Why do two people see the same experimental results?

[wmikewells]

Before quantum mechanics hit the scene, I would expect most physicists (and people) would have answered the above question with the basic assumption:

Two observers see the same result in an experiment because we live in the same universe.

Today, most people (and some physicists) would probably still agree with that answer. However, with all the interpretations of quantum mechanics, I would guess that there would be additional responses from physicists such as but not limited to:

1. The question is outside the scope of legitimate questions that physics can answer.
2. Observers don't live in the same universe (for example, each possible outcome creates a new universe).
3. Observers don't observe the same thing (for example, each observation is an imperfect approximation of a mathematical universe).

So my questions are:

• Am I correct that this assumption is now "up for grabs" and being questioned?
• If so, are the above responses an accurate summary of current responses?
• Are there any other responses besides the ones above?
• Has anyone attempted to experimentally show a difference in observations (response 3 above)?

I have no agenda with these questions. I am just trying to wrap my head around quantum mechanics and its corresponding interpretations.

 

[craigi]

Before quantum mechanics hit the scene, I would expect most physicists (and people) would have answered the above question with the basic assumption:

Two observers see the same result in an experiment because we live in the same universe.

Today, most people (and some physicists) would probably still agree with that answer. However, with all the interpretations of quantum mechanics, I would guess that there would be additional responses from physicists such as but not limited to:

1. The question is outside the scope of legitimate questions that physics can answer.
2. Observers don't live in the same universe (for example, each possible outcome creates a new universe).
3. Observers don't observe the same thing (for example, each observation is an imperfect approximation of a mathematical universe).

So my questions are:

• Am I correct that this assumption is now "up for grabs" and being questioned?
• If so, are the above responses an accurate summary of current responses?
• Are there any other responses besides the ones above?
• Has anyone attempted to experimentally show a difference in observations (response 3 above)?

I have no agenda with these questions. I am just trying to wrap my head around quantum mechanics and its corresponding interpretations.

There is another answer, but I'd rather phrase it in the form of a question.

What would it be like to live in a universe where others have completely different experiences? If you were to ask them what their experiences were would they even hear your question? Would you see them? How would you know they were there?

 

[wmikewells]

There is another answer, but I'd rather phrase it in the form of a question.

What would it be like to live in a universe where others have different experiences? If you were to ask them what their experiences were would they even hear your question?

Thank you for the question.

I don't think a single universe where two observers see different things and those measurements cause different results is possible. I would guess that there is some fundamental law or mechanism that prevents it. However, I don't think the assumption can stand on its own without explanation or exploration, especially with the proposition of multiverses and parallel universes.

I may be wrong in seeing that a current direction of quantum mechanics is to push beyond a "single universe" to explain quantum mechanical features. If so, let me know. Or if you have another interpretation, I would be interested in reading about it.

 

[naima]

There is no problem if they do not interact. they may see what they want. When they meet and interact they always agree.
Maybe something occurs during this interaction.
It is the point of view of Rovelli.

 

[craigi]

Thank you for the question.

I don't think a single universe where two observers see different things and those measurements cause different results is possible. I would guess that there is some fundamental law or mechanism that prevents it. However, I don't think the assumption can stand on its own without explanation or exploration, especially with the proposition of multiverses and parallel universes.

I may be wrong in seeing that a current direction of quantum mechanics is to push beyond a "single universe" to explain quantum mechanical features. If so, let me know. Or if you have another interpretation, I would be interested in reading about it.

I wouldn't say that there's a current push beyond a single universe coming from a desire to interpret QM. Everett proposed the Many Worlds Interpretation over 50 years ago. The current interest in the idea of a multiverse seems to be coming from cosmology.

 

[wmikewells]

There is no problem if they do not interact. they may see what they want. When they meet and interact they always agree.
Maybe something occurs during this interaction.
It is the point of view of Rovelli.

Interesting. Thanks.

 

[UltrafastPED]

Before quantum mechanics hit the scene, I would expect most physicists (and people) would have answered the above question ...

The "interpretation" of QM makes no difference: everybody calculates the same set of probabilities for the results of a well-defined experiment.

Thus if you make enough trials your results should should differ by statistically insignificant amounts from each other. If this is not the case, somebody has a problem with their setup/instrumentation/etc.

 

[wmikewells]

I wouldn't say that there's a current push beyond a single universe coming from a desire to interpret QM. Everett proposed the Many Worlds Interpretation over 50 years ago. The current interest in the idea of a multiverse seems to be coming from cosmology.

I guess some are still extending a theoretical multiverse theory (Tagmark's Our Mathematical Universe) based on extra dimensions instead of positional distance. I have not read it yet, but that was the basis for response 3 in my original post (if I understood a synopsis of the book correctly).

 

[wmikewells]

The "interpretation" of QM makes no difference: everybody calculates the same set of probabilities for the results of a well-defined experiment.

Thus if you make enough trials your results should should differ by statistically insignificant amounts from each other. If this is not the case, somebody has a problem with their setup/instrumentation/etc.

I may have misused the word "interpretation". I was using it in the sense of "Copenhagen interpretation", which is one explanation of what happens when we take a measurement. I was not referring to the actual experimental results or the mathematics that predict the experimental results.

 

[bhobba]

Am I correct that this assumption is now "up for grabs" and being questioned?

Popularizations can give the impression that conciousness causes collapse is a mainstream interpretation that physicists worry about. It isn't. Most interpretations such as Copenhagen, Ensemble, DBB etc assume the existence of a world out there independent of observation. This conciousness causes collapse view never held much of a sway and now is very much a backwater view since one of the high priests of it, Wigner, abandoned it when he heard about some early work of Zureck on Decoherence; without going into the detail of how Decoherence throws a spanner in the works of the reason for conciousness being involved in the first place.

Basically what you have nutted out is one reason why that interpretation is downright crazy anyway, and part of the reason its very backwater.

The modern view is this. Decocerence leads to what is known as effective collapse which occurs independent of human conciousness etc. If you consider the collapse actual, and their is simply no way to tell the difference between actual and effective (apparent is another word used), then all these issues simply disappear.

If you want to pursue the issue further check out (see section 3.2):
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Thanks
Bill

 

[bhobba]

I may have misused the word "interpretation". I was using it in the sense of "Copenhagen interpretation", which is one explanation of what happens when we take a measurement. I was not referring to the actual experimental results or the mathematics that predict the experimental results.

Actually Copenhagen doesn't explain what happens when a measurement occurs. It simply assumes the existence of a classical common-sense everyday world and we know about quantum systems by measurements, observations etc etc that appear in that world. It also has a particular view of a quantum state along the lines of the Baysean view of probabilities that distinguishes it from similar interpretations like the ensemble interpretation:
http://en.wikipedia.org/wiki/Copenhagen_interpretation

Within and of itself its a perfectly valid interpretation that pretty much solves all issues. It's 'blemish' however (note the careful use of the word blemish - it does not disprove Copenhagen) is how does a theory about things like measurement etc that appear in an assumed classical world explain that classical world?

Much progress has been made in solving that but some issues remain and research is ongoing.

Thanks
Bill

 

[wmikewells]

Popularizations can give the impression that consciousness causes collapse is a mainstream interpretation that physicists worry about. ...

The modern view is this. Decoherence leads to what is known as effective collapse which occurs independent of human consciousness etc. If you consider the collapse actual, and their is simply no way to tell the difference between actual and effective (apparent is another word used), then all these issues simply disappear.

If you want to pursue the issue further check out (see section 3.2):
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Thanks
Bill

Thanks for the reference. I am wading in the shallows, and this looks to be over my head, but not in the deep end. So I will get as much as I can from it.

 

[wmikewells]

Actually Copenhagen doesn't explain what happens when a measurement occurs. It simply assumes the existence of a classical common-sense everyday world and we know about quantum systems by measurements, observations etc etc that appear in that world. It also has a particular view of a quantum state along the lines of the Baysean view of probabilities that distinguishes it from similar interpretations like the ensemble interpretation:
http://en.wikipedia.org/wiki/Copenhagen_interpretation

Within and of itself its a perfectly valid interpretation that pretty much solves all issues. It's 'blemish' however (note the careful use of the word blemish - it does not disprove Copenhagen) is how does a theory about things like measurement etc that appear in an assumed classical world explain that classical world?

Much progress has been made in solving that but some issues remain and research is ongoing.

Thanks
Bill

Again thanks for the reference. This is much more accessible. Wikipedia is all over the map in terms of its articles and the level of expertise each requires.

I hope the blemish does not turn into a sore that turns into an infection.

 

[craigi]

Thank you for the question.

I don't think a single universe where two observers see different things and those measurements cause different results is possible. I would guess that there is some fundamental law or mechanism that prevents it.

I'm not sure if you noticed this, but you actually answered your own question here (which was the purpose of my question). If the only possible universe, (for an observer to be in, at least), is one where observers see the same results then it should be no surprise to find yourself in such a universe. As for a fundamental law, it doesn't get more fundamental than this. It's just a simple piece of logic. It might, at first, seem like a circular argument but it's not.

You don't even need physics for it. It's firmly rooted in metaphysics and no physical discovery or model can change it. I'm sure if you trawl the history of metaphysics, you'll find a very similar argument, attributed to philiosophers from long ago, including theologians.

If you're interested in this line of reasoning, then you should read about the Anthropic Principle:
http://en.wikipedia.org/wiki/Anthropic_principle

There is a degree of controvesy associated with it. It's accepted amongst many high profile physicists, including Susskind and Tegmark, for example, though I've seen Turok and Smolin become quite agitated when asked to discuss it. They seem to share the view that it has nothing to contribute to the field of physics, though I haven't yet fully understood why they hold this view.

In my opinion, many questions that we currently consider unanswerable, will have accepted answers following this principle.

 

[naima]

The modern view is this. Decocerence leads to what is known as effective collapse which occurs independent of human conciousness etc. If you consider the collapse actual, and their is simply no way to tell the difference between actual and effective (apparent is another word used), then all these issues simply disappear.

If you want to pursue the issue further check out (see section 3.2):
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Thanks
Bill

I found no "effective collapse" and no "apparent collapse" in this link.
What is it?

 

[naima]

The modern view is this. Decocerence leads to what is known as effective collapse which occurs independent of human conciousness etc. If you consider the collapse actual, and their is simply no way to tell the difference between actual and effective (apparent is another word used), then all these issues simply disappear.

If you want to pursue the issue further check out (see section 3.2):
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Thanks
Bill

I found no "effective collapse" and no "apparent collapse" in this link.
What is it?

 

[bhobba]

I found no "effective collapse" and no "apparent collapse" in this link.
What is it?

Decoherence transforms a superposition into an improper mixed state. If it was a proper mixed state collapse would have occurred. But it isn't. There is no way to tell the difference, and that is what is meant by effective or apparent.

See section 1.2.3 for a discussion.

Thanks
Bill

 

[craigi]

I guess some are still extending a theoretical multiverse theory (Tagmark's Our Mathematical Universe) based on extra dimensions instead of positional distance. I have not read it yet, but that was the basis for response 3 in my original post (if I understood a synopsis of the book correctly).

I haven't read it either, but Tegmark's ideas are exactly the kind of thinking I'm alluding to. I really should read it, but I fear it would kill the fun.

I suspect he argues that in an infinite universe all the laws of nature need to do is facilitate correlation and all that we observe can in principle, be explained with statistics and probability theory.

 

[naima]

Decoherence transforms a superposition into an improper mixed state. If it was a proper mixed state collapse would have occurred. But it isn't. There is no way to tell the difference, and that is what is meant by effective or apparent.

I can undestand that collapse can give a mixed state.
Are there measurements that give as an outcome a mixed state (POVM?) if yes can you give examples?

 

[wmikewells]

There is a degree of controversy associated with it. It's accepted amongst many high profile physicists, including Susskind and Tegmark, for example, though I've seen Turok and Smolin become quite agitated when asked to discuss it. They seem to share the view that it has nothing to contribute to the field of physics, though I haven't yet fully understood why they hold this view.

In my opinion, many questions that we currently consider unanswerable, will have accepted answers following this principle.

Thanks for not force feeding me dogma.

If you open a can of worms, best be prepared to eat them. I guess that is why I asked my original questions. I was wondering if a can of worms was truly opened and how the mess was being addressed. It sounds like there is not really that much of a mess.

I would agree that certain assumptions have to be made and accepted (either implicitly or explicitly) in order to even do physics (or any science). If not, nothing would get done.

However, I would probably fall into the "renegade" camp that maintains that things can get interesting when assumptions are questioned for a good reason. Just look at the assumption that "time is constant". Where would we be if that was not questioned.

Even though questioning assumptions can be productive, I am also not ready to discard a single universe that we share and that is independent of observers. I would think that the following points at the very least would have to addressed (not that I am an expert) before taking such a huge step.

1. There are rules to define a universe, its boundaries, and how it behaves.
2. There exists well defined exceptions to a single universe
3. There is causality between universes
4. There is an explanation for why two observers see the same thing
5. There is an explanation for how causality is maintained when two observers do not see the same thing (if possible)
6. There is a well defined physical explanation for consciousness

That seems like a tall order. I guess the current assumptions will be kept for the foreseeable future.

 

[craigi]

Thanks for not force feeding me dogma.

If you open a can of worms, best be prepared to eat them. I guess that is why I asked my original questions. I was wondering if a can of worms was truly opened and how the mess was being addressed. It sounds like there is not really that much of a mess.

I would agree that certain assumptions have to be made and accepted (either implicitly or explicitly) in order to even do physics (or any science). If not, nothing would get done.

However, I would probably fall into the "renegade" camp that maintains that things can get interesting when assumptions are questioned for a good reason. Just look at the assumption that "time is constant". Where would we be if that was not questioned.

Even though questioning assumptions can be productive, I am also not ready to discard a single universe that we share and that is independent of observers. I would think that the following points at the very least would have to addressed (not that I am an expert) before taking such a huge step.

1. There are rules to define a universe, its boundaries, and how it behaves.
2. There exists well defined exceptions to a single universe
3. There is causality between universes
4. There is an explanation for why two observers see the same thing
5. There is an explanation for how causality is maintained when two observers do not see the same thing (if possible)
6. There is a well defined physical explanation for consciousness

That seems like a tall order. I guess the current assumptions will be kept for the foreseeable future.

I think they can be answered consistently and I'd love to offer answers to them but I'd be crossing the line into personal theory, which is in breach of forum rules.

I can't answer them with any certaintly from the perspetives of Tegmark or Susskind, but I'm sure that they go a long way towards answering them in their books.

 

[bhobba]

I can undestand that collapse can give a mixed state.
Are there measurements that give as an outcome a mixed state (POVM?) if yes can you give examples?

A POVM is a different concept to a mixed state. Its a concept associated with generalised measurement theory and generalises the usual Von Neuman measurements based on resolutions of the identity and the spectral theorem:
http://red.csie.ntu.edu.tw/QC/peng/slides3.pdf

If you want to investigate POVM's further, like I said before, its used in generalised measurement theory - its not an area I am really into though.

You may be asking is there measurements that are a proper mixed state. Of course - but they would be very contrived eg you measure a quantum system someone has randomly selected for you.

Thanks
Bill

 

[atyy]

A POVM is a different concept to a mixed state. Its a concept associated with generalised measurement theory and generalises the usual Von Neuman measurements based on resolutions of the identity and the spectral theorem:
http://red.csie.ntu.edu.tw/QC/peng/slides3.pdf

If you want to investigate POVM's further, like I said before, its used in generalised measurement theory - its not an area I am really into though.

You may be asking is there measurements that are a proper mixed state. Of course - but they would be very contrived eg you measure a quantum system someone has randomly selected for you.

Thanks
Bill

Conceptually, a POVM is a little related to a improper mixed states. An improper mixed state is when the entire system is pure, and we just see a part of the system. Similarly, a POVM can be imagined that one did a projective measurement on the entire system, but one only sees a part of the system.

 

[bhobba]

There are rules to define a universe, its boundaries, and how it behaves.

Well first you have to show there is more than one universe. That such exist is far from certain.

There is an explanation for why two observers see the same thing

That's easy - what exists out there is independent of us. The issue comes from those that doubt it for philosophical discussion. That's fine if you like that sort of thing - but as you probably have guessed its not my bag, and I don' t think its really what most physicists are into either.

There is a well defined physical explanation for consciousness

That's really something for cognitive scientists - not physicists.

Thanks
Bill

 

[bhobba]

An improper mixed state is when the entire system is pure, and we just see a part of the system.

Sorry Atyy - don't see that at all.

You will need to give the detail.

The second bit about POVM's being projections of resolutions of the identity in higher dimensional space is well known of course.

Thanks
Bill

 

[atyy]

Sorry Atyy - don't see that at all.

You will need to give the detail.

The second bit about POVM's being projections of resolutions of the identity in higher dimensional space is well known of course.

Thanks
Bill

I just mean that the improper mixed state is the reduced density matrix of the subsystem.

 

[craigi]

That's really something for cognitive scientists - not physicists.

I differ on this.

We don't need to go to the lengths of Von Neumann, Wigner or Penrose to see that it's impossible to completely remove ourselves from the system that we're observing, on both a theoretical and experimental level.

The role of consciousness in our models of nature is something that many, if not all, highly respected theoretical physicsts are currently taking seriously, while considering it to be nothing more than the product of the constituent elementary particles.

I think a good start for this subject is the Fine Tuning problem.

 

[bhobba]

We don't need to go to the lengths of Von Neumann, Wigner or Penrose to see that it's impossible to completely remove ourselves from the system that we're observing, on both a theoretical and experimental level.

Can't follow you there. Take bog standard Copenhagen - it assumes exactly that ie the world out there and observations exist independent of an observer..

But this is really getting into philosophy territory and is not suitable for these forums.

Thanks
Bill

 

[wmikewells]

I'm not sure if you noticed this, but you actually answered your own question here (which was the purpose of my question). If the only possible universe, (for an observer to be in, at least), is one where observers see the same results then it should be no surprise to find yourself in such a universe. As for a fundamental law, it doesn't get more fundamental than this. It's just a simple piece of logic. It might, at first, seem like a circular argument but it's not.

You don't even need physics for it. It's firmly rooted in metaphysics and no physical discovery or model can change it. I'm sure if you trawl the history of metaphysics, you'll find a very similar argument, attributed to philosophers from long ago, including theologians.

If you're interested in this line of reasoning, then you should read about the Anthropic Principle:
http://en.wikipedia.org/wiki/Anthropic_principle

Alright, Socrates. I fell into your trap.

Sorry, but I am going to have to side with Smolin on the antrhopic principle. I cannot agree that the anthropic principle is sufficient to establish the assumption that two observers see the same thing because of a single universe. I think it has more to do with how humans reason and how we come up with primitive objects.

For example, if we ask Albert and Max to determine if there is a kookaburra sitting in the old gum tree, there are four possible determinations:

Albert determines the kookaburra is in the tree
Albert determines the kookaburra is not in the tree
Max determines the kookaburra is in the tree
Max determines the kookaburra is not in the tree

In our universe, the only combinations we observe are 1 & 3 and 2 & 4. Consequently, we negate 2 & 3 and 1 & 4. Using simple logic from the negation, we create the rules "1 if and only if 3" and "2 if and only if 4". A baby intuitively does this. Just consistently and secretly switch on a light when your 9 month old raises her hand in her crib (or does some other cute thing).

What gets interesting is when we ask why we don't observe 2 & 3 (and 1 & 4). That is the realm for adult humans (and scientists). We can't prove 2 & 3 will never happen using observation because we would have to be immortal to do so. If we can deduce the negation of 2 & 3 from other first principles, we have our rule. However, if we cannot make a deduction, we posit the existence of objects with certain properties to get the negation. Hence, we have things like Newton's gravity and our universe (not that they have the same efficacy or that there is something fallacious in doing so). It is a necessary starting point.

Which brings me to my mistrust of the anthropic principle (sorry for the long-winded discourse). Primitive objects are temporary resting places. They are subject to evolution and revolution. I would not be surprised if the next revolution in physics (quantum gravity?) messed with our current notion of the universe. So, to say "the universe appears shared and independent to humans because it really is" tends to eliminate the possibility of change. Please let me know if I interpreted your invocation of the anthropic principle correctly (although I don't think that is what the anthropic principle says).

So, our universe is shareable and independent. Could humans observe in a universe that was not shareable or that was dependent on observation? Possibly but I cannot conceive what it would be like to live in it.

 

[wmikewells]

Well first you have to show there is more than one universe. That such exist is far from certain.

That's easy - what exists out there is independent of us. The issue comes from those that doubt it for philosophical discussion. That's fine if you like that sort of thing - but as you probably have guessed its not my bag, and I don' t think its really what most physicists are into either.

That's really something for cognitive scientists - not physicists.

Thanks for giving me a long leash. I'll try not to abuse it.

I listed those points not because I am a renegade when it comes to the "two observers - one universe" assumption. I still believe in that assumption. It is just the challenge I think a physicist (or anyone) would face if he or she challenged that premise. It would force a physicist to answer a lot of non-physics type questions.

 

[craigi]

Alright, Socrates. I fell into your trap.

I'm aware of the Socratic method, but my intent wasn't to use it to convince you of the answer to your question. I don't consider it to be a particularly fair method of dialogue. It just turns out that some points are easier to make as questions.

Which brings me to my mistrust of the anthropic principle (sorry for the long-winded discourse). Primitive objects are temporary resting places. They are subject to evolution and revolution. I would not be surprised if the next revolution in physics (quantum gravity?) messed with our current notion of the universe. So, to say "the universe appears shared and independent to humans because it really is" tends to eliminate the possibility of change. Please let me know if I interpreted your invocation of the anthropic principle correctly (although I don't think that is what the anthropic principle says).

As, I think you've already guessed, you've misinterpretation of the Anthropic Principle. You've actually interpreted it as tautology and you're certainly not the first to do so.

I listed those points not because I am a renegade when it comes to the "two observers - one universe" assumption.

It's not clear why you consider that physics makes this assumption. I can think of no laws in physics that treat this axiomatically.

The term "observers" is most prominently used in Special Relatively, which predicts that observers don't actually see the same thing.

It has also been used in Quantum Physics, but as Bill painstakingly points out on this forum, many writers deliberatlely avoid this term, to avoid confusion. That said, QM does still leave the concept of objective realism, as opposed to subjective realism, open to interpretation. That is to say that the QM formalism is agnostic about whether 2 observers see the same thing.

 

[Khashishi]

You and I see the same results of an experiment because the version of You that I see must be consistent with the experimental result which I also see.

 

[craigi]

You and I see the same results of an experiment because the version of You that I see must be consistent with the experimental result which I also see.

I'd say that this is correct, but interpretation specific. It sounds like a mixture of CH and MWI terminology to me.

We could ask why is this the case and under both interpretations, decoherence provides at least a partial answer.

 

[wmikewells]

As, I think you've already guessed, you've misinterpretation of the Anthropic Principle. You've actually interpreted it as tautology and you're certainly not the first to do so.

What form of the Anthropic Principle do you invoke and why did you invoke it?

 

[bhobba]

You and I see the same results of an experiment because the version of You that I see must be consistent with the experimental result which I also see.

This is one of the issues here.

Feynman has commented on this sort of thing. Most certainly everyday experience indicates the world is like that ie observations are the same for any observer, but science does not DEMAND it is like that.

Some people say things must be such and such for science to even exist - scientists however are generally a bit less dogmatic on such things. I know I am.

I am very anti conciousness causes collapse because of its many attendant problems - but a valid scientific theory it certainly is. The world may be like that with its very weird subjectivism - but that is not what science is about - weirdness is not the deciding factor.

Thanks
Bill