Particles and waves in your basement

April 30, 2007 11:16 AM

Demonstrate one of the weirdest quantum effects in your home using a laser pointer, some tinfoil, a piece of wire, and a $7 polarizer. The device, called a quantum eraser, operates in a way very similar to the famously mind-blowing double slit experiment that was voted the most beautiful experiment in physics.
posted by blahblahblah (49 comments total)

A little more information:

More on quantum erasers.

A somewhat condescending but clear interactive tutorial on the two slit experiment.

Also, a caveat: the link to "double slit experiment" in the FPP goes to a Google video drawn from "What the Bleep Do We Know" which is mostly New Age weirdness, but their explanation of the details of the two slit experiment are quite good, if you ignore their views of the implications.
posted by blahblahblah at 11:28 AM on April 30, 2007

The double slit experiment makes my brain hurt. Not because it's hard to understand the experiment, but because it means the universe is one insane place. The behavior of photons as they pass through slits depends on what they encounter after they pass through the slits? Buh?
posted by Justinian at 11:33 AM on April 30, 2007

one of these days someone is going to figure out how to destroy the work using nothing but a tissue, two strips of duct tape, a nine volt battery and a can of soda... you just watch.
posted by edgeways at 11:35 AM on April 30, 2007

The behavior of photons as they pass through slits depends on what they encounter after they pass through the slits

You are only forced into this conclusion if you reject the many worlds explanation.
posted by DU at 11:35 AM on April 30, 2007

See also: Bob the Angry Flower and Schrodinger's Fridge.
posted by unixrat at 11:36 AM on April 30, 2007 [4 favorites]

interestingly enough, experiments like this also have the effect of putting my brain into a superposition of two states - total confusion and total disorientation.
posted by the painkiller at 11:39 AM on April 30, 2007

Excuse me. This is the most beautiful experiment.
posted by Terminal Verbosity at 11:40 AM on April 30, 2007 [2 favorites]

one of these days someone is going to figure out how to destroy the work using nothing but a tissue, two strips of duct tape, a nine volt battery and a can of soda

That Macgyver's one shifty trouble-making mofo.
posted by dazed_one at 11:42 AM on April 30, 2007

The light patterns that you will see if you conduct the experiment successfully can be accounted for by considering the light to be a classical wave, with no quantum mechanics involved.

I feel cheated.
posted by malaprohibita at 12:09 PM on April 30, 2007

Uh I just watched the WtBDWKnow video ....lemme reframe

1. some model suggest electron have an e>0 (but very little) mass (demonstrated ?) therefore when it hits the screen it is deflected away, unless it goes throught the slits.

2. if by some means (what means?) we manage to shoot one single electron toward two slits and

2a and we observe _only_ the result on the screen, we will find a wavelike interference pattern forming over time, suggesting that the single electrons _must_ be behaving like waves, otherwise no wavelike interference pattern would form

3a if we "observe" (how the hell ?) the _single_ electron trying to ascertain which slit it is going to pass through and shoot a number of electrons & observe them, the wavelike interference no longer forms ! but only a 2 slit pattern

Now how would I record this without watching the electrons ? I'd use some kind of material that is altered by the electron...like photons on a film , if a photon its the film the fil m is changed in at least one point (the collision point)

BUT if I do the same experiment and "observe" (how?!) the electron...the experiment form only two slits.

Got it right ?
posted by elpapacito at 12:10 PM on April 30, 2007

That article was really poorly written
posted by delmoi at 12:10 PM on April 30, 2007

a bad fpp would be a link to the quantum eraser page, and that's it. And that page is cool, but not exactly idiot proof, either.

what would a good fpp of this look like? why, it would look precisely like this fpp. thanks, blahblahblah. I'm going to go favorite this, now.
posted by shmegegge at 12:15 PM on April 30, 2007

This is just God playing dice.
posted by F Mackenzie at 12:15 PM on April 30, 2007

One of the contributors described watching small plastic bags circulating in wind pockets, commenting that "sometimes there's so much beauty in the world, I just can't take it".

That whooshing noise you hear is the sound of that cultural reference flying safely overhead, undetected by the author of the last article.
posted by quarter waters and a bag of chips at 12:19 PM on April 30, 2007 [3 favorites]

This is just God playing dice.

The greatest trick the Devil ever pulled was getting the world to roll a natch-20 to disbelieve.
posted by cortex at 12:19 PM on April 30, 2007

Feynman’s lectures on QED (quantum electrodynamics) [streaming Real Media]
posted by ijoshua at 12:37 PM on April 30, 2007 [3 favorites]

tiny question:

the idea behind observing one of the slits, (and that in some way relating to the particles hitting the screen in a 2 line pattern rather than a wave interference pattern) implies that being able to determine which slit the particle went through (for whatever reason) causes the particle to behave like matter, rather than a wave, right? one theory, as wikipedia describes it, is that the particle would take every possible pathway between its origin and its first point of detection, correct? since measuring one of two slits, would automatically indicate where the particle had gone EVEN if it hadn't gone through the slit with a detector installed, then the particle's first point of detection will still be either slit, because not showing up on one slit's detector necessarily means it went through the other slit. am i getting this right?

now, what happens if you have a setup with 3 slits, and a detector only installed in one of them?
posted by shmegegge at 12:38 PM on April 30, 2007

BUT if I do the same experiment and "observe" (how?!) the electron...the experiment form only two slits.

That's pretty much how the whole movie is. It relies on rhetorical trickery to make you feel like you understand the "trick" of the universe.

You've caught on to one of them: There is no such thing as observing a photon passing by. You can either let the photon pass unhindered, thus never truly knowing if it indeed did pass, or you can alter its path and detect that it would have passed this way, if you hadn't been there.

There is no way to "observe" the photon passing and just let it go where it was going.
posted by odinsdream at 12:40 PM on April 30, 2007

You know how everybody laughs now at goofy, naive ideas like the world standing on the backs of turtles, or space being filled with ether, or personality being determined by head bumps?

That's how people will look back on our current quantum mechanics theories 100 years from now. "Hey, look, we get peculiar results from this one experiment… the only rational explanation is that there are infinite universes, time runs backwards, and subatomic particles care whether we look at them or not."

Mark my words, they'll figure out that things like the double-slit paradox are based on a simple oversight or misunderstanding, and all the massive towers of speculation that have been built on top of it will look extremely silly.

(I Am Not A Quantum Physicist.)
posted by designbot at 12:47 PM on April 30, 2007

There are ways of thinking about this that are less head-hurting.

1) Can an electron (or photon) interfere with itself? Yes, if it takes every possible path, rather than just one path.

2) Therefore, it won't interfere with itself if there really is only one possible path.

3) So why does the act of observation limit it to just one path? Because your observation has interfered with the pathways - basically, you stuck something in its way.

The head-hurtiness comes from thinking that the ways we interact with the world on the macrocosmic level with simply "scale down". But it doesn't work that way. We are used to thinking that objects only take one path to there destination, and that observation is a passive act, because on a large scale that is a reasonable way to describe how things generally work.

In fact, *you* also behave as a wave as well as a particle, with a wavelength and intereference patterns. They're simply so small compared to you that you never notice their effects. When you get down to objects that are tiny enough to be affected by their own wavelengths, it matters a lot more.

To observe something on a large scale, a photon has to bounce off of it and hit your eye. Doesn't matter much if that object is a baseball, so it seems like the object has not been affected. It has, just not enough for you to notice. But what happens to something tiny if you try to bounce something off of it which is the same size?

Moving baseballs have a measurable wavelength. If you just made the baseball and bat tiny enough, you would see the intereference pattern in a two-slit experiment.

(IANAS, so sorry if my admittedly already simplified explanation here turns out to be plain old inaccurate.)
posted by kyrademon at 1:04 PM on April 30, 2007

(Designbot, you may have been joking, but the theories of quantum mechanics have been shown to have predictive accuracy all over the place, and are the basis for many demonstrably working technological innovations.)
posted by kyrademon at 1:05 PM on April 30, 2007

My brain got the Blue Screen of Death from thinking about this too much.
posted by Rock Steady at 1:12 PM on April 30, 2007

YATTA!!!
posted by ZachsMind at 1:19 PM on April 30, 2007

(I Am Not A Quantum Physicist.)

I'll say.

Personally, I don't laugh at the goofy, naive ideas of past cultures. They were doing the best they can with the tools and observations available to them. We're doing the same today.

In the 1930s, Einstein, Podolsky, and Rosen wrote a paper proposing that QM wasn't a complete theory because it could not resolve a particular paradox. Thirty years later, John Bell wrote about why they were wrong. These things are always like that- attack and defense. Laughing at Einstein because he was wrong seems counterproductive.

The double slit experiment is in fact over two hundred years old. It's not new information and neither is it controversial. The added levels of interest regarding QM are new and interesting and quite possibly could be proven to be false. That doesn't mean that we'll discount QM as a theory or that it wouldn't be useful. Newtonian physics is still useful for many things, just not stringent scientific models.

Being wrong means we can be more right in the future, every time.

I don't think very many actual quantum physicists pat themselves on the back saying, "We've solved it all this time!" and i've known a few.
posted by thethirdman at 1:21 PM on April 30, 2007 [4 favorites]

(I Am Not A Quantum Physicist.)

I'll say.

That came out meaner than I meant it. I wasn't trying to call you out specifically, designbot, I just meant that I am in general frustrated with people making leaps about QM when they don't really know about it.

Really I'm just mad at the "What The Bleep..." folks...
posted by thethirdman at 1:29 PM on April 30, 2007

Just so you know, "What The Bleep..." was funded by a freaky cult and most of the contributing scientists got hoodwinked. (I still think it's a fun, so-so empowering movie, though, Marlee Matlin... sigh...)
posted by Skwirl at 1:37 PM on April 30, 2007

Ahhh the More on Quantum eraser link appears to be more complicated, it is :) BUT it at least explains better

1. entangled photons ! (sexy!) Such are two distinct (?) photons which have this in common : they have a property, but if Photon A has this property like "white" then photon B must have this property like "black" (and vice versa) ..so that we reliably know that if photon A is black therefore photon B must be white

But some think they are not photon A & B ..but the same photon ? A bell state ? Ok this isn't clear to me.

2. I shoot the two entangled photons, but if I place a two piece quarter wave polarizer in front of the slits the distribution will no longer look like "interference pattern" but pretty much like some kind of "normal" distribution.

The pattern will remain the same for an x polarized beam, a y polarized beam, a left or a right circularly polarized beam.

WTF ?! Why the hell is the distribution turned into a +- a "normal" ? The quarter plates aren't supposed to affect the distribution

Lo and behold, If i get another polarizer in the path of the photon B the distribution is back to an interf pattern.
posted by elpapacito at 1:39 PM on April 30, 2007

now, what happens if you have a setup with 3 slits, and a detector only installed in one of them?

An unsatisfying answer.

The big warning when thinking about this is to separate interpretation from results. The weirdness of quantum physics, which is demonstrated in my post, and the way it violates our conventional intuition is very real. What is all means is a different question, one without a clear answer, and one that many physicists don't worry about that much. You might be interested in reading about the standard Copenhagen Interpretation, but there are many other well grounded theories as well, such as the Many Worlds Intepretation.
posted by blahblahblah at 1:50 PM on April 30, 2007 [1 favorite]

reading feynman's QED book and learning about the double-slit experiment was definitely one of those mind-blowing, life-altering moments that you can't really recover from. It is so completely foreign to our own experiences and yet seemingly inescapable. the humility one should gain by learning about experiments like these is very valuable. We humans aren't gods (yet), as much as we try to pretend.

designbot, I agree that our theories could one day be shown on the level of "fire, water, ether" kind of stuff. We seem to be equivalent to a guy sitting in a hut in africa with no fire trying to figure out if the earth is flat while not being able to travel more than a few miles in any direction. I think there is no simple solution as we understand it. A simple and elegant solution (if such exists) would require major technological advances, or a significant paradigm shift in the way we think. While they might be simple or obvious after the fact, there is nothing trivial about their discovery.
posted by ryanfou at 1:50 PM on April 30, 2007

and the double slit experiment might be the most beautiful experiment in physics, but it won the vote with a staggering 20 votes, so let's just remember that.
posted by ryanfou at 1:56 PM on April 30, 2007

Sorry if my qualification of What the Bleep was not strong enough. To be clear, it is total bunkum , and its interpretations of quantum mechanics (and everything else) are wrong. But they made the most engaging video of the two slit experiment, which is why I included it. Please do not view this as an endorsement of the film.
posted by blahblahblah at 1:57 PM on April 30, 2007

The universe is a fun place, innit? :)
posted by zoogleplex at 2:05 PM on April 30, 2007

First off, I'm a quantum physicist (well, graduate student), and I've asked other quantum physicists (graduate students) how much they knew about quantum erasers. Everyone, so far, has said zip, zero, nada, okay maybe a little-- but not enough to stand before a crowd and speak up about it. So, don't feel bad about being confused or saying something incorrect. It's better to be wrong and know it than to think you know what you're talking about and never say it-- since this influences future judgments. Upon describing the interference effects shown in the SciAm article, the story from the physicist sampling changes somewhat.

What they are talking about in that article are "interference effects," which are the very reason for even accepting quantum mechanics in the first place. This is why the double-slit experiments for massive particles are so fascinating!

What the quantum eraser "does" is restore a fundamental interference after a measurement has been taken. This is not clear from the SciAm article, and it can be implemented in many seemingly unrelated ways depending on what we mean when we say measurement.

What's odd is that this can be done without sending the quanta back through the experiment again. We merely erase the measurement information by applying some other operation or measurement. This is useful in cases where we want to use the same quanta in its original unmeasured state more than once.

Classically, you would simply make copies, and use each copy for something else. In quantum mechanics, this is IMPOSSIBLE. You cannot "copy" an arbitrary quantum state-- reasons to follow if there's interest. So, if you want to use the same quanta for many different measurements, you have to undo the effects of the previous measurements.

In the SciAm article, the filters on each side of the pin is a measurement that must be undone to see the previous interference. It's undone by applying that diagonal filter. Thinking about information you gain about the path of the photon at each stage in the experiment should convince you that the final filter erases the effect of the previous filters. This, I think, should be enough to clear up what a "quantum eraser" is. The GREAT part of the whole thing is that it works for any arbitrary state that's sent through the experiment, and so it's a result of fundamental importance. :-)

Cheers.

p.s. Quanta are whatever you are describing quantum mechanically: photons, electrons, blahblahblah ;-)
posted by quanta and qualia at 2:18 PM on April 30, 2007 [3 favorites]

The big warning when thinking about this is to separate interpretation from results. The weirdness of quantum physics, which is demonstrated in my post, and the way it violates our conventional intuition is very real. What is all means is a different question, one without a clear answer,

I suppose what I'm getting at is that a 3 slit setup with only one detector on one slit would put an interpretation to the test. the link you provided, near as I can parse the science jargon, seems to be about a 3 slit setup with a detector on 2 of the slits. that isn't a comparable setup, to my mind, because you'd still be able to determine the outcome for all 3 slits. the question that was asked in the setup you provided was exactly what i was thinking of, but the answer provided seemed to misunderstand the question.
posted by shmegegge at 2:32 PM on April 30, 2007

I’ve plan to have already done this experiment last week tomorrow.

(it probably looks different in other dimensions)

Metafilter: the final filter erases the effect of the previous filters.
posted by Smedleyman at 2:35 PM on April 30, 2007

Metafilter: the final filter erases the effect of the previous filters.

I don't usually care for tagline gags but that one rocked. Way to quip, Smedleyman.
posted by maryh at 2:46 PM on April 30, 2007

Another nice experiment needs three polariser filters.

First put two on top of each other, with their polarisations crossed, so no (or maybe very little) light gets through.

Then slide the third one in between, at a 45 degree angle.

Try to explain that one to your mum...
posted by Djinh at 2:49 PM on April 30, 2007

If you look at What The Bleep for what it actually is: proof that Marlee Matlin is an amazing talent and can outdo anyone from Julia Roberts to Dame Edna, it's actually a lot of fun to watch. Granted, it's funded by the same kinda people who talked John Travolta into that L. Ron Hubbard movie debacle, but so what? Marlee Matlin on screen! hubba hubba!

And I embrace the many worlds theory of quantum mechanics. I believe 30 years from now we'll all take that as a given, and laugh at the people of our past for thinking so linearly.
posted by ZachsMind at 2:52 PM on April 30, 2007

reasons to follow if there's interest

Man is there ever interest, q&q.
posted by cortex at 3:09 PM on April 30, 2007

I like that experiment where the guy puts a beaker or something similar into a tank of oil and the beaker becomes invisible because of some sort of light thing going on.
posted by juiceCake at 4:03 PM on April 30, 2007

/thanks maryh
posted by Smedleyman at 4:43 PM on April 30, 2007

Although I am serious about the ‘other dimensions’ thing. I suspect there’s something funky happening in other dimensions and we’re not perceiving what that is.
Not a scientist m’self either, but I have read Flatland. And I’m fairly up on QM (although only in layman’s terms and from a hobbyist perspective)
posted by Smedleyman at 4:50 PM on April 30, 2007

I stopped trying to understand science when my AP Chemistry teacher told me that an orbiting electron could be in two places at one time. Things like this don't do much to encourage me.


But they're still awesome.
posted by PhatLobley at 5:03 PM on April 30, 2007

Schroedinger's cat is may be watching you all bloviate.
posted by rob511 at 5:32 PM on April 30, 2007 [1 favorite]

I am in general frustrated with people making leaps about QM when they don't really know about it.

Fenyman quipped that no one fully understands QM.

He also pointed out that (despite it being preposterous) QM is one of the most tested theories in science - and science has yet to disprove it.
posted by three blind mice at 8:40 PM on April 30, 2007

I stopped trying to understand science when my AP Chemistry teacher told me that an orbiting electron could be in two places at one time.

That doesn't sound as bad as what I remember from somewhere: "An electron exists in a cloud of probability around the nucleus. It's everywhere and nowhere all at once". Am I remembering this correctly, or did I just mash up two entirely different concepts?

(After searching, the first part seems fairly common.)
posted by IronLizard at 12:02 AM on May 1, 2007

QM seems to make more sense if you accept that there is no one concrete "universe", but bunches of probability... how to say... everything, if seperated into single quanta, would naturally be in individual random states, but as they group up and interact, the extremes of possibilities become ever more unlikely.
Allowing quanta (or small systems) to become seperate from the larger island of probability (more or less the one that defines our macro-scale environment) restores the likelihood of outlying probability, so we are more likely to see the quirkiness inherent in the system.
Whether or not probability coherence has bidirectional time effects is a very interesting question, I wouldn't be suprised if it holds true.

IronLizard, the first thing that comes to mind is a pair of pendulums on a shared plank of wood - while they are interacting via the vibrations in the floor, there will be some correspondence so you can infer the position of one from the other. But if you seperate them enough so there is no longer detectable forces shared between them, then the non-observed pendulum will become unpredictable, like the electron. If there is no information leaking into our "island of probability" of what position or speed the pendulum is at, then it is effectively at a superposition of states until information is shared with our "i.o.p.". That's a very basic QM analogy though, doesn't explain the double slit experiment.
posted by Tzarius at 2:19 AM on May 1, 2007

cortex: Man is there ever interest, q&q. = a |ground> + b |excited> where a^2 and b^2 are the probability of measuring the ground and excited energy levels (think floors).

This is the only equation I'll use, and it's merely to emphasize that the information we would need to copy for an arbitrary wave function is the a and b coefficients, which can be anything so long as there squares add up to 1.

So, it turns out that we (physically) cannot develop machinery to distinguish between all of these possible states and can only make copies for a and b equal to 1 or 0. The easiest proof I've seen of this would entail a few mathematical equations and some background in quantum circuits. However, the best "reason" I can give for the no-cloning theorem is that the a and b values are not knowable and not transferable without many, many measurements and/or destroying the state. (all of this explanation might make those in the "know" cringe, but i hope it's at least a helpful introduction.)
posted by quanta and qualia at 5:17 PM on May 1, 2007

Okay, That didn't work very well. Something happened with the post, my apologies, I only signed up this week for metafilter. My basic quantum analogy before the "= a |ground> + b |excited>" stuff was to think about the simplest quantum state as being a building with two floors. The 2nd floor is the excited level, and classically we would be able to determine whether we were on the 1st or second floor at all times, but for quantum mechanical systems you must treat the object you're discussing as a wave function occupying both floors. There are interference effects that come from this assumption. We are led to a description using |Psi> = a |ground state> + b |excited state>, where a^2 and b^2 are the probabilities of measuring the object on the first or second floor, respectively. (SORRY IF THIS IS A REPOST)
posted by quanta and qualia at 5:24 PM on May 1, 2007