism is of entanglement, independent of MWI, explains the 2-slit experiment better. The photon travels like a wave at all points until measurement. Maybe the photon's various positions can be seen as all those positions being entangled with itself. So when one position is determined, all the others are likewise determined.

I have also wondered and maybe you can help me. But if something travels the speed of light, wouldn't it from it's vantage point experience 0 time? So all of the photon's positions and trajectory exist in a now-state. So of course, if there is a change, the photon in what seems to be the past to us is just responding in the present without any time from it's vantage point elapsing.

If so, quantum computers might one day utilize time the past and future in it's calculations rather than just the present.

 

[sol invictus]

Well, let me try to clarify a little based on my limited understanding. I am open to some ideas on MWI or a many-minds interpretation. I just don't think it preserves causality and locality in the same way.

It does - unlike any other interpretation of QM. That's why I like it.

Take the 2-slit experiment. You measure to see what way the photon went and so it travels in particle-like. It seems to know before you measure it that you are going to and so takes a particular path.

The MWI is that it still takes a wave-like path as well, but not in our universe but in another and so tries to explain away how it seems to know the measurement. That's my understanding at least.

That's not really accurate. According to QM in any interpretation, before the measurement the photon's wavefunction is spread across both paths. If you like, you can think of there being two worlds, one where the photon follows one path and one where it follows the other.

According to the most vanilla interpretation (Copenhagen), when you measure (at the slit) you force the photon to "choose". More formally, according to Copenhagen the effect of the measurement is that you (randomly) project onto one or the other of the two worlds, throw away the other, and re-normalize the one you keep.

So measurement is the source of non-determinism in the standard interpretation. It's also the source of non-locality (not in that example, but it would be in another where there is entanglement over a distance).

According to many worlds, the measurement in one world finds that the photon went through one slit, and in the other that it went through the other slit. No non-determinism, and no non-locality.

Well, that means every time a photon takes a which-way path, there has to be a whole different universe with people acting differently. They have to be there already or perhaps the argument is there are no people there at all. Regardless, it has to preexist the collapse. The collapse doesn't cause it. As you say, there is no split. So MWI is more of a theory that all possible universes exist in all likelihood.

That's more or less correct, with the emphasis on all possible universes. Impossible universes - like those that violate the laws of physics - aren't there.

There are those making that argument, but if so, then universes exist where causality and locality are violated, and so why not this one?

None of them violate causality or locality, because those are part of the laws of physics.

On the entanglement issue, I don't think you are doing it justice. The delayed-choice quantum eraser and related experiments suggest photons can collapse into a particle-like which-way path and then back again, and the entangled particles do the same.

I'm quite familiar with those experiments. The MWI gives one a way to understand very clearly what is happening in them and what they indicate - they're actually much less confusing thought of through its lens. Again, there is nothing non-local (or even non-deterministic) in any of them - if you're willing to give up the idea that there is only one "world".

So in MWI, are we able to switch between universes back and forth? Plus, the connection is still independent of space and time. Even under MWI, the entangled particles react to the measurement of those not entangled. Maybe the argument is the universe, multiverse reacts or rather preexists, but there is still a mechanism independent of distance and space involved, and so there is still a violation of locality and likely linear causality too.

Nope. Be a little more specific about which experiment and which measurements, and I'll show you why not.

So MWI just does not preserve locality and causality. If it preserves locality, it violates causality. Zeilinger, I believe, makes this point in one of his papers.

Again, no. It quite explicitly violates neither. It cannot, because it follows directly from local relativistic quantum field theory.

 

[randman]

None of them violate causality or locality, because those are part of the laws of physics.

That remains to be seen. Certainly physicists like Zeilinger and others argue otherwise.

 

[randman]

Here's one of the experiments. Apologize for using wiki but it's handy. You can assess the paper from the citation. I wouldn't, of course, rely on the wiki claims in the article to be correct.

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

The earlier quantum eraser experiment actually shows what I am talking about just fine but this one adds a twist I found interesting.

I will try to look up a more formal explanation of the ideas on MWI that I ran across that might do a more complete job here.

I also still don't see how entanglement is not a violation of locality in MWI.

Lastly, isn't the standard idea of QM not that the photon exists actually in all possible locations as much as it exists as a potential or a possible for one of those paths but it's not actually in "discrete form." That's what I've read from quantum researchers but fell back to an older description of the 2-slit experiment and now that I think about it, might have to go back and consider it all again.

But some quantum physics researchers of note clearly say things, Wheeler does for example, that a very basic concept in quantum physics is the particle has no "discrete form" until observation (measurement as we're discussing since that can be a loaded word and separate concept).

 

[randman]

This isn't the exact paper I was looking for but it's worth noting Zeiinger's work showing a violation of local realism is also leading to potential real-world applications.

Besides its significance in tests of quantum mechanics versus local realism, the methods developed in the experiment also have many useful applications in the field of quantum information.

http://www.drchinese.com/David/Bell-MultiPhotonGHZ.pdf

I suppose you would say the violation of local realism is only illusory due to MWI, but I still don't see how that is. Working with the belief of entanglement as a violation of local realism is producing results.

I know local realism is not the same as locality per se and I am trying to get my head around it completely but isn't the basis of the experiment the use of entanglement as a non-local system? And violating local realism seems more of an upending of what some think of as "the laws of physics" than just locality.

Maybe you can explain it to me better? How can local realism and locality be preserved?

Here's a quote from a media source. Still looking for another paper I read though the one linked to may do.

They found that, just as in the realizations of Bell's thought experiment, Leggett's inequality is violated – thus stressing the quantum-mechanical assertion that reality does not exist when we're not observing it. "Our study shows that 'just' giving up the concept of locality would not be enough to obtain a more complete description of quantum mechanics," Aspelmeyer told Physics Web. "You would also have to give up certain intuitive features of realism."

http://physicsworld.com/cws/article/news/27640

I think it's clear they think both local realism and locality cannot be preserved.

.

 

[randman]

Ok, doing some more thinking on your post. So in the 2-slit experiment, when we choose to measure the which-way path, we see the path and all the other paths are taken in other universes (presumably with people there also measuring the particle right?).

So since the particle collapses, it does so in all the universes and there is not a universe with someone running the experiment without trying to measure the which-way path so it travels in a wave-like pattern. The interference pattern is gone for the whole multi-verse.

Is that it?

If not, how can an interference pattern emerge since the collapse so to speak has occurred? Can the people running the experiment in some universe without detecting the which-way path exist? If they did, they'd be observing the particle take all the paths in the multiverse and creating an interference pattern and so I don't see how that helps MWI solve the problem.

Hope that's right?

Now here's the tricky part. You'd have to people running the same experiment and determining the which-way path and so a huge number of alternate universes with presumably the same person essentially in each of them. This suggests the act of choosing here to measure the photon one way or another then is indeed causal to whether these universes exist, hence the split.

But the idea as I understand it is that those universes are already there prior to someone making the choice, which suggests a violation of causality in some sense. Either way, the universe or multiverse seems to know the choice will be made just as the photon seems to know that. The problem then remains the same unless one says the choice causes the split.

if that's the case, how and why? Why would making a choice to observe or measure something cause a split? That seems to get into the concept of consciousness being causal to which universe one is in. Otherwise, why would just a mechanical apparatus being set up that can tell which-way a particle travels cause a new alternate universe. What's the mechanism there?

Now, of course, the MWI could be true, but it doesn't seem to answer the problems initially discovered in QM.

Moreover, it suggests by choosing something, we can make a choice about the past universe we will be in. From out reference, the past universe is still something partly dictated by present choices. MWI may well say that alternate universe has always been there but by choosing whether to measure the which-way path or not, we choose whether to either create or create the possibility of existing in an alternate universe. Of course, we wouldn't notice anything different, but it's still a present action that affects the past.

If we choose to not measure the which-way path, we have chosen to live in a universe where the particle, in the past, travelled as a wave producing an interference problem.

If we choose to measure the which-way path, we choose to live in a universe with a past where the particle took a specific path.

Either way, from our vantage point, the present choice determines which past history we live in!

That doesn't mean the multiverse is not correct. It just doesn't preserve in our universe causality and local realism, although locality may just be more a matter of our perspective, but even then conceiving of locality being violated would still work. You'd just have to say the way the multiverse works allows us from our vantage point to work with systems, say in entanglement, that appear non-local.

In entanglement, you could say the available options are all there, but either way, by choosing to act in a certain way, there is a non-local effect from our vantage point.

 

[Perpetual Student]

The Bohm interpretation explicitly violates locality, which makes it almost certainly inconsistent with relativity. Since special relativity (specifically, relativistic quantum field theory) is the most precisely tested idea in the history of science, that's a pretty serious problem.

Why would violation of locality make it inconsistent with special relativity? That is not one of the several "objections" discussed in the link I provided nor do I find anything about that objection elsewhere.