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The Double Slit Experiment Explained
- Thread starter Skwim
- Start date
One of the things I really, really dislike about this video is that it gets 'observing' wrong.
When we 'take a look' at the double slit experiment to see 'which slit' information, we need to have a *way* to look. That means sending *another* particle in to interact with the electrons going through the slits.
But, if you interact with the electrons, you affect how they move. And that changes the pattern on the other side.
In particular, to be able to get 'which slit' information, you need to interact with something that has a wavelength small enough to distinguish the slits. But it is always the case that smaller wavelength for a given particle means higher energy (and thereby more influence when interacting).
So, if you try to look with light with a low wavelength, the influence on the electrons is not much, but you cannot get 'which slit' information. If you decrease the wavelength enough to get 'which slit' information, the light interacts with the electrons enough to eliminate the interference pattern. if you turn the low wavelength light down enough to not affect too many electrons, you miss enough and the interference pattern emerges again.
The same type of thing happens in the quantum eraser experiment, but a more thorough analysis is needed than is given in that video. There is no time reversal in the quantum eraser.
When we 'take a look' at the double slit experiment to see 'which slit' information, we need to have a *way* to look. That means sending *another* particle in to interact with the electrons going through the slits.
But, if you interact with the electrons, you affect how they move. And that changes the pattern on the other side.
In particular, to be able to get 'which slit' information, you need to interact with something that has a wavelength small enough to distinguish the slits. But it is always the case that smaller wavelength for a given particle means higher energy (and thereby more influence when interacting).
So, if you try to look with light with a low wavelength, the influence on the electrons is not much, but you cannot get 'which slit' information. If you decrease the wavelength enough to get 'which slit' information, the light interacts with the electrons enough to eliminate the interference pattern. if you turn the low wavelength light down enough to not affect too many electrons, you miss enough and the interference pattern emerges again.
The same type of thing happens in the quantum eraser experiment, but a more thorough analysis is needed than is given in that video. There is no time reversal in the quantum eraser.
So you're saying any attempt to determine the position of the particle interferes with and collapses the wave?
Being that we can't really see the particle, though that's kind of implied by having the eye observe. Any method we'd use to detect the particle is an interference to the particle which causes the wave function to collapse?
This doesn't seem particularly magical. Unless I'm missing something. Any detection along the path prior to hitting the screen collapses the wave function. It's not like it is going backwards to time, just wherever the particle is interfered with it collapses.
Can't be this simple though as I'm sure scientists would have had this figured out.
There's only one problem with your argument, & it's a big one.
The analysis behind it is far above the heads of others here.
To avoid shaming others, I'll put it in a spoiler for only you to see.
The cartoon physicist is wearing a cape.
You aren't. I recommend a caped avatar.
You aren't. I recommend a caped avatar.
ratiocinator
Lightly seared on the reality grill.
Not again!
This is an extract from a woo-peddling film called What the Bleep Do We Know!? The implication that conscious observation is important is not supported by any evidence at all.
it goes deeper than that. Both cases: where there is 'which slit' information, and when there is not, are described by wave functions. It's just that the interaction with the light used to 'see' also has to be taken into account and that affects the wavefunction. So it isn't so much 'collapse' of the wave function as it is 'changes' to that wave function.
Another aspect is that the light usually used is not coherent: it is randomized to some extent, which also affects what can and cannot be measured.
What 'collapses' a wavefunction is, according to QM, an interaction with a sufficiently complex environment. We can even time such collapses in some cases. This is described pretty well by decoherence theory, which currently finds its main application in attempts to design quantum computers (where we want to prevent collapse until the calculation is over).
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Oh, another aspect that isn't mentioned in the video is that things in QM are ultimately all probabilistic and not deterministic. This is in strong contrast to Newtonian physics and to the intuitions that many people have.
For example, in entanglement, it is the *correlation* of probabilities that it transmitted. That is why a measurement on one side is correlated to a result on the other side. There is no causal connection.
But, again, QM is not a causal theory. It is a probabilistic one. There are even results that show that some observations *cannot* be described by causal connections unless those connections move much faster than light (which has problems because of relativity).
many of the 'paradoxes' of quantum mechanics come from attempting to think about the quantum world using classical notions of what a particle is and what a wave is. A quantum particle is a strange mixture of both classical notions. The advantage is that ALL quantum particles have 'both' of these 'aspects': electrons, photons, protons, etc. All show what would classically be described as wave and as particle behavior. In all cases, the wave describes the probability of detecting the particle.
The 'collapse' due to measurement has to do with the fact that after a measurement, the particle is described by a *new* wave function that has properties determined by the type of measurement with a specific value for that measurement. But *which* value is not determined prior to the measurement. Only probabilities are.
Skwim
Veteran Member
Interesting; however, I didn't see a thing connecting the video to What the Bleep Do We Know!? Got a link or something?
Not again!
This is an extract from a woo-peddling film called What the Bleep Do We Know!? The implication that conscious observation is important is not supported by any evidence at all.
.
If you do a search for the movie, full version, this is about 33 minutes into it.
Beautiful!!!
Almost like "God said, watched and the electrons took form as He expected".
The Knower of all knowers, knew what He was doing.Thank you!!
Twilight Hue
Twilight, not bright nor dark, good nor bad.
Oh thank god.
I thought it was another pseudoscience vid of Dr Quantum from "What the Bleep"
....
...
Drat.
Heyo
Veteran Member
Actually, if there was an universal observer or "knower", there wouldn't be any interference. Interference only happens when nobody is looking.Beautiful!!!
Almost like "God said, watched and the electrons took form as He expected".
Thank you!!
Ergo, there is no observer.
God* disproved by QM.
(God*, in this case, defined as universal observer.)
Heyo
Veteran Member
You usually have a better understanding of QM than me so you need to explain your last sentence.
We have a combination of quantum effects in the Delayed Choice Quantum Eraser Experiment, Particle-Wave-Duality (with interference and "wave function collapse") and Entanglement.
The entanglement is responsible for the correlation of outcomes at sensor D0 and the others. But the entangled particle/wave couldn't "know" which path it would take. So it seems as if the information of the second path traveled back in time to determine the outcome at D0.
This is not a new result of the DCQEE, it was already seen in the Alain Aspect Experiment. Either causality or locality is broken.
That's how I understood it up to now. Where am I wrong?
Heyo
Veteran Member
Right. The entanglement correlates the results of the two sides. But there is only *one* wave function and that wave function works for the entire apparatus.
Next, *every* detector collapses that wave function to some degree, but potentially not all the way. So, a measurement can give a definite value at one point, be correlated, but still not give a definite value at another point.
Next, if you look at only the D0 detector, and don't worry about the results in the others, the result is that you get an interference pattern. The 'paradox' comes when you correlate the results at D0 and those at the other detectors.
So, you get a detection at D0. What can still happen? Well, we don't have 'which slit' information, so we only know there was a detection at D0. Nothing else. The 'collapsed' wave function after that detection is *still* a superposition of wave functions from the two slits.
Now, detectors D3 and D4 *only* detect from a single slit. So, if they are hit, the component of the wave function that is relevant is from that slit and that slit only. Those detectors collapsed the wave function to give that information. And that means that when we *select out* those detections at D0 that correspond to detections at D3 or D4, we won't see an interference pattern: the component of the wave function that would produce it is selected away *after the experiment is run*.
On the other hand, detections at D2 do NOT have 'which slit' information, which means that the component of the wave function after that still has component from both slits and so will show an interference pattern. The D2 collapse is to an interference state.
One crucial thing here is that, just like in the Aspect experiment, things at D0 look random: there is an interference pattern. It is only when you put the information together that you see these correlations. Everything happens in a forward time direction and through propagation of probabilities and correlations through the wave function. Furthermore, each measurement 'collapses' the wave function to some degree, but not completely.
Right. QM is a non-causal, but local theory. it is also not a realist theory: particles do NOT have definite properties at all times: they have *probabilities* of different values for properties. And those probabilities can be *correlated* by entanglement.
The wave functions propagate locally, but the value of any measurement is undetermined ahead of time, although *correlations* are. The correlation between the two sides of the apparatus is made at that initial prism that splits the beams and is propagated to both sides. When a measurement is made, we know some *part* of the wave function, but not all of it.
I really like this guy's videos. He addresses the important points and clearly knows his stuff.
Actually. no.
In that there are actual objects that you can see, suggests that there is an observer.
Which makes me wonder, if two people are looking and expecting a different result, perhaps the interference comes when man interferes with what God expects?
causing the chaos in this world? 
Thief
Rogue Theologian
And if you are seeing them, then *you* are the observer.
More generally, it is NOT consciousness that is required to 'collapse' a wave function, but merely interaction with a complex environment. The environment serves as the 'observer'.
Which makes me wonder, if two people are looking and expecting a different result, perhaps the interference comes when man interferes with what God expects?
Expectations have nothing to do with this. In fact, the first people to observe some of these effects were quite surprised by them. the actual results go against the intuitions most people initially have. Even Einstein's intuition on these matters was *wrong*.
Heyo
Veteran Member
He should as a professor of physics. (Matt O'Dowd) His stuff is right at the edge of what I can watch as "edutainment". Anything more in depth would require work. but I guess it goes wide over the head of most of this audience.