The Quantum Eraser
I apologize in advance to most of you reading this. This won't happen often, I assure you.
Ok, my current tally is 1 perky post, 2 melacholy posts, and 3 sports posts (one of which doubled as a "soap-box post"), which means it is time for a nerdy post. However, to my less mathematically inclined readers, please do not be discouraged, as I will try not to get technical here, and the experiment I'm going to describe is truly mind-blowing.
One of the most fundamental, underlying principles governing quantum mechanics is that all objects exhibit both wave and particle features. For objects of everyday size, the wave features are negligible for reasons I will not delve into here. However, for an object like a photon (the fundamental "packet" of light), wave and particle features are on almost equal footing. One of the experiments that helped to show this was an experiment in which beams of light (or photons) were shot through two slits. If you fire the photons through just one slit (either one), they produce some light on a screen on the other side, as expected. However, when you fire them through both at the same time, instead of adding up their intensities, which seems intuitive once you know that light is made up of little tiny particles called photons, they behave like waves and generate an "interference pattern", which can be thought of like crossing water waves adding their intensities and cancelling each other out in some spots. On the screen is a pattern of alternating light and dark bands.
Even if you slow down the photons and fire, say, one every ten seconds, they generate the same interference pattern, which is amazing since none of the individual photons can interact with any of the others -- they just know, somehow, where the crests and troughs are for the interacting wave pattern. Wave nature can also be thought of as the particle being sort of a lot of places at once (not that we just don't know where it is, but experimental evidence has shown over and over that an electron or a photon or any particle is described not as being here or here, but sort of spread out in and being sort of here and sort of here and sort of there, etc.).
That is, until you look at it. When you measure a particle, somehow, by some mechanism not understood by modern physics, the particle suddenly collapses to a specific position. The interaction of the measurement somehow forces the particle to lose its wave-like nature. By repeating the same experiment described above, only putting sensors on the slits to see which one the photon passes through forces the light to behave like a particle. The interference pattern disappears as soon as you do this. However, as soon as you turn off the sensors, the interference pattern magically appears again.
However, the amazing part is that if you muddle the sensor information down the line in the experiment such that it is no longer useful, the interference pattern mysteriously reappears. If the sensors are down-convertors, which take one photon in and launch two photons out (one in the original direction and one in a separate beam to tell you "the photon went through this sensor"), this experiment is easy to describe. The "which-path" information is given by the down-convertor's beam. However, if, the two output beams from the down-convertors are fed into a mixer which jumbles them up down the line such that no one could get the "which-path" information, the interference pattern reappears on the screen. This is incredible, since the interference pattern shouldn't seem to care whether or not you look at the "which-path" information, and since the jumbling could occur far, far away, which would make the mixing of the beams happen after the photons actually hit the screen.
This experiment challenges fundamental assumptions that we have about physics and how the world works. We assume that the universe is causal (events happening now can only be affected by past events), but this experiment seems to suggest that, at best, our model of causality is far too simple. With this experiment, and several other, more famous ones, our assumption that the universe is local (stuff happening here can't affect stuff happening over there until "information" such as light or gravity waves has had time to travel the separation) is severely shaken. Also, it is almost as if quantum mechanics shows that things don't really exist until you interact with them. Does the moon exist when no one is looking at it? Some of the more liberal quantum philosophers would say no.
Quantum mechanics has a lot of other crazy stuff, but I think I'll limit myself to just one today. I got the description for this experiment from "The Fabric of the Cosmos", by Brian Greene, and if, by some miracle, this post interested you, you should definitely read it. Anyway, if our universe truly is held together by something as nutty as quantum mechanics, I'm just glad I believe in God...
Ok, my current tally is 1 perky post, 2 melacholy posts, and 3 sports posts (one of which doubled as a "soap-box post"), which means it is time for a nerdy post. However, to my less mathematically inclined readers, please do not be discouraged, as I will try not to get technical here, and the experiment I'm going to describe is truly mind-blowing.
One of the most fundamental, underlying principles governing quantum mechanics is that all objects exhibit both wave and particle features. For objects of everyday size, the wave features are negligible for reasons I will not delve into here. However, for an object like a photon (the fundamental "packet" of light), wave and particle features are on almost equal footing. One of the experiments that helped to show this was an experiment in which beams of light (or photons) were shot through two slits. If you fire the photons through just one slit (either one), they produce some light on a screen on the other side, as expected. However, when you fire them through both at the same time, instead of adding up their intensities, which seems intuitive once you know that light is made up of little tiny particles called photons, they behave like waves and generate an "interference pattern", which can be thought of like crossing water waves adding their intensities and cancelling each other out in some spots. On the screen is a pattern of alternating light and dark bands.
Even if you slow down the photons and fire, say, one every ten seconds, they generate the same interference pattern, which is amazing since none of the individual photons can interact with any of the others -- they just know, somehow, where the crests and troughs are for the interacting wave pattern. Wave nature can also be thought of as the particle being sort of a lot of places at once (not that we just don't know where it is, but experimental evidence has shown over and over that an electron or a photon or any particle is described not as being here or here, but sort of spread out in and being sort of here and sort of here and sort of there, etc.).
That is, until you look at it. When you measure a particle, somehow, by some mechanism not understood by modern physics, the particle suddenly collapses to a specific position. The interaction of the measurement somehow forces the particle to lose its wave-like nature. By repeating the same experiment described above, only putting sensors on the slits to see which one the photon passes through forces the light to behave like a particle. The interference pattern disappears as soon as you do this. However, as soon as you turn off the sensors, the interference pattern magically appears again.
However, the amazing part is that if you muddle the sensor information down the line in the experiment such that it is no longer useful, the interference pattern mysteriously reappears. If the sensors are down-convertors, which take one photon in and launch two photons out (one in the original direction and one in a separate beam to tell you "the photon went through this sensor"), this experiment is easy to describe. The "which-path" information is given by the down-convertor's beam. However, if, the two output beams from the down-convertors are fed into a mixer which jumbles them up down the line such that no one could get the "which-path" information, the interference pattern reappears on the screen. This is incredible, since the interference pattern shouldn't seem to care whether or not you look at the "which-path" information, and since the jumbling could occur far, far away, which would make the mixing of the beams happen after the photons actually hit the screen.
This experiment challenges fundamental assumptions that we have about physics and how the world works. We assume that the universe is causal (events happening now can only be affected by past events), but this experiment seems to suggest that, at best, our model of causality is far too simple. With this experiment, and several other, more famous ones, our assumption that the universe is local (stuff happening here can't affect stuff happening over there until "information" such as light or gravity waves has had time to travel the separation) is severely shaken. Also, it is almost as if quantum mechanics shows that things don't really exist until you interact with them. Does the moon exist when no one is looking at it? Some of the more liberal quantum philosophers would say no.
Quantum mechanics has a lot of other crazy stuff, but I think I'll limit myself to just one today. I got the description for this experiment from "The Fabric of the Cosmos", by Brian Greene, and if, by some miracle, this post interested you, you should definitely read it. Anyway, if our universe truly is held together by something as nutty as quantum mechanics, I'm just glad I believe in God...
posted by Abe at 12:08 PM
3 Comments:
Well Abe, good read. I learned something I did not know before. Very interesting stuff. I still would like to get that book from you that you were telling me about earlier in the summer. Anyways...later.
So chaos theory IS correct. "A butterfly can flap its wings in Peking and in Central Park you get rain instead of sunshine." Oh yeah, good layman explanation of that experiment, and I had never heard the jumbling of the signal effect before. Light should just make up its mind instead of being a confused little photon. And I suppose this will spark the question, are you born a wave, or do you choose to be a wave?
Yes, this is an interesting experiment. I don't know quantum well enough to claim I understand it completely. But anyway, the person who wrote the original paper on the Quantum Eraser, Marlan Scully, is a professor at Texas A&M (perhaps the best in the department). He has done a lot of work on Hidden Variables/Causality/Locality/EPR-Bell Correlations in Quantum Mechanics if you are interested in delving into the physics literature. In short, he does not believe that locality (and therefore causality) must be relinquished. Like I said, I don't understand this stuff . . . yet.
Post a Comment
<< Home