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Entanglement and observations

  1. Oct 7, 2007 #1
    I just had one of those lightbulb thoughts a few minutes ago reading someones post. Does observing some particle in an superposed state entangle you with that particle? It makes perfect sense to me right now..

    Its definately like entanglement, even if it isnt. Observing the first observers reaction instantly gives you the state of the particle (and vise versa). Wouldnt this mean superposition is relative depending on whos observing? For example if someone did a quantum coin flip to determine whether to spread a deadly virus or not Then everyone on earth would be in a superposition of dead and alive to the rest of the universe, right?
    Last edited: Oct 7, 2007
  2. jcsd
  3. Oct 7, 2007 #2
    I agree with this mostly. I've come to the same conclusions.

    Isn't the many world interpretation also part of this? I don't see how it could make sense to say that the observer is in superposition of having measured different outcomes without the many world interpretation. The observer doesn't see himself being in superposition, so there should be, from the observer's point of view, different worlds where he has obtained different results. Only from the point of view of somebody who has not been involved in the measurement, does it make sense to say that the observer is in superposition.
  4. Oct 7, 2007 #3
    The act of observation destroys the state of entanglement. This is why entanglement is so good for quantum cryptography. The act of receiving the data that an entangled particle has is a process in itself (generally requiring multiple entanglements). Also entanglement works at the speed of light. By the time that process is complete, the state is unknown. And by the time your brain perceives the sensory information that tells you about the particles state, it will be even further along its path. Here's an interesting link I found on it: <http://calitreview.com/2007/03/30/the-strange-world-of-quantum-entanglement/>
  5. Oct 7, 2007 #4
    Yes -- this is the basis of the relational interpretation. We say that reality is defined by what observers can agree on when they exchange/compare information. Note that this is strictly a weaker definition of reality than the usual Copenhagen interpretation. Thus, measurement is nothing more or less than simply entangling yourself with the system. A 2nd observer can see that you've entangled yourself, but could still not know anything about the system, except that if she were to measure you and the system separately, there would be a correlation.

    See http://plato.stanford.edu/entries/qm-relational/

    I think this is the easiest view for those who are skeptical of absolute reality to begin with. Notice that it doesn't say *everything* is subjective -- different observers will agree with each others' observations when they interact/communicate (which is a quantum process itself -- mutual observation).

    My personal crackpot theory is that the probabilistic nature of QM is due to the impossibility of knowing one's own state, plus a lower limit to how much you much disturb a system if you want to gain information about it (the nature scale set by \hbar).
  6. Oct 7, 2007 #5
    no kidding. However what Im saying is that observation only destroys the state of superposition/entanglement for the observer. The fact that observing a state entangles you with it explains this perfectly. Now in macroscopic systems, you observe a particle and become entangled with it. Then all the air around u gets entangled with you and the particle and etc until everything in the world is entangled with this particle. Therefore it seems like the act of observation simply decoheres a system when it fact it only decohers it for the observer and the observers of the observer.
  7. Oct 7, 2007 #6
    In this debate you have to be weary of what 'observe' means. When quantum physicists talk about observation, they talk about the small amount of energy exerted on a physical system that is needed to extract the information, such as a photon bouncing off a phosphorous atom and breaking down the rhodopsin in your photo receptors which is then transferred as an ionic impulse into your brain. Now entanglement is a very delicate event that is destroyed by entropy, which is why it's rare (actually only entanglement that lasts for a while is rare, entanglement for small fractions of a second is very common). For the laboratory superposition that is what I assume you're talking about, there must be virtually no interaction between the entangled particles, otherwise superposition only lasts a few seconds (or less).If everything in the world was entangled, there would be a violation in the laws of thermodynamics (specifically the second one that deals with entropy). For a system to maintain entanglement, energy must be exerted to reduce entropy, which in turn increases entropy is some distant system. An over simplification of the process to find out what the state of an entangled system is that there must be more than one system entangled with it. Thus, only one aspect of system is compromised when energy is exerted onto it. The act of entangling other particles with it once more exerts energy, which could potentially destroy the system, so there is a limited amount of times that you can extract the data. Also, increasing the distance increases the entropy since once more, energy is exerted onto the system, adding further complications.
    Last edited: Oct 7, 2007
  8. Oct 7, 2007 #7
    You guys are all kidding right? I see how you guys are going off into metaphysics and I wonder if you are serious.

    "entanglement" is really mutual exclusivity disguised in weird language. Let's take an example. If you know that Mr Smith has two children, one a boy and the other a girl, the the children are "entangled". What this means is that if you meet one of the children who turns out to be a boy, you will immediately be able to tell that the other is a girl.

    Before you meet the first child, if you were to calculate the probability that the child is boy or girl, it would be 50:50 (boy:girl) provided you have no other information. However the probability that the second child will be a girl is not independent of the probability of the first observation, because both are not mutually exclusive. Some how QM decided to invent it's own term "entanglement" which get's a lot of people really confused.

    The probability of one of random children being boy or girl is 50:50 irrespective of if it's the first observation, or the second observation, simply because we have no information to "entangle" them in our minds. By knowing that the two kids are from the same parent who has only one boy and one girl, entangles them.

    The same applies to quantum systems. Two photons from the same source will be entangled by the property of the source they share.

    Coming to think of it, "entanglement" is a clever way of hiding the hidden variable shared by both systems when we have no idea about the nature of the hidden variable.
  9. Oct 7, 2007 #8
    Quantum entanglement is a quantum mechanical event that causes two particle to essentially be part of one entity. Read the link I posted.
  10. Oct 7, 2007 #9
    First, quantum entanglement is not an event, it is a description.
    Second, saying the two particles are part of one entity is misleading. Siblings can be part of an entity called a family, yet they are separate individuals. Two particles being entangled is analogous to two people being related. In reality all that is happening is that the two particles have common or correlated properties.

    Quantum entanglement is essentially equivalent to a hidden variable theory since Bell's theorem has been effectively shown to be wrong.
    Last edited: Oct 7, 2007
  11. Oct 7, 2007 #10
    mn4j, rather than debate off-topic technical issues, I think we can both agree that the claims you are making contradict mainstream modern physics, yes? As such, can you understand why such claims are inappropriate (i.e., anti-conducive to others understanding modern physics) in threads not specifically intended to investigate alternative views?
  12. Oct 7, 2007 #11
    yea I think your just wrong about this mn4j. What your describing is a classical view of entanglement. Quantum entanglement is defined as a multi-particle superposition. i.e. |0>|0> + |1>|1> is an entanglement between two qubits where they have equal values. The system is in a superposition between |0>|0> and |1>|1>, it isnt in one or the other.

    As for the entropy argument, Im not sure that disproves what Im saying. If you view the earth as isolated, any quantum measurement that effected everything on earth would put earth into an entangled state with what was being measured relative to everything outside the earth. If you dont like assuming the earth is isolated from the rest of the universe, imagine a sphere around the earth whose radius grows with the speed of light. Nothing outside of the earth could possibly know the outcome of the measurement.

    looking at this from the MWI, the area of the universe that is affected by the measurement splits while the rest of the universe doesnt. In our localized sphere, the 2nd law of thermodynamics isnt violated, since from our point of view the particle has been measured and we have been effected by that measurement (we split into one of the possible universes).
  13. Oct 8, 2007 #12
    Apparently you are not following the literature very much. It is a fact that Bell's theorem has been disproved, maybe your textbook does not say that yet.

    Hess and Philipp (2000) Bell's theorem and the problem of decidability between the views of Einstein and Bohr, PNAS vol.98 no.25 pp14228-14233
    The authors find that:
    In case you don't recall, Bell's theorem was supposed to prove that Quantum Entanglement was not the same as a hidden variable theorem. This paper shows that Bell's theorem is wrong.

    I've quoted just one example, there are several others in case you are still in doubt.

    If by mainstream physics you mean "the religion of blindly following ideas without any due diligence on their historical basis" then you are right. I'm not mainstream.
  14. Oct 8, 2007 #13
    I think you are wrong. Quantum superposition is a an epistemological property not an ontological one. By trying to make it an ontological one, you are committing the mind-project fallacy -- the most prevalent error of reasoning in modern physics.

    Using epistemological statements ontologically, makes absolutely no sense and is equivalent to saying:

    What happens if I boil my political opinions to 50 degrees and subtract my love for cigarettes?.

    A good example is:
    "I don't know the exact state of the system, therefore the system exists in all possible states at once"

    In case you don't see it yet, and are tempted to agree with the above statement, remember that by saying the system exists in all possible states at once, you are saying that the system has no specific state and by definition, any attempt to measure a state of the system is a contradiction. This is only made worse by then positing that by observing "the state" (which we've established is meaningless), the system collapses into a particular state. What has essentially happened is that the individual has projected a deficiency of his brain (ignorance of the particular state of the system), to a deficiency of nature (indeterminacy of the state of the system), and has projected a change in his brain (obtaining new information about the state of the system) to an actually physical process happening in nature (the indeterminate system collapsing into a singular value). --- The mind-projection fallacy at it's best or rather worst.

    I'll encourage anyone here to investigate for themselves the difference between "epistemological statements" and "ontological statements"

    The best article I have found on the subject particularly as applied to QM is the following:
    Jaynes, E. T., 1990, Probability in Quantum Theory
  15. Oct 9, 2007 #14
    Refuted (PNAS 99 14632).

    That difference is basically moot (or philosophical) since physicists are always open to the possibility that new experiments will disprove their current theories, and as such, stating "the universe is a Lorentzian four-manifold" is merely shorthand for "measurements agreed with a Lorentzian four-manifold model".
  16. Oct 9, 2007 #15
    The original authors rebutted that 'refutation' three years ago. Neither Gill nor anyone else has attempted a response.

    Breakdown of Bell's theorem for certain objective local parameter spaces
    PNAS | February 17, 2004 | vol. 101 | no. 7 | 1799-1805

    Last edited: Oct 9, 2007
  17. Oct 9, 2007 #16
    mn4j, according to QM (which is experimentally proven) superposition and entanglement are fundamentally different than hidden variables.

    "I don't know the exact state of the system, therefore the system exists in all possible states at once"

    Superposition is proven by the fact that a particle can interfere with itself, not by the fact that we cant predict its properties.

    Either way, can you please take this argument to another thread because my original question hasn't been answered and I think this is completely irrelevant. Im asking for what quantum mechanics says on the topic, not your personal view. Especially when it has been shown that no hidden variable theory works.
  18. Oct 9, 2007 #17
    QM is not a physical theory but a mathematical model. If you have a theory with an infinite number of variables, you can fit any experimental data through it. That does not mean the data has validated the theory.

    If only you'll get over your religious beliefs and read the articles on Bell's theorem, you'll see for yourself that Quantum superposition is in effect a hidden variable theorem. Quantum superposition hangs on the validity of bell's theorem. Bell's theorem IS the supposed proof that no hidden variable theorem works. The articles I posted prove that Bell's theorem is wrong. Therefore, Quantum superposition hangs on thin air.
    There has NEVER been an experiment proving that a particle can interfere with itself. It is simply a postulate.
    Your question has been answered. The answer is "The question is a meaningless mix of ontological and epistemological statements"
    Last edited: Oct 9, 2007
  19. Oct 9, 2007 #18
    theres nothing wrong with the question. Its simply does an observer entangle with the observed state. The observer could be a photon or w/e, it doesn't have to be a macroscopic object.

    how can you say the double slit experiment with electrons DOESNT prove that a particle can interfere with itself? Unless your just being a pain and trying to say that experimental evidence is different from a proof, I dont understand what you mean by that.

    Also, Ill admit I didn't read the papers you posted (havnt had time). However that other guy posted saying those papers have been refuted:
    (I havnt read that either).

    Also, how can you equate believing in a generally accepted scientific theory to religion? I know that scientists accept these theories, and I know that scientists require proof (or experiment evidence). Its also ridiculous to expect someone to verify every single theory they read about. Other than the simple experiments they teach in school, I dont have time to try to reprove quantum mechanics (if I had time Id do it). Ive read enough about it to believe its "true" (like you said its a mathematical model it doesnt explain whats actually going on). However as far as a particle interfering with itself, I dont see how any other conclusion can be drawn from observing an interference pattern when sending single electrons through a double slit.
  20. Oct 10, 2007 #19
    Clearly you need a paradigm shift to be able to see what I'm talking about. And when you do see it, you will understand why most of the things you are posting here make no sense at all. Let me try to illustrate to you the difference between ontological statements and epistemological statements.

    Imagine I call you up on the phone and tell you I have a coin in my hand and I'm going to toss it. Then I toss it. You actually hear as the coin drops and settles to a stop. Then I ask you, what is the outcome, heads or tails? What will you say. The correct answer will be to say you don't know, which is exactly the same thing but more precise to say that there is a 0.5 probability that the outcome is heads and 0.5 probability that the outcome is tails.

    If you say it is "both heads and tails", or "neither heads no tails", I would think you are just being stupid because I look down and see clearly the state of the coin. This clearly tells you that there is a difference between epistemological statements and ontological ones. For the person who has observed the outcome, their observation is an ontological statement. For the person who is yet to observe the outcome, their statement is epistemological.

    - epistemological: the probability of the outcome being a "head" is 0.5
    - ontological: the outcome IS a "head", or the probability of the outcome being a "head" is 1.0

    As you see, the two statements appear to contradict each other but they are both correct in their contexts. It would be wrong for a person who has not observed the outcome and thus is making an epistemological statement, to suggest without any extra information that the probability for "head" is 1.0, even though ontologically that is the correct answer. Therefore, it is nonsensical to interpret a statement that was made epistemologically in an ontological manner.

    Every time somebody says the "the coin IS in a superposition of head and tails" that is what they are doing. It makes absolutely no difference whether you are talking about macroscopic objects, or photons and electrons. Every time a person uses wavefunction collapse as a real physical process happening at observation, that is, with the unstated assumption that something is actually happening to the coin or photon when it is observed, they commit the same error. Everytime somebody says there are two universes such that in one the coin is heads and the other is tails, commits this error.

    This is so fundamental, I dare say your future as a scientist (as opposed to a phenomenologist) hangs on you understanding this difference.

    You've got the sequence wrong.

    In the first paper, the authors showed that Bell's theorem was inaccurate.
    In the second paper, the authors attempted a rebuttal to the first paper.
    In the third paper, the original authors showed that the rebuttal in the second paper was based on fautly mathematics and reiterated more clearly their position showing exactly where the second paper's authors as well as Bell himself were wrong.
    It's been 4 years, since and nobody has attempted a response to the third paper, not even the authors of the second paper.

    Because it is not scientific and is mostly vain speculation.

    You should read some physics history. There are a lot of things that are commonly accepted but are unscientific. Unfortunately, you have to read the articles and be convinced by them rather than accept the claims at face value.

    You don't have to, there are many renowned scientists that have successfully challenged some of these commonly accepted dogma. You don't have to accept their claims at face value. You have to read it for yourself.

    The mathematical system of "epicycles" was developed 1000s of years ago to explain the motion of the planets and the solar system. It appeared to work, some people like Galileo, Kepler, Corpernicus challenged it, and many thought they were crazy for rejecting a commonly accepted theory. It turns out they were right.

    QM and QED are analogous to epicycles. They are mathematical tools that appear to work but are far from describing what actually happens. The sooner you realize this the sooner you can put your mental effort to an attempt at developing something better.

    As far as concerns an electron interfering with itself, you claim that sending electrons through a slit system one at a time and obtaining a pattern means the electron interferes with itself? It doesn't, a single electron produces a single spec on the screen not a diffraction pattern. The ensemble of electrons passing through the slits, produce the diffraction pattern. The only thing this experiment proves is that the slits sort out the ensemble of electrons passing through them into a pattern. It says nothing about the mechanism of the sorting although it is evident that the sorting is based on a property of the electrons themselves. There is no evidence of any "interference".
  21. Oct 10, 2007 #20
    A new question

    If one particle is entangled with a second particle, then the second particle is entangled with another one, then what is the relationship between the first particle and the third one?If the answer is just entanglement, then what is the essential function of quantum repeater?
    Thanks for replying:)
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