MelissaA
Brenton
Shilpa Karhik
A Bluffer's Guide to Quantum Theory
June 29th 2006 02:58
For the next week or so I am going to include a post I wrote a long time ago, to get a break from my normal fun facts. It was about quantum theory which I now write about all the time, so it’s not such a break. I call it the bluffers guide to Quantum Theory. While it is a weeks worth of my posting time I have decided to include it all in one piece to make it easier to read.
So here it goes (if you don’t like it, blame it on the past me rather than the present me).
Quantum Theory may not be a fun fact...but it is fun (for those of us who are masochists).
What follows is not really so much of a bluffer's guide to quantum, but more an attempt to explain some basic concepts of the theory.
***
Quantum Mechanics is about small things: your little baby brother? The skimpy clothing you can't help but stare at at clubs? The atrophied brain of an arts student (Don't worry - I'm an arts student, really, so I don't mean it)? Maybe we should be thinking a little smaller, like at the atomic level.
Probabilities, not certainties: Quantum Theory was invented by a sinister scientist who aimed to make the laws of nature too complex to understand without years of study. This scientist and his cruel cohorts decided that it would be too simple if particles occupied a distinct position in space and so they decided that a particle, until observed, only has a probability of being in a certain place, rather than actually being there.
This sounds a little baffling at first, especially the bit about observation. Indeed, what quantum theory says is not that we don't know where a particle is because we can't see it, but rather that the particle is not in a single place, there is (for example) a 50% chance it is in one place, a 25% chance that its in another and so on and it occupies all of these places at once in the forms of a probability wave function. As soon as the particle is observed, though, it suddenly occupies only one position.
One strange effect of this is quantum tunneling. This means that a scientist can observe a particle in a container with no holes in it (not even tiny ones). He can then stop observing the particle and it can magically appear on the other side of the container. This effect is called quantum tunneling, but it really happens because the particles position is based on probability. I've shown this on the diagram below.
So lets say that the particle was at position A 50% of the time, and position B the other 50% of the time, then if you looked at it two times it could appear to move through the walls.
This is then connected to the Heisenberg uncertainty principle, which basically says that the more you know about the position of a particle the less you can know about its speed and vice versa. Heisenberg is one of the chief demons that the Quantum cult worships. Beware. (If you want to know why this is so then write a comment and I'll provide more detail...right now I'm feeling lazy).
The Double Slit Experiment: Now the irreconcilably evil demons set about to make things worse. For years physicists had argued over whether light was a wave or a particle. Quantum's evil practitioners decided to make it not one or the other, but both.
To prove this they set up an experiment. This experiment contained a light source which was shone onto a board that had two holes (or slits) in it. Behind these holes was a light detector. When the light was shone it created an interference pattern (the sort of pattern you would see if you threw two stones into a pond. The two waves would meet each other and interfere). They then performed the experiment with a light source that emitted only one particle of light at a time. This particle still interfered with itself. For this reason the light must be a wave as well as a particle or the interference couldn't have taken place.
The experiment gets even more interesting (confusing), when you place a light detector on each of the slits, but once again, laziness commands. Leave a comment if you want to know more.
Schrödinger's cat: This is the name of a though experiment designed by someone trying to discredit the evil of quantum. This relies on the fact that quantum theory claims particles can be in a superposition of states. What this means is that before someone observes a particle it not only could be in multiple places, but it actually IS in all these places at once. Only when the particle is observed does it exist in one location.
Superposition is an important feature of quantum theory. The theory says that if you take a nucleus with a 50% chance of decaying in one hour then after that hour the nucleus is both decayed and not decayed at the same time...until the nucleus is observed.
When a nucleus decays it releases a particle and Schrodinger asks what would happen if a cat was kept in a box with such a decaying nucleus. This box would also contain a detector that would release poison when a particle is detected. When the nucleus decays a particle is released, the detector is set off, the poison is release and the cat dies. However, as the nucleus is both decayed and not decayed is the cat both alive and not alive.
If so this seems a little strange, but then so does all of quantum. Regardless, quantum theory needs to find a way to relate the quantum world (small things) to the classical world (everyday size things). The copenhagen interpretation (the most popular view at the moment), says that a system becomes one thing or the other when it is observed, but the concept of observation is not well defined.
This fiendish cat does not spell the end of quantum, but it sure has made the quantum priests scurry about for answers.
And that's it: Well, that's it from me, after all, I'm not even a science student. As an art student I hope that anyone out there who spots a mistake in my Bluffer's Guide will post a comment so that all will know how foolish I am.
Cheers,
Adam
So here it goes (if you don’t like it, blame it on the past me rather than the present me).
Quantum Theory may not be a fun fact...but it is fun (for those of us who are masochists).
What follows is not really so much of a bluffer's guide to quantum, but more an attempt to explain some basic concepts of the theory.
***
Quantum Mechanics is about small things: your little baby brother? The skimpy clothing you can't help but stare at at clubs? The atrophied brain of an arts student (Don't worry - I'm an arts student, really, so I don't mean it)? Maybe we should be thinking a little smaller, like at the atomic level.
Probabilities, not certainties: Quantum Theory was invented by a sinister scientist who aimed to make the laws of nature too complex to understand without years of study. This scientist and his cruel cohorts decided that it would be too simple if particles occupied a distinct position in space and so they decided that a particle, until observed, only has a probability of being in a certain place, rather than actually being there.
This sounds a little baffling at first, especially the bit about observation. Indeed, what quantum theory says is not that we don't know where a particle is because we can't see it, but rather that the particle is not in a single place, there is (for example) a 50% chance it is in one place, a 25% chance that its in another and so on and it occupies all of these places at once in the forms of a probability wave function. As soon as the particle is observed, though, it suddenly occupies only one position.
One strange effect of this is quantum tunneling. This means that a scientist can observe a particle in a container with no holes in it (not even tiny ones). He can then stop observing the particle and it can magically appear on the other side of the container. This effect is called quantum tunneling, but it really happens because the particles position is based on probability. I've shown this on the diagram below.
So lets say that the particle was at position A 50% of the time, and position B the other 50% of the time, then if you looked at it two times it could appear to move through the walls.
This is then connected to the Heisenberg uncertainty principle, which basically says that the more you know about the position of a particle the less you can know about its speed and vice versa. Heisenberg is one of the chief demons that the Quantum cult worships. Beware. (If you want to know why this is so then write a comment and I'll provide more detail...right now I'm feeling lazy).
The Double Slit Experiment: Now the irreconcilably evil demons set about to make things worse. For years physicists had argued over whether light was a wave or a particle. Quantum's evil practitioners decided to make it not one or the other, but both.
To prove this they set up an experiment. This experiment contained a light source which was shone onto a board that had two holes (or slits) in it. Behind these holes was a light detector. When the light was shone it created an interference pattern (the sort of pattern you would see if you threw two stones into a pond. The two waves would meet each other and interfere). They then performed the experiment with a light source that emitted only one particle of light at a time. This particle still interfered with itself. For this reason the light must be a wave as well as a particle or the interference couldn't have taken place.
The experiment gets even more interesting (confusing), when you place a light detector on each of the slits, but once again, laziness commands. Leave a comment if you want to know more.
Schrödinger's cat: This is the name of a though experiment designed by someone trying to discredit the evil of quantum. This relies on the fact that quantum theory claims particles can be in a superposition of states. What this means is that before someone observes a particle it not only could be in multiple places, but it actually IS in all these places at once. Only when the particle is observed does it exist in one location.
Superposition is an important feature of quantum theory. The theory says that if you take a nucleus with a 50% chance of decaying in one hour then after that hour the nucleus is both decayed and not decayed at the same time...until the nucleus is observed.
When a nucleus decays it releases a particle and Schrodinger asks what would happen if a cat was kept in a box with such a decaying nucleus. This box would also contain a detector that would release poison when a particle is detected. When the nucleus decays a particle is released, the detector is set off, the poison is release and the cat dies. However, as the nucleus is both decayed and not decayed is the cat both alive and not alive.
If so this seems a little strange, but then so does all of quantum. Regardless, quantum theory needs to find a way to relate the quantum world (small things) to the classical world (everyday size things). The copenhagen interpretation (the most popular view at the moment), says that a system becomes one thing or the other when it is observed, but the concept of observation is not well defined.
This fiendish cat does not spell the end of quantum, but it sure has made the quantum priests scurry about for answers.
And that's it: Well, that's it from me, after all, I'm not even a science student. As an art student I hope that anyone out there who spots a mistake in my Bluffer's Guide will post a comment so that all will know how foolish I am.
Cheers,
Adam
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