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ThorGoLucky
Snuggle Wolf
USA
1486 Posts |
 Posted - 04/14/2014 : 08:55:36
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Over the years, I have posted comments at YouTube videos of the double-slit experiment where it misleads that passive observation by humans is all that's needed for the wave interference pattern to collapse into a shotgun pattern.
Someone asked how the particles can be detected at the slit(s). In know that for photons, polarization filters can be used, but what is the method for detecting which slit an electron went through?
Here's a clip from the What the Bleep Movie, where it starts out as an excellent demonstration of the double-slit experiment, but then goes off into pseudoscience.
http://www.youtube.com/watch?v=fwXQjRBLwsQ
If all it takes is human observation to collapse the wave, we would never observe the wave interference pattern to begin with!
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Dave W.
Info Junkie
USA
26020 Posts |
Posted - 04/14/2014 : 09:26:50 [Permalink]
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Google electron detectors, maybe?
Per Heisenberg, I don't know how one could actually detect the position of an electron in a DSE without destroying the interference pattern, but it's primarily a thought experiment anyway. |
- Dave W. (Private Msg, EMail)
Evidently, I rock!
Why not question something for a change?
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Boron10
Religion Moderator
USA
1266 Posts |
Posted - 04/14/2014 : 11:20:14 [Permalink]
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There are several ways to measure the passage of electrons. However, to address the larger, unasked, question first: The location of the measurement is key. If you measure the electrons solely at the end of the line, you'll get an interference pattern because the probability wave of each electron goes through both slits. If you measure the passage of electrons through your slits *and* at the end of the line, you'll see the "shotgun" pattern because the probability wave of each electron only travels through one slit. Here is a great explanation: DR. QUANTUM - DOUBLE SLIT EXPERIMENT.
To answer the question directly asked: there are many ways to detect electrons. At the end of the line, for example, you can use some kind of phosphorescent film, like the inside screen of a CRT, that traps the signal like photographic film. At the slits it's a little more complicated. Of course, anything you could use to measure the passage of an electron will affect the electron. If you try to bounce a photon off it, the electron's momentum will change. If you set up an apparatus that traps, counts, and emits an electron; well then, you've redesigned your experiment, and you now just have two independent electron guns where you used to have slits. You can try to measure the magnetic field generated by the moving charge, but it will be *very* small and quite difficult to detect. |
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ThorGoLucky
Snuggle Wolf
USA
1486 Posts |
Posted - 04/14/2014 : 13:57:32 [Permalink]
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Thanks, B10. That Dr. Quantum video is from the "What the Bleep Do We Know" movie. It starts out as an excellent demonstration, but then it deceptively shows a passive observer of electrons going by and goes off into mystical crap about consciousness altering reality.
I was asked how using photon polarization filters at the slits could cause the wave to collapse. How does that affect the photon's momentum, energy or position? It seems that it would passively allow photons of a certain polarization to pass through.
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Boron10
Religion Moderator
USA
1266 Posts |
Posted - 04/15/2014 : 08:30:17 [Permalink]
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Originally posted by ThorGoLucky
Thanks, B10. That Dr. Quantum video is from the "What the Bleep Do We Know" movie. It starts out as an excellent demonstration, but then it deceptively shows a passive observer of electrons going by and goes off into mystical crap about consciousness altering reality. |
I (regrettably) saw that movie in the theaters -- a friend thought I would like it. I've seen the Dr. Quantum video in various other places, and apparently had forgotten its role in the movie. I just googled the name and linked to it, apparently missed the opening credits.
The point remains, though, that the simple act of observing forces decoherence. There is a lot of speculation about why that may be; I will present the simplest explanation: If you can observe an electron going through a particular slit, then your experiment has changed to be equivalent to two electron guns, one at each slit. Of course they won't interfere with each other. You would have the same results with photons.| I was asked how using photon polarization filters at the slits could cause the wave to collapse. How does that affect the photon's momentum, energy or position? It seems that it would passively allow photons of a certain polarization to pass through. |
Yes, it would passively only allow certain polarization to pass through; but differently polarized photons won't really interfere with each other, so you'd end up with the same result. Either you use the same polarization direction on each slit, whereby you've added no information about which slit was used; or you use a different filter on each, and they won't interfere with each other (or, rather, with themselves, which is what's actually happening).
Edited to add: Just remember, there is no physical way to passively observe something. The act of seeing something involves bouncing photons off it, etc. -B10 |
| Edited by - Boron10 on 04/15/2014 11:16:18 |
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Dave W.
Info Junkie
USA
26020 Posts |
Posted - 04/15/2014 : 13:57:01 [Permalink]
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"Observing" is the confusing term. It implies a sentient being seeing something happen.
In the context of a DSE, an "observation" is any interaction between two (or more) particles. |
- Dave W. (Private Msg, EMail)
Evidently, I rock!
Why not question something for a change?
Visit Dave's Psoriasis Info, too. |
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Boron10
Religion Moderator
USA
1266 Posts |
Posted - 04/15/2014 : 20:09:51 [Permalink]
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Originally posted by Dave W.
"Observing" is the confusing term. It implies a sentient being seeing something happen.
In the context of a DSE, an "observation" is any interaction between two (or more) particles.
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but keep in mind that description isn't unique to the Double Slit Experiment. It is an unavoidable consequence of reality that "observation" necessarily implies, fundamentally, interactions among fields. |
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ThorGoLucky
Snuggle Wolf
USA
1486 Posts |
Posted - 04/19/2014 : 13:49:37 [Permalink]
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| To be clear, if the same polarization filters are placed on the slits, the wave/interference pattern will occur, ja? |
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Boron10
Religion Moderator
USA
1266 Posts |
Posted - 04/21/2014 : 08:47:38 [Permalink]
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Originally posted by ThorGoLucky
To be clear, if the same polarization filters are placed on the slits, the wave/interference pattern will occur, ja? |
Yes. If the outgoing photons are identically polarized, they will still interfere with each other. Why?
By the way, I spoke with one of the professors here (considered the best theoretical physicist on campus) about your spacetime/"plankies" question.
Me: Is spacetime quantized?
Prof: Nobody knows. A lot of people have been trying to find that out.
Me: Ok. So, supposing it is quantized. We can then assume the quantum unit of spacetime is related to Planck length and Planck time, right?
Prof: Sure.
Me: But we know the universe is expanding. Does that mean we would be somehow generating more "plankies" or would the "plankies" somehow be expanding?
Prof: Wow, that's a really good question.
He then pointed out that Dirac (one of the founders of Quantum Mechanics) spent a significant part of his life trying to determine if the fundamental constants in physics were, in fact, constant, or if they changed over time.
http://en.wikipedia.org/wiki/Dirac_large_numbers_hypothesis
I asked him if they did change over time, would anyone notice? |
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Dr. Mabuse
Septic Fiend
Sweden
9687 Posts |
Posted - 04/21/2014 : 12:04:57 [Permalink]
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Originally posted by Boron10
Me: But we know the universe is expanding. Does that mean we would be somehow generating more "plankies" or would the "plankies" somehow be expanding?
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I thought about that during one of the previous chats I hosted. I remember saying that if spacetime is quantized, then evidence of this should be visible in the shift of the red-shift spectra of distant supernovas and quasars, as discreet steps in the shift itself. However, detecting these discreet steps might be difficult.
Prof: Wow, that's a really good question.
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I thought so too. How do you like my proposed sollution/experiment to find out?
One of my astronomy-friends mentioned a while ago that he read that scientists are leaning toward time being quantized. My question then would be, is space and time connected into spacetime in such a way as to allow for time to be quantized while space is continous?
My impression is that Dirac got so excited by his discovery of G=1/t (the gravitational constant is the inverse of the Universe's current age) that he temporarily forgot that correlation does not equal causation, and that the correlation he found was purely incidental.
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Boron10
Religion Moderator
USA
1266 Posts |
Posted - 04/21/2014 : 14:12:59 [Permalink]
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Originally posted by Dr. Mabuse
I remember saying that if spacetime is quantized, then evidence of this should be visible in the shift of the red-shift spectra of distant supernovas and quasars, as discreet steps in the shift itself. |
Seems plausible, but I do not believe we yet have equipment sensitive enough to detect this.| However, detecting these discreet steps might be difficult. |
No kidding!| One of my astronomy-friends mentioned a while ago that he read that scientists are leaning toward time being quantized. My question then would be, is space and time connected into spacetime in such a way as to allow for time to be quantized while space is continous? |
No. If Einstein was right, and so far as we can tell he was, the time dimension is not special. And, anything that significantly affecting time would have to similarly affect space. Otherwise we'd also have to throw out Maxwell's equations.| My impression is that Dirac got so excited by his discovery of G=1/t (the gravitational constant is the inverse of the Universe's current age) that he temporarily forgot that correlation does not equal causation, and that the correlation he found was purely incidental. |
That basically sums up my impression, too. He seems to have supposed that, since the age of the universe is about the same as the ratio between the electric and gravitational constants, and is quite a large number, there might be some significance there. So far as we know he was wrong.
Edited for formatting -- B10 |
| Edited by - Boron10 on 04/21/2014 14:13:59 |
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Dave W.
Info Junkie
USA
26020 Posts |
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ThorGoLucky
Snuggle Wolf
USA
1486 Posts |
Posted - 04/26/2014 : 00:02:28 [Permalink]
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Originally posted by Boron10
Originally posted by ThorGoLucky
To be clear, if the same polarization filters are placed on the slits, the wave/interference pattern will occur, ja? |
Yes. If the outgoing photons are identically polarized, they will still interfere with each other. Why? |
Why? I was checking if what I said in a YouTube comment was reasonable, thanks.
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Request for info: electron detection methods
