Talk:Quantum entanglement/Archive 1

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transmitting information

(13 Feb 2003) Though I have read the discussion below about transmitting (and not transmitting) information, I am afraid I still don't "get it." Perhaps my brain stumbles on the meaning of "classical" information. To my average mind, "information" would include the fact that the state of a paired photon had changed. If that sort of "information" is detectable, it seems fairly easy to imagine that such transmissions could be scaled up and such transaction could be used to transmit any sort of "information."

What raised the issue is the latest of several New Scientist articles, http://www.newscientist.com/news/news.jsp?id=ns99993384, mentioning entanglement being used to transmit information. I confess I don't fully understand the articles.

In quantum mechanics, the only "information" you can obtain is by performing a measurement. In this case, this is the measurement of "spin". Nothing else can tell you anything about the system, including whether or not "the state of a paired photon had changed." As for quantum teleportation, see that article. -- CYD

I don't fully understand the article either (I'm not a quantum physicist). But is it possible to "scramble" a particle that has already been observed so that its state is again uncertain? Because this would open up a whole new can of worms.

I think the author of the unsigned comment above may have stepped on his/her own shoelaces. The issue of "no information transmitted" is not mysterious. Let's say that A goes off to Alpha Centauri to play the futures market. Other people need to wait for a four year transmission of laser telegraph or radio to know how much of what Earth wants to buy. Every buyer gets that information at the same time, so nobody has an advantage. If one guy can get information four years earlier than anybody else then he can buy or sell futures accordingly. If he has an ansible he is home free. But one hasn't been invented yet. If he can take an entangled particle with him, he thinks, then he can experimentally determine its state at a predetermined time and before that time somebody on Earth will have performed an operation that will make it "spin up" for buy and "spin down" for sell. But entanglement does not work that way because the person on Earth cannot force his particle to show up as he chooses. All he can do is to perform a measurement, and it will be either a "spin up" or a "spin down." He will know that his counterpart will get the opposite state, but they knew that much from the get-go. If they are experimental physicists they can check out what really happened. As soon as the guy on Earth makes his experiment he sends off a radio message saying, "I got a 'spin-up.' Did you get a 'spin-down'?" His counterpart responds, "I did get a 'spin-down'. Far out!" But no useful information about futures purchases has been sent or received.P0M (talk) 07:43, 23 June 2008 (UTC)

Yes. You use some device to observe (the previous state of) the particle. That device scrambles the state of the particle, so that the state (after the observation) is again uncertain. But that device also causes quantum decoherence, so the state (after the observation) is no longer entangled with the state of the other object. Is there some way we could make this more clear in the article? --DavidCary 18:36, 5 January 2006 (UTC)

I think that by giving a concrete example is probably a better way to understand an abstract concept. A good quantum physics text book should provide many solved examples for the readers. A solved example is helpful to me, but it would be a tedious job for the writer :) -- Justin545 (talk) 02:23, 25 January 2008 (UTC)

SPDC

Amir Aczel's book mentions Parametric Down-Conversion; he also mentions how Aspect was unable to create a completely random source and had to create a periodic source from the lasers. That note in fact bothered me; how is it possible to claim complete collapse of a wave function when the source comes from continuous lasing. Ancheta Wis 20:26, 17 Aug 2004 (UTC)

Aspect did not use SPDC, he used an atomic "radiative cascade", stimulated not by periodic laser pulses but by two continuous laser beams. It sounds as if Aczel got it a bit wrong. I don't pretend to know anything much about quantum theory, so can't comment on the collapse of the wave function. I have my own ideas about what really happened in Aspect's experiments, having read his PhD thesis, which gives a lot more detail than the published reports. See some of my ideas in http://arxiv.org/abs/quant-ph/9711044, or my page on local hidden variable theory.

Caroline Thompson 20:45, 17 Aug 2004 (UTC)

Disintangling Bell's theorem and quantum entanglement

I did some work trying to distangle Bell's theorem from the topic of quantum entanglement. It's really easy to observe quantum entanglement. It's also really easy to come up with a local realist theory that explains quantum entanglement. However, that theory will give different predictions than QM, and the hard part is to observe those differences in an airtight way.

Caroline Thompson 10:03, 21 Aug 2004 (UTC) I don't think experiments will ever be airtight, but a combination of experiments and theory should eventually do the trick. Experiments could easily put the QM predictions in trouble. It's not that I would not expect QM to come up with some modification that would cover any experiment. It is a very flexible theory. If, however, you change the conditions for a Bell test experiment, it is very much easier to modify the corresponding hidden variable model than the quantum mechanical one. As it stands, the basic QM model does not cover easily (or covers but unless modified makes wrong predictions for):

Failure of rotational invariance

Asymmetries of various kinds

Variations in beam intensities

Use of different makes and/or settings of photodetectors

Indeed, one of my main grumbles is that one cannot find published the basic facts about the behaviour of photodetectors: how they really respond to changes in input intensity. Perhaps I have not looked hard enough, but the impression I get is that it is universally assumed that the response is exactly proportional — this despite known facts about dark counts when the input is zero and saturation when it is too intense.

Put briefly. Demonstrating that quantum entanglement occurs is easy. Demonstrating that quantum entanglement occurs in a way that local realist theories cannot explain is a much more difficult. This is an important distinction because I don't think that any of the experimental papers cited here did the latter. Also, I don't think that any of the real world applications of entanglement have anything to do with Bell's test (i.e. quantum cryptography would work fine even if QM was wrong in its details.)

Caroline Thompson 10:03, 21 Aug 2004 (UTC)Yes, I know, and in fact even some of the things they do in relation to the development of quantum computing does not really need entanglement. Ordinary correlations would do.

Keep in mind that it took people about 40 years to figure out that you couldn't explain entanglement with a local realist theory.

Roadrunner 07:44, 21 Aug 2004 (UTC)

idea for faster-than-light communication

I have an idea: Since by adding/taking any energy into one entangled particle would add/take away from the other, this would warp the fabric of gravitational spacetime. If you did this, you'd be able to measure this (perferably) infintesimal gravitational pull by placing a network of lasers and ultra-sensitive sensors, you might tell if data is sent. since this is minutely economical, you'dd want it in a sattelite or something of that sort. Iamdalto 09:13, 3 Sep 2004 (UTC)

I think this is a great idea. The government must fund it. —Preceding unsigned comment added by 200.122.104.131 (talk) 20:58, 1 January 2008 (UTC)

cause of entanglement

i dont understand why if a particle is observed to be spin up the other one must be down, what causes that?

Because that's the way it actually woks when you try it in real life. QM is used as the explanation for how this "real life" works.linas 21:41, 5 January 2006 (UTC)

Cause of entanglement #2

I don't understand why quantum entanglement would necessitate "spooky action at a distance"... Isn't it possible that the "entangled" photons are just behaving deterministically and thus appear to be interconnected without actually being so? Rōnin 02:41, 10 December 2005 (UTC)

No, if they did behave deterministacally, it would violate Bell's inequality. This idea you propose is called local realism, and Bell's inequalit demonstrates that lcal realism is incompativble with QM. 21:39, 5 January 2006 (UTC)

That isn't quite what I'm proposing. I'm asking whether the two photons could simply behaving deterministically, which isn't incompatible with the idea of local realism, but isn't dependent on it either. Imagine that I have a green ball and a yellow ball and hide them under two cups. If I discover the green ball under one cup, I can assume that there will be a yellow ball under the other because the two balls always have a different value, without invoking quantum mechanics. One could imagine that something similar could happen when one splits photons. I realize there may be good reasons for assuming the photons are really interacting across a distance, and it's those reasons that I'm inquiring about. Rōnin 22:33, 10 January 2006 (UTC)

This is exactly what "local realism" is. Local realism says that, under one cup, the ball "really is yellow", but we won't know till we look. I'm not sure what the article on local realism states; if its not clear, then I guess it should be fixed. Maybe this needs to be clarified hree too? Anyway, this is why non-physcists like local realism: QM states that the ball really really is green and yellow "at the same time", until you look, and some people really hate that. linas 00:29, 11 January 2006 (UTC)

Slight correction, linas: the ball cannot be said to be either green or yellow till you look at it. Dave Kielpinski 14:32, 11 January 2006 (UTC)

But does anyone know why the explanation provided by quantum mechanics is preferred over the one I just proposed? Stating that it's incompatible with quantum mechanics is rather obvious, but it doesn't provide any explanation. Rōnin 14:19, 14 January 2006 (UTC)

Because local realism predicts different outcomes than QM for certain experiments; those experiments have been performed, and QM "won". The two predictions are compared in Bell's inequality. Your green/yellow shell-game example predicts the same answer as QM when the polarizers are aligned; but if they are off at a different angle, the predictions differ significantly. linas 20:05, 14 January 2006 (UTC)

Ah, thank you. What we need is obviously a ban on unaligned polarizers, then. ;) Anyway, thanks for humouring me. This issue is such an interesting one. Rōnin 17:17, 15 January 2006 (UTC)

Unanswered questions?

Maybe these will be considered silly questions, but I really want to understand this concept:

1. Are the entangled particles considered to be in a state of quantum superposition before a measurement on any of them has been conducted?

2. I found some confusion on this subjects by non-physicists - if one of the particles, after being measured, is having it's quantum state altered (if actually possible or only theoretically), will the entangled particle reflect that change?

Thank you! Nate --85.64.39.51 23:51, 30 April 2006 (UTC)

Reply:

1. superposition should not be confused with entanglement. any quantum state is, in general, a superposition.

entanglement is possible only in a composite system. superposition says quantum states are elements of a linear space. entanglement has to do with property of tensor product of linear spaces. not the same.

2. not quite clear what you're asking. take the simplest case. if you have a separable pure state $P\otimes Q$, and you perform a local measurement on, say , the second subsystem. then Q would, in general, collapse to some eigenstate of the observable you're measuring. clearly the composite state now changes as well.

Mct mht 00:15, 1 May 2006 (UTC)

Thanks for the reply. I got the picture on the first issue, apparently I have misunderstood the Quantum State term.

2. I have explored several websites and there were quite a few that made this kind of assumption about Quantum Entanglement:

Assuming you have a pair of entangled particles. Now one of the particles has been measured, therefore we now know the value of the other entangled particle as well. But then (after the measurements have been done) we go and change the properties of one of those particles. Will the other entangled particle reflect that new change in properties?

Nate --85.64.39.51 00:53, 1 May 2006 (UTC)

more technical details and context is needed in your question for a precise answer. the phenomenon you speak of is not an assumption. it's a physical fact. take a typical example. say a composite system is in a bell state(therefore maximally entangled) and a local measurement(say in the z-axis of the first subsystem) is performed. part of reason that, after that measurement, you can infer from the state of first subsystem that of the second is that, after measurement, the composite system collapses to a product state, i.e. separable. once the composite system is in a separable state, i.e. once entanglement is broken, what you do locally to one will not effect the other. Mct mht 01:31, 1 May 2006 (UTC)

Ok now I understand perfectly. So In fact the process of collapsing the system to a product state by it's nature breaks the entanglement. This should be noted in the article. Browsing the net I found that many simple-minded persons tend to misunderstand this "entanglement" as some kind of "supernatural" link between the particles where they can "read each other's mind" on a regular basis, which obviously is not the case ;)

Nate --85.64.39.51 01:56, 1 May 2006 (UTC)

the existence of entanglement simply follows from the fact that, given two linear spaces $V_{1}$ and $V_{2}$, not every vector in the tensor product $V_{1}\otimes V_{2}$ takes the form $v_{1}\otimes v_{2}$ for some $v_{i}\in V_{i}$. those nonproduct states are called entangled. Mct mht 02:12, 1 May 2006 (UTC)

Question to Which I Can't Find the Answer Elsewhere

Say you have streams (A and B) of polarization-entangled photon pairs. You put a polarizer at 0 degrees in the path of A and one at 90 degrees in the path of B, then measure the correlations of pass / no pass between each pair.

Does classical physics predict a 50% correlation?

What does QM predict? What is the simplest explanation for it?

What if the angles are 0 degrees and 45 degrees instead?

As I understand it, the interesting thing is that classical physics says that there should be no correlation between the two particles what-so-ever! In other words, whether or not one particle makes it through one of the filters should have no bearing on the other, but that's not what happens. There is no framework for entanglement in classical physics, at least not of the quantum kind, for once particle A passes through the filter, if it passes first, it immediately affects particle B. I don't remember the exact percentage, but I think the idea is that if particle A passes through the filter, particle B will either pass or not pass 50% of the time as well (depending on the polarization). Hope that helps, hopefully someone more knowledgeable in this area than I will be able to explain it better. -- itistoday (Talk) 04:02, 20 December 2006 (UTC)

Actually, let me revise that statement. In your example, if photon traveling down path A passes through the filter, then the other photon will not pass through the filter when the angles are 90 degrees. When they are 0 and 45... I'm not sure, but I think that there will be a 50% chance, or whatever is given by that cosine polarization-filter formula. -- itistoday (Talk) 21:17, 16 October 2007 (UTC)

not an experimental physicist but here's a reply. in classical physics, there is no such thing as two entangled particles---think of two ping pong balls, whatever you do to one will not affect the other.

as for the question, say your entangled pair is in the state (|0>_A |0>_B + |1>_A|1>_B)/ √2, where |0> and |1> are two orthogonal polarizations: 0 and 90 degrees. suppose, as you say, a 0 degree polarizer in put in the path of A. this amounts to a local measurement on the subsystem A. the possible outcomes of this measurement are easy to list: |0>|0>, |0>|1>, |1>|0>, and |1>|1>. (notice that irrespective of the outcome, the pair would be no longer entangled after this measurement).

the probabilities of each outcome are dictated by the initial state. in our case, we get 50% |0>|0> and 50% |1>|1>. so there's 50% chance that particle A will either pass or not pass. in case particle A passes, we know for certain that particle B has "collapsed" to the |0> state, i.e. it is polarized in the 0 degree direction. (the fact that the initial state is entangled might be worth emphasising. we have only interacted with particle A, but due to entanglement, the state of particle B has now "changed.")

now, still assuming A passes, if you put a 90 degree polarizer in the path of B, QM predicts that it will not pass, with 100% probability. the case when A doesn't pass is analogous: particle B will pass a 90 degree polarizer with 100% probability. Mct mht 07:34, 20 December 2006 (UTC)

This is a multi-paragraph reply

In classical physics, there is no such thing as entangled photons. In the classical view, a photon has an angle of polarization and its odds of passing through a polarizer are cos^2(theta) where theta is the difference in angle between the photon and the polarizer. If the photon passes, then thereafter its own polarization angle is the same as the polarizer. You could in theory set up an experiment where a source spits out pairs of photons that have identical but random polarization. This is the classical physics analogy to entangled photons that causes so much confusion. If you were to do that, and pass each photon through a polarizer set at the same angle, you'd get no correlation in results. Basically it would boil down to each photon having a 50% chance of passing through. This is the probability you get if you average the probability over all possible angles for the photons. So, 25% of the time they would both pass through, 50% of the time only one would pass through, and 25% of the time they would both be absorbed.

When the photons become entangled, then QM's prediction of the rate of correlation of the measurement results changes to cos^2(theta) where theta is the difference in angle between the two *polarizers*. So, when both polarizers are at the same angle, and the photons are entangled, the correlations are 100% (the cosine squared of 0 is 1) -- 50% of the time they both pass through and 50% of the time they are both absorbed.

The difference between the two sets of results is what Bell's Inequality is. The results with entangled photons mean we have to break at least one key assumption about how the universe works in order to make sense of it. Either the future affects the past, or paradox is possible, or the universe is deterministic. Why? Well, picture it. Say your pairs of photons are entangled and your measurement angles are the same. If you measure A and it passes through, then you know B will pass its detector, or will have passed through if it was already measured. This means that your measurement of A will have had to somehow guaranteed that B will pass through its detector. Either your measurement of A propagated backwards in time to when the entanglement was created, or it somehow instantaneously changed B faster than light. —Preceding unsigned comment added by 207.114.255.2 (talk) 18:56, 16 October 2007 (UTC)

While true, one could perhaps make the claim that you are comparing the "entangled photons" model versus the semi-classical case of non-entangled photons. If light is considered entirelly classical (as in classical electrodynamics) then the formula indicates the proportion of light going through the polarizer and not the probability of individual photons going through. When performing actual experiments the result that would come out of this third case should perhaps also be taken into consideration.Agge1000 18:55, 26 October 2007 (UTC)

Superluminal Communication

Can someone see if they can find any problem with this paragraph and the idea:

For example, in a Bell-state Quantum Eraser (see joot.com), if the polarizing lens is rotated, the interference field will instantly disappear at both detectors. Thus, it may be possible for two people at huge distances to communicate at superluminal speeds if they have a midpoint that constantly transmits polarized, entangled beams at both detectors. The only limitation would be the time it takes to start the communicator, which would be half the time it takes for light to travel between the two detectors. Afterwards, instantaneous communication could be possible by an alternating stream of interference fields and noninterference fields, like Morse code.

Awesomepenguin 02:24, 2 January 2007 (UTC)

Hi, sorry, but without reading any of that I can tell you it's not possible, no amount of justification will let you send information faster than the speed of light. Trust me, it's been tried by very very smart people, and all have failed. In addition to violating several fundamental physical theories, it would also create a paradox by allowing you to send somebody an answer before they asked you the question. Check out the book "Schrodinger's Rabbits" for more. -- itistoday (Talk) 03:38, 2 January 2007 (UTC)

: Whhhyyyyyyy? :-)

: Seriously, if polarising light changes it's quantum state, then if you polarize one entangled photon the other will be polarised too...? It might be theoretically impossible, but from an engineering point of view it sounds extremely plausible. What it boils down to for me is that if you can deliberately change the spin / quantum state of an entangled object, then you can use that to transmit information... unless of course the act of measuring it's entangled twin also destroys the information you've "sent". Probably does, actually. Is this correct?

: Please don't hit me, I'm a layman!

: --Badnewswade 00:48, 27 April 2007 (UTC)

It does seem kinda strange, doesn't it? But then Quantum Physics always does to we laymen. For example, I don't know nearly enough to know WHY you could send a message to the past. Mostly, I think it's the way the explanations have been, to be frank, dumbed down; the way it's presented, you do something to one particle and the 'entangled' particle does exactly the opposite at precisely the same time with no known anything connecting them (here of course lies the confusion. A layman could, for instance, visualize a doorknob turning and so the opposite doorknob turns in response, without any intervening machinery. Those of you who, unlike me, understand quantum mechanics (or who, like myself and Socrates, understand we know nothing about it) can see the problems inherent in this mindset.) Clearly, this explanation is flawed, because it appears action done to one particle can also affect the entangled particle before it happens, which is no where mentioned in the explanation given to laymen. Bah! I should take a class. I'd probably be LESS confused than I would be with the laymen explanations. I don't even want to think about what sending a message to the past could do it it were possible, unless we're in the whole branching worlds simulation. Jachra 06:34, 5 September 2007 (UTC)

Poe's entanglement

The first mention of entanglement in literature was made by Edgar Allan Poe in his November 1838 short story A Predicament. In it, the protagonist manages to have her head stuck in a church steeple's large clock face, with the sharp minute hand slicing her neck.

My eyes, from the cruel pressure of the machine, were absolutely starting from their sockets. While I was thinking how I should possibly manage without them, one actually tumbled out of my head, and, rolling down the steep side of the steeple, lodged in a rain gutter which ran along the eaves of the main building. The loss of the eye was not so much as the insolent air of independence and contempt with which it regarded me after it was out. There it lay in the gutter just under my nose, and the airs it gave itself would have been ridiculous had they not been disgusting. Such a winking and blinking were never before seen. This behaviour on the part of my eye in the gutter was not only irritating on account of its manifest insolence and shameful ingratitude, but was also exceedingly inconvenient on account of the sympathy which always exists between two eyes of the same head, however far apart. I was forced, in a manner, to wink and blink, whether I would or not, in exact concert with the scoundrelly thing that lay just under my nose.

Here, two unattached objects (eyes) act together while being separated by a distance, without regard to the position of the observer. This merits inclusion in the Quantum entanglement article, as both are products of the human imagination.Lestrade 19:29, 22 August 2007 (UTC)Lestrade

Although interesting - I don't really think this is appropriate to include in an article about 'quantum' entanglement. PhySusie (talk) 16:59, 17 January 2008 (UTC)

Why tensor product?

In section Pure States:

Consider two noninteracting systems $A$ and $B$, with respective Hilbert spaces $H_{A}$ and $H_{B}$. The Hilbert space of the composite system is the tensor product

$H_{A}\otimes H_{B}.$

My questions are:

1. Why the composite Hilbert space is from tensor product $H_{A}\otimes H_{B}.$, but not cartesian product$H_{A}\times H_{B}.$?

2. Is the tensor product a consequence which can be derived from Schrodinger equation?

Thank you!

Justin545 (talk) 00:26, 26 January 2008 (UTC)

Composite System and Tensor Product

It always be true the tensor product is accounted for the concept of composite quantum systems, quantum entanglement especially. As well, it is a big deal with respect to quantum computation. The massive Hilbert space of the composite system dramatically boosts the power of quantum computers. According to postulate of quantum mechanics, the Hilbert space of a composite system is the Hilbert space tensor product of the state spaces associated with the subsystems. But it's rare to see any article which can point out where such a postulate comes from. Is the postulate due to the overwhelming, experimental evidence? Is it a derivational consequence from fundamental quantum theory?

It's difficult to convince me of the ability and the power of quantum computation if no one can tell how the composite quantum system relates to the tensor product. Hopefully, the postulate came from the derivational consequence of quantum theory rather than just from the experimental evidence. After reviewed the original EPR paper, I came up with an idea. So I tried to explain it myself. Although the explanation is very likely to be wrong and even seems naive and optimistic, I would like to put it here to see if anyone could give some advice or correction to my faults. For simplicity, the following assumes all relevant state spaces are finite dimensional.

For a composite system of two particles, the wave function is

$\Psi (x_{1},x_{2})$ (1)

where $x_{1}$ and $x_{2}$ are respective positions of the first and the second particles. Similar to the way Separation of variables for solving PDE, if the wave function can be separated into multiplication of two functions such that

$\Psi (x_{1},x_{2})=U(x_{1})V(x_{2})$ (2)

As a result, the functions $U(x_{1})$ and $V(x_{2})$ can be viewed as wave functions for the first and the second particles, respectively. Furthermore, $U(x_{1})$ and $V(x_{2})$ are in Hilbert spaces $H_{A}$ and $H_{B}$, respectively. Therefore, the two functions can be expanded by their related basis such that

$U(x_{1})=\sum _{i}a_{i}|i\rangle _{A}$ (3)

$V(x_{2})=\sum _{j}b_{j}|j\rangle _{B}$ (4)

where $\{|i\rangle _{A}\}$ and $\{|j\rangle _{B}\}$ are respective sets of basis for $H_{A}$ and $H_{B}$. Substitute (3) and (4) into (2), we have

$\Psi (x_{1},x_{2})=\left(\sum _{i}a_{i}|i\rangle _{A}\right)\left(\sum _{j}b_{j}|j\rangle _{B}\right)$

$=\left(a_{1}|1\rangle _{A}+a_{2}|2\rangle _{A}+...+a_{m}|m\rangle _{A}\right)\left(b_{1}|1\rangle _{B}+b_{2}|2\rangle _{B}+...+b_{n}|n\rangle _{B}\right)$

$=\sum _{i,j}a_{i}b_{j}|i\rangle _{A}|j\rangle _{B}$ (5)

Since $|i\rangle _{A}$ and $|j\rangle _{B}$ are in different Hilber spaces, their multiplication is equivalent to their tensor product. Thus

$|i\rangle _{A}|j\rangle _{B}=|i\rangle _{A}\otimes |j\rangle _{B}$ (6)

Substitute (6) into (5), we have

$\Psi (x_{1},x_{2})=\sum _{i,j}a_{i}b_{j}\left(|i\rangle _{A}\otimes |j\rangle _{B}\right)=\sum _{i,j}a_{i}b_{j}|ij\rangle _{AB}$ (7)

That (7) is a state or vector in Hilber space $H_{A}\otimes H_{B}$. And (7) can be generalized to systems that involve more than two particles or subsystems. However, it is problematic such as

1. The method of the separation of variables can not guarantees to be the solution for every class of PDE. Likewise, not all wave function of form (1) can be separated into multiplication of two functions of form (2).
2. Even if the wave function (1) could be separated to the form of (2) "mathematically", but does it make physical sense to say that the functions $U(x_{1})$ and $V(x_{2})$ are two "wave functions" which are the component systems of $\Psi (x_{1},x_{2})$?

Well, I am neither a mathematician nor a physicist. I don't mean to offend or mislead someone with my words. I am just hoping to get more clue by this discussion.

Justin545 (talk) 10:12, 22 February 2008 (UTC)

Some questions

I would like to ask the august people present here some questions? I remember that string theory supposes that there are additional dimensions to our familiar three, yet that those are collapsed in a point.

A point is a mathematical idea that was long borrowed by physicists to simplify some calculations. One of the speculations introduced by quantum theory is that there is a limit to how small anything could be. There being a dimension assumes that there can be movements and measurements in that dimension. If you can find Flatland or George Gamow's One, Two, Three... Infinity, you can see discussions of what life would be like for a two-dimensional creature.

The question that people have asked string theorists is why, if there are more than four dimensions, there are no signs of movement or difference of locations in those additional dimensions. One answer proposed is that the dimensions might be so limited in extent that they would approximate a point, i.e., they might be so limited that it would be virtually impossible to measure them. If you and I started out side by side a mile from the Washington Monument, and then I climbed a long ladder 90 degrees doon in the fifth dimension and made the same measurement then I ought to find a slightly longer distance to the monument since I would be measuring the hypotenuse of a right triangle while you would be measuring the long leg of the same triangle. But if my ladder could only be an inch long then it would be hard to discover any difference in length experimentally.P0M (talk) 01:42, 14 May 2008 (UTC)

Thank's for the input. I´ll try not to sound very stupit, but why would we be able to discern the precense of additional spatial dimensions? If we think about the classical two dimensional creatures, how would they notice the precense of a three dimensional being or precense in they´r wiscinity? Say a person, i.e. one of us, would be standing on a flat plane, a two dimensional being would only perceive a two dimensional outline of the bottom of our feet, presumably. If we would be walking, we might to theyr perceptions appear,,,that is the tiny aspect of us they´d be able to perceive...to apparate and disapparate so to speak without regard to the rules in which they were used to things and familiar phenomena to be operating under. Tink about it, a line in theyr path would be an obstackle for them, which they would need to get around, while we would simply step over thus to them appear to disappear and then appearing where to them it would seem illogical for us to be appearing. Wouldn´t a two dimensional being be quite incapable of perceiving additional spatial dimensions, and might the same not hold for us? What I´m implying is that additional phenomena might be all around us without us being able to perceive them except in a very hapsard manner...somewhat in the manner of the inability of a two dimensional being to oberve how logical walking is for us. It would be very easy for us to stand abow theyr plane and thus to be completelly unobservable for them. Perhaps four or even five dimensional precenses or beings can do similar to us, perhaps they are.

One of the things that makes us believe that three spatial dimensions may not be enough to explain everything is that the Universe seems to be expanding, but it is not growing "at the edges." Every thing is getting farther away from every other thing. Imagine that our two dimensional creatures were on a balloon that was slowly expanding, but that the "planets" on this sphere were more like solid buttons. The buttons would stay the same size, but the distances between them would increase.

Indeed, the rational mind insists the universe must be inflating into something. That it must be a part of something waster, in some manner.

One way that a two-dimensional creature might decide that something odd was going on would be that, if s/he lived on a spherical surface the angles of a triangle would equal 180 over small distances, but would equal more than 180 degrees if, e.g., the triangle had its bottom formed by the equator and its two sides met at the north pole.

We can hope that means can be found to indirectly confirm the existence of additional dimensions.

Some people think that there may be more than three dimensions and that we may move in all of them, but say that if the possible range of travel in the additional dimensions is very small you would not expect to be able to notice. All that they are saying is that maybe there is something to look for, reason to think that mathematical ideas and string theories may not be fantasy, etc. P0M (talk) 07:47, 15 May 2008 (UTC)

I understand that mathematicians have created mathematical descriptions of four dimensional shapes. That may give grounds to hope, as mathematics frequently are decates ahead of technological trends, that technological means may eventually be discovered to confirm the existence of additional dimensions all around us.

There also does exist brane theory, which proposes that brains moving through some additional spatial dimension are causing our universe, or other universes as well, to come into being as they move about through that additonal dimension every now and a then touching. Now, if we can accept the presence of additional dimensions,the question might be quite relevant how that affects quantum systems.

This view is highly speculative. Can you document its source?P0M (talk) 01:42, 14 May 2008 (UTC)

Please excuse that I don´t have a book to go by, but this [1] is one of the linked articles available discussing it in a language the public can understand.

I)Might particles actually be moving in more dimensions than three, hence their movements being potentially fully measurable and predictable in four or more dimensions?

We already know that particles (and everything else) move in four dimensions, and that what we really have to measure is the movement of things in space and time if we are not going to have trouble understanding things like time dilation. The same reasoning would apply to movement in more dimensions. If we had some way of affecting a photon so that its speed from point A to point B was different from an un-messed-with photon traveling from point A to point B, then we might suspect that unbeknownst to us it was following a different path. A two-dimensional creature on the surface of a sphere might shoot off a photon that would follow the surface of the sphere and travel a great circle path from A to B at the speed c, and the somehow shoot off another photon that would go through the sphere (assuming it to be transparent of course) and get there in a shorter time. "Old fashioned" science fiction spoke of warping space to accomplish the same feat in our universe. P0M (talk) 01:42, 14 May 2008 (UTC)

Clearly I was talking about spatial dimensions. But think about a three dimensional person walking over a plane populated by two dimensional beings. Two dimensional beings are presumably used to the need to circumnavigate lines that obstruct theyr path, while to us such a line might only be a slight obstruction that can be overstepped, climbed over or othervice overcome beyond the sight of the two dimensional beings. To them, as we are appearing/disappearing to theyr sences not according to any rules they are familiar with, our path might seem wholly random as aspects of our path are invisible to them, while to our three dimensional perceptions our path is wholly logical and reasonable as we are unlike them able to perceive all of it but they only the aspects of our path which touches theyr plane. This is more in the way of the kind of analogy I´m talking about. The suggestion is that a particle may, if it´s movements in additional spatial dimensions can be accounted for, in fact be wholly predictable in its movements even though to our three dimensional perception capabilities its movements appear wholly random as there are aspect of its path we simply are not equipped to perceive. In other words that the randomness is only an illution due to our limited perception capabilites.

In other words might the presence of uncertainty only be due to our inability to measure their states through additional spatial dimensions? This assumes that higher spatial dimensions are not in a collapsed state.

I don't see how that is going to work. Using the above analogy, suppose that the two-dimensional creature didn't have good control of which path his photons would take. Then there would be uncertainty in how long a light signal would take. It would really mess up their physics because from their original naive point of view the speed of light would be different on different occasions. For all they knew the variations in the speed of light might be analogous to the variations in the speeds of horses or racing pigeons. P0M (talk) 01:42, 14 May 2008 (UTC)

I prefer the analogy of a three dimensional being walking across a plane populated with two dimensional beings. In the case of the light, the two dimensional being would only perceive some of the light being emitted. For all what we know a light cone is only the three dimensional wisible part. As far as I can see, as the two dimensional beings are unable ot observe a three dimensional light cone, just as well would we be unable to observe if a lightcone has a shape in an additional spatial dimension.

Of course all of this stuff is speculative. The Flatland discussions are intended to help us imagine how things could operate in higher dimensions. String theory is perhaps the first place where multiple higher dimensions may have real significance. If we really are in a Universe of many dimensions we may find ourselves stymied in investigating it very detailed ways unless we find ways to operate outside our own dimension. How could a two dimensional creature fire something into the third dimension? It's pretty hard to see a direct way. Suppose that a two-dimensional creature lived on a flat two-dimensional surface but managed, somehow, to do something like drawing a circle on that space and then causing the area within the circle to increase in area without permitting the circumference to increase. The only way that could happen would be for the surface to bulge into the third dimension. Any creature within the circle might notice that triangles no longer had a sum of 180 degrees or other such phenomena.

As we appear to be able to mathematically decripe some four dimensional shapes, that perhaps indicates that in the future it may become possible to create a device to observe in an additional dimension. Perhaps that´s all it would take, i.e. travel into a fourth dimension wouldn´t be necessary. Apparently dark matter is all around us. As far as we can tell, it forms no objects that can be observed. Maybe, the precense of dark matter is another indirect indication of the precense of additional dimensions as after all if matter of some kind has formed objects/structures that manifest only from a perspective of more than three dimensions, they presumably as a result would be pretty much invisible to our current means of observation with the sole exception of gravitical influences.

I'm not sure what this discussion has to do with entanglement. The entanglement phenomena seem to take place without anything traveling (or with something traveling at infinite speed), rather than something getting from place to place sooner than expected due to having gone through some kind of space warp. Discussion of entanglement put into question our assumptions about locality both in space and in time. Things that are "located in different places" and that are "located at different times" may interact. P0M (talk) 07:47, 15 May 2008 (UTC)

Hmm, think of it as establishing the groundwork. I think that additional dimensions are needed to explain entanglement. If for the sake of argument we accept the precense of additional dimensions, i.e. the 4th., 5th. and so on, and moreover that matter propably exists in those additional dimensions which only manifests to our observations so far through gravitational influences, that for one might explain why this dark matter interacts litle with our familiar matter as it may simply be moving around the three dimensional solids we are familiar with through an additional dimension of travel. If matter exists in those dimensions, it means particles exist there and probably that familiar particles interact with those dimensions as well...meaning that they move presumably also in a fourth dimension as well as our familiar three, and perhaps even more dimensions. Now, in the case of entanglement, I wonder if that phenomena may not even involve a 5th. dimension. I suggested earlier that if our universe may appear to be a three dimensional brane from a fourth dimensional perspective, it may even appear as simply a point from a fifth dimensional perspective meaning that through a fifth dimensional perspective all points within our own might be in the same place so to speak...remember that one dimension down from our perspective is flatland and two dimensions down from our perspective is a point. My thinking is that entanglement works in the way that the entagled particles essentially become one,i.e. that they have effectivelly become a single particle, being connected through a 5th. dimension where all points within our universe are together in one. That would explain the apparent instantaneous effect independent seeming from our observed distance. Hmm, I wonder what that would affect the 'no-cloning theorem.' According to my idea passing information independent of distance observed to us through the means of entanglement would neither be 'transmission' nor would it either be 'cloning' as no copy would be made of a particle, rather two particles would become one so that observed condition of the said particle at what to us appears to be the one end would inevitably be the same at the other. Whichever party would make the observation would decide the state of the merged particle.

II)Alternativelly, if higher dimensions are collapsed in a point, truly, from our perspective, might that be different from the perspective of a higher dimension? In other words might our universe be only a single point from the perspective of a higher dimension?

A straight line, no matter how long, looks like a point when seen from right on. But the logic of the situation means that it is in the lower dimensions that tesseracts get compressed into cubes within cubes, that cubes get compressed into squares within squares with their vertices connected, and that squares get compressed into single lines or one or two points. (Make a square out of soda straws and light it from inside and outside and then watch the shadows you can create.) P0M (talk) 01:42, 14 May 2008 (UTC)

It´s indeed a touchy, but how would our universe appear from a 4 dimensional perspective or 5 dimensional perspective. I have read the idea that from a 4 dimensional perspective our universe might appear to be a brane and moreover that there may be more such even infinite numbers.

That proposes the idea, that if a higher dimension is a factor in the movements of quantum particles, then by moving in that they may appear in different places within our universe without actually being traveling across the apparent to us spatial distance somewhat like they were traveling through a wormhole, say if we are a dot to the perspective of the higher dimension...then perhaps all dots within our universe touch that same dot in the higher dimension. Perhaps then superluminal communication to our appearance wouldn´t actually be superluminal as the particles wouldn´t be moving in three dimensional space through that distance. Is this just insane rambling?

One of the ideas that you seem to be circling around, and one that has to do with string theory, is that what kind of fundamental particle is observed depends on how a single string is vibrating. If you only have two dimensions (plus time) to move around in there are very limited ways that a "string" (actually a line in this case) could vibrate. If you have three dimensions then the dancing string can be more athletic. It can go upstage and downstage as well as going side to side and up and down. But I think the string theorists were not able to come up with enough vibrational forms that way, so they asked how many dimensions would be required to form a coherent theory that would account for the phenomena that are here to be observed. So we could see something as "just a point" that was enthusiastically bopping back and forth along a line in the Nth dimension, just as a two-dimensional creature could see a sewing needle poking in and out of the membrane he lived on as a motionless little circle. P0M (talk) 01:42, 14 May 2008 (UTC)

—Preceding unsigned comment added by 194.144.20.188 (talk) 00:26, 14 May 2008 (UTC)

Yes, that is the general idea. I´ve been wondering wether there aren´t actually 4 dimensional precenses around us. Think about it, how would they appear to us. Wouldn´t we only be able to perceive them very fleetingly, and apparently quite randomly, somewhat in the manner of a two dimensional creature to which our appearances would indeed appear very fleeting, random and outside the rules of behavour they´re use to. If there are four dimensional appearances randomly, as far as we can perceive them, appearing/disappearing, without behaving in a way that appears reasonable to our standards...wouldn´t most of us simply think it an illusion and clean reject the whole idea?

Here is a thought experiment that may further your thinking about dimensions and physical changes. I'll leave working out the experimental details to others. Suppose that you have a rotating disk. It happens to be rotating clockwise. Its axle is attached to the wall of a transparent cylinder that is about as wide as the disk's diameter. Next you perform the following operation: You rotate the cylinder half a turn in either direction, d=½. Now the disk is pointing away from you, and you perceive it as spinning counter-clockwise. To restore its original spinning mode (from your point of view), you perform that operation a second time, d=1. Each of those operations is equivalent to flipping a spinning top. Next you replace the ordinary cylinder with a Möbius strip. (Make a long strip of paper, give it half a twist, and paste the ends together.) When you rotate this modified holder the full distance, d=1, as you rotated the original cylinder to get its original spin back, you will find that you have changed the spin from clockwise to counter-clockwise. So to get the original spin orientation back, you have to perform the same operation four times rather than twice. Even more unexpectedly, if you view it at its near and far locations, equivalent to flipping the top, you will see the sequence cw, ccw, ccw, and cw. If it were an apparently regular top and it happened to be spinning clockwise before you flipped it the first time, you would fine it spinning counter-clockwise the next time. O.K. so far, so you flip it again, and it is still spinning counter-clockwise. Then you flip it a third time, and this time it seems to meet your normal expectations and it comes up spinning clockwise again. Maybe you would go away satisfied that you had just failed to flip it the second time you tried... but then you try it again. The Möbius strip is a curiosity because both sides are the same side. There is a three-dimensional analog called a Klein's bottle. Do not store your wine in a Klein's bottle. It may turn hallucinogenic. ;-) P0M (talk) 03:01, 14 May 2008 (UTC)

This section seems to be something stuck in by the writer without regard to context. Probably it should have been put in at the bottom of the page. P0M (talk) 03:01, 14 May 2008 (UTC)

Ontological Theory & Entanglement

What is the relationship between these two theories? Entanglements seems much closer to Bohm & Hiley's Ontological Theory of Quantum Mechanics than the Copenhagen Interpretation but I have read that no-one takes the Ontological Theory seriously. Bohm is mentioned in this article but it is not clear what his relationship with Entanglement is. Can anyone explain?

Also, does Entanglement now make the Copenhagen Interpretation redundant?

ThePeg (talk) 22:48, 17 November 2008 (UTC)

In my understanding, all the Copenhagen group did was to curb their own impulses when it came to projecting macro-world observations onto the quantum domain. They said, basically, we can only talk about what we observe. It is puzzling to me that the article starts with an argument about what is "really" going one rather than describing what the observable phenomena are.

The work of Dr. Bell has not gained complete acceptance. I think there are still some hold-outs who think that his conclusions ought to be able to be refuted. So far they have not succeeded. If he is right, then it is my understanding that there cannot be unobserved determinations that are carried with quantum scale entities that otherwise appear to be in an indeterminate state until an interaction with the environment forces them to take some determinate state. If you believe that every entangled particle has its unobservable but determinate characteristics, then it is no wonder that after a long period of time has passed and two photons have arrived at different detection screens at different times, one will turn out to have, e.g., vertical polarization and the other will turn out to have horizontal polarization. If Bell is right, however, they cannot be pre-determined because if that were the case then the statistics would work out in a different way. (Brian Greene describes this stuff very well.)

So let's go back to the experiment and question our observations and our conclusions. The first thing we have is a laser. What do we know about lasers? If we apply an electrical potential to the device we observe a screen at some distance from the laser getting lit up. With improvements in technology we can make things so that throwing an electrical switch results in a single photon getting recorded on a sensitive detection device. So all we know is that we do something on one end and get a certain result on the other end. What happens in between is "in the black box." One of the surprising things that was discovered early on was that if you try to poke around inside that black box all you do is to move the effective detection screen closer to the laser. What happens between the laser and whatever you did to try to get inside the black box is just in a smaller black box.

Now we put a BBO in front of a laser. All we know about it for sure is that it is a crystal grown from a certain combination of atoms of different elements. We can try to understand what is going on after we discover that it is doing something interesting. What people have discovered, which must have been a surprise in itself, is that when a photon hits the BBO crystal it "disappears" and two photons emerge going in two different directions that are sort of extensions of the original path. That is interesting in itself. It is a little like the "day glo" type effects where ultra-violet light is down-converted to visible light making something seem unnaturally bright for the ambient illumination. But then when the BBO photons are investigated we see that they are always coordinated as to polarizations and other characteristics.

But in making the above description I have added in a bunch of my own usually unconscious assumptions. It's the same kind of assumption that someone encountering echoes for the first time might make about the sources of the sounds: it appears that there are two people yelling, one near at hand and one responding from a distance. Because different spots, one on each of two detectors, get lit up at times that are related to the distance from the BBO by the speed of light, we assume that there are two things. Having decided, on the basis of our knowledge of billiards and similar macro phenomena, that there are two things that arrive at the two places, then we cannot easily explain to ourselves how two things that are remote from each other are coordinated in their interactions with the environment.

If your model of the universe does not make the assumption of discrete entities then how does the experiment look? We do whatever we did before to get one spot lighted up on a detector screen, but we put in a BBO, and then we get two spots lighted up in two places at half the energy and differentiated polarity, etc. We didn't know what was going on between the laser and the detection screen in the first place, and now we don't know what was going on between the laser and the two detections screen either, except that there is a crystal involved and some correlations between what appears on one detection screen and what appears on another detection screen.

If there were something that one could do to determine beforehand which of the entangled photons was going to turn out to be vertically polarized and which was going to turn out to be horizontally polarized, then there would be some point in talking about "hidden variables." But if all you can do is to say that if there were hidden variables then we could predict the outcomes, but we don't have access to the variables because they are hidden, then we are just playing games with words. And we would still have to deal with the Bell inequalities.

Our current understanding of photons is that a photon is produced when an electron falls from one "orbit" to a lower "orbit," and that when a photon is absorbed an electron is boosted from a lower "orbit" to a higher "orbit." In molecules, electrons do not necessarily belong to one atom or another atom. In crystals, electrons may have much more tenuous relations to position within the crystal. But there appear to be some major differences between absorption of a photon by elemental hydrogen and absorption of a photon by a crystal. What happens within the crystal and how would we ever know what happens? It would appear that a photon enters the crystal, something happens to one or more electrons, and then it is as though two electrons fell into lower "orbits" in coordination with each other and produced a photon that is the same thing in different places and going in different directions, or you might say that they produced two photons that are linked outside of our space-time. I don't know whether anybody has tried to figure out the configurations of electrons and their energy levels in a BBO. I guess it is theoretically possible to do. I'd certainly like to learn of it if it has been done. P0M (talk) 06:06, 18 November 2008 (UTC)

Some sources that may be useful:

http://qopt.phys.msu.su/english/history.html

I've had a quick look at spontaneous parametric down-conversion articles. It may be that the BBO and other crystals function as wave guides that do interesting things to the frequencies and amplitudes of photons, so there may not be absorption and re-emission as I had mentioned above. I find it difficult to visualize/conceptualize how one wave front can "bend" in the middle and thereafter progress in two directions -- but probably that is not the right analogy anyway. P0M (talk) 10:37, 18 November 2008 (UTC)

early plea for improvement

For example, it is possible to prepare two particles in a single quantum state such that when one is observed to be spin-up, the other one will always be observed to be spin-down and vice versa, this despite the fact that it is impossible to predict, according to quantum mechanics, which set of measurements will be observed.

Spins involve rotary motion around a axial direction vector. So you can't measure spin unless you have a defined axial direction vector, which has to be identical with that of the compared location of interest. So your measuring would have to carry a gyroscope to keep track of direction. But let's say you can. Then you have to know which is the top and which is the bottom if the spinning entity, because a spinning object spins one way when viewed from one axial direction and the other way when viewed from the other. And a single spinning entity can create 2 spinning appendages (one on each end) and each one can have either spin, depending on which end is its top. And note that when two equal but opposite spinning objects cancel each other, nothing physical is lost except the identity of the spinning entities' constituents.WFPM (talk) 23:36, 28 April 2010 (UTC)See User talk:FyzixFighter Quantum teleportation WFPM (talk) 00:28, 29 April 2010 (UTC)

Extra Dimension explanation

I was thinking that this has been a closed problem as a proof for an extra dimension. To explain in flat-lander logic, a flatlander might see 2 lines behaving into relation with each other, because they are a projection of a 3D shape into their world. If your an architect the idea is that rotating a 3D part would also change front-view, side-view and top-view of a drawing. Well back to the flatlander he sees 2 lines moving but not conected, he cannot imagine a shape that connects both lines to something of a 3D shape. A flatlander is able to move a single 2D line and he can see it does effect the other line as well (because he pushes in reality a 3D shape). a 4D shape could explain such a similair behaviour for us 3D-landers. There we wouldnt see the logic of moving something that moves as well another part of itself far away, to us there is distance between it, in 4D it can be just one shape. Other problems like double split experiment are explainable as well with an extra dimension. Also there a lot of theories of extra dimensions in other fields of physics so i was thinking this was solved, am i wrong on this ?? —Preceding unsigned comment added by 82.217.115.160 (talk) 19:55, 5 May 2010 (UTC)

Actually, I think this is one possible model for understanding what is going on.

Suppose that two things can have x, y, z, and t separation, but can have 0 separation on some other dimension w, or can have some separation in the w dimension also. On the assumption that there is 0 separation in the w dimension, there is no problem in understanding the changes involved in the experiment. On the assumption there there is some separation in the w dimension, there must be a single "object" that has extension in the w dimension. So in that case, turning one end of the "rod" produces an equal change in the other end of the "rod." But we know nothing about the characteristics of this "rod." Is this "rod" perfectly rigid? P0M (talk) 00:02, 8 May 2010 (UTC)

Quantum Entanglement

This is my opinion. “”All things physical to the furthest ends of the universe, are made of atoms. There are a few more than 100 atoms that combine to make everything physical. This includes gases, liquids and solids. All atoms are composed of protons, neutrons and electrons. Positive, neutral and negative charges of energy. It is the positive and neutral charges that make up the nucleus. This is the stuff we power cities with, and can blow them up as well. "Quantum entanglement", is the interconnectedness of nuclear energy, as well at some foundational level electrons are interconnected. It is easier to envision this, as frequencies, like signals being sent from a transmitter to a receiver. Scientific studies have proven, that you can think about a plant that is hooked up to and EKG, on the other side of the globe, or even to the furthest extent of the known universe, and the plant reacts instantaneously, not an instant before, not an instant after, but the very instant that you consider it. this proves a couple of things, one is the fact that thought is quicker than light,"I have heard it takes light one second to go around the globe three times", it also proves, "Quantum Entanglement",the interconnectedness of all things physical, at a foundational level. Here is something that will help you to understand, when you click to stones together, you get a spark. What has happened here, is you have broken an atom open. The positive and neutral charges, "the nucleus", are entrapped by up to seven shells of electrons. This directly correlates with the seven colors of the light spectrum. So, when you knock two rocks together, you break that bondage, that allows the nucleus to escape. It will naturally overwhelm as many atoms as it can, as it tries to come back to source. This is the same principle that develops into procreation. At a foundational level, everything is trying to come back to source. The ideal is this, "Quantum energy", is saying, "mass body of energy". It was scientifically proven in 1960s, that the nucleus of any, and all atoms, is a particle of light. So when you say quantum energy, you are talking about a solid body of light particles. This solid body becomes entrapped, to develop into everything physical. As I am a Christian, I correlate, "Quantum Energy", with what is termed, "Body of Christ", "Christ Is the Light of the World". —Preceding unsigned comment added by RevDrLee (talk • contribs) 03:37, 7 July 2010 (UTC)

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