Thanks. And if a detector is placed on one of the slits, the interference pattern disappears?
Doesn’t this experiment support the wave nature of matter? A single quanta interferes with itself because it is a wave. Couldn’t you just say that when location is measured as a discrete spot on a phospor or instrument, that is not actually a particle, but rather just a point of connection with further quantum systems?
If your detector blocks the slit, then the pattern disappears, because no light from that slit gets through to interfere. But knowing which slit a photon passes through won’t erase an interference pattern.
Actually, any measurement sufficient to tell which slit a photon (or electron or whatever we’re using) passed through will also change the quantum state sufficiently to not produce an interference pattern.
mea maxima culpa
On the scientific, but less mainstream side, biocentrism also explains the double slit experiment result. Biocentrism, applied to Schrodingers cat, would not postulate the cat to be chance percentages alive or dead, no more then light is a wavicle. Biocentrism says: properties of Schrodingers cat, or light, don’t exist outside a human mind contemplating (testing) them. (Or the human mind interpreting the results of the non-human test instruments - no matter how you conduct the experiment, in the end there is a mind involved.)
Therefore the properties of the human mind are more determining what we perceive, then the properties of the experiment.
Google biocentrism and be prepared to have your mind blown.
Such an explanation has a very difficult time accounting for how counterintuitive quantum mechanics is. If the only reality were what we thought it was, wouldn’t we have thought up a quantum mechanics that makes more sense?
I am reminded of various “quantum eraser” experiments where one throws in various configurations of beam splitters, prisms, mirrors, non-linear crystals, etc., after the double-slit, but the end result is always that when photons could have only passed through a specific slit (or whatever that means in this case, since we know even single photons go through both slits simultaneously), the interference pattern disappears. This is might be thought of alongside the possibility of interaction-free measurements, like the famous “quantum bomb detector”.
Well, I expected to get a bunch of New Age woo. Didn’t get that, but had a hard time getting an actual explaination of what biocentrism actually proposes. Mostly I got a bunch of reviews of a book on the subject.
But anyway, if I understand this theory correctly, there would never be any such thing as an overthrow of a scientific paradigm. The majority of scientists think the world works in a certain way and that would determine how the world works. But then perhaps I’m misunderstanding this theory.
Your terms are a little vague. The universe is fully quantum all of the time, so sure, in that sense particles are connections with further quantum systems.
Maybe the confusion here is what a particle really is in QM. Particles in QM are no longer the hard little billiard balls they were in classical mechanics. They aren’t really physical objects at all: instead, they are the representation of a measurement on the system. The answer to a measurement of position (as is the case of a photon hitting a screen) can only be a single point, and so that’s what we see. We call it a particle because it looks like a little ball hit a point on the screen. But really the photon’s wavefunction hit everywhere.
Note that measurement inevitably selects one of the possible outcomes. It’s impossible to overstate how fundamental this is to quantum mechanics (though IMO, it’s better illustrated via spin experiments, not the double slit experiment). The quantum state will evolve further, but only consistently with the results of the observation you performed.
This is fundamentally why observing which slit the photon went through kills interference. Once you know which slit it was, you know it could not have been both slits. And since it only went through one slit, there’s no longer any interference.
That really doesn’t help much, though. After all, the theory itself doesn’t tell us what a measurement is; the same interaction can be described both by applying the usual Schrödinger dynamics, and the collapse dynamics, and both will be appropriate in different contexts.
Besides, there’s nothing about quantum mechanics that prevents us from thinking of particles as localized physical objects—that’s just what Bohmian mechanics does.
The outcome of a position measurement need not be a single point at all. Just think of measuring the position of a water wave by having it impact on a wall: you get the whole wall wet, not just a single spot. That this isn’t what happens in quantum mechanics—that, in other words, something that has to be described as being wave-like in one context fails to be describable in such terms in another—is what the whole kerfluffle is all about.
Again, that depends on the context in which you describe it. If you make a measurement within a sealed lab, then your observation of the system going forward will always be such as to be only compatible with the outcome you observe; if somebody models the whole lab, however, they’ll have to take into account every possible outcome you could’ve observed. Otherwise, you’ll end up with something like Wigner’s interpretation where it’s somehow the experimenter’s consciousness that collapses the wave function; but as far as quantum mechanics is concerned, consciousness—or conscious systems—simply isn’t anything special, or indeed, meaningful.
This just restates the mystery, though. Why would it be the case that what we know affects what happens?
Bohmian Mechanics is great if you want to add a bunch of unnecessary cruft to your theory just because you can’t believe the universe really works this way. It doesn’t really explain the weird stuff, anyway. It’s fine, but so are the other interpretations of QM, including ones where there is no physical particle.
This is where the quantum part comes in. Normally, waves aren’t quantized.
The consciousness aspect isn’t relevant. What is relevant is that our warm meat-brains are very close to classical, and only store classical data. So our memories will only ever be of the particle in one of its many possible discrete states. A quantum computer could (in principle) record the quantum state in its entirety. Could a conscious quantum AI exist? Maybe.
Of course, our brains are still quantum mechanical, and so with sufficient isolation a Schrödinger’s cat scenario is possible. Though my opinion is that there’s inherently enough background noise in the universe to make this not actually possible in reality. And in any case, living me and about-to-expire me still only remember a single history.
This, again, is better justified with the example of quantum spin. I’ll try to write something up later, but for now I’ll emphasize that it’s utterly fundamental to QM. The very nature of a measurement in QM is that it modifies the state of the system it’s measuring.
All right, the spin thing I’ve been talking about.
We’re all familiar with the classical example of a spinning top. The spin, as we’ll call it (more properly called angular momentum) can be measured along an axis, and is basically the amount (and distribution) of mass whirling around multiplied by the rate of whirling. We can choose a direction for the whirling–say, counter-clockwise is positive and clockwise negative.
So a top spinning in the normal way, and counter-clockwise, measured along the Z axis (that is, the axis pointing up) would have some positive amount of spin. Change directions, but keep the rate the same, and you get the negative of the number. What if we measure along the X axis? Well, it’s not spinning at all in that direction–so zero. Maybe the top falls over eventually, in which case there will be a small amount of spin along X. What amount an intermediate axis, like halfway between X and Z? The spin will likewise be intermediate–if you look down the axis, there is still matter whirling around, but in a tight elliptical patter, which makes for a lower spin.
All totally consistent and sound. So how is QM different?
Particles in QM also have a notion of spin, which like the classical version is just a name for angular momentum. But QM spin is quantized, and can only take on discrete values. What does that mean?
For an electron, it means that the spin can only ever be +1 and -1. That’s it. It can’t be 0.7. It can’t be -3.14. And it can’t be 0. There are only two values it can be, neither of them zero. It doesn’t matter which axis you measure, what state the particle is in, or anything else. +1 and -1 are the only possible results of the measurement.
There is no use in asking how this can be. It just is. Experiment after experiment have confirmed it. It is one of the single strangest facts of the universe that I know of. And the only way to make progress in explaining this fact is to set aside the feeling that it is somehow wrong for the universe to behave this way.
It may not be readily apparent how weird this is, so let’s do some thought experiments. We have a particle, and we measure it along the Z axis. We’ll say that the reading is +1. We can do this same reading over and over, and it will always give the same answer of +1. We can measure along -Z, and get the result of -1. Maybe it’s spinning upright like a top?
To test, we measure along X. Again, we get +1. Are we in trouble? Not necessarily: maybe it’s spinning on an axis halfway between X and Z. By the laws of trigonometry, the amount of spin should be something like +1.4 on an XZ axis. So we measure again.
The result is -1. What the hell? It’s the wrong magnitude and the wrong sign. We need to approach this more diligently.
We do some more careful measurements. We start by measuring on the Z axis until we get an answer of +1. It sometimes takes a couple of tries, but we’ll ignore that for now. Then we measure along X (and then Y, in a later set of experiments). We find that if Z started as +1, then when we measure later along X, there is a 50%/50% mix between +1 and -1. The result is random and evenly distributed. The same is true of Z-Y. But Z-Z measurements are always the same.
You think that maybe our measurements are inherently unreliable. That there’s some underlying real spin direction, but there’s some inherent noise that gives us the wrong answer sometimes. But then, how are Z-Z experiments 100% reliable? You persist, and try Z-X-X-X experiments. The first X is random, but then later ones confirm the first measurement.
There’s no way to explain this except that measurements inherently change the state of the system. It’s not a matter of just being delicate enough–it’s the very nature of measurement that’s different in QM.
The problem, fundamentally, is that quantized measurements can conflict with each other. Classical spin is continuous–if we measure Z as +1, then what does it measure just a little off to the side? Just a little less; maybe +0.98. And then measure a little farther off, and get +0.96. It all works out. But if the answer is always +1 or -1, how does one resolve the conflict? We ruled out that the measurements were noisy. The only alternative is the measurements are correct each time we perform them. And the only way that’s true is that if there’s a conflict between the old and the new, then it must have been the measurement itself that put us into the new state.
Again, no one knows how this can be true. It just is. The universe doesn’t have to care that we don’t like it.
Why not a parve?
Sure, but the fact that it gives a perfectly consistent picture of quantum mechanics does allow us to check what we’re actually licensed to conclude from its phenomenology. Case in point:
Most of the above is fine (leaving aside the issue that it’s more usual to assign a spin +/- 1/2 to an electron; a spin-1 particle can, after all, have spin 0), but this doesn’t follow. It’s easy to see using Bohmian mechanics. There, a particle is just a dot within a wave packet, say. Now, something like a Stern-Gerlach experiment to measure spin will split this packet (approximately) into two; only one of which will contain the particle. So, that we always just get either of two values is readily explainable. And so’s the other phenomenology: re-measuring the wave packet containing the particle will reveal its presence again and again; measuring it in another direction will just yield another splitting of the wave packet, with the particle ending up in either part. That’s sufficient to account for the full phenomenology, and doesn’t introduce any state change upon measurement—measurement merely reveals in which ‘part’ of the wave packet the particle ended up.
But the question is how something like a classical outcome ever occurs. By having to appeal to classicality, you never get around to actually addressing the issue.
And it’s in principle possible to carry out an interference measurement on the whole lab, and measure the interference fringes; the presence of these fringes will then reveal that the experimenter in the lab can’t be in either of the states (sees electron spin up) or (sees electron spin down), as there wouldn’t be any interference in that case. So if you postulate that there couldn’t even in principle be such interference terms, you’re actually arguing for a modification of quantum theory—either something like a spontaneous collapse theory, or something like Wigner’s ‘Consciousness Causes Collapse’.
This wouldn’t help. You’d merely set up correlations with the noise; so, you could just include that in the measurement, and come to the same conclusion.
I don’t get the debate over particle and wave duality.
For a long time, I have thought that the phenomena being discussed are very nicely described by fields. Field theory. Quantum field theory, in this case, quantum electrodynamics.
That is, all of the things we observe in these experiments are phenomena that happen in the (quantum) electromagnetic field.
I don’t find the idea of a relativistic quantum electromagnetic field any more strange than the idea of a classical Newtonian gravitational field. Even though most of the mathematics are way over my head. In a Newtonian gravitational field, you can derive formulas that describe what happens. So you can in a relativistic quantum electromagnetic field. Neither am I bothered by living in a Minkowski space, going through a Lorenz transformation on my way to work, seeing electrons behave according to spinor mathematics, etc.
Still the whole wave particle duality debate has continued raging long after quantum electrodynamics was discovered (invented?).
What am I missing? We’re just seeing the field working the way it should.
Yup, you can treat it as a field, with waves and excitations in the field, and do all of the calculations (which we know how to do), and get results consistent with all of the experiments we’ve done. In that sense, we understand quantum mechanics just fine. But it still doesn’t “feel right” to us. Which, of course, just tells us something about the limitations of our brains, not anything about how the world really works: Being able to “understand” quantum mechanics wouldn’t help us to avoid predators, or to find ripe fruit, or to jockey for status in our social groups, so evolution didn’t give us brains capable of doing that.
So I went back to the Wiki article about Wave–particle duality
Reading that page ISTM that mostly old people (that is, people who were old in 1920!) and crazy people seem to have a need to come up with an “interpretation” of quantum mechanics.
The correct interpretation is actually mentioned on that page amid all the nonsense. Twice. Like when it says, “Following the development of quantum field theory the ambiguity disappeared…” so there does seem to be agreement that the whole “debate” is already settled.
Now only for the lay press to catch up.
Quantum field theory presents its own set of interpretational troubles, though, and few think they’re any simpler than those of QM. For instance, the field of QFT is not very much like that of e. g. classical electrodynamics. Classical fields assign a value of a physical quantity to any point in space; the way that quantity varies over space (and through time) then includes all the physics. A simple example would be temperature: there’s a single number assigned to every point, and that’s that.
Quantum fields aren’t like that. At all. What’s assigned there to every spacetime point isn’t the value of a physical quantity, but rather, an operator. An operator is something that takes a mathematical object (a state vector, in this case) and returns another—performing a certain transformation (something like a generalized rotation, for instance). These operators are observables in quantum physics—their expectation values dictate what sorts of values are presented in measurements. The quantum field configuration then isn’t the state of a physical system, but something that acts on the state, which is a wholly abstract notion.
So, consequently, there are various proposals as to how to interpret this sort of situation—one being (various versions of) a field interpretation, and another being (various versions of) a particle interpretation. There are other, more abstract proposals, including the idea that QFT isn’t really about things, as such, but perhaps rather about relations, or maybe about bundles of properties, or something like that.
At any rate, nobody seems yet to have found a convincing way to appeal to QFT to make the interpretation of QM any easier; you may get different issues, but that’s not to say they’re any more straightforward.
It is somewhat of a fallacy to account philosophically for classical outcomes when-- as all experiments show!-- all the outcomes are quantum.