Quantum Physics Quantum Physics Help Forum  2Likes  1 Post By mattlock
 1 Post By Pmb
Jul 8th 2017, 11:06 PM

#1  Junior Member
Join Date: Jul 2017
Posts: 1
 Understanding Quantum Entanglement
I often read articles on Wikipedia or Scientific America etc on topics related to Quantum physics. In particular, they talk about spooky action at a distance and paradoxes.
Please forgive my ignorance, but I don't understand what the paradox is, and I'm hoping someone can explain it to me without getting overly technical.
To ground the discussion I'm pasting from wiki:
" For example, if a pair of particles are generated in such a way that their total spin is known to be zero, and one particle is found to have clockwise spin on a certain axis, the spin of the other particle, measured on the same axis, will be found to be counterclockwise, as to be expected due to their entanglement. However, this behavior gives rise to paradoxical effects: any measurement of a property of a particle can be seen as acting on that particle (e.g., by collapsing a number of superposed states) and will change the original quantum property by some unknown amount; and in the case of entangled particles, such a measurement will be on the entangled system as a whole. It thus appears that one particle of an entangled pair "knows" what measurement has been performed on the other, and with what outcome, even though there is no known means for such information to be communicated between the particles, which at the time of measurement may be separated by arbitrarily large distances."
In my mind I simply see two particles, one which always had positive spin, and the other always had negative, we just hasn't yet measured this. I don't see where communication between the particles comes in to play.
As is probably clear, I have a very limited background in physics; but I like to read about it when I have spare time. Thanks for your patience 
 
Jul 14th 2017, 07:46 PM

#2  Junior Member
Join Date: May 2017
Posts: 12

It's called theoretical physics for a reason; many arguments have not been settled as of yet.
Check out this video;
Pay particular attention to the part about the experiment that set out to prove Einstein correct, and the two photons that are emitted from the calcium at the same time, as they are 'entangled.'

 
Jul 15th 2017, 08:59 AM

#3  Senior Member
Join Date: Aug 2010
Posts: 336

Originally Posted by GoneAwayFishing I often read articles on Wikipedia or Scientific America etc on topics related to Quantum physics. In particular, they talk about spooky action at a distance and paradoxes.
Please forgive my ignorance, but I don't understand what the paradox is, and I'm hoping someone can explain it to me without getting overly technical.
To ground the discussion I'm pasting from wiki:
" For example, if a pair of particles are generated in such a way that their total spin is known to be zero, and one particle is found to have clockwise spin on a certain axis, the spin of the other particle, measured on the same axis, will be found to be counterclockwise, as to be expected due to their entanglement. However, this behavior gives rise to paradoxical effects: any measurement of a property of a particle can be seen as acting on that particle (e.g., by collapsing a number of superposed states) and will change the original quantum property by some unknown amount; and in the case of entangled particles, such a measurement will be on the entangled system as a whole. It thus appears that one particle of an entangled pair "knows" what measurement has been performed on the other, and with what outcome, even though there is no known means for such information to be communicated between the particles, which at the time of measurement may be separated by arbitrarily large distances."
In my mind I simply see two particles, one which always had positive spin, and the other always had negative, we just hasn't yet measured this 
This is the problem. In quantum physics, and especially quantum field theory, this is not true. A particle, such that some property has not been measured, is NOT in a specific state of that property that we just don't know. It is in superposition of states. It literally does NOT have a specific value for that property until you measure it.
That sounds weird but, then, Quantum Physics [b]is/b] weird! There have been plenty of experiments done whose results could only happen if the property were not in such a "superposition of states".
I don't see where communication between the particles comes in to play.
As is probably clear, I have a very limited background in physics; but I like to read about it when I have spare time. Thanks for your patience  
 
Jul 15th 2017, 06:45 PM

#4  Junior Member
Join Date: May 2017
Posts: 12

First of all, most people do not understand quantum physics, and all of it's implications; myself included.
Quantum Theory, and it is still a theory, as it has not as yet been proven, is based on Bohr's math done in the late 19th century. So, the entire construct is, or was, purely mathematical and/or logical in physics terms.
But it does pass as a principal to a certain degree, since every time it has been tested, physically and mathematically, it passes.
When Bohr did his studies, he determined (per the example in the video) that the two photons that were created would have a certain amount of energy (the spin) but he could not determine whether the energy would be positive or negative; ie; clockwise or counter clockwise. But he did know that the amounts of energy would be equal and opposite and connected (quantum entanglement.)
Keep in mind, at the time of his studies, molecules had not been proven to exist, let alone atoms. Special relativity also came later, derived from his work.
Bohr also believed, later, that we could never prove quantum theory because is exists in another universe, level of reality, or dimension, and there was a 'wall,' either physical or perceptual that we would not be able to pass. This concept was derived from his continued work with Einstein.
Consider, Einstein never believed in this entanglement, so, if he had difficulty understanding it, or accepting, it's not surprising that you and i find the concept difficult to understand. Some of these things i either accept or reject and move on.

 
Jul 16th 2017, 01:59 PM

#5  Physics Team
Join Date: Apr 2009 Location: Boston's North Shore
Posts: 1,548

Originally Posted by mattlock First of all, most people do not understand quantum physics, and all of it's implications; myself included. 
This is a great point to make a philosophical observation:
What does it mean to when someone says "I understand this"?
To most people it means that there is no mystery to it for them. When a scientist says he understands something it can mean one or two things (1) there is no mystery to it for them or (2) he knows the laws, theory and how to do the math and the rest is cake.
It is in respect to (1) that Richard Feynman said, in The Feynman Lectures on Physics, vol III, p. 189 (1965)
On the other hand, I think I can safely say that nobody understands quantum mechanics. So do not take the lecture too seriously, feeling that you really have to understand in terms of some model what I am going to describe, but just relax and enjoy it.

I highly recommend reading the context in which that was taken from at https://en.wikiquote.org/wiki/Quantum_mechanics
Originally Posted by mattlock Quantum Theory, and it is still a theory, as it has not as yet been proven, ... 
This is a common misunderstanding. All too often you'll hear people say "It's just a theory" as if that was a valid argument. In fact that statement is famous for being quite invalid.
I highly recommend reading the following article from Scientific American at https://www.scientificamerican.com/a...sciencewords/
I also recommend reading the definition of Scientific Theory https://www.livescience.com/21491wh...oftheory.html
It's also a common misconception that science is about proving things when in fact its not. No scientific theory can be proven. Please watch the following video on the subject by Alan Guth http://www.newenglandphysics.org/com...an_Guth_04.mp4
Originally Posted by mattlock ..., is based on Bohr's math done in the late 19th century. So, the entire construct is, or was, purely mathematical and/or logical in physics terms. 
That's quite incorrect. Bohr didn't have a theory of quantum mechanics, at least not in the literal sense. The called his model of the atom a quantum theory. However he didn't have a real quantum theory in the sense that such a theory described all particle dynamics. And all laws of physics are described in mathematical terms because math is the language of physics. Here you make it sound as the math and logical description does not "really" tell us what can be known of nature on the atomic and subatomic level, which is far from true.
Originally Posted by mattlock But it does pass as a principal to a certain degree, since every time it has been tested, physically and mathematically, it passes. 
To a "certain degree"? In fact it passes to the level which we have the ability to measure it.
But in one sense you're correct, in that this is related to the logic of science is inductive rather than deductive.
Originally Posted by mattlock When Bohr did his studies, he determined (per the example in the video) that the two photons that were created would have a certain amount of energy (the spin) ... 
I think that you're confused here. Energy and spin are not the same thing. Energy is a scalar quantity whereas spin is a vector quantity. By the way, the energy of a photon is always taken as positive.
Note: The absolute value of the energy of a closed system is defined only as being constant. It can have any real value. What corresponds to physical reality is that the value is constant. In fact when one calculates the total energy of a particle next to an infinite line of mass (i.e. the sum of the gravitational potential energy and the kinetic energy) one must include that additive constant which in most all other cases one can set to zero. In this case it can't be done.
Originally Posted by mattlock Keep in mind, at the time of his studies, molecules had not been proven to exist, let alone atoms. 
Einstein’s 1905 paper on Brownian Motion confirmed the atomic theory of matter and it was that paper which is taken as the argument after which there was no longer any serious doubt of the existence of atoms. It's been said that
"This is viewed by many as the first proof that atoms actually exist." but I hate to use the term "proof".
Originally Posted by mattlock Special relativity also came later, derived from his work. 
Where did you get that impression from? Einstein is credited with creating relativity in his 1905 paper entitled On the Electrodynamics of Moving Bodies which had nothing to do with anything Bohr ever did up to that point. The paper is online here: https://www.fourmilab.ch/etexts/einstein/specrel/www/
Notice that nowhere in that paper is Bohr or anything he did mentioned used or referenced in that paper. I doubt Einstein even heard of Bohr at that time. The first mention I can find of Bohr's work by Einstein was in 1913, i.e. 8 years after the publication of special relativity.
See page 373 at: https://books.google.com/books?id=OA...tachel&f=false
Originally Posted by mattlock Bohr also believed, later, that we could never prove quantum theory .. 
May I also ask where you got this impression from? Again, physicists know that they can't prove any theory and Bohr was a first rate physicist.
Originally Posted by mattlock Consider, Einstein never believed in this entanglement, s.. 
Since when? Also, what do you mean by "never believed"?
Originally Posted by mattlock Some of these things i either accept or reject and move on. 
Why do you think that's a good idea?
Last edited by Pmb; Jul 16th 2017 at 06:35 PM.
Reason: Corrected an error

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