Quantum entanglement and hidden variables

[PainterGuy]

Hi,

I have a basic understanding of quantum physics. I was reading a Wikipedia article on hidden variables, https://en.wikipedia.org/wiki/Hidden-variable_theory . The article says the following.

In physics, hidden-variable theories are proposals to provide explanations of quantum mechanical phenomena through the introduction of (possibly unobservable) hypothetical entities. The existence of fundamental indeterminacy for some measurements is assumed as part of the mathematical formulation of quantum mechanics; moreover, bounds for indeterminacy can be expressed in a quantitative form by the Heisenberg uncertainty principle. Most hidden-variable theories are attempts to avoid quantum indeterminacy, but possibly at the expense of requiring the existence of nonlocal interactions.

Albert Einstein objected to aspects of quantum mechanics, and famously declared "I am convinced God does not play dice". Einstein, Podolsky, and Rosen argued while assuming local causality that quantum mechanics is an incomplete description of reality. Bell's theorem and the related experiments have subsequently ruled out nearly all local hidden variable theories.

One notable non-local hidden-variable theory is the De Broglie–Bohm theory.

I was confused about the words "local" and "nonlocal" in the quote above so I checked out another Wikipedia article, https://en.wikipedia.org/wiki/Principle_of_locality . The quote below has been taken from the mentioned article.

In physics, the principle of locality states that an object is influenced directly only by its immediate surroundings. A theory that includes the principle of locality is said to be a "local theory".

Question:
The quantum entanglement phenomenon is also non-local where one quantum particle can influence the other particle instantaneously which could be located thousands of miles away.

How is this phenomenon explained? Is it only taken as 'natural fact' without any resort to underlying mechanism which is responsible for this instantaneous interaction? Or, it is accepted that there are some unknown features or variables in nature, experimentally not-yet-discovered, which are responsible for this entanglement?

I'd appreciate if you could keep it simple though I do agree this topic is difficult to simplify. Thank you for your help, in advance! Helpful link:
1: https://www.scientificamerican.com/article/how-einstein-revealed-the-universe-s-strange-nonlocality

 

[Hyperfine]

Question:
The quantum entanglement phenomenon is also non-local where one quantum particle can influence the other particle instantaneously which could be located thousands of miles away.

How is this phenomenon explained? Is it only taken as 'natural fact' without any resort to underlying mechanism which is responsible for this instantaneous interaction? Or, it is accepted that there are some unknown features or variables in nature, experimentally not-yet-discovered, which are responsible for this entanglement?

The one quantum particle does not influence the other one instantaneously via some mechanism that exchanges information. The system cannot be considered as a pair of separate, distinct particles. Both members of the entangled pair must be considered together as an inseparable pair rather than two individual particles.

That is about as basic as I can make it, as requested.

 

[PeroK]

Question:
The quantum entanglement phenomenon is also non-local where one quantum particle can influence the other particle instantaneously which could be located thousands of miles away.

As above, QM predicts correlation across a non-local system. Not influence of one system on another.

How is this phenomenon explained? Is it only taken as 'natural fact' without any resort to underlying mechanism which is responsible for this instantaneous interaction?

There are no mechanisms in physics, as such. Here's an interesting example from this site recently. You have a positive charge at the origin and a negative charge on the x-axis. What is the electric field further along the x-axis, beyond the negative charge? A student recently assumed that the negative charge would block the electric field from the positive charge at the origin. It was explained that electric fields pass right through intervening charges. What is the mechanism for this he asked?

More generally, what is the mechanism for local phenomena? How does a particle in an EM or gravitational field know to react to the field in a certain way? If it follows the gradient of the potential, how does it know the gradient? Again, there is no rational mechanism for how this happens.

My point is that QM is not as different in this respect as many people would like to imply.

Or, it is accepted that there are some unknown features or variables in nature, experimentally not-yet-discovered, which are responsible for this entanglement?

There may be a deeper theory, but there may not. But, even if there were a deeper theory, I suggest it would have its own fundamental assumptions and lack of mechanisms to explain these.

Helpful link:
1: https://www.scientificamerican.com/article/how-einstein-revealed-the-universe-s-strange-nonlocality

This is a decidedly unhelpful link. The more you read of popular science journalism, the less you understand about QM as a robust academic branch of physics.

PS after over 900 posts on PF, you should know better than to link to something like this!

 

[PainterGuy]

Thank you for the help!

There are no mechanisms in physics, as such. Here's an interesting example from this site recently. You have a positive charge at the origin and a negative charge on the x-axis. What is the electric field further along the x-axis, beyond the negative charge? A student recently assumed that the negative charge would block the electric field from the positive charge at the origin. It was explained that electric fields pass right through intervening charges. What is the mechanism for this he asked?

About the charge example. I have always thought of a positive charge as the source and a negative charge as the sink. If a negative charge is lying along the positive x-axis and the positive charge is situated at the origin, I think that some of the the field lines emanating from the positive charge will converge onto the negative charge.

Source: University Physics, Young, Freedman, 13th ed. pg. 709

More generally, what is the mechanism for local phenomena? How does a particle in an EM or gravitational field know to react to the field in a certain way? If it follows the gradient of the potential, how does it know the gradient? Again, there is no rational mechanism for how this happens.

There may be a deeper theory, but there may not. But, even if there were a deeper theory, I suggest it would have its own fundamental assumptions and lack of mechanisms to explain these.

I get your point. Thanks.

This is a decidedly unhelpful link. The more you read of popular science journalism, the less you understand about QM as a robust academic branch of physics.

If I trusted popular science that much, I wouldn't be asking you for help. Yes, for a beginner, sometimes these popular science articles can make one interested to search further.

Thank you for your time!

 

[Demystifier]

How is this phenomenon explained?

Here is a simple analogy. In Newtonian gravity, a massive particle acts on another massive particle far away, instantaneously. How is this phenomenon explained, within Newtonian gravity?

 

[PainterGuy]

Here is a simple analogy. In Newtonian gravity, a massive particle acts on another massive particle far away, instantaneously. How is this phenomenon explained, within Newtonian gravity?

Thank you.

It is just assumed that nature acts in some mysterious way. Though, I think that Newton himself knew that this is not really true.

 

[Bob Walance]

How is this phenomenon explained?

In the 'Many Worlds' interpretation of quantum physics, both entangled particles will exist in separate worlds when a measurement is performed.
For example, in the case of a simple Bell pair where the wave function has 50% probability of finding the two particles with spin up and 50% probability of finding them spin down, when a measurement is performed on the particle(s) then worlds branch. One world has the spin-up particles, and another world has the spin-down particles. The two pairs exist separately, independently and non-interactively.
From Sean Carroll's book 'Something Deeply Hidden' (page 105), "The correlations don't come about because of any kind of influence being transmitted faster than light, but because of the branching of the wave function into different worlds, in which correlated things happen."

To me, 'Many Worlds' is much nicer in this regard. "Spooky actions at a distance" and hidden variables are no longer required.
Is this the way that our world(s) really operate? We will probably never be able to know, but it is an interesting concept to ponder.

 

[PeroK]

To me, 'Many Worlds' is much nicer in this regard. "Spooky actions at a distance" and hidden variables are no longer required.

You've misunderstood that "Many Worlds" is not a different theory from QM, but only an interpretation of QM. There is no "spooky action at a distance" in QM (except that the ghost of Einstein's reservations about QM still cast a shadow seventy years after his death). Einstein, I suspect, would have been horrified by the Many World's Interpretation (MWI). And, there are no "hidden variables" in QM.

Sean Carroll is a great physicist, but he takes his proletyzing zeal about MWI too far, in my opinion.

From Sean Carroll's book 'Something Deeply Hidden' (page 105),

Note that this is a popular science book and, therefore, not a valid reference for QM on this forum, I'm sorry to say.