Entangled!
The subatomic world of quantum reality is strange, not like the everyday world we are accustomed to at all. But it underlies our perceived world and has profound (but disputed) implications for metaphysics, so please bear with me for a little while as I try to explain some of it.
By “quantum reality” I mean things and events that are quite tiny, less than about 100 nanometers long. They are called “quantum,” from a Latin word meaning “how much,” because the magnitudes of certain properties at this level can take on only discrete, not continuous, values. For example electrons orbit their nuclei only at certain discrete distances, not in between, so the electron is said to be quantized. So is light. You may have heard that light behaves sometimes as a wave and sometimes as a particle. The particle aspect of light is the photon, a quantum unit of light.
We can’t see electrons or photons, of course, but we can detect them through instrumentation, and their properties and behavior can be described mathematically by a formula called the “wave function.” Under certain circumstances the wave function divides into two or more pairs or branches, each with its own consequences. Each of these branches represents a potential future or a potential version of reality. When observed, only one of these branches is perceived; that is, only one of the potential futures becomes the actual perceived present. And you can’t tell in advance which one it will be. Things and events at this level are indeterminate, meaning that the outcomes of events cannot be predicted in advance, except in statistical terms. An initial configuration of things and forces does not determine a subsequent configuration. Mathematics can describe the probability of a range of outcomes, but cannot predict a single outcome.
Here is an example. The Stern-Gerlach experiment, named after the scientists who first performed it, consists of sending a series of electrons through a magnetic field, which deflects them. On the other side of the field from the emitter is a recording medium, which registers where the electron hits the medium. Each electron is detected at one of two places on the medium, depending on a property it has called “spin.” One finding of this experiment is that electrons are detected in only two places rather than in a range between them. Thus, an electron’s spin can take only two values; it is quantized. This corroborates the quantum nature of reality at this level.
Another finding is quantum indeterminacy: you cannot predict in advance where the electron will be detected. Given a great number of electrons and the known characteristics of the magnetic field, you can predict the relative number of impressions at each detection point. But there is only a probability, not an absolute certainty, that any single electron will end up in one place or another. An electron is not like a billiard ball. If you know the mass of two billiard balls, the amount of force and its direction applied to one, and the angle at which it hits the second, you can predict in what direction and how fast the second ball will travel. Not so with quanta.
This is weird, but it gets even weirder.
A subatomic particle called a pion decays and emits two photons, traveling in opposite directions. Each photon, like an electron, has spin, and you can measure spin in different directions. Think of a globe with a horizontal axis. As you look at it, it can spin so the surface you see goes up, or it can spin so the surface you see goes down. So the globe can be in one of two states, spin-up or spin-down.(1) Now imagine that it has two more axes, each at right angles to the others. We can call the axes X, Y and Z. The photon, unlike a globe, can spin along any of these axes, but along only one at a time. So we have three things to detect, X-spin, Y-spin and Z-spin, each of which can have one of two states, up or down. Thus there are six possible states: X-up, X-down, Y-up, Y-down, Z-up and Z-down.
The photon is a quantum object; before you measure it, its state is indeterminate. There is no way of telling, before you take a measurement, which kind of spin it has along any given axis. If lots and lots of pions decay and emit photons, we know statistically that half of the photons in each direction will be in state up when measured on the X axis and half will be in state down. But there is no way to tell in advance for a given photon which one it will be.
And you can measure only one axis at a time. Once you measure one axis, the others are indeterminate. Imagine several detectors in a line so that the photon goes through one and then another and then another, and so forth. If the first one measures X-spin and the second one does also, the second one will always agree with the first. So you know that, once measured, the X-spin stays the same. If the first one measures X-spin and the second one measures Y-spin, the Y-spin is indeterminate until you measure it. Half the time it will be up and half the time down, but you can’t know in advance which it will be for any particular photon. If a third detector again measures X-spin, that X-spin might or might not agree with the first measurement. (Yes, this is weird. As I said, Nature works differently at the quantum level from how it works at the classical level.)
When you measure one of the pair of photons – call it A – and then measure the other one – call it B –, they are always opposite. If photon A is X-up, you know for certain that photon B is X-down. If photon A is X-down, you know for certain that photon B is X-up. This is true no matter how far apart they are, a millimeter or thousands of kilometers. This is true even if the measurements are made simultaneously, so that there would be no chance of a signal traveling from A to B. This is true even if they are so far apart that light would not have time to travel from A to B between the time you measure A and the time someone (not you, because you are too far away) measures B, so that there is absolutely no way a signal could travel from one to the other.
Imagine two observers, typically called Alice and Bob. Alice observes the A photons and Bob observes the B photons. They are too far apart to communicate with each other, and they have not decided their observational strategy in advance, so neither knows exactly what aspect of each photon, X, Y or Z, the other will measure. After the experiment is over, they get together to compare notes. They find that when Alice observed X-up, maybe Bob observed Z-down, and when Alice observed Y-down, maybe Bob observed X-up, and so forth. But whenever they happened to observe the same aspect, the observations were correlated. Every time Alice observed X-up, if Bob observed X, it was X-down. Every time Alice observed X-down, if Bob observed X, it was X-up, without fail. And this is true whether Alice observed before Bob did, or Bob observed first or they both observed at the same time.
So here is the question: How does photon B “know” that Alice is observing X-up so that when Bob observes X, it must be X-down?
You might object that it is not mysterious. Suppose you take a coin and carefully slice it in half along the circumference so that one piece has the heads side and the other has the tails side. If you put each half in an envelope and shuffle the envelopes and then open one and it contains heads, you would know without looking that the other one contains tails. But quantum objects are not like that. They don’t exist as heads or tails (up or down) until they are detected. They have only a probability of being one or the other. To use the lingo, they are in a “superposition” of states. Only when a quantum object is detected does it unambiguously take on one property or another.
Albert Einstein and two colleagues, Podolsky and Rosen, developed a thought experiment that, they believed, proved that quantum theory was incomplete. Quantum theory says that you can’t know with certainty two different properties of the same quantum object, for instance its position and its momentum, or its X-spin and its Y-spin. The more closely you pin down one, the less precisely you know the other. But in this case you could theoretically know both X-spin and Y-spin. If Alice observes X-up and Bob observes Y-up, then we know that Alice’s photon is both X-up and Y-down, and we know that Bob’s photon is both X-down and Y-up. This is known as the EPR Paradox, the paradox being that even though theory says you can’t know two properties with certainty, here is a way you can. Einstein thought this proved that something, a hidden variable of some kind, one that we do not yet know about, determines the outcome, and that quantum indeterminacy was bogus.
Since then researchers have proved mathematically and experimentally that quantum theory is correct and that Einstein was wrong. Unfortunately, the math quickly gets very complex, and I am not competent to understand it, much less explain it. The gist of it is that classical (determinate) statistics say one thing about how often you would find combinations of properties, such as X-up, Y-up and Z-up, but actual experiment finds a different distribution. The results of the experiment do not agree with classical assumptions, but they do agree with quantum assumptions, so something about the classical assumptions must be wrong.
The primary assumption violated is called “locality,” meaning that what happens at one place can’t instantaneously affect what happens someplace else. Locality says there has to be some connection between them, some impetus traveling from one to the other. But in this case measuring photon A does in fact instantaneously affect the measurement of photon B. We appear to have what Einstein called “spooky action at a distance.”
Except it’s not action. No signal, impulse, stimulus or data of any sort is transmitted between the two. Instead the two photons appear to be aspects of the same thing. Each member of the pair is described by the same quantum mechanical wave function, and when it “collapses” into something determinate, both aspects become determinate at the same time. They don’t communicate; they are not transmitting information. They are connected, even though physically separate. In the lingo, they are “entangled.”
Last time we saw instances in the biological world where disparate physical elements act as one. Here is an instance at the very foundation of physical reality.
We have to be careful when interpreting quantum physics. The observed facts are unequivocal and repeatable, but what it all means is something else entirely. That quantum objects are sometimes entangled does not prove the mystical intuition that all is one, no matter how many new-age aficionados would like to believe so. But it does open the possibility.
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Notes
(1) Having no spin is not an option. The pion was at rest, having no angular momentum. When it splits, the child photons go in an opposite directions and have opposite spin. The sum of their spin equals zero, the same as the initial pion.
References
Blanton, John, et. al. “Does Bell’s Inequality rule out local theories of quantum mechanics?” Online publication, URL = http://math.ucr.edu/home/baez/physics/Quantum/bells_inequality.html as of 25 August 2011.
Felder, Gary. “Spooky Action at a Distance: An Explanation of Bell’s Theorem.” Online publication, URL = http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/bell.html as of 25 August 2011.
Harrison, David M. “Bell’s Theorem.” Online publication, URL = http://www.upscale.utoronto.ca/PVB/Harrison/BellsTheorem/BellsTheorem.html as of 22 August 2011.
Harrison, David M. “The Stern-Gerlach Experiment, Electron Spin, and Correlation Experiments.” On-line publication: URL = http://www.upscale.utoronto.ca/GeneralInterest/Harrison/SternGerlach/SternGerlach.html as of 29 August 2007.
Harrison, David M. “Bell’s Theorem.” Online publication, URL = http://www.upscale.utoronto.ca/PVB/Harrison/BellsTheorem/BellsTheorem.html as of 24 August 2011.
National Science Teachers Association. “The Stern-Gerlach Experiment.” On-line publication, URL = http://www.if.ufrgs.br/~betz/quantum/SGtext.htm as of 29 August 2007.
Rothman, Tony, and Sudarshan, George. Doubt and Certainty. NY: Helix Books (Perseus Book Group), 1998.
Thinkquest Library. “Bell’s Inequality and The EPR Paradox.” Online publication, URL = http://library.thinkquest.org/C008537/cool/bellsinequality/bellsinequality.html as of 24 August 2011.
Wikipedia. “Bell’s Theorem.” Online publication, URL = http://en.wikipedia.org/wiki/Bell’s_Theorem as of 19 September 2011.
Wikipedia. “EPR Paradox.” Online publication, URL = http://en.wikipedia.org/wiki/EPR_paradox as of 24 August 2011.
Wikipedia. “Interpretations of quantum mechanics.” Online publication, URL = http://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics as of 20 September 2011.
Wikipedia. “Introduction to quantum mechanics.” Online publication, URL = http://en.wikipedia.org/wiki/Introduction_to_quantum_mechanics as of 24 August 2011.
Wikipedia. “Pion.” Online publication, URL = http://en.wikipedia.org/wiki/Pion as of 24 August 2011.
Wikipedia. “Quantum entanglement.” Online publication, URL = http://en.wikipedia.org/wiki/Quantum_entanglement as of 24 August 2011.
Wikipedia. “Stern-Gerlach experiment.” On-line publication, URL = http://en.wikipedia.org/wiki/Stern–Gerlach_experiment as of 29 August 2007.
if you aren’t familiar w/ his work, check out Arny Mindell
aamindell.net
Arny uses quantum physics ideas and metaphors in his Process Work theory and tools for working with individuals, groups, inner work, etc.
He’s a fun and great teacher and human!!
Really a brilliantly simple presentation. Good on you!
One philosophical/psychological aspect of the EPR experiments is that they made it clear that when physicists are compelled by the evidence to choose between the principles of locality and causality, they will retain causality and abandon locality. Some are still holding out hope that the principle of locality can be patched up, perhaps through unexpected implications of hidden higher dimensions (through which the ‘distance’ might be much shorter for entangled particles). But clearly things are not as simple as our evolved 4D space-time perceptual/imaginative facilities would lead us to believe.
I’m sorry, but this doesn’t resemble philosophy as I understand the word to mean in English. Philosophy is about questioning assumptions and authorities to reach the most important questions (which is why Socrates is held up as the first philosopher). What I read above is the pronouncements of authorities new and old, the assumptions and propositions of Experts (in Greek: Sophists) going back hundreds or more years. It IS empirical science. It is NOT “philosophy”.
Once this distinction is admitted, another point rises: prominence. Science is respected. Philosophy is not. Therefore, if science concludes that an axiom of logic is false, logic must be pointless. More specifically, if science concludes that a proposition (the cat is dead) can be both true and false in the same sense, then we can safely flush the identity axiom into the toilet, and let logic circle down with it.
Interesting eh? Of course, since logic and reason are pointless, of what value is anything you or I have written? Without identity, maybe we’re saying the same thing, or opposite things, or both at once.
I don’t like telling people they are wrong because it generally doesn’t help much. But in this case, I feel compelled to point out several errors in what you say.
Socrates was not the first philosopher. There were numerous pre-Socratics, such as Anaximander, Anaximenes, Pythagoras, Heraclitus, Parmenides and others.
Sophists are not the same as experts. Not all experts are sophists and not all sophists are experts. An expert is someone who knows a lot about a subject. Sophists taught young Greek noblemen how to prevail in public discourse. (See “Alain de Botton Dodges the Question.”) It helped a sophist do his job if he was an expert in fields such as rhetoric and persuasion, but the concepts are not the same.
Philosophy is not empirical science, but it certainly includes an examination of the implications and meaning of the results of empirical science. Traditionally philosophy concerns three questions:
My blog post “Entangled!” has to do with the first of these questions, so it is too philosophy.
Science is much more than the pronouncements of authorities. Science is first of all a method for finding out things and secondarily a list of all the things we have found out, what we think is true. What we think is true has changed over the years, as more evidence has been discovered; but the methods of science – careful observation and investigation of empirical evidence, posing of hypotheses and then testing them, continual peer review, and so forth – have not. The findings of quantum physics are extremely well documented and tested, so it seems a bit disingenuous to call them mere “pronouncements.”
Science does not conclude that an axiom of logic is false. It relies quite heavily on logic. Take, for instance, the proof that Einstein was wrong:
This is a well-known logical argument called modus tollens: If P is true, then Q is true. Q is not true. Hence, P is not true. See http://en.wikipedia.org/wiki/Modus_tollens.
Science does not say that the proposition “the cat is dead” is or can be both true and false at the same time. I gather that you are referring to Erwin Schroedinger’s famous thought experiment. A cat is locked in a sealed box with a radioactive substance, and the substance has a 50-50 chance that one of its atoms will decay in a hour and hence (via a Rube-Goldberg-like contraption) kill the cat. Nobody can see into the box. So is the cat dead or alive before we open the box and look? The answer is, the cat is either dead or alive but we don’t know which. That is a far cry from saying that the cat is both dead and alive. Schroedinger’s point was that those who asserted that the cat is both dead and alive were ridiculous. In other words, he affirmed the principle of logic that you seem to think he denied.
Science does say that a quantum object can have a certain probability of being detected at more than one place. Before it is detected the probabilities are superposed, but not its actual existence. When it is detected, thereby becoming actual and not just probable, the object is at one or the other place, not both.
Neither philosophy nor science assert that logic and reason are pointless.
Hi Bill,
I’ll let our disagreement about what the words “philosophy” and “sophist” mean stand — as you said in your email, there’s little point debating such things.
However, I’m very glad to see that Schroedinger disagreed with what is generally understood about the story, as well as what is presentily written in Wikipedia under the entry for Schroedinger’s cat. I suggest you go edit it, for the edification of those, like myself, who read the article. As a fan of economics, I’m well away of the disconnect between popular understandings of issues in a field, and the “insiders” consensus about it.
Regardless of Schroedinger’s opinion, what did you mean when you said “They don’t exist as heads or tails (up or down) until they are detected.”. Did you mean “We don’t know whether it’s heads or tails until it’s detected?” similar to when you said “The answer is, the cat is either dead or alive but we don’t know which.” or did you follow the popular understanding that “the coins are neither heads nor tails”. In other words, is indeterminancy is a epistimological state, or a metaphysical one. Schroedinger aside, what is your opinion? If an espistimological state, then my complaint about that part of your article boils down to a very strange choice of words. If a metaphysical one, then I’m going to need a hair or two split for me before I see the post cleared of my charge of nihilism.
Have a fine weekend.
I am not sure what you are saying about the Wikipedia entry. Wikipedia (as of today, 26 September 2011) says “Schrödinger did not wish to promote the idea of dead-and-alive cats as a serious possibility; quite the reverse, the paradox is a classic reductio ad absurdum.” (http://en.wikipedia.org/wiki/Schrödinger's_cat)
Regarding the epistemological and ontological status of the findings of quantum physics, that is an important and fascinating topic, one which deserves more than I can do in a reply to a reply, so I will defer it to a whole blog post.
Extremely well done, Bill. A brilliant explication of a complex set of ideas.