6 ms·

> By applying a magnetic field, this spin points up, down, or simultaneously points both up and down to form a qubit. Is there any real physical evidence that

> By applying a magnetic field, this spin points up, down, or simultaneously points both up and down to form a qubit. Is there any real physical evidence that an electron can be both states at the same time, outside the mathematical framework of QM?

Florin_Andrei 10y ago
That whole thinking - that states are well defined, and can be only either one or the other - that's the classic world approximation, it's not reality. You firmly believe in it because your crucial early years, when you were crawling on the floor at your parents' house, were spent in a classic-world-approximation universe. That's why it's so hard for you (and pretty much everyone else) to let go of it. The quantum phenomena are the more fundamental reality. This classic universe that you think you understand is just an epiphenomenon. Just foam floating on the ocean. The fundamental reality of the electron is the wave function. States, and everything else, emerge from it. It's not a corpuscle. It's not a wave. It does not behave like ping-pong balls, or like waves in a pond, although it shares a few characteristics. Fundamentally, it's something different. The only way to know that entirely different thing is via the mathematics of quantum mechanics - and whatever intuition you can derive from it after you do the math. There is no equivalent for that stuff at the human scale of things.
- paulddraper 10y ago
  Very much this. This should be prologue to any introduction QM book/class/whatever. --- Building on this idea, "an electron can be both states at the same time" isn't necessarily the right way to think about it. I mean, it's sort of right, but relies on an intuitive understand of what the heck that even means. And QM reality is grounded not in our human intuition, but in the math of wave functions. If we associate "state up" with a wave function concentrated one way and "state down" concentrated the other way and "both states" as a mixture, then okay. We give foreign concepts familiar descriptions all the time. (Like, maybe, telling someone that a byte is a letter...it's kind of true, for some concept of letter.) But the true fundamentals of QM are not particles or waves or discrete states, but wave functions and Hilbert space.
j1vms 10y ago
> Is there any real physical evidence that an electron can be both states at the same time, outside the mathematical framework of QM? Another way to respond to your question is to consider that the transistors inside the machine you used to post your comment were designed based on QM models of the electron that essentially follow from the Schrodinger Equation (e.g. via Fermi-Dirac statistics [0]). As far as has been known since 1947, without QM it is extremely unlikely humans would ever have discovered how to build solid state semiconductor devices like the transistor. A key intrinsic feature of QM is superposition. The double-slit experiment is a famous experiment that provides "tangible" evidence for superposition. [1] [0] https://en.wikipedia.org/wiki/Fermi%E2%80%93Dirac_statistics https://en.wikipedia.org/wiki/Fermi%E2%80%93Dirac_statistics [1] https://en.wikipedia.org/wiki/Double-slit_experiment https://en.wikipedia.org/wiki/Double-slit_experiment
mablap 10y ago
Electron diffraction implies wave behaviour, and wave behaviour implies the superposition principle. Is that good enough evidence or are you looking at experiments measuring entanglement in electrons?
deleted 10y ago
[deleted]
gone35 10y ago
Excess correlations.
sullyj3 10y ago
You can't think of an electron as a little ball whose behavior is described by a weird quantum mathematical formula. The universe is really made of quantum mechanics. The electron is actually an excitation in the wave function, a fundamentally quantum object, of which "tiny ball" happens to be a lossy and flawed, but not altogether terrible description.
valarauca1 10y ago
Yes. Mathematical frameworks that assume Quantum Systems are deterministic (only being in state A or B) during unobserved periods have never been validated by experiment. It is great if you have a model, but if that model fails to agree with experimental results.
- SomeStupidPoint 10y ago
  But as I understand it, this is because of technical lock-in: physicists had to choose between non-locality and indeterminism, chose indeterminism, got forced in to non-locality for other reasons, and never went back to re-examine the model, because it was getting decent computational results, and it would be a lot of work to rebuild. I've never seen a solid argument for indeterminism being anything but an extraneous assumption in QM. Entanglement (and other behaviors at low temperature) necessitates non-locality, which means the most parsimonious model would also use that, rather than indeterminism, to resolve Bell's inequality. So is there any reason besides "it predicts good enough" and "technical lockin" to keep using models with indeterminism? (Physics actually seems a hodgepodge of random philosophical assumptions no one bothered to ever review because they're deep in the model. Not unlike code suffering from code rot.)
  - malkarouri 10y ago
    (Physics actually seems a hodgepodge of random philosophical assumptions no one bothered to ever review because they're deep in the model. Not unlike code suffering from code rot.) This..
  - codethief 10y ago
    Entanglement doesn't require non-locality.
    - SomeStupidPoint 10y ago
      Does it not? Can you explain how entanglement works as an entirely local phenomenon?
deckar01 10y ago
> The Stern–Gerlach experiment showed that the spatial orientation of angular momentum is quantized. https://en.m.wikipedia.org/wiki/Stern–Gerlach_experiment https://en.m.wikipedia.org/wiki/Stern–Gerlach_experiment
- powertower 10y ago
  Going over it, it looks like the experiment found that spin is discrete, either -1/2 or +1/2. And nothing in between.
  - valarauca1 10y ago
    Read the entire section of Basic Theory and Description.
    - powertower 10y ago
      I did, and it states that electrons can hold one of two spin values. But within the mathematical framework of QM, that value is unknown before measurement - so it is ambiguous - which is interpreted (by QM) as a possible superposition of both values. The physical experiment itself only detected spin up or spin down. And that is the main question, how can we know this [electron superposition state] is not just a product of the framework being used, with no real counterpart in reality?
      - gji 10y ago
        The real evidence of this is from the last experiment under "Sequential Experiments". Essentially, you only select particles with spin-up in the z direction, then you select particles with spin-up in the x direction. So the resulting particles should be up in both the z and x direction. But if you measure in the z direction again, you find an equal distribution in up and down, indicating that you can't simply treat the spin as both pointed in the x and z direction, and that measuring in the x direction has scrambled the previously well-defined z direction. This experiment shows that measuring the spin direction in different axes does not commute. Measuring in one direction scrambles the other, which is equivalent to saying measuring in x then z is not the same as measuring in z then x. This fact is inherently related to the notion of a superposition. If a particle's spin direction is well-defined in one measurement basis, it is not well-defined in another - meaning it is in a superposition state in that measurement basis. You might ask - why can't I describe the system after measuring in the x-direction as just a random mixture of up and down in the z-direction? Physicists use something called a density matrix to describe systems that have both some degree of quantum superposition and classical randomness. One way to measure the degree to which some stream of particles is a random mixture or not is to interfere particles in that stream with each other. In the Stern-Gerlach experiment, after measuring in the x-direction, if the information in the z-direction was actually simply randomly scrambled, the probability that any two particles from the stream are truly identical is 1/2. If the particles are all identically in a superposition state, then any two particles will always be identical. You can actually test the indistinguishability of two particles by doing an interference experiment. One very nice example of this is this experiment: https://arxiv.org/pdf/1312.7182.pdf https://arxiv.org/pdf/1312.7182.pdf Two atoms were trapped next to each other using lasers. If these atoms have the same spin, they're indistinguishable. If they have different spin, then they are distinguishable, and won't interfere with each other. In fig. 3, you can see varying levels of interference depending on how well-aligned the spins are.
aji 10y ago
there are certain experimental results in quantum mechanics that only make sense if you accept things like superposition and entanglement. for example: https://www.youtube.com/watch?v=v657Ylwh-_k https://www.youtube.com/watch?v=v657Ylwh-_k
justinpombrio 10y ago
Personally, I think "simultaneously points both up and down" is a terrible way to describe superpositions. So let me avoid that phrase, and see if I can rephrase your question. Would it be fair to say that you're asking this question? > Is there any real physical evidence that an electron can be in a superposition of two states, that can only be explained using quantum mechanics? Surprisingly, the answer to this question is yes. The evidence is the Bell experiment[1]. [1] https://en.wikipedia.org/wiki/Bell_test_experiments https://en.wikipedia.org/wiki/Bell_test_experiments
- SomeStupidPoint 10y ago
  As I understand it, Bell's test only shows indeterminism or non-locality. Since we believe non-locality for other reasons, how does it support indeterminism?
  - AlexCoventry 10y ago
    What are the other reasons for believing non-locality? (I'm a fan, since the main objection I see to non-locality is that observations can't be fully explained in terms of local factors, and to me that still seems preferable to the Copenhagen interpretation that observations have a completely inexplicable component. But I don't know of any hard evidence for non-locality.)
    - SomeStupidPoint 10y ago
      The main use of non-locality I've seen is in relation to (fused) anyons carrying quantum numbers held non-locally over their constituent parts. I expect we see other non-local effects in things like very low temperature condensates and probably stuff like superconductors and superfluids (since the math of the vortex structure is similar to anyons). Of course, entanglement can be interpreted as non-locality, where what happens to one particle inside of a bounded area depends on the fate of its entangled partner, outside of the box. The real reason to use non-locality in place of indeterminism is that it provides a clear research avenue to combine the QM effects (based on information locality) with GR (which is about the causal relations between things). QM would really just be telling us that the actual causal net doesn't look like it does from macro scale. It transitions both frameworks to be about causal nets, instead of a weird mishmash that's hard to combine. Of course, that's if we can do QM as pure non-locality, rather than indeterminism. I just think it's strange that it seemingly hasn't been investigated. Ed: I guess it has been investigated some by say, Bohm, but I feel like science (the institution) gives undue weight to the interpretation they were taught in school, even when it's both arbitrary and constraining. It's like everyone screaming that of course there's exactly one parallel line, that's what you're taught in school and have been since the Greeks, right? Well, maybe non-Euclidean geometry models some things better. Similarly, I think scientists baked some arbitrary design and philosophy choices in early, then never bothered to explore other approaches, because they were able to compute decent results.
GFK_of_xmaspast 10y ago
What non-QM real physical evidence do you have for an electron existing at all?
- mablap 10y ago
  https://en.wikipedia.org/wiki/Oil_drop_experiment https://en.wikipedia.org/wiki/Oil_drop_experiment
  - GFK_of_xmaspast 10y ago
    That's discretization of electric charge, which is not necessarily the same thing as "an electron".
    - mablap 10y ago
      Electrons are defined in part by having unit (negative) electric charge. What other conclusion would you make from that experiment?
      - GFK_of_xmaspast 10y ago
        Why does it mean electrons "exist" in any sense instead of being a model that could explain the data? And if such objects did exist, why could they not follow classical mechanics?
        mablap 10y ago
        Meh, I could also ask you to provide evidence that you're not a p-zombie... There's an undivisible thing (particle) that has undivisible charge (electron) which exhibits properties and behaviour that could not be explained clasically. How do you explain classically (i.e. Newton's laws and no limits on observables uncertainty) electron diffraction and discretized spin?
        GFK_of_xmaspast 10y ago
abdullahkhalids 10y ago
> electron can be both states at the same time This is a very simplified colloquial way of stating a concept that also makes it seem mysterious. The state of an electron is described by a vector 'v' in a 2D complex vector space. Now, to actually talk about the vector concretely you should choose a basis. Suppose your basis is one in which the vector 'v' is not parallel to any of the basis vectors. Then you can say the vector v is "somewhere in between basis vectors e1 and e2". No magic in the way I have stated it. There is a little bit of fake magic when you start thinking about the random measurement results you get when you measure the electron in state 'v'. But these effects can be simulated in a classical deterministic universe. The real magic of quantum mechanics is in entanglement which can't be simulated in a classical deterministic universe. But that is a much longer story. Now to answer your question, yes there is lots of physical evidence that this non-deterministic description of systems is a good model of reality. This comment box is too small to state it. People are currently in the process of determining if this probabilistic description is a good one outside of the mathematical framework of QM. See http://www.nature.com/news/quantum-physics-what-is-really-real-1.17585 http://www.nature.com/news/quantum-physics-what-is-really-re...
- Sonthun 10y ago
  "This comment box is too small to state it." Can you write it in the margin instead?
  - amelius 10y ago
    Also, the comment box has a handle at the lower right, which you can drag to make the comment box bigger.
deleted 10y ago
[deleted]

Florin_Andrei 10y ago

That whole thinking - that states are well defined, and can be only either one or the other - that's the classic world approximation, it's not reality. You firmly believe in it because your crucial early years, when you were crawling on the floor at your parents' house, were spent in a classic-world-approximation universe. That's why it's so hard for you (and pretty much everyone else) to let go of it. The quantum phenomena are the more fundamental reality. This classic universe that you think you understand is just an epiphenomenon. Just foam floating on the ocean. The fundamental reality of the electron is the wave function. States, and everything else, emerge from it. It's not a corpuscle. It's not a wave. It does not behave like ping-pong balls, or like waves in a pond, although it shares a few characteristics. Fundamentally, it's something different. The only way to know that entirely different thing is via the mathematics of quantum mechanics - and whatever intuition you can derive from it after you do the math. There is no equivalent for that stuff at the human scale of things.

paulddraper 10y ago
Very much this. This should be prologue to any introduction QM book/class/whatever. --- Building on this idea, "an electron can be both states at the same time" isn't necessarily the right way to think about it. I mean, it's sort of right, but relies on an intuitive understand of what the heck that even means. And QM reality is grounded not in our human intuition, but in the math of wave functions. If we associate "state up" with a wave function concentrated one way and "state down" concentrated the other way and "both states" as a mixture, then okay. We give foreign concepts familiar descriptions all the time. (Like, maybe, telling someone that a byte is a letter...it's kind of true, for some concept of letter.) But the true fundamentals of QM are not particles or waves or discrete states, but wave functions and Hilbert space.

j1vms 10y ago

> Is there any real physical evidence that an electron can be both states at the same time, outside the mathematical framework of QM? Another way to respond to your question is to consider that the transistors inside the machine you used to post your comment were designed based on QM models of the electron that essentially follow from the Schrodinger Equation (e.g. via Fermi-Dirac statistics [0]). As far as has been known since 1947, without QM it is extremely unlikely humans would ever have discovered how to build solid state semiconductor devices like the transistor. A key intrinsic feature of QM is superposition. The double-slit experiment is a famous experiment that provides "tangible" evidence for superposition. [1] [0] https://en.wikipedia.org/wiki/Fermi%E2%80%93Dirac_statistics https://en.wikipedia.org/wiki/Fermi%E2%80%93Dirac_statistics [1] https://en.wikipedia.org/wiki/Double-slit_experiment https://en.wikipedia.org/wiki/Double-slit_experiment

mablap 10y ago

Electron diffraction implies wave behaviour, and wave behaviour implies the superposition principle. Is that good enough evidence or are you looking at experiments measuring entanglement in electrons?

deleted 10y ago

[deleted]

gone35 10y ago

Excess correlations.

sullyj3 10y ago

You can't think of an electron as a little ball whose behavior is described by a weird quantum mathematical formula. The universe is really made of quantum mechanics. The electron is actually an excitation in the wave function, a fundamentally quantum object, of which "tiny ball" happens to be a lossy and flawed, but not altogether terrible description.

valarauca1 10y ago

Yes. Mathematical frameworks that assume Quantum Systems are deterministic (only being in state A or B) during unobserved periods have never been validated by experiment. It is great if you have a model, but if that model fails to agree with experimental results.

SomeStupidPoint 10y ago
But as I understand it, this is because of technical lock-in: physicists had to choose between non-locality and indeterminism, chose indeterminism, got forced in to non-locality for other reasons, and never went back to re-examine the model, because it was getting decent computational results, and it would be a lot of work to rebuild. I've never seen a solid argument for indeterminism being anything but an extraneous assumption in QM. Entanglement (and other behaviors at low temperature) necessitates non-locality, which means the most parsimonious model would also use that, rather than indeterminism, to resolve Bell's inequality. So is there any reason besides "it predicts good enough" and "technical lockin" to keep using models with indeterminism? (Physics actually seems a hodgepodge of random philosophical assumptions no one bothered to ever review because they're deep in the model. Not unlike code suffering from code rot.)
- malkarouri 10y ago
  (Physics actually seems a hodgepodge of random philosophical assumptions no one bothered to ever review because they're deep in the model. Not unlike code suffering from code rot.) This..
- codethief 10y ago
  Entanglement doesn't require non-locality.
  - SomeStupidPoint 10y ago
    Does it not? Can you explain how entanglement works as an entirely local phenomenon?

deckar01 10y ago

> The Stern–Gerlach experiment showed that the spatial orientation of angular momentum is quantized. https://en.m.wikipedia.org/wiki/Stern–Gerlach_experiment https://en.m.wikipedia.org/wiki/Stern–Gerlach_experiment

powertower 10y ago
Going over it, it looks like the experiment found that spin is discrete, either -1/2 or +1/2. And nothing in between.
- valarauca1 10y ago
  Read the entire section of Basic Theory and Description.
  - powertower 10y ago
    I did, and it states that electrons can hold one of two spin values. But within the mathematical framework of QM, that value is unknown before measurement - so it is ambiguous - which is interpreted (by QM) as a possible superposition of both values. The physical experiment itself only detected spin up or spin down. And that is the main question, how can we know this [electron superposition state] is not just a product of the framework being used, with no real counterpart in reality?
    - gji 10y ago
      The real evidence of this is from the last experiment under "Sequential Experiments". Essentially, you only select particles with spin-up in the z direction, then you select particles with spin-up in the x direction. So the resulting particles should be up in both the z and x direction. But if you measure in the z direction again, you find an equal distribution in up and down, indicating that you can't simply treat the spin as both pointed in the x and z direction, and that measuring in the x direction has scrambled the previously well-defined z direction. This experiment shows that measuring the spin direction in different axes does not commute. Measuring in one direction scrambles the other, which is equivalent to saying measuring in x then z is not the same as measuring in z then x. This fact is inherently related to the notion of a superposition. If a particle's spin direction is well-defined in one measurement basis, it is not well-defined in another - meaning it is in a superposition state in that measurement basis. You might ask - why can't I describe the system after measuring in the x-direction as just a random mixture of up and down in the z-direction? Physicists use something called a density matrix to describe systems that have both some degree of quantum superposition and classical randomness. One way to measure the degree to which some stream of particles is a random mixture or not is to interfere particles in that stream with each other. In the Stern-Gerlach experiment, after measuring in the x-direction, if the information in the z-direction was actually simply randomly scrambled, the probability that any two particles from the stream are truly identical is 1/2. If the particles are all identically in a superposition state, then any two particles will always be identical. You can actually test the indistinguishability of two particles by doing an interference experiment. One very nice example of this is this experiment: https://arxiv.org/pdf/1312.7182.pdf https://arxiv.org/pdf/1312.7182.pdf Two atoms were trapped next to each other using lasers. If these atoms have the same spin, they're indistinguishable. If they have different spin, then they are distinguishable, and won't interfere with each other. In fig. 3, you can see varying levels of interference depending on how well-aligned the spins are.

aji 10y ago

there are certain experimental results in quantum mechanics that only make sense if you accept things like superposition and entanglement. for example: https://www.youtube.com/watch?v=v657Ylwh-_k https://www.youtube.com/watch?v=v657Ylwh-_k

justinpombrio 10y ago

Personally, I think "simultaneously points both up and down" is a terrible way to describe superpositions. So let me avoid that phrase, and see if I can rephrase your question. Would it be fair to say that you're asking this question? > Is there any real physical evidence that an electron can be in a superposition of two states, that can only be explained using quantum mechanics? Surprisingly, the answer to this question is yes. The evidence is the Bell experiment[1]. [1] https://en.wikipedia.org/wiki/Bell_test_experiments https://en.wikipedia.org/wiki/Bell_test_experiments

SomeStupidPoint 10y ago
As I understand it, Bell's test only shows indeterminism or non-locality. Since we believe non-locality for other reasons, how does it support indeterminism?
- AlexCoventry 10y ago
  What are the other reasons for believing non-locality? (I'm a fan, since the main objection I see to non-locality is that observations can't be fully explained in terms of local factors, and to me that still seems preferable to the Copenhagen interpretation that observations have a completely inexplicable component. But I don't know of any hard evidence for non-locality.)
  - SomeStupidPoint 10y ago
    The main use of non-locality I've seen is in relation to (fused) anyons carrying quantum numbers held non-locally over their constituent parts. I expect we see other non-local effects in things like very low temperature condensates and probably stuff like superconductors and superfluids (since the math of the vortex structure is similar to anyons). Of course, entanglement can be interpreted as non-locality, where what happens to one particle inside of a bounded area depends on the fate of its entangled partner, outside of the box. The real reason to use non-locality in place of indeterminism is that it provides a clear research avenue to combine the QM effects (based on information locality) with GR (which is about the causal relations between things). QM would really just be telling us that the actual causal net doesn't look like it does from macro scale. It transitions both frameworks to be about causal nets, instead of a weird mishmash that's hard to combine. Of course, that's if we can do QM as pure non-locality, rather than indeterminism. I just think it's strange that it seemingly hasn't been investigated. Ed: I guess it has been investigated some by say, Bohm, but I feel like science (the institution) gives undue weight to the interpretation they were taught in school, even when it's both arbitrary and constraining. It's like everyone screaming that of course there's exactly one parallel line, that's what you're taught in school and have been since the Greeks, right? Well, maybe non-Euclidean geometry models some things better. Similarly, I think scientists baked some arbitrary design and philosophy choices in early, then never bothered to explore other approaches, because they were able to compute decent results.

GFK_of_xmaspast 10y ago

What non-QM real physical evidence do you have for an electron existing at all?

mablap 10y ago
https://en.wikipedia.org/wiki/Oil_drop_experiment https://en.wikipedia.org/wiki/Oil_drop_experiment
- GFK_of_xmaspast 10y ago
  That's discretization of electric charge, which is not necessarily the same thing as "an electron".
  - mablap 10y ago
    Electrons are defined in part by having unit (negative) electric charge. What other conclusion would you make from that experiment?
    - GFK_of_xmaspast 10y ago
      Why does it mean electrons "exist" in any sense instead of being a model that could explain the data? And if such objects did exist, why could they not follow classical mechanics?
      - mablap 10y ago
        Meh, I could also ask you to provide evidence that you're not a p-zombie... There's an undivisible thing (particle) that has undivisible charge (electron) which exhibits properties and behaviour that could not be explained clasically. How do you explain classically (i.e. Newton's laws and no limits on observables uncertainty) electron diffraction and discretized spin?
        GFK_of_xmaspast 10y ago

abdullahkhalids 10y ago

> electron can be both states at the same time This is a very simplified colloquial way of stating a concept that also makes it seem mysterious. The state of an electron is described by a vector 'v' in a 2D complex vector space. Now, to actually talk about the vector concretely you should choose a basis. Suppose your basis is one in which the vector 'v' is not parallel to any of the basis vectors. Then you can say the vector v is "somewhere in between basis vectors e1 and e2". No magic in the way I have stated it. There is a little bit of fake magic when you start thinking about the random measurement results you get when you measure the electron in state 'v'. But these effects can be simulated in a classical deterministic universe. The real magic of quantum mechanics is in entanglement which can't be simulated in a classical deterministic universe. But that is a much longer story. Now to answer your question, yes there is lots of physical evidence that this non-deterministic description of systems is a good model of reality. This comment box is too small to state it. People are currently in the process of determining if this probabilistic description is a good one outside of the mathematical framework of QM. See http://www.nature.com/news/quantum-physics-what-is-really-real-1.17585 http://www.nature.com/news/quantum-physics-what-is-really-re...

Sonthun 10y ago
"This comment box is too small to state it." Can you write it in the margin instead?
- amelius 10y ago
  Also, the comment box has a handle at the lower right, which you can drag to make the comment box bigger.

deleted 10y ago

[deleted]