July 29, 2004 6:36 AM   Subscribe

The physicist Shariah Afshar has used a beautifully simple experiment, which no-one seems to have thought of before, to disprove Bohr's principle of complementarity, something which has been pretty much unchallenged for 80 years. He may also have gone some way towards showing that there is no such thing as a photon, and that Einstein's Nobel prize should be revoked. So, big stuff. What do you physicists think?
posted by Pretty_Generic (34 comments total)
Why should Einstein's Nobel be revoked? He won the Nobel for his work on the Photoelectric Effect, which, while related to quantum dynamics, isn't really affected by this experiment. Furthermore, Einstein always hated QD, and tried to disprove the Principle of Complementarity -- should this result hold, it would, if anything, bolster Einstein's reputation.

It's not a sin to be wrong in science. Even if General and Special Relativity were later disproved or supplanted, the advances we made based on that work would leave that work still worthy of our acclaim. Indeed, we've found, on the micro and macro scales, that Newtonian Mechanics isn't correct -- yet we still hail Newton for his work, and use his equations.

This is an interesting result. But it's also a very new result. Until independently observed and examined, we can't declare one of the pillars of QD fallen. There may well be another, as yet undiscovered, effect that leads to this result and preserves complementarity.
posted by eriko at 6:58 AM on July 29, 2004

Thanks! I was trying to get a hold of this article as soon as I saw it on New Scientist online.
posted by Gyan at 7:06 AM on July 29, 2004

The text of this post brings very much to mind a paper I wrote once on the stature of genius and science. I found a common thread amongst popular science articles about pretty much any controversial physics research - "It's going to bring down Einstein! Challenging the establishment! David and Goliath! The little physicist that could!" I've seen much worse examples of it, but bringing Einstein (the most prominent figure associated with the idea of physicist) into a topic that has little to do with him is not an unusual tack. Casting a scientific theory as a deep lie that remains unchallenged implies some sort of old guard conspiracy to silence detractors.
posted by jacquilynne at 7:17 AM on July 29, 2004

I'll say one thing: upon hearing of this experiment, I wasn't shocked.
posted by hammurderer at 7:27 AM on July 29, 2004

I think my post only says what the article says, although of course popular science journals are not above hyperbole. Unfortunately it's not yet available online for free. If someone has it and wants to scan it in...
posted by Pretty_Generic at 7:42 AM on July 29, 2004

Complementarity underdefined.
Look at this experiment not as a traditional particle/wave model, the way a newtonian physicist would, but as a packet model, as a quantum physicist would.

For example, "light cannot be particles and waves at the same time", assumes linear time independent of space. But if space-time is a single thing, as believed in much of quantum mechanics theory, waves and particles may *not* be occupying the same space-time, *even if they appear to* from an outside perspective.

If you look at a particle or a wave travelling from point 'A' to point 'B', in the traditional model, it will take a given time at a given speed to follow its course.

But particles and waves are idealized extremes of real energy which is made of packets--having properties of both particles and waves. More energy==more wave-"like".

And when an energy packet travels between point 'A' and point 'B', it *occupies ALL points in between for the duration of its travel*. Until it arrives at its destination, it is *everywhere* between 'A' and 'B'.

So, if you interfere with its progress through observation, you can observe particles *or* waves *or* both, and you can do it any time during the transit period. That is, you can make your observation right in front of the emitter, or right in front of the receiver. Because until the instant when the packet finishes its arrival, it is everywhere in between.

And since it is in every possible space in between 'A' and 'B', it is in every possible *time* of the transit between the two.
posted by kablam at 7:51 AM on July 29, 2004

From the article:
The photon detectors in Afshar's experiment "click" when they detect a photon. But if there is no photon, what are they seeing? It comes down to the interpretation of Einstein's photoelectric effect, the experiment that "proved" the existence of the photon - and won him the 1921 Nobel prize. Afshar says the American physicist Willis Lamb and others have explained these particle-like clicks as a result of the interaction of unquantised electromagnetic waves and quantised matter particles in the detector. So althought Einstein was right to doubt Bohr's complementarity, he was "right for the wrong reasons", Afshar says. "In order to declare Einstein the winner of the Bohr-Einstein debate, we must take back his Nobel prize. We have no other choice but to declare the idea of Einstein's photon dead."
Of course, I don't think anyone's seriously talking about actually revoking the prize. They're just rejecting Einstein's interpretation of the experiment.
posted by Pretty_Generic at 7:52 AM on July 29, 2004

Here's a sci.physics.research post pointing out an obvious problem with the experiment: the wires scatter the photons, forming in effect a second wall with multiple wide slits, and re-introducing uncertainty and duality.
posted by nicwolff at 8:04 AM on July 29, 2004

I'm not in any position to comment on this particular experiment. I will say, however, that 90% of the time in cases like these reported in the popular press, the result is either deeply suspect (and lately disproved) or simply not a big deal. If you have friends who are physicists (as I do—I have a lot of scientist friends but most seem to be physicists), and you hang out with them a lot (as I used to), and you bring up these things reported in the popular press, you'll almost invariably get a sigh in response. That's partly a scientists's natural condescension, but partly an inevitably consequence of the fact that science journalism is Very Very Bad.

As to the philosophy (and culture) of science mentioned in the above discussion, I am in a position to comment on that. Unfortunately, the subject always gets very contentious so I'll avoid detail and just say that the popular imagination of how science "works" is very, very wrong. However, the science culture's imagination of how science works is also very wrong. Both are highly mythologized. But in no case would, for example, anything happen that would lead to Einstein's Nobel to be "revoked".

Finally, a good rule of thumb is that unless you've had at least one or two graudate courses in QM (I haven't), pretend that you know absolutely nothing about it and never discuss it. Ever. Everyone is better off that way, I promise.
posted by Ethereal Bligh at 8:27 AM on July 29, 2004

it's a sweet experiment, but it's not obvious to me that it shows new physics. but then i'm not that good a physicist... my intuition is that there will be some cool interaction between the slits and the other light source that "just happen" to correct everything.

and, of course, the bit about einstein's nobel prize is just populist crap to make the masses think they're not as stupid as bitter experience has taught them. "heh, i always knew einstein was wrong, but no-one listened to me" etc etc.
posted by andrew cooke at 8:27 AM on July 29, 2004

(incidentally, i thought this was much more interesting, but have so far failed to excite anyone else about it - maybe it was cool a couple of years ago).
posted by andrew cooke at 8:30 AM on July 29, 2004

Nicwolff: there was a discussion about that thread on the Kathryn Cramer website linked above. They seem to think that the Google Groups poster misunderstood the description of the results. To paraphrase the last comment on that site:

The experiment involves three different processes.
1) Create a standard double slit, but add a lens so that two photodetectors each detect light from a single slit. Observe the EM flux everywhere, and find that all light is indeed focused on those two detectors.

2) Pick just one location where the double-slit setup would create wave-like negative interference, and place one metal wire at that location. Then cover up one slit, and measure EM flux everywhere. You'll find that some photons collide with that wire and get scattered, and that less light will hit the photodetector.

3) Having established that light particles get scattered by that single wire, uncover the first slit. Now, measure the flux everywhere and find that no light gets scattered. That is, all light hits those two photodectors.

As for myself, I think that the experiment generates a contradiction based on the following two observations:
1) the photon detector only detects "particles" of light, and not "waves".
2) when the two slits are open and a wire is placed at a negative interference spot, that wire does not disturb light propagation -- a phenomenon that shows light behaving as a wave.

I still don't understand the conclusion that is reached, or how it is reached. But I do know one thing: it's not about "which-way information", but simply about particle collisions and wave interference.
posted by hammurderer at 8:32 AM on July 29, 2004

maybe it would help non-physicists to know that one was always taught, as a physics student, that if you could count the photons going through one slit then the interference pattern would disappear (in the usual double slit experiment). this experiment is trying to show that isn't the case, i believe, but (1) it's not exactly equivalent and (2) while the assertion above is a strong meme, i don't know that it's necessarily correct.

as i said, i don't really understand qm well enough. but i think the connection with the "truism" above is why this experiment "feels" interesting.
posted by andrew cooke at 8:39 AM on July 29, 2004

Yeah, don't know enough about physics to interpret this experiment, but talking about taking back Einstein's prize seems like grandstanding to me.

Nobel Prizes tend to reward "breakthrough" theories that tend to be useful for later theories. Even if Einstein was wrong about the ultimate interpretation, his description of the photoelectric effect was the best going for 80 years and opened up quite a few doors for experimentation in science and technology. The fact that a later theory is better, does not make the earlier ones less revolutionary.

After all, we still give credit to Copernicus for the modern solar system model, even though putting the sun at the center was the only thing he got right.
posted by KirkJobSluder at 9:48 AM on July 29, 2004

I'm not quite getting the "principle of complementarity" here, and I have a basic understanding of the double-slit experiment.

Here's what confuses me: In the standard double-slit experiment, you can send through one photon at a time, and you see one photon at a time hit the target, at one specific place. If you do this many many times, and count up where all the individual photons hit, they create the standard wave-interference pattern--no photons hit where there is negative interference, etc.

The one-photon-at-a-time-double-slit experiment has been known for some time. But isn't this an example of light behaving as a wave and as a particle in the same experiment? How is this consistent with the principle of complementarity as stated in the link from the original post? I'm not seeing that Ashfar's experiment contradicts complementarity any more than the old standard does.
posted by DevilsAdvocate at 9:59 AM on July 29, 2004

So when there is just one slit open and wires at the interference point, the light behaves just as a particle and less photons hit the detectors.
But, when both slits are open light then behaves as a wave of particles by interfering and missing the wires altogether and all photons hitting the detectors?

I get the experiment, I suppose...

I've also seen double pinhole experiments that seem to prove the existence of multiple universes similerly well but we havn't yet accepted them as concrete either...

It almost seems as if the act of observation is making this possible and we have no way to prove if that's how things really behave.
posted by LoopSouth at 10:08 AM on July 29, 2004

Here's the true explanation:

It Just Is.

which is to say that there are aspects of our universe which are unknowable, and we're just going to have to accept it...
posted by five fresh fish at 10:50 AM on July 29, 2004

That's nice, fff. Even if there are things that are unknowable, how do we know when we've reached the limit of the knowable? And if we don't know whether we've reached that limit or not, shouldn't we keep striving for greater understanding?
posted by DevilsAdvocate at 10:55 AM on July 29, 2004

Oh, hell, we don't ever stop striving! I say we go for it in a big way!

I read something the other day bitching about how the only reason to explore space was to explore space. Bloody fine idea, I think. Why the heck not explore space... as long as we're also focusing on fixing problems on our own planet, yah.
posted by five fresh fish at 11:33 AM on July 29, 2004

fff: ironically, quantum experiments may help us to explore space.
For instance, the J.S. Bell theory asserts that space can mean nothing to subatomic particles. When you have a paired set of particles with the same spin, and you separate them *at any distance*, then change the spin on one of them, the other will instantly change its spin, too. No matter how far they are away from each other, a foot or a parsec.
Already an "un-tappable" communications system is being built using this theory. Why not put some future version on a space probe for real-time communications across the solar system or beyond? "Faster than light?" (well, sort of).

And, if you can take quantum effects up to our newtonian scale, you could, as example, do something like fly a parsec in a spaceship in an hour, at 10mph. And since at 10mph, you are nowhere near the speed of light, you hardly notice any relativistic effects. So it's one hour later where you are, and one hour later back on Earth. Tickets, please.
posted by kablam at 11:52 AM on July 29, 2004

you could, as example, do something like fly a parsec in a spaceship in an hour, at 10mph.

That's not going to set any records on the Kessel Run.
posted by COBRA! at 11:57 AM on July 29, 2004

i think you can make a somewhat stringer statement than that - if you're at all serious about exploring space that isn't mind-numbingly claustrophobically close to our own planet, and particularly about somehow saving something that is "us" before we fuck the earth completely, you stop pissing around with stupid rockets and put more effort into biology, artificial intelligence and quantum mechanics in the hope of finding some way to either overcome or work-around the huge distance (and hence time) scales of inter-stellar and inter-galactic travel.
posted by andrew cooke at 12:54 PM on July 29, 2004

The experiment obviously needs to be replicated a few times by others, but if it stands up the outcome is consistent with only a subset of all of the possible interpretations of quantum theory, so he'll get a ‘clever’ rubber stamp and a gold star... the fact that this supports an interpretation that I quite like leads me to say: yay.
posted by snarfodox at 3:18 PM on July 29, 2004

Well, I've had quite a bit more than a couple of graduate courses in QM (though by no means an expert) and I'll wait to see this published properly, because as it is presented it doesn't make sense to me... From the New Scientist diagram I fail to see how this supports what it's supposed to support... When you have one hole open, light from that hole behaves as a particle (as it is supposed to in standard Copenhagen-flavoured QM) and thus is scattered off the wires... Then if you re-open the second hole, light behaves as a wave and is not affected by wires placed on the minima of the interference pattern... again as it is supposed to... This seems like a fancy way to do the standard two-slit experiment ...
Where does one conclude that "wave interference is present even when one is determining through which pinhole a photon passes"?... Where was the interference?
What would the Copenhagen interpretation predict differently? What am I missing?
I guess we'll have to wait for a proper publication in a peer-reviewed journal to find out... (or someone more perceptive than me to point out the obvious detail I'm ignoring...)
posted by talos at 4:23 PM on July 29, 2004

An "update":


Dear Friends,

I have a wonderful news! I just received an e-mail from my team at Rowan University: The single-photon experiment confirms my earlier findings! Bohr and Copenhagen are history!!!

Thanks for your help and support.

Best regards.
Shahriar S. Afshar

posted by Gyan at 5:02 PM on July 29, 2004

Wow if this turns out to be true, how cool to see it being bloged in real time as the results come in from diffrent experimenters.

What does all this mean the signifigance, other than Bohr being wrong?
posted by stbalbach at 6:30 PM on July 29, 2004

Well, reductionism doesn't exactly recrudesce. Far from it. And, I always thought Bohr's approach a bit fuzzy, if quite pragmatic.

But, I am not a scientist working in the field or in a related field and - in fact - am not even a scientist at all. I don't even have a basic college degree. I do, however, have a GED. [BIANASWITFOIARFAIFANEASAAIDEHABCDIDHHAGED ]
posted by troutfishing at 10:33 PM on July 29, 2004

This comes to me in e-mail from Ry Herman, whom I don't know and who has a Hotmail address, but who seems well-informed and lucid on the subject:


Anyway, what's interesting about the experiment isn't that light is being observed acting like a wave in transit and as a particle when it hits the detector - that happens all the time, and isn't surprising at all. What's surprising is that, if you can "follow" the light for the whole time - that is to say, you know which slit each photon through and where they're going to end up - they should act like particles the whole time, and there should therefore be no interference pattern. To quote a standard textbook (Introductory Quantum Mechanics, by Richard Liboff):

"We conclude that if it is possible to observe which slit [photons] go through, their interference pattern is destroyed. In observing the position of the [photons], their wave quality (e.g., interference-producing mechanism) diminishes."

(Liboff used electrons instead of photons as an example, but the principle is the same.)

This is determined by the Heisenberg uncertainty principle, which is very closely linked to Bohr's idea of complementarity. This concept is poorly explained by the article - what it actually states, basically, is that if the photon exists within a well-defined locality of space, momentum will be ill-defined, and it will act like a particle. If it doesn't exist within a well-defined locality, momentum can be defined more precisely and it acts more like a wave (although it will still always register on a detector in a particle-like way.) So, if you know which slit it goes through (locality defined), it should act like a particle - no interference pattern.

What Afshar has done is set up a double-slit experiment where, because a lens directs the photon streams to different detectors, he knows which photon came from where. So they should be behaving like particles the whole time. Then he plunks down a set of wires right where there would be interference nulls if the photons were being wave-like. If the photons are behaving as particles, like they should, photons should hit the wires, scatter, and the image at each lens should get all fuzzy, as it does with a non-interfering single slit. Instead, nothing much happens, which is what you'd expect to happen if the wires were in the null points of a wave interference pattern. This is a very interesting result.

Now, there's an obvious problem with this, which was brought up by the article you link to. Putting a set of wires in like that is essentially creating a whole new set of slits after the first pair, which could be effectively destroying the "we know where each photon comes from" phenomenon. One way to test this might be with a single-photon experiment, which is probably what's being referred to in the link in Gyan's "update" post; if you fire single electrons at each slit, and every time they show up at the detector they're supposed to even if the wires are there, that's supporting evidence that the wires aren't making things screwy (single photons should behave exactly as photon streams do, including with regard to interference patterns.)

So, it looks like this is a pretty interesting experiment, since it seems to contradict predictions made by Bohr, Heisenberg, and others which have stood up to a lot of tests and experimentation. I can understand why he might be excited about it. However, it'll require a lot more investigation and experimentation to determine exactly what's going on, and it hardly invalidates Einstein, Bohr, or quantum mechanics in general - but it might be a step towards correcting, amplifying, or even contradicting some of their work in areas which are still poorly understood. It's neat stuff - I wish the article had explained it better.
posted by nicwolff at 6:39 AM on July 30, 2004

thanks - that makes things a lot clearer.
posted by andrew cooke at 8:58 AM on July 30, 2004

nicwolff - thanks for that. So THAT's the significance. It seems pretty major to me, a puny layman.
posted by troutfishing at 9:49 AM on July 30, 2004

Very nicely written, nicwolff. Thx.
posted by five fresh fish at 1:14 PM on July 30, 2004

I didn't write it, fish! My new pal Ry did, but he can't get an account here because, uh, NT suxxors and Jrun errors make baby Haughey cry, so he mailed it to me and I posted it. He's only a well-informed amateur, but apparently his girlfriend is an astrophysicist working in adaptive optics and she checked his work.

(cooke... trout... fish... damn, now I'm hungry.)
posted by nicwolff at 2:16 PM on July 30, 2004

Here is John Cramer's description of the Afshar experiment from Kathryn Cramer's blog... I now see the problem:

Behind the plane where the interference pattern forms, Afshar places a lens that forms an image of each pinhole at a second plane. A light flash observed at image #1 on this plane indicates unambiguously that a photon of light has passed through pinhole #1, and a flash at image #2 similarly indicates that the photon has passed through pinhole #2. Observation of the photon flashes therefore provides particle path which-way information, as described by Bohr. According to the Copenhagen Interpretation, in this situation all wave-mode interference effects must be excluded.

(This is in agreement with Ry Herman's description posted above). Unambiguously? Why? He's "hiding" the interference pattern behind the diffraction pattern... If he had a screen where the detectors are, he would see strong intensity maxima at the "image#1" and "image#2" sites (surrounded by a diffraction pattern) and also much weaker intensity maxima (and minima) in between. The photons from the two sources always interfere when both slits are open. You have no unambiguous way of knowing that the photon landing on image 1, came indeed from source #1...The Copenhagen interpretation would predict exactly what is described in the experiment...

Again I'm ready and willing to stand corrected (I've left serious physics some time ago), because the only way Cramer (and a whole lot of others) isn't seeing something that even I can see, is if he's temporarily blinded by wishful thinking... Still I can't see how he can assume "unambiguousness" in the circumstances described...
posted by talos at 3:59 PM on July 30, 2004

At best, I don't see how this disproves Bohr, as much as it might add to his theory. In other words, if you theorize that "gravity" exists, then postulate it exists *because* the sky pushes down on apples, if your *because* is disproven, it doesn't mean that gravity doesn't exist. The new theory, that gravity exists because the Earth sucks apples down, while better than the first hypothesis, is still inaccurate; and yet gravity still exists.

Much debate in Quantum Theory comes from taking one element and putting it in perspective to Newtonian Theory, not in relation to other Quantum Theory. One apple in a pile of oranges, instead of a pile of apples.

That is, Quantum Theory states that particles and waves are *ideals*, and that energy exists in *packets*. So what do you expect when you do a Quantum experiment, then try to describe its elements in terms of particles and waves? If it is a particle, then it should do 'x', but if it is a wave, then it should do 'y'. But if it is neither a particle nor a wave, but shares characteristics of both, why in the world would you limit your outcome to just 'x' or 'y'? I would at least anticipate finding 'x' or 'y' or 'x'+'y' or 'z'. Maybe even 'x'-'y' or 'y'-'x', or some combination of 'x', 'y', and 'z'.
posted by kablam at 7:16 PM on July 30, 2004

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