Can superposition be tested by gravity?

[cryptist]

Can we test superposition by gravitational effects of particles? For example by measuring gravitational interaction effects of electrons in double slit experiment?

I know that such a weak interaction cannot be tested at this time since experimental devices cannot measure such a small interactions. But is it possible in the future?

And what we would expect? Two gravitational fields for an electron in double slit? (One is from left slit electron, one is from right slit electron, because of superposition)

 

[Vanadium 50]

It's been done with neutrons. Maybe 30 years ago.

 

[cryptist]

Can you give any link of an article, please? Do you know the results? Since superposition principle has been disputed today, then there must be no concrete results?

 

[alxm]

Since superposition principle has been disputed today, then there must be no concrete results?

Who disputes the superposition principle, and in what sense?

The superposition principle is a broad mathematical concept that applies to all linear systems and linear differential equations, e.g. the classical wave equation. I don't see how that would be subject to experimental verification or disputation. It would certainly be surprising to learn that overtones of a classical wave shouldn't occur at the same time. I don't really see how anyone could dispute that the linear Schrödinger equation is, in fact, linear - and therefore that the superposition principle applies to it. I don't know that anyone's disputing the S.E. itself, and I don't believe it's logically/mathematically possible for the S.E. to be true if the superposition principle is not.

I can only assume you mean disputing various interpretations of what these superpositions are supposed to represent physically. As anything related to the wave function's physical interpretation, that's still largely an open question as far as I know, or we wouldn't be calling them 'interpretations'.

 

[cryptist]

Yes, I am talking about different interpretations of the quantum theory. There is no consensus on one interpretation, and there is no consensus on superposition principle. Most physicists just accept it as a mathematical fact and not arguing its physical meaning. I think there might be alternative ideas instead of superposition. Anyway, can you give a referance that proves "Superposition of states is true, and this is the experimental verification of that." I think no one can. And I am asking about its testing with gravity.

So, can anyone show me any article about that, if it was done?

 

[Galap]

Heres a link to a neutron article. Not the best article, but it was what I could dig up.

http://physicsworld.com/cws/article/news/3525

 

[cryptist]

But it is not testing superposition. Revealing quantum effects and finding particles on well defined certain states does not mean there is superposition before the measurement. Since, quantized quantum effects and superposition are different things. Is there any argument that says we detect the gravitational effect of one particle in two different locations in this experiment?

 

[Vanadium 50]

one particle in two different locations

That's not superposition, and that's not what QM says. In QM, you will never measure a particle being in two different locations at the same time. The closest you will come is with a large number of identically-prepared systems you might find a particle at one position some fraction of the time and another position some other fraction of the time.

 

[cryptist]

Ok, I know we can't measure a particle being in two different locations at the same time, since measurement causes collapse of wavefunction and destroy superposition states to one eigenstate. But actually I am talking about before the measurement process. (Superposition happens in that process right?) Is there any way to test superposition without disturbing the particle's superposition states? After all I couldn't understand how one can test superposition otherwise?

 

[Vanadium 50]

I think you have a misunderstanding of what superposition is, and what measurement is. "Telling where a particle is before the measurement process" is nonsensical, since the "telling where the particle is" is itself measurement.

 

[yoda jedi]

Yes, I am talking about different interpretations of the quantum theory. There is no consensus on one interpretation, and there is no consensus on superposition principle. Most physicists just accept it as a mathematical fact and not arguing its physical meaning. I think there might be alternative ideas instead of superposition. Anyway, can you give a referance that proves "Superposition of states is true, and this is the experimental verification of that." I think no one can. And I am asking about its testing with gravity.

So, can anyone show me any article about that, if it was done?

gravity can broke superposition (collapse the wave function) on some models
lke Penrose Interpretation (Roger Penrose) not tested yet and Self Gravity (Tejinder Sinhg) ongoing experiment.
but superposition exist, is just broken on some models or theories (Objective Collapses and Nonlinear Quantum Mechanics)

 

[cryptist]

Quantum superposition refers to the quantum mechanical property of a particle to occupy all of its possible quantum states simultaneously. (the first line from http://en.wikipedia.org/wiki/Quantum_superposition)

In measurement process we have two parts. Before the measurement and after the measurement. (Since collapse is isntantaneous there is no during measurement) It is clear that because of the wave function collapse, after the measurement a system cannot be in superposition. So superposition state must be before the measurement. So, to test superposition principle and to look whether particle is in its all possible states or not, we have to look its before the measurement behaviour, haven't we? How can we test superposition in any oyher way?

 

[Vanadium 50]

"Where a particle" is something which requires a measurement. Asking "where is it when we aren't measuring it" is an impossible question.

 

[cryptist]

"Where a particle" is something which requires a measurement.

Because, before the measurement particle in superposition, right? But where do we know that? What if there is no superposition? Is there an evidence of that?

Asking "where is it when we aren't measuring it" is an impossible question.

According to the Copenhagen Interpretation. But it is an interpretation, so it is not a rule or fact. If superposition tested by experiments, then I guess there would have not been any other interpretations.

 

[yoda jedi]

...well
there is an ad hoc introduction of probabilities to explain quantum measurement, but now i have no time to delve in it, but i will come again.

 

[Vanadium 50]

According to the Copenhagen Interpretation.

This has nothing to do with any interpretation or even QM.

Suppose I have a theory. The Washington Monument is only in Washington when "someone is looking at it" - i.e. if there is some measurement of it's position, whether by someone looking at it, or by its casting a shadow, or gravity, or seeing it on television, or whatever - it's in DC. Otherwise it instantly teleports to Mars, returning the instant someone starts looking at it again. I maintain that this theory is not only unfalsified, but also unfalsifiable.

A statement on where an object is when we are not measuring its position is not science, since we can only test a theory of where an object is by measuring it's position. Otherwise we quickly find ourselves wrapped in logical knots like "where it is when we don't know where it is"

 

[cryptist]

As far as I know, in de Broglie-Bohm interpretation there is no superposition principle and the trajectory of the particle is deterministic also before the measurement.

If a theory is unfalsifiable, then it is also unverifiable. I think we are talking about assumptions here. Not observed facts.

As a result, I guess there is no evidence of superposition and there will not be in the future also. (Since it is an untouchable process) However, still I am wondering how gravity will behave under superposition states of particles.

 

[Vanadium 50]

We have plenty of evidence for superposition. However, what you seem to be calling superposition is not what physicists call superposition.

 

[cryptist]

We have plenty of evidence for superposition.

If there is really an evidence, one evidence would be fine I think. Can you give a link?

However, what you seem to be calling superposition is not what physicists call superposition.

What is the difference?

 

[Vanadium 50]

We already explained what the difference is. See the messages in response to #7,

 

[chronon]

The double slit experiment is evidence for superposition. If you send one particle at a time through slits then the fact that you get an interference pattern indicates that the particle went through both of the slits.

In general if you do a measurement to see where the particle is, then you will find it in a definite position. If you do a measurement to see what sort of superposition it is in (which tends to be harder), then you will see it in a superposition.

 

[ZapperZ]

As far as I know, in de Broglie-Bohm interpretation there is no superposition principle and the trajectory of the particle is deterministic also before the measurement.

If a theory is unfalsifiable, then it is also unverifiable. I think we are talking about assumptions here. Not observed facts.

As a result, I guess there is no evidence of superposition and there will not be in the future also. (Since it is an untouchable process) However, still I am wondering how gravity will behave under superposition states of particles.

Do a search, in this forum, of the Delft/Stony Brook SQUID experiments, which I've talked about a gazillion times. While you are doing that, do a google search on "bonding-antibonding state" that we commonly observe in chemistry and solid-state physics.

Superposition is one of the earliest well-established phenomenon in QM, which was why Schrodinger decided to illustrate how weird it is using his cat analogy. There are plenty of empirical evidence for it. What people are trying to do now (and this is true with the Delft/Stony Brook experiments) is no longer trying to verify it experimentally, but trying to extend the SIZE of the entity that is in a superposition. The Stony Brook experiment, for example, has managed to create an entity consisting of 10^11 particles in a superposition of states!

Why you would want this to be tested "by gravity" is something I'm clueless about.

Zz.

 

[yoda jedi]

maybe at 10^{15} atoms or more, gravity can collapse superposition but in any case exist superposition at lower magnitudes. between other experiments, has been proposed an experiment, called FELIX (Free-orbit Experiment with Laser Interfometry X-Rays) to test the idea.

 

[ZapperZ]

maybe at 10^{15} atoms or more, gravity can collapse superposition but in any case exist superposition at lower magnitudes. between other experiments, has been proposed an experiment, called FELIX (Free-orbit Experiment with Laser Interfometry X-Rays) to test the idea.

But isn't this rather off-topic? The question isn't whether superposition can be collapsed via gravity, a la Roger Penrose's suggestion. The topic is whether (i) superposition has been empirically verified and (ii) it can be "tested by gravity", not collapsed by gravity.

Zz.

 

[yoda jedi]

But isn't this rather off-topic? The question isn't whether superposition can be collapsed via gravity, a la Roger Penrose's suggestion. The topic is whether (i) superposition has been empirically verified and (ii) it can be "tested by gravity", not collapsed by gravity.

Zz.

if we admit its existence...

 

[ZapperZ]

if we admit its existence...

... then the sky is blue.

Zz.

 

[cryptist]

I don't think we understand each other, maybe I cannot explain it very well.

Superposition of states is before the measurement, right?

Can we talk about a superposition of any system of states after the measurement? I think we cannot, since there is collapse or decoherence after the measurement. This is what all quantum mechanics books says.

Then, I am saying that if measurement causes the collapse of wave function, and without a measurement we couldn't know the state of any system, how we can test its superposition of states before our measurement? How could it be possible?

 

[tom.stoer]

Superposition of states is before the measurement, right?

...

Then, I am saying that if measurement causes the collapse of wave function, and without a measurement we couldn't know the state of any system, how we can test its superposition of states before our measurement?

Every theory introduces different entities, some of them are physically measurable, some of them not. Superposition is not measurable physically as in the very end what you detect is not a superposition of particle states but a single particle in a detector at a certain location.

There was a debate between Heisenberg and Einstein whether it is possible to construct a theory based on physically measurable entities. Heisenberg's attempt was to eliminate not only orbits from quantum mechanics, but everything that is not measurable in principle. Einstein denied this possibility and introduced the idea that it's just the other way round: instead using measurable entities for constructing the theory he proposed to let the theory "decide" which entity is measurable or not.

I think that has been done since with great success. Quantum mechanics and even quantum field theory (and GR as well) are based on mathematical entities that are not subject to physical experiments directly. But the theories do provide some rules which entities have a chance of being measured in principle.

The question is this: if you have too equally well testable / verifiable / falsifiable (but not falsified!) theories, which one is better? According to Ockhams razor scientists do believe that the simpler theory "with less overhead" is to be preferred (of course it's sometimes a matter of taste what "simpler" means).

Regarding quantum mechanics there is a mathematical apparatus which can partially be related to physical experiments and which can sometimes be interpreted ontologically ("a particle is a ..."). Now we have to be careful how to ask the question:

a) can a quantum system be in a superposition of different classical states? (can a quantum particle be in two different locations in a superposition before being measured?)
b) shall a quantum system be described by a superposition od states?

Question a) is the ontological version and - to be honest - nobody can answer this; it is interesting philosophically but irrelevant physically

Question b) is the phenomenological version; the answer is "yes, as long as the result agrees with experiment". From that time on it's a matter of taste if you believe in the superposition or if you try to construct a different theory w/o superposition but based on something else.

The majority of physicists believe in superpositions b/c the resulting theory is rather successfull (it is e.g. possible to explain the writing of this post on my laptop and the sending to the server by this theory) and b/c it explains a huge number of facts rather "naturally".

Different interpretations (even Copenhagen and decoherence are different interpretations; and decoherence is not fully sufficient w/o many-worlds) based on equivalent formalisms are identically physical. All these interpretations rely on the superposition principle as it is a well known fact in the classical world (electromagnetism) and b/c it can be transported easily into the quantum mechanical formalism.

I hope this clarifies why most physicists decided to believe in superposition.

Regarding evidence: nearly all qm calculations and experiments over the last 80years rely on superposition; there is not one single experimental result indicating that something is wrong with this principle.

 

[cryptist]

At last! Thank you tom.stoer for your answer and explanation.

So, superposition was not tested directly and also could not be. (at least in the near future) However, all experiments works with superposition principle very well. Ok.

it is a well known fact in the classical world (electromagnetism)

But what do you mean by that?

 

[ZapperZ]

I don't think we understand each other, maybe I cannot explain it very well.

Superposition of states is before the measurement, right?

Can we talk about a superposition of any system of states after the measurement? I think we cannot, since there is collapse or decoherence after the measurement. This is what all quantum mechanics books says.

Then, I am saying that if measurement causes the collapse of wave function, and without a measurement we couldn't know the state of any system, how we can test its superposition of states before our measurement? How could it be possible?

You need to learn a bit of quantum mechanics, especially on what it means when two operators do not commute.

If I make a measurement represented by an observable A, and B is another observable that does not commute with A, all the superposition of the system represented by the observable B are STILL preserved! No "collapse" is done here.

You ought to read those papers first and figure out how they were able to detect the superposition phenomena. Baring that, read the press releases, at the very least!

http://physicsworld.com/cws/article/news/2815
http://physicsworld.com/cws/article/print/525
http://physicsworld.com/cws/article/news/42019

Zz.

 

[cryptist]

You need to learn a bit of quantum mechanics

:) I know quantum mechanics. (at least as undergraduate level) But I'm realist and I believe we do not know too much about the laws of the universe ;)

 

[ZapperZ]

:) I know quantum mechanics. (at least as undergraduate level) But I'm realist and I believe we do not know too much about the laws of the universe ;)

But one must know SOME things to be able to discuss things. If you lack certain parts of it, how are you going to understand the phenomenon? This is such a case in point. You were already focused on a misunderstanding that a measurement "collapses" everything, when in fact, this is not the case. It is not a coincidence that the commutation relationship between observables, i.e. [A,B] is often called the "First Quantization".

Presumably, since that was the ONLY thing that you quoted out of my post, that you've finally accepted not only the info that I gave you, but also have learned about the various experiments producing results in favor of the reality of superposition.

Zz.

 

[Demystifier]

Asking "where is it when we aren't measuring it" is an impossible question.

There are no impossible questions; there are only questions that cannot be answered by given epistemological tools. But questions that cannot be answered by one epistemological tool perhaps can be answered by another epistemological tool. In particular, questions that cannot be answered by a measurement (e.g., where is the Sun during the night) can be answered by a theory (e.g., Newton theory of gravity). The mistake that some scientists make is to assume that measurement is the ONLY epistemological tool. But it is not! Therefore, "where is it when we aren't measuring it" is a question that CAN be answered - by a theory.

 

[cryptist]

you've finally accepted

In fact I never said I do or do not believe superposition or your information. I just questioned its testability, and you gave some links of articles. Thank you very much about that.

 

[yoda jedi]

Every theory introduces different entities, some of them are physically measurable, some of them not. Superposition is not measurable physically as in the very end what you detect is not a superposition of particle states but a single particle in a detector at a certain location...

congratulations. very incisive.

Regarding quantum mechanics there is a mathematical apparatus which can partially be related to physical experiments and which can sometimes be interpreted ontologically* ("a particle is a ..."). Now we have to be careful how to ask the question:

a) can a quantum system be in a superposition of different classical states? (can a quantum particle be in two different locations in a superposition before being measured?)b) shall a quantum system be described by a superposition od states?

Question a) is the ontological version and - to be honest - nobody can answer this; it is interesting philosophically but irrelevant physically

Question b) is the phenomenological version; the answer is "yes, as long as the result agrees with experiment". From that time on it's a matter of taste if you believe in the superposition or if you try to construct a different theory w/o superposition but based on something else.

The majority of physicists believe in superpositions b/c the resulting theory is rather successfull (it is e.g. possible to explain the writing of this post on my laptop and the sending to the server by this theory) and b/c it explains a huge number of facts rather "naturally".

*maybe


Ontological Vs Epistemic, Theory

For you cryptist a good read:

Einstein, incompleteness, and the epistemic view of quantum states
http://arxiv.org/PS_cache/arxiv/pdf/0706/0706.2661v1.pdf

:) I know quantum mechanics. (at least as undergraduate level) But I'm realist and I believe we do not know too much about the laws of the universe ;)

me too.

I believe we do not know too much about the laws of the universe ;)

a lot ! we have to learn a lot.