Specifically, I’m interested in the quantum version of the double-slit experiment. Here’s how I understand it:
Single electrons are fired one-at-a-time at a barrier with two slits, on the other side of which is a screen which registers/records the location of electron that hits it. As more electrons are fired, an alternating pattern of “hit and miss” regions forms on the screen, suggesting the electrons are forming an interference pattern after passing through the slits, similar to the pattern one would expect were the experiment conducted with waves, such as water.
But electrons are particles, and thus one would expect a different pattern to form. The resulting hypothesis: the interference pattern is a result of each electron’s superpositional possibilities interfering with eachother (pardon me if I’m mucking up the terminology)–each single electron, because it is subject to the laws of quantum physics, is literally interfering with itself.
So at this point, a sensor is used to determine if it is possible to detect which slit each electron passes through. When this sensor is added to the experiment, not only is it observed that each electron that makes it through the barrier passes through one slit OR the other, but that the resulting pattern on the screen changes entirely to one that would be expected from particles and not waves. In other words, without the sensor, the screen displays a pattern which indicates a wave-like interference pattern; with the sensor, the screen displays a pattern indicative of particles. And this discrepancy is attributed to the Observer Effect: at the moment the electron is observed by the sensor, all the possible paths the electron could take, which were held in superposition, collapse, and the electron takes a single path.
Again, my understanding: please correct me if I have something wrong, but I think I’ve at least hit all the major points correctly. So here are my questions:
Does the experiment prove, or only suggest that, without the sensor, each single electron could actually be passing through both slits (as well as neither)?
Has the basic paradox superposition presents (how can something be two things at once?) been explained, or must it be taken as an article of faith? (I understand that someone like me, without the grounding in math and physics necessary to comprehend such an explanation, must take it as an article of faith–but it seems like scientists would have big a problem with that).
Can the observer effect be attributed to some physical mechanism (in this case, some form of radiation or other energy from the sensor affecting the electron)? And if not, aren’t we basically left with the conclusion that the electron is somehow “sentient” or at the very least “aware” it is being observed?
Theoretically, if the human eye could directly observe an electron, would we still see what the sensor “sees”? In other words, does the observer effect apply to something as simple as eyesight?
The only part of this I can address is the sensor issue. All of the capacitive and proximity sensors that I sell create a field and what they would actually “see” is the disturbance of this field as the electron passes though it. As for what eyes see, they see light, so to “see” an electron, you have to bounce light off it. Either way, the mechanism of sensing affects the object sensed.
I’ll do the really easy answers, but I have about one minute so I’ll let the physics types answer 2.
Yes, the electron - and all particles, for that matter - does go through both slits.
There is no physical mechanism. It’s part of the wave nature of particles. They are not little balls. They are quantum entities. No, the electron is not in any sense sentient or aware.
We see by particle interaction exactly like any other sensor. There is nothing special about a living observer.
This not the basic paradox of superposition. Electrons are not little balls of matter they are essentially alien and unlike anything we observe at a macroscopic (i.e. every day) level, in certain circumstances they behave like waves and a wave being in two places is not paradoxical. People draw pictures of little moons orbiting little planets but this is just analogy to make up us feel better.
This IS the basic paradox of superposition. Quantum Physics tells us that it is not some physical by-product of the observation, it is the observation itself that causes the collapse of the wave function. Despite the rather crazy meta-physical implications of this thats what it evidence tells us.
The ball representation of an electron is misleading, I’m sure. But so is the fact that we call it a particle, describe how it spins, and have measurements of its radius.
Yes, this is where I think I’m missing something, because most discussions of this experiment focus on the quandary of an electron behaving both as a particle and a wave. Which to me is downright mundane compared to the fact that the same electron is passing through two different slits! I mean a wave is long, right? Long enough such that different parts of the same wave hit both slots. That’s not what’s happening with the electron.
So the fact that it sometimes behaves as a wave seems like it’s just a symptom of the fact that it can be in two different places at once! But probably there’s more to it than that. Still: call it a particle, call it a wave, call it a wartivle if you like. Whatever it is, how can it be two places at the same time? It seems like (again, just from what I’ve read) that would be the amazing part of the equation, not the fact of its dual behavior.
OK, except that the sensor creates a field in order to detect disturbances of that field; the human eye is receive-only, right? The eye wouldn’t be bouncing light off the electron and then reading it back like a sonar; it would be picking up the light from some other source bouncing off the electron.
Or think of it this way: what if the sensor simply generated the field and didn’t have any mechanism to detect disturbances within it? Would it still be observing?
Yes indeed. Properties like spin are misleading. They have nothing to do with similarly named properties in macroscopic particles (i.e. pool balls).
The “wave” does indeed “hit” both slots but that does not mean the photon is in two places at once. The crazy messed up thing is the “wave” is not a physical vibration in a medium (like a water or sound wave) but a probabilty function. Unobservered the particle passed through NEITHER slit, there is a probabilty function that describes the possiblity of it passing though either slit (and indeed any point in the universe though it vanishingly unlikely it is anywhere else but those two slits). Quantum Physics says the wave function does not collapse into a physical “particle” until we observe that it has gone through slit A or B (again it is not some physical by-product of the observation or equipment used that causes this it is the ACT OF OBSERVATION)
This is the paradox, this is why “anybody who thinks they understand quantum physics is wrong.” This is ridiculousness that Schrodinger was highlighting with his cat thought experiment. For a long time quantum physics was thought of as a useful tool that does not actual accurately represent reality (rather like describing Brownian Motion statisitically , the individual collision of atoms against the particles is not actually random , but in large enough numbers they can be considered so) but all the evidence indicates this is not the case. The behaviour of the quantum world (and hence of course the world generally, as you and I both have wave functions just very very thin ones) really is that nonsensical.
Remember that the observer collapsing the wave front is one mathematical interpretation of quantum mechanics, rather than a physical picture of what actually happens. They are other mathematically equivalent interpretations, such as the many worlds interpretation, in which the whole question of the observer collapsing the wave front never enters the picture. You are trying to make physical sense of a phenomenon that has no ordinary analogy.
Second, as griffin1977 says, you are wrong to think of a wave going through the slits. The electron, a quantum thing unto itself, goes through the slits. In fact, it’s not even precisely correct to say that the probability function goes through the slits. As Feynman demonstrated the math can interpreted as stating that the election takes every possible path between the source and the screen. His sum-over-paths method eliminates the need for an probability wave to be associated with the electron.
Third, all one needs is a source of illumination and a receiver for the experiment to work. Whether the illumination comes from the receiver makes no difference. Whether the receiver is an eye or electronic or mechanical makes no difference. But take away either part and there is no experiment.
So if I’m understanding correctly, then act of “observation” is almost equivalent to the act of making a choice–in this case, to observe something forces the “universe” to make a choice.
I didn’t think about this before, but in the experiment with the sensor, the sensor is actually the second instrument that observes each electron–the instrument that fires the electron at the barrier has to be observe the electron before it can fire it, right? We know the electron is at a specific point A when it’s fired at the barrier; we know the electon is in a specific point B when it passes through one of the slits (if we’re using the sensor); and we know it’s at point C when it hits the screen.
By placing the sensor at the barrier, we’re basically asserting that the electron can only take only one of all possible paths through the barrier. And like magic, the universe behaves as though we are correct and chooses one of those possibilities. When we don’t bother to observe that that is actually happening, the universe behaves as it could take all possible paths through the barrier at the same time. But that’s not really what’s happening? Or is it? Or is it that we just can’t ever catch the universe in the act? Every time we look, it’s behaving; every time we turn our back, it’s right back to this high strangeness. And we’re really just ok with the fact this makes no sense at all?
Yes, my bad, I should really have said the “Copenhagen Interpretation says” “Quantum Physics says…”
But the “Copenhagen Interpretation” is still the basis of our understanding of Quantum Mechanics and other interpretations (such as “Many Worlds”, or Feynmann’s “sum over histories” are just as mind-bending , and in some cases flawed). It is the Copenhagen Interpretation that has been successfully used throughout physics for 80 years, without worrying about the meta-physical implications, (but those meta-physical implications were, and are, still there)
The observer collapsing the wave is the Cophenhagen Interpretation, I take it. My basic interpretation of the difference between the two Interpretations is this: in the Cophenhagen I., God really does play dice with the universe; in the Many Worlds I., God plays with a six-sided universe. Here’s a big part of my problem, from wiki page on Cophenagen:
Meaningless?! Really? I don’t see how that could be true. Unless by “meaningless” they just mean “unknowable.”
As far as we can work out by means of experimentation and anaylsis that IS what is happening. Initially it was assumed that thing “Couldn’t” work that way underneath and more sensible underlying model would appear (Einstein spent alot of his later life trying to undermine Quantum Physics by trying to find this underlying reality). But every subsequent experiment has demonstrated that this IS how things work underneath.
We aren’t but we just have to put up with it. The difficulty so many great scientists (Schrodinger, Einstein, etc) accepting it is a testament to how little sense this makes underneath.
It just means the strict Copenhagen Interpretation takes an “Occams Razor” approach, it can be used as a tool to predict real world behaviour without worrying about these details. But they don’t go away.
No, it merely illuminates an actionthat the universe has already made.
We only know where the electron is after we’ve observed it. We do not know that the electron is a specific point B. We infer that from the result.
What do you mean by sense? Does it make sense to you that the earth is revolving around the sun instead of the obvious motion the sun makes? Does it make sense to you that your yard is actually part of a two-dimensional curve rather than falt? Does it make sense to you that your radio can pick up sounds turned into electric impulses thousands of miles away? Your idea of “sense” has no meaning whatever.
The universe is not obliged to make “sense” to humans whose senses have led them to evolve in a world which only corresponds to a tiny fraction of the overall possibilities of energy and matter. We see an infinitesimal slice of the electromagnetic spectrum. We deal with a minute range of times and masses. We’re not even made of the type of material that 96% of the universe is made of.
What hubris to think that the mathematical underpinnings of this invisible universe must make sense to you. We all need to conform our brains with the realities of the universe, not make demands that it conform to our limitations.
At the risk of getting in GD territory thats not my understanding of how the various interpretations work. The “choice” has NOT been made until the observation. In “many worlds” (as I understand it, my knowledge is sketchy) there are several universes each one with a difference choice and we “choose” a universe when we make an observation, in CI the particle exists as only as probility wave until the observation is made. An extension of the original “Schrodigners Cat” thought experiment invovles “Schrodingers Kittens” the same set up as the original but now we have two boxes with two kittens, if the quantum event releases the poison in one box the other kitten remains up harmed. We put the boxes on spacecraft and travel to opposite ends of the universe, when one box is opened the wave function collapses (as in the original thought experiement) the difference here is that wave function in the other collapses IMMEDIATELY, but not until the other box is opened. The bizarre thing about this thought experiment is the equivalent REAL experienment has actually carried out (not as far as I know with Kittens ), an slit-like experiment on solar radiation demonstrated this does in fact happen.
A variation on the double slit experiment invovles putting a detector in one of the slits. In this case the wave function is collapsed and the fringes disappear.
That was beautiful. As I read it, I had a vision of Christopher Lee shouting down to me from the top of Isengard with storm clouds swirling all around.
I tell you I demand nothing, sir! I actually find it positively delightful that it doesn’t make sense to me personally. I was just asking in case, you know, there was anyone around with the hubris to think that they did. And if it there isn’t, well, I’m delighted all the more.
), an slit-like experiment on solar radiation demonstrated this does in fact happen.