Last night I watched an episode of one of my favorite TV shows, PBS's Closer to Truth, which deals with scientific perspectives on questions of philosophy and theology. The episode was called “Why Is the Quantum So Weird?” From Scientific American magazine to popular pseudoscience books like The Secret, we are told that the world of the very small is extraordinarily strange, counterintuitive, unlike anything we can relate to in everyday life — where particles can seem to be in two places at once, go backward in time, “tunnel” through impermeable barriers, etc. We know that on very small scales, these quantum phenomena do occur, and in fact quantum mechanics establishes the theoretical basis behind everything from transistors to quantum computing. So, why is the quantum world so strange?
The episode provided an excellent run-down of quantum theory, but it didn’t provide a satisfying answer to the question. That’s because it’s not the right question. We should be asking, Why is the ordinary world not weird? Because this is a question we actually have an answer for.
Assigning a value-judgment word such as “weird” to quantum phenomena betrays how biased we humans can be. We expect things to behave the same on all scales, large and small, because that’s how a physically consistent universe should be. If a tennis ball can be in only one place at once, we assume that the same must be true of an electron. In fact, today many physicists agree that the world does behave the same on all scales; however, this behavior is most accurately described by the laws of quantum mechanics — even the behavior of the entire universe as a whole. (This is the scientific basis for the parallel universes of the famous “many worlds interpretation.”) In other words, the whole entire universe on all scales is “weird.” So why does it make sense to us? Why do we never see evidence of a tennis ball being in two places at once, or passing through a brick wall, as we do for subatomic particles?
The answer relates to something called quantum decoherence. Discovered in the late 1980s, decoherence refers to the loss of coherence, which is the property of a quantum system (such as an electron) that can give it an uncertain, blurry or smeared out physical description. An electron in a coherent state can be in superposition, meaning that its precise location, momentum, spin, etc., is undefined or blurred out: It appears to possess many values for these things at once. (Most people learn about this bit of quantum weirdness by way of the electron cloud that surrounds an atom's nucleus, but free electrons and other particles have this property as well.) However, if that electron encounters an electron detector, the system of the electron and the detector will undergo decoherence, and the electron will appear to suddenly “snap” into one definite state. You often hear this process described as the collapse of the wavefunction, although that phrase is becoming increasingly archaic among the physics crowd.
Decoherence causes ordinary macroscopic objects to behave differently than subatomic particles; unlike electrons or photons, they always exist in definite places and follow well-understood and predictable or classical laws of motion. To experimentally prove that decoherence is responsible, just take an object and put it into a coherent state of superposition, and keep it that way — prevent decoherence from happening. To achieve this feat, though, there’s one thing you need to do: The object must be completely removed, or decoupled, from interaction with its environment. For example, it needs to be kept incredibly cold, at a fraction of a degree above absolute zero. This is because the moment the object starts getting hit with photons of heat or light, those photons begin to “observe” the object. In doing so, they carry away enough information about the superposition that the object appears to collapse into one definite state, with astonishing speed. Decoherence ensures that anything that’s directly observed in any way at all cannot remain in a state of superposition. Even though everything in the universe obeys those “weird” laws of quantum behavior — all the time — whenever there’s any kind of observation going on, decoherence destroys that quantum weirdness. In the process, it creates a world that makes sense.
Actually, decoherence only destroys the weird aspect of nature; it doesn’t destroy the alternate states represented by a superposition, or change anything about the way quantum mechanics works. If a tennis ball in a quantum superposition undergoes decoherence, information describing those potential alternate states still technically exists in the world. It’s just that it has been irreversibly lost to the chaos of the environment, and like Humpty Dumpty, no amount of effort will be able to restore it. The “blurry” aspect of a tennis ball that that has undergone decoherence is a little like the kinetic energy of a car with the brakes applied: It isn’t destroyed altogether, but only gets dissipated into the environment. Once this happens, the probability of any alternate state reappearing — for the alternate positions or momenta of all of the ball’s particles to randomly reconstitute themselves, allowing us to see a second ball — becomes vanishingly tiny.
So the next time you’re playing tennis, and you hit one definite ball back to a definite spot on the court, you can thank quantum decoherence for making it possible.
Showing posts with label superposition. Show all posts
Showing posts with label superposition. Show all posts
Thursday, January 27, 2011
Friday, December 3, 2010
Superposition: Not That Strange (1/29/2010)
This was originally posted on a horrible site called Myspace. When Myspace underwent a redesign in Fall 2010, hundreds of insightful reader comments that had been left over the years were lost. I have since deleted my account there.
Quantum mechanics is considered one of the weirdest areas in modern physics, and one of the weirder aspects of quantum mechanics is the idea of superposition. Superposition is the theoretical coexistence or "overlaying" of more than one state of an object, where that object's state is unknown. For example, when a photon of light has not been measured for location, its location is said to be a superposition of all its possible locations. Similarly, a radioactive atom that may or may not have decayed is said to be a superposition of its decayed and undecayed states — that is, until we look to see if the atom has decayed or not, at which time it can be described as being in one state or the other. This is not an intuitive concept on the human scale, as best illustrated by Edwin Schrödinger's famous thought experiment:
Imagine a cat in a box along with a radioactive atom, plus a mechanism whereby poisonous gas is released if the atom decays. If the atom has a 50/50 chance of decaying during the hour-long experiment, does that mean the cat is in a state of superposition — i.e., simultaneously both alive and dead — right before the experimenter opens the box? In fact, Schrödinger used this thought experiment to show that it's silly to consider a macroscopic object like an animal being in superposition like this. A cat that's both alive and dead? Yeah, right!
And yet, some interpretations of quantum mechanics — including my favorite, the relational interpretation — suggest that yes, a cat in that situation is, in fact, "both alive and dead." So how can we wrap our brains around this notion?
I have a 16-year-old cat, Pokey. (You may know her as Miss Delilah.) Let's say she has a 50/50 chance of living to age 20. How do I presently describe her state on January 29, 2014? You guessed it: The 2014 cat, as described today, is both alive and dead. As I write this, her future state is a superposition of two states, simply because her future is uncertain to me. Superposition doesn't seem strange at all when viewed this way, because we're used to things in the future being uncertain. It's only strange to think of an object as being in two "overlapping" states at once — but you don't need to think of it so literally. A better description is that from our (present) perspective, the future cat is in a probability state, where there's a 50% chance of 2014 Pokey being alive and and a 50% chance of her being dead.
Similarly,* Schrödinger's cat isn't somehow a ghostly overlap of alive-and-dead cats in that box. From the perspective of the experimenter, the animal simply dwells in a probability state whose final outcome has yet to be discovered. You could say that even though an hour has elapsed and the experiment is over, the state of the cat remains in the experimenter's future, and therefore it's uncertain and/or in superposition.
This view of superposition isn't just my twisting of QM theory to make it more palatable to human intuition; it's fully consistent with the relational interpretation. Consider the three main tenets of RQM:
1. We cannot attribute any absolute states or properties to any object, in and of itself. It would be wrong to say, "The apple has an absolute velocity, spin, color, etc., independent of other objects or observers." Rather, states and properties can only be defined in terms of interactions between things — whether they be microscopic or macroscopic, inanimate or living, observing or not observing. RQM makes no distinctions among these. Our observations of the world consist only of our interactions with objects in the world (e.g., our perception of an apple appearing to be the way it is), not any absolute properties of the objects themselves.
2. Two observers can have different, but equally accurate, descriptions of one object or system, depending on the nature of their respective interactions with that system.
3. Our description of any system depends specifically upon the information transferred or extracted during our interaction with the system.
The key here is that relational quantum mechanics is a theory about information. If we're involved in an interaction where we acquire information about an object's momentum, for example, the property of momentum then becomes defined by us for that object; before that, it remains undefined or uncertain to us. In the case of Schrödinger's cat, at the end of the hour, the cat has received information about the state of the atom, and that is why the cat — from its own perspective — is either alive or dead. Meanwhile, though, the experimenter has no information about what went on inside the box, so from his or her perspective, the cat is in superposition, i.e., its state is uncertain.
In the future, literally everything is uncertain to us, dwelling in probability states only.** The total lack of information from the future means that everything about it must remain undefined. Even the surest bet we know, that the sun will rise tomorrow, is a probability; there is a
tiny but non-zero chance that the Earth's rotation will be halted by an
asteroid impact before then.***
And this may be a stretch, but you can apply the same principle to the very distant past: How did the first reproducing life form, our earliest ancestor, come about? We have absolutely no direct information about this event, so the best we can do is offer potential scenarios and gauge their respective probabilities. You could say that from our current perspective, the various possible earliest life forms exist in superposition — but "uncertain" feels a lot more natural.
Perhaps that's the most confusing thing about superposition: the word itself. It conjures up an image of overlapping, partially transparent alternate versions of an object. It's no surprise that students and researchers alike have been uneasy about the concept for 80-some years. But superposition is merely uncertainty based on a lack of available information. That's all it is.
* One can well argue that traditional superposition is a mathematically "real" state of affairs for an atom or even a cat, whereas my conception of "future" superposition is a metaphorical extrapolation. While it's definitely an extrapolation, there are fewer differences between these cases than you might expect. Mathematical uncertainty is mathematical uncertainty, such descriptions having no direct bearing on the actual nature of the objects themselves. But that's a topic for another day.
** I floated this idea in an earlier essay, where I argued that time appears to flow in one direction because information only comes from the past, never the future. Even though the "arrow of time" is often explained thermodynamically, where the inevitable increase in disorder (entropy) points in only one direction, that explanation doesn't shed much light on the phenomenon in conscious observers of a definite and unidirectional "flow" of time. Is it a coincidence that both arrows point in the same direction? No — entropy is a key element in quantum information theory.
*** Someone once asked me that if the "realness" of objects depends so strongly on our observations, do the table and chairs in his dining room go away when he goes to bed? It's not that they "go" anywhere; it's just that as soon as information on them stops being collected, they lapse into an increasingly uncertain probability state. There is a small but non-zero chance that his house will be emptied by robbers, a flood will wash out the downstairs, etc., and this goes up the longer he sleeps.
Quantum mechanics is considered one of the weirdest areas in modern physics, and one of the weirder aspects of quantum mechanics is the idea of superposition. Superposition is the theoretical coexistence or "overlaying" of more than one state of an object, where that object's state is unknown. For example, when a photon of light has not been measured for location, its location is said to be a superposition of all its possible locations. Similarly, a radioactive atom that may or may not have decayed is said to be a superposition of its decayed and undecayed states — that is, until we look to see if the atom has decayed or not, at which time it can be described as being in one state or the other. This is not an intuitive concept on the human scale, as best illustrated by Edwin Schrödinger's famous thought experiment:
Imagine a cat in a box along with a radioactive atom, plus a mechanism whereby poisonous gas is released if the atom decays. If the atom has a 50/50 chance of decaying during the hour-long experiment, does that mean the cat is in a state of superposition — i.e., simultaneously both alive and dead — right before the experimenter opens the box? In fact, Schrödinger used this thought experiment to show that it's silly to consider a macroscopic object like an animal being in superposition like this. A cat that's both alive and dead? Yeah, right!
And yet, some interpretations of quantum mechanics — including my favorite, the relational interpretation — suggest that yes, a cat in that situation is, in fact, "both alive and dead." So how can we wrap our brains around this notion?
I have a 16-year-old cat, Pokey. (You may know her as Miss Delilah.) Let's say she has a 50/50 chance of living to age 20. How do I presently describe her state on January 29, 2014? You guessed it: The 2014 cat, as described today, is both alive and dead. As I write this, her future state is a superposition of two states, simply because her future is uncertain to me. Superposition doesn't seem strange at all when viewed this way, because we're used to things in the future being uncertain. It's only strange to think of an object as being in two "overlapping" states at once — but you don't need to think of it so literally. A better description is that from our (present) perspective, the future cat is in a probability state, where there's a 50% chance of 2014 Pokey being alive and and a 50% chance of her being dead.
Similarly,* Schrödinger's cat isn't somehow a ghostly overlap of alive-and-dead cats in that box. From the perspective of the experimenter, the animal simply dwells in a probability state whose final outcome has yet to be discovered. You could say that even though an hour has elapsed and the experiment is over, the state of the cat remains in the experimenter's future, and therefore it's uncertain and/or in superposition.
This view of superposition isn't just my twisting of QM theory to make it more palatable to human intuition; it's fully consistent with the relational interpretation. Consider the three main tenets of RQM:
1. We cannot attribute any absolute states or properties to any object, in and of itself. It would be wrong to say, "The apple has an absolute velocity, spin, color, etc., independent of other objects or observers." Rather, states and properties can only be defined in terms of interactions between things — whether they be microscopic or macroscopic, inanimate or living, observing or not observing. RQM makes no distinctions among these. Our observations of the world consist only of our interactions with objects in the world (e.g., our perception of an apple appearing to be the way it is), not any absolute properties of the objects themselves.
2. Two observers can have different, but equally accurate, descriptions of one object or system, depending on the nature of their respective interactions with that system.
3. Our description of any system depends specifically upon the information transferred or extracted during our interaction with the system.
The key here is that relational quantum mechanics is a theory about information. If we're involved in an interaction where we acquire information about an object's momentum, for example, the property of momentum then becomes defined by us for that object; before that, it remains undefined or uncertain to us. In the case of Schrödinger's cat, at the end of the hour, the cat has received information about the state of the atom, and that is why the cat — from its own perspective — is either alive or dead. Meanwhile, though, the experimenter has no information about what went on inside the box, so from his or her perspective, the cat is in superposition, i.e., its state is uncertain.
In the future, literally everything is uncertain to us, dwelling in probability states only.** The total lack of information from the future means that everything about it must remain undefined. Even the surest bet we know, that the sun will rise tomorrow, is a probability; there is a
tiny but non-zero chance that the Earth's rotation will be halted by an
asteroid impact before then.***
And this may be a stretch, but you can apply the same principle to the very distant past: How did the first reproducing life form, our earliest ancestor, come about? We have absolutely no direct information about this event, so the best we can do is offer potential scenarios and gauge their respective probabilities. You could say that from our current perspective, the various possible earliest life forms exist in superposition — but "uncertain" feels a lot more natural.
Perhaps that's the most confusing thing about superposition: the word itself. It conjures up an image of overlapping, partially transparent alternate versions of an object. It's no surprise that students and researchers alike have been uneasy about the concept for 80-some years. But superposition is merely uncertainty based on a lack of available information. That's all it is.
* One can well argue that traditional superposition is a mathematically "real" state of affairs for an atom or even a cat, whereas my conception of "future" superposition is a metaphorical extrapolation. While it's definitely an extrapolation, there are fewer differences between these cases than you might expect. Mathematical uncertainty is mathematical uncertainty, such descriptions having no direct bearing on the actual nature of the objects themselves. But that's a topic for another day.
** I floated this idea in an earlier essay, where I argued that time appears to flow in one direction because information only comes from the past, never the future. Even though the "arrow of time" is often explained thermodynamically, where the inevitable increase in disorder (entropy) points in only one direction, that explanation doesn't shed much light on the phenomenon in conscious observers of a definite and unidirectional "flow" of time. Is it a coincidence that both arrows point in the same direction? No — entropy is a key element in quantum information theory.
*** Someone once asked me that if the "realness" of objects depends so strongly on our observations, do the table and chairs in his dining room go away when he goes to bed? It's not that they "go" anywhere; it's just that as soon as information on them stops being collected, they lapse into an increasingly uncertain probability state. There is a small but non-zero chance that his house will be emptied by robbers, a flood will wash out the downstairs, etc., and this goes up the longer he sleeps.
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