ready29003, the site you give is good, but I fear you’re interpretation of what the word “erased” means might need to be revised. The “erasure” does not occur by throwing away data, but rather by changing the experimental set-up… in effect, ALTERING the detector! It has nothing to do with the “measurement” being taken, but rather whether and how the detector is set-up to take measurements.
ready29003, the site you give is good, but I fear your interpretation of what the word “erased” means might need to be revised. The “erasure” does not occur by throwing away data, but rather by changing the experimental set-up… in effect, ALTERING the detector! It has nothing to do with the “measurement” being taken, but rather whether and how the detector is set-up to take measurements.
"Measurements, once made, can also be erased, altering the outcome of an experiment that has already occurred. "
how can you change the experimental setup for an experiment that has already occurred?
RickJay and jawdirk, measurement effecting the observed phenomena is pretty subtle stuff. We’re not sure WHY measurement tends to affect the world in this way… it just does. All I can tell you to reassure you is that the mathematical formalism works out. Believe me, physicists love to come up with potential ways around these quandaries (that’ where such double-slit experiments first get dreamed up). They think of all sorts of way to “trick” nature into giving a result that will falsify quantum mechanics. Time and again, nature sticks its tongue out at the scientists and says, “nyah, nyah, nyah, nyah, nyah… you can’t get around it!” Same goes for sending information faster than the speed of light… we just can’t figure out a way to do it.
That doesn’t mean that in the future someone will figure out how some more basic mechanism works to cause these phenomenon, but for now we are left with basically describing the world and saying how these interference or non-interference patterns exist depending upon where detectors are placed. By the way, the site that ready29003 lists has given a good refutation of simply the view that the observer naively effects the measurements… it’s a bit more fundamental than that. Basically, the wavefunctions can exist as superpositions only when a measurement isn’t made regardless of the actual mechanisms of the measurement.
On preview, I see that ready29003 is asking about changing the experimental setup after the experiment has occurred. The way to think about this is putting a detector in between the screen and the slits that will allow you to erase the detection effects that occur with detectors at the slits. Again, the site ready29003 gives us has a good explanation of one way this is done. (I leave it to the reader to find it!)
Now, I may have to give the quote everybody has to hear when in dealing with QM, “Confused? Good!”
I think what the original post would be referring to is the Quantum Eraser Experiment or its variation, Wheeler’s Delayed Choice Experiment. It seems you know already that if the slits are equipped to detect whether or not a particle passes through then we do not get an interference pattern, however it seems that when the detector is left on, but NOT collecting data an interference pattern emerges. If thats not weird enough, check this out. If we COLLECT the data and look at it after the experiment then we do not see interference, but if we erase the data without looking at it then we get interference. Its like the photons either: A) know whether or not were going to look and position themselves accordingly or B) position themselves accordingly after we look, but did so before we looked. (Ow… my head hurts) I’m not going to pretend I understand the fundamentals of Quantum Physics, as I do not. I am simply interested in and read a lot about it. I found the following links interesting, and you might as well. The second link is a YouTube video. Its part 6/8 which he discusses Delayed Choice, but all 8 videos are interesting and explains the the Double Slit Experiment as a whole on a very elementary and easy-to-understand level. ENJOY!!
Check out these links:
http://www.bottomlayer.com/bottom/basic_delayed_choice.htm
This is part 6/8 of this video, but the whole thing is very interesting and I highly recommend it for a simple explanation of the Double Slit Experiment as a whole.
http://www.youtube.com/watch?v=QBOaXcG3sJ0&feature=related
I’m extremely interested in this. I came in at the same point of query as the OP. I also came across the Youtube stuff quoted above.
Is it fair to say that the lecturer shown (he claims that you can keep the detectors on, and then discard the results without looking at them, and still produce an interference pattern), is completely wrong?
The guy in post #25 was a one-and-done poster-doubt he’ll be back to answer your question.
I seem to remember there being some experiment where something was observed without collapsing the waveform. It had to do vibration and absolute zero. Anyone know what I’m talking about?
This looks very interesting. Thanks.
If the collapse of the wave function due to measurement is interpreted as a loss of coherence between the measurement system and the object being measured, wouldn’t it be possible for an observer to still be in superposition with the system as a whole?
Or in other words, just because decoherence occurs between the measument apparatus and the electron (for example), that is no reason for decoherence occuring for other systems as well.
So a setup in which the results of the detector are never observed/measured might still produce an interference pattern?
You are correct: the lecturer is completely wrong. Jawdirk got it exactly right in his last post:
Again, it’s not recording or looking at or doing anything with the data that removes the interference pattern: it’s the physical interaction between the electron and the detector. We could build a machine that only interacts with the electron, and makes no attempt to detect the result in any way (take a standard detector and break off all the parts that detect the results of the photon-electron collision). That will still remove the interference pattern, even though there’s no way a human could ever know the result of the photon-electron interaction.
That conclusively and concisely answers my query - I thank you :).
I suppose I still only have your word against the lecturers, so to that extent I’m like the particle/waveform/entity - caught between two states. But to be honest - at this point - I believe your version.
Collapse does not equal decoherence. Just from a matehmatical view they are not the same.
You have already figured out for yourself a vital point (though you haven’t expressed it well- the observer can no longer be represented as a quantum mechanical system, only part of one, so it’s wrong to talk about the observer being in a superpostion of states), the larger system still is in a state of superpostion.
Now that’s not to say you cannot use some subtle arguments to say that wavefunctions appear to collapse due to decoherence, but that still leaves a lot of questions unanwsered. The Many Worlds Interpretation plus decoherence has a lot of things to be said fot it, but it is not the panacea to the the quantum mechanical measurement problem/the conceptual problems of quantum mechanics.
The point of view that there is no collapse throws up problems of it’s own e.g. if the larger system which the observer-experiment sysetm is in is still in a state of superpostion why does the observer observe only one result? What does probabilty mean in such an interpretation?
Interaction is the same as measurement (from a quantum mechanical measurement theory pov)? That’s a pretty bold statement and I don’t think you’d find many who’d agree.
I don’t which lecturer is being referred to, but the are you saying the delayed choice experiment is incorrect?
The lecturer is Ross Rhodes, and yes, the implication being made on this thread is that the delayed choice experiment is invalid (if I’ve understood things correctly).
Link to the lecture below (as originally posted by dan.greene):
It seems odd as the delayed choice experiment is a prediction of quantum mechanics. If we could prove that the delayed choice wasn’t really a delayed choice then we’d be disproving quantum mechanics (now of course that’s not to say for quantum mechanics to be correct the delayed choice has to be a true delayed choice, but quantum mechanics at the very least predicts you can’t prove it isn’t).
I hope this is not too much of a hijack because i would really like to understand this. My only knowledge of quantum mechanics is from stuff i’ve picked up online so i suspect i’m way out of my league.
I understand decoherence to be an alternative interpretation of what we observe when we refer to wave function collapse. That is, the wave function does not actually collapse but only appears to do so? The wave function collapse in terms of (de)coherence is that coherence between the observer and the quantum object is lost.
Before the experimental setup (measuring device and the electron) is observed it may still be in a superposition of states and thus produce an interference pattern? Only when the observer interacts with the system is the coherence lost, since he is then part of the same quantum system (the experiment and the observer are now in a superposition of states.)
Or is the interaction between the quantum object and the measuring system enough to loose coherence and thus give the appearance of a collapsed wave? I would guess not if the delayed choice theory is valid.
I suspect that this is very close to being meta-physical since we have no way of knowing the actual mechanisms behind wave function collapse.
I read the link at the utoronto site linked above and liked it a lot. Very layman-friendly experiment. I have a question about the photon collision explanation offered above:
From the experimental design, would not photons strike the electrons as they were traveling to the slits? Or do photons not penetrate through a vacuum? Because we can see through a clear container that’s had all the particles sucked out of it, can’t we? Why are the light bulb (measurement tool’s) photons different? Or am I misunderstanding this? Would no photons interact with the electrons if we did not measure them with the light bulb?
I’m also kinda confused because we aren’t measuring which slit the electron chose until after it comes out, so the electron could not change its behavior (choose a different slit or act as a particle instead of a wave) unless it knew it was going to be measured in the future. Isn’t there a causality violation here?
Another question re: the uncertainty principle. How can we measure the velocity of an electron without knowing its position? How can we be sure we are measuring the right electron if we don’t know where it is? Maybe we’re measuring the velocity of an adjacent electron instead. How can we be sure?