View Full Version : Quantum Mysteries - The Infamous Double-Slit Experiment

Plot Device
09-06-2008, 09:38 PM
Okay, I admit I know very little about quantum anything (quantum theory, quantum physics, quantum mechanics, etc). So if the following information turns out to be a real yawn-a-minute example of "Geez, Plot Device! We already knew about that!" then please excuse my ignorance.

But I simply had to post the following YouTube link, and then ask everybody here:




According to this video clip about particle theory, particles somehow "know" when we are watching them, and so the particles then change their behavior accordingly. Almost like the ultimate example of "a watched pot never boils."


Plot Device
09-08-2008, 09:21 AM
Okay, so I guess that after 36 hours, 15 views of this thread, and not one reply, I can assume one of the following:

a) I am mis-understanding the purpose of the Science Fact sub-forum, and people are politely ignoring a thread that doesn't even belong here
b) eveyone already knows quantum mechanics inside and out and I am now coming across as a real dope for NOT knowing as much as everyone else
c) no one has an answer
d) nobody really bothers with this sub-forum

Whatever the reason for the relentless symphony of crickets, I am a little stunned.

09-08-2008, 09:49 AM
I love this about quantum physics.

Does the tree make a noise if we aren't there to hear? Perhaps not!

I would love to see what happens when the tech gets powerful enough to measure the electrons from a great distance. Will they still behave differently? Does space enter into the equation at all?

How can measuring the action change the result, while measuring the result appears to change nothing?

Things like this give me hope that there are still frontiers in the world to explore.


PD, this forum is a little slower than others.

09-08-2008, 10:07 AM
the act observing may not be a passive activity on the microcosmic level. Our eyes are interacting with light waves to enable us to see. So in observing light we may be engaging in a relationship with it and effecting its composition, the same as if our eyeballs were magnets and we were studying magnetic rays. As soon as we turned to examine the rays they would be altered. // Pay little mind to this explanation which is probably 99.9% erroneous.

A. Hamilton
09-08-2008, 10:11 AM
well, you certainly gave me an hour or so of pondering, that's how long I've been perusing Dr Quantum on YouTube since I read your post. this stuff hurts my brain, yet it's so very fascinating. I've always loved time travel stories, but they always leave me spinning in circles, as the explanations are usually vague. but this quantum stuff opens so many doors, still spinning in there though, LOL.

09-08-2008, 10:21 AM
Okay, so I guess that after 36 hours, 15 views of this thread, and not one reply, I can assume one of the following:

a) I am mis-understanding the purpose of the Science Fact sub-forum, and people are politely ignoring a thread that doesn't even belong here
b) eveyone already knows quantum mechanics inside and out and I am now coming across as a real dope for NOT knowing as much as everyone else
c) no one has an answer
d) nobody really bothers with this sub-forum

Whatever the reason for the relentless symphony of crickets, I am a little stunned.

Ummmm sort of B? I'm pretty sure I learned this in high school physics. Maybe it's just me? (The fact that it's been turned into a cartoon seems to support my hypothesis.)

Mac H.
09-08-2008, 10:21 AM
It may be that it is pretty familiar ground these days, and people always make it sound cooler than it really is.

For example, compare :

Cool Version:
.. particles somehow "know" when we are watching them, and so the particles then change their behavior accordingly


More boring reality: We need not actually "do the looking." We do not have to detect the light flashes and ascertain which way each particle went. It suffices that the information is available in the flashes and could have been observed in that way.

In other words, it depends on what information is available, not whether the particles 'is observed'. Much less cool.

The last quote is from an article describing how you can perform a do-it-yourself experiment on the subject: http://www.sciam.com/article.cfm?id=E20B77CB-E7F2-99DF-33669D92032DFF8C

All you need is a rubber band, a laser pointer, a paper clip, a piece of tin foil and a polariser.


09-10-2008, 09:34 PM
It's an incredibly interesting experiment. A conscious observer is certainly not necessary, but what's actually going on is not known, as far as I understand things.

09-11-2008, 01:23 AM
I studied QT stuff in both Physics and Philosophy classes. I guess I'm a little spent over the whole thing :)

Actually, though, it has a heavy impact on the entire body of science, philosophically speaking. We strive to remove the scientist from the experiment, making them purely an outside observer, not influencing the experiment in any way (Longino, keep to yourself on this one). Yet the Double-Slit experiment, taken outside of its narrow context and applied broadly, implies that there is no way to remove the scientist from the experiment--that by the very act of experimenting and observing, they are affecting the results. I won't get into the wonky time-traveling and future-affects-the-past stuff.

Yeah. I've done this topic to death.

09-21-2008, 07:26 PM
Yes, this stuff is very interesting. The main thing you want to be careful with is projecting quantum physics onto a "real world" scale. The What the Bleep films try to use quantum physics as a justification for all sorts of things that are better categorized as uh... religion? pseudo-science at BEST? These things are being studied on a mini-mini-mini-microscopic level by real scientists--a scale so incredibly small that the rules of traditional physics seem to fall to pieces. But... quantum events have not yet been shown to have any influence whatsoever on OUR level of "reality," which is why we still use the rules of Einstein and co (pre-quantum) for stuff on a normal or large scale.

...Of course, this doesn't mean we can't pretend for the sake of our fiction :tongue

(I am not at all a science person, but I learned a ton from Physics for the Rest of Us (http://www.amazon.com/Physics-Rest-Us-Roger-Jones/dp/0809237164) . I highly recommend it!)

09-24-2008, 10:55 AM
That's a great point katzen.

What's true at the atomic level, is often not true at the macroscopic level. There's a reason why newtonian physics are still pretty much the bread and butter of most engineering design. While at the extremes of speed, or size Newtonian physics tend to break down, but for everything in between it's a pretty dang good approximation.

10-12-2008, 09:56 PM
This is the "collapse of the wavefunction" problem.

The photon is big enough to travel through both slits, but hitting the photographic film (or some other detector) makes it interact with only a very small spot.

As to what causes this collapse, the Copenhagen interpretation states that it is the process of being observed. This has led to a lot of mystical woo-woo about what great powers we have, how wishing will make it so.

But a lot of QM effects are rather removed physically from a human observer, so physicists have explored alternate possibilities. One of the most promising of these is that it is the complexity of the photographic film that causes the collapse. And that would also be true of other detectors. However, the advocates of that hypothesis are still not completely successful in demonstrating it.

11-17-2008, 02:56 PM
Man I love Science.

The funny thing about Quantum Mechanics is that it describes what happens in OUR universe. So does Newtonian Physics and Relativity. Eventhough QM applies to the MICROSCOPIC world, and the others to the MACROSCOPIC world, it all applies to what happens in OUR collective reality. And, the bitch of the whole thing is that QM and Relativity+Newtonian Physics don't work well with each other.

Let's take a black hole. Now, you have an object, like a sun, that has collapsed upon itself until it is the size of an atom. When that much matter compresses to such a small point, you have a black hole. Now, in measuring that black hole, do you use Relativity or QM? Do you use the measurements of large macroscopic objects that account for things like gravity, or do you use measurements of small microscopic objects like atoms? Well, the wierd thing is that neither sufficiently does the job. You get non-sensical answers.

The thing you have to understand about science is that it isn't complete and probably never will be. When one question is answered, dozens of more questions abound.

The amazing thing about writing science fiction is that you get to use science, which describes a universe, or multiple universes, where any and all things are possible. You do want to be careful so as to be exact about what we DO know to be true about the universe. But, at a certain point our limited understanding breaks down and thats where the realm of fantasy comes into play.

If you were to apply the QM world to the real world, anything and everything is possible. You would have a wierd world of infinite probabilities. A guy goes to the bar and asks for a grape soda, well, he may not get a grape soda. There's merely a probability that he WILL get a grape soda even though he specifically asked for grape (sounds like the drive through at a fast food joint.) A guy's walking down a hall and he may fall through the floor.

For a better understanding of this stuff, I suggest watching Dr. Brian Greene's film "Elegant Universe". You should be able to find it still available on PBS's Nova website. It's quite long, but simple enough a child could understand it. It takes you step by step through our growing understanding of the universe for Galileo and on. If you're a science junky like me, then you'll be drooling as you watch this.

Plus, something else you have to consider when writing Sci-Fi. What we understand to be true about the universe via science today could be thrown out of the window tomorrow. Like how we laugh at people who used to think the world was flat. People 100 years from now will probably look back at Hawking and joke about his narrow vision of the universe.

So, if you're writing about a universe that is bound by our current understandings of physics, then you do have to be careful. If you're writing about a universe that is unbound by our current understanding of phyiscs, one which has developed its own perspective on science and has antiquated our own, then you don't have to be so careful.

11-17-2008, 05:18 PM
I'm reading The Fabric of the Cosmos by Brian Greene. It's the history and bare bones basics of physics up to the present day. My brain gets a bit overheated, so I have to read it in small doses, but it's absolutely amazing.

11-23-2008, 07:12 AM
Yeah. Elegant Universe was the first video I watched concerning relativity and QM. I literally sat with my mouth open as Relativity was explained. I couldn't believe scientists believe in such aberant ideas. It goes to show, though, that the universe is far beyond our comprehension.

11-23-2008, 09:09 AM
I didn't answer before, becase I didn't see this thread before. So here's my response.

Okay, the first thing you should know is this clip is from the movie "What The Bleep Do We Know? (with Greek letters replacing some of the English-alphabet letters). Website for the movie is here (at just a glance, it's hugely self-congratulatory)

The Wikipedia entry explains most of what I'd want to say about it:
http://en.wikipedia.org/wiki/What_the_Bleep_Do_We_Know!%3F (http://en.wikipedia.org/wiki/What_the_Bleep_Do_We_Know%21%3F)

And just so you get it, the movie features the Robert Palmer song "Addicted To Love" choreographed to some characters in it, you should watch this video (or at least a few seconds of it, enough to see that the women dance and "play" the instruments):
Okay, the start of this clip is the part of the film I found so highly amusing (caution, this first part has sort of a PG rating):
If you keep watching that clip, you'll see that the film is also partly a "documentary." The woman who talks at 5:30 is J.Z. Knight, a new-age author, head of the Ramta new-age religion/cult/whatever, and the apparent driving force behind this movie.

This was a bit of a controversial movie, and here's just one review of it that rips it to shreds, from Scientific American, where several scientists who were interviewed for the movie say they were wildly misquoted and their words edited out of context):

The review there is the first part of a larger conversation with this other guy, Stuart Hameroff, who was interviewed in the movie and did some work with Roger Penrose. I read Penrose's book "The Emperor's New Mind (http://www.amazon.com/Emperors-New-Mind-Concerning-Computers/dp/0198519737)" many years ago when it came out and which I think is an excellent introduction to quantum physics, but his conclusion near the end of the book concludes that quantum effects on neurons in the brain are an essential part of human consciousness, and thus he claims it is theoretically impossible to create a huiman-like intelligence using traditional computer approaches to artificial intelligence (no matter how much faster or how much more memory the computer has than current ones). I''ve always been doubtful of that, and thought he made a weak argument out of it. I'm not neccesarily claiming that ("traditional" Von-Neuman architecture) computers will someday have computer-like intelligence, but that Penrose's reasoning for claiming they won't is weak.

But other than that, he's a very well respected scientist...

Brian Greene is also very good, though in the book "The Elegant Universe" (I saw the Nova specal that whats-his-name mentioned, it's also very good, but the book goes more in depth) he not only covers quantum physics (which was pretty much worked out in the first half of the 20th century), he also goes into Superstring Theory (often abbreviated to String Theory), an even more bizarre idea than this quantum stuff, and it's only about 30 years old, and only become to be widely discussed in the last 20 years.

I saw the movie (after reading that review), and found the "Addicted to love" part highly amusing. and ironically, the animated parts seemed to be the more accurate parts of it. There's another part that spends just a couple of minutes listing the six findings of quantum physics that I thought was an interesting summary. But PLEASE don't go solely by this movie for your info on quantum physics.

Another book on quantum physics I enjoyed reading about 25-30 years ago is "In Search of Shrodinger's Cat (http://www.amazon.com/Search-Schr%C3%83%C2%B6dingers-Cat-Quantum-Physics/dp/0553342533)."

11-23-2008, 09:48 AM
I suppose there could be two explanations (or more). The first would be the straight wave collapse triggered by observation. Here I'd imagine that the probabilities of which slit for passage continue to play out until their premature and needless observation shows the result to have been 50/50. For any other phenomenom in which the odds were not so evenly split, the observation would cause the cloud of probabilies to collapse into a single and definite choice. Schroedinger's cat would yield similar results, if we expand the experiment to a design of 100 cats or so, and add the measured variable of food consumed.

The second most likely explanation is the many worlds model, which yields such possibility as quantum suicide (actually a form of immortality). Perhaps worlds is not as accurate a description as dimensions, or better yet, fractal scales.

My personal belief is that the two are combined into a third alternative, in which the cloud or wave function plays out not in many worlds, but in the ten or eleven likely dimensions (using characteristics other than actual randomness), of which we perceive three or four. This perception is the triggering observation.

That....or we're stuck in a holographic universe in which this detail manifests awkwardly due to polarization or something.

....or demons did it.